Tracking Soil Temperature and Moisture in a Multi-Factor Climate Experiment in Temperate Grassland: Do Climate Manipulation Methods Produce their Intended Effects?

Passive open-top chambers (OTCs) and rainout shelters (RSs) have been used for over two decades to manipulate temperature and water availability in experiments on plant communities. These types of manipulations have been independently evaluated; however, as experiments become more complex and multiple factors are evaluated the potential for unknown or undesirable treatment effects increases. We present the effects of temperature manipulations (with OTCs), water manipulations (with RSs and water additions), and a clipping treatment, implemented in a fully factorial design, on soil moisture and temperature over 2 years in a temperate grassland. Temperature was increased 0.2°C by OTCs. Soil volumetric water content was reduced 3% by RSs and increased 2% by watering. However, clipping vegetation, treatment interactions, and weather conditions also affected soil temperature and moisture. For example, in OTCs RSs increased the temperature an additional 0.4°C, watering lowered it 0.4°C, and clipping raised temperature 2°C. Similarly, changes in soil moisture due to the RSs decreased VWC by 3% and increased 1% by clipping whereas soil moisture due to watering was reduced 1% by the OTCs and clipping. We also found that OTCs are more effective at raising temperatures on cooler days when soil temperatures are below 16.3°C. Our results suggest that all treatment types generally affect soil variables in predicable ways, but use of such devices should be adopted with caution, as they do not act independently, or exclusively, on the target variables.     

Dynamic and complex microclimate responses to warming and grazing manipulations

Synthesis efforts that identify patterns of ecosystem response to a suite of warming manipulations can make important contributions to climate change science. However, cross‐study comparisons are impeded by the paucity of detailed analyses of how passive warming and other manipulations affect microclimate. Here we document the independent and combined effects of a common passive warming manipulation, open‐top chambers (OTCs), and a simulated widespread land use, clipping, on microclimate on the Tibetan Plateau. OTCs consistently elevated growing season averaged mean daily air temperature by 1.0–2.0°C, maximum daily air temperature by 2.1–7.3°C and the diurnal air temperature range by 1.9–6.5°C, with mixed effects on minimum daily air temperature, and mean daily soil temperature and moisture. These OTC effects on microclimate differ from reported effects of a common active warming method, infrared heating, which has more consistent effects on soil than on air temperature. There were significant interannual and intragrowing season differences in OTC effects on microclimate. For example, while OTCs had mixed effects on growing season averaged soil temperatures, OTCs consistently elevated soil temperature by approximately 1.0°C early in the growing season. Nonadditive interactions between OTCs and clipping were also present: OTCs in clipped plots generally elevated air and soil temperatures more than OTCs in nonclipped plots. Moreover, site factors dynamically interacted with microclimate and with the efficacy of the OTC manipulations. These findings highlight the need to understand differential microclimate effects between warming methods, within warming method across ecosystem sites, within warming method crossed with other treatments, and within sites over various timescales. Methods, sites and scales are potential explanatory variables and covariables in climate warming experiments. Consideration of this variability among and between experimental warming studies will lead to greater understanding and better prediction of ecosystem response to anthropogenic climate warming.     

Effects of increased temperature on plant communities depend on landscape location and precipitation

Global climate change is affecting and will continue to affect ecosystems worldwide. Specifically, temperature and precipitation are both expected to shift globally, and their separate and interactive effects will likely affect ecosystems differentially depending on current temperature, precipitation regimes, and other biotic and environmental factors. It is not currently understood how the effects of increasing temperature on plant communities may depend on either precipitation or where communities lie on soil moisture gradients. Such knowledge would play a crucial role in increasing our predictive ability for future effects of climate change in different systems. To this end, we conducted a multi‐factor global change experiment at two locations, differing in temperature, moisture, aspect, and plant community composition, on the same slope in the northern Mongolian steppe. The natural differences in temperature and moisture between locations served as a point of comparison for the experimental manipulations of temperature and precipitation. We conducted two separate experiments, one examining the effect of climate manipulation via open‐top chambers (OTCs) across the two different slope locations, the other a factorial OTC by watering experiment at one of the two locations. By combining these experiments, we were able to assess how OTCs impact plant productivity and diversity across a natural and manipulated range of soil moisture. We found that warming effects were context dependent, with the greatest negative impacts of warming on diversity in the warmer, drier upper slope location and in the unwatered plots. Our study is an important step in understanding how global change will affect ecosystems across multiple scales and locations.     

The use of open-top chambers in forests for evaluating warming effects on herbaceous understorey plants

Open-top chambers (OTCs) are widely used experimental warming devices in open-field ecosystems such as tundra and alpine heath. However, knowledge of their performance in temperate deciduous forest ecosystems is largely lacking. The application of OTCs in forests might become important in the future since the effects of climate warming on growth, reproduction, and future distribution of understorey forest herbs have rarely been investigated. Therefore, polycarbonate OTCs covered with (OTCs+GF) and without permeable polypropylene GardenFleece (OTCs−GF) were installed in a temperate deciduous forest to create an experimental warming gradient. Short-term responses in phenology, growth, and reproduction of a model understorey forest herb (Anemone nemorosa L.) to OTC installation were determined. In a second growing season, an in-depth study of multiple abiotic conditions inside OTCs−GF was performed. Both OTCs+GF and OTCs−GF raised air and soil temperature in a realistic manner (ca. +0.4°C to +1.15°C), but OTCs−GF only in the leafless period (up to +1.5°C monthly average soil temperature). The early flowering forest herb A. nemorosa also showed a clear phenotypic response to OTC installation. Based on these facts and the large ecological drawbacks associated with OTCs+GF (mostly in connection with a higher relative air humidity and a lower light quantity) and very modest abiotic changes in OTCs−GF, we encourage the use of OTCs−GF in deciduous forest ecosystems for evaluating climate-warming effects on early flowering understorey forest herbs. There is also a potential to use this warming method on later flowering species, but this needs further research.     

Effects of open-top passive warming chambers on soil respiration in the semi-arid steppe to taiga forest transition zone in Northern Mongolia

The response of soil respiration to warming has been poorly studied in regions at higher latitude with low precipitation. We manipulated air temperature, soil temperature and soil moisture using passive, open-top chambers (OTCs) in three different ecosystem settings in close proximity (boreal forest, riparian area, and semi-arid steppe) to investigate how environmental factors would affect soil respiration in these different ecosystems, anticipating that soil respiration would increase in response to the chamber treatment. The results indicated that OTCs significantly increased air and soil temperature in areas with open canopy and short-statured vegetation (i.e., steppe areas) but not in forest. OTCs also affected soil moisture, but the direction of change depended on the ecosystem, and the magnitude of change was highly variable. Generally, OTCs did not affect soil respiration in steppe and riparian areas. Although soil respiration was slightly greater in OTCs placed in the forest, the difference was not statistically significant. Analyses of relationships between soil respiration and environmental variables suggested that different factors controlled soil respiration in the different ecosystems. Competing effects analysis using a model selection approach and regression analyses (e.g., Q₁₀) demonstrated that soil respiration in the forest was more sensitive to warming, while soil respiration in the steppe was more sensitive to soil moisture. The differing responses and controlling factors among these neighboring forest, riparian and steppe ecosystems in Northern Mongolia highlight the importance of taking into account potential biome shifts in C cycling modeling to generate more accurate predictions of landscape-scale responses to anticipated climate change.     

Climate change and grazing interact to alter flowering patterns in the Mongolian steppe

Socio-economic changes threaten nomadic pastoralism across the world, changing traditional grazing patterns. Such land-use changes will co-occur with climate change, and while both are potentially important determinants of future ecosystem functioning, interactions between them remain poorly understood. We investigated the effects of grazing by large herbivores and climate manipulation using open-top chambers (OTCs) on flower number and flowering species richness in mountain steppe of northern Mongolia. In this region, sedentary pastoralism is replacing nomadic pastoralism, and temperature is predicted to increase. Grazing and OTCs interacted to affect forb flowering richness, which was reduced following grazing removal, and reduced by OTCs in grazed plots only. This interaction was directly linked to the soil moisture and temperature environments created by the experimental treatments: most species flowered when both soil moisture and temperature levels were high (i.e. in grazed plots without OTCs), while fewer species flowered when either temperature, or moisture, or both, were low. Removal of grazing increased the average number of graminoid flowers produced at peak flowering in Year 1, but otherwise grazing removal and OTCs did not affect community-level flower composition. Of four abundant graminoid species examined individually, three showed increased flower number with grazing removal, while one showed the reverse. Four abundant forb species showed no significant response to either treatment. Our results highlight how climate change effects on mountain steppe could be contingent on land-use, and that studies designed to understand ecosystem response to climate change should incorporate co-occurring drivers of change, such as altered grazing regimes.     

