Influence of Slope Steepness and Aspect on Pinus torreyana Juvenile Propagation

Pinus torreyana is an endemic pine tree native to Santa Rosa Island (SRI), California that had historically faced diminished populations due to sheep grazing and intensive ranching (Wells 1996). The National Park Service (NPS) gained full jurisdiction over SRI and eradicated non-native ungulate grazers in 2011. Since then, Pinus torreyana populations have been relieved of many of the stressors hindering their growth in their four remnant groves. Through my analysis of the geographic information system (GIS) maps of the groves, I came to suspect that the trees might be propagating in a manner associated with the slope steepness of the land. Prior researchers had found that slope steepness influenced distribution of some tree species, such as Pinus tabulaeformis (Song 2017). If there is a correlation between steepness and juvenile propagation then the NPS could use the information in efforts regarding the conservation of this species. To test whether slope steepness or aspect influences Pinus torreyana, I counted saplings and seedlings within 44 100m2 plots within the four groves that Travis Hall utilized in his Capstone in 2014-2015, binning my sites by categories of slope steepness (0-10°, 11-20°, 21-30°, etc.). Most recruitment occurred at intermediary slope angles, between 21-30°. Any future restoration or management work should target these intermediate hillside regions for Pinus torreyana efforts. Directing efforts to these regions are likely to be the most effective path towards expanding remnant populations and fostering additional stands in our coming era of increasing climactic uncertainty.