Multiscale predictors of small tree survival across a heterogeneous tropical landscape

Abstract

Uncertainties about controls on tree mortality make forest responses to land-use and climate change difficult to predict. We tracked biomass of tree functional groups in tropical forest inventories across Puerto Rico and the U.S. Virgin Islands, and with random forests we ranked 86 potential predictors of small tree survival (young or mature stems 2.5–12.6 cm diameter at breast height). Forests span dry to cloud forests, range in age, geology and past land use and experienced severe drought and storms. When excluding species as a predictor, top predictors are tree crown ratio and height, two to three species traits and stand to regional factors reflecting local disturbance and the system state (widespread recovery, drought, hurricanes). Native species, and species with denser wood, taller maximum height, or medium typical height survive longer, but short trees and species survive hurricanes better. Trees survive longer in older stands and with less disturbed canopies, harsher geoclimates (dry, edaphically dry, e.g., serpentine substrates, and highest-elevation cloud forest), or in intervals removed from hurricanes. Satellite image phenology and bands, even from past decades, are top predictors, being sensitive to vegetation type and disturbance. Covariation between stand-level species traits and geoclimate, disturbance and neighboring species types may explain why most neighbor variables, including introduced vs. native species, had low or no importance, despite univariate correlations with survival. As forests recovered from a hurricane in 1998 and earlier deforestation, small trees of introduced species, which on average have lighter wood, died at twice the rate of natives. After hurricanes in 2017, the total biomass of trees ≥12.7 cm dbh of the introduced species Spathodea campanulata spiked, suggesting that more frequent hurricanes might perpetuate this light-wooded species commonness. If hurricane recovery favors light-wooded species while drought favors others, climate change influences on forest composition and ecosystem services may depend on the frequency and severity of extreme climate events.

Aridity and forest age mediate landscape scale patterns of tropical forest resistance to cyclonic storms

“…3). These results link, for the first time, the well-documented speciesspecific patterns of hurricane resistance in relation to higher WD and shorter height with landscape scale responses across aridity gradients obtained from the FIA plot network and airborne LiDAR (Zimmerman et al 1994;Ostertag et al 2005;Ogle et al 2006;Lin et al 2018;Uriarte et al 2019;Feng et al 2020;Hall et al 2020;Helmer et al 2023). We showed here that aridity and forest age gradients, plant communities dominated by species with high WD tend to be shorter in stature and have overall greater resistance to hurricane winds (Fig.…”

“…In Northeast Puerto Rico, older forest stands might not or will never reach a stable state due to frequent disturbances favoring plant communities with a mosaic of both early succession and late succession specialist tree species (Uriarte et al 2009). On the other hand, it might be possible that these patterns are shaped by the widespread presence of non-native species that also tend to have lighter wood, making them less resistant to hurricanes (Helmer et al 2018(Helmer et al , 2023. In contrast to results from intensively studied, old forest sites, here younger forests (with widespread species introductions)…”

“…From these maps, we used average canopy height values of 1-m pixels in each 0.065 ha plot and the proportion of pixels with forest cover for canopy cover. To determine stand age, we used data combining landcover maps from the years 1951-2000 with plot status (forest or nonforest) since then (Helmer et al 2023).…”

“…First, exposure to hurricane winds plays an important role in determining the damage during storms (McLaren et al 2019;Feng et al 2020;Zhang et al 2022). This was evidenced by the higher increase in MRdead/alive in forests closer to the hurricane track and in lowland flat areas compared with steep slopes, some of which may be more protected from winds (McLaren et al 2019;Zhang et al 2022;Helmer et al 2023). Second, older forest stands showed greater increases in mortality.…”