气候变暖与富营养化交互作用对浅水湖泊水-气界面N2O通量的影响

研究通过构建中尺度控温围隔模拟系统, 模拟21世纪末气候变化与富营养化趋势, 探讨未来气候变暖与富营养化趋势下浅水湖泊水-气界面N₂O交换过程的响应特征及机制。结果表明: (1)恒定与波动升温引起的代谢过程及生物间相互作用的改变显著促进了水-气界面间N₂O的排放及年累积释放量, 而磷的添加可能因为影响了水体中反硝化代谢的效率而削弱了水-气界面N₂O排放及年累积释放量; (2)实验期间随季节转换, 控制系统内优势的初级生产者由水生植物转变为浮游植物, 水体中有机质含量亦不断积累, 研究结果表明季节变化及初级生产者转换均对水-气界面N₂O排放量的增加起到了显著促进作用。在气候变化与富营养化趋势下浅水湖泊水-气界面的N₂O交换过程主要受到水体中氮磷含量及其比例的变化、水生植物与浮游植物的转换及有机质的积累过程的影响。因此, 气候变暖(恒定和波动升温)能够促进湖泊N₂O排放量的上升, 而变暖和营养盐的交互作用会使水-气界面N₂O交换更加复杂。     

东北地区5个物种潜在栖息地变化与优化保护规划

气候变化广泛影响着物种多样性及其分布变迁。优化模型模拟结果,获取气候变化影响下的优先保护区域将为制定应对气候变化的物种保护政策或行动提供理论依据,提升保护绩效。选取东北地区五种代表性动物,包括黑熊(Ursus thibetanus)、驼鹿(Alces alces)、水獭(Lutra lutra)、紫貂(Martes zibellina)及黑嘴松鸡(Tetrao parvirostris);结合最大熵模型(Maxent)模拟在不同RCP情景下未来3个年代(2030s,2050s,2070s)的物种潜在栖息地。根据九个常用气候模式的评价结果,获取东北地区合适的气候模式,了解气候变化对物种潜在栖息地的影响,同时开展物种保护规划,识别保护空缺,为应对气候变化、保持生物多样性提供支持。结果显示,在气候变化背景下物种潜在栖息地面积整体呈现下降趋势,但不同气候模式之间存在差异;评价结果推荐CCSM4、Nor ESM1-M、Had GEM2-AO及GFDL-CM3气候模式,推荐在东北地区使用以上气候模式进行物种未来潜在分布的研究。5个物种潜在栖息地平均面积变化率分别为-62.16%,-73.93%,-78.46%(2030s,2050s,2070s)。综合5个重点保护物种的保护优先区,大兴安岭的呼中、汗马与额尔古纳国家级自然保护区,延边地区的天佛指山、老爷岭东北虎、珲春东北虎与汪清原麝国家级自然保护区,长白山国家级自然保护区是气候变化下物种保护的热点区域。     

气候变化对“山水林田湖草”重大生态工程的影响

“山水林田湖草”重大生态保护与修复工程是中国对复杂生态环境治理的重要探索。鉴于目前大多数重大生态工程未系统地考虑气候变化对重大生态工程的影响问题,针对气候变化对高寒草地、北方林区以及风沙源区生态的影响,以实施了多年的三江源生态保护工程、三北防护林工程和京津风沙源治理工程为重点,分析了重大生态工程对全球气候变化的响应,解构了重大生态工程与气候的反馈关系和影响程度,指出了中国“山水林田湖草”重大生态工程实施过程中可能存在的问题,并给出了应对建议。结果表明：（1）气候变化对重大生态工程影响研究不足,尤其是涉及区域生态系统结构、功能、生物多样性与脆弱性等方面以及气候变化在工程实施效果贡献率的研究。（2）缺乏有效区分气候变化和工程实施效果的评估方法。目前能够定量识别气候变化对生态系统恢复的试验和方法比较少见,且缺乏对气候变化未来风险预估,导致制订的措施不能有效适应气候变化从而达到生态效益的最大化。所以在今后设计和实施“山水林田湖草”重大生态保护与修复工程时,要充分考虑自然规律、地理地带性差异和气候因素对生态系统的影响,并且加强建设生态综合监测和工程评价体系,根据未来不同的气候变化情景制订不同的措施,并依据气候动态预估来适当调整措施,使得生态工程能发挥出最大效益,以保证修复工程的系统性、区域性和整体性。     

