Spatial Clustering and Ecological Crowding of Valley Oak (Quercus lobata, Née) Associated with Shifts in Recruitment Establishment Sites in Southern California

Abstract

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.

A New Approach for Interpreting the Morisita Index of Aggregation through Quadrat Size

“…Differences in I Mr values do not have a clear interpretation other than "higher" or "lower", and analyzing differences in the index generated with different quadrat sizes is even more confounding. For example, Hayes and Donnell [30] applied I M and I Mr to valley oak stem locations at the same sites examined in the present paper, and they found clear evidence of clustering at all three sites, for multiple quadrat sizes. Their results, however, demonstrate the difficulty of comparing the index values across sites since only relative statements about differences in the degree and intensity of crowding could be made and only when the sites are analyzed using the same quadrat size.…”

“…Further, since the rMax curves are based on maximum I Mr values and the r number of stems for that maximum value, using the I Mr curves to choose our quadrat size assures that the definition of clustering is based on an empirical limit of r stems being clustered using the quadrat size chosen. While Hayes and Donnell [30] examined I Mr values at the three sites using multiple quadrat sizes for analysis, interpretation was constrained by the necessity of having results from each quadrat size on different scales, and the clearest interpretation of differences among sites was possible only when the same quadrat size was used for all three sites. Applying our final measure of clustering, d M (see Figure 8), we showed a new approach to solve the problem of scale in terms of a single quadrat size.…”

“…The three sites, Cheeseboro Canyon (CHE), Paramount Ranch (PAR), and Malibu Creek State Park (MAC), are part of the Santa Monica Mountains National Recreation Area and are managed by the US National Park Service (CHE and PAR) and California Department of Parks and Recreation (MAC). The sites' histories and their valley oak communities have been described and analyzed by Thomas [26], McLaughlin and Zavaleta [27], Hayes [28], Hayes and Donnelly [29], Hayes and Donnell [30]. Valley oak is a deciduous oak endemic to California that has been, and is expected to continue, experiencing changes in spatial distribution at local and regional scales in response to land use and climate change [27][28][29][31][32][33].…”