Low-intensity Fire Regime Research Articles

Range-Wide Assessment of Recent Longleaf Pine (Pinus palustris Mill.) Area and Regeneration Trends

Longleaf pine (Pinus palustris Mill.) is a conifer historically associated with an open forest ecosystem that extended across much of the coastal plain of the Southeastern United States. It now exists mainly in isolated fragments following the conversion of forests and the long-term disruption of the low-intensity fire regime upon which the species depends. Recent decades have seen efforts to restore longleaf pine forests by government and private landowners. This was reflected in analyses of national forest inventory data during two time periods, ca. 2009–2015 and 2016–2021, that showed increases in the estimated number of longleaf pine trees, the area of the longleaf pine forest type, and the number and area of planted longleaf pine, along with growth in mean plot-level longleaf pine carbon and importance value. At the same time, we found a decrease in the overall forest area containing longleaf pine, manifested across a variety of other forest types. These results point to a dynamic through which forests dominated by longleaf pine are becoming more widespread via restoration, while forests in which the species is a less important component are transitioning to other forest types or land uses. We also detected a decrease over time in the estimated number of longleaf seedlings across most states and forest types and a decline in naturally regenerated longleaf pine. To further assess regeneration trends in longleaf pine, we calculated the estimated proportion of small trees (seedlings and saplings) for the entire species and for seed zone sub-populations. We found a species-wide decrease in the proportion of small trees, from 82.1 percent to 75.1 percent. This reduction was most pronounced along the edges of the species distribution and could indicate less sustainable levels of regeneration in some areas. These results underscore the challenges of facilitating natural regeneration in this important species.

Consistently heterogeneous structures observed at multiple spatial scales across fire-intact reference sites

Yellow pine and mixed-conifer (YPMC) forests of California’s Sierra Nevada have experienced widespread fire suppression for over a century, resulting in ingrowth and densification of trees, heavy fuel accumulation, and shifts in species composition. Under warmer and drier climates, these forests are primed for stand-replacing fires and severe drought mortality, requiring management interventions to improve their resilience and mitigate future impacts. Observations from functioning frequent-fire systems (e.g., contemporary reference sites) can provide key insights about pattern-process relationships in fire-intact systems, which can be used to inform regional management efforts. In this study, we used airborne lidar data to quantify and compare forest structure at multiple spatial scales between contemporary reference sites (i.e., forests with a restored frequent, low-intensity fire regime) and control sites (i.e., typical fire-suppressed forests). We evaluated structures at the neighborhood- (∼1 ha), site- (∼100–1,000 ha), and among-site- (∼10,000–100,000 ha) levels. In reference sites, high proportions of individual trees, small clumps of 2–4 trees, and open space formed mostly open canopy structures at the neighborhood-level, and patches of these neighborhood-level structures were arranged in heterogeneous spatial patterns within sites. We observed low variability in site-level structures among reference sites, indicating a stabilizing effect of frequent, low-intensity fire across broad, ecosystem scales. In fire-suppressed control sites, edaphic factors and other non-fire disturbances occasionally produced heterogeneity at the neighborhood- and site-level, but the degree of heterogeneity was not consistent across sites. Structural patterns in contemporary reference sites suggest improved resilience to future disturbances and climate change, and increased provisioning of ecosystem services relative to control sites. We suggest applying these metrics to help inform multi-scale and multi-resource management in Sierra Nevada forests.

Mechanical treatments and prescribed burning can reintroduce low-severity fire in southern Australian temperate sclerophyll forests

The establishment of sustainable, low-intensity fire regimes is a pressing global challenge given escalating risk of wildfire driven by climate change. Globally, colonialism and industrialisation have disrupted traditional fire management, such as Indigenous patch burning and silvo-pastoral practices, leading to substantial build-up of fuel and increased fire risk. The disruption of fire regimes in southeastern Tasmania has led to dense even-aged regrowth in wet forests that are prone to crown fires, and dense Allocasuarina-dominated understoreys in dry forests that burn at high intensities.Here, we investigated the effectiveness of several fire management interventions at reducing fire risk. These interventions involved prescribed burning or mechanical understorey removal techniques. We focused on wet and dry Eucalyptus-dominated sclerophyll forests on the slopes of kunanyi/Mt. Wellington in Hobart, Tasmania, Australia. We modelled potential fire behaviour in these treated wet and dry forests using fire behaviour equations based on measurements of fuel load, vegetation structure, understorey microclimate and regional meteorological data.We found that (a) fuel treatments were effective in wet and dry forests in reducing fuel load, though each targeted different layers, (b) both mechanical treatments and prescribed burning resulted in slightly drier, and hence more fire prone understorey microclimate, and (c) all treatments reduced predicted subsequent fire severity by roughly 2–4 fold. Our results highlight the importance of reducing fuel loads, even though fuel treatments make forest microclimates drier, and hence fuel more flammable.Our finding of the effectiveness of mechanical treatments in lowering fire risk enables managers to reduce fuels without the risk of uncontrolled fires and smoke pollution that is associated with prescribed burning. Understanding the economic and ecological costs and benefits of mechanic treatment compared to prescribed burning requires further research.

