陆地生态系统凋落物分解对全球气候变暖的响应

陆地生态系统凋落物分解是全球碳收支的一个重要组成部分, 主要受气候、凋落物质量和土壤生物群落的综合控制。科学家们普遍认为全球气候变化将对陆地生态系统凋落物分解产生复杂而深远的影响。该文结合凋落物分解试验的常用方法——缩微试验、原位模拟实验和自然环境梯度实验, 归纳现有研究结果, 意在揭示全球气候变化对陆地生态系统凋落物分解的直接影响(温度对凋落物分解速率的影响)和间接影响(温度对凋落物质量、土壤微生物群落及植被型的影响)的普遍规律。各种研究方法都表明: 在水分条件理想的情况下, 温度升高往往能加快凋落物的分解速率; 原位模拟实验中, 凋落物分解速率因物种、增温方法和地理方位而异; 全球气候变化能改变凋落物质量, 但可能不会在短期内影响凋落物的分解速率; 凋落物质量和可分解性的种间差异远大于增温所引发的表型响应差异, 那么, 气候变化所引发的植物群落结构和物种组成的变化将对陆地生态系统凋落物分解产生更强烈的影响; 土壤生物群落如何响应全球气候变化, 进而怎样影响凋落物分解过程, 这些都还存在着极大的不确定性。     

Global climate change and litter decomposition: more frequent fire slows decomposition and increases the functional importance of invertebrates

Of the many mechanisms by which global climate change may alter ecosystem processes perhaps the least known and insidious is altered disturbance regimes. We used a field-based experiment to examine the climate change scenario of more frequent fires with altered invertebrate assemblages on the decomposition of Eucalyptus leaves. Our design comprised three fire regimes [long-term fire exclusion (FE), long-term frequent burning (FB) and FE altered to FB (FEFB)] and two litter bag mesh sizes (8.0 and 0.2 mm) that either permitted or denied access to the leaf litter by most invertebrates. We found a significant interaction effect between fire regime and mesh size in losses of litter mass and net carbon (C). Compared with the regime of FE, with more frequent burning (FB and FEFB) the pace of decomposition was slowed by 41% (when access to litter by most invertebrates is not impeded). For the regime of FE, denying access to leaf litter by most invertebrates did not alter the pace of decomposition. Conversely, under regimes of frequently burning, restricting access to the litter by most invertebrates altered the pace of decomposition by 46%. Similar results were found for net C. For net losses of nitrogen (N), no interaction effects between fire regime and mesh size were detected, although both main effects were significant. Our results show that by modifying disturbance regimes such as fire frequency, global climate change has the potential to modify the mechanism by which ecosystems function. With more FB, decomposition is driven not only by fire regime induced changes in substrate quality and/or physiochemical conditions but through the interaction of disturbance regime with animal assemblages mediating ecosystem processes.     

Chemistry and toughness predict leaf litter decomposition rates over a wide spectrum of functional types and taxa in central Argentina

Litter decomposition, a major determinant of ecosystem functioning, is strongly influenced by the litter quality of different species. We aimed at (1) relating interspecific variation in leaf litter decomposition rate to the functional types different species belong to; and (2) understanding the chemical and/or physical basis for such variation and its robustness to environmental factors. We selected 52 Angiosperms from a climatic gradient in central-western Argentina, representing the widest range of functional types and habitats published so far. Ten litter samples of each species were simultaneously buried for 9 weeks during the 1996 summer in an experimental decomposition bed. Decomposition rate was defined as the percentage of dry mass loss after incubation. Chemical litter quality was measured as carbon (C) content, nitrogen (N) content, and C-to-N ratio. Since tensile strength of litter and living leaves were strongly correlated, the latter was chosen as an indicator of physical litter quality. A subset of 15 species representing different functional types was also incubated in England for 15 weeks, following a similar experimental procedure. Litter C-to-N and leaf tensile strength of the leaves showed the strongest negative associations with decomposition rate, both at the species and at the functional-type level. Decomposition rates of the same species in Argentina and in England were strongly correlated. This reinforces previous evidence that species rankings in terms of litter decomposition rates are robust to methodological and environmental factors. This paper has shown new evidence of plant control over the turnover of organic matter through litter quality, and confirms, over a broad spectrum of functional types, general models of resource allocation. The strong correlations between leaf tensile strength – a trait that is easy and quick to measure in a large number of species – decomposition rate, and C-to-N ratio indicate that leaf tensile strength can be useful in linking plant quality to decomposition patterns at the ecosystem level. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of hydrologic changes on aboveground production and surface water chemistry in two boreal peatlands in Alberta: Implications for global warming

