BIOTROPICA 39(2): 177–185 2007 10.1111/j.1744-7429.2006.00244.x Effects of Wind on the Allometry of Two Species of Plants in an Elfin Cloud Forest Roberto A. Cordero 1 Department of Biology, University of Puerto Rico, R´ ıo Piedras, Puerto Rico 00931-3360, U.S.A. Ned Fetcher 2 and Janice Voltzow Department of Biology, University of Scranton, Scranton, Pennsylvania 18510-4625, U.S.A. ABSTRACT Thigmomorphogenesis includes the effects of mechanical perturbation on plant growth. To test whether thigmomorphogenesis is evident at different scales within plants, we investigated the effect of wind on allometric relationships between specific plant parts. We chose two species from the elfin cloud forest of Puerto Rico that have contrasting growth habits, the shrub Clibadiun erosum (Asteraceae) and the palm Prestoea acuminata var. montana (Arecaceae), and subjected them to barrier-protected and wind-exposed treatments. For C. erosum, we compared the allometry of stems and branches against three allometric models that predict that plant height or branch length increases at the 1, 2/3, and 1/2 power of stem diameter. Only the geometric similarity model (scaling exponent of 1) seemed to hold when plants were exposed to the wind. We found relatively fewer leaves per number of branches produced and fewer leaves per increment of branch diameter in the plants of C. erosum exposed to the wind. Mean petiole length ratios (petiole length/basal radius) of P. acuminata were higher on leaves of barrier-protected plants for both simple and compound leaves, indicating that petioles were stouter and mechanically safer in the wind-exposed plants. We suggest that alteration of the allometric relationships of plant parts, organs, or plant modules (stems and branches of C. erosum and leaves of P. acuminata) and alteration of the number of plant parts (leaves and branches of C. erosum) are adaptive responses resulting from the mechanical perturbation induced by wind in the elfin forest. Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp. Key words: Clibadium erosum; leaf shape; Prestoea acuminata var. montana; Puerto Rico; scaling; thigmomorphogenesis. IN TERRESTRIAL ENVIRONMENTS AND ESPECIALLY IN MOUNTAINS, forces caused by wind are the most ubiquitous and important cause of dynamic loading (Grace 1977). Mechanical stimulation has been tested in many vascular plants, producing reduced stem elonga- tion and an increase in stem diameter (Grace 1977). This response, called thigmomorphogenesis by Jaffe (1980), involves the motion- induced reallocation of growth (Niklas 1992). From an ecological point of view, thigmomorphogenesis may have adaptive significance (Jaffe 1980; King 1981, 1991; Lawton 1982). It may also represent a developmental response to mechanical stress suffered by plants in windy cloud forest (Lawton 1982). Field observations suggest that the effect of wind on trees and plants of tropical and sub- tropical cloud forests is to produce a reduction in height and an increase in diameter (Lawton 1982, Cavelier & Mej´ ıa 1990, Dupuy et al. 1993). Lawton (1984) found that species that were more common on ridge crests had greater wood density. In general, plant diameter increases with exposure to wind (Telewski 1995), although Henry and Thomas (2002) found a decrease in diameter of Abutilon theophrasti when exposed to wind in a greenhouse. In another green- house study, Pruyn et al. (2000) found that mechanical perturbation caused increases in flexural rigidity due largely to an increase in cross- sectional area. Working with sunflowers, Smith and Ennos (2003) showed that the effect of wind on mechanical properties could be attributed to mechanical perturbation rather than the passage of air over the surface of the plant. Thigmomorphogenesis is likely to Received 11 November 2005; revision accepted 10 April 2006. 1 Present address: P.O. Box 597, Curridabat 2300, San Jos´ e, Costa Rica; 2 Corresponding author; Present address: College of Science and Engineering, Wilkes University, Wilkes-Barre, PA 18766; e-mail: nedfetcher@wilkes.edu be especially important in windy environments such as montane terrain. Wind microclimates can vary dramatically, with important consequences for the vegetation (Clark et al. 1998). Although these studies strongly suggest that wind should affect allometry of cloud forest species, this idea has hardly been tested experimentally with plants from the cloud forest. An exception is the work of Cordero (1999) on saplings of Cecropia schreberiana, which demonstrated drastic changes in allometry, architecture, stem biomechanics, and gas exchange as a result of exposure to wind. In a prior study (Fetcher et al. 2000), protection from wind had no significant effects on measures of growth such as total biomass, aboveground biomass, and total leaf area of Clibadium erosum, Prestoea acuminata var. montana, and Psychotria berteriana. In this paper, the effect of wind on allometric relationships between specific plant parts is analyzed on two of those species: the shrub C. erosum (Ws.) DC. (Asteraceae) and the palm P. acuminata (Willde- now) H. E. Moore var. montana (Graham) Henderson & Galeano (Arecaceae), chosen because of their contrasting growth habits. We hypothesized that thigmomorphogenesis could become evident at a scale different from that of the whole plant when subjected to barrier-protected and wind-exposed treatments. Reinforcement of branches has been reported for Abies fraseri under wind or mechanically induced perturbations (Telewski & Jaffe 1986). Three models have been proposed to explain girth in- creases that are disproportionately greater for plants subjected to mechanical stimulation. The first is the elastic similarity model, also called the “elastic stiffness model” (Niklas 1992). This model, proposed by McMahon and Kronauer (1976), assumes that a branch acts as a cantilevered beam that is fixed at one end. If a branch C  2006 The Author(s) Journal compilation C  2006 by The Association for Tropical Biology and Conservation 177