极端降水和极端干旱事件对草原生态系统的影响

当前人类活动的加剧显著地影响着全球大气循环的格局。大气循环的多个模型均预测未来全球气候变化的显著特征是极端降水事件和极端干旱事件发生的频率会显著增加。水分是干旱、半干旱区草原植物生长发育的限制性资源, 而草原生态系统是陆地生态系统中对降水格局变化非常敏感的系统。但是, 关于极端降水事件和极端干旱事件对草原生态系统结构和功能的影响还是以分散的个案研究为主, 甚至关于极端气候事件的定义迄今也不尽相同。为此, 该文在分析极端气候事件定义及其研究方法的基础上, 总结了极端降水事件和极端干旱事件对草原生态系统土壤水分和养分状况、植物生长发育和生理特性、群落结构、生产力和碳循环过程的影响, 并提出了未来极端气候事件研究中应重点关注的5个重要方向, 以及控制试验研究的2个关键科学问题, 对开展全球变化背景下草原生态系统对极端气候事件响应机制的研究具有指导意义。     

How disturbance,competition, and dispersal interact to prevent tree range boundaries from keeping pace with climate change

Climate change is expected to cause geographic shifts in tree species' ranges, but such shifts may not keep pace with climate changes because seed dispersal distances are often limited and competition‐induced changes in community composition can be relatively slow. Disturbances may speed changes in community composition, but the interactions among climate change, disturbance and competitive interactions to produce range shifts are poorly understood. We used a physiologically based mechanistic landscape model to study these interactions in the northeastern United States. We designed a series of disturbance scenarios to represent varied disturbance regimes in terms of both disturbance extent and intensity. We simulated forest succession by incorporating climate change under a high‐emissions future, disturbances, seed dispersal, and competition using the landscape model parameterized with forest inventory data. Tree species range boundary shifts in the next century were quantified as the change in the location of the 5th (the trailing edge) and 95th (the leading edge) percentiles of the spatial distribution of simulated species. Simulated tree species range boundary shifts in New England over the next century were far below (usually <20 km) that required to track the velocity of temperature change (usually more than 110 km over 100 years) under a high‐emissions scenario. Simulated species` ranges shifted northward at both the leading edge (northern boundary) and trailing edge (southern boundary). Disturbances may expedite species' recruitment into new sites, but they had little effect on the velocity of simulated range boundary shifts. Range shifts at the trailing edge tended to be associated with photosynthetic capacity, competitive ability for light and seed dispersal ability, whereas shifts at the leading edge were associated only with photosynthetic capacity and competition for light. This study underscores the importance of understanding the role of interspecific competition and disturbance when studying tree range shifts.     

The role of the brown-eared bulbulHypsypetes amaurotis as a seed dispersal agent

The role of the brown-eared bulbul,Hypsypetes amaurotis, as a dispersal agent for seeds of fruiting plants was studied by field observations for two years, in parallel with laboratory experiments on seed germination. Bulbuls consumed fruits of 53 species from 24 plant families. The fruits of these plants had similar color and size, and these characteristics were likely to enhance the feeding efficiency of the frugivore. Laboratory experiments on 20 food plant species demonstrated that: (1) no seeds were injured by passing through the bulbul's gut; (2) seeds that had passed through the bulbul's gut were still able to germinate and (3) fruit pulp reduced germination ability. When pulp was removed by passing through bulbul's gut, or by hand, germination was improved. An estimate of the home range of six bulbuls suggested that they may transport seeds for at least 300 m.     

