Hotspots of biotic compositional change in lakes along vast latitudinal transects in northern Canada

Ecotones mark zones of rapid change in ecological structure at various spatial scales. They are believed to be particularly susceptible to shifts caused by environmental transformation, making them key regions for studying the effects of global change. Here, we explored the variation in assemblage structure of aquatic primary producer and consumer communities across latitudinal transects in northeastern North America (Québec‐Labrador) to identify spatial patterns in biodiversity that indicated the location of transition zones across the landscape. We analyzed species richness and the cumulative rate of compositional change (expressed as beta‐diversity) of diatoms and chironomids to detect any abrupt shifts in the rate of spatial taxonomic turnover. We used principal coordinates analysis to estimate community turnover with latitude, then applied piecewise linear regression to assess the position of ecotones. Statistically significant changes in assemblage composition occurred at 52 and 55°N, corresponding to the transition between closed‐ and open‐crown forest, and to the southern onset of the forest tundra (i.e., the forest limit), respectively. The spatial distribution of ecotones was most strongly related to air temperature for chironomids and to vegetation‐ and soil‐related chemical attributes of lake water for diatoms, including dissolved organic carbon content and water color. Lakes at mid‐ to high‐latitudes currently face pressures from rapidly rising temperatures, accompanied by large increases in organic carbon inputs from their catchments, often leading to browning and its associated effects. The biota at the base of food webs in lakes located in transition zones are disproportionately affected by the cascading effects of these multi‐factorial changes, concurrent with pronounced terrestrial greening observed in these regions. Similar patterns of biotic shifts have been observed along alpine aquatic transects, indicating the potential for widespread restructuring of cold, high‐altitude and high‐latitude freshwater communities due to global change.     

Trait‐based diatom functional diversity as an appropriate tool for understanding the effects of environmental changes in soda pans

Saline lakes, among the most seriously endangered ecosystems, are threatened due to climate change and human activities. One valuable feature of these environments is that they constitute areas of high biodiversity. Ecologists are, therefore, under great pressure to improve their understanding of the effects of natural and anthropogenic disturbances on the biodiversity of saline lakes. In this study, a total of 257 samples from 32 soda pans in Central Europe between 2006 and 2015 were examined. The effects of environmental variables and of geographical and limnoecological factors on functional diversity were analyzed. Furthermore, the explanatory power of the trait‐based approach was assessed, and the applicability of the indices for biomonitoring purposes was determined. It was found that low habitat heterogeneity and harsh environments lead to the selection of a small number of suitable traits, and consequently, to a naturally low level of functional diversity. Anthropogenic activities enhance diversity at functional level due to the shift toward freshwater characteristics. On the regional scale, the effects of the region and status (natural, degraded, reconstructed) on diatom functional diversity were significant and more pronounced than that of the environmental and other limnoecological factors. The degree of variance found in functional diversity ascribed to environmental variables is five times greater in the case of the application of a trait‐based approach, than when a taxonomic one is employed in the literature. Each of the tested functional diversity indices was sensitive to the most important environmental variables. Furthermore, these were type‐specific and proved to be more complex indicators than taxonomic metrics. It is possible to suggest four functional diversity indices (FGR, FRic, FDis, and FDiv) which emphasize their independence from substrate and seasonal variations for ecological status assessment and conservation planning.     

Effects of land-use change on functional and taxonomic diversity of Neotropical bats

