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Harvard Forest Symposium Abstract 2005

  • Title: Soil Warming at Harvard Forest: Effects on Biogeochemistry and Forest Trees
  • Primary Author: Jacqueline Mohan (University of Georgia)
  • Additional Authors: Joe Blanchard (Marine Biological Laboratory); Elizabeth Burrows (Rutgers University); Jerry Melillo (Marine Biological Laboratory); Paul Steudler (Marine Biological Laboratory); (Not Specified)
  • Abstract:

    Will temperate forests of the globe shift from being net “sinks” of atmospheric carbon dioxide (CO2) to becoming sources of CO2, forming a positive feedback that could increase the rate of climate warming? Global temperatures are rising and an increase of up to 5.5 oC by the end of this century is predicted for the New England region. Two of the major results of our original soil warming study at Harvard Forest were that: (1) warming stimulated the decay of a labile soil carbon pool thus increasing the rate of CO2 efflux to the atmosphere, and (2) warming increased the availability of soil nitrogen (N) to plants. Due to the small size of the original warming plots important questions we have not been able to answer are: Has the increase in available nitrogen led to an increase in growth of the trees - and if so, what is the balance between the carbon lost from the soil and the carbon stored in the vegetation in response to soil warming? We are now addressing these questions with a new, larger warming experiment in Harvard Forest. This study is one of the few warming experiments in the world to take place in a natural forest setting. Pre-treatment data was collected in 2001 and 2002, and the soil warming treatment commenced May 2003 in a 900 m2 forest tract which, by way of buried heating cables, is warmed to 5 oC above an adjacent control plot.


    We found that soil CO2 efflux increased significantly with heating in both 2003 and 2004 (p<0.001 from repeated-measures ANOVA taking into account pre-treatment differences between plots; Figure 1). Although in pre-treatment year 2002 the mean CO2 flux from the to-be-heated plot was 6% less than the control plot, by 2003 heated fluxes were 22% higher and remained 11% higher in 2004. After standardizing net N mineralization rates for differences in pre-treatments rates, we found they were also significantly higher (p<0.001) in the heated plot in 2003 and 2004 (Figure 2).


    In both 2003 and 2004 we observed species-specific changes in foliar N concentrations of canopy trees. Yellow birch and red maple exhibited higher %N and lower C:N ratios with soil warming in 2004, while white ash and red oak were less responsive. In addition, this year we’re observing for the first time changes in aboveground growth of canopy dominants. Over the 2001 to 2004 time period, dominant trees in the heated plot grew on average 12% larger, compared to a 6% increase for control trees (p=0.037; Figure 3a). Much of the increase in heated trees’ biomass occurred during the summer of 2004. Although canopy dominants grew faster with heating, they also displayed higher mortality rates so that by 2004 only 91% of the heated versus 100% of the control trees survived (p=0.08; Figure 3b).


    Understory tree seedlings and saplings continue to grow faster under heated conditions. While mean relative growth rates in the control versus heated plots were virtually identical in 2001 and 2002, heated trees grew about 50% faster than control individuals in both 2003 and 2004. Part of this increase in yearly growth may be explained by earlier bud break of heated trees in the spring, effectively lengthening the growing season for these individuals (by 27 April 2004, heated juvenile trees were 39±3% leafed out, whereas control trees averaged 27±3%; p=0.01).


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