Spatial patterns of regeneration in managed uneven-aged ponderosa pine/Douglas-fir forests of Western Montana, USA

In uneven-aged ponderosa pine (Pinus ponderosa Dougl.) and Douglas-fir (Pseudotsuga menziesii var. glauca (Mirbel) Franco) stands managed by individual tree selection, the recruitment of new sapling cohorts necessary to sustain structure and composition is generally assumed to occur within canopy openings created by harvest. However, few studies have examined the spatial patterns of new regeneration in relation to reserve trees in the overstory in this forest type. In this study, we analyzed the spatial distributions and associations between saplings and overstory trees using univariate and bivariate transformations of Ripley's L-functions. Furthermore, we introduced a ratio-based index to quantify the strength of bivariate spatial associations, which allowed us to further evaluate potential causes for the observed patterns. We found that saplings of both ponderosa pine and Douglas-fir established predominantly in clumps, and that saplings of the two species generally had a positive association with one another at short scale (up to 5 m). We found no evidence to suggest that saplings of ponderosa pine and Douglas-fir occupied separate spatial locations in these stands. We also found mixed results for bivariate associations between saplings of both species and overstory trees, where both positive and negative spatial associations occurred depending on the putative moisture level of plots indicated by habitat type group. The strength of positive spatial association between saplings and medium-sized overstory trees appeared to decrease as plot moisture-level increased, suggesting that sapling establishment and survival might be facilitated by shading on moisture-limited sites.