Biogeography of fire regimes in western U.S. conifer forests: A trait‐based approach

Abstract

Aim Functional traits are a crucial link between species distributions and the ecosystem processes that structure those species’ niches. Concurrent increases in the availability of functional trait data and our ability to model species distributions present an opportunity to develop functional trait biogeography (i.e., the mapping of functional traits across space). Functional trait biogeography can improve process‐based predictions about the resistance of certain species assemblages to changing environmental conditions across landscape scales. We illustrate this concept by developing the first trait‐based, quantitative ranking of fire resistance (adult tree survival) in North American conifer species and mapping that fire resistance across space. Location and time period Western continental USA, present day. Major taxa studied Twenty‐nine common conifer tree species. Methods We compiled six traits for each species: three relating to tree morphology and three relating to litter flammability. We combined these traits into a single fire resistance score and used community‐weighted averaging to estimate the fire resistance scores of different forest communities, using interpolated species distribution and relative abundance data. Results Species associated historically with frequent fire have high fire resistance scores (e.g., Pinus ponderosa), reflected by thick bark, tall crowns and flammable litter. Species associated with subalpine or arid conditions have low fire resistance scores (e.g., Picea engelmannii and Pinus edulis), reflected by thin bark, short stature, poor self‐pruning and low litter flammability. A map of forest community fire resistance across the western USA reveals agreement with independent assessments of historical fire regimes, while also identifying areas where community‐wide species traits might be mismatched with historical fire regimes. Main conclusions Quantifying the functional traits that confer resistance to tree‐killing fire provides a direct link between ecosystem disturbance and community resistance. Understanding this link is crucial to evaluation of the long‐term resilience of different forest types under dynamic fire regimes. Our work represents the first known spatial representation of fire resistance traits at a regional scale and, as such, provides a link between functional traits and biogeography relevant to a critical ecosystem process.

Rainfall continentality, via the winter Gams angle, provides a new dimension to biogeographical distributions in the western United States

“…The results of our study support their conclusion—conceptual and mathematical models that do not independently resolve VPD and soil moisture limitations (and thus rainfall continentality and low water balance) will not adequately capture the magnitude of ecosystem response to increasing climate stress. In conclusion, we argue that the Gams‐angle index proposed here to disentangle water balance from rainfall continentality has exceptional potential for predicting species responses to climate change, as well as contributing to fundamental plant biogeography (Bell et al, 2014; Stevens et al., 2020; Violle et al., 2014).…”

The distribution of woody species in relation to climate and fire in Yosemite National Park, California, USA

“…Stevens et al 2020 considered the two pines to be resistant to frequent fires, and Thode et al 2011 placed them in the low and low to moderate fire severity regime types. Abies concolor is a shade-tolerant species and has moderate resistance to fire (Stevens et al (2020). Fires tend to kill the less fire resistant and more shade-tolerant tree species in the understory of pine stands such as smaller diameter Abies concolor, Calocedrus decurrens, and Pseudotsuga menziesii, thereby perpetuating the overstory species.…”

“…These forests experience heterogeneous fire, and patches of unburned, low severity, moderate severity, and high severity are intermixed based on landscape position (Jeronimo et al 2019), fuel heterogeneity (Cansler et al 2019), or the stochasticity of fire behavior (Jeronimo et al 2020). Species can possess either distinct fire adaptations (Stevens et al 2020) or can persist in fire refugia (sensu Kolden et al 2012;Meddens et al 2018). A notable feature of lower montane forests is fire heterogeneity at all scales, thus permitting some species without direct adaptations to fire to persist in small transient unburned areas (Blomdahl et al 2019).…”

A trait‐based approach to assessing resistance and resilience to wildfire in two iconic North American conifers

“…resistance; Werner & Prior, 2013). Similarly, in conifer forests throughout western North America, communities with a history of more frequent, low‐severity fire have traits such as thick bark and self‐pruning that promote fire resistance (Stevens, Kling, Schwilk, Varner, & Kane, 2020). Allocation to bark and other fire‐defensive traits is less advantageous when fires burn at higher severity.…”

“…Pinus ponderosa and P. menziesii are useful study species because they co‐occur across vast areas and exhibit important differences in physiological tolerances and life history. For example, P. menziesii has intermediate shade and fire tolerance, whereas P. ponderosa is highly shade‐intolerant and fire‐tolerant (Bigelow, North, & Salk, 2011; Buechling et al., 2017; Stevens et al., 2020). Within our study area in the southern Rocky Mountains, USA, historical fire regimes of P. ponderosa var.…”