Andrew N. Parsons

Antarctic climate cooling and terrestrial ecosystem response

The average air temperature at the Earths surface has increased by 0.06 °C per decade during the 20th century, and by 0.19 °C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarcticas 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 °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.

Comparative responses of phenology and reproductive development to simulated environmental change in sub-Arctic and high Arctic plants

The effects of temperature, precipitation and nutrient perturbations, and their interactions, are being assessed on two contrasting arctic ecosystems to simulate impacts of climate change. One, a high arctic polar semi-desert community, is characterised by a sparse, low and aggregated vegetation cover where plant proliferation is by seedlings, whereas the other, a sub-arctic dwarf shrub heath, is characterised by a complete vegetation cover of erect, clonal dwarf shrubs which spread vegetatively. The developmental processes of seed production were shown to be highly sensitive, even within one growing season, to specific environmental perturbations which differed between sites

Long-term experimental manipulation of winter snow regime and summer temperature in arctic and alpine tundra

Three 60 m long, 2·8 m high snowfences have been erected to study long-term effects of changing winter snow conditions on arctic and alpine tundra. This paper describes the experimental design and short-term effects. Open-top fiberglass warming chambers are placed along the experimental snow gradients and in controls areas outside the fences; each warming plot is paired with an unwarmed plot. The purpose of the experiment is to examine short- and long-term changes to the integrated physical-biological systems under simultaneous changes of winter snow regime and summer temperature, as part of the Long-Term Ecological Research network and the International Tundra Experiment. The sites were at Niwot Ridge, Colorado, a temperate high altitude site in the Colorado Rockies, and Toolik Lake, Alaska, a high-latitude site. Initial results indicate that although experimental designs are essentially identical at the arctic and alpine sites, experimental effects are different. The drift at Niwot Ridge lasts much longer than do the Toolik Lake drifts, so that the Niwot Ridge fence affects both summer and winter conditions, whereas the Toolik Lake fence affects primarily winter conditions. The temperature experiment also differs in effect between the sites. Although the average temperature increase at the two sites is similar (daily increase 1·5 °C at Toolik and 1·9 °C at Niwot Ridge), at Toolik Lake there is only minor diurnal variation, whereas at Niwot Ridge the daytime increases are extreme on sunny days (as much as 7–10 °C), and minimum nighttime temperatures in the chambers are often slightly cooler than ambient (by about 1 °C). The experimental drifts resulted in wintertime increases in temperature and CO2 flux. Temperatures under the deep drifts were much more consistent and warmer than in control areas, and at Niwot Ridge remained very close to 0 °C all winter. These increased temperatures were likely responsible for observed increases in system carbon loss. Initial changes to the aboveground biotic system included an increase in growth in response to both snow and warming, despite a reduced growing season. This is expected to be a transient response that will eventually be replaced by reduced growth. At least one species, Kobresia myosuroides, had almost completely died at Niwot Ridge three years after fence construction, whereas other species were increasing. We expect in both the short- and long-term to see the strongest effects of snow at the Niwot Ridge site, and stronger effects of temperature at Toolik Lake. Copyright

Relationships at the aboveground-belowground interface: Plants, soil biota, and soil processes

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.

Growth responses of four sub-Arctic dwarf shrubs to simulated environmental change

Vegetative responses of Empetrum hermaphroditum, Vaccinium vitis-idaea, V. uliginosum 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). Total above-ground biomass was largely unresponsive to the perturbations due to dilution of current seasons growth by material produced in previous years. 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 (...)

Environmental constraints on the growth, photosynthesis and reproductive development of Dryas octopetala at a high Arctic polar semi-desert, Svalbard

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.

Responses of plant litter decomposition and nitrogen mineralisation to simulated environmental change in a high arctic polar semi-desert and a subarctic dwarf shrub heath

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.

Soil Carbon Dioxide Flux in Antarctic Dry Valley Ecosystems

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.

VARIATION IN BIOGEOCHEMISTRY AND SOIL BIODIVERSITY ACROSS SPATIAL SCALES IN A POLAR DESERT ECOSYSTEM

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.

Differential growth, allocation and photosynthetic responses of Polygonum viviparum to simulated environmental change at a high Arctic polar semi-desert

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 (P n ). The objectives were to determine whether P. viviparum demonstrates conservative or opportunistic responses to increased temperature, water supply and nutrient availability, to assess whether vegetative and reproductive development show differential sensitivity, and thus whether allocation patterns are altered, and to evaluate whether changes in rate of photosynthesis underlied any changes in growth and allocation (...)