Plant Functional Groups and Species Contribute to Ecological Resilience a Decade After Woodland Expansion Treatments

Woody plant expansions are altering ecosystem structure and function, as well as fire regimes, around the globe. Tree-reduction treatments are widely implemented in expanding woodlands to reduce fuel loads, increase ecological resilience, and improve habitat, but few studies have measured treatment outcomes over long timescales or large geographic areas. The Sagebrush Treatment Evaluation Project (SageSTEP) evaluated the ecological effects of prescribed fire and cut-and-leave treatments in sagebrush communities experiencing tree expansion in North American cold desert shrublands. We used 10 yr of data from the SageSTEP network to test how treatments interacted with pre-treatment tree dominance, soil climate, and time since treatment to affect plant functional groups and dominant species. Non-sprouting shrub (Artemisia spp.), sprouting shrub, perennial graminoid, and annual grass responses depended on tree dominance and soil climate, and responses were related to the dominant species\u27 life-history traits. Sites with warm and dry soils showed increased perennial graminoid but reduced Artemisia shrub cover across the tree dominance gradient after prescribed burning, while sites with cool and moist soils showed favorable post-burn responses for both functional types, particularly at low to moderate tree dominance. Cut-and-leave treatments sustained or increased native perennial plant functional groups and experienced smaller increases in exotic annual plants in both soil climates across the tree dominance gradient. Both treatments reduced biocrust cover. Selecting appropriate tree-reduction treatments to achieve desired long-term outcomes requires consideration of dominant species, site environmental conditions, and the degree of woodland expansion. Careful selection of management treatments will reduce the likelihood of undesirable consequences to the ecosystem

Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration. © 2023 the Author(s)

Vegetation dynamics at the woodland-shrubland interface: Role of climate, disturbance, and species interactions

The boundary between woodlands and shrublands delineates the distribution of the tree biome in many regions across the globe. Woodlands and shrublands interface at multiple spatial scales, and many ecological processes operate at different spatial scales to determine the position of the woodland-shrubland boundary. The overall objective of this dissertation was to examine processes affecting vegetation dynamics at the woodland-shrubland interface in the western United States, at spatial scales ranging from biomes to individual plants. In Chapter 1, I examined broad-scale drivers of the position of lower treeline in the Intermountain West, using vegetation classifications derived from remote sensing imagery. I found that pinyon-juniper woodlands are broadly limited by water balance and will likely be sensitive to climate change, but that lower treelines at more northern latitudes are functionally constrained by permanent landscape features, land use and disturbance. In Chapters 2 and 3, I characterized post-fire plant community trajectories in an experimental network of prescribed fire treatments at the interface between pinyon-juniper woodlands and sagebrush shrublands in the Great Basin. I found that plant community responses to burning were strongly stratified along gradients of elevation and pre-fire tree cover, and that resistance to fire-induced invasion of the annual grass Bromus tectorum (cheatgrass) was low on sites that were relatively warm and dry (lower elevations) and on sites that lacked perennial understory species before burning (high pre-fire tree cover). Seeding perennial species after burning decreased invasibility in sites with low resistance, increasing perennial cover while reducing the abundance of invasive plants. In Chapter 4, I used field experiments to examine the interaction between Artemisia tridentata (big sagebrush) and Pinus monophylla (singleleaf pinyon pine), the dominant species within respective shrubland and woodland types that interface over broad environmental gradients in the Great Basin. I found that the effect of A. tridentata on P. monophylla shifted from strongly positive (facilitative) toward neutral after the vegetative transition from juvenile to adult foliage in P. monophylla. The timing of the ontogenetic shift did not vary across an elevational gradient, suggesting that this shrub-tree interaction may be relatively insensitive to increasing temperatures. Taken together, this research illustrates the dynamic link between woodland and shrubland ecosystems, and suggests that future dynamics at their interface will be complex and scale dependent

Managing for ecological resilience of pinyon–juniper ecosystems during an era of woodland contraction

Abstract Dryland woodland ecosystems worldwide have experienced widespread drought‐ and heat‐related tree mortality events coupled with extreme wildfire behavior. In contrast to other forest types where the emphasis has been on the silvicultural enhancement of ecosystem resilience and restoration of structural heterogeneity, limited frameworks are available for management to improve drought resilience in semiarid woodlands. This challenge is especially acute in pinyon–juniper woodlands, a dominant vegetation type across western North America that has experienced extensive tree die‐off over the past several decades while simultaneously undergoing expansion in portions of its range. In this paper, we describe the critical and urgent need to manage for future drought resilience in these highly vulnerable ecosystems and synthesize the current state of knowledge on how to enhance woodland resilience to drought coupled with high temperatures and associated disturbances. We present a landscape prioritization framework for guiding management goals and practices that requires prioritization of efforts based on the need for action and the probability of a positive outcome. Four guiding factors include historical woodland structure and drivers of long‐term landscape change, current vegetation structure and composition, future climate suitability, and habitat and resource value. In summarizing the strength of evidence supporting our recommendations, we identify critical knowledge gaps and highlight the importance of adaptive management strategies that reflect current uncertainties. This will ultimately allow for improved management of diverse semiarid woodland ecosystems that are undergoing substantial changes due to past and present land use, biological invasions, and climate change

Interacting effects of climate and landscape physiography on pinon pine growth using an individual-based approach

Forest and woodland ecosystems play a crucial role in the global carbon cycle and may be strongly affected by changing climate. Here, we use an individual-based approach to model pinon pine (Pinus edulis) radial growth responses to climate across gradients of environmental stress. We sampled pinon pine trees at 24 sites across southwestern Colorado that varied in soil available water capacity (AWC), elevation, and latitude, obtaining a total of 552 pinon pine tree ring series. We used linear mixed-effect models to assess pinon pine growth responses to climate and site-level environmental stress (30-year mean cumulative climatic water deficit [CWD] and soil AWC). Using a similar modeling approach, we also determined long-term growth trends across our gradients of environmental stress. Pinon pine growth was strongly positively associated with winter precipitation. Summer vapor pressure deficit (VPD) was strongly negatively associated with pinon pine growth during years of low winter precipitation, whereas summer VPD had no effect on pinon pine growth during years of high winter precipitation. The strength of the relationship between the annual climatic variables (winter precipitation and summer VPD) and pinon pine growth was also influenced by site-level environmental stress, suggesting that the sensitivity of woodland ecosystems to changing climate will vary across the landscape due to differences in local physiographic conditions. Trees at sites with lower CWDs were more responsive to summer VPD, showing greater reductions in growth rates during warmer years. Trees at sites with greater soil AWC were more responsive to winter precipitation, showing higher growth rates during years of high precipitation. Pinon pine growth rates declined moderately over the past century across our study area, suggesting that recent increases in aridity have resulted in long- term growth declines

Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration