Integrating plant physiology into simulation of fire behavior and effects.

Wildfires are a global crisis, but current fire models fail to capture vegetation response to changing climate. With drought and elevated temperature increasing the importance of vegetation dynamics to fire behavior, and the advent of next generation models capable of capturing increasingly complex physical processes, we provide a renewed focus on representation of woody vegetation in fire models. Currently, the most advanced representations of fire behavior and biophysical fire effects are found in distinct classes of fine-scale models and do not capture variation in live fuel (i.e. living plant) properties. We demonstrate that plant water and carbon dynamics, which influence combustion and heat transfer into the plant and often dictate plant survival, provide the mechanistic linkage between fire behavior and effects. Our conceptual framework linking remotely sensed estimates of plant water and carbon to fine-scale models of fire behavior and effects could be a critical first step toward improving the fidelity of the coarse scale models that are now relied upon for global fire forecasting. This process-based approach will be essential to capturing the influence of physiological responses to drought and warming on live fuel conditions, strengthening the science needed to guide fire managers in an uncertain future.

A simplified and affordable approach to forest monitoring using single terrestrial laser scans and transect sampling.

Traditional forestry, ecology, and fuels monitoring methods can be costly and error-prone, and are often used beyond their original assumptions due to difficulty or unavailability of more appropriate methods. These traditional methods tend to be rigid and may not be useful for detecting new ecological changes or required data at modern levels of precision [1]. The integration of Terrestrial Laser Scanning (TLS) methods into forest monitoring strategies can cost effectively standardize data collection, improve efficiency, and reduce error, with datasets that can easily be analyzed to better inform management decisions. Affordable (sub-$20K) off-the-shelf TLS units-such as the Leica BLK360- have been used commercially in the built environment but have untapped potential in the natural world for monitoring. Here, we provide a methodology that successfully integrates LiDAR scanning with existing monitoring methods. This new method:•Allows for simplified and quick extraction of forestry, fuels and ecological vegetation variables from a single TLS point cloud and quick transect sampling.•Streamlines the data collection process, removes sampling bias, and produces data that can be easily processed to provide inputs for models and decision support frameworks.•Is adaptable to integrate additional or new environmental measurements.

Tree crown injury from wildland fires: causes, measurement and ecological and physiological consequences.

The dead foliage of scorched crowns is one of the most conspicuous signatures of wildland fires. Globally, crown scorch from fires in savannas, woodlands and forests causes tree stress and death across diverse taxa. The term crown scorch, however, is inconsistently and ambiguously defined in the literature, causing confusion and conflicting interpretation of results. Furthermore, the underlying mechanisms causing foliage death from fire are poorly understood. The consequences of crown scorch - alterations in physiological, biogeochemical and ecological processes and ecosystem recovery pathways - remain largely unexamined. Most research on the topic assumes the mechanism of leaf and bud death is exposure to lethal air temperatures, with few direct measurements of lethal heating thresholds. Notable information gaps include how energy transfer injures and kills leaves and buds, how nutrients, carbohydrates, and hormones respond, and what physiological consequences lead to mortality. We clarify definitions to encourage use of unified terminology for foliage and bud necrosis resulting from fire. We review the current understanding of the physical mechanisms driving foliar injury, discuss the physiological responses, and explore novel ecological consequences of crown injury from fire. From these elements, we propose research needs for the increasingly interdisciplinary study of fire effects.

Effects of canopy midstory management and fuel moisture on wildfire behavior.

Increasing trends in wildfire severity can partly be attributed to fire exclusion in the past century which led to higher fuel accumulation. Mechanical thinning and prescribed burns are effective techniques to manage fuel loads and to establish a higher degree of control over future fire risk, while restoring fire prone landscapes to their natural states of succession. However, given the complexity of interactions between fine scale fuel heterogeneity and wind, it is difficult to assess the success of thinning operations and prescribed burns. The present work addresses this issue systematically by simulating a simple fire line and propagating through a vegetative environment where the midstory has been cleared in different degrees, leading to a canopy with almost no midstory, another with a sparse midstory and another with a dense midstory. The simulations are conducted for these three canopies under two different conditions, where the fuel moisture is high and where it is low. These six sets of simulations show widely different fire behavior, in terms of fire intensity, spread rate and consumption. To understand the physical mechanisms that lead to these differences, detailed analyses are conducted to look at wind patterns, mean flow and turbulent fluxes of momentum and energy. The analyses also lead to improved understanding of processes leading to high intensity crowning behavior in presence of a dense midstory. Moreover, this work highlights the importance of considering fine scale fuel heterogeneity, seasonality, wind effects and the associated fire-canopy-atmosphere interactions while considering prescribed burns and forest management operations.

