We studied the impacts of climate variability on low‐elevation forests in the U.S. northern Rocky Mountains by quantifying how post‐fire tree regeneration and radial growth varied with growing‐season climate. We reconstructed post‐fire regeneration and radial growth rates of
Forest resilience to climate change is a global concern given the potential effects of increased disturbance activity, warming temperatures and increased moisture stress on plants. We used a multi‐regional dataset of 1485 sites across 52 wildfires from the US Rocky Mountains to ask if and how changing climate over the last several decades impacted post‐fire tree regeneration, a key indicator of forest resilience. Results highlight significant decreases in tree regeneration in the 21st century. Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985–1999, suggesting increasingly unfavourable post‐fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non‐forests after wildfires. Major climate‐induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future.
more » « less- NSF-PAR ID:
- 10048067
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Ecology Letters
- Volume:
- 21
- Issue:
- 2
- ISSN:
- 1461-023X
- Page Range / eLocation ID:
- p. 243-252
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Pinus ponderosa andPseudotsuga menziesii at 33 sites that burned between 1992 and 2007, by aging seedlings at the root–shoot boundary. We also measured radial growth in adult trees from 12 additional sites that burned between 1900 and 1990. To quantify the relationship between climate and regeneration, we characterized seasonal climate before, during, and after recruitment pulses using superposed epoch analysis. To quantify growth sensitivity to climate, we performed moving regression analysis for each species and for juvenile and adult life stages. Climatic conditions favoring regeneration and tree growth differed between species. Water deficit and temperature were significantly lower than average during recruitment pulses of ponderosa pine, suggesting that germination‐year climate limits regeneration. Growing degree days were significantly higher than average during years with Douglas‐fir recruitment pulses, but water deficit was significantly lower one year following pulses, suggesting moisture sensitivity in two‐year‐old seedlings. Growth was also sensitive to water deficit, but effects varied between life stages, species, and through time, with juvenile ponderosa pine growth more sensitive to climate than adult growth and juvenile Douglas‐fir growth. Increasing water deficit corresponded with reduced adult growth of both species. Increases in maximum temperature and water deficit corresponded with increases in juvenile growth of both species in the early 20th century but strong reductions in growth for juvenile ponderosa pine in recent decades. Changing sensitivity of growth to climate suggests that increased temperature and water deficit may be pushing these species toward the edge of their climatic tolerances. Our study demonstrates increased vulnerability of dry mixed‐conifer forests to post‐fire regeneration failures and decreased growth as temperatures and drought increase. Shifts toward unfavorable conditions for regeneration and juvenile growth may alter the composition and resilience of low‐elevation forests to future climate and fire activity. -
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Abstract Aim Climate warming is increasing fire activity in many of Earth’s forested ecosystems. Because fire is a catalyst for change, investigation of post‐fire vegetation response is critical to understanding the potential for future conversions from forest to non‐forest vegetation types. We characterized the influences of climate and terrain on post‐fire tree regeneration and assessed how these biophysical factors might shape future vulnerability to wildfire‐driven forest conversion.
Location Montane forests, Rocky Mountains, USA.
Time period 1981–2099.
Taxa studied Pinus ponderosa ;Pseudotsuga menziesii .Methods We developed a database of dendrochronological samples (
n = 717) and plots (n = 1,301) in post‐fire environments spanning a range of topoclimatic settings. We then used statistical models to predict annual post‐fire seedling establishment suitability and total post‐fire seedling abundance from a suite of biophysical correlates. Finally, we reconstructed recent trends in post‐fire recovery and projected future dynamics using three general circulation models (GCMs) under moderate and extreme CO2emission scenarios.Results Though growing season (April–September) precipitation during the recent period (1981–2015) was positively associated with suitability for post‐fire tree seedling establishment, future (2021–2099) trends in precipitation were widely variable among GCMs, leading to mixed projections of future establishment suitability. In contrast, climatic water deficit (CWD), which is indicative of warm, dry conditions, was negatively associated with post‐fire seedling abundance during the recent period and was projected to increase throughout the southern Rocky Mountains in the future. Our findings suggest that future increases in CWD and an increased frequency of extreme drought years will substantially reduce post‐fire seedling densities.
