Cr. Keyes et al., Overstory and shrub influences on seedling recruitment patterns in an old-growth ponderosa pine stand, NW SCI, 75(3), 2001, pp. 204-210
Seedling recruitment dynamics in old-growth ponderosa pine forests are poor
ly understood, but in very dry environments these processes are believed to
be especially influenced by facilitation and competition from adjacent veg
etation. Seedling recruitment in an old-growth ponderosa pine stand in cent
ral Oregon was studied to determine its relationship with overstory trees a
nd 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 germinant and seedling density was observed when
calculated at a very local scale (5-m radius). Seedling and germinant densi
ties were also adversely affected by adjacent saplings, However, they were
positively associated with shrubs, indicating that the facilitative influen
ce of shrub presence exceeds its competitive effect for these regeneration
stages. Unlike germinants and seedlings, saplings were most strongly relate
d to shrubs, and this relationship was negative. Spatial analysis revealed
significant aggregation of germinants, seedlings, and saplings, with the in
tensity of aggregation decreasing from smaller to larger size classes. Thes
e results suggest that regeneration is most sensitive to adjacent vegetatio
n 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 t
ransforms toward a more regular pattern as stand development progresses.