Fire regimes in North American forests are diverse and modern fire records are often too short to... more Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree‐ring fire scars provide valuable perspectives on fire regimes, including centuries‐long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree‐ring fire‐scar network (NAFSN), which contains 2562 sites, >37,000 fire‐scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000‐m elevation and across a range of topographic settings that vary by ecoregion. Mul...
Fire is a powerful natural disturbance influencing vegetation patterns across landscapes. Recent ... more Fire is a powerful natural disturbance influencing vegetation patterns across landscapes. Recent transitions from mixed-species forests to post-fire shrublands after severe wildfire is an increasingly prevalent phenomenon in pine-oak and conifer forest ecosystems in southwestern North America. However, we know little about how variation in fire severity influences other common forest types in the region. In this study, we evaluated fire-induced changes in woody plant community composition and forest structure in Chiricahua Mountains in southeastern Arizona in the United States that hosts a diverse set of vegetation types. Cluster analysis of the pre-fire vegetation data identified three dominant pre-fire vegetation types including juniper woodland, piñon forest, and pine-oak forest. All vegetation types experienced significant tree mortality across a wide range of size classes and species, from forests to shrublands. The magnitude of change within sample plots varied with fire sever...
Spatial patterns of vegetation change and fire severity are influenced by fire exclusion, topogra... more Spatial patterns of vegetation change and fire severity are influenced by fire exclusion, topography and weather conditions during a fire. Since the late nineteenth century, fire exclusion has increased vegetation cover which could influence fire severity and post-fire vegetation composition. We use field measurements and remote sensing of a 2011 wildfire to answer the following questions: (1) How did twentieth century vegetation change influence fire severity patterns? (2) What were the key drivers of wildfire severity? (3) Did initial post-fire development indicate stability or a shift in woody plant composition. Repeat aerial photography and pre and post fire field measurements were used to quantify twentieth century vegetation change and measure wildfire effects. Spatial controls on 2011 fire severity were determined using geospatial layers of vegetation type and change, topography, fire weather, daily fire extent, and fire severity. Vegetation changes since 1935 increased area of closed woodlands and shrublands and the 2011 fire reversed this trend and increased heterogeneity of vegetation types. Vegetation in plots experienced a post-fire shift towards resprouting species. At the landscape scale, terrain and weather were the main variables controlling fire severity with modest contribution by vegetation type and areas with vegetation change since 1935. Our findings indicate that wildfire severity in this landscape is strongly influenced by weather and terrain and wildfires reverse the effects of fire exclusion. Using terrain based landscape compartments for prescribed and wildfire management would help maintain the heterogeneous effects of the 2011 fire into future.
Along the U.S.‐Mexico border, an aggressive non‐native grass, giant cane (Arundo donax), grows in... more Along the U.S.‐Mexico border, an aggressive non‐native grass, giant cane (Arundo donax), grows in dense, nearly impenetrable stands along hundreds of kilometers of the Rio Grande/Bravo (RGB). Between 2008 and 2018, a diverse, multisector binational‐team repeatedly treated giant cane with prescribed fire and herbicide along 90 km of this binational river to restore aquatic and riparian habitat and native plant community composition. The large geographic scale, binational management response, treatment methods used, and development of a long‐term monitoring program to quantify treatment impacts on the RGB's riparian plant community underscore the unique aspects of this effort. Results of this decade‐long management experiment indicate that (i) the combination of a primary treatment of giant cane (using prescribed fire followed 4–6 weeks later by herbicide treatment of regrowth) and a secondary treatment (spot treatment of regrowth one or more years following primary treatment) was...