Impact of Fire History on the Structure of a Temperate Forest in Northern Mexico

Understanding the ecological role of fire in forests is essential for proper management and conservation programs. The objectives of this study were: (1) to reconstruct the history of fires in a temperate forest in Sierra Madre Occidental; and (2) to interpret the impacts of fire and climate on forest structure. Sixty tree cross-sections with fire scars were analyzed, and descriptive statistics of fire history were generated. Additionally, growth cores were analyzed, and the ages of trees of different diameter categories were calculated. The synchrony between fire history and tree establishment was determined, and precipitation and Palmer Drought Severity Index (PDSI) values were correlated with the number of trees established per year. The presence of 137 fire scars was determined, which allowed the reconstruction of 41 fire events over the period 1855–2019; however, only the period 1940–2015 was used to compare tree recruitment, as tree establishment was detected in this period. The mean fire interval (MFI) was 2.28 years in general, and 12.17 years for extensive fires. As regards vegetation, a continuous recruitment pattern was observed, typical of a frequent low-intensity fire regime, although peak regeneration occurred after extensive fires. The correlation analysis showed that the number of trees established per year was influenced by the wet conditions that occurred in December of the previous year and the dry conditions in September and October of the previous year. This finding demonstrates the historical influence of fire and climate on the structure of the current stand in the study area. Therefore, the present study highlights the importance of including fire in forest management programs, considering the natural fire regime to which the species in this ecosystem are already adapted.

Modeling Post-Fire Tree Mortality Using a Logistic Regression Method within a Forest Landscape Model

Fire is a multi-scale process that is an important component in determining ecosystem age structures and successional trajectories across forested landscapes. In order to address questions regarding fire effects over large spatial scales and long temporal scales researchers often employ forest landscape models which can model fire as a spatially explicit disturbance. Within forest landscape models site-level fire effects are often simplified to the species, functional type, or cohort level due to time or computational resource limitations. In this study we used a subset of publicly available U.S. Forest Service forest inventory data (FIA) to estimate short-term fire effects on tree densities across multiple stem diameter classes in two ecological sections in the central and southern United States. We found that FIA plots where low-intensity fires occurred within the preceding five years in the Ozark Highlands ecological section had significantly reduced stem densities in the two smallest diameter classes and in the Gulf Coastal Plains and Flatwoods fire reduced stem densities in the three smallest diameter classes. Using an independent subset of FIA plots we then parameterized and calibrated a forest landscape model to simulate site-level fire effects using a logistic regression based method and compare the results to previous methods of modeling fire effects. When representative landscapes from both study areas were simulated under a low-intensity fire regime using a forest landscape model the logistic regression probability method of modeling fire effects produced a similar reduction in stem densities while the previous age-cohort method overestimated density reductions across diameter classes. A more realistic representation of fire effects, particularly in low intensity fire regimes, increases the utility of forest landscape models as tools for planning and management.

Seed heat tolerance and germination of six legume species native to a fire-prone longleaf pine forest

