Risk of natural disturbances makes future contribution of Canada's forests to the global carbon cycle highly uncertain

A large carbon sink in northern land surfaces inferred from global carbon cycle inversion models led to concerns during Kyoto Protocol negotiations that countries might be able to avoid efforts to reduce fossil fuel emissions by claiming large sinks in their managed forests. The greenhouse gas balance of Canada's managed forest is strongly affected by naturally occurring fire with high interannual variability in the area burned and by cyclical insect outbreaks. Taking these stochastic future disturbances into account, we used the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) to project that the managed forests of Canada could be a source of between 30 and 245 Mt CO(2)e yr(-1) during the first Kyoto Protocol commitment period (2008-2012). The recent transition from sink to source is the result of large insect outbreaks. The wide range in the predicted greenhouse gas balance (215 Mt CO(2)e yr(-1)) is equivalent to nearly 30% of Canada's emissions in 2005. The increasing impact of natural disturbances, the two major insect outbreaks, and the Kyoto Protocol accounting rules all contributed to Canada's decision not to elect forest management. In Canada, future efforts to influence the carbon balance through forest management could be overwhelmed by natural disturbances. Similar circumstances may arise elsewhere if global change increases natural disturbance rates. Future climate mitigation agreements that do not account for and protect against the impacts of natural disturbances, for example, by accounting for forest management benefits relative to baselines, will fail to encourage changes in forest management aimed at mitigating climate change.     

A large carbon sink in the woody biomass of Northern forests

The terrestrial carbon sink, as of yet unidentified, represents 15-30% of annual global emissions of carbon from fossil fuels and industrial activities. Some of the missing carbon is sequestered in vegetation biomass and, under the Kyoto Protocol of the United Nations Framework Convention on Climate Change, industrialized nations can use certain forest biomass sinks to meet their greenhouse gas emissions reduction commitments. Therefore, we analyzed 19 years of data from remote-sensing spacecraft and forest inventories to identify the size and location of such sinks. The results, which cover the years 1981-1999, reveal a picture of biomass carbon gains in Eurasian boreal and North American temperate forests and losses in some Canadian boreal forests. For the 1.42 billion hectares of Northern forests, roughly above the 30th parallel, we estimate the biomass sink to be 0.68 +/- 0.34 billion tons carbon per year, of which nearly 70% is in Eurasia, in proportion to its forest area and in disproportion to its biomass carbon pool. The relatively high spatial resolution of these estimates permits direct validation with ground data and contributes to a monitoring program of forest biomass sinks under the Kyoto protocol.     

An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker

We present an estimate of net CO(2) exchange between the terrestrial biosphere and the atmosphere across North America for every week in the period 2000 through 2005. This estimate is derived from a set of 28,000 CO(2) mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO(2) called CarbonTracker. By design, the surface fluxes produced in CarbonTracker are consistent with the recent history of CO(2) in the atmosphere and provide constraints on the net carbon flux independent from national inventories derived from accounting efforts. We find the North American terrestrial biosphere to have absorbed -0.65 PgC/yr (1 petagram = 10(15) g; negative signs are used for carbon sinks) averaged over the period studied, partly offsetting the estimated 1.85 PgC/yr release by fossil fuel burning and cement manufacturing. Uncertainty on this estimate is derived from a set of sensitivity experiments and places the sink within a range of -0.4 to -1.0 PgC/yr. The estimated sink is located mainly in the deciduous forests along the East Coast (32%) and the boreal coniferous forests (22%). Terrestrial uptake fell to -0.32 PgC/yr during the large-scale drought of 2002, suggesting sensitivity of the contemporary carbon sinks to climate extremes. CarbonTracker results are in excellent agreement with a wide collection of carbon inventories that form the basis of the first North American State of the Carbon Cycle Report (SOCCR), to be released in 2007. All CarbonTracker results are freely available at http://carbontracker.noaa.gov.     

