Divergence of Arctic shrub growth associated with sea ice decline

Arctic sea ice extent (SIE) is declining at an accelerating rate with a wide range of ecological consequences. However, determining sea ice effects on tundra vegetation remains a challenge. In this study, we examined the universality or lack thereof in tundra shrub growth responses to changes in SIE and summer climate across the Pan-Arctic, taking advantage of 23 tundra shrub-ring chronologies from 19 widely distributed sites (56°N to 83°N). We show a clear divergence in shrub growth responses to SIE that began in the mid-1990s, with 39% of the chronologies showing declines and 57% showing increases in radial growth (decreasers and increasers, respectively). Structural equation models revealed that declining SIE was associated with rising air temperature and precipitation for increasers and with increasingly dry conditions for decreasers. Decreasers tended to be from areas of the Arctic with lower summer precipitation and their growth decline was related to decreases in the standardized precipitation evapotranspiration index. Our findings suggest that moisture limitation, associated with declining SIE, might inhibit the positive effects of warming on shrub growth over a considerable part of the terrestrial Arctic, thereby complicating predictions of vegetation change and future tundra productivity.

Arctic sea ice extent (SIE) is decreasing at an accelerating rate (1–5), with a seasonally ice-free Arctic Ocean expected within a few decades (6). Sea ice decline has elicited major changes in local climates and large-scale atmospheric circulation (7), extending beyond the regions of in situ sea ice changes (8). This includes the impact of winter SIE on upper-level atmospheric flow and subsequent summer air temperature, precipitation, and even soil moisture (9). While this rapid change in the physical system is occurring, the mechanisms by which Arctic sea ice interacts with biological systems are still largely unknown, especially in terrestrial systems (10). For this reason, the study of sea ice effects on Arctic biota has recently been classified as a crisis discipline (2).The effects of rapidly diminishing SIE on Arctic terrestrial ecosystems, such as changes in shrub growth and tundra productivity, are highly uncertain and understudied at the biome level (2, 3). This is due to i) the complex nature of sea ice dynamics and its strong coupling with atmospheric circulation patterns (7, 11) and climate variables, such as temperature (12), precipitation (13), and humidity (14); 2) the spatial scale of the processes, which are characterized by strong regional variation (15, 16); and 3) the dynamic nature of interannual changes in SIE (1). It is important that we improve understanding of sea ice effects on tundra ecosystems, because changes in the productivity and composition of Arctic vegetation have the potential to amplify or dampen trends in air temperature and sea ice extent through effects on land surface-atmosphere exchanges of carbon and energy (17).One of the best-documented vegetative responses to Arctic warming is widespread increased productivity and encroachment of deciduous shrubs into lower-statured tundra (18–20). More recently, several studies (21–25) have highlighted the potential for soil moisture to mediate the response of tundra shrub growth to climate warming. A recent synthesis of tundra shrub-ring data showed that shrub growth was more sensitive to interannual climate variability at sites with greater soil moisture (22). Meanwhile, sampling along a moisture gradient within a landscape of northern Alaska revealed a positive correlation between June air temperature and shrub growth at a riparian site and a distinct June air temperature optimum at a drier upland site (23). In Kangerlussuaq, western Greenland, which is a relatively dry area that is experiencing rapid warming, shrub-ring analysis revealed a decline in growth that coincided with decreasing carbon isotope discrimination, low midsummer xylem water potentials, and strong sensitivity of foliar gas exchange to recent rainfall events, suggesting moisture limitation as an underlying cause (25).Loss of sea ice likely promotes warmer conditions in adjacent terrestrial ecosystems because of the associated dramatic decrease in surface albedo (26). Local warming from sea ice loss can extend several hundred kilometers inland (27–31), and tundra responses to declining sea ice are emerging (10, 32). Examining relationships between sea ice conditions and shrub-ring data throughout the Arctic with explicit consideration of the indirect ways by which sea ice variability can ultimately affect vegetation growth may help assess tundra productivity trajectories. However, few studies have investigated sea ice–shrub growth relationships (33) and none have been conducted at the Pan-Arctic scale. In keeping with widespread observations of increasing shrub abundance, we hypothesized that shrub growth across the tundra biome would be promoted by declining SIE through a positive feedback between declining sea ice and increasing near-surface air temperatures. We expect that one important mechanism through which diminishing sea ice amplifies warming in this context is through greater surface solar absorption in expanding open water areas (34), which leads to local heating and thus favors shrub growth across the tundra biome.Here we report on tundra shrub growth responses to changes in SIE using 23 annually resolved shrub-ring chronologies of Betula and Salix shrubs from 19 sites distributed throughout the Arctic from a latitude of 56°N in eastern Canada to 83°N in northern Greenland (Fig. 1B and SI Appendix, Table S1). In total, we analyzed 641 shrubs with 20,336 growth ring measurements in relation to 1) Pan-Arctic and 2) regional SIE (both monthly and seasonal) and 3) the timing of regional sea ice retreat and freeze-up. Each chronology that was significantly correlated with either Pan-Arctic or regional SIE was aggregated into a specific responder group: increasers, chronologies that were negatively correlated with at least one monthly or seasonal SIE variable; and decreasers, chronologies that were positively correlated with at least one monthly or seasonal SIE variable. In order to test for direct and indirect effects of SIE and summer climate (air temperature, precipitation, and standardized precipitation evapotranspiration index (SPEI)) on shrub growth, we construct piecewise structural equation models (35) (SEMs). We verified the link between each sea ice variable and the growth of individual shrubs by analyzing individual shrub-ring series hierarchically in linear mixed effects (LME) models.Open in a separate windowFig. 1.Divergent growth response of Arctic shrubs linked to sea ice decline across the Pan-Arctic region. (A) Mean increaser (blue) and decreaser (red) chronologies (RWI with SD) in comparison with seasonal Arctic sea ice extent (black) time series for previous September–October–November (pSON SIE). Vertical dashed lines indicate the common period (1979 to 2008) studied in the synthesis. (B) Geographical locations of 23 shrub-ring chronologies and 641 shrubs in total included in the synthesis with indication of sea ice extent–shrub growth response type.     

