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Soil carbon dioxide (CO2) flux is an integrative measure of ecosystem functioning representing both biotic and physical controls over carbon (C) balance. In the McMurdo Dry Valleys of Antarctica, soil CO2 fluxes (approximately −0.1–0.15 μmol m−2 s−1) are generally low, and negative fluxes (uptake of CO2) are sometimes observed. A combination of biological respiration and physical mechanisms, driven by temperature and mediated by soil moisture and mineralogy, determine CO2 flux and, therefore, soil organic C balance. The physical factors important to CO2 flux are being altered with climate variability in many ecosystems including arid forms such as the Antarctic terrestrial ecosystems, making it critical to understand how climate factors interact with biotic drivers to control soil CO2 fluxes and C balances. We measured soil CO2 flux in experimental field manipulations, microcosm incubations and across natural environmental gradients of soil moisture to estimate biotic soil respiration and abiotic sources of CO2 flux in soils over a range of physical and biotic conditions. We determined that temperature fluctuations were the most important factor influencing diel variation in CO2 flux. Variation within these diel CO2 cycles was explained by differences in soil moisture. Increased temperature (as opposed to temperature fluctuations) had little or no effect on CO2 flux if moisture was not also increased. We conclude that CO2 flux in dry valley soils is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the dry valley soil C cycle. Negative CO2 fluxes in arid soils have recently been identified as potential net C sinks. We demonstrate the potential for arid polar soils to take up CO2, driven largely by abiotic factors associated with climate change. The low levels of CO2 absorption into soils we observed may not constitute a significant sink of atmospheric CO2, but will influence the interpretation of CO2 flux for the dry valley soil C cycle and possibly other arid environments where biotic controls over C cycling are secondary to physical drivers.

Antarctic ecosystems are often considered nearly pristine because levels of anthropogenic disturbance are extremely low there. Nevertheless, over recent decades there has been a rapid increase in the number of people, researchers and tourists, visiting Antarctica. We evaluated, over 10 years, the direct impact of foot traffic on the abundance of soil animals and soil properties in Taylor Valley within the McMurdo Dry Valleys region of Antarctica. We compared soils from minimally disturbed areas with soils from nearby paths that received intermediate and high levels of human foot traffic (i.e., up to approximately 80 passes per year). The nematodes Scottnema lindsayae and Eudorylaimus sp. were the most commonly found animal species, whereas rotifers and tardigrades were found only occasionally. On the highly trampled footpaths, abundance of S. lindsayae and Eudorylaimus sp. was up to 52 and 76% lower, respectively, than in untrampled areas. Moreover, reduction in S. lindsayae abundance was more pronounced after 10 years than 2 years and in the surface soil than in the deeper soil, presumably because of the longer period of disturbance and the greater level of physical disturbance experienced by the surface soil. The ratio of living to dead Eudorylaimus sp. also declined with increased trampling intensity, which is indicative of increased mortality or reduced fecundity. At one site there was evidence that high levels of trampling reduced soil CO2 fluxes, which is related to total biological activity in the soil. Our results show that even low levels of human traffic can significantly affect soil biota in this ecosystem and may alter ecosystem processes, such as carbon cycling. Consequently, management and conservation plans for Antarctic soils should consider the high sensitivity of soil fauna to physical disturbance as human presence in this ecosystem increases. Resumen: Los ecosistemas de la Antartida a menudo son considerados casi pristinos porque los niveles de perturbacion antropogenica son extremadamente bajos. Sin embargo, en las decadas recientes ha habido un rapido incremento en el numero de personas, investigadores y turistas, que visitan la Antartida. Evaluamos, durante 10 anos, el impacto directo del trafico peatonal sobre la abundancia de animales del suelo y las propiedades del suelo en el Valle Taylor dentro de la region de los Valles Secos McMurdy de la Antartida. Comparamos suelos de las areas con minima perturbacion con suelos de veredas cercana que tenian niveles intermedios y altos de trafico peatonal (i.e., hasta ∼80 pases por ano). Los nematodos Scottnema lindsayae y Eudorylaimus sp. fueron las especies animales mas comunes, mientras que solo encontramos rotiferos y tardigrados ocasionalmente. En las veredas con trafico peatonal intenso, la abundancia de S. lindsayae y de Eudorylaimus sp. fue hasta 52% y 76% menor, respectivamente, que en las areas sin trafico. Mas aun, la reduccion en la abundancia de S. lindsayae fue mas pronunciada despues de 10 anos que dos anos y en el suelo superficial que en el suelo profundo, presumiblemente debido a un periodo de perturbacion mas largo y al mayor nivel de perturbacion fisica en el suelo superficial. La proporcion vivos – muertos de Eudorylaimus sp. tambien declino con el incremento de la intensidad del trafico peatonal, lo cual es un indicador de incremento de la mortalidad o reduccion de la fecundidad. En un sitio, hubo evidencia de que los niveles altos de trafico peatonal reducian los flujos de CO2 del suelo, que esta relacionado con una actividad biologica total en el suelo. Nuestros resultados muestran que aun niveles bajos de trafico humano pueden afectar significativamente a la biota en este ecosistema y pueden alterar procesos del ecosistema, como el ciclo del carbono. Consecuentemente, los planes de manejo y de conservacion de los suelos de la Antartida deberian considerar la alta sensibilidad del la fauna del suelo a la perturbacion fisica a medida que incrementa la presencia humana en este ecosistema.

