Pinyon-juniper woodland dynamics: The role of climate and land use in tree recruitment and growth

Over the last century there has been a marked expansion of pinyon-juniper woodlands into grassland and shrubland ecosystems in the arid and semi-arid regions of the West. Although pinyon-juniper populations have fluctuated along elevational and latitudinal gradients with changing climate throughout the Holocene; over the last century, local scale impacts such as livestock grazing, changes in fire regimes, and increasing atmospheric CO₂ concentrations are thought to be more recent drivers of pinyon-juniper woodland distribution. To better understand the role of historical livestock grazing in pinyon-juniper woodland dynamics we examined pinyon stand dynamics on a relict mesa site to a nearby historically grazed site in Grand Staircase-Escalante National Monument.  No differences in pinyon density or basal area were observed across the sites. Stand age structure of pinyons showed peak recruitment occurred during the early 1900s across both sites; 16% and 17% of the pinyon trees on No Man’s Mesa and Deer Springs Point dated to the period 1910-1920, which was a time period of above average precipitation across the Southwest. The occurrence of old trees (> 200 yrs) across all transects provides evidence that pinyons have long been established at these sites and does not constitute expansion of the population into areas where they did not previously exist and that stand replacing fires have been an infrequent event, supporting similar findings in the region that fire return intervals for some pinyon-juniper sites may be > 400 yrs.