Forest's Response to Hurricanes

Photo Credit

Dr. Grizelle González

The historic event of Hurricane Maria in 2017 undoubtedly marked the Puerto Rican society and had major consequences for its forest ecosystems. The Institute’s unique position as a leader of scientific research on the island allowed for a wealth of knowledge to be developed based on the experiences of many scientists that lived through and studied this extreme event. Some of the lessons learned from this event have been summarized in a recent book authored by the Institute’s past director (now Emeritus Scientist) that focuses on planning for resilience under extreme events. 

May the lessons learned through these experiences and shared with the public in these publications by Institute scientists and collaborators continue improving our fundamental relationship with the forests that sustain us.

• The effects of Hurricane Maria and subsequent forest recovery along an elevational gradient at the LEF were further explored using airborne LiDAR data acquired from 2016 to 2020 [Leitold et al., 2021]. This study was conducted in collaboration with NASA scientists and revealed widespread loss of canopy height across the study area. However, this reduction in forest canopy height was of variable magnitude across the landscape, with larger statured forests in the lower elevations suffering more damage than shorter forests above 600 m. While the hurricane opened the canopy releasing light-limited understory vegetation, a combination of fast height growth and lateral crown expansion by surviving individuals and advanced regeneration were able to maintain a heterogeneous canopy with only a small fraction of forest gaps detected in 2018, only one-year post-hurricane.

   

• As a result of the changes in forest structure caused by Hurricane Maria, some forest abiotic properties such as light availability and humidity were drastically affected [Van Beusekom et al., 2020]. The hurricane effects on canopy, understory, and soil microclimate were well-captured thanks to the ongoing satellite and field-based monitoring efforts as part of the Luquillo Long-Term Ecological Research program’s Canopy Trimming Experiment (CTE) in the LEF. In addition to providing the essential infrastructure for capturing the hurricane effects and recovery process, the long-term data derived from the CTE allowed for comparing previous treatment responses to the response of an actual hurricane. Looking into the future, these long-term ecosystem-scale experiments provide an invaluable platform and source of information for improving our understanding of how tropical forest ecosystems will respond to more frequent disturbances.  

   

• A good example of the significant spatial variation of hurricane effects are the regional differences in hurricane-induced litterfall rates that were measured in the field by Institute scientists across four forests found in contrasting life zones [Liu et al., 2018]. While vegetation was severely defoliated after Hurricanes Irma and Maria at all sites, there was a gradient of disturbance that was related to the exposure to the hurricane tracks, as well as the prevailing forest structure. Wet forests sustain higher levels of standing biomass, and thus produce more background litterfall than moist or dry forests. The magnitude of hurricane-induced pulses of litterfall were associated with these differences in forest structure, with higher values in wet forests such as in the LEF and Río Abajo State Forest. Moreover, the significant effect of Hurricane Irma only on the forest in BEW just two weeks before Hurricane Maria highlighted the importance of considering the geographic position relative to the hurricane tracks for understanding the spatial variation observed across the landscape.

   

• In order to better understand the causes of the large spatial variation of Hurricane Maria’s effects, Institute scientists used satellite data to model the effects of hurricane forces on forests across the U.S. Caribbean [Van Beusekom et al., 2018]. These models consider independent variables such as kinetic energy and total rainfall, among other landscape characteristics, to explain the observed spatial variation in greenness loss and landslide occurrence. Higher levels of greenness loss were observed for the Luquillo Experimental Forest (LEF; concomitant to El Yunque National Forest) compared to the entire region, in part due to the steep elevation gradient and rugged topography. Similarly, landslide occurrence—associated with high rainfall and soil clay content— was more frequent in the LEF relative to the rest of the region. An assessment of forest susceptibility to hurricane effects based on these models revealed that both hurricane properties, such as intensity, and landscape characteristics, such as forest cover and fragmentation, were important for determining the future vulnerability of forests in the U.S. Caribbean to hurricane disturbance.