Spatial heterogeneity of light and woody seedling regeneration in tropicalwet forests

Variation in forest canopy structure influences both understory light avail ability and its spatial distribution. Because light is a major environmenta l factor limiting growth and survival of many forest species, its distribut ion may affect stand-level regeneration patterns. We examined spatial patte rning in light availability and seedling regeneration in old-growth, second -growth, and selectively logged stands of tropical moist forest in northeas tern Costa Rica. Our objectives were to determine how the frequency distrib ution and spatial pattern of understory light "microsites" differ among tro pical wet forests; whether patterns of seedling regeneration are linked to spatial patterning of light availability; and whether these relationships d iffer among old-growth, second-growth, and selectively logged forest stands . We used both sensor-based and hemispherical photograph-based methods to m easure light availability along three 130-160 m long transects in each of e ight stands (three old-growth, three second-growth, and two selectively log ged). Woody seedling abundance was assessed at 4 m(2), 25 m(2), and full-st and scales (430 m(2)), and species richness was computed at the 25-m(2) and full-stand levels. Data were analyzed using both conventional parametric a pproaches and spatial statistics. Mean light availability did not differ ma rkedly 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 spe cies richness of woody seedlings. Light availability was significantly spat ially autocorrelated in all stand types, but patch size (analogous to gap s ize) was twice as large in old-growth stands, based on sensor data. Seedlin g abundance was also spatially autocorrelated over greater distances in old -growth than in second-growth stands, often at similar spatial scales to li ght distribution. The selectively logged stands demonstrated spatial autoco rrelation of light and seedling abundance over distances intermediate to th e other two stand types. Despite the similarities in patterns of light and seedling distributions, relationships between woody seedling abundance, spe cies richness, and the three light availability measures were not strong or consistently positive, regardless of whether standard regressions or parti al Mantel tests were applied. Although seedling abundance is likely to be a ffected by a wide variety of factors, the similarities in the scales of spa tial autocorrelation of light and seedling abundance suggest that current s eedling abundance distributions may reflect past patterns of light distribu tion within the stands. Our results confirm the importance of examining spa tial 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 avail ability are needed to establish causal relationships between resource avail ability and stand-level patterns of seedling regeneration.