Terrestrial ecosystems of the Antarctic Dry Valleys are among the most inhospitable soil environments on Earth due to climate and substrate limitations over biota. These ecosystems present a challenge to understanding controls over carbon (C) cycling because likely sources of organic matter are 10 2 –10 4 yrs old and in situ soil respiration is typically less than 1.0 μmol CO 2 m −2 s −1 . In this paper we describe an analytical approach to characterize kinetic pools of labile and recalcitrant soil C, and estimate C turnover in dry valley soils based upon these descriptions. Rate parameters for C turnover were derived from laboratory incubations conducted under a range of soil moistures and temperatures. We developed a C flux and reservoir model using these rate parameters along with published estimates of internal C transformations in soil microbial ecosystems, and a previously described primary production (NPP) model for Antarctic endolithic communities. We found that decomposition in 120 d incubations was well described by double-exponential rate kinetics, and that temperature, moisture and substrate availability significantly influenced observed rates of soil respiration. Simulations of soil C cycling based upon these parameters produced initially high rates of soil respiration following inputs of external organic matter, with mean residence times for C of 10–60 yrs. Soil organic C content equilibrated at 44–140% of observed levels within 1000 yrs. Simulations of equilibrium C were sensitive to NPP, microbial efficiency ( Y ), and the distribution of C inputs into labile and passive pools, indicating that more thorough investigation of microbial influence over the C cycle in dry valley soils is necessary.

Antarctic Dry Valley ecosystems are among the most inhospitable soil ecosystems on earth with simple food webs and nearly undetectable fluxes of carbon (C) and nitrogen (N). Due to the lack of vascular plants, soil organic matter concentrations are extremely low, and it is unclear how much of the contemporary soil C budget is actively cycling or a legacy of paleolake production and sedimentation. While recent work indicates multiple sources of organic matter for dry valley soils, the composition and kinetics of organic pools remain poorly characterized. We examined soil organic matter pools and potential C and N turnover in soils from within six sites located across three hydrological basins of Taylor Valley, Antarctica that differed in surface age, microclimate and proximity to legacy (paleolake) sources of organic matter. We estimated potential C and N mineralization, and rate kinetics using gas exchange and repeated leaching techniques during 90-d incubations of surface soils collected from va...

Desert ecosystems are characterized by distinct spatial patterning in soil biogeochemistry and biodiversity. In the Antarctic Dry Valleys, soil polygons are prominent features of the landscape and may be key units for scaling local ecological information to the greater region. We examined polygon soils in each of the three basins of Taylor Valley, Antarctica. Our objectives were to characterize variability in soil biogeochemistry and biodiversity at local to regional scales, and to test the influence of soil properties upon invertebrate communities. We found that soil biogeochemical properties and biodiversity vary over multiple spatial scales from fine ( 10 km) scales. Differences in biogeochemistry were most pronounced at broad scales among the major lake basins of Taylor Valley corresponding to differences in geology and microclimate, while variation in invertebrate biodiversity and abundance occurred at landscape scales of 10–500 m, and within individual soil polygons. Variation in biogeochemistry and invertebrate communities across these scales reflects the influence of physical processes and landscape development over ecosystem structure in the dry valleys. The development of soil polygons influences the spatial patterning of soil properties such as soil organic matter, salinity, moisture, and invertebrate habitat suitability. Nematode abundance and life history data indicate that polygon interiors are more suitable habitats than soils in the troughs at the edges of polygons. These data suggest that physical processes (i.e., polygon development) and biogeochemistry are important influences on the spatial variability of biotic communities in dry valley soil ecosystems.