Human land-use changes are particularly extensive in tropical regions, representing one of the greatest threats to terrestrial biodiversity and a key research topic in conservation. However, studies considering the effects of different types of anthropogenic disturbance on the functional dimension of biodiversity in human-modified landscapes are rare. Here, we obtained data through an extensive review of peer-reviewed articles and compared 30 Neotropical bat assemblages in well-preserved primary forest and four different human-disturbed habitats in terms of their functional and taxonomic diversity. We found that disturbed habitats that are structurally less similar to primary forest (pasture, cropland, and early-stage secondary forest) were characterized by a lower functional and taxonomic diversity, as well as community-level functional uniqueness. These habitats generally retained fewer species that perform different ecological functions compared to higher-quality landscape matrices, such as agroforestry. According to functional trait composition, different bat ensembles respond differently to landscape change, negatively affecting mainly gleaning insectivorous bats in pasture, narrow-range species in cropland, and heavier animalivorous bats in secondary forest. Although our results highlight the importance of higher-quality matrix habitats to support elevated functional and taxonomic bat diversity, the conservation of bat species that perform different ecological functions in the mosaic of human-modified habitats also depends on the irreplaceable conservation value of well-preserved primary forests. Our study based on a pooled analysis of individual studies provides novel insights into the effects of different human-modified habitats on Neotropical bat assemblages.     

Defoliation of a grass is mediated by the positive effect of dung deposition,moss removal and enhanced soil nutrient contents: results from a reindeer grazing simulation experiment

Herbivory is one of the key drivers shaping plant community dynamics. Herbivores can strongly influence plant productivity directly through defoliation and the return of nutrients in the form of dung and urine, but also indirectly by reducing the abundance of neighbouring plants and inducing changes in soil processes. However, the relative importance of these processes is poorly understood. We, therefore, established a common garden experiment to study plant responses to defoliation, dung addition, moss cover, and the soil legacy of reindeer grazing. We used an arctic tundra grazed by reindeer as our study system, and Festuca ovina, a common grazing‐tolerant grass species as the model species. The soil legacy of reindeer grazing had the strongest effect on plants, and resulted in higher growth in soils originating from previously heavily‐grazed sites. Defoliation also had a strong effect and reduced shoot and root growth and nutrient uptake. Plants did not fully compensate for the tissue lost due to defoliation, even when nutrient availability was high. In contrast, defoliation enhanced plant nitrogen concentrations. Dung addition increased plant production, nitrogen concentrations and nutrient uptake, although the effect was fairly small. Mosses also had a positive effect on aboveground plant production as long as the plants were not defoliated. The presence of a thick moss layer reduced plant growth following defoliation. This study demonstrates that grasses, even though they suffer from defoliation, can tolerate high densities of herbivores when all aspects of herbivores on ecosystems are taken into account. Our results further show that the positive effect of herbivores on plant growth via changes in soil properties is essential for plants to cope with a high grazing pressure. The strong effect of the soil legacy of reindeer grazing reveals that herbivores can have long‐lasting effects on plant productivity and ecosystem functioning after grazing has ceased.     

CO2 efflux from a Mediterranean semi-arid forest soil. I. Seasonality and effects of stoniness

We studied the seasonality of total soil CO₂efflux and labeled C-CO₂ released from ¹⁴Clabeled straw incubated in the H horizon of asemi-arid Mediterranean forest soil. Fieldmeasurements were carried out over 520 days in aseries of reconstructed soil profiles with and withouta gravel layer below the H horizon. We monitored soilclimate and related this to soil CO₂ efflux.Seasonal variations in soil CO₂ efflux in asemiarid Mediterranean forest were mainly related tochanges in soil temperature. In spite of drought, highrespiration rates were observed in mid summer. Highsoil CO₂ efflux in hot and dry episodes wasattributed to increases in soil biological activity.The minimum soil CO₂ efflux occurred in latesummer also under dry conditions, probably related toa decrease in soil biological activity in deephorizons. Biological activity in organic layers waslimited by water potential () in summer and bytemperature in winter. Rewetting a dry soil resultedin large increases in soil CO₂ efflux only at hightemperatures. These large increases represented asignificant contribution to the decomposition oforganic matter in the uppermost horizons. Soilbiological activity in the uppermost horizons was moresensitive to changes in soil and hence tosummer rainstorms than the bulk soil microbialactivity. The presence of a layer of gravel improvedboth moisture and temperature conditions for thedecomposition of organic matter. As a result, soilCO₂ efflux increased in soils containing rockfragments. These effects were especially large for theorganic layers.     

