Stored carbon in longleaf pine ecosystems: The intersection of evolutionary history, physiological function, and ecological process
The allocation of assimilated carbon to storage pools provides a critical carbohydrate buffer when metabolic demands exceed current photosynthetic supply. Despite the importance of carbon storage to the forest carbon cycle, our process-level understanding of the controls on carbon storage pools and fluxes at tree and ecosystem scales remains relatively poor. Recent evidence suggests that stored carbon pools increased even after growth had ceased in trees experiencing drought and that tapping of rubber trees resulted in increased—not decreased—stored carbon pools. These observations suggest a paradigm shift away from the concept that stored carbon pools are a sink of low priority that accumulate passively when photosynthetic inputs exceed demand and toward the concept that these pools are active sinks of high priority. It follows that allocation toward storage—at the expense of growth—is a trait that would be under selective pressure since species that allocate toward storage are more resilient to disturbance. Using the fire-dependent longleaf pine ecosystem in a series of manipulative and observational studies, we explore how stored carbon dynamics are controlled by a combination of evolutionary, physiological, and ecological pressures.
Multiple lines of evidence support the concept that stored carbon dynamics are controlled by a combination of evolutionary, physiological, and ecological pressures. Our manipulative studies revealed that roots of longleaf pine contained large stored carbon pools that maintained belowground metabolism for a year after the photosynthetic carbohydrate supply was restricted. Likewise, the concentration of stored carbon in the smallest, most metabolically active roots was not influenced until nearly one year later. Our observational studies indicate that stored carbon pools differ among closely related pine species with overlapping natural distributions, but evolutionary histories of different disturbance frequencies and thus, different selective pressures on carbon storage. Our comparisons of stored carbon pools between longleaf trees growing under xeric or mesic soil moisture regimes, in combination with other drought studies, indicate that allocation toward storage exhibits plasticity through space and time in response to both short- and long-term variations in resource availability. We expect a continuum of responses to disturbances related to ecological niche and evolutionary adaptation that influence the availability of carbohydrates for metabolic demands, as well as a continuum in stored carbon pools and metabolic buffering capacity among species as well as spatially, temporally, and developmentally within a given species.