Collaborative Research: Do Shifts in Plant Carbon Allocation Facilitate Plant Nutrient Uptake & Primary Productivity in the Face of Oligotrophication in Northern Hardwood Forests?

Carbon (C), nitrogen (N) and phosphorus (P) are three of the most important elements necessary for plant growth. Temperate forests around the world, particularly in North America and Europe, have experienced declines in N availability relative to plant demand since the end of the twentieth century. In forests of the northeastern U.S., documented mechanisms behind these declines include changes in snowmelt timing, soil moisture, resorption back into the plant from leaves, and altered chemistry of litterfall (i.e., leaves that fall from trees). P availability relative to plant demand may be decreasing even more than N, especially if N limitation is driven by increases in plant nutrient consumption. This project will examine the interactions and feedbacks affecting plant N and P availability, and C partitioning in different plant components, to determine their impacts on long-term forest health and productivity. The project will also advance training of the next generation of scientists in integrative biological science, including the application of molecular and bioinformatic approaches to forest biogeochemistry, as well as computational modeling and mechanistic understanding of forest processes underpinning the U.S. bioeconomy. It will also result in the creation of digital and printed fact sheets and bulletins for participating organizations and broad public distribution to a range of audiences including local community members and K-12 students and teachers. The research will test the overarching hypothesis that long-term reductions in N and P availability in soils relative to plant demand lead to increased transfer of C from aboveground to belowground plant tissue. This, in turn, enhances nutrient acquisition from soils and partially offsets this decline in N and P. In the short-term, acquisition of nutrients from soil pools is projected to alleviate limitation of P to a greater degree than N, since microbial and enzymatic processes mobilize P more effectively than N from soil pools. It is expected that in the long-term, increases in plant growth will exceed the capacity of soil pools to supply N and P, leading to sustained nutrient limitation and reduced forest productivity. These hypotheses will be tested using a combination of long-term datasets, targeted field measurements, and process-based modeling for northern temperate forests at Hubbard Brook Experimental Forest in New Hampshire. The work includes use of (1) existing data from four long-term monitoring sites to quantify changes in plant above- and belowground C allocation over time and in relation to shifts in soil N and P and their uptake by plants; (2) targeted measurements at fourteen previously established field sites along an elevation gradient, including stable isotopic tracer and isotope probing of microbial communities; and (3) simulation modeling using artificial intelligence (AI) to represent the belowground C allocation and nutrient uptake processes driving and underlying N and P acquisition and oligotrophication. The project will also include research training at the postdoctoral, graduate student and undergraduate levels, as well as public outreach. These activities will involve multiple engagement meetings among project leaders, students, and members of the New Hampshire Forest Economy Alliance, a coalition of leaders committed to revitalizing the state’s forest-based economy. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.