Host Distribution and Hurricane Damage to an Orchid Population at Toro Negro Forest, Puerto Rico

Abstract

Hurricanes frequently pass through the West Indies and strike Puerto Rico with an average frequency of 30 years, causing negligible to severe disturbance to natural populations of both plants and animals (Wadsworth and Englerth, 1959; L. Walker, unpubl.). Because of the unpredictability of hurricanes, damage to populations may occur without regard to adaptation and survival may be pure luck (Gould, 1989a, b). On the other hand, survival could be correlated with genotype if there is differential survival of individuals adapted to different microhabitats. Herein we relate survivorship of an epiphytic plant to microhabitat variation following the passage of a hurricane. On September 18, 1989, Puerto Rico suffered the direct impact of Hurricane Hugo. The west side eyewall moved over the eastern and northeastern tip of Puerto Rico, and where Hugo first hit the island, maximum sustained winds were 167 km/hr with gusts of 193 km/hr (U.S. Dept. of Commerce, 1989). Forest damage was so severe in the Luquillo Experimental Forest in eastern Puerto Rico that at one site all orchid epiphytes were destroyed (L. Migenis, pers. comm.). Forest damage was progressively less extensive westward and away from the eye of the hurricane (L. Walker, unpubl.). However, locally severe damage occurred as faraway as the Toro Negro Forest Reserve in the Cordillera Central, nearly 100 km southwest of the path of the hurricane. On November 12, 1989 we assessed the immediate effects of the hurricane on a population of Comparettia falcata Poeppig & Endlicher, an obligate epiphytic orchid that occurs in the Greater Antilles, Mexico, and Central and tropical South America (Liogier and Martorell, 1982). Comparettia falcata is locally common in the Toro Negro Forest Reserve (located between 18°07'30” N and 18°15'00” N, and 66°30'00” W and 66°37'30” W). Most of the reserve lies within the subtropical lowland montane wet forest (Ewel and Whitmore, 1973). One hundred eighty seven plants of C. falcata had been marked prior to the storm for another study. All plants were censused and categorized as to the type of damage, if any, they had suffered. Host species (phorophytes) were identified, and their position (i.e., understory, overstory) and support diameter were recorded. The occurrence of severe phorophyte defoliation (26-100%) resulting from the physical force of the wind was recorded, as was the frequency of branch breakage, snapping of tree trunks, and uprooting among host plants. The most common phorophytes of C. falcata were Syzygium jambos (L.) Alst. and Myrcia deflexa (Poir.) DC. Together they accounted for 63% of the total number of phorophytes. Syzygium jambos and M. deflexa were the dominant tree and shrub species, respectively, in the study area. Twelve other species comprised the remaining 37% of C. falcata hosts (Table 1). As is the case with other Puerto Rican epiphytic orchids (Ackerman et al., 1989; Migenis and Ackerman, unpubl.), C. falcata grows on a variety of phorophytes, including plants not commonly known for their suitability as epiphyte hosts, such as the sawgrass, Scleria canescens Boeck, the only non-woody phorophyte at our site. Extreme defoliation, the most common type of hurricane-caused damage to a forest, was suffered by only 13% of the phorophytes of C. falcata. Syzygium jambos was the most affected host, but in general, severe defoliation was not a common phenomenon among phorophyte species of C. falcata at our Toro Negro study site (Table 1). Branch breakage, snappings of tree trunks, and uprooting of host trees and shrubs were uncommon, maybe in part because all phorophytes but one individual of S. jambos were part of the forest understory vegetation. Only 9% of the hosts suffered any branch damage, about 4% were snapped off, and 3% were uprooted (Table 1). Thus, nearly all of the phorophytes withstood the winds. All four uprooted trees, Fraxinus uhdei (Wensig) Linglesh, Miconia sp., Psychotria aff. berteroana DC, and Syzygium jambos, affected neighboring trees and shrubs. Syzygium jambos and P. aff. berteroana were the host species most susceptible to general storm damage. Thirty three (17.6%) of the individuals of C. falcata were blown off their hosts, or were crushed by windthrown trees or branches, whereas 22 (11.8%) plants were left loosely attached to their phorophytes, hanging by their roots, and one plant (0.5%) had severe leaf damage. One hundred thirty unharmed plants (70. 1%) constituted the remainder of the population. There was no statistical difference in susceptibility to damage between plants epiphytic on twigs (branches 52.5 cm in diameter), where 102 of 115 orchids survived, and those growing on larger diameter substrates, where 44 of 46 survived (G-test of independence with Williams correction, G = 2.04, df = 1, P = 0.15). However, the differences in mortality percentages, 11.3% vs. 4.3% for small versus larger branches, respectively, are suggestive. If plants occupying twigs are significantly different in genotype from those occupying larger diameter substrates, as implied by Chase (1987), and are more likely to perish during extreme circumstances, as suggested by this study, then a natural disaster such as a hurricane could result in natural selection. There may be a tradeoff between those characters favoring establishment on twigs and likeliness to survive extreme, unpredictable environmental conditions.