Overstory and shrub influences on seedling recruitment patterns in an old-growth Ponderosa pine stand

Seedling recruitment dynamics in old-growth ponderosa pine forests are poorly understood, but in very dry environments these processes are believed to be especially influenced by facilitation and competition from adjacent vegetation. Seedling recruitment in an old-growth ponderosa pine stand in central Oregon was studied to determine its relationship with overstory trees and nearby shrubs, and to identify the spatial patterns resulting from these relationships. From tests of significance using Poisson regression it was found that the influence of adjacent vegetation was greatest among smaller size classes of regeneration. The strongest relationship (negative) between overstory basal area and genninant and seedling density was observed when calculated at a very local scale (5-m radius). Seedling and germinant densities were also adversely affected by adjacent saplings. However, they were positively associated with shrubs, indicating that the facilitative influence of shrub presence exceeds its competitive effect for these regeneration stages. Unlike germinants and seedlings, saplings were most strongly related to shrubs, and this relationship was negative. Spatial analysis revealed significant aggregation of germinants, seedlings, and saplings, with the intensity of aggregation decreasing from smaller to larger size classes. These results suggest that regeneration is most sensitive to adjacent vegetation during early stages, that the facilitative influence of shrubs diminishes relative to their competitive effect once regeneration reaches the sapling stage, and that the early aggregated condition of regeneration gradually transforms toward a more regular pattern as stand development progresses.