The effect of wildfire on scattered trees, ‘keystone structures’, in agricultural landscapes

Scattered trees are considered ‘keystone structures’ in many agricultural landscapes worldwide because of the disproportionate effect they have on ecosystem function and biodiversity. Populations of these trees are in decline in many regions. Understanding the processes driving these declines is crucial for better management. Here, we examine the impact of wildfire on populations of this keystone resource. We examined 62 observation plots affected by wildfire and matched with 62 control observation plots where fire was absent. Counts of scattered trees were conducted pre‐fire in 2005 and repeated post‐fire in 2011. Changes in populations were compared between the control and fire‐affected observation plots. Our results show wildfire had a significant local impact, with an average decline of 19.9% in scattered tree populations on burned plots. In contrast, scattered trees increased on average by 5.3% in the control observation plots. The impact of wildfire was amplified (as revealed by greater percentage tree losses) by larger wildfires. Wildfire effects on scattered tree populations are of concern, given a background of other (usually) chronic stressors (often associated with agriculture) and that the frequency and intensity of wildfire are predicted to increase in many landscapes.     

Sampling grain influences trends in vegetation composition and diversity with time since fire in Australian heathland

Scale‐dependency of pattern and process is well‐understood for many ecological communities; however, the influence of spatial scale (sampling grain) in detecting temporal change in communities is less well‐understood. The temperate lowland heathlands of south‐east Australia are one of the most fire‐prone ecosystems on earth. Despite the extensive literature documenting the effect of time since fire on heathlands, we know little about how sampling grain influences trends in vegetation variables over time, and whether these trends are scale‐dependent. Using 3500 ha of heathland in the Gippsland Lakes Coastal Park, south‐east Australia, we investigated how above‐ground species composition and diversity, and trends in these variables with increasing time since fire, were influenced by sampling grain (1 m², 10 m², 100 m², 900 m², 1 ha, 4 ha). Sampling grain influenced patterns detected in vegetation variables and in some instances, significantly affected their relationship with time since fire. Richness decreased with time since fire, with mean richness decreasing at three of the four grains, while total richness decreased at half of the sampled grains. Evenness (J) decreased with increasing time since fire for all grains except 1 m². The decline in diversity (H) with time since fire appeared to be independent of scale, as all grains decreased significantly with increasing time since fire. Community heterogeneity demonstrated a weak response to time since fire across most grains. Changes in composition among young (0–6 years since fire), intermediate (9–19 years) and old (23–27 years) sites were dependent on sampling grain, with all grains exhibiting significant differences in composition, apart from the 1 m² grain and the 100 m² grain (presence/absence data). Overall, species composition, richness, evenness, diversity and community heterogeneity were dependent on the scale at which the vegetation was sampled. In addition, trends in many of these vegetation variables with increasing time since fire were scale‐dependent. This work provides strong evidence that sampling at multiple grains contributes substantially to understanding pattern and process in heathlands.     

Defining the fire trap: Extension of the persistence equilibrium model in mesic savannas

Mesic savannas are dominated by trees that are strong resprouters caught in a frequent fire trap. Persistence within this fire trap has been described by a resprout curve of SizeNext ~ f(Pre‐fire size), defined by the Michaelis‐Menten function. A key feature of this resprout curve is a stable persistence equilibrium that represents the size of individual plants upon which a population will converge over successive inter‐fire time steps under a given fire regime. Here, we contend that such a resprout curve does not adequately describe resprout tree dynamics in frequently burnt mesic savannas because it is constrained to an asymptote. We propose a new framework for modelling the resprout curve, which recognizes that local environmental stochasticity and growth patterns can interact to change the growth response function entirely, and thus more readily reflect the range of feasible resprout responses. Importantly, we define an unstable equilibrium representing the size above which individuals have escaped the fire trap and explore mechanisms that can shift an individual from persistence to escape. Through a case study from northern Australia, we confirm that our framework provides a simple yet practical approach to defining these critical aspects of savanna tree growth dynamics: persistence and escape.     