Responses of an old-field plant community to interacting factors of elevated [CO2], warming, and soil moisture

Aims: The direct effects of atmospheric and climatic change factors—atmospheric[CO₂], air temperature and changes in precipitation—canshape plant community composition and alter ecosystem function.It is essential to understand how these factors interact tomake better predictions about how ecosystems may respond tochange. We investigated the direct and interactive effects of[CO₂], warming and altered soil moisture in open-top chambers(OTCs) enclosing a constructed old-field community to test howthese factors shape plant communities. Materials and methods: The experimental facility in Oak Ridge, TN, USA, made use of4-m diameter OTCs and rain shelters to manipulate [CO₂] (ambient,ambient + 300 ppm), air temperature (ambient, ambient + 3.5°C)and soil moisture (wet, dry). The plant communities within thechambers comprised seven common old-field species, includinggrasses, forbs and legumes. We tracked foliar cover for eachspecies and calculated community richness, evenness and diversityfrom 2003 to 2005. Important findings: This work resulted in three main findings: (1) warming had species-specificeffects on foliar cover that varied through time and were alteredby soil moisture treatments; (2) [CO₂] had little effect onindividual species or the community; (3) diversity, evennessand richness were influenced most by soil moisture, primarilyreflecting the response of one dominant species. We concludethat individualistic species responses to atmospheric and climaticchange can alter community composition and that plant populationsand communities should be considered as part of analyses ofterrestrial ecosystem response to climate change. However, predictionof plant community responses may be difficult given interactionsbetween factors and changes in response through time.     

Variable temperature effects of Open Top Chambers at polar and alpine sites explained by irradiance and snow depth

Environmental manipulation studies are integral to determining biological consequences of climate warming. Open Top Chambers (OTCs) have been widely used to assess summer warming effects on terrestrial biota, with their effects during other seasons normally being given less attention even though chambers are often deployed year‐round. In addition, their effects on temperature extremes and freeze‐thaw events are poorly documented. To provide robust documentation of the microclimatic influences of OTCs throughout the year, we analysed temperature data from 20 studies distributed across polar and alpine regions. The effects of OTCs on mean temperature showed a large range (?0.9 to 2.1 °C) throughout the year, but did not differ significantly between studies. Increases in mean monthly and diurnal temperature were strongly related (R² = 0.70) with irradiance, indicating that PAR can be used to predict the mean warming effect of OTCs. Deeper snow trapped in OTCs also induced higher temperatures at soil/vegetation level. OTC‐induced changes in the frequency of freeze‐thaw events included an increase in autumn and decreases in spring and summer. Frequency of high‐temperature events in OTCs increased in spring, summer and autumn compared with non‐manipulated control plots. Frequency of low‐temperature events was reduced by deeper snow accumulation and higher mean temperatures. The strong interactions identified between aspects of ambient environmental conditions and effects of OTCs suggest that a detailed knowledge of snow depth, temperature and irradiance levels enables us to predict how OTCs will modify the microclimate at a particular site and season. Such predictive power allows a better mechanistic understanding of observed biotic response to experimental warming studies and for more informed design of future experiments. However, a need remains to quantify OTC effects on water availability and wind speed (affecting, for example, drying rates and water stress) in combination with microclimate measurements at organism level.     