植被变化对气候的反馈机制及调节效应

植被通过光合作用固定大气中的CO₂来减缓温室效应，同时植被也通过改变地表能量收支影响温室效应。在过去的气候-植被研究中，大多关注气候变化对植被的影响，而植被对气候反馈的研究相对较少。植被通过调节地表能量收支、水通量等重要地气过程影响局地、区域乃至全球气候，在气候变化中的作用十分重要。因此，需要厘清植被对气候的反馈效应机制及其结果，并识别其地域差异。从生物地球物理和生物地球化学过程两方面分析植被与气候之间的作用机制，对全球及关键区域内植被变化对局地、区域乃至全球的气候反馈效应进行了系统总结：（1）生物地球物理反馈的区域特征明显，生物地球化学反馈则表现在全球尺度上，二者相互作用但难以统一；（2）植被破坏带来的气候影响在气温效应方面与生态系统的类型及地理分布相关：热带森林破坏带来增温效应，北方森林破坏带来降温效应，温带森林破坏则会通过增加森林反照率抵消丢失的固碳降温效应，气温效应表现不明显；（3）当前研究对关键过程机制考虑不够完善，不同研究方法的结果差异较大，且缺乏高质量观测数据的验证；同时考虑生物地球物理和生物地球化学的净气候反馈研究尚无法支撑植树造林对气候变化单一减缓作用的常规理解。本文可为科学评估植树造林对气候变化作用的方向与强度提供理论依据。     

不同林龄、树种落叶松人工林径向生长与气候变化的关系

研究人工林径向生长与气候变化的关系对全球气候变暖背景下人工林合理经营有着重要的意义。该文对在辽东山区广泛栽培的黄花落叶松(Larix olgensis)和日本落叶松(Larix kaempferi)人工林, 运用树木年轮气候学方法建立了辽宁草河口和湾甸子林场落叶松人工林年表, 分析了落叶松径向生长对气候变化的响应以及气候条件、树种、立地条件和林分因子(林龄、密度、蓄积量等)的相对影响程度。结果发现在影响年轮-气候关系的因素中, 气象因子的潜在蒸发散(PET)的影响力最大; 林龄、密度和蓄积量同时也具有重要的影响作用。中龄落叶松人工林径向生长主要与气温呈正相关关系, 成熟落叶松人工林径向生长主要与气温呈负相关关系; 而其他因素, 如树种、立地条件等的影响作用不大。这表明在气候变暖背景下随着林龄增加, 林分会逐渐受到气温升高导致的水分亏缺的限制, 导致明显的生长下降趋势, 因而气候变暖对成熟落叶松人工林威胁更为严重, 所以要注重对成熟林的优先保护, 同时可以预测, 随着东北地区今后气候进一步变暖, 可能将逐步影响到林龄更小的林分的生长, 因此需要进一步研究如何在落叶松人工林经营中采取科学的措施来更好地应对未来气候变化。     

气候变化对水文水资源的影响分析

气候变化已成为最近几十年来最受瞩目的环境问题,全球气候变暖及由此引起的水文地质灾害、水资源短缺已成为国际、国内学术研究的热点。水资源是地球上最宝贵的自然资源,也是人类乃至整个生物圈赖以生存的基础。研究气候变化对水资源的影响,对于今后水资源的合理开发、利用、管理和保护具有非同寻常的意义。有鉴于此,本文分析和总结了近代到现代气候变化趋势以及对水文水资源的影响。     