The McMurdo Dry Valleys is the largest of the ice-free areas in Antarctica. Precipitation events in excess of 1 cm of snow accumulation are rare. During the winter, snow is transported by strong katabatic winds blowing from the polar plateau, and deposited into the lee of topographic features (e.g., stream channels and other topographic depressions). At the start of the austral summer (early October), as much as 10% of the valley soils may be covered by distributed snow patches. Because liquid water is the primary driver of biological, physical, and chemical processes in this polar desert, quantifying fluxes of water from snow patches is important to understanding the influence of hydrology on soil biology and nutrient cycling. During the austral summer of 1999–2000, four snow patches that had developed during the previous winter in Taylor Valley were studied. We measured snow-patch area, depth, and snow water equivalent, as well as subnivian (under snow) and nearby exposed (control) soil tempera...

Interactions at the aboveground-below ground interface provide important feedbacks that regulate ecosystem processes. Organisms within soil food webs are involved in processes of decomposition and nutrient mineralization, and their abundance and activity have been linked to plant ecophysiological traits such as species identity and the quality and quantity of plant tissue. We tested aboveground-below ground diversity relationships in a naturally developed plant community of native tallgrass prairie by taking soil samples from beneath naturally established grass tillers of chosen characteristics (e.g., homogeneous vs. heterogeneous plant combinations or C-4 vs. C-3 photosynthetic pathway) without imposing any disturbances to existing plant-soil relationships. The goal of this study was to elucidate the consequences, for soil microbiota (microflora phospholipid fatty acids, protozoa, and nematode functional groups) and for C and N mineralization, of plant community properties such as species richness, resource quality, resource heterogeneity, species identity, and presence of exotics. None of the biotic or abiotic soil variables was related to plant resource heterogeneity. Protozoa were not responsive to any of the plant community traits. Some bacterial and nematode groups were affected by plant characteristics specific to a particular plant species, but no uniform pattern emerged. Invasive and native plants generally were similar with respect to soil variables tested in this study. The lack of clear responses of soil variables to plant community traits indicates that idiosyncratic effects dominate both at the plant and soil biotic level and that generalized plant and soil diversity effects are hard to predict.

Summary 1 Responses of a Scandinavian sub-Arctic dwarf shrub heath community to 9 years of nutrient and temperature treatments were examined. Our objective was to assess the responses of plant and insect herbivore communities to these treatments, and to determine how vegetation responses scale-up to those of a second trophic group. 2 There were strong effects of nutrient addition on the above-ground biomass of both dominant (dwarf shrubs) and subordinate (grasses and mosses) plant functional groups, with responses by the latter being of greater magnitude. Responses to temperature were less frequent and of a smaller magnitude. 3 There were marked changes in the abundance of insect herbivores in response to the treatments. Changes in the above-ground biomass of subordinate plant species had a greater impact on the composition of the insect herbivore community than the smaller responses of dominant dwarf shrubs. For example, the abundance of a moss-feeding Heteropteran in fertilized plots was as little as 6% that of controls, while Homoptera specializing on grasses were over 400% more abundant. In addition, gramnivorous taxa (the Delphacidae) were present only in those plots that received nutrients. 4 Despite some species-specific effects of the perturbations on the quality of dwarf shrub annual shoots (defined as the concentrations of nitrogen and phenolic compounds), little change in leaf herbivory was observed. Insect herbivores removed less than 1% of annual biomass from dominant dwarf shrub species. 5 It is proposed that insect community change was driven by subordinate plant groups and not by the dominant dwarf shrub species, suggesting a wider importance of subordinate species for community structure.

