Spatial distribution of regeneration patches in an old-growth Pinus jeffreyi-mixed conifer forest in northwestern Mexico

Question: What are the shape, abundance, size, and structural characteristics of sapling patches in an old-growth Pinus jeffreyi-mixed conifer forest with a relatively intact disturbance regime? Location: Sierra San Pedro Martir, Baja California, Mexico. Methods: Regeneration was quantified by sampling sapling patches on seven 1200 m permanently marked line transects. For all patches intersected, tree size, species, age, and patch canopy cover were recorded. Patch structural characteristics were statistically compared to data from unbiased forest inventory plots. The minimum regeneration patch was defined as three saplings in a 49-m(2) area. Results: We sampled 54 sapling regeneration patches. Patch size varied from 6.6 to 674.8 m(2). A small portion of the forest was in patches (patch fraction = 3.8%, patch abundance = 8.5 per ha). The majority of the patches were small; 64.8% were less than the mean patch area of 100.1 m2. Patches were shaped as a rhombus. For all forest characteristics, mean values inside patches were significantly different than those from the unbiased forest inventory plots. Conclusions: These forests have a fine-grained pattern of regeneration patches. Our largest patch size of 674.8 m(2) is the smallest reported in Pinus jeffreyi, P. ponderosa, and mixed conifer forests; other studies have probably had more difficulty delineating regeneration patches because of forest ingrowth from fire exclusion. Frequent fire, irregular seed crops, and seed dispersal by small mammals and birds could create this regeneration patch regime. High variation in nutrient availability after fire could also contribute to increased stand patchiness.