Plant and ecosystem response to long term experimental warming and nutrient additions in arctic shrub tundra

Background/Question/Methods

Warming arctic temperatures can drive changes in ecosystem structure and function directly by stimulating plant growth or indirectly by stimulating microbial decomposition of organic matter and releasing more nutrients for plant uptake and growth. The arctic tundra biome includes a diverse array of vegetation communities which, due to the variety of plant functional types that may dominate, may differ in their response to environmental change. We examined the plant and ecosystem response from the longest running (18 years) nutrient and warming experiment in Alaskan arctic riparian shrub tundra ecosystem to understand controls over plant productivity and carbon (C) and nitrogen (N) storage in shrub tundra ecosystems in order to make inferences about how these systems will respond to environmental change. We hypothesized that productivity, C and N pools in plant biomass would increase more strongly and consistently to increased nutrient availability than they would to a 1-3 °C increase in air temperature. In addition, we predicted that increasing plant productivity in response to nutrient addition would lead to an increase in C and N pools in soil organic matter due to increased inputs of plant litter.  

Results/Conclusions

We found that total aboveground biomass increased by 98, 81, and 326% in the nutrient addition, warming, and nutrient additions plus warming treatments relative to the control (F₃,₁₆ = 5.5, P < 0.01). Deciduous shrubs showed the greatest increase in biomass while forb, graminoid, and evergreen shrub biomass decreased with treatments. The aboveground N pool increased with nutrient plus warming treatment and this was marginally significant (F_3,4 = 4.5, p = 0.09) and mainly attributed to increases in live shoots, woody standing dead, and litter. Surprisingly, there was no detectable change in soil C and N pools across any of the treatments. The dominant functional group, deciduous shrubs, increased allocation to aboveground stems but decreased allocation to belowground stems with nutrient additions plus warming. In almost all cases, the effects of environmental manipulations were more pronounced in the nutrient plus warming treatments. A future arctic that is both warmer and more nutrient rich has the potentially to alter riparian shrub ecosystems by increasing total deciduous shrub biomass and greater allocation to woody tissue that has a longer residence time and should be considered when making predictions about the response of arctic vegetation to future climate change.