A comparison of bee communities between primary and mature secondary forests in the longleaf pine ecosystem.

Much of the once-dominant longleaf pine (Pinus palustris Mill.) ecosystem has been lost from the Coastal Plain of the southeastern United States and only a few scattered remnants of primary forest remain. Despite much interest in understanding and restoring this ecosystem, relatively few studies have attempted to characterize or assess the conservation status of the longleaf bee fauna. The objective of this study was to compare the diversity and composition of bee communities between primary and mature secondary (>100 years old) fire-maintained forests in Georgia and Florida. We used colored pan traps to sample bees at three primary and four secondary locations divided between two regions characterized by sandy (Eglin Air Force Base) or clayey (Red Hills) soils. There were no overall differences between primary and secondary forests in bee richness, diversity, evenness or abundance. Community composition differed among locations but we found no evidence that primary remnants provide critical habitat to sensitive bee species.

Simulating Groundcover Community Assembly in a Frequently Burned Ecosystem Using a Simple Neutral Model.

Fire is a keystone process that drives patterns of biodiversity globally. In frequently burned fire-dependent ecosystems, surface fire regimes allow for the coexistence of high plant diversity at fine scales even where soils are uniform. The mechanisms on how fire impacts groundcover community dynamics are, however, poorly understood. Because fire can act as a stochastic agent of mortality, we hypothesized that a neutral mechanism might be responsible for maintaining plant diversity. We used the demographic parameters of the unified neutral theory of biodiversity (UNTB) as a foundation to model groundcover species richness, using a southeastern US pine woodland as an example. We followed the fate of over 7,000 individuals of 123 plant species for 4 years and two prescribed burns in frequently burned Pinus palustris sites in northwest FL, USA. Using these empirical data and UNTB-based assumptions, we developed two parsimonious autonomous agent models, which were distinct by spatially explicit and implicit local recruitment processes. Using a parameter sensitivity test, we examined how empirical estimates, input species frequency distributions, and community size affected output species richness. We found that dispersal limitation was the most influential parameter, followed by mortality and birth, and that these parameters varied based on scale of the frequency distributions. Overall, these nominal parameters were useful for simulating fine-scale groundcover communities, although further empirical analysis of richness patterns, particularly related to fine-scale burn severity, is needed. This modeling framework can be utilized to examine our premise that localized groundcover assemblages are neutral communities at high fire frequencies, as well as to examine the extent to which niche-based dynamics determine community dynamics when fire frequency is altered.

Allometry of the pyrophytic Aristida in fire-maintained longleaf pine-wiregrass ecosystems.

PREMISE OF THE STUDY: Aboveground biomass (AGB) of herbaceous vegetation is a primary source of fuel in frequent surface fires that maintain grasslands, savannas, and woodlands. Methods for nondestructively estimating AGB are required to understand the mechanisms by which fuels affect fire behavior and the effects of time since the last burn. We developed allometric equations to estimate AGB in wiregrass (Aristida beyrichiana/A. stricta), a dominant bunchgrass in Pinus palustris ecosystems and a key species for ecological restoration. METHODS: We collected wiregrass from North Carolina to Florida, across a range of time-since-last burn and site types. We tested 32 mixed effect models to see which predictors were best at predicting live, dead, and total AGB. We also examined how time since burn (TSB) affected the live-to-dead ratio (LDR) using regression. KEY RESULTS: Wiregrass AGB was found to increase with increasing latitude (relative to tussock volume), possibly due to an increase in precipitation, and was greater on more fertile clay soils and flatwoods than on sandy soils. The LDR decreased as a power function with TSB, resulting in rapid accumulation of dead, highly flammable, biomass in the fire-free period. CONCLUSIONS: Greater biomass will support fires of higher intensity. Our models can be useful in the parameterization of future physics-based models to predict fire behavior. Understanding the environmental variables that influence the allometry of wiregrass should help increase the precision of AGB estimates and the subsequent effects on fire behavior and effects on neighboring vegetation.

The Precision Problem in Conservation and Restoration.

Within the varied contexts of environmental policy, conservation of imperilled species populations, and restoration of damaged habitats, an emphasis on idealized optimal conditions has led to increasingly specific targets for management. Overly-precise conservation targets can reduce habitat variability at multiple scales, with unintended consequences for future ecological resilience. We describe this dilemma in the context of endangered species management, stream restoration, and climate-change adaptation. Inappropriate application of conservation targets can be expensive, with marginal conservation benefit. Reduced habitat variability can limit options for managers trying to balance competing objectives with limited resources. Conservation policies should embrace habitat variability, expand decision-space appropriately, and support adaptation to local circumstances to increase ecological resilience in a rapidly changing world.