Recognition of spatial heterogeneity of fire at fine scales is emerging, particularly in ecosystems characterized by frequent, low-intensity fire regimes. Differences in heat flux associated with variation in fuel and moisture conditions create microsites that affect survivorship and establishment of species. We studied the mechanisms by which fire affects seed germination using exposure of seeds to fire surrogates (moist and dry heat). Tolerance (survival) and germination responses of six perennial, herbaceous legume species common to the fire-prone longleaf pine–wiregrass ecosystem of the southeastern USA were examined the following heat treatments. Moist heat was more effective in stimulating germination than dry heat flux for most species examined. We also compared intrinsic seed properties (relative seed coat hardness, percent moisture, and seed mass) among species relative to their heat tolerance and heat-stimulated germination responses. Seed coat hardness was closely associated with the probability of dry and moist heat-stimulated germination. Variation among species in optimal germination conditions and degree of heat tolerance likely reflects selection for specific microsites among a potentially diverse suite of conditions associated with a low-intensity fire regime. Fire-stimulated germination, coupled with characteristics of seed dormancy and longevity in the soil, likely fosters favorable recruitment opportunities in restoration situations aimed at reintroducing a frequently prescribed burn regime to a relict longleaf pine site. In a restoration context in which externally available seed pool inputs are limited, this regenerative mechanism may provide a significant source of recruitment for vegetative recovery in a post-fire landscape.

Historical high-severity fire patches in mixed-conifer forests

Two ends of the fire regime spectrum are a frequent low-intensity fire regime and an infrequent high-intensity fire regime, but intermediate fire regimes combine high- and low-severity fire over space and time. We used fire-scar and tree-age data to reconstruct fire regime attributes of mixed-conifer and aspen forests in the North Rim area of Grand Canyon National Park, with a goal of estimating patch sizes of historical high-severity fire and comparing them with modern patch sizes. We used three methods based on (i) aspen groves, (ii) even-aged stands, and (iii) inverse distance weighting, to estimate occurrence and patch sizes of historical high-severity fire. Evidence of high-severity fire was common in the 1800s, and high-severity fire years were associated with drought. High-severity fire patch sizes likely ranged from 10−1 to 102 ha. However, the forest is quite young, and we cannot rule out a historical large high-severity fire that could have reinitiated much of the 1400 ha study area. Fire scars, which are indicative of low-severity fire, were also common. Historical fire was likely heterogeneous across the landscape. Maintaining heterogeneity of fire severity, size, and frequency would promote heterogeneity of forest structure and composition and resilience to future disturbances.

The effect of land use on aboveground biomass and soil quality indicators in spontaneous forests and agroforests of eastern Amazonia

This study unites physicochemical indicators of aboveground vegetation, litter layer and topsoil (0–20 cm) in contrasting vegetation types commonly found in the eastern Amazonia. We compare three agroforestry systems (enriched fallows, homegardens and commercial plantations) with three spontaneous forest types (young and old secondary forests and mature rainforests) via one-way ANOVA, linear and non-linear regressions and multivariate analyses. Agroforests had significantly lower understory biomass when compared with young secondary forest. Commercial plantation agroforests had higher topsoil pH and Ca-contents and homegardens had higher K-contents and P-availability hotspots, as revealed by their higher variance and single very high values. Agroforests and spontaneous forests were similar in their litter biomass (both leaves and twigs) and C:N ratio, and in soil organic matter and P contents. The overstory negatively impacted the understory (r 2 = 0.20, p < 0.05) and the understory correlated significantly with the litter layer (r 2 = 0.11, p < 0.07). By contrast, there were no direct relationships between overstory and the litter layer, pointing to a major discontinuity between vegetation and topsoil. Principal component analysis depicted a successional sequence of systems, with homegardens closest to mature rainforests. According to co-inertia analysis, plant biomass was more strongly related to topsoil in spontaneous forests than in agroforests. Altogether, agroforests were similar to mature rainforests in a wide range of variables of the vegetation, litter and topsoil, and co-inertia analysis indicated that agroforestry management can alter this continuum. Our results point to an outstanding position of homegardens in the study region, with higher aboveground biomass and elevated nutrient availability which may have been caused by the traditional sweep-and-burn low-intensity fire regime prevalent throughout Amazonia and beyond.

Fire history of Atlas cedar (Cedrus atlantica Manetti) in Mount Chélia, northern Algeria

The first fire history of the Cedrus atlantica forest of Chélia in northern Algeria is reconstructed from samples of 14 and 12 dead fire-scarred trees from lower and upper elevation stands respectively. Superposed epoch analysis is applied to summarize mean climate conditions before, during, and after sets of fire years in both stands. Prior to 1850, C. atlantica forests in both sites were characterized by high frequency and low intensity fire regimes. The sharp decline in the fire frequency after the mid-nineteenth century coincided with the promulgation of the first laws governing land and forest use in Algeria. Most fire events occurred in the summer, during the latter part of the growing season. No significant relationships were found between an October–June precipitation reconstruction and fire events. This could be due to anthropogenic and topographic factors. The findings of this study provide baseline knowledge on the ecological role of fire in the C. atlantica forest. This information is vital to support ongoing ecosystem management efforts in the region.