Effects of experimental season of prescribed fire and nutrient addition on structure and function of previously grazed grassland

Aims Understanding the drivers of grassland structure and function following livestock removal will inform grassland restoration and management. Here, we investigated the effects of fire and nutrient addition on structure and function in a subtropical semi-native grassland recently released from grazing in south-central Florida. We examined responses of soil nutrients, plant tissue nutrients, biomass of live, standing dead and litter, and plant species composition to experimental annual prescribed fire applied during different seasons (wet season vs. dry season), and nutrient additions (N, P and N + P) over 9 years.Methods Experimental plots were set up in a randomized block split-plot design, with season of prescribed fire as the main treatment and nutrient addition as the subplot treatment. Species cover data were collected annually from 2002 to 2011 and plant tissue and plant biomass data were collected in 2002–2006 and 2011. Soil nutrients were analyzed in 2004, 2006 and 2011.Important findings Soil total phosphorus (P) levels increased substantially with P addition but were not influenced by prescribed fire. Addition of P and N led to increased P and N concentrations in live plant tissues, but prescribed fire reduced N in live tissue. Levels of tissue N were higher in all plots at the beginning of the experiment, an effect that was likely due to grazing activity prior to removal of livestock. Plant tissue N steadily declined over time in all plots, with annually burned plots declining faster than unburned plots. Prescribed fire was an important driver of standing dead and litter biomass and was important for maintaining grass biomass and percent cover. Nutrient addition was also important: the addition of both N and P was associated with greater live biomass and woody forbs. Removal of grazing, lack of prescribed fire, and addition of N + P led to a reduction of grass biomass and a large increase in biomass of a woody forb. Annual prescribed fire promoted N loss from the system by reducing standing dead and litter, but maintained desirable biomass of grasses.     

Restoring plant species diversity and community composition in a ponderosa pine-bunchgrass ecosystem

Monitoring of ecological restoration treatments often focuses on changes in community structure and function. We suggest that long-term changes in community composition also need to be explicitly considered when evaluating the success of restoration treatments. In 1992, we initiated an experiment in a ponderosa pine-bunchgrass ecosystem to evaluate responses to restoration treatments: (a) thinning the overstory vegetation (‘thinning’), (b) thinning plus forest floor manipulation with periodic prescribed burning (‘composite’), and (c) untreated ‘control.’ Treatments were further stratified by forest patch type: presettlement tree clumps (trees that established prior to the onset of fire exclusion in 1876), patches of retained postsettlement trees, patches where all postsettlement trees were removed, and remnant grass openings. Species richness did not differ among treatments for 10 years, but was highest in the composite treatment in 11th and 12th year after initial treatment. Community composition diverged among treatments 5 years after initial treatment, and compositional changes were greatest in the composite treatment. Species richness and composition differed among patch types prior to treatment. Remnant grass patches were the most diverse and presettlement patches were the least diverse. Following treatment, species richness in the postsettlement removed and retained patches, gradually approached levels found in remnant grass patches. Compositional differences among patch types changed a little by 2005. Species richness at the 2 m² scale increased only where the overstory was thinned and the understory was burned. However, these changes may not be detectable for many years, and can vary temporally in response to events such as severe droughts. Nonnative species establishment may be reduced by scheduling longer burn intervals or by refraining from burning where fuel loads are not hazardous, though these options may hinder goals of increasing diversity. Restoring species diversity and community composition continues to be more difficult than restoring ecosystem structure and function.     

Ecological Engineering by a Mega-Grazer: White Rhino Impacts on a South African Savanna

Herbivory can change the structure and spatial heterogeneity of vegetation. We ask whether all species of grazers in a savanna ecosystem can have this effect or whether megaherbivores (>1000 kg) have a ‘special’ role that cannot be replicated by other species of grazers. We performed a replicated landscape scale experiment that examined the effects of White Rhino on the grass sward, on other species of grazing mammals and on the movement of fire through the landscape. White Rhino maintained short grass (‘lawn’) patches in mesic areas (∼750 mm pa) with increases in grass sward height when they were removed. Other species of grazers were unable to maintain short grass communities when White Rhino were removed. In semi-arid areas (∼600 mm pa) other, smaller grazers were able to maintain short grass communities in the absence of White Rhino and sward height did not increase. White Rhino removals affected fire by increasing fuel loads and fuel continuity. This resulted in larger, less patchy fires. We propose that the White Rhino acts as an influential ecosystem engineer, creating and maintaining short grass swards, which alter habitat for other grazers and change the fire regime. These results indicate the existence of context-dependent facilitation between White Rhino and other grazers in mesic, but not in semi-arid, savannas. Such top down effects on the ecosystem may have been much more widespread before the extinction of large grazers in the Pleistocene.     

