Above- and belowground community linkages in boreal peatlands

Plant Ecology - In boreal peatlands, the aboveground (plant) and belowground (microbial) communities are acutely linked because the whole soil profile is partially decomposed plant matter (peat),...     

Rhizosphere microbial community and its response to plant species and soil history

The plant rhizosphere is a dynamic environment in which many parameters may influence the population structure, diversity and activity of the microbial community. Two important factors determining the structure of microbial community present in the vicinity of plant roots are plant species and soil type. In the present study we assessed the structure of microbial communities in response to four plant species (i.e. maize (Zea mays L.), oat (Avena sativa L.), barley (Hordeum vulgare L.) and commercial grass mix) planted in soil with different land use history (i.e. arable land under crop rotation, maize monoculture and permanent grassland). Both factors, plant species and land use history, showed clear effects on microbial community and diversity as determined by PCR-DGGE fingerprinting with universal and group-specific bacterial primers. Moreover, we explored the rhizosphere effect of these plant species on the abundance of bacterial antagonists of the potato pathogen Rhizoctonia solani AG3. The data showed that the abundance and taxonomic composition of antagonists differed clearly between the different plants. The highest percentages of antagonists were found in maize and grass rhizosphere. When antagonistic Pseudomonas populations were compared, the highest, abundance and diversity of antagonists were detected in barley and oat rhizospheres, as compared to maize and grass rhizosphere. The results obtained in our study demonstrate clearly that plant species and soil type are two important factors affecting the structure of total bacterial, Pseudomonas and Bacillus community.     

Disentangling direct and indirect effects of water table drawdown on above‐ and belowground plant litter decomposition: consequences for accumulation of organic matter in boreal peatlands

Pristine peatlands are carbon (C)‐accumulating wetland ecosystems sustained by a high water table (WT) and consequent anoxia that slows down decomposition. Persistent WT drawdown as a response to climate and/or land‐use change affects decomposition either directly through environmental factors such as increased oxygenation, or indirectly through changes in plant community composition. This study attempts to disentangle the direct and indirect effects of WT drawdown by measuring the relative importance of environmental parameters (WT depth, temperature, soil chemistry) and litter type and/or litter chemical quality on the 2‐year decomposition rates of above‐ and belowground litter (altogether 39 litter types). Consequences for organic matter accumulation were estimated based on the annual litter production. The study sites were chosen to form a three‐stage chronosequence from pristine (undrained) to short‐term (years) and long‐term (decades) WT drawdown conditions at three nutrient regimes. The direct effects of WT drawdown were overruled by the indirect effects through changes in litter type composition and production. Short‐term responses to WT drawdown were small. In long‐term, dramatically increased litter inputs resulted in large accumulation of organic matter in spite of increased decomposition rates. Furthermore, the quality of the accumulated matter greatly changed from that accumulated in pristine conditions. Our results show that the shift in vegetation composition as a response to climate and/or land‐use change is the main factor affecting peatland ecosystem C cycle, and thus dynamic vegetation is a necessity in any model applied for estimating responses of C fluxes to changing environment. We provide possible grouping of litter types into plant functional types that the models could utilize. Furthermore, our results clearly show a drop in soil summer temperature as a response to WT drawdown when an initially open peatland converts into a forest ecosystem, which has not yet been considered in the existing models.     

Water level drawdown makes boreal peatland vegetation more responsive to weather conditions

Climate warming and projected increase in summer droughts puts northern peatlands under pressure by subjecting them to a combination of gradual drying and extreme weather events. The combined effect of those on peatland functions is poorly known. Here, we studied the impact of long-term water level drawdown (WLD) and contrasting weather conditions on leaf phenology and biomass production of ground level vegetation in boreal peatlands. Data were collected during two contrasting growing seasons from a WLD experiment including a rich and a poor fen and an ombrotrophic bog. Results showed that WLD had a strong effect on both leaf area development and biomass production, and these responses differed between peatland types. In the poor fen and the bog, WLD increased plant growth, while in the rich fen, WLD reduced the growth of ground level vegetation. Plant groups differed in their response, as WLD reduced the growth of graminoids, while shrubs and tree seedlings benefited from it. In addition, the vegetation adjusted to the lower WTs, was more responsive to short-term climatic variations. The warmer summer resulted in a greater maximum and earlier peaking of leaf area index, and greater biomass production by vascular plants and Sphagnum mosses at WLD sites. In particular, graminoids benefitted from the warmer conditions. The change towards greater production in the WLD sites in general and during the warmer weather in particular, was related to the observed transition in plant functional type composition towards arboreal vegetation.     