A nested age within stage-structured model was developed to simulate Scottnema lindsayae population dynamics in soils of the McMurdo Dry Valleys, and used to evaluate responses of this species to a range of annual temperature regimes. Simulations predicted that many sites are unlikely to support S. lindsayae, consistent with field observations showing that many soils in the dry valleys lack nematodes. The simulated coefficient of population growth (λ) varied as a roughly linear function of cumulative annual degree-days (>0°C), providing a possible index to site suitability for S. lindsayae that could be used in conjunction with remotely sensed energy data to estimate species distributions. Low temperatures, coupled with long lifespan and slow development of this species, may make dry-valley soil systems sensitive to perturbations that disrupt population dynamics. Indeed, sensitivity analysis indicated that factors affecting adult longevity were the most important determinants of model behavior, further emphasizing the potential impacts of disturbance on local populations.

The average air temperature at the Earth's surface has increased by 0.06 degrees C per decade during the 20th century, and by 0.19 degrees C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarctica's peninsula region over the second half of the 20th century. Although previous reports suggest slight recent continental warming, our spatial analysis of Antarctic meteorological data demonstrates a net cooling on the Antarctic continent between 1966 and 2000, particularly during summer and autumn. The McMurdo Dry Valleys have cooled by 0.7 degrees C per decade between 1986 and 2000, with similar pronounced seasonal trends. Summer cooling is particularly important to Antarctic terrestrial ecosystems that are poised at the interface of ice and water. Here we present data from the dry valleys representing evidence of rapid terrestrial ecosystem response to climate cooling in Antarctica, including decreased primary productivity of lakes (6-9% per year) and declining numbers of soil invertebrates (more than 10% per year). Continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change.

We have examined organismic responses of Dryas octopetala to simulated changes in the summer climate at four tundra sites as part of the International Tundra Experiment (ITEX). Our study sites are located in the High Arctic, on Svalbard, Norway, in the Low Arctic at Abisko, Sweden, and at Toolik Lake, Alaska, USA and our temperate alpine site is at Niwot Ridge, Colorado, USA. These sites represent a range of tundra temperature and precipitation regimes, being generally cold and dry in the High Arctic and warmer and wetter at Toolik Lake and Niwot Ridge. Results from our studies indicate organismic attributes such as flowering shoot length varies by 30% between low and high arctic populations and that experimental warming results in significant increases in shoot height at three of four sites. We find that phenological development of Dryas is accelerated under experimentally warmed conditions which corresponds with a lengthening of the growing season in autumn, greater degrees of seed set and a higher likelihood of colonization of bare ground. We also observe that Dryas dominated ecosystems which are exposed to experimental manipulations are capable of exhibiting net carbon sequestration in late autumn, and that Dryas photosynthesis and green leaf biomass is significantly greater under warmer as opposed to ambient temperature conditions. Dryas leaf nitrogen is also significantly lowered under warmer conditions resulting in senescent leaves having a higher C:N ratio than those under ambient conditions. Together these findings indicate that Dryas phenology and carbon flux may be altered to the greatest degree in spring and again in autumn by higher summer temperatures and that simultaneously both positive and negative feedback effects may result from changes in plant and ecosystem performance.

We investigated the potential effects of global climate change on arctic tundra vegetation used as caribou forage. A total of 96 experimental plots was established at six sites on the coastal plain of the Arctic National Wildlife Refuge, Alaska, in 1993 and 1994. We erected snow-fences to increase the amount of snow deposition, and therefore delay the date of the snowmelt on 48 plots (referred to as increased snow/late melting plots). We used black mesh netting on the surface of the snow to increase the rate of melting on 24 plots; the remaining 24 plots served as controls. In July 1994, we collected green leaves from Eriophorum vaginatum, Salix planifolia, and Betula nana and analysed these samples for total carbon and total nitrogen content. Ratios of carbon to nitrogen differed among treatments for all three species. Generally, C:N ratios for B. nana and E. vaginatum on increased snow/late melting plots were lower than on control plots. C:N ratios for S. planifolia on increased snow/late melting plots did not differ from controls, but were lower than on plots which started to melt early. These results may be due to the timing of nitrogen translocation from leaf and stem tissue into storage organs, or due to an increase in available nitrogen input to the system. Further sampling is needed to adequately determine the mechanism responsible for increased nitrogen content of caribou forage in areas with increased amount of snow and delayed snowmelt.

