Long-Term Response of Plants at Barrow and Atqasuk to ITEX Warming Experiments

Robert D. Hollister, Patrick J. Webber , Steven P. Rewa, and Craig E. Tweedie

Task 4:
Describe community changes in relation to temperature.

Changes in species composition are likely to occur with warming due to differences in competitive interactions. These changes are likely to be slow due to the long lived nature and wide tolerance ranges of tundra plants. Vegetational change in the region have been describe by the AEL (Arctic Ecology Laboratory) under the FAV (Forecasting Arctic Vegetation) project. Therefore, it is important to make a distinction between changes in species composition due to the natural environment and chamber response (Figure I). We have documented small yet significant changes in species composition in the chambers, and these changes are in the same order of magnitude as changes due to the environment (Figure M). The relationship between sites and the overall change in response to the environment and chambers is shown with an ordination of the plots (Figure N).

Figure N - Ordination of the community composition of the MSU ITEX plots. The mean (symbol) and range (line) dimensional value for each treatment at each sampling for each site are derived from a Correspondence Analysis. The temperature and moisture axes were secondarily derived based on intuition. Figure M - Comparison of the magnitude of environmental, direct and long-term chamber effects on relative cover of growth forms at each MSU ITEX site.

Figure L - The conceptual diagram of the experimental design and analysis. Mathematically the compositional difference due to the environment can be estimated by calculating the difference between the control plots at time 2 with time 1 (E=CT2-CT1). Likewise the direct chamber response can be estimated by calculating the difference between the OTC plot with the control plot at time 1 (D=ET1-CT1) if it is assumed that there are no landscape differences (L=0). Finally the sustained chamber response can be estimated by calculating the difference between the difference in the control plots over time with the difference between the chamber plots over time (S=(ET2-ET1)-(CT2-CT1)).

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Figure N - Ordination of the community composition of the MSU ITEX plots. The mean (symbol) and range (line) dimensional value for each treatment at each sampling for each site are derived from a Correspondence Analysis. The temperature and moisture axes were secondarily derived based on intuition.	Figure M - Comparison of the magnitude of environmental, direct and long-term chamber effects on relative cover of growth forms at each MSU ITEX site.


Figure L - The conceptual diagram of the experimental design and analysis. Mathematically the compositional difference due to the environment can be estimated by calculating the difference between the control plots at time 2 with time 1 (E=CT2-CT1). Likewise the direct chamber response can be estimated by calculating the difference between the OTC plot with the control plot at time 1 (D=ET1-CT1) if it is assumed that there are no landscape differences (L=0). Finally the sustained chamber response can be estimated by calculating the difference between the difference in the control plots over time with the difference between the chamber plots over time (S=(ET2-ET1)-(CT2-CT1)).