In an effort to identify the influence oflight and temperature on the physiology and leaf structu... more In an effort to identify the influence oflight and temperature on the physiology and leaf structural characteristics of three species of Quercus from Coahuila, Mexico, we measured a comprehensive suite of plant traits as functions of light and temperature intensity. We tested the hypotheses that 1) species' physiological responses to light and temperature were related to their distributions in their native habitats; and 2) that species' physiological responses corresponded to similar variation in leaf anatomical and morphological traits. Quercus sideroxyla was adapted to high elevation forest overstories as evidenced by its high photosynthetic rate, transpiration rate, relative water content (RWC), leaf density (LD), and thick palisade and spongy parenchyma. Quercus rugosa displayed typical characteristics of a forest understory species including a low photosynthetic rate and light saturation point, thick spongy parenchyma tissue and high RWC, leaf density, and leaf mass per unit area. Quercus laceyi was adapted to hot, dry sites based on its lower RWC and LD, intermediate photosynthetic rate, thick cuticle and upper epidermis, and low transpiration rates at high temperatures. Our results suggest that the physiological and structural adaptations of Mexican oaks to changing environmental conditions across resource gradients are key regulators of plant community structure.
Dam removal is an increasingly common stream restoration tool. Yet, removing dams from small stre... more Dam removal is an increasingly common stream restoration tool. Yet, removing dams from small streams also represents a major disturbance to rivers that can have varied impacts on environmental conditions and aquatic biota. We examined the effects of dam removal on the structure, function, and composition of benthic macroinvertebrate (BMI) communities in a temperate New England stream. We examined the effects of dam removal over the dam removal time-series using linear mixed effects models, autoregressive models, non-metric multidimensional scaling, and indicator and similarity analyses. The results indicated that the dam removal stimulated major shifts in BMI community structure and composition above and below the dam, and that the BMI communities are becoming more similar over time. The mixed model analysis revealed that BMI functional groups and diversity were significantly influenced by sample site and several BMI groups also experienced significant interactions between site and dam stage (P < 0.05), while the multivariate analyses revealed that community structure continues to differ among sites, even three years after dam removal. Our findings indicate that stream restoration through dam removal can have site-specific influences on BMI communities, that interactions among BMI taxa are important determinants of the post-dam removal community, and that the post-dam-removal BMI community continues to be in a state of reorganization.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
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...
Piñon-juniper (PJ) woodlands comprise an important and extensive dry-site forest cover-type that ... more Piñon-juniper (PJ) woodlands comprise an important and extensive dry-site forest cover-type that has historically experienced high spatiotemporal variation in fire frequency and extent. We re-measured a network of permanent monitoring plots in a fire-suppressed PJ site the Davis Mountains of West Texas after prescribed fire (Rx) treatments and three subsequent wildfires to 1) quantify wildfire effects on PJ woodland stand structure and fuel loadings, and 2) evaluate the effects of Rx fires followed by wildfire. Although fire weather was extreme during the wildfire years, all three wildfires burned as low-severity fire events. Simultaneous autoregressive modeling revealed that total tree density and basal area declined significantly (P < 0.05) over the time-series, while surface fuel loadings increased significantly (P < 0.05), likely in response to fuel inputs from fire-induced tree mortality. However, tree mortality largely occurred in smaller tree size-classes (i.e. < 25 cm diameter at breast height). Neither fire severity, nor changes in stand structure varied significantly over the time-series among recent fire history types (i.e. no fire, Rx fire only, Rx fire + wildfire, or wildfire only) indicating that Rx fires had little effect on subsequent wildfire effects on forest structure or species composition. The low-severity nature of these recent wildfires under severe fire weather and in the wake of almost a century of fire suppression suggests that just because fires have been absent from a site for decades contemporary wildfires will not always result in fire regime characteristics that differ from historical fires in PJ-dominated areas.
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.
Wetlands provide myriad ecosystem services, yet the C-cycling of vegetation within interior fresh... more Wetlands provide myriad ecosystem services, yet the C-cycling of vegetation within interior freshwater tidal wetlands remains poorly understood. To this end, we estimated species'-specific plant carbon-fixation rates for the six dominant wetland plant species in a large temperate freshwater wetland in Connecticut, USA. We integrated field C-fixation rates for dominant marsh plant species with satellite-derived leaf area index and wetland aerial extent data to: 1) quantify seasonal and species-level differences in wetland plant C-fixation rates; and 2) estimate whole-marsh emergent aquatic plant C-fixation rates over the growing season. Photosynthetic rates differed significantly by species and month (P < 0.05). Acorus calamus had the highest photosynthetic rate between May and September, and Acer saccharinum had the lowest. By integrating field photosynthetic data with wetland aerial extents, we estimated that the total annual C uptake by the vegetation in this wetland, which was 2868 Mg C. Herbaceous vegetation contributed to most of that stock (herbaceous vegetation ¼ 2099.2 Mg C, forest ¼ 769.6 Mg C), although soil respiration likely offset those numbers substantially. Our results demonstrate the importance of short-term above-ground freshwater wetland C-fixation, and that the emergent vegetative component of these wetland systems are key components of the tidal freshwater wetland C cycle.
