Disturbance is a universal feature of terrestrial ecosystems that reorganizes soil and biomass pools, typically in a configuration away from the natural mode of variability. For example, disturbances such as fire or hurricanes transfer a substantial amount of carbon and nitrogen from live vegetation to dead litter, thereby altering ecosystem function. As another example, reforestation of abandoned agricultural fields can be considered a disturbance to the mature forest dynamics. Seeding, fertilization, harvest, and other intensive management practices lead to an initial non-forest state that precedes reforestation (i.e., the transition from the disturbed to the mature state). In this project, we address the modeling of these initial transients in ecosystem dynamics and, in particular, the conditions for non-linear disturbance responses in soil and plant carbon and nitrogen pools during reforestation of abandoned agricultural fields.

To date, we have developed 2 models of varying complexity with observations collected during reforestation of the Calhoun Experimental Forest in the South Carolina Piedmont. The data reveal distinct oscillations between plant and soil nitrogen pools, commonly referred to as “overshoot.” First, using a system of five coupled ordinary differential equations to describe the tree and soil carbon-nitrogen dynamics, we show that this non-linear behavior occurs during initial transients, even when model stability analysis suggests the underlying model dynamics are not complex. Second, we simplify the five-pool carbon-nitrogen model to only two nitrogen pools and analytically derive the conditions for overshoot at Calhoun from this reduced-order model. These conditions suggest a strong dependence on the system initial conditions, which reflect the agricultural land use legacy. These models can be parameterized and applied across soil, vegetation, and climate types to further analyze the occurrence of non-linear initial transients during ecosystem transitions.