Abstract Recent increases in wildfire severity are converting pine-oak (Pinus-Quercus) woodland t... more Abstract Recent increases in wildfire severity are converting pine-oak (Pinus-Quercus) woodland to oak shrubland in many sites in the southwestern United States. A key mechanism underlying this transition is the vigorous resprouting capacity of oaks compared to low regeneration rates in pines following wildfire. Differences among species in leaf physiological characteristics may also contribute to such vegetation type conversions, especially in the context of recent increased regional aridity. To that end, we evaluated variation in leaf functional traits in post-fire recruits five years after the 2011 Horseshoe Two in the Chiricahua Mountains, Arizona, USA. We measured a suite of functional traits in two pines (Pinus engelmannii and P. leiophylla) and two oaks (Quercus hypoleucoides and Q. arizonica), including leaf gas exchange, leaf pigment concentrations, leaf spectral reflectance, and wood xylem δ13C, δ18O isotopes, and integrated water use efficiency (iWUE). The four study species displayed a range of plant functional traits tied to desiccation tolerance. P. engelmannii seedling recruits presented invariably low maximum photosynthetic rate (Amax), stomatal conductance (gs), and transpiration (E), and high relative leaf water content during the peak of the May-June drought, a response strongly indicative of a desiccation-avoidant leaf that prioritizes water conservation over carbon fixation in arid conditions. In contrast, resprouts of both oaks displayed leaf desiccation tolerance, with high levels of gs and E and low relative leaf water content. Gas exchange in P. leiophylla resprouts was more similar to the oaks, except for relative water content, which was similar to that of P. engelmannii. Compared to the other two species, P. engelmannii and Q. hypoleucoides exhibited significantly higher δ13C, δ18O and iWUE values—a pattern that appears to have arisen from different mechanisms in the two species. In desiccation-avoidant P. engelmannii, low instantaneous gs and high δ18O suggested that low stomatal conductance rather than high photosynthetic rates led to high iWUE, whereas lower δ18O and very high pigment concentrations in Q. hypoleucoides suggest a more important role for Amax in this species. Neither fire severity nor physiography influenced the physiological responses in any of the four species, suggesting that the expression of resprout and seedling functional traits is fixed. These results raise the possibility that differences in leaf physiological characteristics contribute to the regeneration success of some oak species and the poor seedling establishment of obligate seeder pines, such as P. engelmannii, after high-severity wildfire in the American Southwest.
Fire regimes in North American forests are diverse and modern fire records are often too short to... more Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree‐ring fire scars provide valuable perspectives on fire regimes, including centuries‐long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree‐ring fire‐scar network (NAFSN), which contains 2562 sites, >37,000 fire‐scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000‐m elevation and across a range of topographic settings that vary by ecoregion. Mul...
Fire is a powerful natural disturbance influencing vegetation patterns across landscapes. Recent ... more Fire is a powerful natural disturbance influencing vegetation patterns across landscapes. Recent transitions from mixed-species forests to post-fire shrublands after severe wildfire is an increasingly prevalent phenomenon in pine-oak and conifer forest ecosystems in southwestern North America. However, we know little about how variation in fire severity influences other common forest types in the region. In this study, we evaluated fire-induced changes in woody plant community composition and forest structure in Chiricahua Mountains in southeastern Arizona in the United States that hosts a diverse set of vegetation types. Cluster analysis of the pre-fire vegetation data identified three dominant pre-fire vegetation types including juniper woodland, piñon forest, and pine-oak forest. All vegetation types experienced significant tree mortality across a wide range of size classes and species, from forests to shrublands. The magnitude of change within sample plots varied with fire sever...
Spatial patterns of vegetation change and fire severity are influenced by fire exclusion, topogra... more Spatial patterns of vegetation change and fire severity are influenced by fire exclusion, topography and weather conditions during a fire. Since the late nineteenth century, fire exclusion has increased vegetation cover which could influence fire severity and post-fire vegetation composition. We use field measurements and remote sensing of a 2011 wildfire to answer the following questions: (1) How did twentieth century vegetation change influence fire severity patterns? (2) What were the key drivers of wildfire severity? (3) Did initial post-fire development indicate stability or a shift in woody plant composition. Repeat aerial photography and pre and post fire field measurements were used to quantify twentieth century vegetation change and measure wildfire effects. Spatial controls on 2011 fire severity were determined using geospatial layers of vegetation type and change, topography, fire weather, daily fire extent, and fire severity. Vegetation changes since 1935 increased area of closed woodlands and shrublands and the 2011 fire reversed this trend and increased heterogeneity of vegetation types. Vegetation in plots experienced a post-fire shift towards resprouting species. At the landscape scale, terrain and weather were the main variables controlling fire severity with modest contribution by vegetation type and areas with vegetation change since 1935. Our findings indicate that wildfire severity in this landscape is strongly influenced by weather and terrain and wildfires reverse the effects of fire exclusion. Using terrain based landscape compartments for prescribed and wildfire management would help maintain the heterogeneous effects of the 2011 fire into future.
