Impacts of winter climate change on growing season plant nutrient uptake and productivity in northern forest ecosystems

Annual variation in winter temperatures and snowfall can have a significant impact on the subsequent growing season in northern forest ecosystems. For example, periods with a reduced or lack of snow pack can increase the incidence of soil frost even when the ambient temperature is higher than average. Climate warming over the past several decades has resulted in a reduced winter snow pack throughout the northeastern U.S. and this trend is likely to continue into the future. Because snow pack serves as an effective ground insulator, this trend is expected to result in a greater incidence of soil frost. Previous studies at the Hubbard Brook Experimental Forest in New Hampshire have shown that periods of soil frost result in increased fine root mortality, as well as increased carbon and nitrogen losses. We hypothesize that an increased depth and duration of soil frost also results in a decrease in forest productivity and a reduction in nutrient uptake by dominant woody plants. We are testing this hypothesis using a combination of laboratory and field manipulations, as well as model experiments. This research will provide important insight into the ability of northeastern U.S. forests to retain high inputs of atmospheric nitrogen in the face of future climate change.

Graduate students: Annie Socci, Andrew Reinmann

Collaborators: John Campbell, Peter Groffman

The interaction between coastal fog inputs and redwood tree utilization of nutrients and water from fog

Fog deposition from the ocean contains a range of important nutrients and ions that can influence terrestrial ecosystem functions. Nitrogen (N) is present in fog, sometimes in high concentrations and often in much higher concentrations than normally found in rainwater. A defining feature of the redwood forest in coastal California is the presence of fog in the summer months. In this Mediterranean climate region, the fog provides water in a time when there is typically no rainfall.

In this project, we hypothesize that fog water provides redwood trees with N during the summer months when they would otherwise not be getting any nutrients. We are using natural abundance techniques to determine where redwood trees obtain their nutrients from throughout the year. This information will help us better understand the interaction between nitrogen and water cycles of California coastal redwood forests.

Collaborators: Kathie Weathers, Holly Ewing, Todd Dawson, Mary Firestone

Effects of calcium depletion on nutrient uptake by dominant tree species of northeastern forests

Acid rain, in the form of nitrogen and sulfur deposition, has led to significant base cation depletion in forests throughout the northeastern U.S. Widespread, but patchy, sugar maple mortality has been associated with deficiencies in base cation nutrition such as calcium, which predisposes trees to secondary stresses such as winter freezing and insect defoliation. Previous research shows that under current levels of nitrogen deposition, the forest floor of sugar maple stands retains significantly less nitrogen compared to stands of other dominant tree species. One possible consequence of calcium depletion could be reduced nitrogen uptake by sugar maple and other dominant tree species, a development that could lead to reduced vigor of trees, a reduction in the size of the plant sink for incoming nitrogen, further losses of nitrogen from forests, and greater losses of calcium. We began our study at the Hubbard Brook Experimental Forest in summer 2006 to examine the relationship between soil cation depletion and plant uptake of nutrients.

Collaborator: Linda Pardo

More Detailed Project Summary

Interaction between plant species composition and nitrogen retention in temperate forests

Human activity has doubled the amount of nitrogen (N) naturally fixed in the atmosphere. As a result, forests are receiving greater atmospheric N inputs than ever before. Furthermore, tree species composition of many forests is changing in response to plant diseases, competition with introduced plant species and global warming.

Understanding the combined effects of increased N inputs and changes in plant species composition on forest N cycling is critical to our understanding of forest biogeochemistry and nutrient budgets. It is also important to understand controls on N retention because N lost from forested watersheds can lead to cation leaching, nutrient imbalances within trees, acidification of stream water and eutrophication of estuaries and coastal waters.

Our work in temperate forests of the eastern U.S. examines the influence of dominant tree species on forest N retention and loss, using both enriched and natural abundance 15N stable isotopes. We aim to determine the mechanisms by which different tree species affect forest N retention. We are also investigating the use of natural abundance δ15N isotope signatures of plants as indicators of relative forest N cycling rates among different tree species' stands.

Collaborators: Gary Lovett, Kathie Weathers, Mary Arthur, Todd Dawson, Linda Pardo

Controls on nitrogen (N) retention and loss in tropical forest ecosystems

This project aims to characterize plant and microbial influences on forest nitrogen (N) cycling in tropical rainforests of Puerto Rico. Less research has focused on N cycling in tropical forests than in temperate forests. As a result, N conservation mechanisms of humid tropical forests are not well understood. We need to understand how N cycles within tropical forests because anthropogenic N inputs are expected to significantly increase there in the future.

We use enriched 15NH4+ and 15NO3- stable isotopic tracers to determine how plant and soil micro-organisms interact and either retain or lose N. We hope to clarify the role of N cycling in tropical forests via assimilatory (plant and microbial uptake), dissimilatory (microbial denitrification and dissimilatory NO3- reduction to NH4+) and leaching processes.

Collaborators: Whendee Silver and Mary Firestone

Effects of land-use change on nutrient cycling

Changes in land use are significantly affecting nutrient cycling and plant species composition in many parts of the world. This has prompted a considerable amount of research into how conversion of land from forest to agriculture is altering plant communities and soil nutrient cycling. Most studies focus on the conversion of tropical forest to pasture. However, we also need to understand the impact of forest conversion to other agricultural uses because it is common. To achieve that understanding, we have compared soil nitrogen (N) and carbon cycling across sites with different land use legacies. We plan to continue this work in the future.

Collaborators: Allison Power, Alex Flecker and Peter Groffman