Effect of increasing CO2 on the terrestrial carbon cycle

Feedbacks from the terrestrial carbon cycle significantly affect future climate change. The CO₂ concentration dependence of global terrestrial carbon storage is one of the largest and most uncertain feedbacks. Theory predicts the CO₂ effect should have a tropical maximum, but a large terrestrial sink has been contradicted by analyses of atmospheric CO₂ that do not show large tropical uptake. Our results, however, show significant tropical uptake and, combining tropical and extratropical fluxes, suggest that up to 60% of the present-day terrestrial sink is caused by increasing atmospheric CO₂. This conclusion is consistent with a validated subset of atmospheric analyses, but uncertainty remains. Improved model diagnostics and new space-based observations can reduce the uncertainty of tropical and temperate zone carbon flux estimates. This analysis supports a significant feedback to future atmospheric CO₂ concentrations from carbon uptake in terrestrial ecosystems caused by rising atmospheric CO₂ concentrations. This feedback will have substantial tropical contributions, but the magnitude of future carbon uptake by tropical forests also depends on how they respond to climate change and requires their protection from deforestation.In projections of future climate, the carbon cycle is second only to physical climate sensitivity itself in contributing uncertainty (1). Earth system model uncertainty has increased as more mechanisms have been incorporated into a growing number of increasingly sophisticated models. Terrestrial ecosystem feedbacks to atmospheric CO₂ concentration result from two mechanisms, direct effects of CO₂ on photosynthesis and effects of climate change on photosynthesis, respiration, and disturbance (2). The CO₂ effect, used here to describe the effect of increasing atmospheric CO₂ on terrestrial carbon storage by increasing photosynthetic rates, is also known as the β effect (3, 4). The effects of CO₂ on carbon uptake occur at the enyzmatic and stomatal scales but impact the global carbon cycle.The CO₂ effect on terrestrial carbon storage is a key potential negative feedback to future climate, and in models of the present, it is the largest carbon cycle feedback (5, 6). In simulations of the next century, the CO₂ effect is four times larger than the climate effect on terrestrial carbon storage and twice as uncertain (4). Land use also creates large fluxes, but these are not driven by CO₂ or climate directly and so are not feedbacks. In models of the future, the biosphere operates as a net sink, reducing the climate impact of fossil fuel and deforestation emissions, until positive feedbacks from climate change [reduced productivity, increased respiration, or dieback (7)] and land use emissions exceed the CO₂ effect. The magnitude of this negative feedback is crucial to simulating future climate, but because observational constraints on the CO₂ effect are limited, the effects of CO₂ remain controversial. The effects of CO₂ are known mainly from small-scale experimental studies, ranging from single-leaf experiments through to ecosystem-scale experiments with a spatial scale of hundreds of meters (8), but predictions from theory of a large tropical effect of CO₂ have appeared to be inconsistent with global patterns of atmospheric CO₂ (6).Photosynthesis increases with increasing CO₂ following a Michaelis−Menton curve, and this effect grows stronger at higher temperatures, implying, all else being equal, larger effects in warmer climates (9–11), especially in the tropics. Many factors control the relationship between increased photosynthetic rate and carbon storage, including how fixed carbon is allocated to plant tissues and soils with different residence times, the development of progressive nitrogen limitation, interactions with water or light limitation, and many other biological responses (12). Theory and experiments agree in suggesting a CO₂-driven net sink that should be roughly proportional to overall productivity (13) leading to a large sink in the tropics, a prediction that should be testable with global observations (11).     

Ecological contingency in the effects of climatic warming on forest herb communities

Downscaling from the predictions of general climate models is critical to current strategies for mitigating species loss caused by climate change. A key impediment to this downscaling is that we lack a fully developed understanding of how variation in physical, biological, or land-use characteristics mediates the effects of climate change on ecological communities within regions. We analyzed change in understory herb communities over a 60-y period (1949/1951-2007/2009) in a complex montane landscape (the Siskiyou Mountains, Oregon) where mean temperatures have increased 2 °C since 1948, similar to projections for other terrestrial communities. Our 185 sites included primary and secondary-growth lower montane forests (500-1.200 m above sea level) and primary upper montane to subalpine forests (1,500-2,100 m above sea level). In lower montane forests, regardless of land-use history, we found multiple herb-community changes consistent with an effectively drier climate, including lower mean specific leaf area, lower relative cover by species of northern biogeographic affinity, and greater compositional resemblance to communities in southerly topographic positions. At higher elevations we found qualitatively different and more modest changes, including increases in herbs of northern biogeographic affinity and in forest canopy cover. Our results provide community-level validation of predicted nonlinearities in climate change effects.     

A rapid upward shift of a forest ecotone during 40 years of warming in the Green Mountains of Vermont

Detecting latitudinal range shifts of forest trees in response to recent climate change is difficult because of slow demographic rates and limited dispersal but may be facilitated by spatially compressed climatic zones along elevation gradients in montane environments. We resurveyed forest plots established in 1964 along elevation transects in the Green Mountains (Vermont) to examine whether a shift had occurred in the location of the northern hardwood-boreal forest ecotone (NBE) from 1964 to 2004. We found a 19% increase in dominance of northern hardwoods from 70% in 1964 to 89% in 2004 in the lower half of the NBE. This shift was driven by a decrease (up to 76%) in boreal and increase (up to 16%) in northern hardwood basal area within the lower portions of the ecotone. We used aerial photographs and satellite imagery to estimate a 91- to 119-m upslope shift in the upper limits of the NBE from 1962 to 2005. The upward shift is consistent with regional climatic change during the same period; interpolating climate data to the NBE showed a 1.1 degrees C increase in annual temperature, which would predict a 208-m upslope movement of the ecotone, along with a 34% increase in precipitation. The rapid upward movement of the NBE indicates little inertia to climatically induced range shifts in montane forests; the upslope shift may have been accelerated by high turnover in canopy trees that provided opportunities for ingrowth of lower elevation species. Our results indicate that high-elevation forests may be jeopardized by climate change sooner than anticipated.     

Recent burning of boreal forests exceeds fire regime limits of the past 10,000 years

Wildfire activity in boreal forests is anticipated to increase dramatically, with far-reaching ecological and socioeconomic consequences. Paleorecords are indispensible for elucidating boreal fire regime dynamics under changing climate, because fire return intervals and successional cycles in these ecosystems occur over decadal to centennial timescales. We present charcoal records from 14 lakes in the Yukon Flats of interior Alaska, one of the most flammable ecoregions of the boreal forest biome, to infer causes and consequences of fire regime change over the past 10,000 y. Strong correspondence between charcoal-inferred and observational fire records shows the fidelity of sedimentary charcoal records as archives of past fire regimes. Fire frequency and area burned increased ∼6,000–3,000 y ago, probably as a result of elevated landscape flammability associated with increased Picea mariana in the regional vegetation. During the Medieval Climate Anomaly (MCA; ∼1,000–500 cal B.P.), the period most similar to recent decades, warm and dry climatic conditions resulted in peak biomass burning, but severe fires favored less-flammable deciduous vegetation, such that fire frequency remained relatively stationary. These results suggest that boreal forests can sustain high-severity fire regimes for centuries under warm and dry conditions, with vegetation feedbacks modulating climate–fire linkages. The apparent limit to MCA burning has been surpassed by the regional fire regime of recent decades, which is characterized by exceptionally high fire frequency and biomass burning. This extreme combination suggests a transition to a unique regime of unprecedented fire activity. However, vegetation dynamics similar to feedbacks that occurred during the MCA may stabilize the fire regime, despite additional warming.     