Passive open-top devices have been proposed as a method to experimentally increase temperature in high-latitude ecosystems. There is, however, little documentation on the efficacy of these devices. This paper examines the performance of four open-top chambers for altering temperature at six sites in the Arctic and Antarctica. Most of the heating effect was due to daytime warming above ambient; occasional night-time cooling below ambient, especially of air temperatures, depressed mean daily temperature. The mean daily temperatures at four arctic sites were generally increased by 1.2–1.8 °C; but occasionally, temperature depressions also occurred. Under optimal conditions at the antarctic site (dry soils, no vegetation, high radiation) mean daily soil temperatures were increased by +2.2 °C (–10 cm) to +5.2 °C (0 cm). Protection from wind may play a more important role than temperature per se in providing a favourable environment for plant growth within open-top devices. Wind speed had a generally negative impact on mean daily temperature. Daily global radiation was both positively and negatively related to chamber temperature response. The effect of chambers on snow accumulation was variable with the Alexandra Fjord site showing an increased accumulation in chambers but no difference in the date of snowmelt, while at Latnjajaure in a deep snowfall site, snowmelt occurred 1–2 weeks earlier in chambers, potentially increasing the growing season. Selection of a passive temperature-enhancing system requires balancing the temperature enhancement desired against potential unwanted ecological effects such as chamber overheating and altered light, moisture, and wind. In general, the more closed the temperature-enhancing system, the higher is the temperature enhancement, but the larger are the unwanted ecological effects. Open-top chambers alter temperature significantly and minimize most unwanted ecological effects; as a consequence, these chambers are a useful tool for studying the response of high-latitude ecosystems to warming.

A prototype design for a cloud and mist collector is presented. The collector is a relatively simple and inexpensive design and excludes precipitation, making it suitable for long term collection of chemical data related to occult deposition. Chemical data obtained from weekly samples taken during an 18 month period using the occult collector and an adjacent bulk collector are presented.

The Antarctic dry valleys of southern Victoria Land are extreme desert environments where abiotic factors, such as temperature gradients, parent material, and soil water dynamics, may have a significant influence on soil carbon dioxide (CO2) flux. Previous measurements of soil respiration have demonstrated very low rates of CO2 efflux, barely above detection limits. We employed a modified infrared gas-analyzer system that enabled detection of smaller changes in CO2 concentration in the field than previously possible. We measured diel CO2 fluxes and monitored soil microclimate at three sites in Taylor Valley. Soil CO2 flux ranged from −0.1 to 0.15 μmol m−2 s−1. At two of the three sites, we detected a physically driven flux associated with diel variability in soil temperature. At these sites, CO2 uptake (negative flux) was associated with dropping soil temperatures, whereas CO2 evolution (positive flux) was associated with increases in soil temperature. These observations are corroborated by laboratory experiments that suggest that CO2 flux is influenced by physically driven processes. We discuss four potential mechanisms that may contribute to physically driven gas exchange. Our results suggest there are strong interactions between biological and abiotic controls over soil CO2 flux in terrestrial ecosystems of the Antarctic dry valleys, and that the magnitude of either may dominate depending on the soil environment and biological activity.

Opportunities exist in high Arctic polar semidesert communities for colonisation of unvegetated ground by long-lived clonal plants such as Dryas octopetala. This can be achieved by lateral spread of vegetative ramets, or by sexual reproduction and seedling recruitment. The objectives of this study were (1) to determine whether these two means of proliferation show differential sensitivity to contrasting components of the abiotic environment (temperature, soil nutrient and water availability) and (2) to evaluate the potential for D. octopetala to respond to climate change by an increase in cover and biomass in polar semi-desert communities. Factorial environmental manipulations of growing season temperature, soil nutrient and water status were conducted over 3 years at a polar semi-desert community in Svalbard, Norway (78°56.12′N, 11°50.4′E) and both clonal and sexual reproductive performance, together with instantaneous net photosynthesis (Pn), were recorded during the third season (1993). D. octopetala capitalised rapidly on an amelioration in the availability of inorganic nutrients (N, P and K) by an expansion in leaf area and biomass supported by increased Pn per unit leaf weight, and by apparent luxury uptake of nutrients (particularly P). Several facets of sexual reproductive development and seed viability were markedly improved by elevated temperatures or soil nutrient availability. Thus although D. octopetala is a long-lived clonal plant, with many traits characteristic of stress resistance syndrome, it showed considerable phenotypic plasticity in response to environmental manipulations. The results support the hypothesis that clonal growth confers survival potential during unfavourable years, together with the ability to capitalise on nutrient flushes and recycle nutrients internally. Continued investment in sexual reproduction ensures that seed setting is successful during favourable years, even if these occur infrequently. Cimate warming in the high Arctic could thus result in changes in the cover, biomass and the relative significance of clonal versus sexual proliferation of D. octopetala (and thus the genetic diversity of the population) but the long-term responses will probably be constrained by lack of available nutrients.