Anomalous,extreme weather disrupts obligate seed dispersal mutualism: snow in a subtropical forest ecosystem

Ongoing global climate change is predicted to increase the frequency and magnitude of extreme weather events, impacting population dynamics and community structure. There is, however, a critical lack of case studies considering how climatic perturbations affect biotic interactions. Here, we document how an obligate seed dispersal mutualism was disrupted by a temporally anomalous and meteorologically extreme interlude of unseasonably frigid weather, with accompanying snowstorms, in subtropical China, during January–February 2008. Based on the analysis of 5892 fecal samples (representing six mammalian seed dispersers), this event caused a substantial disruption to the relative seed dispersal function for the raisin tree Hovenia dulcis from prestorm 6.29 (2006) and 11.47 (2007), down to 0.35 during the storm (2008). Crucially, this was due to impacts on mammalian seed dispersers and not due to a paucity of fruit, where 4.63 fruit per branch were available in January 2008, vs. 3.73 in 2006 and 3.58 in 2007. An induced dietary shift occurred among omnivorous carnivores during this event, from the consumption fruit to small mammals and birds, reducing their role in seed dispersal substantially. Induced range shift extinguished the functionality of herbivorous mammals completely, however, seed dispersal function was compensated in part by three omnivorous carnivores during poststorm years, and thus while the mutualism remained intact it was enacted by a narrower assemblage of species, rendering the system more vulnerable to extrinsic perturbations. The storm's extended effects also had anthropogenic corollaries – migrating ungulates becoming exposed to heightened levels of illegal hunting – causing long‐term modification to the seed dispersal community and mutualism dynamics. Furthermore, degraded forests proved especially vulnerable to the storm's effects. Considering increasing climate variability and anthropogenic disturbance, the impacts of such massive, aberrant events warrant conservation concern, while affording unique insights into the stability of mutualisms and the processes that structure biodiversity and mediate ecosystem dynamics.     

Reduced germination success of temperate grassland seeds sown in dung: consequences for post‐dispersal seed fate

• Endozoochory is one of the main drivers shaping temperate grassland communities by maintaining plant populations of its constituents and enabling plants to colonize new habitats. Successful endozoochorous dispersal implies that seeds not only get consumed and survive the digestive tract but are also able to develop into viable seedlings in a dung environment.
• We experimentally assessed the germination probability and timing of 15 annual and perennial temperate European grassland species in cattle and horse dung and in different climatic conditions (greenhouse and outdoor conditions).
• Interspecific variation in germinability and germination timing are found, while life strategy had only an effect on germination timing. We found adverse effects of both cattle and horse dung on the germination characteristics of all tested grassland species, but the effects of cattle dung were more pronounced. In comparison with the control treatment, fewer seeds emerged in dung and more time was needed to germinate. Also, germination metrics clearly differed between the artificial greenhouse and outdoor conditions, with generally a lower germinability in outdoor conditions.
• According to our results, a large cost seems to be associated with endozoochorous dispersal in this stage of the life cycle, as seed dispersal effectiveness strongly depends on the quality of the deposition site with a lowered survival and germination probability when seeds are deposited in dung.

     

The role of the seed bank,seed rain and the timing of disturbance in gap regeneration

Abstract. Question: Does the degree and timing of disturbance contribute significantly to the pattern and process of regeneration in plant communities as a consequence of the availability and number of species of propagules present? Location: Acid grassland at 230 m a.s.l., eastern Scotland, UK. Methods : Plots were surface disturbed or had their soil profile inverted at monthly intervals at 12 dates during a year. Seed bank and seed rain were assessed at each treatment time. The effect of disturbance intensity and timing on the regenerating vegetation was assessed. Results: Removing the seed bank significantly slowed regeneration, as it contributed 43 % of developing cover after one year where it was present. At an individual seed level, seed in the seed rain had a much higher likelihood of contributing to the regenerating vegetation than a seed in the seed bank. Some species showed a reliance on the seed bank for regeneration, and hence there was a significant difference in the vegetation that developed between plots with the seed bank intact and those where it was removed. Winter disturbed plots (little seed rain) had slower rates of re‐vegetation than summer disturbed plots. Timing had little effect on species composition, though a significantly higher cover of perennial forb species developed on the winter disturbed plots. Conclusion: Removing the contribution of the seed bank had a greater effect on the composition of regenerating vegetation than the effect of seasonal variation on the seed rain.     