Transient climate-carbon simulations of planetary geoengineering

Geoengineering (the intentional modification of Earth's climate) has been proposed as a means of reducing CO2-induced climate warming while greenhouse gas emissions continue. Most proposals involve managing incoming solar radiation such that future greenhouse gas forcing is counteracted by reduced solar forcing. In this study, we assess the transient climate response to geoengineering under a business-as-usual CO2 emissions scenario by using an intermediate-complexity global climate model that includes an interactive carbon cycle. We find that the climate system responds quickly to artificially reduced insolation; hence, there may be little cost to delaying the deployment of geoengineering strategies until such a time as "dangerous" climate change is imminent. Spatial temperature patterns in the geoengineered simulation are comparable with preindustrial temperatures, although this is not true for precipitation. Carbon sinks in the model increase in response to geoengineering. Because geoengineering acts to mask climate warming, there is a direct CO2-driven increase in carbon uptake without an offsetting temperature-driven suppression of carbon sinks. However, this strengthening of carbon sinks, combined with the potential for rapid climate adjustment to changes in solar forcing, leads to serious consequences should geoengineering fail or be stopped abruptly. Such a scenario could lead to very rapid climate change, with warming rates up to 20 times greater than present-day rates. This warming rebound would be larger and more sustained should climate sensitivity prove to be higher than expected. Thus, employing geoengineering schemes with continued carbon emissions could lead to severe risks for the global climate system.     

Permafrost carbon-climate feedbacks accelerate global warming

Permafrost soils contain enormous amounts of organic carbon, which could act as a positive feedback to global climate change due to enhanced respiration rates with warming. We have used a terrestrial ecosystem model that includes permafrost carbon dynamics, inhibition of respiration in frozen soil layers, vertical mixing of soil carbon from surface to permafrost layers, and CH(4) emissions from flooded areas, and which better matches new circumpolar inventories of soil carbon stocks, to explore the potential for carbon-climate feedbacks at high latitudes. Contrary to model results for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), when permafrost processes are included, terrestrial ecosystems north of 60°N could shift from being a sink to a source of CO(2) by the end of the 21st century when forced by a Special Report on Emissions Scenarios (SRES) A2 climate change scenario. Between 1860 and 2100, the model response to combined CO(2) fertilization and climate change changes from a sink of 68 Pg to a 27 + -7 Pg sink to 4 + -18 Pg source, depending on the processes and parameter values used. The integrated change in carbon due to climate change shifts from near zero, which is within the range of previous model estimates, to a climate-induced loss of carbon by ecosystems in the range of 25 + -3 to 85 + -16 Pg C, depending on processes included in the model, with a best estimate of a 62 + -7 Pg C loss. Methane emissions from high-latitude regions are calculated to increase from 34 Tg CH(4)/y to 41-70 Tg CH(4)/y, with increases due to CO(2) fertilization, permafrost thaw, and warming-induced increased CH(4) flux densities partially offset by a reduction in wetland extent.     

The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off

Forest ecosystems store approximately 45% of the carbon found in terrestrial ecosystems, but they are sensitive to climate-induced dieback. Forest die-off constitutes a large uncertainty in projections of climate impacts on terrestrial ecosystems, climate-ecosystem interactions, and carbon-cycle feedbacks. Current understanding of the physiological mechanisms mediating climate-induced forest mortality limits the ability to model or project these threshold events. We report here a direct and in situ study of the mechanisms underlying recent widespread and climate-induced trembling aspen (Populus tremuloides) forest mortality in western North America. We find substantial evidence of hydraulic failure of roots and branches linked to landscape patterns of canopy and root mortality in this species. On the contrary, we find no evidence that drought stress led to depletion of carbohydrate reserves. Our results illuminate proximate mechanisms underpinning recent aspen forest mortality and provide guidance for understanding and projecting forest die-offs under climate change.     

Tropical nighttime warming as a dominant driver of variability in the terrestrial carbon sink

