Abstract:
A long term soil warming experiment at the Harvard Forest in central Massachusetts has given researchers insight into responses of northern temperate forests to changing climate conditions. The field experiment consists of two 30 x 30 m plots (control and heated) in a deciduous forest, where soil temperatures on the heated plot are maintained at 5°C above ambient. The experiment has been operating for seven years, including one year of pretreatment data collection. The large size of the plots has enabled the study of carbon and nitrogen cycling at the ecosystem level. In the first six years of the study both net nitrogen mineralization and net nitrification increased under warmer soil conditions. Soil respiration was significantly higher on the heated plot for the first five years (p < 0.05 for each year); however, in the past two years respiration on the heated plot has fallen to levels more similar the control plot. The average annual warming-induced soil respiration rate over the study period was approximately 1100 kg C/ha. Heating the soil also stimulated carbon storage in the vegetation, which was due to both increased nitrogen availability and a longer growing season. The average annual warming-induced carbon storage in vegetation was approximately 920 kg C/ha. The largest increases in relative growth rate were observed for red maple, Acer rubrum. Since 2004, the relative growth rates of red maples on the heated plot have been significantly higher than on the control plot (p< .05 for all years since 2004). We are interested in determining the mechanism(s) by which red maples thrive in warmer soil conditions. A previous study found that nitrate reductase activity is high for red maples fertilized with nitrate1 and this leads us to suspect that red maples have a greater capacity than other species to use the warming-induced “nitrate niche” observed in this study.
1 Downs, M. R., K. J. Nadelhoffer, J. M. Melillo, and J. D. Aber. l. (1993). Foliar and fine root nitrate reductase activity in seedlings of four forest tree species in relation to nitrogen availability. Trees, 7, 233-236.