高血压大鼠动脉的零应力状态与内皮素的关系

本文通过观察胸主动脉、肾动脉主干张开角的大小 ,研究了SHR高血压建立前后胸主动脉和肾动脉零应力状态的变化 ,并于SHR高血压形成前 ,给予口服型ETA型受体拮抗剂BMS 182 874,研究了内源性ET在SHR主动脉和肾动脉零应力状态变化中的作用。结果发现 ,在未建立高血压的SHR(4周 ) ,主动脉和肾动脉主干的张开角与同龄WKY相比 ,都没有显著的变化。在高血压已建立的SHR(16周 ) ,主动脉和肾动脉主干的张开角分别为 6 9.0± 2 7.0°和 114.2± 42 .2° ,显著大于同龄的WKY大鼠 (4 5 .9± 2 5 .7°和 70 .2± 30 .4°)。给予SHRBMS 182 874(4～ 16周 ) ,SHR主动脉张开角未发生明显变化 (6 2 .8± 2 5 .6°) ,而肾动脉主干的张开角显著减小 (6 6 .6±41.5 4°)。结果表明 ,在已建立高血压的SHR ,主动脉和肾动脉主干都存在非均匀生长 ,其残余应力、应变大于同龄的WKY ,内源性ET在SHR肾动脉零应力状态的改变中起重要作用。     

超声测量胎儿主动脉峡部血流多普勒指标的意义

胎儿宫内缺氧严重威胁胎儿宫内安危,长期以来受到产科医师的关注。早期发现、及时处理和治疗,对降低围生儿死亡率、提高围生医学质量具有重大意义。目前临床上应用彩色多普勒超声测量胎儿的脐动脉、大脑中动脉等各种血流指标,以评估胎儿宫内缺氧的情况。现从简单、方便、快捷、有效等各方面,探讨超声测量胎儿主动脉峡部血流多普勒指标对早期发现胎儿宫内缺氧的意义。     

主动脉弓腔内修复术后卒中的防治研究进展

传统开放性主动脉弓修复手术存在较高的围术期并发症和死亡风险。主动脉弓腔内修复术（EAAR）为开放手术高风险患者提供了微创治疗选择。然而,与传统开放手术相比,EAAR仍具有较高的卒中风险。分支技术是腔内治疗主动脉弓部疾病最具应用前景的技术之一,尽管其早期卒中风险略高于传统开放手术,但对于高风险患者来说,这样的风险是可以接受的。导致术后卒中的主要原因包括固体栓塞、气体栓塞和脑灌注不足。术前评估、围术期监测、药物预防和优化术中操作是预防EAAR术后卒中发生的关键策略。对于已经发生卒中的患者,及时诊断和评估、药物治疗和必要的手术干预是治疗的基石,而多学科有效协作对于改善患者的病情和预后亦尤为重要。目前,EAAR术后卒中的防治仍有很大的研究空间,因此,笔者就EAAR术后早期卒中的发生率、发生机制、危险因素以及预防和治疗策略方面进行论述,以期为临床工作提供思路。     

N-甲基小檗胺对依Ca~(2+)性内皮源松弛因子释放的影响

在无Ca~(2+)krebs-Henseleit液中,Ach诱发兔主动脉松弛受抑制,恢复Ca~(2+)后抑制解除。用NA0.5μmol/L诱发收缩反应,NMBA50、100μmol/L,Ver1、10μmol/L对肌条的Ach松弛均呈抑制,而且随剂量增加,抑制作用增强。结果提示兔主动脉EDRF释放与Ca~(2+)有关,NMBA及Ver对EDRF释放有抑制作用。     

Debranching Solutions in Endografting for Complex Thoracic Aortic Dissections

Background

Conventional surgical repair of thoracic aortic dissections is a challenge due to mortality and morbidity risks.