区域种植业气候适宜度及其对种植活动的响应——以四川省盐亭县为例

在分析盐亭县近63年来(1950—2012)种植业生产发展的基础上,选取该县农村社会经济条件相对稳定的近32年(1981—2012)为研究时段。运用农业生态气候适宜度方法,依据水稻、红薯、玉米、小麦和油菜等5种主要作物生育期的光、热、水等气候条件,分别估算各种作物的资源适宜指数、效能适宜指数和利用指数,构建小尺度区域种植业气候适宜度模型和种植活动对区域种植业气候适宜度的影响度模型,进行小尺度区域种植业气候适宜度以及种植活动对种植业气候适宜度的影响度估算,并对种植业生产对气候变化的适应进行探讨。研究结果表明,(1)近32年来盐亭县大春作物的平均资源适宜指数、效能适宜指数和利用指数(分别为0.578、0.281和48.37%)均大于小春作物(分别为0.304、0.128和42.24%),大春作物的气候适宜度高于小春作物,且作物间的气候适宜度差异较大。(2)受季风气候波动的影响,该县作物气候适宜度有明显的年际波动;该县近32年来气候变化对大春作物气候适宜度有轻微不利影响,而对小春作物气候适宜度趋于有利。(3)盐亭县近32年来种植业平均的资源适宜指数为0.466、效能适宜指数为0.212、利用指数为45.49%;受5种作物资源适宜指数、效能适宜指数,以及作物播种面积与产量年际波动的综合影响,该县种植业气候适宜度亦有明显的年际波动;气候变化对该县种植业气候适宜度总体上有不利影响。(4)近32年来该县种植活动对种植业气候适宜度的影响度平均值为0.00092,其年际波动较大。通过作物种植组合结构的调整,在20世纪90年代中期前对种植业气候适宜度的提高有微弱的正向影响,对气候变化有一定程度的适应;而后期则有负向作用。     

基于暴露度-恢复力-敏感度的城市适应气候变化能力评估与特征分析

城市是人口和社会经济活动最密集的地方,随着城市化进程和气候变化的发展,城市地区面临的气候风险和影响日益凸显。提升城市适应气候变化能力已成为城市应对气候变化挑战最重要的任务和途径。通过梳理和评价我国城市适应气候变化能力及其关键要素,以期为区域适应政策的制定和实施提供科学依据。基于IPCC适应能力评价框架,构建了基于暴露度-敏感度-恢复力的城市适应气候变化能力评估框架,进而筛选了19项指标,将指标划分为适应气候变化能力对应的5个等级,以熵权法赋权重;采用集对分析方法,评估我国286个地级市的适应气候变化能力水平,并分析了主要限制因素。结果显示,我国东部的适应能力整体高于西部地区,适应能力较低的区域主要集中在西北的甘肃陕西部分城市、华中的两湖和江西等城市以及西南的广西云南等城市;城市适应能力的各项限制要素主要表现为,适应能力高主要为暴露度-恢复力-敏感度的(低-高-低)的组合;适应能力低则分别包括暴露度-恢复力-敏感度(高-高-高)、(低-低-低)和(高-低-低)3种组合。提高城市适应气候变化能力,对西部西北的甘肃-陕西等城市,重点在于提升应对气候变化的恢复力,例如建立良好的灾后恢复与应急系统等;对于华中、西南等城市则以提高气候风险的防御能力为主。     

