Homeostatic adjustment
of a loblolly pine forest
under elevated atmospheric
carbon dioxide

Terrestrial ecosystems, particularly forest ecosystems, are important in regulating atmospheric CO2 through the balanced effects of photosynthesis and respiration. Many experiments indicate that increased CO2 will stimulate plant growth and suggest that this increase in growth will sequester CO2 thereby slowing its rate of increase. While some studies show an enduring growth stimulation by elevated CO2, others indicate that the growth enhancement decreases with time, and under nutrient-limited conditions typical of most forests there may be no growth stimulation. A central question in "climate change" research remains whether the initial photosynthetic and growth enhancement observed for tree seedlings and saplings with a doubling of CO2 will be sustained for large trees, and hence forested ecosystems, experiencing the full suite of forest ecosystem processes. The objective of our proposed research is to determine if a time-dependent decline in the CO2-stimulation of growth occurs in an intact forest ecosystem.

We have identified three broad mechanisms operating at the tree level that are homeostatic in that they may serve to maintain a proper balance of tissue carbohydrate and nutrient status but at a potentially lower growth rate. The mechanisms are 1) down-regulation of photosynthesis resulting from source/sink constraints; 2) increased allocation to support structure and roots with concomitant increases in maintenance respiration; and 3) plant nutrient imbalance as growth exceeds the delivery of limiting nutrients. We are studying these processes by estimating a) changes in leaf-level photosynthetic capacity and photosynthetic enzymes, b) changes in the allocation of biomass to above- and below-ground structures c) alterations in respiration rates, d) changes in foliage dynamics and nutrient levels, and e) carbon budgets of individuals and loblolly pine stands. These mechanisms are interdependent and represent homeostatic adjustment of trees to altered resource states that directly influence carbon cycling and storage in a forest ecosystem.

We recently completed the first full growing season of the FACE study under the treatment conditions. During the first year we observed a significant growth stimulation for pine trees and little evidence of down-regulation of photosynthesis. A single year of CO2 treatment may not be a sufficient time period to draw firm conclusions about the homeostatic mechanisms outlined in this proposal. We are requesting three additional years of funding to document long-term changes in the growth stimulation as well as the timing and magnitude of "down regulation" of loblolly pine under CO2 enrichment. We expect that the responses of this forest to elevated CO2 will become transient as reallocation of biomass and nitrogen in trees is initiated, as the full complement of foliage is developed under the CO2 treatment, and as ecosystem feedbacks associated with altered litter chemistry begin to express their influence on nutrient cycling in this system. The mechanistic examination of these processes over the course of this study will enhance our understanding of potential ecosystem interactions and feedbacks and strengthen our ability to predict long-term responses of forests to global change.

Some notes on statistical analysis at FACTS-1 regarding RBAI (relative basal area increment), biomass increment, NPP and litterfall [RTF format].

For more information, including ESA abstracts and results, upcoming publications, and collaborations please visit Dave Moore's website.