Tritrophic interactions at a community level: effects of host plant species quality on bird predation of caterpillars

Effects of plant traits on herbivore-carnivore interactions are well documented in component communities but are not well understood at the level of large, complex communities. We report on a 2-year field experiment testing mechanisms by which variation in food quality among eight temperate forest tree species alters avian suppression of an assemblage of dietary generalist caterpillars. Plant quality and bird effects varied dramatically among tree species; high-quality plants yielded herbivores of 50% greater mass than those on low-quality plants, and bird effects ranged from near 0% to 97% reductions in caterpillar density. We also find evidence for two mechanisms linking host plant quality to bird effects. If caterpillar density was statistically controlled for, birds had relatively strong effects on the herbivores of low-quality plants, as predicted by the slow-growth/high-mortality hypothesis. At the same time, caterpillar density increased with plant quality, and bird effects were density dependent. Consequently, the net effect of birds was strongest on the herbivores of high-quality plants, a dynamic we call the high-performance/high-mortality hypothesis. Host plant quality thus changes highly generalized herbivore-carnivore interactions by two complementary but opposing mechanisms. These results highlight the interrelatedness of plant-herbivore and herbivore-carnivore interactions and thus the importance of a tritrophic perspective.     

Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands

Facilitation among plants mediated by grazers occurs when an unpalatable plant extends its protection against grazing to another plant. This type of indirect facilitation impacts species coexistence and ecosystem functioning in a large array of ecosystems worldwide. It has nonetheless generally been understudied so far in comparison with the role played by direct facilitation among plants. We aimed at providing original data on indirect facilitation at the community scale to determine the extent to which indirect facilitation mediated by grazers can shape plant communities. Such experimental data are expected to contribute to refining the conceptual framework on plant–plant–herbivore interactions in stressful environments. We set up a 2‐year grazing exclusion experiment in tropical alpine peatlands in Bolivia. Those ecosystems depend entirely on a few, structuring cushion‐forming plants (hereafter referred to as “nurse” species), in which associated plant communities develop. Fences have been set over two nurse species with different strategies to cope with grazing (direct vs. indirect defenses), which are expected to lead to different intensities of indirect facilitation for the associated communities. We collected functional traits which are known to vary according to grazing pressure (LDMC, leaf thickness, and maximum height), on both the nurse and their associated plant communities in grazed (and therefore indirect facilitation as well) and ungrazed conditions. We found that the effect of indirectly facilitated on the associated plant communities depended on the functional trait considered. Indirect facilitation decreased the effects of grazing on species relative abundance, mean LDMC, and the convergence of the maximum height distribution of the associated communities, but did not affect mean height or cover. The identity of the nurse species and grazing jointly affected the structure of the associated plant community through indirect facilitation. Our results together with the existing literature suggest that the “grazer–nurse–beneficiary” interaction module can be more complex than expected when evaluated in the field.     

三峡水库消落带植物群落组成及物种多样性

研究消落带植物群落多样性的梯度变化和空间分布特征,可为认识和保护消落带生物多样性提供依据。通过对三峡库区澎溪河开县段的植被样方调查,研究消落带植物群落物种多样性的梯度变化和空间分布特征。结果表明:(1)各群落优势种和伴生种的种类和优势度等组成特征变化明显;(2)群落物种丰富度随高程增加而呈先升后降的格局;(3)α多样性与β多样性起伏较大,其中Shannon多样性和Simpson优势度的变化格局相反,其沿高程的曲线变化分别呈M和W型,Pielou均匀度的变化格局基本与丰富度一致,呈先升后降的趋势;相异性指数和Cody指数呈先降后升的格局,最小值均出现在中间高程,最大值出现在较高高程。分析认为,消落带α多样性的变化来自于高程差异造成的淹水时间的差异而导致的生境差异;β多样性变化原因较为复杂,需进一步研究。     

Fluxes of nitrous oxide from boreal peatlands as affected by peatland type,water table level and nitrification capacity

Peat soils with high nitrogen content are potential sources of nitrous oxide (N₂O). Fluxes of nitrous oxide were measuredin situ on nine virgin and ten drained peatlands of different hydrology and nutrient status. Numbers of nitrifying bacteria were estimated in different layers of the peat profiles with a most-probable-number technique. Nitrification potentials were determined in soil slurries of pH 4 and 6 from the profiles of six peat soils. Many virgin peatlands showed low N₂O uptake. Lowering of the water table generally increased the average fluxes of N₂O from the soils, although more in minerotrophic (nutrient rich) than in ombrotrophic (nutrient poor) sites. Ammonium oxidizing bacteria were found on only two sites but nitrite oxidizers were detected in almost all peat profiles. More nitrite oxidizers were found in drained than in virgin peat profiles. Nitrification was enhanced after lowering of the water table in minerotrophic peat but not in ombrotrophic peat. The N₂O fluxes correlated positively with the numbers of nitrite oxidizers, nitrification potential, N, P and Ca content and pH of the soil and negatively with the level of water table (expressed as negative values) and K content of the soil.     