孟印缅地区农田生产力脆弱性变化及气候影响机制——基于1982-2015年GIMMS3g植被指数

农田生产力对气候变化的敏感性决定了其脆弱性,全球气候变暖及极端气候频发将严重影响农业粮食生产,进而将可能影响区域粮食安全。科学评估农田生产力脆弱性并分析其气候影响机制有助于积极应对气候变化,保障区域粮食安全,具有重要的现实和科学意义。以"一带一路"区域的"孟中印缅经济走廊"为研究区,基于1982-2015年卫星遥感数据的归一化植被指数,根据IPCC脆弱性定义,采用年际变率及其变化趋势计算农田生产力对气候变化的敏感性、适应性和脆弱性指数,分时段分析研究区农田生态系统脆弱性空间格局变化及气候影响机制。结果表明：（1）较之1982-2000年,2000-2015年期间研究区农田脆弱性程度略有提升,高度和极度脆弱面积略有增加（分别增加0.42%和1.12%）,但其分布格局发生北移。（2）年降水、年平均气温和年辐射与年累积NDVI间线性回归分析表明,孟加拉和缅甸地区与气候因素显著相关的区域面积在本国农田面积中的比例分别增加21.3%和16.7%,而印度地区减少10.5%,全区减少8.1%;（3）线性回归方程的复相关系数（R²）表征气候变化的解释能力,整个研究区增加12%,其中印度气候解释能力从48%提升至64%,增加16%。（4）农田生产力脆弱性受气候影响的范围略有减小,但影响程度增大,且存在较大的区域性差异;高温和降水季节不均引发的旱涝灾害是农田高脆弱度形成的两个关键气候因素。为该地区农业应对气候变化适应性管理措施的提出及决策提供了科学依据,有效支撑"一带一路"建设;也为其他地区应用卫星遥感开展脆弱性研究提供了方法参考,为生态系统对全球变化响应研究提供重要知识参考。     

不同林龄、树种落叶松人工林径向生长与气候变化的关系

研究人工林径向生长与气候变化的关系对全球气候变暖背景下人工林合理经营有着重要的意义。该文对在辽东山区广泛栽培的黄花落叶松(Larix olgensis)和日本落叶松(Larix kaempferi)人工林,运用树木年轮气候学方法建立了辽宁草河口和湾甸子林场落叶松人工林年表,分析了落叶松径向生长对气候变化的响应以及气候条件、树种、立地条件和林分因子(林龄、密度、蓄积量等)的相对影响程度。结果发现在影响年轮-气候关系的因素中,气象因子的潜在蒸发散(PET)的影响力最大;林龄、密度和蓄积量同时也具有重要的影响作用。中龄落叶松人工林径向生长主要与气温呈正相关关系,成熟落叶松人工林径向生长主要与气温呈负相关关系;而其他因素,如树种、立地条件等的影响作用不大。这表明在气候变暖背景下随着林龄增加,林分会逐渐受到气温升高导致的水分亏缺的限制,导致明显的生长下降趋势,因而气候变暖对成熟落叶松人工林威胁更为严重,所以要注重对成熟林的优先保护,同时可以预测,随着东北地区今后气候进一步变暖,可能将逐步影响到林龄更小的林分的生长,因此需要进一步研究如何在落叶松人工林经营中采取科学的措施来更好地应对未来气候变化。     

森林生态系统水源涵养服务功能解析

森林水源涵养功能作为森林生态系统服务的一个重要方面,在国内的研究已有数十年的历史。多数研究中将森林水源涵养功能狭义地等同于森林对降雨的拦蓄能力或对径流的调节能力,而忽略了其对各水文过程的整体性作用以及对气象和土壤因子的综合性影响。此外,在不同研究区域中,对于森林水源涵养功能的理解和研究方法往往一概而论,并未考虑研究区域的自然条件特性和社会经济特征,这使得研究结果的区域针对性不足,限制了对其进一步的应用推广。针对上述问题,在全面分析森林水源涵养的含义和特征之后,定义了不同区域的水源涵养功能内涵和主导服务,并给出了不同区域森林水源涵养功能的适宜计算方法,以期为不同区域森林水源涵养功能的全面提升和水源涵养林生态修复工程提供科学依据。     