Demographic responses of two endemic plants to sandhill restoration on the Lake Wales Ridge1

Balancing land management and restoration goals for ecosystems and for individual species can create challenges. Here we describe the responses of two federally endangered plants to experimental restoration of fire-suppressed xeric longleaf pine/wiregrass (sandhill) habitat on the Lake Wales Ridge in south-central Florida. We compared responses to prescribed fire with (saw & burn treatment) and without (burn-only treatment) prior felling of the oak subcanopy to an untreated control. We conducted a 7-year study of the demography of two endemic plants, scrub buckwheat (Eriogonum longifolium var. gnaphalifolium) and scrub plum (Prunus geniculata). Scrub buckwheat responded positively to the burn treatments, with high survival, positive growth, and increased flowering in both treatments. Likewise, scrub plum had high survival in all treatments, although severely burned, fire-consumed plants were more likely to die. Fire benefits scrub plum by increasing flowering 2–3 years after fire. These results for these two endemic species are consistent with vegetation studies in long-unburned Lake Wales Ridge sandhills in suggesting that chainsaw felling of the subcanopy can be an effective pre-burn treatment and, when followed by fire, can be useful in “speeding up” restoration. The ultimate goal of sandhill restoration, the return of a frequent low-intensity fire regime, will benefit scrub buckwheat and scrub plum. In this case, management for ecosystem goals benefits two of its rare component species.

Vegetation and fire history since the Late Pleistocene from the Trinity Mountains, northwestern California, USA

A 267-cm sediment core spanning the past c. 15 200 cal. yr was recovered from Mumbo Lake, in the Trinity Mountains of northwestern California's Klamath Region. Plant macrofossils and pollen detail local and extra-local vegetation history, while high-resolution charcoal analysis provides details on local fire history. For the first c. 3000 years, climate was colder and drier than present, and supported an open, subalpine parkland vegetation, with low fire frequencies and fuel biomass. From c. 12 100 to 9800 cal. yr BP increasing moisture and soil development led to a woodland community with three new pine species invading the basin. Fire frequencies remained low, but individual fires may have been more intense because of increased fuel loads. Between c. 9800 and 7200 cal. yr BP, climate warmed and dried considerably, allowing for the expansion of oak and other chaparral species. Fire frequencies increased in the early Holocene, but low charcoal accumulation rates suggest a frequent, relatively low-intensity fire regime. From c. 7200 to 3800 cal. yr BP, the climate became cooler and moister again. Many conifer species appeared for the first time, although chaparral species maintained a strong presence. The fire record shows a dramatic increase in charcoal accumulation rates as well as an increase in fire frequency. From c. 3800 cal. yr BP to present, more conifer species enter the record, and abundance of chaparral species gradually diminishes to present levels.

INITIAL CARBON, NITROGEN, AND PHOSPHORUS FLUXES FOLLOWING PONDEROSA PINE RESTORATION TREATMENTS

Southwestern ponderosa pine forests were dramatically altered by fire regime disruption that accompanied Euro-American settlement in the 1800s. Major changes include increased tree density, diminished herbaceous cover, and a shift from a frequent low-intensity fire regime to a stand-replacing fire regime. Ecological restoration via thinning and prescribed burning is being widely applied to return forests to the pre-settlement condition, but the effects of restoration on ecosystem function are unknown. We measured carbon (C), nitrogen (N), and phosphorus (P) fluxes during the first two years after the implementation of a replicated field experiment comparing thinning and composite (thinning, forest floor fuel reduction, and prescribed burning) restoration treatments to untreated controls in a ponderosa pine forest in northern Arizona, USA. Total net primary productivity (260 g C·m−2·yr−1) was similar among treatments because a 30–50% decrease in pine foliage and fine-root production in restored ecosystems was balanced by greater wood, coarse root, and herbaceous production. Herbaceous plants accounted for <20% of total plant C, N, and P uptake in the controls but from 25% to 70% in restored plots. Total plant N uptake was ∼3 g N·m−2·yr−1 in all treatments, but net N mineralization was just one-half and two-thirds of this value in the control and composite restoration, respectively. Element flux rates in controls generally declined more in a drought year than rates in restoration treatments. In this ponderosa pine forest, ecological restoration that emulated pre-settlement stand structure and fire characteristics had a small effect on plant C, N, and P fluxes at the whole ecosystem level because lower pine foliage and fine-root fluxes in treated plots (compared to controls) were approximately balanced by higher fluxes in wood and herbaceous plants.

