Spatial Heterogeneity of Light and Woody Seedling Regeneration in Tropical Wet Forests

Abstract

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 m 2 , 25 m 2 , and full-stand 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 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 dynami...

Light gradient partitioning by tropical tree seedlings in the absence of canopy gaps

“…Studies of the frequency distribution of light availability in tropical forests suggest that the gap versus nongap paradigm overlooks the continuum of light levels beneath closed canopy (Lieberman et al 1989;Clark et al 1996;Brown and Jennings 1998;Nicotra et al 1999;Montgomery and Chazdon 2001). Within such forests, the frequency distribution of light is highly skewed towards environments with less than 2% full sun (Smith et al 1992;Clark et al 1996;Connell et al 1997;Nicotra et al 1999).…”

“…Within such forests, the frequency distribution of light is highly skewed towards environments with less than 2% full sun (Smith et al 1992;Clark et al 1996;Connell et al 1997;Nicotra et al 1999). Although the drastic changes in the resource environment produced by gap formation play an undisputed role in tree life-history, more subtle changes in magnitude and spatial distribution of light beneath closed canopies are also likely to influence long-term survivorship and recruitment of seedlings composing advanced regeneration.…”

Geometric Effects of an Inhomogeneous Sea Ice Cover on the under Ice Light Field

“…They provide crucial input data for various disciplines. Inherent optical properties are often derived from vertical profiles across various disciplines from atmospheric shortwave radiation measurements (e.g., Ohmura et al, 1998) over rain forest ecology (e.g., Nicotra et al, 1999) to ocean optics (e.g., Antoine et al, 2014). In most of those cases the approach of horizontally infinite homogenous layers is a sufficiently good approximation of reality.…”

Interferência da disponibilidade de luz na resposta à adubação de plantios de enriquecimento com leguminosas arbóreas

“…A interação entre esses fatores resulta em valores de disponibilidade de luz no sub-bosque em torno de 2% da luz total incidente (Nicotra et al, 1999;Rüger et al, 2009). Nessas condições, as plantas beneficiam-se apenas da luz difusa (Oliveira et al, 2007), o que pode afetar seu crescimento e desenvolvimento (Schulze, 2008).…”

“…A luz disponível no sub-bosque da capoeira foi semelhante à relatada em áreas de floresta primária (Montgomery & Chazdon, 2002), tendo variado significativamente entre os tratamentos de fertilização (Tabela 2). No sub-bosque das florestas tropicais úmidas, na ausência de clareiras, a luminosidade é sempre baixa, independentemente do estádio sucessional da vegetação (Nicotra et al, 1999), e pode limitar o crescimento das plantas.…”