The terrestrial biosphere is currently a strong carbon (C) sink but may switch to a source in the 21st century as climate-driven losses exceed CO₂-driven C gains, thereby accelerating global warming. Although it has long been recognized that tropical climate plays a critical role in regulating interannual climate variability, the causal link between changes in temperature and precipitation and terrestrial processes remains uncertain. Here, we combine atmospheric mass balance, remote sensing-modeled datasets of vegetation C uptake, and climate datasets to characterize the temporal variability of the terrestrial C sink and determine the dominant climate drivers of this variability. We show that the interannual variability of global land C sink has grown by 50–100% over the past 50 y. We further find that interannual land C sink variability is most strongly linked to tropical nighttime warming, likely through respiration. This apparent sensitivity of respiration to nighttime temperatures, which are projected to increase faster than global average temperatures, suggests that C stored in tropical forests may be vulnerable to future warming.Terrestrial ecosystems have been a substantial net sink of anthropogenic carbon (C) emissions since the 1960s (1–4), but the terrestrial C sink could switch to a C source in the 21st century, resulting in a positive C cycle-climate feedback that would accelerate global surface warming with potentially major consequences for the biosphere (5–7). The interannual variability of the terrestrial C sink can help constrain our understanding of C/climate feedbacks and identify regions and mechanisms of the terrestrial C cycle that are most sensitive to climate parameters, shedding light on the future of the sink and its possible transition to a source (8). Currently, several major drivers have been shown to be correlated with the interannual variability of the terrestrial C sink, including (i) tropical temperature, which is tightly coupled to interannual variability in the atmospheric growth rate (AGR) of CO₂ (8, 9); (ii) tropical drought stress, including major droughts in the Amazon (10–12), which has been suggested to underlie increasing sensitivity of the AGR to tropical temperature over the period from 1959–2010 (13); (iii) temperature and precipitation variability in semiarid regions (14, 15); and (iv) average minimum daily (hereafter “nighttime”) temperatures, which studies of several local field sites in the tropics have found play a major role in interannual productivity (16–18).Determining the mechanism underlying the interannual variability of the terrestrial C sink, including the relative roles of precipitation vs. temperature stress and their effects on gross primary productivity (GPP) vs. total respiration (both autotrophic and heterotrophic; R), is critical to predict the sink’s future and to improve Earth system models. Here, we quantify changes in the interannual variability of the terrestrial C sink over the past half-century and then statistically evaluate four hypotheses that the variability of the terrestrial sink is most strongly influenced by (i) tropical mean temperature, (ii) tropical precipitation, (iii) precipitation and temperature in semiarid regions, and (iv) nighttime tropical temperatures. We combine multiple simulations from an atmospheric mass balance of the land C sink [net ecosystem exchange (NEE)] from 1959 to 2010, remote sensing-modeled datasets of vegetation greenness and GPP from 1982 to 2010, and global gridded climate datasets to constrain globally the fundamental equation NEE = GPP − R and the relative sensitivities of each component to temperature and precipitation. We draw on a combination of model selection and partial correlation analysis to provide relative likelihood estimates of each driver and to account for covariation between predictor variables (e.g., tropical mean temperature vs. nighttime temperature).     

Biogeographic variation in evergreen conifer needle longevity and impacts on boreal forest carbon cycle projections

Leaf life span is an important plant trait associated with interspecific variation in leaf, organismal, and ecosystem processes. We hypothesized that intraspecific variation in gymnosperm needle traits with latitude reflects both selection and acclimation for traits adaptive to the associated temperature and moisture gradient. This hypothesis was supported, because across 127 sites along a 2,160-km gradient in North America individuals of Picea glauca, Picea mariana, Pinus banksiana, and Abies balsamea had longer needle life span and lower tissue nitrogen concentration with decreasing mean annual temperature. Similar patterns were noted for Pinus sylvestris across a north–south gradient in Europe. These differences highlight needle longevity as an adaptive feature important to ecological success of boreal conifers across broad climatic ranges. Additionally, differences in leaf life span directly affect annual foliage turnover rate, which along with needle physiology partially regulates carbon cycling through effects on gross primary production and net canopy carbon export. However, most, if not all, global land surface models parameterize needle longevity of boreal evergreen forests as if it were a constant. We incorporated temperature-dependent needle longevity and %nitrogen, and biomass allocation, into a land surface model, Community Atmosphere Biosphere Land Exchange, to assess their impacts on carbon cycling processes. Incorporating realistic parameterization of these variables improved predictions of canopy leaf area index and gross primary production compared with observations from flux sites. Finally, increasingly low foliage turnover and biomass fraction toward the cold far north indicate that a surprisingly small fraction of new biomass is allocated to foliage under such conditions.The boreal forest is an enormous terrestrial biome, characterized by long, cold winters, low rates of productivity and nutrient cycling (1–4), and low richness of the dominant tree species. All of these tree species have large geographic ranges (5), and ecological theory suggests that populations should vary both genotypically and phenotypically in response to such variation. In fact, it has been long recognized that terrestrial plants in general, and boreal conifers specifically, exhibit ecotypic population differentiation that reflects influence of site origin on a variety of chemical, morphological, physiological, and allometric traits (6–12). It is also well known that biogeographic variation in the environment influences the phenotypic traits plastically realized by species across their ranges. Both phenotypic and genotypic processes therefore contribute to observed variation in traits across environmental gradients. Hence, across the boreal biome, conditions that lead to slow growth, such as short growing season, cool summer temperatures, and low soil nutrient supply (1, 2, 13), should be reflected by shifts toward more conservative traits (such as long leaf life span and low nutrient concentrations) considered to be advantageous under such conditions (14, 15). However, although evidence that ecotypic variation occurs is abundant (7, 8, 10), comprehensive characterization of ecologically important intraspecific variation across biogeographic gradients is lacking (16).Global land surface models that simulate carbon cycling of the world’s terrestrial biomes face distinct challenges for both species-poor biomes, such as the boreal forest, and species-rich biomes, such as tropical rain forests. For the latter, the challenge is to sufficiently well parameterize leaf and canopy properties given enormous species diversity within and among sites (e.g., >500 species for <1,000 individuals in a given several-hectare area, with large species turnover at 100-km scale). In contrast, for boreal forests, the task is seemingly easy: Only a small number of species dominate the entire boreal forest in each continent. Moreover, the same genera dominate the circumboreal region. However, although leaf and canopy properties vary with climate and geographic location, this variation is usually ignored in biome-scale or global models, because of a lack of systematic understanding of that variation.Despite long recognition of ecotypic variation in boreal conifers, certain broad-scale aspects of physiological macroecology remain poorly quantified. One trait known to differ intraspecifically in evergreen conifers with climate and latitude is the needle life span (NL) (synonymous with needle longevity) (17, 18). In general, individuals growing in colder locations tend to have longer NL (17–19), but important patterns and consequences are unknown, such as the following. (i) What is the shape of the NL relation to geographic climate variation? (ii) Are proportional shifts in NL for a given climate gradient similar among species? (iii) Do other leaf traits often associated interspecifically with leaf life span (e.g., refs. 15 and 20) covary with NL intraspecifically in a similar fashion? (iv) What are the consequences of geographic patterns of needle traits to ecosystem and regional carbon cycling? To improve our understanding of these issues, we measured needle traits of the four dominant evergreen conifers from Minnesota to northern Canada, compiled similar data for the dominant evergreen species in Eurasia (Pinus sylvestris), characterized the relations of needle traits with climate variables for all five species, and incorporated such findings into a land surface model [Community Atmosphere Biosphere Land Exchange (CABLE)] that simulated biome-wide carbon and nitrogen cycling.We sampled foliage from naturally grown trees (range of height 2.5–5 m) at 127 sites ranging from Minnesota to Ontario, Manitoba, Saskatchewan, Alberta, and the Northwest Territories, in central Canada. We sampled sunlit upper canopy branches of trees ranging from 2.5 to 5 m in height, to standardize tree size and age across sites, as well as the light environment of the sampled branches. The latter was of paramount concern, because needle longevity in the crown of a conifer increases systematically with shading (by as much as 50–60% for spruce and fir). The four species were present (and sampled) at 83 (Pinus banksiana), 50 (Picea mariana), 45 (Picea glauca), and 21 (Abies balsamea) of the sites, respectively. We also use a literature compilation for Pinus sylvestris across northern Fenno-Scandinavia to extend our analysis beyond North America. Because several climatic factors covary with latitude and are hypothesized to be the mechanism behind the latitudinal pattern, we focus our analyses and interpretation on climate, not latitude. Additionally, it is difficult to ascertain which of the related climate metrics (including annual and seasonal measures of temperature and precipitation) are responsible for the observed latitudinal patterns (Methods and Supporting Information). Because mean annual temperature (MAT) was identified in the model selection process as the climate metric that explained the most variance in NL, we use it in the results presented herein. However, we discuss the ways in which multiple aspects of climate, and associated soil resource availability, can combine to influence these results, below and in Supporting Information.     