Aboveground net primary production (NPP) and surface water chemistryvariables were monitored in a lacustrine sedge fen and a bog for four years.There were no significant differences in precipitation, mean growing seasonannual temperature, and number of growing degree days from 1991 to 1994. Themean annual water levels in the lacustrine sedge fen differed significantly,whereas they were similar in the bog during these four years. We measured 15surface water variables in the lacustrine sedge fen and the bog, and foundthat only two correlated significantly with water level fluctuations. In thelacustrine sedge fen, calcium correlated positively (r²= 0.56) and nitrate correlated negatively (r² =0.20) with water levels. In the bog, potassium correlated positively(r² = 0.88) and total dissolved phosphorus correlatednegatively (r² = 0.62) with water levels. The remainingchemical variables showed no significant correlations with water levelfluctuations. Net primary production of the different vegetation strataappeared to respond to different environmental variables. In the lacustrinesedge fen, graminoid production was explained to a significant degree bywater levels (r² = 0.53), whereas shrub production wasexplained to a significant degree by surface water chemistry variables, suchas nitrate (r² = 0.74) and total phosphorus(r² = 0.22). In the bog, temperature was the onlyvariable that explained moss production to a significant degree(r² = 0.71), whereas ammonium explained graminoidproduction (r² = 0.66) and soluble reactive phosphorusexplained shrub production to significant degrees (r² =0.71). There are few direct data on the impact of climatic warming in borealwetlands, although paleoecological and 2×CO₂ model datahave provided some indications of past and possibly future changes invegetation composition, respectively. Our results suggest that thelacustrine sedge fen may succeed to a bog dominated by Sphagnum spp. andPicea mariana, whereas the bog may succeed to an upland-type forestecosystem. This revised version was published online in July 2006 with corrections to the Cover Date.     

Solar UVB and warming affect decomposition and earthworms in a fen ecosystem in Tierra del Fuego, Argentina

Combined effects of co-occurring global climate changes on ecosystem responses are generally poorly understood. Here, we present results from a 2-year field experiment in a Carex fen ecosystem on the southernmost tip of South America, where we examined the effects of solar ultraviolet B (UVB, 280–315 nm) and warming on above- and belowground plant production, C : N ratios, decomposition rates and earthworm population sizes. Solar UVB radiation was manipulated using transparent plastic filter films to create a near-ambient (90% of ambient UVB) or a reduced solar UVB treatment (15% of ambient UVB). The warming treatment was imposed passively by wrapping the same filter material around the plots resulting in a mean air and soil temperature increase of about 1.2 °C. Aboveground plant production was not affected by warming, and marginally reduced at near-ambient UVB only in the second season. Aboveground plant biomass also tended to have a lower C : N ratio under near-ambient UVB and was differently affected at the two temperatures (marginal UVB × temperature interaction). Leaf decomposition of one dominant sedge species ( Carex curta ) tended to be faster at near-ambient UVB than at reduced UVB. Leaf decomposition of a codominant species ( Carex decidua ) was significantly faster at near-ambient UVB; root decomposition of this species tended to be lower at increased temperature and interacted with UVB. We found, for the first time in a field experiment that epigeic earthworm density and biomass was 36% decreased by warming but remained unaffected by UVB radiation. Our results show that present-day solar UVB radiation and modest warming can adversely affect ecosystem functioning and engineers of this fen. However, results on plant biomass production also showed that treatment manipulations of co-occurring global change factors can be overridden by the local climatic situation in a given study year.     

Changes in chromosomal polymorphism and global warming: The case of Drosophila subobscura from Apatin (Serbia)

In this study, chromosomal inversion polymorphism data for a natural population of Drosophila subobscura from a swampy region near the town of Apatin (Serbia) were compared with data for the same population collected approximately 15 years earlier. The pattern of chromosomal inversion polymorphism changed over time. There were significant increases in the frequency of characteristic southern latitude ("warm" adapted) chromosomal arrangements and significant decreases in the frequency of characteristic northern latitude ("cold" adapted) chromosomal arrangements in the O and U chromosomes. The chromosomal arrangements O(3+4) and O(3+4) (+) (22) (derived from the O(3+4) arrangement) showed significant increases in 2008 and 2009 with regard to the 1994 sample. There was also a significant increase (～50%) in the U(1) (+) (2) arrangement, while U(1+8) (+) (2) (a typical southern arrangement) was detected for the first time. Since the Apatin swampy population of D. subobscura has existed for a long time in a stable habitat with high humidity that has not been changed by man our results indicate that natural selection has produced chromosomal changes in response to the increase in temperature that has occurred in the Balkan Peninsula of central southeastern European.     

