Long‐term survivorship of single‐needle pinyon (Pinus monophylla) in mixed‐conifer ecosystems of the Great Basin, USA

Abstract

We examined stand structure and development of mixed‐conifer ecosystems in the south‐central Great Basin where pinyon (Pinus monophylla) and juniper (Juniperus osteosperma) are found together with other species, such as ponderosa pine (Pinus ponderosa), with particular emphasis on pre‐ and post‐settlement conditions. Two study sites (Mt. Irish and the Clover Mountains) were selected based on their relative proximity but different topographic conditions and modern fire regime. Vegetation analysis was conducted on 12 0.1‐ha plots per site, where all woody stems were mapped, measured, and cored for age determination, as well as on a systematic grid at 175‐m intervals where tree measurements and increment cores were obtained. At Mt. Irish the total basal area was 14.5 m2 ha−1 and the tree density was 324 stems ha−1, while at the Clover Mountains they were 13.8 m2 ha−1 and 342 stems ha−1, respectively. Pre‐settlement trees were found throughout each site, with maximum tree ages of 400 years or more, and older individuals being particularly common at Mt. Irish. Density of pinyon pine at both sites more than doubled since the 1800s, with peak survivorship occurring in the early 1900s at Mt. Irish and extending into the mid‐1900s at the Clover Mountains. Other tree species, including ponderosa pine, juniper, and white fir (Abies concolor), which have been present over the past few centuries in these stands, have not experienced the large population increase shown by single‐needle pinyon, with the exception of ponderosa pine at the Clover Mountains between the late 1800s and early 1900s. Pinyon mortality was <10% at both sites, even after the early 2000s drought, in agreement with other published studies of Pinus monophylla population dynamics. The presence of old individuals across the landscape indicated that pinyon populations have grown in density without invading new areas. Because wildfire regime and land‐use changes were not identical between the two study sites, and increases of pinyon‐juniper populations have occurred in other Great Basin areas at about the same time, climate was the most likely driver. Therefore, pinyon‐juniper woodlands, which have recently experienced dramatic episodes of climate‐related dieoffs in regions where Pinus edulis is present, have not been negatively impacted by climate in the Great Basin, where the pinyon species is Pinus monophylla.