BAER Soil Burn Severity Maps Do Not Measure Fire Effects to Vegetation: A Comment on Odion and Hanson (2006)

Abstract We comment on a recent Ecosystem paper by Odion and Hanson (Ecosystems 9:1177–1189, 2006), in which the authors claim that high severity fire is rare in the Sierra Nevada under current conditions. Odion and Hanson’s results are predicated on BAER soil burn severity maps, which are based primarily on fire effects to soil, not vegetation. Odion and Hanson, and we fear others as well, are misinformed as to the nature of the BAER severity mapping process, and proper applications of BAER soil burn severity maps. By comparing the BAER soil burn severity maps to a true vegetation burn severity measure (RdNBR) calibrated by field data, we show that the area in the high soil burn severity class for the three fires analyzed by Odion and Hanson is substantially less than the area of stand-replacing fire, and that BAER maps—especially hand-derived maps such as those from two of the three fires—also greatly underestimate the heterogeneity in vegetation burn severity on burned landscapes. We also show that, contrary to Odion and Hanson’s claims, Fire Return Interval Departure (FRID) is strongly correlated with fire severity in conifer stands within the perimeter of the McNally Fire.     

The Effect of Fire Reintroduction on Endemic and Rare Plants of a Southeastern Glade Ecosystem

Open habitats dominated by herbaceous plants on thin, rocky soils occur within the forests of eastern North America. Although these habitats vary in origin, structure, geology, and species composition, all contribute greatly to regional biodiversity by harboring endemic and/or rare plants. Little is known about how disturbances affect plant populations in these ecosystems. Fire once was a frequent natural disturbance in the Ketona dolomite glades of Alabama, an ecosystem harboring eight endemic taxa and numerous other species of conservation concern. We designed an experiment to determine how the reintroduction of fire into the glades and surrounding longleaf pine forests affects populations of rare glade plant species. Experimental and control plots were established within the glades. Experimental plots were burned in April 2004, and all plots were surveyed during two subsequent growing seasons (2004 and 2005). Populations of three of 14 species of conservation concern declined significantly after the initial fire but recovered the next year. Among other herbaceous species, only five and two differed in population size in 2004 and 2005, respectively. In 2004, more species were more abundant in control than burned plots, but this difference was not detected in 2005. Multivariate community‐level analyses of species presence–absence suggested that the effects of fire were negligible by the 2005 survey. Populations of young trees that had invaded the glades declined dramatically as a result of treatment fires. These results suggest that the reintroduction of fire will not harm glade species and may help prevent encroachment of the surrounding forest.     

Simulating climate change impacts on fire frequency and vegetation dynamics in a Mediterranean-type ecosystem

The impacts of climate change on Mediterranean‐type ecosystems may result from complex interactions between direct effects on water stress and subsequent modifications in flammability and fire regime leading to changes in standing biomass and plant species composition. We analysed these interrelations through a simulation approach combining scenarios of climate change developed from GCM results and a multispecies functional model for vegetation dynamics, SIERRA. A fire risk procedure based on weekly estimates of vegetation water stress has been implemented. Using climate data from 1960 to 1997, simulations of a typical maquis woodland community have been performed as baseline and compared with two climate scenarios: a change in the rainfall regime alone, and changes in both rainfall and air temperature. Climate changes are defined by an increase in temperature, particularly in summer, and a change in the rainfall pattern leading to a decrease in low rainfall events, and an increase in intense rainfall events. The results illustrate the lack of drastic changes in the succession process, but highlight modifications in the water budget and in the length of the drought periods. Water stress lower than expected regarding statistics on the current climate is simulated, emphasizing a long‐term new equilibrium of vegetation to summer drought but with a higher sensibility to rare events. Regarding fire frequency, climate changes tend to decrease the time interval between two successive fires from 20 to 16 years for the maquis shrubland and from 72 to 62 years in the forested stages. This increase in fire frequency leads to shrub‐dominated landscapes, which accentuates the yield of water by additional deep drainage and runoff.