A dynamic reference model: a framework for assessing biodiversity restoration goals in a fire-dependent ecosystem.

The use of reference models as templates of historical or natural conditions to assess restoration progress is inherently logical; however, difficulties occur in application because of the need to incorporate temporal variation in ecosystems caused by disturbance and succession, as well as seasonal, interannual, or decadal variability. The landscape-scale restoration of the globally threatened and fire-dependent longleaf pine ecosystem in the southeastern United States is an example in which restoration efforts are even more complicated by the limited availability of extant reference sites. This study uses the dynamic reference conceptual framework to assess the direction and rate of recovery with respect to biodiversity restoration goals using a 15-year vegetation data set from an experimental restoration treatment in fire-excluded, hardwood-encroached longleaf pine sandhills. We compared ground-cover vegetation response to midstory hardwood removal through herbicide application, mechanical removal, and fire only. Nonmetric multidimensional scaling ordinations and proportional similarity analyses suggest that, while vegetation changed in all treatments over time, no differences in species composition or hardwood density in the ground cover were attributable to hardwood reduction treatments after 15 years with frequent prescribed fire. Furthermore, the results of this study indicate that considerable variability is associated with reference sites over time. Sites identified in 1994 as attainable restoration targets had become a moving target themselves, changing in magnitude consistent with alterations in restoration plots attributable to treatment effects and shaped by the modest increase in fire frequency imposed since 1998. In a broad restoration context, this study demonstrates a conceptual framework to better understand and integrate the range of spatial and temporal variation associated with the best available reference sites. It also illustrates a practical tool for statistically defining reference sites and for measuring restoration success in continually changing conditions that should be widely applicable to other ecosystems and restoration goals.

Bird assemblage response to restoration of fire-suppressed longleaf pine sandhills.

The ecological restoration of fire-suppressed habitats may require a multifaceted approach. Removal of hardwood trees together with reintroduction of fire has been suggested as a method of restoring fire-suppressed longleaf pine (Pinus palustris) forests; however, this strategy, although widespread, has not been evaluated on large spatial and temporal scales. We used a landscape-scale experimental design to examine how bird assemblages in fire-suppressed longleaf pine sandhills responded to fire alone or fire following mechanical removal or herbicide application to reduce hardwood levels. Individual treatments were compared to fire-suppressed controls and reference sites. After initial treatment, all sites were managed with prescribed fire, on an approximately two- to three-year interval, for over a decade. Nonmetric multidimensional scaling ordinations suggested that avian assemblages on sites that experienced any form of hardwood removal differed from assemblages on both fire-suppressed sites and reference sites 3-4 years after treatment (i.e., early posttreatment). After >10 years of prescribed burning on all sites (i.e., late posttreatment), only assemblages at sites treated with herbicide were indistinguishable from assemblages at reference sites. By the end of the study, individual species that were once indicators of reference sites no longer contributed to making reference sites unique. Occupancy modeling of these indicator species also demonstrated increasing similarity across treatments over time. Overall, although we documented long-term and variable assemblage-level change, our results indicate occupancy for birds considered longleaf pine specialists was similar at treatment and reference sites after over a decade of prescribed burning, regardless of initial method of hardwood removal. In other words, based on the response of species highly associated with the habitat, we found no justification for the added cost and effort of fire surrogates; fire alone was sufficient to restore these species.

Reptile assemblage response to restoration of fire-suppressed longleaf pine sandhills.

Measuring the effects of ecological restoration on wildlife assemblages requires study on broad temporal and spatial scales. Longleaf pine (Pinus palustris) forests are imperiled due to fire suppression and subsequent invasion by hardwood trees. We employed a landscape-scale, randomized-block design to identify how reptile assemblages initially responded to restoration treatments including removal of hardwood trees via mechanical methods (felling and girdling), application of herbicides, or prescribed burning alone. Then, we examined reptile assemblages after all sites experienced more than a decade of prescribed burning at two- to thee-year return intervals. Data were collected concurrently at reference sites chosen to represent target conditions for restoration. Reptile assemblages changed most rapidly in response to prescribed burning, but reptile assemblages at all sites, including reference sites, were generally indistinguishable by the end of the study. Thus, we suggest that prescribed burning in longleaf pine forests over long time periods is an effective strategy for restoring reptile assemblages to the reference condition. Application of herbicides or mechanical removal of hardwood trees provided no apparent benefit to reptiles beyond what was achieved by prescribed fire alone.

Toward a mechanism for eastern North American forest mesophication: differential litter drying across 17 species.