微地形改造的生态环境效应研究进展

干旱缺水和生境不良使世界上许多地区的植被恢复和生态改善面临困难。为了提高植被成活率、遏制土壤侵蚀和土地退化态势,国内外许多重点地区都开展了多种微地形改造与下垫面整地措施,使得地表生境和植被状况得到一定改善。但微地形改造对生态环境影响的基础研究仍严重滞后于实践的客观需求,许多关键效应和科学机理不明。本文系统梳理和总结了国内外学者在不同生态系统类型区和自然地理单元上开展的相关研究。认为微地形改造对土壤属性和微生境、降雨入渗和水蚀过程、植被恢复的效果及其生态服务功能发挥等多个方面都有重要影响,并综述了相关研究进展。同时指出当前微地形改造研究中存在的突出问题。包括科学分类标准有待系统化、实地量化技术相对滞后、微地形改造的水文效应有待强化、影响植被恢复的机理不明等若干重要局限。建议应进一步加强微地形改造的分类体系研发、发展微地形改造方式的定量刻画技术;设立野外定位站,跟踪监测其长期效应,并加强不同微地形改造措施的生态环境效应对比,为科学筛选和优化下垫面改造技术、服务区域生态改善和应对气候变化提供科学依据。     

水源涵养与水文调节:和而不同的陆地生态系统水文服务

水与生态系统的关系是重要的科学问题,并且受到社会广泛关注。水源涵养和水文调节都是陆地生态系统所能提供的水文服务,并从生态水文和水资源角度把生态系统的健康和完整性与人类社会的持续发展紧密联系起来,从而也要求动态和综合的视角加以深入研究。基于生态水文过程原理,对水源涵养和水文调节的概念进行了辨析,认为前者是后者的有机组成部分,相对具体、应用中务求精确明晰;后者则更具包容性、客观性和广泛适用性。文章进一步简要分析了当前国内外生态系统水源涵养和水文调节服务的主导评估方法,结果表明,水源涵养的评估以储水量法为主,而水文调节则以基于降水和蒸散的水量平衡法及综合模型法(如SWAT)为主。从生态系统服务相互作用的角度考量,实际上水源涵养和水文调节及其相关的其它服务类型(如固碳、土壤保持、生物生产、淡水供给等)存在着复杂的动态权衡或协同关系,在科学研究和生态系统管理实践中必须统筹考虑,以确保对科学问题的准确把握和促进"水-生态-社会系统"的高效、可持续发展。为此,必须加强对生态水文过程的长期观测和实验研究,并且关注空间异质性及尺度效应、时间动态性和利益相关者需求的多维性。     

干旱与湿润区流域时变水热耦合参数的归因分析

气候变化和下垫面条件的改变共同影响着流域的水热耦合状况,定量剖析其影响对研究变化环境下流域水量平衡和能量分配具有重要意义。以往多数研究只做了定性分析,对不同影响因素的定量贡献研究甚少。基于此,以干旱区无定河流域和湿润区汉江上游为研究区域,在Budyko框架下,估算了干旱与湿润区流域的时变水热耦合参数,并通过逐步多元回归模型、敏感性和贡献率分析,定量归因了水热耦合控制参数的演变,并将干旱与湿润区流域做了对比分析。结果表明：干旱与湿润区流域的水热耦合参数在1970-2013年间均呈显著上升趋势;不同气候区水热耦合参数变化的主导因子不同,干旱区无定河参数对降水和植被覆盖度（NDVI）具有较高的敏感性,且NDVI的变化主导着参数的变化（贡献率为89.5%）;湿润区汉江上游参数的变化对气温、有效灌溉面积（EIA）和NDVI更敏感,其中,EIA主导着参数的变化（贡献率为83.1%）;总体来说,下垫面的变化是无定河和汉江上游水热状况改变的驱动因素。     

The problems of sustainable water use in the Mediterranean and research requirements for agriculture