Main conclusions This study highlights the key roles of warming and drying in declining forest resilience to wildfire. Moisture stress, driven by macroclimate and topographic setting, will interact with wildfire activity to shape future vegetation patterns throughout the southern Rocky Mountains, USA.
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Abstract Changing climate and disturbance regimes are increasingly challenging the resilience of forest ecosystems around the globe. A powerful indicator for the loss of resilience is regeneration failure, that is, the inability of the prevailing tree species to regenerate after disturbance. Regeneration failure can result from the interplay among disturbance changes (e.g., larger and more frequent fires), altered climate conditions (e.g., increased drought), and functional traits (e.g., method of seed dispersal). This complexity makes projections of regeneration failure challenging. Here we applied a novel simulation approach assimilating data‐driven fire projections with vegetation responses from process modeling by means of deep neural networks. We (i) quantified the future probability of regeneration failure; (ii) identified spatial hotspots of regeneration failure; and (iii) assessed how current forest types differ in their ability to regenerate under future climate and fire. We focused on the Greater Yellowstone Ecosystem (2.9 × 106 ha of forest) in the Rocky Mountains of the USA, which has experienced large wildfires in the past and is expected to undergo drastic changes in climate and fire in the future. We simulated four climate scenarios until 2100 at a fine spatial grain (100 m). Both wildfire activity and unstocked forest area increased substantially throughout the 21st century in all simulated scenarios. By 2100, between 28% and 59% of the forested area failed to regenerate, indicating considerable loss of resilience. Areas disproportionally at risk occurred where fires are not constrained by topography and in valleys aligned with predominant winds. High‐elevation forest types not adapted to fire (i.e.,
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Abstract As 21st‐century climate and disturbance dynamics depart from historic baselines, ecosystem resilience is uncertain. Multiple drivers are changing simultaneously, and interactions among drivers could amplify ecosystem vulnerability to change. Subalpine forests in Greater Yellowstone (Northern Rocky Mountains, USA) were historically resilient to infrequent (100–300 year), severe fire. We sampled paired short‐interval (<30‐year) and long‐interval (>125‐year) post‐fire plots most recently burned between 1988 and 2018 to address two questions: (1) How do short‐interval fire, climate, topography, and distance to unburned live forest edge interact to affect post‐fire forest regeneration? (2) How do forest biomass and fuels vary following short‐interval versus long‐interval severe fires? Mean post‐fire live tree stem density was an order of magnitude lower following short‐interval versus long‐interval fires (3240 vs. 28,741 stems ha−1, respectively). Differences between paired plots were amplified at longer distances to live forest edge. Surprisingly, warmer–drier climate was associated with higher seedling densities even after short‐interval fire, likely relating to regional variation in serotiny of lodgepole pine (
Pinus contorta var.latifolia ). Unlike conifers, density of aspen (Populus tremuloides ), a deciduous resprouter, increased with short‐interval versus long‐interval fires (mean 384 vs. 62 stems ha−1, respectively). Live biomass and canopy fuels remained low nearly 30 years after short‐interval fire, in contrast with rapid recovery after long‐interval fire, suggesting that future burn severity may be reduced for several decades following reburns. Short‐interval plots also had half as much dead woody biomass compared with long‐interval plots (60 vs. 121 Mg ha−1), primarily due to the absence of large snags. Our results suggest differences in tree regeneration following short‐interval versus long‐interval fires will be especially pronounced where serotiny was high historically. Propagule limitation will also interact with short‐interval fires to diminish tree regeneration but lessen subsequent burn severity. Amplifying driver interactions are likely to threaten forest resilience under expected trajectories of a future fire. -
Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.more » « less