Mild experimental climate warming induces metabolic impairment and massive mortalities in southern African quartz field succulents

Current thermal regimes for many southern African succulent species of the subfamily Ruschioideae, which rapidly diversified during the cooler Pleistocene period, may be close to their tolerable extremes, and likely exceeded with anticipated future climate warming. This hypothesis was tested by exposing succulent species of different size and architecture to differently elevated temperatures approximating future African climate scenarios (2.5–3.8 °C increases in mean annual daily temperature maxima) using transparent hexagonal open-top chambers of different heights. Air temperatures, soil water potentials and amounts of fog and dew precipitation were monitored hourly in the differently heated open-top chambers and ambient environment, and changes in species leaf densities and canopy covers precisely determined in these chambers and ambient environment from high resolution digital images taken at 3-monthly intervals spanning a 12-month monitoring period. Photochemical efficiencies and activities of the photosynthetic enzyme Rubisco were also measured in one widespread dwarf succulent species following 2-h exposures of its populations in a forced draft oven to eight different heat intensities (range: 40–54 °C), the highest closely matching the temperature extreme of 54.8 °C recorded in the most intensively heated open-top chambers. After 12-months warming, all succulent species displayed massively (up to 90.2%) reduced leaf densities and canopy covers in the differently heated open-top chambers, with small sparsely branched species comprising single leaf pairs per axis exhibiting much greater reductions than large, shrubby or creeping species with multiple leaves. Noteworthy, was that fog and dew precipitation levels and soil water potentials at the centres of the least intensively heated chambers did not differ significantly from those in the ambient environment, even during the critical dry summer and early autumn seasons. However, leaf density and canopy reductions in these chambers were of similar magnitude to those in the most intensively heated chambers where fog and dew precipitation levels and soil water potentials were significantly reduced. These findings identified elevated temperatures as the principal cause of the observed massive reductions in leaf density and canopy cover and supported the hypothesis that mild anthropogenic warming could exceed the thermal thresholds of many southern African quartz field succulents leading to metabolic impairment. This impairment explained by an observed loss in the catalytic efficiency of Rubisco at daytime temperature extremes exceeding 54 °C, preceded by a decrease in PSII electron transport commencing at temperatures much lower than the threshold for Rubisco de-activation.     

Influence of simulated warming using OTC on physiological–biochemical characteristics of Elymus nutans in alpine meadow on Qinghai-Tibetan plateau

Elymus nutans Griseb. is a typical important plant species in the alpine meadow of Qinghai-Tibetan plateau. To examine the effects of temperature elevation on its physiological and chemical characteristics, a simulation study was conducted in situ with open-top chambers (OTC) followed the method of International Tundra Experiment (ITEX) from November 2002 to September 2007, and these OTCs were designed five kinds of size with bottom diameters of 0.85, 1.15, 1.45, 1.75, 2.05 m so as to rise different air temperatures. The air temperature inside OTCs increased by 2.68, 1.57, 1.20, 1.07 and 0.69 °C with increase of OTC diameter compared with ambient air. We found that with increase of air temperature, the soluble sugar content and SOD (superoxide dismutase) activity in leaves of E. nutans increased first, and then decreased, whereas, the soluble protein content and GSH (Glutathione) content decreased first and increased then, the chlorophyll a and total chlorophyll contents were decreased, but the contents of chlorophyll b were higher than that of control. Increased temperature enhanced the above-ground biomass and blade height of E. nutans. These results indicated that elevated temperature had significant and complicated effects on physiological–biochemical characteristics of E. nutans on Qinghai-Tibet plateau, when the temperature increased within the range of 0.69–1.57 °C, it may have positive effects on plant growth and development, and E. nutans could adapt even develop defensive strategy to the changes of a certain ecological environment changes.     

