Climate-Induced Annual Variation in Canopy Tree Growth in a Costa Rican Tropical Rain Forest

Abstract

1 Annual growth was measured over eight consecutive years (1984-92) for saplings and adults of a diverse group of nonpioneer tree species in a lowland neotropical rain forest (La Selva Biological Station, Costa Rica). The study species included five emergents and one canopy species. 2 Adult diameter increments varied markedly from year to year for all species. In the year of highest growth for individual species, mean increments were 25-112% greater than in the year of least growth. These among-year differences were significant for five of the six species. 3 The variation in adult growth rates showed strong temporal concordance across species. For all six species, mean adult diameter increments were higher than average in 1985 and were lower than average in 1986 and 1988. 4 Marked year to year growth differences were also shown by juvenile trees of these species. In the class > 50 cm tall and < 1 cm in diameter, mean diameter increments were 3-10 times greater in the year of highest growth than in the year of least growth. In each of the other two juvenile size classes (1-10 cm and 10-30 cm in diameter), two species showed highly significant among-year growth variation. 5 Within all three juvenile size classes, the year to year growth variation showed significant temporal concordance across species. As for adults, 1985 and/or 1984 were among the two highest growth years. 6 That these patterns of interyear growth differences were shared across tree species and between saplings and adults demonstrates significant impacts of climatic variability on the productivity of this 'equable' ecosystem. 7 Annual rainfall was not correlated with the annual growth patterns. Although the two years of highest overall growth, 1984 and 1985, were exceptionally dry, adult trees of two species showed their greatest growth in one of the highest rainfall years (1990). 8 Recent studies suggest that photosynthetically active radiation (PAR) controls productivity of tropical wet forests. Recent data from La Selva show strong year to year variation in monthly PAR. Demonstration of a causal relationship will, however, require concurrent measurement of tree growth, PAR, and other climatic factors. 9 Marked temporal variation in tree growth in tropical wet forests will have pervasive consequences for many processes in these complex ecosystems. Moreover, the degree of climatic sensitivity found in this study indicates that tropical rain forests could be strongly affected by global climate change.