International Research Fellowship Program: Does the Foliar Nitrogen Isoptope Variability in Rain Forest Trees Reflect Niche Partitioning, Biophysical Processes or Climate?

Mayor Jordan R, Gainesville FL

Investigators

Abstract

The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award will support a 24-month research fellowship by Dr. Jordan Mayor to work with Dr. Benjamin Turner at the Smithsonian Tropical Research Institute in Panama. The availability of nitrogen (N) to plants is a primary control over forest productivity and ecosystem dynamics throughout much of the world. Tropical forests are presumed to be relatively N-rich yet their growth and adaptations respond to natural and anthropogenic changes in N-cycling and climate. Due to the complexity of the N cycle and the various types of plant responses to N limitations, there is a strong research need to use the ratio of heavy to light stable isotope measurements of N to integrate both biogeochemical processes and plant community responses to fluctuations in N availability. However, to be confident in the use and interpretation of foliar N isotope ratios, underlying mechanistic processes must be better understood. The PI, while housed within the Soils Biogeochemistry laboratory of Dr. Benjamin Turner, will meet the above research need by measuring stable isotope ratios from all three forms of plant available soil N, along with tree root and leaf tissue, across three experimental systems throughout Panamanian rainforest. The experimental systems were selected to target the main hypothesized controls over plant N isotope values: precipitation, soil nutrient bioavailability, and the type of root-associated mycorrhizal fungi. Each of the three experimental systems is designed to target specific hypotheses and, when combined, provide for a unified theory on the causes of variability in tree N isotope patterns in tropical forests. Human activities can increase N availability in ecosystems either by increasing N inputs through fertilizer addition or atmospheric deposition, or by speeding up internal recycling of N as a result of altered climate or landscape degradation. This joint research will inform the interpretation of relatively simple foliar analyses so that complex, time-integrated differences in N-cycles can be better understood and changes to them easily detected. In a time of rapid global change, these natural indexes will undoubtedly prove useful to forest monitoring efforts. In addition to the benefits of differentiating the underlying causes of tree N isotope ratios globally, these data will also permit testing for tree specialization on specific forms of soil N - a hypothesized mechanism for permitting the coexistence, rather than competitive exclusion, of otherwise similar tree species in biodiverse rainforest.