Fall 2012
HOW MUCH WATER does a forest drink? Give Heidi Asbjornsen a tree and a heat ratio sensor and she'll give you an answer.
Using three separate probes inserted into sapwood, the sensor emits a pulse of heat and then takes two temperature readings. The combined measurements provide a tree's water flow velocity. Add a few additional "whole tree physical measurements," do a bit of multiplication to scale things up and, voilà, you have the estimated total water usage for the trees in the forest.
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| Heidi Asbjornsen installs heat ratio sensors in sapwood to measure water flow velocity. Photos by Michelle Day, UNH-EOS. |
It is a powerful scientific tool, and when Asbjornsen arrived at UNH from Iowa State University in 2010 as part of an eight-faculty, interdisciplinary, agro-ecosystems cluster hire, she added a new dimension within the Earth Systems Research Center (ESRC) that immediately infused previous research with new life.
That previous research, led by Scott Ollinger and detailed in the story "Synergistic Science" in this issue of Spheres, used remotely sensed techniques to establish the relationship between leaf nitrogen content, carbon sequestration, and albedo. Now, Ollinger, Jingfeng Xiao, and Asbjornsen are collaborating on a new NASA project that adds the role of water to the mix.
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Says Asbjornsen, an associate professor with a joint appointment in EOS and the UNH department of natural resources and the environment (NREN), "Once you've established these strong patterns based on observations, for example, between foliar nitrogen and spectral data, the next question is, what are the underlying reasons or mechanisms that explain these patterns?"
She adds, "To understand these mechanisms, it's important to measure the leaf- and plant-level physiological processes, such as water and carbon uptake. Then, scaling information from leaves to landscapes requires directly linking information about plant physiology to larger scale measurements obtained from remote sensing and flux towers."
Working in the disciplines of ecophysiology—an experimental science that seeks to describe the physiological mechanisms underlying ecological observations—and ecohydrology, Asbjornsen's ground-level, leaf-to-whole-plant research will complement the larger scale remote sensing and flux tower work done by Ollinger and Xiao.
She'll use tree cores and stable isotope analysis to look at growth over time, going back at least 50 years to determine how trees responded to climate variability.
"We can extract wood from individual tree rings and analyze it for stable isotopes of carbon-13, which serves as an indicator of water use efficiency," Asbjornsen says.
Water use efficiency is a measure of a plant's ability to simultaneously take in or "fix" atmospheric carbon dioxide through tiny leaf pores called stomata without losing too much water during the process of photosynthesis. Among other things, it is an important indicator of the capacity of trees and forests to maintain their productivity during environmentally stressful periods, such as extreme drought events or heat waves.
"There is an inherent trade-off between carbon fixation and water use," notes Asbjornsen, "and when vegetation is changed on a landscape, for example a forest is converted to crops, this can change the balance."
Agro-ecosystem trade-offs
Analyzing these inherent trade-offs when landscapes are significantly altered is part and parcel of work Asbjornsen is doing as a member of a group of UNH faculty members brought together under the recent sustainable agro-ecosystem cluster hire through the College of Life Sciences and Agriculture. The eight faculty members, including assistant professor Wil Wollheim of the ESRC and NREN, are, among other things, working as a group investigating various environmental implications of increasing food production in New Hampshire and New England.
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| Asbjornsen on the páramo—an alpine tundra ecosystem—in Colombia this past summer. Photo by L. Bruijnzeel, Free University-Amsterdam. |
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This initiative encompasses analyzing how changes in land use could affect water resources, water quality, regulation of hydrologic flows (flooding and water scarcity), as well as other ecosystem services such as carbon sequestration.
"My work in Iowa was more agriculturally focused and included research in prairie systems as well as savannah/woodlands and how the integration of these perennial vegetative covers within agriculturally dominated landscapes can help reduce negative environmental impacts," says Asbjornsen. "Here, the situation is kind of flipped because the landscape is primarily forested."
Indeed, compared to Midwest agriculture, the Northeast is a mere bump on the landscape and part of the initial work for the agro-ecosystem group is to get a better understanding of current and potential future trajectories of agriculture in New England and how the public at large perceives and values these changes—the latter being anything but clear, according to Asbjornsen and others on the project.
"The first step for us is to establish the relevance and importance of agriculture and agricultural expansion in New England," Asbjornsen says. "We need to understand the implications and potential environmental impacts of increased local food production."
With these data in hand, the researchers will then look into how landscapes could be designed and configured to maximize agricultural production while at the same time maintaining key ecosystem services that ensure water and soil quality and biodiversity, for example.
To this end, establishing an agro-ecosystem experimental station on UNH land similar to the Hubbard Brook Experimental Forest in New Hampshire's White Mountains is being considered. The long-term ecosystem study at Hubbard Brook has involved many researchers from different fields of expertise including hydrology and water quality, vegetation dynamics, soils, nutrients, biogeochemistry, etc.
"Part of the challenge is that, compared to a purely forested ecosystem, we'd need a very different type of experimental framework to answer questions in a mixed agro-ecosystem setting," says Asbjornsen.
For example, that framework would require being able to look at watersheds with different configurations of agricultural crop production and forests that could be experimentally manipulated.
The upshot of the group's work is to evaluate the potential impacts of expanding food production in the Northeast so that decision makers can accurately assess the situation as public demand grows for more locally grown food. This evaluation will analyze expected trade-offs between key ecosystem services (e.g., carbon sequestration, nutrient cycling and retention, and hydrologic flow regulation) under different land use change scenarios.
"It's possible that the growing demand for local products will result in more intensive agricultural production and/or conversion of existing forests to cropland or pasture. Thus, before embarking on this path, it's important that society first understands the potential impacts of different land use practices on multiple ecosystem services," Asbjornsen says. "If we want to increase food production in 30 years by, say, 50 percent, what will the specific trade-offs be? We need to understand those and justify them before we actually go down that road."