In recent years, increased wildfire activity and climate change have raised concern among scienti... more In recent years, increased wildfire activity and climate change have raised concern among scientists and land managers regarding current and future vegetation patterns in post-burn landscapes. We surveyed conifer regeneration 8-15 years after fire in six burn areas in the lower montane zone of the Col-orado Front Range. We sampled across a broad range of elevations, aspects, and fire severities and found that densities of ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) are generally low, although areas of abundant regeneration do occur. Conifer regeneration was most limited in xeric settings , including more southerly aspects and elevations closer to lower treeline. Additionally, fewer juvenile conifers occurred at greater distances from mature, live trees indicating that seed source as well as topocli-matic setting limits post-fire tree regeneration. Projecting the extent of future forest cover is uncertain due to the possibility of future pulses of tree establishment and unknown depletion rates of existing seedling populations. However, current patterns of post-fire seedling establishment suggest that vegetation composition and structure may differ notably from historic patterns and that lower density stands and even non-forested communities may persist in some areas of these burns long after fire, especially in xeric settings or where no nearby seed source remains.
Climate warming is contributing to increases in wildfire activity throughout the western United S... more Climate warming is contributing to increases in wildfire activity throughout the western United States, leading to potentially long-lasting shifts in vegetation. The response of forest ecosystems to wildfire is thus a crucial indicator of future vegetation trajectories, and these responses are contingent upon factors such as seed availability, interannual climate variability , average climate, and other components of the physical environment. To better understand variation in resilience to wildfire across vulnerable dry forests, we surveyed conifer seedling densities in 15 recent (1988-2010) wildfires and characterized temporal variation in seed cone production and seedling establishment. We then predicted postfire seedling densities at a 30-m resolution within each fire perimeter using downscaled climate data, monthly water balance models, and maps of surviving forest cover. Widespread ponderosa pine (Pinus pon-derosa) seed cone production occurred at least twice following each fire surveyed, and pulses of conifer seedling establishment coincided with years of above-average moisture availability. Ponderosa pine and Douglas-fir (Pseudotsuga menziesii) seedling densities were higher on more mesic sites and adjacent to surviving trees, though there were also important interspecific differences , likely attributable to drought and shade tolerance. We estimated that postfire seedling densities in 42% (for ponderosa pine) and 69% (for Douglas-fir) of the total burned area were below the lowest reported historical tree densities in these forests. Spatial models demonstrated that an absence of mature conifers (particularly in the interior of large, high-severity patches) limited seedling densities in many areas, but 30-yr average actual evapotranspiration and climatic water deficit limited densities on marginal sites. A better understanding of the limitations to postfire forest recovery will refine models of vegetation dynamics and will help to improve strategies of adaptation to a warming climate and shifting fire activity.
Dendroecology is the science that dates tree rings to their exact calendar year of formation to s... more Dendroecology is the science that dates tree rings to their exact calendar year of formation to study processes that influence forest ecology (e.g., Speer 2010 [1], Amoroso et al., 2017 [2]). Reconstruction of past fire regimes is a core application of dendroecology, linking fire history to population dynamics and climate effects on tree growth and survivorship. Since the early 20th century when dendrochronologists recognized that tree rings retained fire scars (e.g., Figure 1), and hence a record of past fires, they have conducted studies worldwide to reconstruct [2] the historical range and variability of fire regimes (e.g., frequency, severity, seasonality, spatial extent), [3] the influence of fire regimes on forest structure
Proceedings of the National Academy of Sciences, 2019
Climate change is increasing fire activity in the western United States, which has the potential ... more Climate change is increasing fire activity in the western United States, which has the potential to accelerate climate-induced shifts in vegetation communities. Wildfire can catalyze vegetation change by killing adult trees that could otherwise persist in climate conditions no longer suitable for seedling establishment and survival. Recently documented declines in postfire conifer recruitment in the western United States may be an example of this phenomenon. However, the role of annual climate variation and its interaction with long-term climate trends in driving these changes is poorly resolved. Here we examine the relationship between annual climate and postfire tree regeneration of two dominant , low-elevation conifers (ponderosa pine and Douglas-fir) using annually resolved establishment dates from 2,935 destructively sampled trees from 33 wildfires across four regions in the western United States. We show that regeneration had a nonlinear response to annual climate conditions, with distinct thresholds for recruitment based on vapor pressure deficit, soil moisture, and maximum surface temperature. At dry sites across our study region, seasonal to annual climate conditions over the past 20 years have crossed these thresholds, such that conditions have become increasingly unsuitable for regeneration. High fire severity and low seed availability further reduced the probability of postfire regeneration. Together, our results demonstrate that climate change combined with high severity fire is leading to increasingly fewer opportunities for seedlings to establish after wildfires and may lead to ecosystem transitions in low-elevation ponderosa pine and Douglas-fir forests across the western United States.
