Ph.D. Thesis Defense Announcement

Forest Recovery from Hurricane Disturbances: The Influence of Changing Climate

Jiaying Zhang

Dr. Rafael L. Bras

Dr. Jingfeng Wang (CEE), Dr. Emanuele Di Lorenzo (EAS), Dr. Tamara Heartsill Scalley (USDA), Dr. Marcos Longo (LBNL)

Date & Time: Wednesday, September 8, 2021, 3:00 pm - 5:00 pm Eastern

Location: Virtual via BlueJeans https://bluejeans.com/369912945/5097

Hurricanes are a major disturbance to tropical forests. The forest structure and composition are affected by the immediate damages and mortality caused by the disturbance and altered by the subsequent recovery via species succession and competition. To understand the mortality and recovery, we use census observations at Bisley Experimental Watersheds (BEW) in Puerto Rico to study the mortality after hurricane Hugo in 1989 and after hurricane Maria in 2017 and the subsequent recovery of the forest after hurricane Hugo between 1989 and 2014 (the last census before hurricane Maria). We found that hurricane-induced mortality varied with species and stem sizes. Specifically, palms are wind-resistant and had the lowest mortality, followed by mid and late successional trees. Early successional trees had the highest mortality. Small stems were protected and had the lowest mortality compared to medium and large stems in a large-stem dominant forest, but they were exposed and had the highest mortality in a small-stem dominated forest. In the succession recovery of the forest after hurricane Hugo, palms had the lowest background mortality and the highest recruitment rate, which make them superior competitors in the forest.

We implement a hurricane disturbance module, which accounts for both the immediate mortality and subsequent recovery of each species and stem size class, in the ecosystem demography model (ED2). We calibrate the model to properly represent the stem density, aboveground biomass, species composition and size structure of the forest in the 25 years of recovery from hurricane Hugo. Then we use the calibrated model to study the impact of 1) initial forest state, 2) climate conditions (e.g., temperature, precipitation, CO2 concentration, etc.), and 3) hurricane severity (frequency and intensity) on the recovery of forest biomass and species composition. The simulation results show that a single hurricane disturbance on a forest with

wind-resistant initial state will result in a higher aboveground biomass level after 100 years of recovery compared to a less wind-resistant initial state. Species composition and size structure at recovery are not as dependent on initial state. However, frequent and intense hurricane disturbances in the future will decrease the aboveground biomass accumulation and alter the species composition. Specifically, frequent and intense hurricane disturbances will increase the abundance of palms and early

successional trees but decrease the abundance of late successional trees. The effects will be enhanced with more frequent and intense hurricanes. Warmer climates will enhance the aboveground biomass accumulation but will have smaller effects on the species composition and size structure of the forest in comparison to hurricane disturbances. The biomass accumulation from warmer climates cannot compensate for the biomass loss due to hurricane disturbances.

In summary, we have demonstrated that 1) the state of the forest at the time of disturbance has effects on the recovery of the forest, especially on the biomass accumulation, but less effect on the species composition and size structure; 2) The severity of the hurricane disturbance has significant impacts on the biomass accumulation, species composition and size structure of the forest; 3) Climate warming will promote biomass, but not sufficiently to counteract biomass reduction from hurricane disturbances; 4) Palms will become more and more abundant in forests that are subject to frequent hurricane disturbances.