Contrasting long-term alpine and subalpine precipitation trends in a mid-latitude North American mountain system,Colorado Front Range,USA

Background: Long-term climate trends in mountain systems often vary strongly with elevation.

Aims: To evaluate elevation dependence in long-term precipitation trends in subalpine forest and alpine tundra zones of a mid-continental, mid-latitude North American mountain system and to relate such dependence to atmospheric circulation patterns.

Methods: We contrasted 59-year (1952–2010) precipitation records of two high-elevation climate stations on Niwot Ridge, Colorado Front Range, Rocky Mountains, USA. The sites, one in forest (3022 m a.s.l.) and the other in alpine tundra (3739 m), are closely located (within 7 km horizontally, ca. 700 m vertically), but differ with respect to proximity to the mountain-system crest (the Continental Divide).

Results: The sites exhibited significant differences in annual and seasonal precipitation trends, which depended strongly on their elevation and distance from the Continental Divide. Annual precipitation increased by 60 mm (+6%) per decade at the alpine site, with no significant change at the subalpine site. Seasonally, trends at the alpine site were dominated by increases in winter, which we suggest resulted from an increase in orographically generated precipitation over the Divide, driven by upper-air (700 hPa) north-westerly flow. Such a change was not evident at the subalpine site, which is less affected by orographic precipitation on north-westerly flow.

Conclusions: Elevation dependence in precipitation trends appears to have arisen from a change in upper-air flow from predominantly south-westerly to north-westerly. Dependence of precipitation trends on topographic position and season has complex implications for the ecology and hydrology of Niwot Ridge and adjacent watersheds, involving interactions among physical processes (e.g. snowpack dynamics) and biotic responses (e.g. in phenologies and ecosystem productivity).     

New nitrogen uptake strategy: specialized snow roots

The evolution of plants has yielded a wealth of adaptations for the acquisition of key mineral nutrients. These include the structure, physiology and positioning of root systems. We report the discovery of specialized snow roots as a plant strategy to cope with the very short season for nutrient uptake and growth in alpine snow-beds, i.e. patches in the landscape that remain snow-covered well into the summer. We provide anatomical, chemical and experimental ¹⁵N isotope tracking evidence that the Caucasian snow-bed plant Corydalis conorhiza forms extensive networks of specialized above-ground roots, which grow against gravity to acquire nitrogen directly from within snow packs. Snow roots capture nitrogen that would otherwise partly run off down-slope over a frozen surface, thereby helping to nourish these alpine ecosystems. Climate warming is changing and will change mountain snow regimes, while large-scale anthropogenic N deposition has increased snow N contents. These global changes are likely to impact on the distribution, abundance and functional significance of snow roots.     

河漫滩湿地生态阈值——以二卡自然保护区为例

以典型河漫滩湿地 —— 二卡自然保护区为研究区域,进行生态阈值研究。根据湿地补给河流 —— 海拉尔河的月均流量频率分布特征,选取58.72 m³/s(P=74.6%)、119.26 m³/s(P=44.4%)、190.35 m³/s(P=23.8%)分别代表河流的低、中、高径流期,分析各径流期湿地分布特征。结果表明,低、中径流期内草本沼泽及盐化沼泽所占比例较高,而高径流期则以季节性河流湿地及湖泊湿地为主。低径流期内,作为湿地中心区的湖泊湿地景观破碎度出现一个较明显的跃变,由中径流期的0.57增加到1.37。此时的湿地状态可近似的作为区域的生态阈值,即维持湿地面积占全区域面积的43.03%。而中、高径流期湿地面积比例可近似作为保护区湿地面积的适宜值和理想值,分别为53.66%和69.53%。其研究结果为二卡自然保护区的管理及保护提供支持,并为河漫滩湿地研究提供思路。