Frequent Low-intensity Fires Research Articles

Wildfire-vegetation-climate-human interactions in the central Taiwan region during 17.3–2.0 cal kyr BP, inferred from sediments of Toushe Basin

In this study, a long-term relationship amongst wildfire, vegetation, climate, and human in the central Taiwan region, spanning from 17.3 to 2.0 cal kyr BP, is explored. To investigate this relationship, multiple proxy approaches were employed, including charcoal (CHAR and fire frequency), pollen data (Liew et al., 2006), magnetic susceptibility, and the carbon isotopic composition of organic carbon, along with the elemental ratio of total organic carbon to total nitrogen in the core sediments of the Toushe Basin, central Taiwan. From 17.3 to 14.5 cal kyr BP, the Toushe Basin experienced drier climate conditions, leading to high-intensity fires and reduced area of wetland. Between 14.5 and 11.5 cal kyr BP, a wetter climate corresponded with low-intensity fires and an increase in tree prevalence. From 11.5 to 5.0 cal kyr BP, the climate remained relatively stable with peaks of extreme wet phases, with frequent low-intensity fires promoting herbaceous plant growth. During the period from 5.0 to 2.0 cal kyr BP, wetter conditions persisted, but increased human activities likely intensified high-intensity fires, resulting in reduced tree and herbaceous plant populations and an expansion of wetlands, leading to complex impacts on the region's ecological dynamics. The pollen diversity index (PDI) and pollen richness index (PRI) showed that ecological diversity in the Toushe Basin decreased with high-intensity fires and increased with low-intensity fires, highlighting the complex interactions between natural and human factors during 17.3–2.0 cal kyr BP. In addition, this study showed a strong link between El-Niño events and increased wildfire activity in the Toushe Basin. El-Niño conditions, with reduced rainfall and increased dryness, likely heightened wildfire frequencies and intensity by lowering moisture levels and ignition thresholds. Increased lightning during these periods further exacerbated wildfires. This connection highlights El-Niño ‘s role in driving high-intensity fires, impacting the region's vegetation and ecological dynamics.

Terrestrial Laser Scan Metrics Predict Surface Vegetation Biomass and Consumption in a Frequently Burned Southeastern U.S. Ecosystem

Fire-prone landscapes found throughout the world are increasingly managed with prescribed fire for a variety of objectives. These frequent low-intensity fires directly impact lower forest strata, and thus estimating surface fuels or understory vegetation is essential for planning, evaluating, and monitoring management strategies and studying fire behavior and effects. Traditional fuel estimation methods can be applied to stand-level and canopy fuel loading; however, local-scale understory biomass remains challenging because of complex within-stand heterogeneity and fast recovery post-fire. Previous studies have demonstrated how single location terrestrial laser scanning (TLS) can be used to estimate plot-level vegetation characteristics and the impacts of prescribed fire. To build upon this methodology, co-located single TLS scans and physical biomass measurements were used to generate linear models for predicting understory vegetation and fuel biomass, as well as consumption by fire in a southeastern U.S. pineland. A variable selection method was used to select the six most important TLS-derived structural metrics for each linear model, where the model fit ranged in R2 from 0.61 to 0.74. This study highlights prospects for efficiently estimating vegetation and fuel characteristics that are relevant to prescribed burning via the integration of a single-scan TLS method that is adaptable by managers and relevant for coupled fire–atmosphere models.

Carbon starvation is absent regardless of season of burn in Liquidambar styraciflua L.

In the Southeastern United States, the function and stability of pine ecosystems depends on frequent low intensity fires. One of the critical effects of frequent fire is the inhibition of hardwood competition by removing above-ground stems (top-kill). Previous long-term studies have correlated early growing season burns with higher understory mortality when compared to dormant season burns. Seasonal differences in mortality from insufficient carbon reserves in roots after leaf flush were posited as the mechanism but this was never explicitly tested. In this experiment, we burned two-year-old pot-grown sweetgums (Liquidambar styracifluaL.) in the dormant season (February), growing season (May), and compared their physiological responses to plants that were unburned. By the end of the first post-burn growing season, mortality was higher after the dormant season burn than the growing season burn and unburned trees had nearly twice the mass of either the burned treatments. Tissue starch concentrations were similar across all treatments highlighting the resiliency of sweetgum carbon stocks to recover from above-ground stem removal regardless of season. We showed the critical importance of top-killing stems regardless of season of burn. Our results further suggest that dormant season fires can be more lethal to young sweetgum, possibly due to susceptibility of tender new shoots to freezing damage.

