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 Exploring relationships among water use efficiency, canopy nitrogen and carbon cycling across North American ecosystems to improve land surface models

NASA, Terrestrial Ecology (TE), $780,205, 2012-2015

PI Scott Ollinger, Co-Is Heidi Asbjornsen and Jingfeng Xiao

Cycles of carbon (C), nitrogen (N) and water in terrestrial ecosystems are tightly coupled through a shared set of biophysical processes. The N status of plants is a key control on C assimilation, and both affect water fluxes from land to atmosphere.  In turn, water availability is one of the most important controls on plant growth globally and can set an upper bound on the availability of N.  How ecosystems respond to changes in the availability of water and its temporal variability will likely be among the most important effects of climate change and one of the most challenging for models to capture.  

Recent analyses have demonstrated significant, broad-scale relationships between canopy %N and both whole-canopy photosynthesis and shortwave canopy albedo. These results stem from the constraints imposed by acquisition of multiple resources, which drive plants toward functional convergence among a variety of leaf and canopy level traits. The resulting patterns suggest a greater degree of coupling between C, N and energy balances than has previously been considered and provide a basis for using canopy spectral data to improve understanding of ecosystem-climate interactions.

To date, efforts to develop similar scaling approaches for plant water use and water use efficiency (WUE) have lagged, despite abundant evidence suggesting strong relationships between WUE and plant C and N status. Contributing factors include: (1) incongruency of the spatial and temporal scales at which ET and WUE are derived, (2) incomplete understanding of the mechanisms driving WUE across species and ecosystems, (3) the challenge of relating species-level measurements to water fluxes over diverse canopies, and (4) a scarcity of analysis that have considered WUE in relation to plant traits and plant functional types across space and time.

Based on results from our prior NASA-funded research, combined with documented relationships between C and water cycles, the objectives of the proposed work are to examine how evapotranspiration (ET) and WUE in forests relate to variation in foliar N and canopy photosynthesis and to use the resulting information to improve a model of ecosystem C, N and H2O cycles.  We also propose that the vulnerability of ecosystems to altered precipitation amount and variability is influenced by the N status of vegetation and by the diversity of species present. We plan to achieve these goals through the following objectives:
1.       Build on a previous, NASA-supported analysis of hyperspectral remote sensing and C flux data to determine how ET and WUE relate to canopy nitrogen, carbon assimilation and albedo in North American ecosystems. 
2.       Examine the degree to which WUE and its relation with N availability over space and time can be estimated using 13C, 18O and 15N isotope measurements in leaf and woody tissues.
3.       Examine the degree to which ecosystem response to climate variability is influenced by species diversity and the N status of species present.
4.       Evaluate canopy spectral properties related to WUE, canopy water content and species diversity using high spectral resolution aircraft remote sensing. This will be repeated after degrading imaging spectrometer spatial resolution to 60 m in order to evaluate future applications using HyspIRI.
5.       Integrate results from objectives 1-4 to determine how WUE varies with C, N and energy fluxes, along with canopy reflectance, and use the results to improve a continental upscaling analysis of C fluxes, ET and WUE.
6.       Use results from objective 5 to constrain parameters in a widely used ecosystem model and run the model forward to evaluate changes in C-N-H2O interactions under scenarios of future climate and altered biodiversity.