Nutrient Cycling in Response to Forest Growth under Free-Air CO2 Enrichment (FACE)

Forest ecosystems exchange a large amount of C with the atmopshere on an annual basis, and C storage in forests helps to mitigate the rise in the concentration of atmospheric CO2.  Using an experimental forest system in North Carolina, we are studying how soil nutrient availability controls the productivity response of a warm-temperate forest to elevated CO2.  The project uses free-air CO2 enrichment (see web site at allowing the forest to grow under high CO2 in the face of natural variations in climate and soil fertility.  To date (2005) this forest has shown a sustained stimulation in forest productivity under elevated CO2.  However, the magnitude and sustainability of this initial stimulation is tightly regulated by soil N availability.  Research in my lab is currently using a wide variety of techniques to measure changes in soil N availability and forest productivity under elevated CO2.

Amino Acid Cycling in Temperate Forest Ecosystems

Nitrogen is the element most commonly limiting the productivity of terrestrial ecosystems.  Traditional conceptual models assume that plants only take up inorganic forms of nitrogen (NH4+ and NO3-).  Recent research in arctic, alpine and boreal ecosystems suggest that certain organic forms of N (i.e., amino acids) can also contribute to plant-N nutrition.  Until now, however, there were no comparable data from temperate forests.  Research in my lab shows that organic N in the form of amino acids is a critical component of the soil-N cycle and that temperate forest tree species are likely to assimilate amino acids in the field.  There are as yet many unanswered questions in community and ecosystem ecology related to the importance of different forms of N, and these questions a major focus of current work in my lab.


Atmospheric Deposition and the Composition of Temperate Forests

Human activities have increased the deposition of N to northeastern US forests and simultaneously decreased base cation availability in soil (i.e., the availability of calcium and magnesium).  While these changes in the chemistry of the atmosphere and soil are correlated with changes in the species composition of temperate forests, few studies have experimentally modified the availability of these resources simultaneously to
determine their effects on forest composition.  We recently completed a four-year seedling transplant experiment (May 2005). We planted white pine, red maple, sugar maple and red oak seedlings on soils of contrasting parent material, in the forest understory and in experimental gaps, to which we added N and calcium in factorial combinations.  We are now analyzing these data and finding large inter-specific differences in the response of tree seedlings to excees N and calcium loss with implications for forest composition.  While this experiment is treminated this research has generated plenty of new hypotheses that need testing.

Invasive Species and the Alteration of Soil Nutrient Availability

Invasive species can rapidly dominate the understory of temperate forests.  Their invasion is highly correlated with the loss of native biological diversity and in many cases have large economic costs.  While most research has focused on plant competition as the mechanism underlying the success of invasive species, very little research has focused on the ecosystem consuquences.  We are studying the effect of one such species--Garlic Mustard--on nutrient cycling in temperate forests.  Garlic Mustard belongs to a family of plants that has a wide variety of secondary compounds that can act as biofumigants and alter the function of soil microbial communities.  Since soil microbes are fundamental to soil nutrient cycling, alterations in their activity can have profound effects on the productivity and function of ecosystems.  We have found that garlic mustard increases soil N and P availabilty and the rate of leaf litter decomposition.  We are currently investigating the effects of garlic mustard root exudates on the growth of native species (hebaceous species and tree seedlings).  More information at:

Tree Species Effects on Soil Nutrient Cycling

Tree species, through their occupancy of a site, can have a profound effect on nutrient cycling and nutrient availability.  The diversity of tree species found in temperate forests implies that patterns of ecosystem N retention and loss will, in part, depend upon the composition of a forest stand.  Work in my lab has focused on the role of different tree species in regulating soil-N cycling.  We are now expanding this research to focus not only on inorganic forms but also on organic forms of N.

Nitrogen and Phosphorus Control over Temperate Forest Productivity

Conceptual models for pristine ecosystems argue that the productivity of temperate forests is N limited, not phosphorus (P) limited.  However decades of atmospheric N deposition have in many cases lead to N saturation (i.e., N availability in excess of biological demand).  Could it now be that the productivity of temperate forests has become more strongly P limited?  To address this question, we have established experimental forest plots on three different soils types where we are adding N and P alone and in combination.  We have instrumented the trees with dendrometer bands (to measure tree growth) and collect aboveground litterfall annually.  With these data we will determine whether forest productivity remains N limited or whether P availability is increasingly regulting the productivity (and hence C storage capacity) of temperate forests.  This project has plenty of room for involvement by graduate students/post-docs and conceptual expansion.

The Effects of Polyphenolic Compounds on Soil Nitrogen Cycling

Nearly all plants species on Earth have polyphenolic compounds in their leaves.  From an evolutionary perspective, these polyphenolic compounds are thought to serve as a defense mechanism from herbivory.  Yet these very same compounds can also affect the activity of soil microbes by decreasing substrate avaialbility during the decomposition process.  This suggests that polyphenols may not only affect the activity of herbivores but that they may also control soil nutrient availability.  We have purifed the tannins (one type of polyphenolic compound) from the leaves of sugar maple, red oak and eastern hemlock and applied them directly to soil.  We are finding that tannins from different tree species have widely differing effects on soil N cycling--from stimulating N cycling to inhibiting N cycling (including organic forms).  We have only scratched the surface of this novel controlling mechanism for soil-N cycling and are now pursuing many additional avenues of resarch.