Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, United States

Abstract

Many forests in dry mountain regions are characterized by a lower elevational treeline. Understanding the controls on the position of lower treeline is important for predicting future forest distributional shifts in response to global environmental change. Lower treelines currently at their climate limit are expected to be more sensitive to changing climate, whereas lower treelines constrained by non-climatic factors are less likely to respond directly to climate change but may be sensitive to other global change agents. In this study, we used existing vegetation classifications to map lower treelines for our 1.7 million km 2 study region in the U. S. Intermountain West. We modeled topoclimatic drivers of lower treeline position for each of three dominant forest types to identify topoclimatically limited treelines. We then used spatial data of edaphic properties, recent fire, and land use to identify lower treelines potentially constrained above their ecophysiological limits by non-climatic processes. We found that the lower treeline ecotone of pinyon-juniper woodlands is largely limited by topoclimate and is likely to be sensitive to increasing temperatures and associated droughts, though these effects may be heterogeneously distributed across the landscape. In contrast, dry mixed-conifer lower treelines in the northern portion of the study area rarely reached their modeled topoclimatic limit, suggesting that non-climatic processes, including fire and land use, constrain the lower treeline above its ecophysiological limits in this forest type. Our results suggest that much of the lower treeline in the Intermountain West is currently climate limited and will thus be sensitive to ongoing climate changes. Lower treelines in other arid or semi-arid mountainous regions around the globe may also be strongly sensitive to climate, though treeline response to climate change will be mediated at the local scale by soil properties, biotic interactions, and natural or anthropogenic disturbances. Our regional study of lower treeline provides a framework for identifying the drivers of lower treeline formation and allows for more robust projections of future treeline dynamics, which are needed to anticipate shifting global distributions of the forest biome.

Dry forest decline is driven by both declining recruitment and increasing mortality in response to warm, dry conditions

“…While climate change is expected to alter the composition and structure of forests globally (Anderegg et al, 2020; McDowell et al, 2020), the loss of low elevation, dry woodlands is particularly notable as declines in pinyon, juniper, and other low elevation species would likely lead to a larger state‐change from forested ecosystems to grass or shrublands (Mátyás, 2010; Mátyás et al, 2018; McDowell et al, 2016; Urza et al, 2020). Historically, dry woodlands have expanded into lower elevations in the western United States and globally throughout much of the 19th and 20th centuries driven by domestic grazing, fire suppression, climate fluctuations, and other land use practices (Miller et al, 2019; Scholes & Archer, 1997).…”

“…This concern has been particularly acute in many dryland forest regions (Mátyás et al, 2018; Petrie et al, 2017; Williams et al, 2010), which cover > 10 million km 2 across the globe (Bastin et al, 2017). For example, pinyon‐juniper woodlands in the western United States have experienced some of the most notable mortality events (Breshears et al, 2005; Campbell et al, 2020; Meddens & Hicke, 2014), and typically make up the warmest, driest limit of forests in this region (Urza et al, 2020). If warming, drying, or increasing disturbance lead to permanent declines of dominant tree species in these regions, declining species are unlikely to be replaced by other trees and instead may lead to ecosystem state changes from forests to shrub and grasslands, with lasting implications for ecosystem services and biodiversity (Anderegg et al, 2013; McDowell et al, 2016; Reichstein et al, 2013).…”

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

“…In contrast, management efforts should prioritize reforestation efforts for populations projected to remain in climatically suitable areas and potential expansion areas at the leading edge (i.e., where the climate is projected to become suitable within the next century) in areas prioritized for woodland occurrence. Notably, there are considerable challenges with determining trailing edge compared with leading edge populations, which can be assessed using bioclimate envelope or niche modeling (e.g., Rehfeldt et al, 2015; Urza et al, 2020), evidence of recent tree mortality and canopy dieback (Flake & Weisberg, 2019), and recent recruitment (Redmond et al, 2015, 2018). Habitat and resource value : Landscape prioritization must also consider the importance of pinyon–juniper ecosystems for habitat, biodiversity conservation, and the provision of cultural resources.…”

Early pine root anatomy and primary and lateral root formation are affected by container size: implications in dry-summer climates

“…Indeed, while P. ponderosa has typically regenerated after fire and progeny present more growth on burned than unburned sites (Owen et al 2020), the spatial magnitude of recent fires, which dramatically increases distances between areas with viable seed sources, reduces natural regeneration potential (Korb et al 2019). Moreover, changing climate, with warmer temperatures at lower elevations, is shifting lower treelines upward in elevation (Urza et al 2020), witnessed by more P. ponderosa regeneration at higher elevations than lower elevations (Korb et al 2019). The combination of warmer, drier climate with continued uncharacteristically large and intense fires is projected to decrease post-fire natural regeneration (Rodman et al 2020).…”