Summary 1 Vegetative responses of Empetrum hermaphroditum, Vaccinium vitis-idaea, V. uli ginosum and V. myrtillus to environmental change (temperature (T), water (W) and fertilizer (F)) were investigated in a factorial field perturbation study in sub-Arctic Sweden over two growing seasons (1991 and 1992). 2 Total above-ground biomass was largely unresponsive to the perturbations due to dilution of current season's growth by material produced in previous years. 3 The mass of shoot material produced in 1991, increased in response to F within 11 weeks of the start of the experiment in the two evergreen species (V. vitis-idaea and E. hermaphroditum), but not in the only deciduous species (V. uliginosum) measured that year. All three species studied in 1991 were unresponsive to T and W. 4 In all four species the mass of shoot material produced in 1992 showed the greatest response to F. The order of sensitivity was V. myrtillus > V. uliginosum > V. vitis idaea > E. hermaphroditum. T treatments also resulted in greater shoot mass (V. vitis idaea > E. hermaphroditum > V. myrtillus > V. uliginosum). No significant responses to W alone were observed. 5 T and F frequently interacted synergistically on the shoot characteristics measured in 1992. 6 The treatments affected the biomass allocation of the species differently, and this relates to their growth habit. Greater stem growth was observed in V. uliginosum and E. hermaphroditum, both of which spread laterally by producing long above-ground shoots. Greater leaf growth was observed in V. vitis-idaea and V. myrtillus, which spread laterally by rhizomes.

A high arctic polar semi-desert community is characterised by a sparse, low and aggregated vegetation cover where plant proliferation is by seedlings. A sub-arctic dwarf shrub heath is characterised by a complete vegetation cover of erect, clonal dwarf shrubs which spread vegetatively. At the polar semi-desert site, there was a striking effect of temperature enhancement treatments on phenology and seed-setting of Dryas octopetala ssp. octopetala, with almost no seed-setting occurring in plots experiencing ambient temperatures. There was no significant effects of temperature enhancement alone on fruit production of Empetrum hermaphroditum at the sub-Arctic dwarf shrub heath site, although fruit production was significantly influenced by application of nutrients and/or water. The response of a dominant high arctic dwarf shrub to increased temperature suggests that any climate warming may stimulate seed-set. This could be particularly important in the high Arctic where colonisation can proceed in areas dominated by bare ground and where genetic recombination may be needed to generate tolerance to predicted changes of great magnitude. In the sub-Arctic, however, the closed vegetation is dominated by clonally-proliferating species and recruitment from seedlings is rare. Plant fitness will increase here in response to any increased vegetative growth resulting from higher nutrient availability in warmer organic soils. -from Authors

Integrative ecophysiological and vegetative responses of Dryas octopetala were measured in response to field perturbations of temperature, precipitation and their interactions in a polar semi-desert in Svalbard, Norway (79°N, 12°E). Leaf carbon isotope discrimination (Δ), total leaf nitrogen concentration and leaf development were determined for photosynthetic leaves collected during the last week of August 1991, after one season of manipulations. Individual leaf weight and the total mass of leaf tissue were significantly lower when water was added, irrespective of temperature regime. Leaf carbon isotope discrimination and estimated long-term c i/c avalues (the ratio of CO2 concentration in leaf intercellular spaces to that in the atmosphere) were significantly higher under all three field manipulation treatments, and Δ was significantly reduced when Dryas was grown under drought conditions in a related greenhouse study. Nitrogen concentrations of plants from the field experiment were significantly lower under warmed conditions regardless of water regime. Our results indicate that changes in environmental conditions in high arctic settings will result in alterations of Dryas leaf gas exchange, as expressed by increases in carbon isotope discrimination, which may be accompanied by shifts in leaf nitrogen content and leaf biomass.

Doran et al. reply — Turner et al. do not find fault with our main focus — the rapid ecological response to recent cooling in the McMurdo Dry Valleys. The essence of their comment is that the spatial interpolation of the Antarctic continental data set (our Fig. 2) does not provide a meaningful picture of recent temperature trends. Although any interpolation is open to question, we note the following points.