The reconstruction of past forest dynamics over the last 13,500 years in SW Sweden

Evidence for unbroken continuity of tree taxa over the last c. 13,500 years is presented from a biodiversity ‘hotspot’ nature reserve in south-west Sweden. Forest composition, continuity, fire and disturbance events are reconstructed using palaeoecological methods. A lake record reveals that Pinus sylvestris, Betula spp., Salix spp., Populus tremula and Hippophae rhamnoides were the initial trees scattered in a semi-open, steppe environment. This developed into forest with Pinus, Betula, Corylus, Alnus, Ulmus and Populus with evidence for frequent fires. Deciduous trees became more significant as fires became less frequent and Quercus, Fraxinus and Tilia expanded. Fire frequencies increased again in the Bronze Age probably associated with anthropogenic use of the forest, and the first Fagus sylvatica pollen was recorded. Burning continued through the Iron Age, but charcoal is briefly absent for a period often referred to as the ‘Late Iron Age Lull’. The forest re-expanded with successions involving Juniperus, but with an altered composition from the earlier mixed deciduous community, to one dominated by Fagus. This is coincident with the first pollen records for Picea abies. The early Holocene mixed forest with frequent low-intensity fires is potentially associated with the greatest diversity of red-listed insect species. Forest continuity and the fragmented reservoir populations of old deciduous trees in the Fagus-dominated forest today are likely to have been critical in preserving the present-day, species-rich, rare epiphytic flora, wood-inhabiting fungi and invertebrate communities. As many of these forest fragments may become more vulnerable with future climate change, tree diversity with some disturbance may become essential for survival of the endangered saproxylic species.

The role of holm oak edges in the control of disturbance and conservation of plant diversity in fire-prone landscapes

Holm oak (Quercus rotundifolia) woodlands have been able to persist in fire-prone landscapes in northern Portugal where they seem to play a relevant role in community and landscape level patterns and processes, namely in the control of disturbance events and in the maintenance of biodiversity. In this research we tested the following hypotheses: (i) holm oak woodlands affect fire behavior, mainly by disrupting and delaying fire spread; (ii) disruption and delaying of fire spread is due to fire extinction at the edge of woodlands; and (iii) plant diversity is favored by frequent low-intensity fires as a result of the control of fire spread by holm oak woodland edges. We sampled herbaceous, shrub and tree structure and herbaceous plant composition across edges of holm oak woodlands with known recent contact with fire. Using BehavePlus we converted structural data into fuel models and simulated fire behavior for the locations sampled along the exterior–interior gradient of the woodlands. Our results indicate that holm oak woodlands play a relevant role in the control of fire spread and in the maintenance of plant diversity. We found differences in simulated maximum rate of fire spread (ROS) and fireline intensity (FLI) between outside and inside woodlands as well as a 100% probability of fire extinction when fuel moisture was >12% and wind speed at 10m aboveground was <40kmh−1, supporting the self-extinction hypothesis. We found also that richness and abundance of herbaceous plants varied between the interior and exterior of the woodland, with higher values outside the woodland, and that plant composition along the interior–exterior gradient had distinct patterns. These variables and the pattern of plant distribution were also significantly affected by time since last fire event. We conclude that holm oak woodlands affect fire behavior by interrupting fire spread at the edge zone and that fire-created edges affect herbaceous plant species richness, abundance and distribution by maintaining habitat diversity, which increased plant diversity at patch and landscape levels. Based on our results and other evidence, we developed a conceptual model of the dynamics of fire, landscape structure, and plant diversity in fire-prone landscapes where holm oak patches persist.

Physiological responses of ponderosa pine in western Montana to thinning, prescribed fire and burning season

Low-elevation ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) forests of the northern Rocky Mountains historically experienced frequent low-intensity fires that maintained open uneven-aged stands. A century of fire exclusion has contributed to denser ponderosa pine forests with greater competition for resources, higher tree stress and greater risk of insect attack and stand-destroying fire. Active management intended to restore a semblance of the more sustainable historic stand structure and composition includes selective thinning and prescribed fire. However, little is known about the relative effects of these management practices on the physiological performance of ponderosa pine. We measured soil water and nitrogen availability, physiological performance and wood radial increment of second growth ponderosa pine trees at the Lick Creek Experimental Site in the Bitterroot National Forest, Montana, 8 and 9 years after the application of four treatments: thinning only; thinning followed by prescribed fire in the spring; thinning followed by prescribed fire in the fall; and untreated controls. Volumetric soil water content and resin capsule ammonium did not differ among treatments. Resin capsule nitrate in the control treatment was similar to that in all other treatments, although burned treatments had lower nitrate relative to the thinned-only treatment. Trees of similar size and canopy condition in the three thinned treatments (with and without fire) displayed higher leaf-area-based photosynthetic rate, stomatal conductance and mid-morning leaf water potential in June and July, and higher wood radial increment relative to trees in control units. Specific leaf area, mass-based leaf nitrogen content and carbon isotope discrimination did not vary among treatments. Our results suggest that, despite minimal differences in soil resource availability, trees in managed units where basal area was reduced had improved gas exchange and growth compared with trees in unmanaged units. Prescribed fire (either in the spring or in the fall) in addition to thinning, had no measurable effect on the mid-term physiological performance and wood growth of second growth ponderosa pine.