Along the U.S.‐Mexico border, an aggressive non‐native grass, giant cane (Arundo donax), grows in... more Along the U.S.‐Mexico border, an aggressive non‐native grass, giant cane (Arundo donax), grows in dense, nearly impenetrable stands along hundreds of kilometers of the Rio Grande/Bravo (RGB). Between 2008 and 2018, a diverse, multisector binational‐team repeatedly treated giant cane with prescribed fire and herbicide along 90 km of this binational river to restore aquatic and riparian habitat and native plant community composition. The large geographic scale, binational management response, treatment methods used, and development of a long‐term monitoring program to quantify treatment impacts on the RGB's riparian plant community underscore the unique aspects of this effort. Results of this decade‐long management experiment indicate that (i) the combination of a primary treatment of giant cane (using prescribed fire followed 4–6 weeks later by herbicide treatment of regrowth) and a secondary treatment (spot treatment of regrowth one or more years following primary treatment) was...
Abstract Recent increases in wildfire severity are converting pine-oak (Pinus-Quercus) woodland t... more Abstract Recent increases in wildfire severity are converting pine-oak (Pinus-Quercus) woodland to oak shrubland in many sites in the southwestern United States. A key mechanism underlying this transition is the vigorous resprouting capacity of oaks compared to low regeneration rates in pines following wildfire. Differences among species in leaf physiological characteristics may also contribute to such vegetation type conversions, especially in the context of recent increased regional aridity. To that end, we evaluated variation in leaf functional traits in post-fire recruits five years after the 2011 Horseshoe Two in the Chiricahua Mountains, Arizona, USA. We measured a suite of functional traits in two pines (Pinus engelmannii and P. leiophylla) and two oaks (Quercus hypoleucoides and Q. arizonica), including leaf gas exchange, leaf pigment concentrations, leaf spectral reflectance, and wood xylem δ13C, δ18O isotopes, and integrated water use efficiency (iWUE). The four study species displayed a range of plant functional traits tied to desiccation tolerance. P. engelmannii seedling recruits presented invariably low maximum photosynthetic rate (Amax), stomatal conductance (gs), and transpiration (E), and high relative leaf water content during the peak of the May-June drought, a response strongly indicative of a desiccation-avoidant leaf that prioritizes water conservation over carbon fixation in arid conditions. In contrast, resprouts of both oaks displayed leaf desiccation tolerance, with high levels of gs and E and low relative leaf water content. Gas exchange in P. leiophylla resprouts was more similar to the oaks, except for relative water content, which was similar to that of P. engelmannii. Compared to the other two species, P. engelmannii and Q. hypoleucoides exhibited significantly higher δ13C, δ18O and iWUE values—a pattern that appears to have arisen from different mechanisms in the two species. In desiccation-avoidant P. engelmannii, low instantaneous gs and high δ18O suggested that low stomatal conductance rather than high photosynthetic rates led to high iWUE, whereas lower δ18O and very high pigment concentrations in Q. hypoleucoides suggest a more important role for Amax in this species. Neither fire severity nor physiography influenced the physiological responses in any of the four species, suggesting that the expression of resprout and seedling functional traits is fixed. These results raise the possibility that differences in leaf physiological characteristics contribute to the regeneration success of some oak species and the poor seedling establishment of obligate seeder pines, such as P. engelmannii, after high-severity wildfire in the American Southwest.
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