O3浓度升高和太阳辐射减弱对小麦根际土壤微生物功能多样性的影响

采用开顶箱(OTC)法和遮光网技术,设置100 n L/L臭氧熏气与3个辐射减弱梯度结合,模拟臭氧浓度升高和太阳辐射减弱的复合大气背景。用BIOLOG生态测试板,采用孔平均颜色变化率法(AWCD)测定冬小麦根际土壤微生物利用不同碳源的能力,计算微生物群落多样性指数,对不同碳源的利用率进行了主成分分析。两年试验结果显示,臭氧熏气与太阳辐射减弱复合作用,降低了土壤微生物对碳源的利用速度和利用总量;除了聚合物以外其它碳源利用率显著降低;对土壤微生物多样性没有直接的影响;对碳源降解的抑制效应大于增强的O3与减弱的太阳辐射两因素各自的单独作用。太阳辐射减弱20%,一定程度上增加了对聚合物类的分解。O3熏气条件下太阳辐射减弱,糖类、胺类代谢变异度较高,受环境影响较大。     

A reciprocal transplant experiment within a climatic gradient in a semiarid shrub-steppe ecosystem: effects on bunchgrass growth and reproduction, soil carbon, and soil nitrogen

We investigated the effect of climate change on Poa secunda Presl. and soils in a shrub‐steppe ecosystem in south‐eastern Washington. Intact soil cores containing P. secunda were reciprocally transplanted between two elevations. Plants and soils were examined, respectively, 4.5 and 5 years later. The lower elevation (310 m) site is warmer (28.5 °C air average monthly maximum) and drier (224 mm yr^?1) than the upper elevation (844 m) site (23.5 °C air average monthly maximum, 272 mm yr^?1). Observations were also made on undisturbed plants at both sites. There was no effect of climate change on plant density, shoot biomass, or carbon isotope discrimination in either transplanted plant population. The cooler, wetter environment significantly reduced percent cover and leaf length, while the warmer, drier environment had no effect. Warming and drying reduced percent shoot nitrogen, while the cooler, wetter environment had no effect. Culm density was zero for the lower elevation plants transplanted to the upper site and was 10.3 culms m^?2 at the lower site. There was no effect of warming and drying on the culm density of the upper elevation plants. Culm density of in situ lower elevation plants was greater than that of the in situ upper elevation plants. Warming and drying reduced total soil carbon 32% and total soil nitrogen 40%. The cooler, wetter environment had no effect on total soil C or N. Of the C and N that was lost over time, 64% of both came from the particulate organic matter fraction (POM, > 53 µ m). There was no effect of warming and drying on the upper population of P. secunda while exposing the lower population to the cooler, wetter environment reduced reproductive effort and percent cover. With the warmer and drier conditions that may develop with climate change, total C and N of semiarid soils may decrease with the active fraction of soil C also rapidly decreasing, which may alter ecosystem diversity and function.     

Do open-top chambers overestimate the effects of rising CO2 on plants? An analysis using spring wheat

The microclimate in facilities for studying effects of elevated CO₂ on crops differs from ambient conditions. Open-top chambers (OTCs) increase temperature by 1–3 °C. If temperature and CO₂ interact in their effect on crops, this would limit the value of OTC experiments. Furthermore, interaction of CO₂ and temperature deserves study because increases in atmospheric CO₂ concentration are expected to cause global warming. This paper describes two experiments in which a recently developed cooling system for OTCs was used to analyse the effects of temperature on photosynthesis, growth and yield of spring wheat (Triticum aestivum L., cv. Minaret). Two levels of CO₂ were used (350 and 700 ppm), and two levels of temperature, with cooled OTCs being 1.6–2.4 °C colder than noncooled OTCs. Photosynthetic rates were increased by elevated CO₂, but no effect of temperature was found. Cross-switching CO₂ concentrations as well as determination of A–C_i curves showed that plant photosynthetic capacity after anthesis acclimated to elevated CO₂. The acclimation may be related to the effects of CO₂ on tissue composition: elevated CO₂ decreased leaf nitrogen concentrations and increased sugar content. Calculations of the seasonal mean crop light-use efficiency (LUE) were consistent with the photosynthesis data in that CO₂ increased LUE by 20% on average whereas temperature had no effect. Both elevating CO₂ and cooling increased grain yield, by an average of 11% and 23%, respectively. CO₂ and temperature stimulated yield via different mechanisms: CO₂ increased photosynthetic rate, but decreased crop light interception capacity (LAI), whereas cooling increased grain yield by increasing LAI and extending the growing season with 10 days. The effects of CO₂ and temperature were not additive: the CO₂ effect was about doubled in the noncooled open-top chambers. In most cases, effects on yield were mediated through increased grain density rather than increased individual grain weights. The higher growth response to elevated CO₂ in noncooled vs. cooled OTCs shows that a cooling system may remove a bias towards overestimating crop growth response to CO₂ in open-top chambers.     

