Clare H. Robinson

Responses of tundra plants to experimental warming : Meta-analysis of the international tundra experiment

The International Tundra Experiment (ITEX) is a collaborative, multisite experiment using a common temperature manipulation to examine variability in species response across climatic and geographic gradients of tundra ecosystems. ITEX was designed specifically to examine variability in arctic and alpine species response to increased temperature. We compiled from one to four years of experimental data from 13 different ITEX sites and used meta-analysis to analyze responses of plant phenology, growth, and reproduction to experimental warming. Results indicate that key phenological events such as leaf bud burst and flowering occurred earlier in warmed plots throughout the study period; however, there was little impact on growth cessation at the end of the season. Quantitative measures of vegetative growth were greatest in warmed plots in the early years of the experiment, whereas reproductive effort and success increased in later years. A shift away from vegetative growth and toward reproductive effort and success in the fourth treatment year suggests a shift from the initial response to a secondary response. The change in vegetative response may be due to depletion of stored plant reserves, whereas the lag in reproductive response may be due to the formation of flower buds one to several seasons prior to flowering. Both vegetative and reproductive responses varied among life-forms; herbaceous forms had stronger and more consistent vegetative growth responses than did woody forms. The greater responsiveness of the herbaceous forms may be attributed to their more flexible morphology and to their relatively greater proportion of stored plant reserves. Finally, warmer, low arctic sites produced the strongest growth responses, but colder sites produced a greater reproductive response. Greater resource investment in vegetative growth may be a conservative strategy in the Low Arctic, where there is more competition for light, nutrients, or water, and there may be little opportunity for successful germination or seedling development. In contrast, in the High Arctic, heavy investment in producing seed under a higher temperature scenario may provide an opportunity for species to colonize patches of unvegetated ground. The observed differential response to warming suggests that the primary forces driving the response vary across climatic zones, functional groups, and through time.

PLANT COMMUNITY RESPONSES TO SIMULATED ENVIRONMENTAL CHANGE AT A HIGH ARCTIC POLAR SEMI-DESERT

Impacts of climate change were simulated over five summer seasons in a high arctic polar semi-desert at Ny Alesund, Svalbard, by using polythene tents to increase temperature, and by increasing precipitation and soil nutrient (NPK) availability. The effects of these treatments on vegetation cover were assessed at the start of the 1991, 1993, and 1995 field seasons, and at peak biomass in the same years. Over the first season of the experiment (1991), changes in percentage total living vegetation cover were significantly greater, and changes in dead vegetation cover significantly lower, in the tented treatments. In subsequent seasons, changes in total living cover were also greater under treatments simulating climate change, although the significant factors and interactions were year- specific. Between years, at both the early and mid-season sampling periods, the fertilizer application had the strongest effect on changes in plant cover, significantly decreasing cover of living Dryas octopetala, Saxifraga oppositifolia, and bare ground between 1991 and 1995, while increasing cover of bryophytes, Salix Polaris, Polygonum viviparum, and total dead vegetation. Although cover of D. octopetala was greater during the first three years of fertilizer addition, marked winter injury occurred in this species on fertilized plots during winter 1993-1994. This resulted in reductions in total live cover and D. octopetala cover and an increase in total dead cover (by up to 22%) in watered and fertilized plots between 1991 and 1995. Seedlings of nitrophilous immigrant species were established naturally on bare ground in fertilized plots in the third year of the study and subsequently increased in number, so that after five seasons the community tended more toward bird-cliff vegetation rather than polar semi-desert vegetation. The tent treatment and the simulated increase in summer precipitation had little effect between seasons on the plant community, in com- parison with the fertilizer treatment.

Controls on decomposition and soil nitrogen availability at high latitudes

At high latitudes, decomposition rates and soil nitrogen (N) availability are pivotal in determining ecosystem responses to climate change. The effects of temperature, soil moisture content, resource quality, and saprotrophic fungi as an example of soil organisms, on carbon (C) and N mineralisation are reviewed. The controls on N availability are less well characterised than those on decomposition, and C and net N mineralisation sometimes do not respond to these controls in a parallel manner. Increases in mean summer temperatures of 2–4°C predicted for high latitudes may not necessarily cause greater rates of decomposition and N mineralisation because of concomitant small rises in soil temperature together with interactions between the controls, including interactions of the temperature and moisture content of the substrata with the diversity and function of decomposer fungi. Research on decomposition and soil N availability has been carried out at several scales, at all of which future research remains necessary. It is not clear whether species diversity of decomposer fungi influences decomposition and C and N release above the microscale.

