Extreme drought conditions accompanied by rising temperatures have characterized the American Southwest during the past decade, causing widespread tree mortality in piñon–juniper woodlands. Piñon pine (Pinus edulisEngelm.) mortality is linked primarily to outbreaks of the pinyon ips (Ips confusus(Leconte)) precipitated by drought conditions. Although we searched extensively, no biotic agent was identified as responsible for death inJuniperusL. spp. in this study; hence this mortality was due to direct drought stress. Here we examine the relationship between tree abundance and patterns of mortality in three size classes (seedling/sapling, pre‐reproductive, reproductive) during the recent extended drought in three regions: southwest Colorado, northern New Mexico, and northern Arizona. Piñon mortality varied from 32% to 65%, and juniper mortality from 3% to 10% across the three sites. In all sites, the greatest piñon mortality was in the larger, presumably older, trees. Using logistic regression models, we examined the influence of tree density and basal area on bark beetle infestations (piñon) and direct drought impacts (juniper). In contrast to research carried out early in the drought cycle by other researchers in Arizona, we did not find evidence for greater mortality of piñon and juniper trees in increasingly high density or basal area conditions. We conclude that the severity of this regional drought has masked density‐dependent patterns visible in less severe drought conditions. With climate projections for the American Southwest suggesting increases in aridity and rising temperatures, it is critical that we expand our understanding of stress responses expected in widespread piñon–juniper woodlands.

Six large wildfires have burned in Mesa Verde National Park during the last 15 years, and extensive portions of burns were invaded by non-native plant species. The most threatening weed species include Carduus nutans, Cirsium arvense, and Bromus tectorum, and if untreated, they persist at least 13 years. We investigated patterns of weed distribution to identify plant communities most vulnerable to post-fire weed invasion and created a spatially explicit model to predict the most vulnerable sites. At the scale of the entire park, mature piñon–juniper woodlands growing on two soil series were most vulnerable to post-fire weed invasion; mountain shrublands were the least vulnerable. At a finer scale, greater richness of native species was correlated with greater numbers of non-native species, indicating that habitats with high native biodiversity are at the greatest risk of weed invasion. In unburned areas, weed density increased with greater soil nitrogen and phosphorus, and lower salinity. In burned areas weed density correlated with soil nitrogen status and textural class. We also evaluated the effectiveness of a variety of weed mitigation methods; aerial seeding of targeted high-risk areas with native grasses was the most effective treatment tested. We recommend a conservative mitigation plan using natives grass seed on only the most invasible sites.

Pinus edulis–Juniperus osteosperma (piñon-juniper) woodlands in the southwestern United States are of high conservation value and are threatened by changing climate and increasing frequency of large, severe fires. We followed vegetation development after three recent fires (1989, 1996, and 2000) in Mesa Verde National Park (MEVE). Two types of piñon-juniper vegetation are found in MEVE: sprouting woodlands (SPW), dominated by species that resprout after injury, and obligate seeding woodlands (OSW), dominated by non-sprouting species. SPW stands showed greater resilience than OSW in terms of recovery of pre-fire structure and species composition, and vulnerability to invasion by nonnative plant species. After all three fires, plant cover increased more rapidly in SPW stands; all major pre-fire species were present within 2 y, and fewer nonnative species became established. Plant cover developed more slowly in OSW stands; nonnative plant species proliferated, in places being more abundant than the newly germinating native species. No reestablishment of piñon or juniper trees has been observed. If current trends persist, some portions of the burned SPW may be converted to a persistent shrubland type, while much of the burned OSW may be converted to a persistent, novel herbaceous vegetation type with a large component of nonnative species. Similar changes after fire can be expected in piñon-juniper woodlands like those in MEVE, which are widely distributed throughout the region.

The Mogollon Highlands, Arizona/New Mexico, USA, spans a large biogeographical region of 11 biotic communities, 63 land cover types, and 7 ecoregions. This 11.3 M ha region has high levels of beta diversity across topo-edaphic gradients that span deserts to mountain tops. The main stressors affecting the region’s forests and woodlands include climate change, livestock grazing, and frequent mechanical removals of large amounts of forest biomass for fire concerns. We present an ecoregion conservation assessment for robust conservation area design that factors in appropriate wildfire response to protect communities from increasing threats of climate-induced wildfires spreading into urban areas. We focused mainly on maintaining connectivity for endangered focal species (grizzly bear (Ursus arctos horribilis) and Mexican wolf (Canis lupus baileyi)) along with protecting mature and old-growth (MOG) forests, Piñon (Pinus spp.)–Juniper (Juniperus spp.) Woodlands, and riparian areas. Over half the region is managed by federal agencies where new protected areas can be integrated with tribal co-management and prescribed burning, defensible space, and home hardening to protect communities from the growing threat of climate-induced wildfires. However, just 9% of the study area is currently protected, and even with the inclusion of proposed protected areas, only 24% would be protected, which is below 30 × 30 targets. The potential grizzly bear habitat, wolf habitat connectivity, and MOG forests (1.6 M ha (14.2%) of the study area; 18% protected) are concentrated mainly in the central and eastern portions of the MHE. There were 824 fires (2 to 228,065 ha) from 1984–2021, with 24% overlapping the wildland–urban interface. Regional temperatures have increased by 1.5 °C, with a 16% reduction in precipitation and stream flow since 1970 that under worst-case emission scenarios may increase temperatures another 3 to 8 °C by the century’s end. The unique biodiversity of the MHE can be better maintained in a rapidly changing climate via at least a three-fold increase in protected areas, co-management of focal species with tribes, and strategic use of fuel treatments nearest communities.