Characterization of the overlap area between encinal and pinyon-juniper woodlands.

Abstract

Overlap areas between encinal and pinyon-juniper woodlands on three mountain ranges in southeastern Arizona were studied in terms of selected physiographic (slope, aspect, soil coarse fragments, and soil pH) and vegetative (overstory number of stems, basal area, volume; regeneration counts and stocking) variables. Overlap areas, defined arbitrarily in this study, were those areas where oak and pinyon-juniper types mix in space, but, basal area per acre of one species group must not have equaled nor exceeded 80 percent of the total basal area recorded at the area. The purpose of the study was to find and describe factors and relationships that might account for the occurrences of overlap areas. Findings of the study might be used by natural resource managers to better understand and manage these important woodlands, including the overlap area. Slope and aspect, although not critical factors in the occurrence of overlap areas, must be carefully considered before harvesting trees on warm and steep sites because juniper is known to have higher tolerance for xeric conditions than oak. Hence, tree harvesting operations in overlap areas located on warm, steep slopes might induce the dominance of pinyon-juniper over oak in the area. Overlap areas were stonier at the soil surface than encinal and pinyon-juniper types. If a field manager wanted an overlap area to re-establish itself after tree harvesting operations, then he must look for overlap areas with stony to very stony soil surfaces. Basal area and volume of oak in overlap areas appeared to dominate over those of pinyon-juniper at some study areas. Oak trees apparently were more mature and established than pinyon-juniper trees in the overlap areas. The smaller and perhaps younger pinyon-juniper trees might indicate recent occupation of the area. It should be noted, however, that data from which this inference was based upon involved no long-term tree measurements. Regeneration of oak generally was more abundant than that of pinyon-juniper in the overlap area. If sheer numbers of plantlets or stocking would indicate eventual establishment and dominance of a species in the overlap area at a given time, then oaks might dominate over pinyon-juniper in time.