Juvenile tree growth in relation to light availability in second-growth tropical rain forests

Light is a key environmental factor limiting growth and survival of trees in the subcanopy of wet tropical forests (Davies 2001, Thomas 1996). Light availability varies both vertically and horizontally and affects tree height, crown shape and tree architecture (Bongers & Sterck 1998, Sterck & Bongers 2001, Sterck et al. 1999) in addition to growth and survival (Clark & Clark 1992, 2001). Although many studies of tree seedlings and saplings have shown that growth varies significantly with light availability in tropical wet forests (Clark et al. 1993, Iriarte & Chazdon 2005, King 1991, Kohyama 1991, Montgomery & Chazdon 2002, Oberbauer et al. 1988, 1993; Poorter & Werger 1999, Sterck et al. 1999, Welden et al. 1991), few studies have examined these relationships in size classes above 5 cm dbh (Sterck 1999). King et al. (2005) found that annual increment growth of trees in the 8-20-cm dbh size class in two Asian forests was positively dependent on an index of crown light interception, but no direct measurements of light availability were taken in this study. Due to logistical challenges, few direct measurements of light environments above tree crowns have been made in tropical forests (Sterck & Bongers 2001). To our knowledge, no measurements have been made in second-growth forests. We examined variation in growth rates and crown sizes of trees in the 5-9.9-cm dbh size class in response to variation in light availability above crowns of three species in two second-growth, 1-ha plots at La Selva Biological Station in north-eastern Costa Rica (Chazdon et al. 2005). Trees in this size class establish beneath the pioneer successional canopy and represent the future generation of canopy tree species. Gaps are infrequent in young secondary forests of this region (Montgomery & Chazdon 2001, Nicotra et al. 1999) and tree recruitment into the subcanopy and canopy occurs in the absence of gaps through continuous vertical growth through the understorey and subcanopy. The natural vegetation in the area is classified as Tropical Wet Forest (sensu Holdridge et al. 1971). Both plots are on ultisols derived from weathered volcanic deposits (Sanford et al. 1994). Stands developed on abandoned cattle pastures. In 2004, LEP was 27y old and LSUR was 19 y old; stand age was documented by aerial photographs and personal accounts. In 2004, basal area of trees > 10 cm dbh was greater in the older LEP stand than in the LSUR stand (27.2 and 22.0 m2 ha-1, respectively). We studied three shade-tolerant tree species that were present in both sites and that were 5-9.9 cm dbh in 2003, with a maximum crown height of 10 m; Ocotea leucoxylon (Sw.) Laness. (Lauraceae), Pentaclethra macroloba (Willd.) Kuntze (Fabaceae) and Virola sebifera Aubl. (Myristicaceae). These were the most abundant tree species in this size class in the two study plots. Ocotea leucoxylon is a subcanopy tree, whereas the other two species are canopy trees (Hartshorn & Hammel 1994). This study is part of a long-term project to monitor vegetation dynamics in second-growth tropical forests in north-eastern Costa Rica (Chazdon et al. 2005). In 2004, we sampled light availability for all trees present in the 5-9.9-cm dbh size class in 2003, with the exception of 10 Pentaclethra trees in LSUR that were omitted due to equipment failure. In total, 91 trees were measured (10 of 0. leucoxylon, 64 of P. macroloba and 17 of V. sebifera). We used a red:far-red (R:FR) sensor to evaluate light availability above individual tree crowns, based on the rapid assessment technique of *Current address: Ross University School of Medicine, P.O. Box 266 Roseau, Commonwealth of Dominica, West Indes.