Permafrost thaw causes large carbon loss in boreal peatlands while changes to peat quality are limited

Rapid, ongoing permafrost thaw of peatlands in the discontinuous permafrost zone is exposing a globally significant store of soil carbon (C) to microbial processes. Mineralization and release of this peat C to the atmosphere as greenhouse gases is a potentially important feedback to climate change. Here we investigated the effects of permafrost thaw on peat C at a peatland complex in western Canada. We collected 15 complete peat cores (between 2.7 and 4.5 m deep) along four chronosequences, from elevated permafrost peat plateaus to saturated thermokarst bogs that thawed up to 600 years ago. The peat cores were analysed for peat C storage and peat quality, as indicated by decomposition proxies (FTIR and C/N ratios) and potential decomposability using a 200-day aerobic laboratory incubation. Our results suggest net C loss following thaw, with average total peat C stocks decreasing by ~19.3 ± 7.2 kg C m⁻² over <600 years (~13% loss). Average post-thaw accumulation of new peat at the surface over the same period was ~13.1 ± 2.5 kg C m⁻². We estimate ~19% (±5.8%) of deep peat (>40 cm below surface) C is lost following thaw (average 26 ± 7.9 kg C m⁻² over <600 years). Our FTIR analysis shows peat below the thaw transition in thermokarst bogs is slightly more decomposed than peat of a similar type and age in permafrost plateaus, but we found no significant changes to the quality or lability of deeper peat across the chronosequences. Our incubation results also showed no increase in C mineralization of deep peat across the chronosequences. While these limited changes in peat quality in deeper peat following permafrost thaw highlight uncertainty in the exact mechanisms and processes for C loss, our analysis of peat C stocks shows large C losses following permafrost thaw in peatlands in western Canada.     

Greater variation in boreal plant community composition and community-level traits at local- than regional-scale

Questions

Rapid climate change in northern latitudes is expected to influence plant functional traits of the whole community (community-level traits) through species compositional changes and/or trait plasticity, limiting our ability to anticipate climate warming impacts on northern plant communities. We explored differences in plant community composition and community-level traits within and among four boreal peatland sites and determined whether intra- or interspecific variation drives community-level traits.

Location

Boreal biome of western North America.

Methods

We collected plant community composition and functional trait data along dominant topoedaphic and/or hydrologic gradients at four peatland sites spanning the latitudinal extent of the boreal biome of western North America. We characterized variability in community composition and community-level traits of understorey vascular and moss species both within (local-scale) and among sites (regional-scale).

Results

Against expectations, community-level traits of vascular plant and moss species were generally consistent among sites. Furthermore, interspecific variation was more important in explaining community-level trait variation than intraspecific variation. Within-site variation in both community-level traits and community composition was greater than among-site variation, suggesting that local environmental gradients (canopy density, organic layer thickness, etc.) may be more influential in determining plant community processes than regional-scale gradients.

Conclusions

Given the importance of interspecific variation to within-site shifts in community-level traits and greater variation of community composition within than among sites, we conclude that climate-induced shifts in understorey community composition may not have a strong influence on community-level traits in boreal peatlands unless local-scale environmental gradients are substantially altered.     

Temporal variation and its drivers in the elemental traits of four boreal plant species

4种北方植物元素性状的时间变化及其驱动因素 植物种内性状的变异对个体适合度和草食动物的觅食有重要影响。对于植物来说,在空间维度上的性状变异已经得到了很好的证明。然而,对性状变异的时间维度仍知之甚少,但其可能受生长度日、温度和降水季节差异的影响。本研究的目的是量化4种常见的北方植物物种性状的种内时间变异及其变异的驱动因素。我们对4种常见树种的叶片进行了为期两年的元素和化学计量性状采样(%C, %N, %P, C:N, C:P, N:P)。本研究分两个步骤进行,首先将广义线性模型(GzLM, n = 24)与我们研究的物种的元素和化学计量性状拟合,以测试它们是否随年份而变化。当我们获得时间变异性的证据时,我们拟合第二组GzLMs (n = 8),将温度、生产力和湿度作为解释变量。研究结果显示,除了两个例外,4种北方植物的大部分元素和化学计量性状都没有时间变异性。年份是所有4种植物的碳百分比(R² = 0.47–0.67)以及香脂冷杉(Abies balsamea)中多种元素和化学计量性状(5/8, R² = 0.29–0.67)的重要预测因子。因此,碳百分比的变化与年际差异有关,而与限制北方森林养分的氮和磷关系更大。这些研究结果还表明,由于生活史的差异,针叶树的化学计量比落叶植物的化学计量的年际差异更大。生长度日是第二轮模型中最频繁出现的变量(8/8 次,R² = 0.21–0.41),表明温度是这些性状时间变异的重要驱动因素。     