Potential responses of soil organic carbon to global environmental change

Recent improvements in our understanding of the dynamics of soil carbon have shown that 20-40% of the approximately 1,500 Pg of C stored as organic matter in the upper meter of soils has turnover times of centuries or less. This fast-cycling organic matter is largely comprised of undecomposed plant material and hydrolyzable components associated with mineral surfaces. Turnover times of fast-cycling carbon vary with climate and vegetation, and range from <20 years at low latitudes to >60 years at high latitudes. The amount and turnover time of C in passive soil carbon pools (organic matter strongly stabilized on mineral surfaces with turnover times of millennia and longer) depend on factors like soil maturity and mineralogy, which, in turn, reflect long-term climate conditions. Transient sources or sinks in terrestrial carbon pools result from the time lag between photosynthetic uptake of CO2 by plants and the subsequent return of C to the atmosphere through plant, heterotrophic, and microbial respiration. Differential responses of primary production and respiration to climate change or ecosystem fertilization have the potential to cause significant interrannual to decadal imbalances in terrestrial C storage and release. Rates of carbon storage and release in recently disturbed ecosystems can be much larger than rates in more mature ecosystems. Changes in disturbance frequency and regime resulting from future climate change may be more important than equilibrium responses in determining the carbon balance of terrestrial ecosystems.     

Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought

Climate change is progressively increasing severe drought events in the Northern Hemisphere, causing regional tree die-off events and contributing to the global reduction of the carbon sink efficiency of forests. There is a critical lack of integrated community-wide assessments of drought-induced responses in forests at the macroecological scale, including defoliation, mortality, and food web responses. Here we report a generalized increase in crown defoliation in southern European forests occurring during 1987-2007. Forest tree species have consistently and significantly altered their crown leaf structures, with increased percentages of defoliation in the drier parts of their distributions in response to increased water deficit. We assessed the demographic responses of trees associated with increased defoliation in southern European forests, specifically in the Iberian Peninsula region. We found that defoliation trends are paralleled by significant increases in tree mortality rates in drier areas that are related to tree density and temperature effects. Furthermore, we show that severe drought impacts are associated with sudden changes in insect and fungal defoliation dynamics, creating long-term disruptive effects of drought on food webs. Our results reveal a complex geographical mosaic of species-specific responses to climate change-driven drought pressures on the Iberian Peninsula, with an overwhelmingly predominant trend toward increased drought damage.     

