Temperature responses of leaf dark respiration and their implications for tropical forest carbon balance 

(NSF IOS-1051789, 2011-15)

Background - Each year, humans emit roughly 8 billion metric tons (gigatons, Gt) of carbon (C) to the atmosphere by burning fossil fuels (coal, petroleum, and natural gas), and another 2 GtC by land use conversions (primarily deforestation in the tropics). Of these 10 GtC emitted annually, terrestrial ecosystems absorb roughly 2.5 GtC, thereby reducing the growth rate of carbon dioxide (CO2) in the atmosphere by roughly 25% and mitigating a substantial fraction of human-induced climate change. The global terrestrial C “sink” (i.e., the net sequestration of atmospheric CO2) is not simply a result of photosynthesis (which removes CO2 from the atmosphere and produces O2),because at equilibrium, total ecosystem respiration (metabolic processes that consume O2 and produce CO2) is in balance with photosynthesis. Ecosystem respiration includes respiration by plants and other photosynthetic organisms (autotrophic respiration), as well as respiration by consumers (heterotrophic respiration, which is primarily due to bacteria, fungi, and animals that decompose dead organic matter). Global fluxes of photosynthesis and respiration are each roughly 120 GtC per year. These annual fluxes are large relative to human emissions (10 GtC per year) and are a substantial fraction of the total amount of C in the atmosphere (roughly 800 GtC). Thus, even a small percent change in either photosynthesis or respiration can have significant consequences for the global C cycle and Earth’s climate.

            The primary cause of the global terrestrial C sink is thought to be the stimulation of photosynthesis by rising atmospheric CO2 levels. This phenomenon, commonly referred to as “CO2 fertilization”, reflects the primary role of CO2 in photosynthesis, which converts CO2 into organic forms of C. However, a variety of other mechanisms may also contribute to the terrestrial C sink – including regrowth of previously disturbed temperate forests and longer growing seasons in the northern high latitudes – and there is great uncertainty in how terrestrial ecosystems will respond over coming decades to climate change, rising atmospheric CO2 concentrations, and other environmental changes. For example, the frequency of droughts is expected to increase in the future, and the CO2 fertilization effect may diminish as nitrogen or other nutrients become limiting. This uncertainty hinders prediction and planning in many arenas critical to society, including climate forecasting, sea-level rise, food security, and biodiversity conservation.

            One of the key uncertainties in predicting the future terrestrial C balance is how plant respiration will respond to climate warming. Plant respiration is estimated to consume 30-80% of photosynthetic C uptake in forest ecosystems and is predicted to increase with climate warming due to the temperature dependence of the metabolic reactions required for the maintenance of living tissues. Similarly, respiration rates of soil microbes and decomposition rates of plant litter and soil organic matter will likely increase under a warming climate. Intact tropical forests are thought to account for a substantial fraction of the global terrestrial C sink, but the future of these forests and the ecosystem services they provide (including C sequestration, regulation of the hydrological cycle, forest products, and biodiversity maintenance) are uncertain given the potential sensitivity of tropical forests to future drought, and the expectation that respiration rates of tropical trees will increase more than photosynthetic rates as the climate warms. An increase in respiration relative to photosynthesis would negatively affect the growth rates of tropical trees and/or their capacity to tolerate stress from drought, herbivores, and pathogens.

Our NSF-funded project aims to better understand how respiration rates in tropical forests respond to temperature, including temperature acclimation of tropical tree respiration. In this context, “acclimation” refers to physiological adjustments over weekly or longer time-scales that at least partially compensate for warming-induced respiration increases that would otherwise occur due to the direct effects of temperature on metabolic rates. We used a variety of observational and experimental approaches to study the temperature responses of multiple components of ecosystem respiration at tropical forest sites in Panama, including using a construction crane to execute the first-ever warming experiment in a tropical forest canopy. Our key results are summarized HERE


© Kaoru Kitajima 2016