Litter controls plant community composition in a northern fen

The accumulation of litter or thatch can affect plant community composition by affecting the temperature, nutrient availability, and light availability of the soil environment, thereby forming a potentially important linkage between recent productivity and current ecosystem processes. To investigate the importance of litter on a fen peatland plant community, we conducted a litter addition and removal experiment in a fen in northern Minnesota, USA, between 1998 and 2001. The addition of litter had little effect on fen plant community composition or microenvironmental variables, despite a two-fold increase in litter mass compared to control plots. However, the removal of litter dramatically increased cover of Rhynchospora fusca and R. alba , and reduced cover of Carex exilis . Litter removal also increased availability of light and soil temperature and increased the phosphorus content of aboveground plant biomass. Our results indicate that litter is an important control of plant community composition in this northern fen.     

Response of fish assemblages to water quality in boreal rivers

The relationship between fluvial fish assemblages and environmental variables including water quality was studied by nonmetric multidimensional scaling (NMS) and generalised regression neural network (GRNN). Fish assemblages in the studied boreal rivers in Central Finland showed a higher response to water quality than to other environmental variables such as altitude and river width. Of the water quality parameters, total phosphorus, oxygen saturation in winter and solids had the highest influence on fish assemblages. Brown trout (Salmo trutta L.) responded to water quality strongly by favouring pristine waters. Among other fish species, the responses were lighter, and the relative contribution of environmental variables varied. Percentage of fishless sites, i.e. no catch in electrofishing, was 4.4% when pH ≥ 6, and 38.2% when pH < 6. However, the most effective way to influence the fish assemblage composition, and to raise the fish-based FIFI integrity index value, would be to reduce loading of solids and phosphorus, rather than depressing acidification.     

Permafrost condition determines plant community composition and community‐level foliar functional traits in a boreal peatland

Boreal peatlands are critical ecosystems globally because they house 30%–40% of terrestrial carbon (C), much of which is stored in permafrost soil vulnerable to climate warming‐induced thaw. Permafrost thaw leads to thickening of the active (seasonally thawed) layer and alters nutrient and light availability. These physical changes may influence community‐level plant functional traits through intraspecific trait variation and/or species turnover. As permafrost thaw is expected to cause an efflux of carbon dioxide (CO₂) and methane (CH₄) from the soil to the atmosphere, it is important to understand thaw‐induced changes in plant community productivity to evaluate whether these changes may offset some of the anticipated increases in C emissions. To this end, we collected vascular plant community composition and foliar functional trait data along gradients in aboveground tree biomass and active layer thickness (ALT) in a rapidly thawing boreal peatland, with the expectation that changes in above‐ and belowground conditions are indicative of altered resource availability. We aimed to determine whether community‐level traits vary across these gradients, and whether these changes are dominated by intraspecific trait variation, species turnover, or both. Our results highlight that variability in community‐level traits was largely attributable to species turnover and that both community composition and traits were predominantly driven by ALT. Specifically, thicker active layers associated with permafrost‐free peatlands (i.e., bogs and fens) shifted community composition from slower‐growing evergreen shrubs to faster‐growing graminoids and forbs with a corresponding shift toward more productive trait values. The results from this rapidly thawing peatland suggest that continued warming‐induced permafrost thaw and thermokarst development alter plant community composition and community‐level traits and thus ecosystem productivity. Increased productivity may help to mitigate anticipated CO₂ efflux from thawing permafrost, at least in the short term, though this response may be swamped by increase CH₄ release.     

