Effect of repeated deforestation on vegetation dynamics for phosphorus-limited tropical forests

For some predominately phosphorus (P)-limited ecosystems, vegetation can be sustained under steady state conditions by atmospheric P inputs. The structure of the canopy influences deposition via the ability of the canopy to trap airborne P. This dependence suggests that a positive feedback may exist, which would have important impacts on the process of forest regeneration. One source of disturbance in the tropics is shifting cultivation. Over multiple cycles, studies have shown that shifting cultivation can lead to a large net loss of soil P, with a concomitant decline in forest biomass. In this study a model was developed to assess how shifting cultivation affects vegetation and phosphorus dynamics. This model is applied as a case study to a dry tropical forest system in the Southern Yucatan that is primarily P limited and has experienced shifting cultivation over several decades. Results show that following the second cycle, recovery would only be achieved after 100 years or longer in comparison to similar to 30 years after one cycle. Examining the stable states of the system suggests that two stable states exist and that state changes as brought about by repeated disturbance can cause a shift to the other "low vegetation" stable state. Thus, for predominately P-limited ecosystems undergoing repeated disturbance, the depletion of soil P can significantly affect the long-term ability of the forest vegetation to recover. Results from this study have widespread implications, as 400 million ha of forest are affected by shifting cultivation.