Multiscale topoedaphic heterogeneity increases resilience and resistance of a dominant grassland species to extreme drought and climate change

It is argued that the inclusion of spatially heterogeneous environments in biodiversity reserves will be an effective means of encouraging ecosystem resilience and plant community conservation under climate change. However, the resilience and resistance of plant populations to global change, the specific life‐history traits involved and the spatial scale at which environmentally driven demographic variation is expressed remains largely unknown for most plant groups. Here we address these questions by reporting an empirical investigation into the impacts of an unprecedented 3‐year drought on the demography, population growth rates (λ) and biogeographical distribution of core populations of the perennial grassland species Austrostipa aristiglumis in semiarid Australia. We use life‐history analysis and periodic matrix population models to specifically test the hypothesis that patch‐ and habitat‐scale variation in vital life‐history parameters result in spatial differences in the resilience and resistance of A. aristiglumis populations to extreme drought. We show that the development of critical soil water deficits during drought resulted in collapse of adult A. aristiglumis populations (λ?1), rapid interhabitat phytosociological change and overall contraction towards mesic refugia where populations were both more resistant and resilient to perturbation. Population models, combined with climatic niche analysis, suggest that, even in core areas, a significant reduction in size and habitat range of A. aristiglumis populations is likely under climate change expected this century. Remarkably, however, we show that even minor topographic variation (0.2–3 m) can generate significant variation in demographic parameters that confer population‐level resilience and resistance to drought. Our findings support the hypothesis that extreme climatic events have the capacity to induce rapid, landscape‐level shifts in core plant populations, but that the protection of topographically heterogeneous environments, even at small spatial scales, may play a key role in conserving biodiversity under climate change in the coming century.     

油质体在5种蚁播植物种子散布中的作用

蚁播植物种子上常附着有蚂蚁喜食的油质体,该物体可吸引蚂蚁取食或为蚂蚁提供钳着位点从而影响种子散布。为进一步揭示油质体在种子散布中的作用,在野外研究了4属5种典型蚁播植物包括小花宽瓣黄堇(Corydalis giraldii Fedde)、假刻叶紫堇(C.pseudoincisa C.Y.Wu)、白屈菜(Chelidonium majus L.)、紫花堇菜(Viola grypoceras A.Gray)和柔毛淫羊藿(Epimedium pubescens Maxim.)其相应的搬运蚂蚁即玉米毛蚁(Lasius alienus(Foerster))和尼特纳大头蚁(Pheidole nietneri Emery)对植物完整种子(SE)、去除油质体的种子(S)和粘有人工模拟油质体的种子(S+H)的搬运行为和搬运效率。结果显示,蚂蚁对相同植物3类种子的触碰和检查次数均无显著差异。蚂蚁对种子的尝试搬运次数除玉米毛蚁对小花宽瓣黄堇、假刻叶紫堇和白屈菜去除油质体的种子(S)显著高于完整种子(SE)和粘有人工模拟油质体的种子(S+H)外,其它的均无显著差异。玉米毛蚁对小花宽瓣黄堇、假刻叶紫堇、白屈菜和紫花堇菜以及尼特纳大头蚁对小花宽瓣黄堇、假刻叶紫堇和紫花堇菜完整种子(SE)的搬运效率显著高于粘有人工模拟油质体的种子(S+H)和去除油质体的种子(S),此外,玉米毛蚁对小花宽瓣黄堇粘有人工模拟油质体的种子(S+H)的搬运效率显著高于去除油质体的种子(S)。这说明玉米毛蚁在搬运小花宽瓣黄堇种子过程中油质体不仅起到吸引作用,而且起到钳着位点作用;玉米毛蚁在搬运假刻叶紫堇、白屈菜和紫花堇菜以及尼特纳大头蚁搬运小花宽瓣黄堇、假刻叶紫堇和紫花堇菜种子过程中油质体仅起到吸引作用。研究表明油质体可通过对蚂蚁起吸引作用,或同时起吸引和钳着位点的双重作用等影响种子散布,油质体的影响作用不但取决于植物种类,也与搬运蚂蚁的种类有关。     

Impact of the spatial uncertainty of seed dispersal on tree colonization dynamics in a temperate forest

