Seven-year responses of trees to experimental hurricane effects in a tropical rainforest, Puerto Rico

Abstract

a b s t r a c tWe experimentally manipulated key components of severe hurricane disturbance, canopy openness and detritus deposition, to determine the independent and interactive effects of these components on tree recruitment, forest structure, and diversity in a wet tropical forest in the Luquillo Experimental Forest, Puerto Rico. Canopy openness was increased by trimming branches, and we manipulated debris by adding or subtracting the trimmed materials to the trimming treatments, in a 2 Â 2 factorial design replicated in three blocks. Tree (stems P1 cm diameter at breast height) responses were measured during the 9-year study, which included at least 1 year of pre-manipulation monitoring. When the canopy was trimmed, stem densities increased >2-fold and rates of recruitment increased >25-fold. Deposition of canopy debris did not markedly affect stem densities but did have small yet significant effects on tree basal area. Basal area increased about 10% when debris was added to plots with intact canopies; the other treatments exhibited smaller or no increases in basal area over time. Much of the dynamics of stem densities were due to changes in the smallest size class (1-2.5 cm diameter), which responded with a pulse of recruitment in the canopy trimmed treatments, and a steady loss in plots with intact canopies. The decreases in stem densities in the plots with intact canopies is attributed to observed on-going forest thinning from the last natural severe hurricane disturbance in 1998. Given these repeated hurricane effects, our study enabled an experimental test of the Intermediate Disturbance Hypothesis (IDH), for which we predicted an increase in species diversity in canopy trimmed treatments and a loss of species in the treatments with intact canopies. Measured patterns of diversity gave partial support to the predictions of IDH, although raw species richness of sampled plots fit the predictions better than richness adjusted for differences in stem densities among treatments. Ordination of species responses in the community identified a guild of pioneer species responding to the trimmed treatments, but not the debris additions, amongst substantial background variation in species composition unrelated to the experimental treatments. These results are consistent with a growing consensus that, while trade-offs of resilience and resistance govern many species responses to hurricane disturbance, other environmental and historical factors are equally or more important in governing community dynamics in hurricane-disturbed forests.

Responses to canopy loss and debris deposition in a tropical forest ecosystem: Synthesis from an experimental manipulation simulating effects of hurricane disturbance

“…Instead, debris deposition resulted in a small but significant increase in basal area (ca. 10% increase) relative to plots where debris was not added Zimmerman et al, 2014). The positive effects of debris deposition on primary productivity were also evident in plots near our study site (Walker et al, 1996b); the removal of debris generated by Hurricane Hugo caused a significant decrease in tree diameter increment relative to control plots.…”

“…Although the large increase in small stems in the canopy trimmed plots was correlated with increased basal area (see Fig. 1 in Zimmerman et al, 2014), there was no significant effect of trimming on plotlevel basal area. Instead, debris deposition resulted in a small but significant increase in basal area (ca.…”

“…3; Sharpe and Shiels, 2014). However, fern compositional dynamics in the CTE were unlike those of woody plant species that resulted from treatments in the CTE Zimmerman et al, 2014) or in other tropical hurricane studies (Walker et al, 1996b;Murphy et al, 2008;Comita et al, 2009) in that there were (1) no resident fern species lost under simulated hurricane treatments (open canopy with debris deposition), and (2) resident (non-pioneer) species instead of pioneer species dominated the understory during the 5 years after the experimental hurricane disturbance (Sharpe and Shiels, 2014). Thus, the understory fern community in the LEF is more resistant and resilient to hurricane effects than is the woody plant community.…”

“…(Carlton and Bazzaz, 1998;Cooper-Ellis et al, 1999). Community analysis in the CTE demonstrated that the dominant individuals in the small size classes of the trimmed plots were pioneer species, and the establishment of pioneers in the canopy trimmed plots (3-6 new species on average) resulted in significant increases in woody plant species richness relative to intact canopy plots (Zimmerman et al, 2014). Therefore, following a natural hurricane, it is the effects of canopy opening that stimulate the rapid increase in stems of pioneer species, thereby increasing the species richness of forest stands, and after 2-3 years the same small size classes of pioneers begin thinning in the understory and mid-story as a result of the canopy closure.…”

“…Findings from the CTE indicate that following a hurricane, the increase in stem abundance, and the shift in composition to dominance by early successional species, can be largely explained by the increase in light produced by canopy openness rather than by the increase in debris on the forest floor Zimmerman et al, 2014). The increase in woody plant abundance in the trim plots was almost entirely due to recruitment by the smallest size class (1.0-2.5 cm DBH), which remained at elevated densities in trim plots relative to intact canopy plots for at least 7 years (curve D in Fig.…”