Warming effects on growth and physiology in the seedlings of the two conifers <Emphasis Type="Italic">Picea asperata</Emphasis> and <Emphasis Type="Italic">Abies faxoniana</Emphasis> under two contrasting light conditions

The short-term effects of two levels of air temperature (ambient and warmed) and light (full light and ca. 10% of full light regimes) on the early growth and physiology of Picea asperata and Abies faxoniana seedlings was determined using open-top chambers (OTC). The OTC manipulation increased mean air temperature and soil surface temperature by 0.51°C and 0.34°C under the 60-year plantation, and 0.69°C and 0.41°C under the forest opening, respectively. Warming, with either full-light or low-light conditions, generally caused a significant increase in plant growth, biomass accumulation, and stimulated photosynthetic performance of P. asperata seedlings. However, the warming of A. faxoniana seedlings only significantly increased their growth under low-light conditions, possibly as a result of photoinhibition caused by full light, which may shield and/or impair the effects of warming manipulation, per se, on the growth and physiological performance of A. faxoniana seedlings. In response to warming, P. asperata seedlings allocated relatively more biomass to roots and A. faxoniana more to foliage under similar environments. This might provide A. faxoniana with an adaptive advantage when soil moisture was not limiting and an advantage to P. asperata if substantial moisture stress occurred. Warming markedly increased the efficiency of PSII in terms of the increase in F _v/F _m and photosynthetic pigment concentrations for the two conifer seedlings, but the effects of warming were generally more pronounced under low-light conditions than under full-light conditions. On balance, this study suggested that warming had a beneficial impact on the early growth and development of conifer seedlings, at least in the short term. Consequently, warming may lead to changes in forest regeneration dynamics and species composition for subalpine coniferous ecosystems under future climate change.     

Biotic validation of small open-top chambers in a tundra ecosystem

Small open‐top chambers (OTC) are used widely in ecosystem warming experiments. The efficacy of the open‐top chamber as an analogue of climatic warming is examined. Twenty‐four small OTCs were used to passively warm canopy temperatures in wet meadow tundra at Barrow, Alaska, during two consecutive summers with contrasting surface air‐temperatures. Fortuitously, the seasonal average temperature regime within chambers in the colder year (1995) was similar to the controls of the warmer year (1996); this allowed a comparison of natural vs. chamber warming. All measured plant responses behaved similarly to both year and treatment 68% of the time. A comparison of the populations of the warmer summer's control with the cooler summer's OTC found no statistical difference in 80% of the response variables measured. A meta‐analysis also found no significant difference between the responses of the two populations. These results give empirical biotic validation for the use of the OTC as an analogue of regional climate warming.     

Response to simulated climatic change in an alpine and subarctic pollen-risk strategist, Silene acaulis

The aim of this study was to test if early flowering species respond with increased seed production to climate warming as is predicted for late-flowering seed-risk strategists. Experimental climate warming of about 3°C was applied to two populations of the cushion-forming plant Silene acaulis (L.) Jacq. The experiment was run at one subarctic site and one alpine site for 2 years and 1 year, respectively, using open-top chambers (OTC).
The 2-year temperature enhancement at the subarctic site had a marked effect on the flowering phenology. Cushions inside the OTC started flowering substantially earlier than control cushions. Both the male and female phases developed faster in the OTCs, and maturation of capsules occurred earlier. The cushions also responded positively in reproductive terms and produced more mature seeds and had a higher seed/ovule ratio. After 1 year temperature enhancement at the alpine site there was a weak trend for earlier flowering, but there was no significant difference in seed production or seed/ovule ratio.     