An aggregated distribution of dispersed seeds may influence the colonization process in tree communities via inflated spatial uncertainty. To evaluate this possibility, we studied 10 tree species in a temperate forest: one primarily barochorous, six anemochorous and two endozoochorous species. A statistical model was developed by combining an empirical seed dispersal kernel with a gamma distribution of seedfall density, with parameters that vary with distance. In the probability density, the fitted models showed that seeds of Fagaceae (primarily barochorous) and Betulaceae (anemochorous) were disseminated locally (i.e. within 60 m of a mother tree), whereas seeds of Acer (anemochorous) and endozoochorous species were transported farther. Greater fecundity compensated for the lower probability of seed dispersal over long distances for some species. Spatial uncertainty in seedfall density was much greater within 60 m of a mother tree than farther away, irrespective of dispersal mode, suggesting that seed dispersal is particularly aggregated in the vicinity of mother trees. Simulation results suggested that such seed dispersal patterns could lead to sites in the vicinity of a tree being occupied by other species that disperse seeds from far away. We speculate that this process could promote coexistence by making the colonization rates of the species more similar on average and equalizing species fitness in this temperate forest community.     

Persistent versus transient tree encroachment of temperate peat bogs: effects of climate warming and drought events

Peatlands store approximately 30% of global soil carbon, most in moss‐dominated bogs. Future climatic changes, such as changes in precipitation patterns and warming, are expected to affect peat bog vegetation composition and thereby its long‐term carbon sequestration capacity. Theoretical work suggests that an episode of rapid environmental change is more likely to trigger transitions to alternative ecosystem states than a gradual, but equally large, change in conditions. We used a dynamic vegetation model to explore the impacts of drought events and increased temperature on vegetation composition of temperate peat bogs. We analyzed the consequences of six patterns of summer drought events combined with five temperature scenarios to test whether an open peat bog dominated by moss (Sphagnum) could shift to a tree‐dominated state. Unexpectedly, neither a gradual decrease in the amount of summer precipitation nor the occurrence of a number of extremely dry summers in a row could shift the moss‐dominated peat bog permanently into a tree‐dominated peat bog. The increase in tree biomass during drought events was unable to trigger positive feedbacks that keep the ecosystem in a tree‐dominated state after a return to previous ‘normal’ rainfall conditions. In contrast, temperature increases from 1 °C onward already shifted peat bogs into tree‐dominated ecosystems. In our simulations, drought events facilitated tree establishment, but temperature determined how much tree biomass could develop. Our results suggest that under current climatic conditions, peat bog vegetation is rather resilient to drought events, but very sensitive to temperature increases, indicating that future warming is likely to trigger persistent vegetation shifts.     

The ecology and evolution of the monito del monte,a relict species from the southern South America temperate forests

The arboreal marsupial monito del monte (genus Dromiciops, with two recognized species) is a paradigmatic mammal. It is the sole living representative of the order Microbiotheria, the ancestor lineage of Australian marsupials. Also, this marsupial is the unique frugivorous mammal in the temperate rainforest, being the main seed disperser of several endemic plants of this ecosystem, thus acting as keystone species. Dromiciops is also one of the few hibernating mammals in South America, spending half of the year in a physiological dormancy where metabolism is reduced to 10% of normal levels. This capacity to reduce energy expenditure in winter contrasts with the enormous energy turnover rate they experience in spring and summer. The unique life history strategies of this living Microbiotheria, characterized by an alternation of life in the slow and fast lanes, putatively represent ancestral traits that permitted these cold‐adapted mammals to survive in this environment. Here, we describe the ecological role of this emblematic marsupial, summarizing the ecophysiology of hibernation and sociality, updated phylogeographic relationships, reproductive cycle, trophic relationships, mutualisms, conservation, and threats. This marsupial shows high densities, despite presenting slow reproductive rates, a paradox explained by the unique characteristics of its three‐dimensional habitat. We finally suggest immediate actions to protect these species that may be threatened in the near future due to habitat destruction and climate change.     