Forests synchronize their growth in contrasting Eurasian regions in response to climate warming

Forests play a key role in the carbon balance of terrestrial ecosystems. One of the main uncertainties in global change predictions lies in how the spatiotemporal dynamics of forest productivity will be affected by climate warming. Here we show an increasing influence of climate on the spatial variability of tree growth during the last 120 y, ultimately leading to unprecedented temporal coherence in ring-width records over wide geographical scales (spatial synchrony). Synchrony in growth patterns across cold-constrained (central Siberia) and drought-constrained (Spain) Eurasian conifer forests have peaked in the early 21st century at subcontinental scales (∼1,000 km). Such enhanced synchrony is similar to that observed in trees co-occurring within a stand. In boreal forests, the combined effects of recent warming and increasing intensity of climate extremes are enhancing synchrony through an earlier start of wood formation and a stronger impact of year-to-year fluctuations of growing-season temperatures on growth. In Mediterranean forests, the impact of warming on synchrony is related mainly to an advanced onset of growth and the strengthening of drought-induced growth limitations. Spatial patterns of enhanced synchrony represent early warning signals of climate change impacts on forest ecosystems at subcontinental scales.Understanding how climate change affects forests across multiple spatiotemporal scales is important for anticipating its impacts on terrestrial ecosystems. Increases in atmospheric CO₂ concentration and shifts in phenology (1–3) could favor tree growth by enhancing photosynthesis and extending the effective growing period, respectively (4). Conversely, recent warming could increase respiration rates and, together with increasing heat and drought stresses, exert negative impacts on forest productivity (5, 6). Given the uncertainty as to what extent enhanced carbon uptake could be offset by the detrimental effects of warming on tree performance, the actual consequences of climate change on forest carbon cycling remain under debate. Notably, climate change has a stronger impact on forests constrained by climatic stressors, such as suboptimal temperatures or water shortage (7). As high-resolution repositories of biological responses to the environment, dendrochronological archives can be used to monitor this impact (8).The concept of spatial synchrony in tree growth refers to the extent of coincident changes in ring-width patterns among geographically disjunct tree populations (9). Climatic restrictions tend to strengthen growth–climate relationships, resulting in enhanced common ring-width signals (i.e., more synchronous tree growth). Thus, regional bioclimatic patterns can be delineated by identifying groups of trees whose growth is synchronously driven by certain climatic constraints (10, 11). Previous synthesis studies have provided evidence for globally coherent multispecies responses to climate change in natural systems, including forests, with a focus on the role of increasingly warmer temperatures (12, 13). Indeed, climate has changed markedly over the last decades, prompting an array of physiological reactions in trees that could strengthen growth–climate relationships, thereby enhancing spatial synchrony. Such tree responses may be linked to global shifts in the timing of plant activity (2), drought stress in mid-latitudes (6, 14), or an uncoupling of air and soil thermal regimes in the early growing season (15) and direct heat stress (16) in high latitudes, among other factors. Changing tree growth patterns associated with enhanced synchrony in response to warming have been reported at small geographical scales (<150 km) (14–18, but see ref. 19); however, an extended examination of synchrony patterns is currently lacking for large (i.e., subcontinental) areas.To determine whether climate warming and increased variability (1) lead to more synchronous tree growth, we examined changes in spatial synchrony over the last 120 y across subcontinental areas by using a comprehensive network of 93 ring-width chronologies from six different conifer species across two climatically contrasting Eurasian biomes: boreal forests in central Siberia (n = 45 chronologies) and Mediterranean forests in Spain (n = 48 chronologies) (SI Appendix, Fig. S1 and Table S1). Central Siberia has a severe continental climate with a prolonged cold season, large intra-annual temperature variations, and moderate precipitation. Spain is dominated by a typical Mediterranean climate, with mild (coast) to cool (inland) wet winters and summer droughts. Thus, temperature exerts the main climatic control over productivity in boreal forests, whereas Mediterranean forests are primarily water-limited (SI Appendix, section 1A).Temporal changes in spatial synchrony (hereinafter, â_C) are quantified using a novel mixed model framework (20). This methodology has two fundamental advantages for dendroscience (21) over other alternative approaches useful for interpreting population dynamics in ecology (22) or patterns of environmental synchrony (23): (i) it is capable of dealing with partially overlapping chronologies, yielding valid inferences of spatial synchrony for large areas in which ring-width data are available but covering different time periods, and (ii) it is highly flexible to fit general statistical structures for subdivided groups of chronologies, opening new avenues for interpreting complex spatial patterns through geographic or taxonomic stratification of a target region.We hypothesized that climate warming (1) triggers more synchronous tree growth at subcontinental scales owing to an amplified climatic control of growth, e.g., through higher temperatures in Siberia and decreased water availability in Spain. Our objective was to interpret forest reactions to warming through an alternative approach to model-based assessment or field experimentation. Specifically, this study asked the following questions: (i) is spatial synchrony of tree growth increasing across terrestrial biomes and if so, at what pace?; (ii) how are synchrony patterns related to intraspecific and interspecific responses to climate warming?; and (iii) what are the main climate factors underlying more synchronous forest growth? In ecological theory, it is widely accepted that spatial synchrony influences metapopulation persistence and the likelihood of species extinction (24). As forests are becoming more prone to widespread mortality (25), interpreting long-term synchrony patterns of tree growth may be relevant to identifying broad-scale threshold responses to climate change.     

