Spatial clustering and ecological crowding of valley oak (Quercus lobata, NÉE) associated with shifts in recruitment establishment sites in Southern California

Premise of research. Valley oak (Quercus lobata, Née) has received much attention due to concerns about regeneration failure and, more recently, compositional and structural community changes associated with this foundation species. Changes in the structure and spatial distribution of valley oak stands, as examined in this article, are likely affecting ecological processes and interactions throughout communities where it is found. Methodology. We used quadrat-based methods of spatial analysis, Morisita’s index of aggregation (I_M), and a derived index (I_Mr) to examine patterns of clustering, intensity of crowding, and changes in the probability of crowding as the definition of crowding (stems/area) is changed. Saplings, defined as stems with a diameter at breast height (dbh) greater than 1 cm and less than 10 cm (1 cm ≤ dbh < 10 cm) and adults (dbh ≥ 10 cm) were analyzed using four quadrat sizes. We mapped clusters of stems on the basis of spatial autocorrelation of dbh using Anselin’s local Moran’s I to identify the location of sapling and adult stem clusters. Pivotal results. Adults and saplings were clustered for all quadrat sizes, although the intensity of clustering and crowding differed among sites. Two sites had much higher intensity of clustering and crowding, and saplings were spatially segregated from adults away from open savannah habitat. At a third site, stems were less intensely clustered, and saplings did not show clustering away from adults. The intensity of sapling crowding differed among sites as the definition of crowding changed, revealing that saplings experience greater levels of crowding at two of the sites than at the third. Conclusions. Valley oak regeneration is spatially aggregated and ecologically crowded, indicating a shift to higher-density riparian woodland and increasingly open grassland seen at other sites and in other studies. Land change is an important driver of spatial regeneration patterns and may be playing a role here, although additional work is needed to examine the effects of possible drivers of crowding and the habitat shift. Future work should consider the ecological implications of structural and spatial changes in valley oak stands for associated species and processes.