How resistant (not changing) or resilient (return time after changing) ecosystems are will affect the state of ecosystems in response to climate change, and it is thought that there is an inverse relationship between resistance and resilience. Less resistant ecosystems will be more resilient, and more resistant ecosystems will be less resilient. We measured canopy height of humid forests at low (350 m asl) and high (1000 m asl) elevations in hectare-sized plots in the Luquillo Mountains of Puerto Rico during 20 years that included two severe hurricanes. We expected the canopy height of the low elevation forest to be less resistant (be reduced more by hurricane damage) due tothe vulnerability of its taller, broader-canopied trees, but more resilient (regain pre-hurricane height faster) due to its higher productivity, in comparison with the high elevation forest.
Results/Conclusions
Average canopy height of the low elevation forest before the hurricanes was 21.1 m. Hurricane damage reduced this to 9.3 m (44% of original height). In 20 years average height has returned to 17.8 m (84% of original height). Average canopy height of the high elevation forest before the hurricanes was 4.89 m. Hurricane damage reduced this to 1.92 m (39% of original height). In 20 years average height has returned to 2.24 m (46% of original height). Thus in terms of canopy height over 20 years in these plots, the low and high elevation forests were about equally resistant to hurricane damage (similar % reduction in canopy height), but the low elevation forest was more resilient (greater % recovery of canopy height). This conclusion is consistent with results of a separate study on post-hurricane leaf production, showing faster recovery of a low elevation forest. In the present study of canopy height there is no evident inverse relationship between resistance and resilience. Average return time (during years 1700-1989) of severe hurricanes in the Luquillo Mountains is 50-60 years.Both forests may fully recover pre-hurricane canopy height if not damaged again during 50-60 years, but climate models predict storms of greater severity, which may differentially affect forests that differ in resilience.