Decomposition of root mixtures from high arctic plants : a microcosm study

Abstract Fine roots ( Dryas octopetala L., Carex rupestris All., Saxifraga oppositifolia L. and Salix polaris Wahlenb. were decomposed as single species, and in mixtures with fine roots of D. octopetala , under laboratory conditions. Decomposition was measured as carbon dioxide release and mass loss over 556 d at 6°C. There was no relationship between cumulative CO 2 release and mass loss, suggesting that leaching and loss as fine particulate matter are important in fine root decomposition. In single species comparisons of CO 2 release, roots of C. rupestris decomposed to the greatest extent, perhaps because of their relatively high resource quality, although S. polaris roots showed greatest decay in comparisons using total mass loss. Differences between expected and measured values of root decomposition were evaluated, based on a comparison between the results from the single and mixed species. In C. rupestris + D. octopetala and S. oppositifolia + D. octopetala combinations, there was evidence of `negative interactions which decreased the rate of CO 2 release below that predicted. There were also `positive interactions which increased the percentage loss of root mass in S. oppositifolia + D. octopetala and S. polaris + D. octopetala pairings. The results show that the decomposition of resources in mixtures cannot be predicted readily from the characteristics of the component resources decomposing in isolation.

Nutrient and carbon dioxide release by interacting species of straw-decomposing fungi

Pairs of fungi were incubated on wheat straw in microcosms for 10 weeks. Release of Na+, K+ and NH4+-N was similar from all combinations, but Ca2+, Mg2+ and PO43--P release depended on the species. In Agrocybe gibberosa/Chaetomium globosum and Sphaerobolus stellatus/Chaetomium globosum combinations, there was evidence of interactions which suppressed the predicted rate of phosphate release, and in all the mixed species combinations there were interactions which increased the rate of fungal respiration above that of the more combative fungus in pure culture. ei]{gnR}{fnMerckx}

Resource Capture by Interacting Fungal Colonizers of Straw

Nine species of microfungi and eight basidiomycetes were assessed singly and in pairwise or three-way combination for their potential capacity for primary resource capture of agar media, sterile straw and sterile soil. When single species were ranked by extension rate, results on straw and soil agreed broadly with those from agar media, although the rates were much slower in straw. In pairwise and 3-way interactions on sterile soil, colonization proceeded as predicted from the mycelial extension rates of single species on agar, sterile soil and sterile straw, although extension of some species was significantly slower in combination than singly. There was some agreement between the outcome of pairwise interactions observed on 2% malt extract agar medium and sterile soil of four test species, and on straws in non-sterile soil of Agrocybe gibberosa . The four species could be ranked in combative order: A. gibberosa = Sphaerobolus stellatus ≫ Mucor hiemalis and Chaetomium globosum . Cord formation by basidiomycetes was more pronounced in soil than on agar media, and when in close proximity to other species.

Enzyme production by Mycena galopus mycelium in artificial media and in Picea sitchensis F1 horizon needle litter

Mycena galopus is among the most important leaf litter decomposers in UK coniferous and angiosperm woodlands, having the potential to utilise all the major constituents of plant litter. Even so, the enzyme or combination of enzymes produced by M. galopus responsible for lignin depolymerisation was previously unknown. A range of media from liquid and semi-solid cultures to more natural substrata was tested to determine whether laccase was produced by an isolate of M. galopus, M9053. Malt extract liquid medium (MEL) with 2,5-xylidine favoured laccase production as compared with the same medium containing the inducers veratryl alcohol, veratryl aldehyde, veratric acid, homoveratric acid, vanillic acid or p-anisic acid. A semi-solid medium of cereal bran in phosphate buffer and a solid medium of Picea sitchensis F1 horizon needle litter were also not as effective as MEL with 2,5-xylidine as an inducer. Compared with six other isolates of the same species grown in MEL without inducers, M9053 exhibited rates of laccase activity fairly typical for M. galopus. An isolate from a dark coloured basidiome of M. galopus, but not var. nigra, exhibited the greatest activity while var. candida showed relatively low laccase activity. Marasmius androsaceus exhibited peak laccase production several days later than M. galopus. In addition, a manganese-dependent peroxidase that was responsible for 15% (in MEL culture fluid) and 39% (in needle litter extract III) of ligninolytic activity was produced by M9053. A further peroxidase was found to be the major ligninolytic constituent in MEL extracts (53%), and had a similar contribution to total activity (29%) as laccase (32%) in needle litter fraction III. Mycena galopus produced water- and buffer-extractable mannases and xylanases when grown on needle litter.

Fungal biodiversity in dead leaves of fertilized plants of Dryas octopetala from a high arctic site

At a high arctic, polar semi-desert site (79° N) in Svalbard, nitrogen, phosphorus and potassium fertilizers were added to the soil for 5 yr to simulate increases in decomposition and nutrient mineralization which may occur as a result of increases in soil temperature and soil moisture caused by environmental change. Abundance of decomposer fungal species, isolated from standing-dead leaves of Dryas octopetala, varied between fertilized and unfertilized plots. Fungal biodiversity, as indicated by the Brillouin index, was lower in dead Dryas leaves from the fertilized plots, although more colonies were isolated from leaves of plants which had been fertilized. The fungi were cosmopolitan and not restricted to tundra areas. The dead leaves from the fertilized plots contained more nitrogen and phosphorus than those of the unfertilized plants. Leaves in the fertilized plots appeared to have been killed by winter injury resulting from an extended growing season in an atypically mild and wet autumn, which was followed quickly by extreme subzero temperatures.