Influence of substrate type and microsite availability on the persistence of foxtail pine (Pinus balfouriana, Pinaceae) in the Klamath Mountains, California

The persistence of poor competitors within species-rich assemblages is often tied to habitat heterogeneity. Here, the persistence of foxtail pine (Pinus balfouriana) in the Klamath Mountains of northern California was addressed using a two-step approach. First, the response of foxtail pine to shading from six co-occurring conifers was examined using two morphological indices. Foxtail pine increased the height to the first branch that supported foliage, and this branch was shorter when compared with those on all other sampled conifers, suggesting that foxtail pine is a poor competitor for light. Second, three hypotheses to explain foxtail pine persistence were tested: habitat heterogeneity at large spatial scales (substrate hypothesis), habitat heterogeneity at small spatial scales (microsite hypothesis), and the long lifespan of foxtail pine (successional hypothesis). Habitat heterogeneity at multiple spatial scales favored the persistence of foxtail pine. At large spatial scales, ultramafic substrates affected the importance and competitive abilities of shade-tolerant conifers. At small spatial scales, species richness, species diversity (H'), and stand density were positively correlated with microsite availability. No support was found for the successional hypothesis. Results are subsequently linked with general hypotheses of species coexistence in species-rich assemblages.