Objectives

We analyzed our experience in hybrid aortic arch repair for complex dissections of the aortic arch.

Methods

Between 2009 and 2013, 18 patients (the mean age of 67 ± 8 years-old) underwent hybrid aortic arch repair. The procedural strategy was determined on the individual patient.

Results

Thirteen patients had type I repair using trifurcation and another patient with bifurcation graft. Two patients had type II repair with replacement of the ascending aorta. Two patients received extra-anatomic bypass grafting to left carotid artery allowing covering of zone 1. Stent graft deployment rate was 100%. No patients experienced stroke. One patient with total debranching of the aortic arch following an acute dissection of the proximal arch expired 3 months after TEVAR due to heart failure. There were no early to midterm endoleaks. The median follow-up was 20 ± 8 months with patency rate of 100%.

Conclusion

Various debranching solutions for different complex scenarios of the aortic arch serve as less invasive procedures than conventional open surgery enabling safe and effective treatment of this highly selected subgroup of patients with complex aortic pathologies.     

HO-1参与葛根素对抗高糖诱导的大鼠血管舒张功能下降

目的： 研究葛根素对抗急性高糖刺激引起的血管舒张功能的下降,并分析血红素加氧酶（HO-1）在其中的作用。方法: 采用血管环灌流装置,观察大鼠胸主动脉环的舒张效应。结果: ①与空白对照组（含11 mmol/L葡萄糖）相比,经44 mmol/L葡萄糖（高糖）孵育血管2 h后,主动脉环对ACh引起的内皮依赖性血管舒张反应下降。②葛根素（10-10-10-8 mol/L）与高糖联合孵育,可剂量依赖性地改善高糖诱导的血管ACh舒张反应的下降。③葛根素孵育血管后可引起血管HO-1活性增高;用ZnPPIX抑制HO-1的活性后,葛根素抗高糖损伤的作用被取消。结论: 葛根素可以对抗高糖引起的血管舒张功能的下降,其机制可能是通过诱导HO-1而实现的。     

Ankura主动脉覆膜支架行原位开窗重建弓上分支的应用

背景与目的:胸主动脉腔内修复术(TEVAR)已经成为治疗主动脉弓部病变的首选术式.然而,此术式要求支架近端安全锚定区至少为15 mm,对于锚定区不足者,则通常需重建弓部分支血管以确保手术安全.在目前各种重建技术中,原位开窗技术因其较大程度的保留分支血管以及较低的内漏风险而应用最多.因此,本研究探讨Ankura主动脉覆膜...     

Endovascular stenting for unsuccessful angioplasty of the aorta in aortoarteritis

Purpose: The efficacy and safety of endovascular stent implantation to correct dissection or a suboptimal result after percutaneous transluminal angioplasty (PTA) was evaluated in patients suffering from aortic stenosis due to aortoarteritis. Methods: Twelve children and young adults [aged (mean ± SD) 18.2 ± 8.7 years] underwent stent implantation after PTA of the aorta, seven for obstructive dissection, four for ineffective balloon dilatation, and one for recurrent restenosis. Nine patients underwent implantation of self-expandable stents and three received balloon-expandable Palmaz stents. Results: Stent implantation could be successfully performed in all 12 patients. After stent implantation, the peak systolic pressure gradient decreased from 91 ± 33.5 mmHg to 12.4 ± 12.5 mmHg (p < 0.001). The diameter of the stenosed segment increased from 4.6 ± 0.8 mm to 11.1 ± 1.9 mm (p < 0.001). The dissection was completely covered in all seven patients with dissection. Except for epigastric pain with vomiting in one patient, there was no complication. On follow-up, over 12–57 months (mean 26.8 ± 10.8 months), 11 patients (91.6%) had marked improvement in their blood pressure. Patients with congestive heart failure and claudication also showed improvement. Repeat catheterization in five patients, between 6–30 months (mean 16.8 ± 9.1 months) after stent implantation, showed sustained improvement in four and a fusiform, long segment, intrastent restenosis after 30 months in one child. The stenosis was safely redilated. Conclusion: Endovascular aortic stent implantation is safe and provides good immediate relief in patients with unsatisfactory results after balloon angioplasty. Improvement is sustained in most patients on intermediate-term follow-up.