In an effort to identify the influence oflight and temperature on the physiology and leaf structu... more In an effort to identify the influence oflight and temperature on the physiology and leaf structural characteristics of three species of Quercus from Coahuila, Mexico, we measured a comprehensive suite of plant traits as functions of light and temperature intensity. We tested the hypotheses that 1) species' physiological responses to light and temperature were related to their distributions in their native habitats; and 2) that species' physiological responses corresponded to similar variation in leaf anatomical and morphological traits. Quercus sideroxyla was adapted to high elevation forest overstories as evidenced by its high photosynthetic rate, transpiration rate, relative water content (RWC), leaf density (LD), and thick palisade and spongy parenchyma. Quercus rugosa displayed typical characteristics of a forest understory species including a low photosynthetic rate and light saturation point, thick spongy parenchyma tissue and high RWC, leaf density, and leaf mass per unit area. Quercus laceyi was adapted to hot, dry sites based on its lower RWC and LD, intermediate photosynthetic rate, thick cuticle and upper epidermis, and low transpiration rates at high temperatures. Our results suggest that the physiological and structural adaptations of Mexican oaks to changing environmental conditions across resource gradients are key regulators of plant community structure.
Dam removal is an increasingly common stream restoration tool. Yet, removing dams from small stre... more Dam removal is an increasingly common stream restoration tool. Yet, removing dams from small streams also represents a major disturbance to rivers that can have varied impacts on environmental conditions and aquatic biota. We examined the effects of dam removal on the structure, function, and composition of benthic macroinvertebrate (BMI) communities in a temperate New England stream. We examined the effects of dam removal over the dam removal time-series using linear mixed effects models, autoregressive models, non-metric multidimensional scaling, and indicator and similarity analyses. The results indicated that the dam removal stimulated major shifts in BMI community structure and composition above and below the dam, and that the BMI communities are becoming more similar over time. The mixed model analysis revealed that BMI functional groups and diversity were significantly influenced by sample site and several BMI groups also experienced significant interactions between site and dam stage (P < 0.05), while the multivariate analyses revealed that community structure continues to differ among sites, even three years after dam removal. Our findings indicate that stream restoration through dam removal can have site-specific influences on BMI communities, that interactions among BMI taxa are important determinants of the post-dam removal community, and that the post-dam-removal BMI community continues to be in a state of reorganization.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
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...
Piñon-juniper (PJ) woodlands comprise an important and extensive dry-site forest cover-type that ... more Piñon-juniper (PJ) woodlands comprise an important and extensive dry-site forest cover-type that has historically experienced high spatiotemporal variation in fire frequency and extent. We re-measured a network of permanent monitoring plots in a fire-suppressed PJ site the Davis Mountains of West Texas after prescribed fire (Rx) treatments and three subsequent wildfires to 1) quantify wildfire effects on PJ woodland stand structure and fuel loadings, and 2) evaluate the effects of Rx fires followed by wildfire. Although fire weather was extreme during the wildfire years, all three wildfires burned as low-severity fire events. Simultaneous autoregressive modeling revealed that total tree density and basal area declined significantly (P < 0.05) over the time-series, while surface fuel loadings increased significantly (P < 0.05), likely in response to fuel inputs from fire-induced tree mortality. However, tree mortality largely occurred in smaller tree size-classes (i.e. < 25 cm diameter at breast height). Neither fire severity, nor changes in stand structure varied significantly over the time-series among recent fire history types (i.e. no fire, Rx fire only, Rx fire + wildfire, or wildfire only) indicating that Rx fires had little effect on subsequent wildfire effects on forest structure or species composition. The low-severity nature of these recent wildfires under severe fire weather and in the wake of almost a century of fire suppression suggests that just because fires have been absent from a site for decades contemporary wildfires will not always result in fire regime characteristics that differ from historical fires in PJ-dominated areas.
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.
Wetlands provide myriad ecosystem services, yet the C-cycling of vegetation within interior fresh... more Wetlands provide myriad ecosystem services, yet the C-cycling of vegetation within interior freshwater tidal wetlands remains poorly understood. To this end, we estimated species'-specific plant carbon-fixation rates for the six dominant wetland plant species in a large temperate freshwater wetland in Connecticut, USA. We integrated field C-fixation rates for dominant marsh plant species with satellite-derived leaf area index and wetland aerial extent data to: 1) quantify seasonal and species-level differences in wetland plant C-fixation rates; and 2) estimate whole-marsh emergent aquatic plant C-fixation rates over the growing season. Photosynthetic rates differed significantly by species and month (P < 0.05). Acorus calamus had the highest photosynthetic rate between May and September, and Acer saccharinum had the lowest. By integrating field photosynthetic data with wetland aerial extents, we estimated that the total annual C uptake by the vegetation in this wetland, which was 2868 Mg C. Herbaceous vegetation contributed to most of that stock (herbaceous vegetation ¼ 2099.2 Mg C, forest ¼ 769.6 Mg C), although soil respiration likely offset those numbers substantially. Our results demonstrate the importance of short-term above-ground freshwater wetland C-fixation, and that the emergent vegetative component of these wetland systems are key components of the tidal freshwater wetland C cycle.
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