Forest resilience to climate change is a global concern given the potential effects of increased ... more 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 ecosyst...
Background/Question/Methods Our research examined climatic influences on episodic post-fire conif... more Background/Question/Methods Our research examined climatic influences on episodic post-fire conifer establishment in low-elevation, ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range. Given observations of limited post-fire regeneration in several recent burns of the study area, we examined what role climate variability may play in explaining regeneration success or failure after fire. Our specific research objectives were to: (1) construct a dataset of annually-resolved establishment dates for post-fire ponderosa pine and Douglas-fir (Pseudotsuga menziesii) juveniles, and (2) assess climatic influences on temporal patterns of conifer establishment. We hypothesized that conifer establishment would be concentrated in years of above-average moisture availability and that little or no establishment would occur in drier years. Results/Conclusions We developed a large dataset (n = 413) of annually resolved establishment dates for post-fire juvenile ponderosa pine and Do...
Few tree-ring based fire-history studies have been completed in pine ecosystems of the Southeaste... more Few tree-ring based fire-history studies have been completed in pine ecosystems of the Southeastern Coastal Plain, in part because of difficulties in finding old fire-scarred material. We propose specialized field methods that improve the likelihood of locating fire scars in dead trees (i.e. stumps, snags, and logs). Classic fire-history field methods developed in the southwestern United States involve targeting only trees with evidence of repeated external scarring, but we have found this approach to be less effective in our region given that trees without any external scarring may contain an abundance of buried scars. The buried scars occur primarily near the ground surface and can be sampled by collecting full cross-sections from the bases of old dead trees. We hope our insights foster further fire-history research in the Southeastern Coastal Plain.
In recent years, increased wildfire activity and climate change have raised concern among scienti... more In recent years, increased wildfire activity and climate change have raised concern among scientists and land managers regarding current and future vegetation patterns in post-burn landscapes. We surveyed conifer regeneration 8-15 years after fire in six burn areas in the lower montane zone of the Col-orado Front Range. We sampled across a broad range of elevations, aspects, and fire severities and found that densities of ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) are generally low, although areas of abundant regeneration do occur. Conifer regeneration was most limited in xeric settings , including more southerly aspects and elevations closer to lower treeline. Additionally, fewer juvenile conifers occurred at greater distances from mature, live trees indicating that seed source as well as topocli-matic setting limits post-fire tree regeneration. Projecting the extent of future forest cover is uncertain due to the possibility of future pulses of tree establishment and unknown depletion rates of existing seedling populations. However, current patterns of post-fire seedling establishment suggest that vegetation composition and structure may differ notably from historic patterns and that lower density stands and even non-forested communities may persist in some areas of these burns long after fire, especially in xeric settings or where no nearby seed source remains.
Climate warming is contributing to increases in wildfire activity throughout the western United S... more Climate warming is contributing to increases in wildfire activity throughout the western United States, leading to potentially long-lasting shifts in vegetation. The response of forest ecosystems to wildfire is thus a crucial indicator of future vegetation trajectories, and these responses are contingent upon factors such as seed availability, interannual climate variability , average climate, and other components of the physical environment. To better understand variation in resilience to wildfire across vulnerable dry forests, we surveyed conifer seedling densities in 15 recent (1988-2010) wildfires and characterized temporal variation in seed cone production and seedling establishment. We then predicted postfire seedling densities at a 30-m resolution within each fire perimeter using downscaled climate data, monthly water balance models, and maps of surviving forest cover. Widespread ponderosa pine (Pinus pon-derosa) seed cone production occurred at least twice following each fire surveyed, and pulses of conifer seedling establishment coincided with years of above-average moisture availability. Ponderosa pine and Douglas-fir (Pseudotsuga menziesii) seedling densities were higher on more mesic sites and adjacent to surviving trees, though there were also important interspecific differences , likely attributable to drought and shade tolerance. We estimated that postfire seedling densities in 42% (for ponderosa pine) and 69% (for Douglas-fir) of the total burned area were below the lowest reported historical tree densities in these forests. Spatial models demonstrated that an absence of mature conifers (particularly in the interior of large, high-severity patches) limited seedling densities in many areas, but 30-yr average actual evapotranspiration and climatic water deficit limited densities on marginal sites. A better understanding of the limitations to postfire forest recovery will refine models of vegetation dynamics and will help to improve strategies of adaptation to a warming climate and shifting fire activity.