Impacts of climate change were simulated in two contrasting European arctic ecosystems, a high arctic polar semi-desert and a subarctic dwarf shrub heath, by increasing temperature (using polythene tents), precipitation and soil nutrient (NPK) availability. The effects of these treatments and their interactions on plant litter decomposition and soil nutrient fluxes were assessed. Polythene tents increased air, litter and soil temperatures but reduced litter and soil moisture contents. At both sites, litter decomposition was significantly retarded in the tent treatments due probably to reduced litter moisture contents. The tent treatment had no effect on extractable soil N pools or net total N mineralisation at either site, although the treatment significantly reduced net seasonal nitrification values at the subarctic site. The additional precipitation treatment significantly increased litter decomposition at the dwarf shrub heath site and the net amount of N mineralised at the polar semi-desert site. Litter decomposition was increased, as was net N mineralisation, by the application of nutrients. The results suggest that soil temperature increases of up to 1°C, which may occur by the end of the next century as an effect of a predicted 4°C rise in air temperature, have only small effects on total N mineralisation in the short term in arctic soils.

A sub-arctic heath community dominated by dwarf shrubs has been subjected to three components of environmental change (temperature, water and fertilizer) in a factorial experiment over three growing seasons. Above-ground biomass of the most abundant grass in the ecosystem, Calamagrostis lapponica, was measured after one, two and three seasons, shoot height and flowering frequency were recorded after two and three seasons. There was up to 50 times more above-ground biomass at the end of the second season compared with the end of the first season. Fertiliser had the greatest effect on all parameters, resulting in greater biomass, increased flowering and a greater shoot height. The mean biomass of C. lapponica was greatest in the third season, but still contributed less than 8% of the above-ground biomass in the fertilised plots; the dominant species being ericaceous dwarf shrubs. The response of the grass to temperature and water was not as marked as the response to fertiliser. Temperature increase led to greater shoot height in both seasons whilst in the first season both warmer temperature and water additions resulted in greater biomass. All treatment effects were less in the third season than in the second, perhaps because of competition from the dominant dwarf shrubs which respond more slowly to the perturbations than the grass. The responses of the C. lapponica and the dwarf shrubs to the treatments are compared. If climate change results in greater solute availability in this ecosystem, there may be a marked change in species composition with C. lapponica becoming more prevalent.

Growing season temperatures and precipitation, and soil nutrient status, were increased in situ at a polar semi-desert site in northwest Spitsbergen to simulate the possible impacts of climate change. During the second year of the experiment the responses of a perennial geophyte, Polygonum viviparum, were assessed both by biometric analyses of vegetative and reproductive structures and by measurements of instantaneous net photosynthesis (Pn). The objectives were (1) to determine whether P. viviparum demonstrates conservative or opportunistic responses to increased temperature, water supply and nutrient availability, (2) to assess whether vegetative and reproductive development show differential sensitivity, and thus whether allocation patterns are altered, and (3) to evaluate whether changes in rate of photosynthesis underlied any changes in growth and allocation. Biometric analyses revealed that P. viviparum responded dramatically to the addition of nutrients (N, P, and K, at rates of 5, 5 and 6.3 g m-2 yr'1, respectively), both in terms of increased vegetative development (leaf sizes and corm weights) and also asexual reproductive development (spike lengths, bulbil numbers and weights). The observa tion that annual structures (leaves, reproductive spikes and bulbils), as well as peren nating tissues (underground corms), responded strongly to nutrients suggests that this species exhibits opportunism when nutrient constraints are alleviated. By contrast, increasing the mean growing season air temperatures by 3.50C above the ambient values exerted no significant impact on vegetative parameters, but was associated with significant increases in reproductive development and thus in alloca tion to reproductive structures. Reproductive and vegetative development thus showed differential sensitivity to the abiotic environmental constraints examined here. Measurements of Pn on a 'unit leaf area' basis failed to indicate any significant treatment effects on P. viviparum at this site, although it is concluded that increased leaf growth in the nutrient amended plots maintained optimal photosynthetic rates. Water additions at the site, to simulate the impacts of increased summer precipitation, were not significant as main effects on any of the measured parameters of plant performance.