Spatial heterogeneity of light and woody seedling regeneration in tropical wet forests

Variation in forest canopy structure influences both understory light availability and its spatial distribution. Because light is a major environmental factor limiting growth and survival of many forest species, its distribution may affect stand-level regeneration patterns. We examined spatial patterning in light availability and seedling regeneration in old-growth, second-growth, and selectively logged stands of tropical moist forest in northeastern Costa Rica. Our objectives were to determine how the frequency distribution and spatial pattern of understory light 'microsites' differ among tropical wet forests; whether patterns of seedling regeneration are linked to spatial patterning of light availability; and whether these relationships differ among old-growth, second-growth, and selectively logged forest stands. We used both sensor-based and hemispherical photograph-based methods to measure light availability along three 130-160 m long transects in each of eight stands (three old-growth, three second-growth, and two selectively logged). Woody seedling abundance was assessed at 4 m², 25 m², and full-stand scales (430 m²), and species richness was computed at the 25-m² and full-stand levels. Data were analyzed using both conventional parametric approaches and spatial statistics. Mean light availability did not differ markedly among stand types, but variance and frequency distributions of light availability did. Second-growth stands had significantly higher unweighted canopy openness along solar tracks and a higher frequency of microsites at intermediate light levels. Old-growth stands had greater representation of both low- and high-light microsites, and greater overall variance in light availability. Old-growth stands also had slightly higher abundance and species richness of woody seedlings. Light availability was significantly spatially autocorrelated in all stand types, but patch size (analogous to gap size) was twice as large in old-growth stands, based on sensor data. Seedling abundance was also spatially autocorrelated over greater distances in old-growth than in second-growth stands, often at similar spatial scales to light distribution. The selectively logged stands demonstrated spatial autocorrelation of light and seedling abundance over distances intermediate to the other two stand types. Despite the similarities in patterns of light and seedling distributions, relationships between woody seedling abundance, species richness, and the three light availability measures were not strong or consistently positive, regardless of whether standard regressions or partial Mantel tests were applied. Although seedling abundance is likely to be affected by a wide variety of factors, the similarities in the scales of spatial autocorrelation of light and seedling abundance suggest that current seedling abundance distributions may reflect past patterns of light distribution within the stands. Our results confirm the importance of examining spatial dependence of resource availability in studies of forest dynamics, but they also underscore the limitations of a single period of data collection. Long-term studies as well as experimental manipulations of resource availability are needed to establish causal relationships between resource availability and stand-level patterns of seedling regeneration.