This study addresses the sustainable use of water resources in the Mediterranean basin, particularly in the Southern and Eastern parts of the region, and the many problems generated by water scarcity and misuse. Water economy in the region is beset by two specific problems: high irrigation needs and changes in consumer demands (especially after population shifts from rural to urban areas and because of increasing tourism and industrialisation). The challenges presented by the water crisis are even greater because of growing populations and estimated future climatic changes in the region. The integrated management of limited water resources in the Southern and Eastern parts of the Mediterranean involves several areas of research. Those most directly related with agriculture concern improving water (and nutrient) use in agriculture through the management and breeding of irrigated and rain-fed crops. However, these fields of research address only one face of a multi-factorial equation that affects water sustainability in the region. Thus, other research fields include the design of comprehensive water policies and integrated planning, and technologies for advanced water treatment and re-use. Moreover, local problems and socio-economic aspects must be considered when addressing research issues.     

Changes and Relationships of Climatic and Hydrological Droughts in the Jialing River Basin,China

The comprehensive assessment of climatic and hydrological droughts in terms of their temporal and spatial evolutions is very important for water resources management and social development in the basin scale. To study the spatial and temporal changes of climatic and hydrological droughts and the relationships between them, the SPEI and SDI are adopted to assess the changes and the correlations of climatic and hydrological droughts by selecting the Jialing River basin, China as the research area. The SPEI and SDI at different time scales are assessed both at the entire Jialing River basin and at the regional levels of the three sub basins. The results show that the SPEI and SDI are very suitable for assessing the changes and relationships of climatic and hydrological droughts in large basins. Based on the assessment, for the Jialing River basin, climatic and hydrological droughts have the increasing tendency during recent several decades, and the increasing trend of climatic droughts is significant or extremely significant in the western and northern basin, while hydrological drought has a less significant increasing trend. Additionally, climatic and hydrological droughts tend to increase in the next few years. The results also show that on short time scales, climatic droughts have one or two months lag impact on hydrological droughts in the north-west area of the basin, and have one month lag impact in south-east area of the basin. The assessment of climatic and hydrological droughts based on the SPEI and SDI could be very useful for water resources management and climate change adaptation at large basin scale.     

Decadal change in wetland–woodland boundaries during the late 20th century reflects climatic trends

Wetlands are important and restricted habitats for dependent biota and play vital roles in landscape function, hydrology and carbon sequestration. They are also likely to be one of the most sensitive components of the terrestrial biosphere to global climate change. An understanding of relationships between wetland persistence and climate is imperative for predicting, mitigating and adapting to the impacts of future climate change on wetland extent and function. We investigated whether mire wetlands had contracted, expanded or remained stable during 1960–2000. We chose a study area encompassing a regional climatic gradient in southeastern Australia, specifically to avoid confounding effects of water extraction on wetland hydrology and extent. We first characterized trends in climate by examining data from local weather stations, which showed a slight increase in precipitation and marked decline in pan evaporation over the relevant period. Remote sensing of vegetation boundaries showed a marked lateral expansion of mires during 1961–1998, and a corresponding contraction of woodland. The spatial patterns in vegetation change were consistent with the regional climatic gradient and showed a weaker co‐relationship to fire history. Resource exploitation, wildland fires and autogenic mire development failed to explain the observed expansion of mire vegetation in the absence of climate change. We therefore conclude that the extent of mire wetlands is likely to be sensitive to variation in climatic moisture over decadal time scales. Late 20th‐century trends in climatic moisture may be related primarily to reduced irradiance and/or reduced wind speeds. In the 21st century, however, net climatic moisture in this region is projected to decline. As mires are apparently sensitive to hydrological change, we anticipate lateral contraction of mire boundaries in coming decades as projected climatic drying eventuates. This raises concerns about the future hydrological functions, carbon storage capacity and unique biodiversity of these important ecosystems.     

