Forest Productivity and Ecophysiology
Carbon Cycling in an AgingForest: a Test of the "Respiration" Versus the "GPP" Hypothesis
Intellectual merit:Forest ecosystems are a major part of the terrestrial carbon sink that removes ~30% of anthropogenic C emissions each year. Our preliminary data show that net primary production (NPP) of loblolly pine (Pinus taeda L.) forestsdeclines strongly as they age, diminishing their capacity to sequester atmospheric C. NPP declined by >60% as stands aged, from 14 (~1200 gC m-2 y-1) to 115 (~500 gC m-2 y-1) years old, while leaf area index did not vary over this age range (LAI ~5). The factors governing this age-related decline in NPP are not well understood. The primary objective of the proposed research is to determine how changes in the different components of forest productivity contribute to the age-related decline in NPP. We will test competing hypotheses explaining the age-related decline in NPP by making independent measurements of NPP, gross primary production (GPP), plant respiration (Ra), and total belowground C allocation (TBCA) of loblolly pine stands that vary in age from 14 to 115 years old. The proposed research is novel in its construction of independent estimates of GPP, NPP and Ra. We will use the newly developed canopy conductance approach to estimate canopy photosynthesis in our experimental plots. This method, based on Fick’s law of diffusion, utilizes estimates of canopy conductance derived from measurements of whole-tree water use and measurements of intracellular CO2 concentration in foliage, derived from measurements of stable carbon isotopes and a model of light penetration and leaf conductance (4C-A model). This new approach has been successfully used to calculate GPP in stands of loblolly pine, mixed RockyMountain conifers, and a western poplar plantation. Using a Monte Carlo simulation, we estimate that we have a ≥95% probability of detecting a hypothetical decrease in Ac,net per year of stand age of 5 gC m-2 y-1, or a decrease from 2431 gC m-2 y-1 in a 19 year old stand to 1951 gC m-2 y-1 in a 115 year old stand.
Robust estimates of the response of forest ecosystems through their entire life to the major elements of global change hinge on understanding the mechanisms governing the age-related decline in NPP. A second objective of the proposed research is to examine putative C-based and non-C-based mechanisms causing the age related decline in GPP or NPP. It has been proposed that a reduced ability to transport water in large trees and subsequent reductions in stomatal conductance (the hydraulic limitation hypothesis) or sequestration of N in ever increasing biomass and subsequent reductions in photosynthesis drive the age related decline in GPP. Additional measurements of foliage and soil properties will be made to identify C-based mechanisms that potentially drive these changes in productivity. Alternatively, the decline in NPP may be unrelated to changes in tree C status, and may instead be driven by changes in cell turgor and expansion in large trees, representing a non-C-based mechanism. We will measure age-related changes in C storage in foliage, stems and roots to test this alternative explanation for the decline in NPP.
Broader impacts of the proposed research: The PIs propose three explicit tasks to broaden the societal impact of the proposed research. First, we will implement an active learning module to teach science teachers and secondary school students about the coupling between terrestrial ecosystemsand the Earth’s climate system. In collaboration with teachers and students at UniversityLaboratoryHigh School in Urbana,IL, we will conduct tree growth and litterfall measurements in a local old growth and aggrading hardwood forest, thus providing students with exposure to data collection, statistical analysis, and data interpretation. Oren and Palmroth already have established a three-term seminar course in which they teach high school students from the North Carolina School of Science and Mathematics how to construct 4C-A model components and how to build and deploy sap flux sensors, extract data and analyze them in the 4C-A framework. Our second task is to continue development of interdisciplinary undergraduate teaching modules on global environmental change and the role of forests in the global C cycle for large introductory courses. DeLucia and Thomas teach introductory biology courses that are attended by 500 and 1000 students, respectively. Our third task will be to recruit talented undergraduates, particularly those from facets of society that are underrepresented in science education and research to participate in this research. To enable undergraduates to fully participate in this research the proposed research will be listed with WVU’s McNair Program, Summer Undergraduate Research Experience Program and an NSF-fund REU program and UIUC’s Howard Hughes Program.