Historical warming reduced due to enhanced land carbon uptake

Previous studies have demonstrated the importance of enhanced vegetation growth under future elevated atmospheric CO₂ for 21st century climate warming. Surprisingly no study has completed an analogous assessment for the historical period, during which emissions of greenhouse gases increased rapidly and land-use changes (LUC) dramatically altered terrestrial carbon sources and sinks. Using the Geophysical Fluid Dynamics Laboratory comprehensive Earth System Model ESM2G and a reconstruction of the LUC, we estimate that enhanced vegetation growth has lowered the historical atmospheric CO₂ concentration by 85 ppm, avoiding an additional 0.31 ± 0.06 °C warming. We demonstrate that without enhanced vegetation growth the total residual terrestrial carbon flux (i.e., the net land flux minus LUC flux) would be a source of 65–82 Gt of carbon (GtC) to atmosphere instead of the historical residual carbon sink of 186–192 GtC, a carbon saving of 251–274 GtC.From the preindustrial times to present day, the total cumulative land carbon flux is estimated to be a source of 11 ± 47 Gt of carbon (GtC) to the atmosphere (1). The total historical direct land-use changes (LUC) carbon flux (i.e., only from anthropogenic activities without any effects of environmental change) is estimated at ∼160 GtC (2) with an uncertainty range of ±50% (1). The difference between the former and the latter implies a substantial residual terrestrial carbon sink. According to previous studies, the land was a carbon source to the atmosphere from the preindustrial period to the 1940s (3) and then became a carbon sink, which has steadily increased over the last 50 y (4, 5). For the 1980s and the 1990s the residual terrestrial carbon sink is estimated to be between −3.4 and 0.2 GtC/y and −4.2 and −0.9 GtC/y, respectively (6). One of the leading causes of the increasing residual terrestrial sink is believed to be enhanced vegetation growth under elevated levels of atmospheric CO₂ (i.e., CO₂ fertilization) (4, 6–9).Previous studies have evaluated the implications of enhanced vegetation growth under future elevated atmospheric CO₂ for 21st century climate warming (10–12). Estimating the impact of enhanced vegetation growth on historical, transient climate change is surprisingly difficult: it cannot be simply deduced from the published idealized relationships such as an equilibrium climate sensitivity (13) or the Transient Climate Response to cumulative carbon Emissions (TCRE) (14). For example, analysis of simulations from the Coupled Climate Carbon Cycle Model Intercomparison Project (C4MIP) indicated that the TCRE is not constant from 1900 to 2000 (14). The TCRE does not include effects of any other greenhouse gases (GHGs), aerosols, or changes in historical forcings. The transient rate of temperature increase (15) and strengths of carbon sources and sinks (16, 17) both depend on the rate of change in the atmospheric CO₂ concentration. Moreover, feedbacks among climate, atmospheric CO₂, and land and ocean carbon exchanges must be accounted for in isolating the effect of any one factor. To assess the contribution of enhanced land carbon uptake to historical global warming requires a climate–carbon cycle model (i.e., an Earth System Model, ESM), capable of prognostically simulating the transient physical climate, and the exchanges of CO₂ among land, ocean, and atmosphere, as well as feedbacks between the climate and carbon system. The ESM must be forced by historical fossil fuel emissions and include an internally consistent treatment of historical LUC carbon emissions from agricultural conversion, wood harvesting, and regrowth of the previously used or logged (i.e., secondary) lands. Finally, the assessment needs to isolate the temperature change due to a particular process and show that its impact is significant relative to unforced climate variability.Despite the fact that a number of comprehensive ESMs have been developed and analyzed during the last decade, most previous studies have not simulated historical LUC carbon fluxes in an internally consistent manner. For example, C4MIP (18) prescribed (rather than predicted) LUC CO₂ emissions for both agricultural conversion and logging from a regional bookkeeping model (2) although all land was modeled as unmanaged (i.e., without LUC) when computing land–atmosphere carbon fluxes. With the exception of one recent ESM analysis (ref. 19, with prescribed atmospheric CO₂), most ESM studies (e.g., refs. 14, 18) have not included land cover properties or carbon fluxes on secondary lands from forestry, which currently cover ∼30 million km². Such omission is likely to underestimate the net LUC emissions by 25–35% (20). A previous study with the stand-alone land component (21) of the Geophysical Fluid Dynamics Laboratory (GFDL) comprehensive ESM estimated an LUC carbon source of 45 GtC from wood harvesting and shifting cultivation during the 20th century.     

Benchmark map of forest carbon stocks in tropical regions across three continents

Developing countries are required to produce robust estimates of forest carbon stocks for successful implementation of climate change mitigation policies related to reducing emissions from deforestation and degradation (REDD). Here we present a "benchmark" map of biomass carbon stocks over 2.5 billion ha of forests on three continents, encompassing all tropical forests, for the early 2000s, which will be invaluable for REDD assessments at both project and national scales. We mapped the total carbon stock in live biomass (above- and belowground), using a combination of data from 4,079 in situ inventory plots and satellite light detection and ranging (Lidar) samples of forest structure to estimate carbon storage, plus optical and microwave imagery (1-km resolution) to extrapolate over the landscape. The total biomass carbon stock of forests in the study region is estimated to be 247 Gt C, with 193 Gt C stored aboveground and 54 Gt C stored belowground in roots. Forests in Latin America, sub-Saharan Africa, and Southeast Asia accounted for 49%, 25%, and 26% of the total stock, respectively. By analyzing the errors propagated through the estimation process, uncertainty at the pixel level (100 ha) ranged from ± 6% to ± 53%, but was constrained at the typical project (10,000 ha) and national (>1,000,000 ha) scales at ca. ± 5% and ca. ± 1%, respectively. The benchmark map illustrates regional patterns and provides methodologically comparable estimates of carbon stocks for 75 developing countries where previous assessments were either poor or incomplete.     

Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest

Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem–atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m⁻² (95% CI: 23.2 to 26.7 1 µg ⋅ m⁻²), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export.