Dendroecology is the science that dates tree rings to their exact calendar year of formation to s... more Dendroecology is the science that dates tree rings to their exact calendar year of formation to study processes that influence forest ecology (e.g., Speer 2010 [1], Amoroso et al., 2017 [2]). Reconstruction of past fire regimes is a core application of dendroecology, linking fire history to population dynamics and climate effects on tree growth and survivorship. Since the early 20th century when dendrochronologists recognized that tree rings retained fire scars (e.g., Figure 1), and hence a record of past fires, they have conducted studies worldwide to reconstruct [2] the historical range and variability of fire regimes (e.g., frequency, severity, seasonality, spatial extent), [3] the influence of fire regimes on forest structure
Proceedings of the National Academy of Sciences, 2019
Climate change is increasing fire activity in the western United States, which has the potential ... more Climate change is increasing fire activity in the western United States, which has the potential to accelerate climate-induced shifts in vegetation communities. Wildfire can catalyze vegetation change by killing adult trees that could otherwise persist in climate conditions no longer suitable for seedling establishment and survival. Recently documented declines in postfire conifer recruitment in the western United States may be an example of this phenomenon. However, the role of annual climate variation and its interaction with long-term climate trends in driving these changes is poorly resolved. Here we examine the relationship between annual climate and postfire tree regeneration of two dominant , low-elevation conifers (ponderosa pine and Douglas-fir) using annually resolved establishment dates from 2,935 destructively sampled trees from 33 wildfires across four regions in the western United States. We show that regeneration had a nonlinear response to annual climate conditions, with distinct thresholds for recruitment based on vapor pressure deficit, soil moisture, and maximum surface temperature. At dry sites across our study region, seasonal to annual climate conditions over the past 20 years have crossed these thresholds, such that conditions have become increasingly unsuitable for regeneration. High fire severity and low seed availability further reduced the probability of postfire regeneration. Together, our results demonstrate that climate change combined with high severity fire is leading to increasingly fewer opportunities for seedlings to establish after wildfires and may lead to ecosystem transitions in low-elevation ponderosa pine and Douglas-fir forests across the western United States.
Forest resilience to climate change is a global concern given the potential effects of increased ... more 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 ecosyst...
Background/Question/Methods Our research examined climatic influences on episodic post-fire conif... more Background/Question/Methods Our research examined climatic influences on episodic post-fire conifer establishment in low-elevation, ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range. Given observations of limited post-fire regeneration in several recent burns of the study area, we examined what role climate variability may play in explaining regeneration success or failure after fire. Our specific research objectives were to: (1) construct a dataset of annually-resolved establishment dates for post-fire ponderosa pine and Douglas-fir (Pseudotsuga menziesii) juveniles, and (2) assess climatic influences on temporal patterns of conifer establishment. We hypothesized that conifer establishment would be concentrated in years of above-average moisture availability and that little or no establishment would occur in drier years. Results/Conclusions We developed a large dataset (n = 413) of annually resolved establishment dates for post-fire juvenile ponderosa pine and Do...
Few tree-ring based fire-history studies have been completed in pine ecosystems of the Southeaste... more Few tree-ring based fire-history studies have been completed in pine ecosystems of the Southeastern Coastal Plain, in part because of difficulties in finding old fire-scarred material. We propose specialized field methods that improve the likelihood of locating fire scars in dead trees (i.e. stumps, snags, and logs). Classic fire-history field methods developed in the southwestern United States involve targeting only trees with evidence of repeated external scarring, but we have found this approach to be less effective in our region given that trees without any external scarring may contain an abundance of buried scars. The buried scars occur primarily near the ground surface and can be sampled by collecting full cross-sections from the bases of old dead trees. We hope our insights foster further fire-history research in the Southeastern Coastal Plain.
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Papers by Monica Rother