Long-term fire exclusion has altered ecological function in many forested ecosystems in North America. The invasion of fire-sensitive tree species into formerly pyrogenic upland forests in the southeastern United States has resulted in dramatic shifts in surface fuels that have been hypothesized to cause reductions in plant community flammability. The mechanism for the reduced flammability or "mesophication" has lacked empirical study. Here we evaluate a potential mechanism of reduced flammability by quantifying moisture retention (response time and initial moisture capacity) of foliar litter beds from 17 southeastern tree species spanning a wide range of fire tolerance. A k-means cluster analysis resulted in four species groups: a rapidly drying cluster of eight species; a five-species group that absorbed little water but desorbed slowly; a two-species group that absorbed substantial moisture but desorbed rapidly; and a two-species cluster that absorbed substantial moisture and dried slowly. Fire-sensitive species were segregated into the slow moisture loss clusters while fire-tolerant species tended to cluster in the rapid drying groups. Principal-components analysis indicated that several leaf characteristics correlated with absorption capacity and drying rates. Thin-leaved species with high surface area : volume absorbed the greatest moisture content, while those with large, curling leaves had the fastest drying rates. The dramatic shifts in litter fuels as a result of invasion by fire-sensitive species generate a positive feedback that reduce the windows of ignition, thereby facilitating the survival, persistence, and continued invasion of fire-sensitive species in the uplands of the southeastern United States.

Interactions among overstory structure, seedling life-history traits, and fire in frequently burned neotropical pine forests.

Fire-dependent pine forests in the Caribbean Basin cover extensive areas in the coastal plain of the Caribbean Sea and Gulf of Mexico and on several islands in the Bahamas Archipelago, Cuba, Hispaniola, and the Honduran Bay islands. These forests are high in conservation value but, unfortunately, remain mostly unprotected. Moreover, even though they are fire dependent, the use of fire for forest management often suffers from poor public perception and is prohibited by law in several countries. In this paper, we describe the fundamental links among fire, forest regeneration, and forest persistence in these ecosystems. We identify two general strategies based on the presence or absence of pine seedling adaptations for fire survival and describe management implications of these two strategies. We also introduce conceptual models describing fire, forest structure, and regeneration strategy linkages.

Forest floor depth mediates understory vigor in xeric Pinus palustris ecosystems.

Longleaf pine (Pinus palustris) woodlands and savannas are among the most frequently burned ecosystems in the world with fire return intervals of 1-10 years. This fire regime has maintained high levels of biodiversity in terms of both species richness and endemism. Land use changes have reduced the area of this ecosystem by >95%, and inadequate fire frequencies threaten many of the remnants today. In the absence of frequent fire, rapid colonization of hardwoods and shrubs occurs, and a broad-leaved midstory develops. This midstory encroachment has been the focus of much research and management concern, largely based on the assumption that the midstory reduces understory plant diversity through direction competition via light interception. The general application of this mechanism of degradation is questionable, however, because midstory density, leaf area, and hardwood species composition vary substantially along a soil moisture gradient from mesic to extremely xeric sites. Reanalysis of recently reported data from xeric longleaf pine communities suggests that the development of the forest floor, a less conspicuous change in forest structure, might cause a decline in plant biodiversity when forests remain unburned. We report here a test of the interactions among fire, litter accumulation, forest floor development, and midstory canopy density on understory plant diversity. Structural equation modeling showed that within xeric sites, forest floor development was the primary factor explaining decreased biodiversity. The only effects of midstory development on biodiversity were those mediated through forest floor development. Boundary line analysis of functional guilds of understory plants showed sensitivity to even minor development of the forest floor in the absence of fire. These results challenge the prevailing management paradigm and suggest that within xeric longleaf pine communities, the primary focus of managed fire regime should be directed toward the restoration of forest floor characteristics rather than the introduction of high-intensity fires used to regulate midstory structure.

The effects of fire regime on legume reproduction in longleaf pine savannas: is a season selective?

The biodiversity of fire-dependent ecosystems is increasingly threatened by habitat fragmentation and fire suppression. Reducing species loss requires that salient features of natural fire regimes be incorporated into managed regimes. Lightning-season burns have been emphasized as the critical component of disturbance regimes that maintain native biodiversity within endangered longleaf pine savannas, the most diverse plant community in North America. Over evolutionary time, lightning-season fire is thought to have exerted strong selection pressure on plant pollinator interaction, flower production, and seedling establishment. In this study, season of burn had no effect on pollination activity of native legumes. Contrary to the conventional paradigm, we found a wide range of reproductive responses among dominant legumes in response to the season of burn treatments, suggesting that a variable fire season, rather than a single season of burn, is appropriate to maintain a greater variety of native species. We propose that varying the components of fire regimes, rather than selecting a particular fire regime, is likely to be important to conserve biodiversity in this and other fire-dependent communities.