Effects of plant traits on ecosystem and regional processes: a conceptual framework for predicting the consequences of global change

Human activities are causing widespread changes in the species composition of natural and managed ecosystems, but the consequences of these changes are poorly understood. This paper presents a conceptual framework for predicting the ecosystem and regional consequences of changes in plant species composition. Changes in species composition have greatest ecological effects when they modify the ecological factors that directly control (and respond to) ecosystem processes. These interactive controls include: functional types of organisms present in the ecosystem; soil resources used by organisms to grow and reproduce; modulators such as microclimate that influence the activity of organisms; disturbance regime; and human activities. Plant traits related to size and growth rate are particularly important because they determine the productive capacity of vegetation and the rates of decomposition and nitrogen mineralization. Because the same plant traits affect most key processes in the cycling of carbon and nutrients, changes in plant traits tend to affect most biogeochemical cycling processes in parallel. Plant traits also have landscape and regional effects through their effects on water and energy exchange and disturbance regime.     

Response of plant species richness and primary productivity in shrublands along a north–south gradient in Europe to seven years of experimental warming and drought: reductions in primary productivity in the heat and drought year of 2003

We used a nonintrusive field experiment carried out at six sites – Wales (UK), Denmark (DK), the Netherlands (NL), Hungary (HU), Sardinia (Italy – IT), and Catalonia (Spain – SP) – along a climatic and latitudinal gradient to examine the response of plant species richness and primary productivity to warming and drought in shrubland ecosystems. The warming treatment raised the plot daily temperature by ca. 1 °C, while the drought treatment led to a reduction in soil moisture at the peak of the growing season that ranged from 26% at the SP site to 82% in the NL site. During the 7 years the experiment lasted (1999–2005), we used the pin‐point method to measure the species composition of plant communities and plant biomass, litterfall, and shoot growth of the dominant plant species at each site. A significantly lower increase in the number of species pin‐pointed per transect was found in the drought plots at the SP site, where the plant community was still in a process of recovering from a forest fire in 1994. No changes in species richness were found at the other sites, which were at a more mature and stable state of succession and, thus less liable to recruitment of new species. The relationship between annual biomass accumulation and temperature of the growing season was positive at the coldest site and negative at the warmest site. The warming treatment tended to increase the aboveground net primary productivity (ANPP) at the northern sites. The relationship between annual biomass accumulation and soil moisture during the growing season was not significant at the wettest sites, but was positive at the driest sites. The drought treatment tended to reduce the ANPP in the NL, HU, IT, and SP sites. The responses to warming were very strongly related to the Gaussen aridity index (stronger responses the lower the aridity), whereas the responses to drought were not. Changes in the annual aboveground biomass accumulation, litterfall, and, thus, the ANPP, mirrored the interannual variation in climate conditions: the most outstanding change was a decrease in biomass accumulation and an increase in litterfall at most sites during the abnormally hot year of 2003. Species richness also tended to decrease in 2003 at all sites except the cold and wet UK site. Species‐specific responses to warming were found in shoot growth: at the SP site, Globularia alypum was not affected, while the other dominant species, Erica multiflora, grew 30% more; at the UK site, Calluna vulgaris tended to grow more in the warming plots, while Empetrum nigrum tended to grow less. Drought treatment decreased plant growth in several studied species, although there were some species such as Pinus halepensis at the SP site or C. vulgaris at the UK site that were not affected. The magnitude of responses to warming and drought thus depended greatly on the differences between sites, years, and species and these multiple plant responses may be expected to have consequences at ecosystem and community level. Decreases in biodiversity and the increase in E. multiflora growth at the SP site as a response to warming challenge the assumption that sensitivity to warming may be less well developed at more southerly latitudes; likewise, the fact that one of the studied shrublands presented negative ANPP as a response to the 2003 heat wave also challenges the hypothesis that future climate warming will lead to an enhancement of plant growth and carbon sequestration in temperate ecosystems. Extreme events may thus change the general trend of increased productivity in response to warming in the colder sites.