Climate warming is strongly altering the timing of season initiation and season length in the Arctic. Phenological activities are among the most sensitive plant responses to climate change and have ...important effects at all levels within the ecosystem. We tested the effects of two experimental treatments, extended growing season via snow removal and extended growing season combined with soil warming, on plant phenology in tussock tundra in Alaska from 1995 through 2003. We specifically monitored the responses of eight species, representing four growth forms: (i) graminoids (Carex bigellowii and Eriophorum vaginatum); (ii) evergreen shrubs (Ledum palustre, Cassiope tetragona, and Vaccinium vitis‐idaea); (iii) deciduous shrubs (Betula nana and Salix pulchra); and (iv) forbs (Polygonum bistorta). Our study answered three questions: (i) Do experimental treatments affect the timing of leaf bud break, flowering, and leaf senescence? (ii) Are responses to treatments species‐specific and growth form‐specific? and (iii) Which environmental factors best predict timing of phenophases? Treatment significantly affected the timing of all three phenophases, although the two experimental treatments did not differ from each other. While phenological events began earlier in the experimental plots relative to the controls, duration of phenophases did not increase. The evergreen shrub, Cassiope tetragona, did not respond to either experimental treatment. While the other species did respond to experimental treatments, the total active period for these species did not increase relative to the control. Air temperature was consistently the best predictor of phenology. Our results imply that some evergreen shrubs (i.e., C. tetragona) will not capitalize on earlier favorable growing conditions, putting them at a competitive disadvantage relative to phenotypically plastic deciduous shrubs. Our findings also suggest that an early onset of the growing season as a result of decreased snow cover will not necessarily result in greater tundra productivity.
How the carbon balance of arctic ecosystems responds to climate warming will depend on the changes in carbon assimilation capacity of tundra plant species. Along with air and soil warming, one of the ...consequences of warming likely to be important for carbon assimilation of tundra plant species is an expected 40% increase in growing season length. We examined the effects of a lengthened growing season and soil warming on the photosynthetic capacity of seven tundra plant species from four growth forms that comprise >90% of the vascular cover of wet tussock tundra. Maximum photosynthetic capacity of these key species was relatively unchanged by the manipulation that significantly altered growing season length, active layer depth, and soil temperatures. Highest photosynthetic rates were found for the forb, Polygonum bistorta, and the lowest for dwarf evergreen shrubs. Seasonal patterns revealed that plants maintained relatively high light-saturated photosynthetic capacity (Amax) values throughout most of the growing season. Interannual variation was significant, but differences were small for most species. The study shows that tundra species operate within a relatively narrow range for maximum photosynthetic capacity with this maximum seldom being reached under ambient conditions. Thus, when evaluating the effects of climate change on tundra ecosystem carbon uptake, species composition and total photosynthetic leaf area should be considered first. These two factors will affect the system carbon exchange capacity during climate warming more so than species-level assimilation capacity.
How the carbon balance of arctic ecosystems responds to climate warming will depend on the changes in carbon assimilation capacity of tundra plant species. Along with air and soil warming, one of the ...consequences of warming likely to be important for carbon assimilation of tundra plant species is an expected 40% increase in growing season length. We examined the effects of a lengthened growing season and soil warming on the photosynthetic capacity of seven tundra plant species from four growth forms that comprise >90% of the vascular cover of wet tussock tundra. Maximum photosynthetic capacity of these key species was relatively unchanged by the manipulation that significantly altered growing season length, active layer depth, and soil temperatures. Highest photosynthetic rates were found for the forb, Polygonum bistorta, and the lowest for dwarf evergreen shrubs. Seasonal patterns revealed that plants maintained relatively high light-saturated photosynthetic capacity ($\text{A}_{\text{max}}$) values throughout most of the growing season. Interannual variation was significant, but differences were small for most species. The study shows that tundra species operate within a relatively narrow range for maximum photosynthetic capacity with this maximum seldom being reached under ambient conditions. Thus, when evaluating the effects of climate change on tundra ecosystem carbon uptake, species composition and total photosynthetic leaf area should be considered first. These two factors will affect the system carbon exchange capacity during climate warming more so than species-level assimilation capacity.
Climate change in the Arctic is predicted to increase plant productivity through decomposition-related enhanced nutrient availability. However, the extent of the increase will depend on whether the ...increased nutrient availability can be sustained. To address this uncertainty, I assessed the response of plant tissue nutrients, litter decomposition rates, and soil nutrient availability to experimental climate warming manipulations, extended growing season and soil warming, over a 7 year period. Overall, the most consistent effect was the year-to-year variability in measured parameters, probably a result of large differences in weather and time of snowmelt. The results of this study emphasize that although plants of arctic environments are specifically adapted to low nutrient availability, they also posses a suite of traits that help to reduce nutrient losses such as slow growth, low tissue concentrations, and low tissue turnover that result in subtle responses to environmental changes.
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