Interacting effects of climate change in winter and the growing season on northern hardwood forests
Human activities such as fossil fuel combustion and deforestation have increased atmospheric concentrations of carbon dioxide, a greenhouse gas and major driver of climate change. Changes in the global carbon cycle have increased the need for improved understanding of the controls on carbon sequestration in terrestrial ecosystems because these systems represent a major, yet vulnerable, sink for carbon. More than half of the global organic carbon is stored in forest ecosystems and northern hardwood forests are considered net carbon sinks, storing carbon at a greater rate than they release it. However, the ability of forests to take up nutrients and store carbon could be altered by future temperature changes. For example, mean annual temperatures for the Northeastern U.S. are projected to increase 2.9-5.4 oC C by the year 2100, which could increase nutrient uptake and carbon storage by trees. However, the increase in temperature will lead to a smaller winter snowpack and increased frequency of soil freeze/thaw cycles, which may offset the positive effects of warming by damaging roots. The overall result could be reduced nutrient uptake and storage of carbon and further elevation of atmospheric concentrations. Understanding the mechanisms and controls underlying changes in nutrient uptake by trees is necessary for improving predictions of how ecosystems will respond as the climate changes. Whereas many studies have evaluated the impacts of climate change on forests within a single season, few have examined the impacts of climate change across seasons and how these impacts interact. These across-season effects could be antagonistic or synergistic, which would not be apparent from examining one season alone.
The goals of this research are to determine the interactive effects of winter and growing season climate on nutrient uptake and carbon sequestration in northern hardwood forests. We established a new ecosystem warming experiment at Hubbard Brook Experimental Forest, NH in summer 2012. Four plots are equipped with heating cable buried 10cm below the soil surface to warm the soils 5 oC above ambient temperatures. Two plots will be warmed throughout the year while the other two will be warmed in the growing season, but turned off in winter with snow manually removed via shoveling to induce soil frost. Two reference plots will experience ambient temperatures through the year. As part of this project, we are training high school teachers in forest ecology and climate change through formal lectures and field exercises. We have also established a new Sap-Net network of maple syrup producers to gain a better understanding of how climate change influences maple syrup production and quality.
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.
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
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.
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.
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.