Mercury is a neurotoxic environmental pollutant distributed via the atmosphere to ecosystems globally (1). In terrestrial ecosystems, dry deposition of atmospheric gaseous elemental mercury (GEM) is considered the dominant source of mercury, accounting for 54 to 94% of mercury loads observed in soils (2–5). Terrestrial GEM deposition propagates through watersheds and ultimately provides a critical source of mercury to freshwater, coastal sediments, and marine biota (6). Yet, direct measurement of GEM dry deposition is lacking over most ecosystem types, in particular over forests, which are considered the largest global atmospheric GEM sink (7, 8). GEM deposition includes direct uptake of atmospheric GEM by plants, which is transferred to soils as litterfall when plants die off and shed leaves or as throughfall when precipitation washes it off from plant surfaces (9). Depending on environmental conditions, underlying soils serve as either additional sinks or as sources (i.e., emissions) of GEM to or from the atmosphere, which complicates quantification of integrated, whole-ecosystem net GEM loadings (10). Globally, dry GEM deposition to terrestrial ecosystems may constitute the largest removal mechanism of atmospheric mercury, currently estimated between 1,500 to 2,145 Mg ⋅ yr⁻¹ (6) and on average may turn over the entire global atmospheric mercury pool of 4,400 to 5,300 Mg (11) every 2 to 3.5 y.Major uncertainties exist in regard to magnitude and seasonality of the dominant terrestrial GEM deposition and their controlling ecological and environmental controls. Data are particularly scarce from forests in which direct GEM exchange measurements are limited largely to summertime (12) or stem from Hg-contaminated sites (13, 14). Direct flux measurements are also required to partition GEM deposition into canopy and forest floor (litter and soil) contributions. In lieu of direct GEM flux measurements, GEM deposition across forests is often inferred from proxy measures such as mercury litterfall collected under canopies (8, 15). Litterfall mercury deposition, however, is not an ideal proxy for GEM dry deposition because it does not capture uptake by large woody tissues, nonvascular plants (i.e., lichen, mosses), and underlying soils, and it does not account for GEM re-emission back to the atmosphere after deposition (7). Here, we employed a micrometeorological flux-gradient method that directly quantifies GEM exchange at the ecosystem level over a 470-d record (May 2019 to August 2020) in a rural temperate forest in Massachusetts. The site is a second-growth forest that is actively accumulating biomass and is approaching standing biomass levels of old-growth stands in the region (16). The flux-gradient method we employed consists of GEM concentration measurements at two heights above the canopy in combination with quantification of atmospheric turbulence to calculate forest-level surface–atmosphere exchanges (17). We here analyze seasonality and diel patterns of GEM exchange, compare its magnitude to other deposition measurements such as litterfall and wet deposition, and assess contributions by the underlying forest floor using a second flux-gradient system deployed under the canopy.     

Re-evaluation of forest biomass carbon stocks and lessons from the world's most carbon-dense forests

From analysis of published global site biomass data (n = 136) from primary forests, we discovered (i) the world's highest known total biomass carbon density (living plus dead) of 1,867 tonnes carbon per ha (average value from 13 sites) occurs in Australian temperate moist Eucalyptus regnans forests, and (ii) average values of the global site biomass data were higher for sampled temperate moist forests (n = 44) than for sampled tropical (n = 36) and boreal (n = 52) forests (n is number of sites per forest biome). Spatially averaged Intergovernmental Panel on Climate Change biome default values are lower than our average site values for temperate moist forests, because the temperate biome contains a diversity of forest ecosystem types that support a range of mature carbon stocks or have a long land-use history with reduced carbon stocks. We describe a framework for identifying forests important for carbon storage based on the factors that account for high biomass carbon densities, including (i) relatively cool temperatures and moderately high precipitation producing rates of fast growth but slow decomposition, and (ii) older forests that are often multiaged and multilayered and have experienced minimal human disturbance. Our results are relevant to negotiations under the United Nations Framework Convention on Climate Change regarding forest conservation, management, and restoration. Conserving forests with large stocks of biomass from deforestation and degradation avoids significant carbon emissions to the atmosphere, irrespective of the source country, and should be among allowable mitigation activities. Similarly, management that allows restoration of a forest's carbon sequestration potential also should be recognized.     

Carbon residence time dominates uncertainty in terrestrial vegetation responses to future climate and atmospheric CO2

Future climate change and increasing atmospheric CO₂ are expected to cause major changes in vegetation structure and function over large fractions of the global land surface. Seven global vegetation models are used to analyze possible responses to future climate simulated by a range of general circulation models run under all four representative concentration pathway scenarios of changing concentrations of greenhouse gases. All 110 simulations predict an increase in global vegetation carbon to 2100, but with substantial variation between vegetation models. For example, at 4 °C of global land surface warming (510–758 ppm of CO₂), vegetation carbon increases by 52–477 Pg C (224 Pg C mean), mainly due to CO₂ fertilization of photosynthesis. Simulations agree on large regional increases across much of the boreal forest, western Amazonia, central Africa, western China, and southeast Asia, with reductions across southwestern North America, central South America, southern Mediterranean areas, southwestern Africa, and southwestern Australia. Four vegetation models display discontinuities across 4 °C of warming, indicating global thresholds in the balance of positive and negative influences on productivity and biomass. In contrast to previous global vegetation model studies, we emphasize the importance of uncertainties in projected changes in carbon residence times. We find, when all seven models are considered for one representative concentration pathway × general circulation model combination, such uncertainties explain 30% more variation in modeled vegetation carbon change than responses of net primary productivity alone, increasing to 151% for non-HYBRID4 models. A change in research priorities away from production and toward structural dynamics and demographic processes is recommended.