Higher temperatures increase growth rates of Rocky Mountain montane tree seedlings

Abstract

Recent observational studies report weak or flat temperature − growth relationships for many tree species in temperate forests. In contrast, distribution limits of trees are strongly shaped by temperature, and studies show marked short-term temperature effects on leaf-level ecophysiology. To better determine the effects of warming on trees, we planted one-year-old seedlings of one lower montane (ponderosa pine), two upper montane (quaking aspen and lodgepole pine), and one subalpine tree species (subalpine fir) in in situ experimental gardens on an elevation gradient in the Rocky Mountains (USA) which span a 6°C range in temperature but have approximately uniform precipitation. Seedlings were lightly watered the first three growing seasons to facilitate establishment, and growth and survivorship were followed for four years. We expected a trade-off between growth and survivorship, as seedlings in high temperatures grow faster (e.g., with a longer growing season), but have higher mortality from heat stress. Compared to the coldest site, aspen (+256% wider, +337% taller), ponderosa pine (+234% wider, 270% taller), and lodgepole pine (+235% wider, 283% taller) all had strikingly higher cumulative diameter and height growth in the warmest site by the end of the study. Linear models of cumulative and annual growth in the montane species showed strong, positive relationships with growing-season temperature, but no significant relationships with growing-season precipitation. In contrast, growth of subalpine fir did not vary significantly with temperature, but increased slightly with higher growing-season precipitation. Accelerated growth did not come at the expense of survivorship in the montane species: cumulative four-year survivorship of the montane species remained robust (71.4-94.4%) in high temperatures, but caused complete mortality of subalpine fir. As long as precipitation remains adequate, these results indicate that warming is likely to strongly increase growth in seedlings of montane species with only modest decreases in survivorship despite higher evapotranspiration, especially in cooler and wetter portions of their current distributions where hydric stress is low. In contrast, warming may negatively affect seedling growth and survival in hotter and drier areas of the Rockies, and warming of +3-6°C may endanger the persistence of subalpine fir over much of its current distribution.

Rocky Mountain forests are poised to recover following bark beetle outbreaks but with altered composition

“…These findings were relatively consistent among species, despite interspecific differences in their environmental tolerances. One experimental study with a subset of our study species shows similar results, wherein juveniles had more rapid height growth on warmer, drier sites (Carroll et al, 2021). Primary productivity (e.g.…”

“…While additional research is needed to understand subalpine tree species performance across abiotic gradients, we infer that low juvenile densities in warm, dry sites could be partially driven by high mortality in such areas (e.g. high fir mortality on dry sites; Carroll et al, 2021). Our finding that climate may have opposing effects on juvenile height growth rates and juvenile densities aligns with broader ecological theory, indicating that individual vital rates (e.g.…”

Resilience of a tropical montane pine forest to fire and severe droughts

“…By 2018, 98% of plots had at least one pine sapling, despite the wide range in conditions (fire severity, tree stand structure and shrub density) and landscape context (patch size, environment) across the study. A wet period immediately after the fire (Figure 3)—when new regeneration from the Pinaceae is generally most sensitive to climate conditions (Andrus et al, 2018; Carroll et al, 2017, 2021; Copenhaver‐Parry et al, 2020; Foster et al, 2020; Goke & Martin, 2022; Harvey et al, 2016a; Kemp et al, 2019; Urza & Sibold, 2017)—may have enhanced pine regeneration across the study and contributed to the ecosystem's ensuing resilience. Increasing temperatures, however, may reduce soil moisture and thus limit pine regeneration (e.g., of Pinus hartwegii on sites exposed to high solar radiation near treelines in tropical Mexico; Astudillo‐Sánchez et al, 2019), potentially reducing the resilience of these ecosystems across much of the tropics.…”

“…By 2018, 98% of plots had at least one pine sapling, despite the wide range in conditions (fire severity, tree stand structure and shrub density) and landscape context (patch size, environment) across the study. A wet period immediately after the fire (Figure 3)-when new regeneration from the Pinaceae is generally most sensitive to climate conditions (Andrus et al, 2018;Carroll et al, 2017Carroll et al, , 2021Copenhaver-Parry et al, 2020;Foster et al, 2020; the study and contributed to the ecosystem's ensuing resilience.…”

Poor acclimation to experimental field drought in subalpine forest tree seedlings

“…Droughts linked to climate change have already caused widespread tree mortality across large areas of many forested regions, with deeply adverse impacts on landscape structure and function ( Van Mantgem et al 2009 ; Allen et al 2010 ; Anderegg et al 2013 ; Clark et al 2016 ). These drought episodes are of particular concern for high-elevation forests in mountainous regions where climate change is accelerated relative to lower elevations ( Beniston et al 1997 ; IPCC 2014 ; Pepin et al 2015 ; Dobrowski and Parks 2016 ) and resistance of forests to environmental shifts is strongly affected by competitive interactions ( Buechling et al 2017 ), divergent species responses ( Carroll et al 2017 , 2021 ) and low phenotypic plasticity due to strong local adaptation to narrow bioclimatic envelopes ( Valladares et al 2007 , 2014 ; Vitasse et al 2013 ; Gugger et al 2015 ). Recent progress in characterizing the underlying causes of drought-induced tree mortality has highlighted the value of plasticity in mitigating drought stress and reducing likelihood of mortality, especially in traits associated with water and carbon regulation strategies ( Richter et al 2012 ; Choat et al 2018 ).…”