Wildfire-resistant biological soil crusts and fire-induced loss of soil stability in Palouse prairies, USA

Frequent low-intensity fires are a natural component of the ecology of the Palouse prairies of northwestern North America. To study the effects of fire upon biological soil crusts (BSCs) occurring in these grasslands, we sampled three burned (in 2000) sites and three unburned sites in the Hell’s Canyon area (OR, USA) ∼1 year post-fire. We measured vascular plant and BSC cover, soil microbe pigmentation, texture and chemistry, and soil surface physical properties (stability and rugosity). Festuca idahoensis was two times more abundant in unburned plots ( P=0.0006), and vascular plant and litter cover were generally higher in unburned plots. At the community scale, there was no difference in the lichen and moss species composition, suggesting much less drastic effects of fire on BSCs than reported in other systems. Soil surface stability (measured using slake value) was significantly lower in burned sites than unburned sites (median value=5 versus 6, P=0.008), a result which is likely due to the greater density of lichens and mosses encountered in the unburned plots. Soil microbe pigmentation was lower in burned plots ( P=0.03), suggesting that the biomass of photosynthetic microbes had decreased; however, the presence of intra- and extracellular pigments in burned soils indicates that microorganisms were not eradicated. Pigments most strongly associated with cyanobacteria were more abundant in unburned sites, suggesting that cyanobacteria may have been more strongly impacted by the fire than other BSC components. Composition of nutrients and surface rugosity did not differ significantly between treatments. We hypothesize that Palouse prairie soil crusts are relatively resistant to wildfire because of low fire intensity and their occupation of space away from the vascular plant fuel load.

Re-introducing fire at the urban/wild-land interface: planning for success

The application of fire in the southern United States continues to increase in complexity due to urban sprawl, air quality issues and regulatory constraints. Many sites suffer from unnaturally high fuel accumulations dueto decades of fire exclusion. The loss of habitat to urbanization and successional changes resulting from the absence of fire increases the importance of restoring and maintaining those remaining acres. The wild-land/urban interface case study we discuss herein includes several fire-adapted plant communities ranging in required fire regime from frequent low-intensity fires to infrequent high-intensity stand replacement fires. This area has experienced extended fire-free periods and includes tightly packed homes in subdivisions developed with no consideration of the potential fOr wild-land fire. Additional smoke-sensitive areas include schools and heavily travelled highways. Such worst-case scenarios exponentially increase the challenges/risks facing fire managers. This case study thus illustrates many of the complex societal issues and technical challenges facing fire managers when planning and conducting restoration burns in the wild-land/urban interface. In fact, it reinforces the notion that, when burning in the wild-land/urban interface, executing the burn often requires less effort than the planning, co-operation and co-ordination necessary prior to ignition.

Changes in southwestern Tasmanian fire regimes since the early 1800s

There have been major changes in the fire regime of southwestern Tasmania over the past 170 years. The fire regime has changed from an Aboriginal fire regime of frequent low-intensity fires in buttongrass moorland (mostly in spring and autumn) with only the occasional high-intensity forest fire, to the early European fire regime of frequent high-intensity fires in all vegetation types, to a regime of low to medium intensity buttongrass moorland fires and finally to the current regime of few fires. These changes in the fire regime resulted in major impacts to the region's fire-sensitive vegetation types during the early European period, while the current low fire frequency across much of southwestern Tasmania has resulted in a large proportion of the region's fire-adapted buttongrass moorland being classified as old-growth. These extensive areas of old-growth buttongrass moorland mean that the potential for another large-scale ecologically damaging wildfire is high and, to avoid this, it would be better to re-introduce a regime oflow-intensity fires into the region.

Regeneration of the Shrub Gaylussacia baccata and Associated Species After Low-Intensity Fire in an Atlantic Coastal Plain Forest

The upland shrub community of the New Jersey Pine Barrens maintains strong compositional and structural stability despite frequent low-intensity fires. To determine the mechanisms by which individual species respond to fire, regrowth of vegetation was monitored for 3 years after fire at a burned site, and at a nearby site that had not been burned. At both sites, experimental treatments simulated various physical aspects of fire, including clipping of stems, removal of leaf litter, and application of fertilizer. Both fire and clipping were followed by multiple resprouting and enhanced growth of the dominant shrub, Gaylussacia baccata, suggesting that sprout growth is controlled by correlative inhibition of adventitious buds. By contrast, neither fertilizer nor litter removal had a significant impact on G. baccata. Other species were only observed at very low numbers, and appeared to act independently of either fire or experimental treatments. Recruitment from seed was not observed in any treatment at either site, despite reports of a large soil seed bank. Thus, compositional stability is maintained through a large reserve of dormant buds on the dominant species, and through a lack of opportunities for associate species. This stability is precarious, however, depending strongly on fire frequency and intensity.

Influence of fire on growth rate, mortality, and butt damage in Mediterranean forest of Western Australia

We investigated the short-term (10 years) and long-term (30–50 years) effect of frequent low-intensity fires, and the long-term effect of single high-intensity fires, on growth rate, mortality and incidence of fire-scar formation in jarrah, the principal timber species of Western Australia. The study was restricted to trees with diameter underbark (at the first measurement) of at least 20 cm. Frequent low-intensity fires were neither detrimental nor beneficial to growth rate and survival of jarrah, and did not appreciably affect incidence of fire-scars. High-intensity fires increased mortality slightly, caused more fire-scars to form, but had differing effects on growth rate. In two forest types, rate of growth was significantly improved but in one other it was significantly reduced.