Small Canopy Gaps Influence Plant Distributions in the Rain Forest Understory’ Jens-Christian Svenning Department of Systematic Botany, University of Aarhus, Herbariet bygn. 137, Universitetsparken, DK-8000 Aarhus C. Denmark ABSTRACT This study tested three hypotheses regarding how plants respond to the spatial heterogeneity in light availability in the rain forest understory: (1) understory plants occur preferentially in the lighter parts of the understory; (2) under- story palms are more shade tolerant than other understory plants; (3) rain forest plants differ in their ontogenetic response to understory light conditions. The study was carried out in old-growth rain forest in the Yasuni National Park, Amazonian Ecuador. The hypotheses were tested by comparing the distributions of 20 plant species (1454 individuals) over microsites with differing degrees of exposure to canopy gaps to the background distribution of these microsites in the forest. The gap exposure of a given microsite was described by an index based on the number and size of gaps in the canopy to which the site was exposed. Two plant height classes were studied: 0.80-2.49 and 2.50- 5 m. The first and third hypotheses were accepted, while the second hypothesis was rejected. The results for the individual species corresponded well with what is known from earlier studies about the ecology of these species or close relatives, suggesting that the patterns observed can be generalized for Neotropical rain forests. Notably, the most abundant species in the study represent several different life history strategies. Thus, abundance in the rain forest understory can be achieved by several different strategies. This suggests that niche differentiation in terms of response to small changes in understory light conditions may be an important factor in the maintenance of the high local plant species richness of tropical rain forests. RESUMEN En la investigaci6n reportada aqui se probaron tres hip6tesis concernientes a c6mo las plantas responden a la heter- ogeneidad espacial en cuanto a la accesibilidad de luz en el sotobosque del bosque pluvial tropical: (1) las plantas del sotobosque se encuentran preferentemente en las Areas del sotobosque mas iluminadas; (2) las palmas del sotobosque son mAs tolerantes a la sombra que otras plantas del sotobosque; (3) las plantas del bosque pluvial tropical son diferentes, con respeto a sus respuestas ontogentticas, a las condiciones de iluminaci6n en el sotobosque. Se realiz6 la investigaci6n en un bosque primario pluvial tropical en el Parque Nacional de Yasunf en la amazonia del Ecuador. Se probaron las hip6tesis comparando las distribuciones de 20 especies de plantas, 1454 individuos en total, en micrositios con diferentes grados de exposicion a claros en el dosel, con la distribuci6n de estos micrositios en el bosque. Se describi6 la exposici6n a claros usando una indice basado en el nlimero y tamaho de 10s claros a 10s que el micrositio estaba expuesto. Se estudiaron dos clases de a ha de las plantas: 0,80-2,49 m y 2,50-5 m. Se aceptaron la primera y la tercera hipbtesis, pero la segunda hip6tesis fue rechazada. Los resultados para las especies individuales correspon- dieron bien con lo que se conece de investigaciones anteriores sobre la ecologia de estas especies o especies emparentadas indicando que se pueden generalizar 10s patrones observados a 10s bosques pluviales Neotropicales en general. Se puede notar que las especies mis abundantes en el estudio representan varias estrategias de historia de vida, por lo tanto, la abundancia en el sotobosque se puede lograr mediante varias estrategias diferentes. Eso indica que la diversificaci6n de nichos en respuesta a pequehos cambios en las condiciones de iluminaci6n en el sotobosque pudiera ser un factor importante en el mantenimiento de la muy aka riqueza local de especies de plantas en 10s bosques pluviales tropicales. Kq words: canopy gaps; Ecuador; lif. history; palms; tropical rain forest; understo y plants. TWO IMPORTANT PARADIGMS IN RAIN FOREST PLANT KOLOGY are that survival and growth of plants in the understory depend highly on light availability and that adaptations to light availability differ among species (e.g., Lieberman et al. 1985, Den- slow 1987, Grubb 1996). Light levels in the closed understory of tropical rain forest are typically low, Received 24 June 1998; revision accepted 15 March 1999. often <2-percent full sunlight (Chazdon & Fetcher 1984, Denslow 1987, MacDougal & Kellman 1992, Clark et al. 1996), and tropical rain forest plants, both pioneer and shade tolerant understory species, are frequently light limited (e.g., Chazdon 1986, Denslow 1987, Welden et al. 1991, Clark et al. 1993, Ellison et al. 1993, Oberbauer et al. 1993, Cunningham 1997, AmCzquita 1998). Denslow (1987) suggested that niche differen- tiation in response to light availability is one of the 252