物种多样性与植物群落的维持机制

物种多样性的形成与维持是生物多样性研究中的核心问题之一,形成了许多假说。剖析了群落物种多样性维持具有增加、减少和稳定共存三方面内容,着重阐明物种稳定共存的内因是物种的生物学与生态学特性差异,外因是生境具有小尺度的差异;群落生境异质性是其物种多样性维持的基础,并影响着群落的组织过程;群落的斑块镶嵌结构源于林隙形成的生境异质性,导致了物种多样性;群落斑块镶嵌结构是群落的基本属性,使群落的结构和功能得以长期维持等论点。最后,还以鼎湖山季风常绿阔叶林为例,对群落斑块镶嵌结构的形成及其对物种多样性维持和群落生产力构成与维持等方面的意义予以说明。     

景观湿地夏季原生动物群落结构与水质关系

2011年夏季(6-8月)对杭州市境内一景观湿地西溪国家湿地公园进行采样, 共得原生动物样品21个。利用浮游生物研究方法对该湿地水域中原生动物的种类组成、群落特征、优势种分布、指示种及生物多样性指数进行了综合分析, 研究了该特征性水域中原生动物的群落特征与水质变动之间的关系。共鉴定原生动物84种, 隶属于3纲45属, 其中鞭毛虫纲61种, 纤毛虫纲14种, 肉足虫纲9种。优势种主要为隐滴虫(Cryptomonas)和眼虫(Euglena), 均属于鞭毛虫纲; 污染性指示种及Margalef等多样性指数均表明水质呈中度污染状态; 在原生动物功能类群中, 光合自养者最多, 腐生者、杂食者和捕食者相对较少; 主要理化指标中TN、TP和COD含量均较高, 且对原生动物群落的影响较大, 是造成群落变化的主要原因。综合评价结果揭示出, 西溪国家湿地公园水体目前仍处于中度污染状况。研究对湿地水域的污染原因进行了分析并提出了防治参考对策。       

Arctic and boreal plant species decline at their southern range limits in the Rocky Mountains

Climate change is predicted to cause a decline in warm‐margin plant populations, but this hypothesis has rarely been tested. Understanding which species and habitats are most likely to be affected is critical for adaptive management and conservation. We monitored the density of 46 populations representing 28 species of arctic‐alpine or boreal plants at the southern margin of their ranges in the Rocky Mountains of Montana, USA, between 1988 and 2014 and analysed population trends and relationships to phylogeny and habitat. Marginal populations declined overall during the past two decades; however, the mean trend for 18 dicot populations was ?5.8% per year, but only ?0.4% per year for the 28 populations of monocots and pteridophytes. Declines in the size of peripheral populations did not differ significantly among tundra, fen and forest habitats. Results of our study support predicted effects of climate change and suggest that vulnerability may depend on phylogeny or associated anatomical/physiological attributes.     

A water level drawdown index for aquatic macrophytes in Nordic lakes

Many northern lakes are regulated to enhance hydropower production and flood protection. This bears hydromorphological pressures which are important factors causing lowered ecological status. Water level fluctuation triggers erosion on the shoreline and, depending on fluctuation range, also affects species composition or disappearance of sensitive aquatic macrophytes. We developed a water level-drawdown index (WI_c) for Nordic lakes using macrophyte data from 73 lakes with varying water level fluctuation in Finland, Norway and Sweden. The index is based on the ratio between sensitive and tolerant macrophyte species. The sensitive and tolerant species are identified based on a percentile approach, analysing the presence or absence of species along the winter drawdown range. The index correlates well with winter drawdown in Finnish and Norwegian lakes with strongest correlations with winter drawdown in storage lakes (lakes regulated for hydroelectric power and with a considerable winter drawdown). The WI_c-index is applicable in low alkalinity, oligotrophic and ice-covered lakes, and is suggested to be a useful tool to identify and designate heavily modified water bodies in Nordic lakes according to the European Water Framework Directive.     

揭示群落结构及其环境响应的联合物种分布模型的研究进展

物种分布模型通常用于基础生态和应用生态研究,用来确定影响生物分布和物种丰富度的因素,量化物种与非生物条件的关系,预测物种对土地利用和气候变化的反应,并确定潜在的保护区.在传统的物种分布模型中,生物的相互作用很少被纳入,而联合物种分布模型(JSDMs)作为近年提出的一种新的可行方法,可以同时考虑环境因素和生物交互作用,因而成为分析生物群落结构和种间相互作用过程的有力工具.JSDMs以物种分布模型(SDMs)为基础,通常采用广义线性回归模型建立物种对环境变量的多变量响应,以随机效应的形式获取物种间的关联,同时结合隐变量模型(LVMs),并基于Laplace近似和马尔科夫蒙脱卡罗模拟的最大似然估计或贝叶斯方法来估算模型参数.本文对JSDMs的产生及理论基础进行归纳总结,重点介绍了不同类型JSDMs的特点及其在现代生态学中的应用,阐述了JSDMs的应用前景、使用过程中存在的问题及发展方向.随着对环境因素与多物种种间关系研究的深入,JSDMs将是今后物种分布模型研究的重点.