Enhanced summer warming reduces fungal decomposer diversity and litter mass loss more strongly in dry than in wet tundra

Many Arctic regions are currently experiencing substantial summer and winter climate changes. Litter decomposition is a fundamental component of ecosystem carbon and nutrient cycles, with fungi being among the primary decomposers. To assess the impacts of seasonal climatic changes on litter fungal communities and their functioning, Betula glandulosa leaf litter was surface‐incubated in two adjacent low Arctic sites with contrasting soil moisture regimes: dry shrub heath and wet sedge tundra at Disko Island, Greenland. At both sites, we investigated the impacts of factorial combinations of enhanced summer warming (using open‐top chambers; OTCs) and deepened snow (using snow fences) on surface litter mass loss, chemistry and fungal decomposer communities after approximately 1 year. Enhanced summer warming significantly restricted litter mass loss by 32% in the dry and 17% in the wet site. Litter moisture content was significantly reduced by summer warming in the dry, but not in the wet site. Likewise, fungal total abundance and diversity were reduced by OTC warming at the dry site, while comparatively modest warming effects were observed in the wet site. These results suggest that increased evapotranspiration in the OTC plots lowered litter moisture content to the point where fungal decomposition activities became inhibited. In contrast, snow addition enhanced fungal abundance in both sites but did not significantly affect litter mass loss rates. Across sites, control plots only shared 15% of their fungal phylotypes, suggesting strong local controls on fungal decomposer community composition. Nevertheless, fungal community functioning (litter decomposition) was negatively affected by warming in both sites. We conclude that although buried soil organic matter decomposition is widely expected to increase with future summer warming, surface litter decay and nutrient turnover rates in both xeric and relatively moist tundra are likely to be significantly restricted by the evaporative drying associated with warmer air temperatures.     

Effects of temperature on performance and phenotypic selection on plant traits in alpine Ranunculus acris

Discovering temperature effects on the performance of tundra plants is important in the light of expected climate change. In this 4-year study on alpine Ranunculus acris, I test the hypothesis that temperature influences flowering phenology, reproductive success, growth, population dynamics, and phenotypic selection on quantitative traits, by experimental warming using open-top chambers (OTCs). Warming significantly advanced flowering phenology in only one season. Seed number and weight were significantly increased by warming during the first three seasons, but not in the fourth. Plants inside OTCs produced bigger leaves than control plants in the fourth season, but leaf number was unaffected by the OTC treatment. Despite increased seed number and weight, the density of flowering plants decreased inside OTCs compared to control plots, possibly because of a higher graminoid cover inside OTCs. Phenotypic-selection regression showed a significant selection differential and gradient in the direction of larger leaf sizes in control plants, whereas no selection on leaf size was detected on warmed plants. The direction and strength of selection on flowering time, flower number, and leaf number did not differ between control and warmed plants. The results suggest that increased reproductive output of R. acris may not be sufficient to maintain current population density under a denser vegetation cover. Received: 1 December 1998 / Accepted: 14 April 1999     

Impact of warming and timing of snow melt on soil microarthropod assemblages associated with Dryas-dominated plant communities on Svalbard

Open Top Chambers (OTCs) were used to measure impacts of predicted global warming on the structure of the invertebrate community of a Dryas octopetala heath in West Spitsbergen. Results from the OTC experiment were compared with natural variation in invertebrate community structure along a snowmelt transect through similar vegetation up the adjacent hillside. Changes along this transect represent the natural response of the invertebrate community to progressively longer and potentially warmer and drier growing seasons. Using MANOVA, ANOVA, Linear Discriminant Analysis and χ² tests, significant differences in community composition were found between OTCs and controls and among stations along the transect. Numbers of cryptostigmatic and predatory mites tended to be higher in the warmer OTC treatment but numbers of the aphid Acyrthosiphon svalbardicum , hymenopterous parasitoids, Symphyta larvae, and weevils were higher in control plots. Most Collembola, including Hypogastrura tullbergi , Lepidocyrtus lignorum and Isotoma anglicana , followed a similar trend to the aphid, but Folsomia bisetosa was more abundant in the OTC treatment. Trends along the transect showed clear parallels with the OTC experiment. However, mite species, particularly Diapterobates notatus , tended to increase in numbers under warming, with several species collectively increasing at the earlier exposed transect stations. Overall, the results suggest that the composition and structure of Arctic invertebrate communities associated with Dryas will change significantly under global warming.