Hurricane disturbance effects on tropical forests; structure, plant dynamics, and carbon storage

Abstract

Tropical cyclones are likely to increase in intensity, cause increased rainfall, and have larger storm surges. Changes in intensity and strength of tropical cyclones potentially have considerable effects on tropical forests. In the research presented in this dissertation, the hurricane disturbance effects on tropical rain forest structure, plant populations, and carbon storage were investigated in the Luquillo Experimental Forest (LEF), Puerto Rico. The overarching goal is to assess the effects of hurricanes on tropical rainforest by using El Yunque National Forest of Puerto Rico as example. It is articulated around three main objectives. 1) measure the forest canopy change through time and on the elevation gradient to see how hurricanes affect forest structure, 2) measure the effect of the canopy opening on the understory plant populations, using two pioneer species, 3) measure the hurricane disturbance effects on aboveground carbon through time in a simulation plot to see how the canopy openness and plant recruitment influence aboveground carbon.<br /> <br /> I had three working hypothesizes for the respective objectives. 1) My first hypothesis was that forests will differ in resistance and/or resilience because of presumed climate differences associated with the elevation gradient. To test that, I used canopy height data, from before Hurricane Hugo to after Hurricane Maria, in three hectare-sized plots at 350, 750, and 1000 m asl, respectively. I compared the maximum canopy heigh through time, made triangulated irregular network before and after hurricanes, plotted standard deviation and coefficient of variations through time for each plot. Then, I computed resistance and resilience through time for each plot and compared the values among the plots. Results indicated the Tabonuco forest seemed to be more resilient. The forest recovered at 91 percent in 2013, 24 years after Hurricane Hugo. They also showed that the Dwarf forest was the least resistant to Hugo, but the most resistant to Hurricane Maria. It seems to be the least resilient among the three plots. 25 years after Hurricane Hugo, measured in 2014, it showed only 48 percent of recovery. 2) My second hypothesis, regarding colonizing pioneer plants, was that average maximum canopy height in 2019 will be more strongly correlated with abundance in 2019 than in 2021 because of the direct overhead light. This early correlation with light is expected because the canopy recovers and shades the understory. I found that plant recruitment relative to canopy height was stronger in 2019 two years after the hurricane than in 2021, four years after the storm. 3) My third hypothesis was that forest regrowth after a simulated hurricane (experimental trimming) would compensate for carbon loss in the period of the study. If this is not true it implies that a predicted increase in frequency of intense hurricanes could eventually reduce aboveground carbon in forests subjected to strong cyclonic storms. I expected that during the 14-year period after canopy trimming, regrowth of branches and stems and stem recruitment stimulated by increased light and trimmed debris would help restore biomass and carbon loss due to trimming. Compared to control plots, in the trimmed plots recruitment of palms and dicot trees increased markedly after trimming, and stem diameters of standing trees increased. This response restored pre-treatment biomass and carbon in the experimental period. However, the data showed that recruitment of small trees adds little to aboveground carbon, compared to the amount in large trees.<br /> <br /> Climate change is likely to alter forest processes. More hurricanes and other disturbances are projected to happen by the middle of the century in tropical regions. In the LEF, the 31-year data set shows substantial effects of hurricanes on forest structure, mainly reduction in the canopy height, and canopy surface damage. The created-gap radically changes light, temperature, soil moisture, and available nutrients to create an environment which favors many species to grow to replace the dead ones. Over the long term, a continued loss of large trees could eventually result in less aboveground carbon stored in this Puerto Rican Forest and in other hurricane-affected tropical forests.