Assessing Tropical Forests' Climatic Sensitivities with Long‐term Data

Analyses relating long‐term records of tree growth to interannual climatic variation at La Selva, Costa Rica have revealed marked forest sensitivities to both temperature and dry‐season intensity ( Clark et al. 2010 ). The tropical‐forest biome is certain to become warmer, and many areas may become drier. Testing the generality of the La Selva findings with similar analyses of field data from diverse forests across the biome will be a valuable next step. Based on our experiences during the La Selva studies, we propose that such assessments will need to address three issues. One is the number of repeat forest measurements. Short series of re‐censuses can be an unreliable basis for assessing climatic sensitivities. For some key climatic factors (e.g., temperature), records consisting of fewer than 10–12 re‐censuses can span limited climatic ranges, producing erratic and largely nonsignificant correlations. Multiyear census intervals exacerbate these data limitations. Second, different types of forest‐growth data call for different analysis approaches. Cohort and tree‐ring records need to be adjusted for ontogenetic growth changes, while stand‐level data require taking into account potentially confounding influences from forest compositional changes, as from succession. Third, a reliable meteorological record is critical. Poor‐quality or internally inconsistent climatic records can fatally corrupt assessments of forest sensitivities. To be usable in such analyses, the meteorological record requires data quality control, gap filling, and adjustments to maintain the record's internal consistency in the face of commonly occurring methods changes (instruments, siting). We illustrate these issues using analyses of the long‐term La Selva records.     

Potential distribution of the invasive freshwater dinoflagellate Ceratium furcoides (Levander) Langhans (Dinophyta) in South America

Dinoflagellates of the genus Ceratium are predominantly found in marine environments, with a few species in inland waters. Over the last decades, the freshwater species Ceratium hirundinella and Ceratium furcoides have colonized and invaded several South American basins. The purpose of this study was to create a distribution model for the invasive dinoflagellate C. furcoides in South America in order to further investigate the basins at potential risk, as well as the environmental conditions that influence its expansion. This species is known to develop blooms due to its mobility, resistance to sedimentation, and optimized use of resources. Although nontoxic, blooms of the species cause many problems to both the natural ecosystems and water users. Potential distribution was predicted by using a maximum entropy algorithm (MaxEnt). Model was run with 101 occurrences obtained from the scientific literature, and climatic, hydrological and topographic variables. The developed model had a very good performance for the study area. The most susceptible areas identified were mainly concentrated in the basins between southeastern Brazil and northeastern Argentina. Besides already affected regions, new potentially suitable areas were identified in temperate regions of South America. The information generated here will be useful for authorities responsible for water and watershed management to monitor the spread of this species and address problems related to its establishment in new environments.     

Consequences of declining snow accumulation for water balance of mid‐latitude dry regions

Widespread documentation of positive winter temperature anomalies, declining snowpack and earlier snow melt in the Northern Hemisphere have raised concerns about the consequences for regional water resources as well as wildfire. A topic that has not been addressed with respect to declining snowpack is effects on ecosystem water balance. Changes in water balance dynamics will be particularly pronounced at low elevations of mid‐latitude dry regions because these areas will be the first to be affected by declining snow as a result of rising temperatures. As a model system, we used simulation experiments to investigate big sagebrush ecosystems that dominate a large fraction of the semiarid western United States. Our results suggest that effects on future ecosystem water balance will increase along a climatic gradient from dry, warm and snow‐poor to wet, cold and snow‐rich. Beyond a threshold within this climatic gradient, predicted consequences for vegetation switched from no change to increasing transpiration. Responses were sensitive to uncertainties in climatic prediction; particularly, a shift of precipitation to the colder season could reduce impacts of a warmer and snow‐poorer future, depending on the degree to which ecosystem phenology tracks precipitation changes. Our results suggest that big sagebrush and other similar semiarid ecosystems could decrease in viability or disappear in dry to medium areas and likely increase only in the snow‐richest areas, i.e. higher elevations and higher latitudes. Unlike cold locations at high elevations or in the arctic, ecosystems at low elevations respond in a different and complex way to future conditions because of opposing effects of increasing water‐limitation and a longer snow‐free season. Outcomes of such nonlinear interactions for future ecosystems will likely include changes in plant composition and productivity, dynamics of water balance, and availability of water resources.