基于NDVI的云南省2009-2010年干旱遗产效应研究

气候变化正导致干旱事件发生的强度、频度显著改变,极端气候事件发生的不确定性直接影响陆地生态系统关键生态过程。我国西南地区在2009-2010年发生百年一遇的极端干旱,目前关于植被生长在长时间尺度对此次干旱事件的响应尚不明确。以云南省为研究区,基于多年Normalized difference vegetation index(NDVI)影像数据以及长时间序列气象资料对此次极端事件的干旱遗产效应开展研究,分析了干旱遗产效应的持续时间以及不同植被类型的响应差异。结果表明:1)云南省植被生长在极端干旱事件发生后受到的抑制时间大约持续1-2年,受影响区域主要集中在遭遇降水严重减少的地区;2)海拔2000 m附近为植被对干旱响应最为敏感的区域,海拔高于4000 m的植被生长几乎未受到干旱影响;3)较之草地和农田,森林植被受到的抑制作用更为强烈。研究揭示了极端干旱对云南省植被生长造成的影响,为该地区未来应对极端干旱并有效开展植被恢复提供理论依据。     

Dry and wet periods drive rapid shifts in community assembly in an estuarine ecosystem

The impacts of changing climate regimes on emergent processes controlling the assembly of ecological communities remain poorly understood. Human alterations to the water cycle in the western United States have resulted in greater interannual variability and more frequent and severe extremes in freshwater flow. The specific mechanisms through which such extremes and climate regime shifts may alter ecological communities have rarely been demonstrated, and baseline information on current impacts of environmental variation is widely lacking for many habitats and communities. Here, we used observations and experiments to show that interannual variation in winter salinity levels in San Francisco Bay controls the mechanisms determining sessile invertebrate community composition during the following summer. We found consistent community changes in response to decadal‐scale dry and wet extremes during a 13‐year period, producing strikingly different communities. Our results match theoretical predictions of major shifts in species composition in response to environmental forcing up to a threshold, beyond which we observed mass mortality and wholesale replacement of the former community. These results provide a window into potential future community changes, with environmental forcing altering communities by shifting the relative influences of the mechanisms controlling species distributions and abundances. We place these results in the context of historical and projected future environmental variation in the San Francisco Bay Estuary.     

Snow damage in a Himalayan forest maintains the dominance of evergreen oaks

Extreme climate events are increasing due to climate change. Heavy snowfall in a Himalayan evergreen oak forest damaged 21% trees and created large treefall gaps. Such occasional events may help define the upper altitudinal boundary of certain species. Oak trees with branches lopped by local communities showed lower damage.     

Seed dispersal and seedling recruitment of trees at different successional stages in a temperate forest in northeastern China

Aims Spatial distribution of adult trees in a forest community is determined by patterns of both seed dispersal and seedling recruitment. The objectives of our study were to understand the processes of seed dispersal and seedling recruitment of dominant tree species in a temperate forest of northeastern China and to identify the factors constraining seed dispersal and seedling establishment at different stages of forest succession.Methods During three summer and autumn sessions between 2006 and 2008, altogether 113080 seeds from 22 different tree species were collected in three large field plots representing different forest types in the Changbai Mountain region of northeastern China. The spatial distribution of seed abundance was analyzed using a Syrjala test. Regeneration success of nine major tree species was assessed using variables defining 'limitations' in 'seeds' and 'seedling establishment'.Important findings We found that seed production fluctuated between years and varied greatly with forest types. Four tree species, Acer spp., Fraxinus mandshurica, Tilia amurensis and Betula spp., had the greatest seed production and the widest range of seed dispersal, whereas Quercus mongolica showed the most sustained seed production pattern. The spatial patterns of seed abundance differed significantly among forest types and years. The tree species investigated in this study differed in the degree of seed limitation, as well as in limitation of seedling establishment. There were both negative and positive correlations between seed density and seedling density, depending on site and parental tree density. Seeds of 16 tree species were found in the Populus davidiana–Betula platyphylla forest (PBF) plot, 11 in the conifer and broad-leaved mixed forest (CBF) plot but only 8 in the broad-leaved-Korean pine mixed forest (BKF) plot. The number of seed-contributing species was not only greater in the secondary forests (CBF and PBF plots) than in the primary forest (BKF plot) but was also more variable during the 3 years of assessment. Results from the correlations between seed density and seedling occurrence and that between parental tree density or seed weight and dispersal limitation confirm our intuitive expectations, i.e. heavy seeds had greater dispersal limitation but higher establishment success than light seeds.     

