||"Forests recovering from disturbance (secondary forests) are strong carbon (C) sinks that play an important
role in the global C cycle. Climate change is likely to alter forest recovery dynamics or even prevent
recovery, and changes in disturbance-recovery dynamics will impact the global C cycle. To improve
understanding of the role of secondary forests in the changing climate system, and facilitate their accurate
representation in earth system models (ESMs), we will create a comprehensive database on C cycling in
secondary forests and use it, together with biogeochemical process modeling in DAYCENT, to address
three questions that are key to understanding the role of secondary forests in the climate system:
1. How and why does C cycling in forests, particularly the net ecosystem C balance (NECB), vary
as a function of ecosystem age? Controversy remains as to whether old-growth forests continue to
sequester C, and what drives observed age-related declines in forest NECB. To provide clarity to
these questions, we will combine data synthesis and analysis with DAYCENT modeling to test the
hypotheses that NECB converges to zero as forests age and that these age-related declines in NECB
are driven primarily by decreases in gross primary production rather than increases in respiration.
2. How does C cycling during forest recovery vary globally with respect to climate? Improved data
on rates of forest recovery globally are critical to understanding the contributions of secondary
forests to the global carbon budget and accurately representing these forests in ESMs. Synthesis and
analysis of global data will allow us to test the hypothesis that rates of carbon sequestration in
secondary forests increase with temperature and precipitation across global scales. Conversely, the
time required for forests to reach certain developmental stages (e.g., peak NECB, steady-state)
decreases with increasing temperature and precipitation.
3. How will expected changes in atmospheric CO2 and climate affect C cycling in secondary
forests? Climate change will alter the dynamics forest recovery following disturbance, likely
affecting both the rate of forest recovery and the final biomass. We will combine meta-analysis of
how secondary forests respond to changes in atmospheric CO2 and temperature with DAYCENT
modeling experiments to test the hypotheses that climate change will substantially alter C cycling in
secondary forests and C cycle responses to climate change will vary with forest age.
Our findings will provide synthetic understanding of C flux and allocation by secondary forests in both
current and future climates. They will contribute to DOE’s mission to “deliver improved scientific data
and models about the potential response of the Earth’s climate and terrestrial biosphere to increased
greenhouse gas levels for policy makers to determine safe levels of greenhouse gases in the
atmosphere”—and to the near- to long-term goals of DOE’s terrestrial science research activity—in three
main ways. First, synthesis and analysis of forest C cycle data—including eddy-covariance data from
AmeriFlux and Free Air CO2 Enrichment (FACE) studies—will provide improved information on C
allocation patterns as forests age and the C cycle forcings associated with secondary forests, thereby
improving data for validating ESMs and quantifying the role of secondary forests in global C inventories.
Second, our analyses and modeling activities will improve process knowledge of the mechanisms driving
forest recovery and how these might best be represented in ESMs. Representation of forest recovery in
ESMs will be critical to the important goal of determining the sign of the terrestrial C cycle feedback.
Finally, our analyses and modeling activities will make significant headway in understanding how forest
recovery might be affected by climate change—a potentially important feedback to the climate system."