Scale-dependent changes in the contributions of biotic and abiotic factors to leaf area index in a natural forest in northeast China

Abstract

The leaf area index (LAI) is one of the most commonly used parameters for quantifying canopy structure and predicting vegetation growth. However, it remains unclear whether the relative contributions of biotic factors and abiotic factors to stand LAI are modulated by the study scale (or plot size). Additionally, whether tree size affects the contributions of biotic factors to the LAI is unknown. In this study, we measured LAI with a LAI-2200 plant canopy analyzer in subplots of three scales (i.e., 10 m × 10 m, 20 m × 20 m and 30 m × 30 m) in a 9 ha old-growth forest plot in northeast China. We also measured biotic factors for large trees (diameter at breast height, DBH > 9 cm) and small trees (DBH ≤ 9 cm) as well as abiotic factors (e.g., soil properties and light availability quantified by hillshade) in each subplot for three scales. With structural equation modeling (SEM), we found that an increase in the basal area of large trees directly reduced the stand LAI at all three scales and indirectly reduced the LAI by limiting the growth of small trees, but increased stand density (or basal area) for small trees increased the stand LAI. The increased species richness of small trees enhanced the LAI at 10 m × 10 m and 20 m × 20 m; in contrast, the increased species richness of large trees only significantly reduced the LAI at 20 m × 20 m. Increases in a soil property (total phosphorus) directly reduced the LAI at all three scales, but increased light increased the LAI. We also found that the contribution of large trees to LAI was higher at smaller scales but that of small trees increased with increasing scale; the contribution of soil properties to the LAI increased with increasing scale, but light made higher contributions at smaller scales. These results clearly suggest that the relative contributions of biotic factors and abiotic factors to stand LAI are scale-dependent and that tree size plays a key role in revealing the relationships between biotic factors and stand LAI in natural forests.