The role of spines in anthropogenic seed dispersal on the Galápagos Islands

1. Dispersal has important ecological and evolutionary consequences for populations, but understanding the role of specific traits in dispersal can be difficult and requires careful experimentation. Moreover, understanding how humans alter dispersal is an important question, especially on oceanic islands where anthropogenic disturbance through species introductions can dramatically alter native ecosystems.
2. In this study, we investigated the functional role of spines in seed dispersal of the plant caltrop (Tribulus cistoides L., Zygophyllaceae) by anthropogenic dispersal agents. We also tested whether humans or wildlife are more important seed dispersers of T. cistoides on the Galápagos.
3. Tribulus cistoides is found on tropical mainland and oceanic island habitats. The dispersal structure of T. cistoides is called a mericarp, and they are typically protected by one pair of upper spines and a second pair of lower spines, but the presence and size of spines varies within and between populations. On the Galápagos, the upper and lower spines protect mericarps from seed predation by Darwin's finches. We tested whether spines play a dual role in dispersal by factorially manipulating the presence/absence of the upper and lower spines to simulate natural variation in mericarp morphology.
4. The upper spines greatly facilitated seed dispersal, whereas the lower spines had no discernible effect on dispersal. The presence of upper spines increased dispersal rate on shoes by pedestrians 23‐fold, on fabrics (e.g., towels) and cars by nearly twofold, and the presence of upper spines increased dispersal distance by cars sixfold. When comparing dispersal rates in habitats with high (roads and foot paths) versus low (arid forest) anthropogenic activity, dispersal rates were demonstrably higher in the habitats with more human activity.
5. These results have important implications for understanding the ecology and evolution of plant dispersal in the Anthropocene. Spines on the fruits of T. cistoides play important functional roles in anthropogenic dispersal, whereas native and introduced wildlife plays a minor role in dispersal on inhabited islands of the Galápagos. Our results imply that seed predators and humans are jointly shaping the ecology and evolution of contemporary populations of T. cistoides on the Galápagos.

     

树木年轮揭示的东灵山主要树种间干旱耐受性差异

近年来北京地区的森林随极端干旱加剧表现出脆弱性特征,为了解气候变化下不同树种的干旱耐受性,选择北京东灵山森林内3个乔木树种(华北落叶松、油松和辽东栎),利用树木年轮生态学方法分析了径向生长与气候的关系,以及对极端干旱事件的抵抗力和弹性。结果表明: 华北落叶松和油松与5—6月气温呈显著负相关,辽东栎与5月气温呈显著负相关;华北落叶松与6月降水量、5—6月和8—9月相对湿度呈显著正相关,油松与6—8月降水量和相对湿度呈显著正相关,辽东栎与2月和5月降水量、5月相对湿度呈显著正相关;所有树种均与当年5—7月标准化降水蒸散指数(SPEI)呈显著正相关。华北落叶松是干旱耐受性最弱的树种,径向生长在所选极端干旱事件中(1994年、2001—2002年和2007年)下降幅度最大(46.6%～69.6%),抵抗力(0.534、0.304、0.530)和弹性(0.686、0.570、0.753)显著低于辽东栎和油松,辽东栎在2007年抵抗力显著高于油松,弹性无显著差异。生长季持续的高温或降水减少引起的极端干旱是树木径向生长下降的主要原因,树种间各异的生理生态策略是干旱耐受性差异的可能原因。研究结果可为未来造林树种选择和森林管护措施制定提供新依据,以在气候压力持续增加背景下维持森林生态系统功能和服务。