Life histories of Mediterranean pines

The life history of Spanish pines and their relation to fire as the main disturbance factor in their ecosystems was analysed. The primary ecological attributes studied were the canopy seed bank (onset of cone production, percentage and persistence of serotinous cones), seed and cone morphology, sprouting and bark thickness. Four ecological groups were separated using multivariate cluster analysis and their life-history characteristics are discussed. Serotiny and early flowering in Pinus halepensis and P. pinaster reflect their evader strategy in relation to fire as this character is advantageous to survive frequent crown fires and to attain successful post-fire recruitment. Late flowering and absence of serotinous cones in P. nigra, P. sylvestris and P. uncinata indicate that their natural forest did not evolve under frequent crown fires. P. canariensis and P. pinea appeared in two single groups because of their sprouting capability and their seed size respectively. Intraspecific variation in P. pinaster was also analysed using the same criteria and high variability was found in its life history traits. A group of P. pinaster populations showed high levels of serotiny and thin bark as a possible adaptation to frequent stand-replacing crown fires. In contrast, a group of non- or weakly-serotinous populations seems to have evolved under a low-intensity fire regime where the best fitness corresponds to thick-barked individuals capable of surviving ground fires. Intermediate strategies were also evident in this species and were discussed in relation to the effect of different fire regimes caused by the understorey vegetation.

Effects of an Intense Wildfire in a Mexican Oak-Pine Forest

Abstract An oak-pine forest burned by intense wildfire in April, 1996, and a companion unburned area were sampled 1 month and 1 yr postfire in La Michilía Biosphere Reserve, Durango, Mexico. Up to 90% of the trees were killed or top-killed in the burned area, but larger trees tended to survive, so basal area was only reduced by 66%. Top-killing was relatively higher among fire-susceptible oaks and lower among fire-resistant pines. However, oaks were strong resprouters both in the canopy and at the base of top-killed trees. Damage codes based on crown scorch and bole char were highly accurate when predicting that a tree would die but substantially overestimated survivors. Most tree regeneration was top-killed in the fire, but oak sprout density was 700% that of the unburned area by 1 yr postfire. Manzanita shrubs also resprouted vigorously. Herbaceous production and cover were lower after the first postfire growing season in the burned area than the unburned area. Woody fuels and forest floor depth were also reduced. Although short-term fire effects indicate that the forest ecosystem has moved closer toward a savanna condition, remnant seed trees and sprouting trees are expected to maintain forest cover. Future herbaceous production is likely to increase in response to overstory mortality. Quantification of fire effects is helpful for supporting short-term management decisions since oak-pine forests cover millions of hectares in northern Mexico. As a long-term management strategy, however, we suggest that restoring the frequent, low-intensity fire regime may be desirable for ecological and economic reasons. For. Sci. 46(1):52-61.

Demography of woody plants in relation to fire: Banksia serrata Lf. and Isopogon anemonifolius (Salisb.) Knight

AbstractBanksia serrata and Isopogon anemonifolius are serotinous resprouters (single‐stemmed tree, multi‐stemmed shrub, respectively) found in forests within the Sydney region. Studies were conducted to predict the population dynamics of these species. Seed production and survival and the accumulation of seed‐bank within cones were estimated in relation to time since fire. Emergence, survival and development of lignotubers were measured in young juveniles and the time taken to reach adulthood was estimated. This information and published data on survival were used to estimate the amount of recruitment of adults and juveniles necessary for stands to remain in a stable state under frequent (&lt;16 years), high and low intensity fire regimes. The effects of longer intervals (up to 80 years) between fires were also estimated. It was predicted that B. serrata populations will decline in numbers when the interval between high intensity fires is &lt;9 years, while under low intensity fires the critical interval was 12–13 years. In I. anemonifolius the predicted intervals were 14 and 16 years, respectively. When fires are timed so that maximum seed‐bank is available (about 30 year interval), it is unlikely that resprouters will dominate communities because the seed‐banks and rates of growth of seedlings of obligate seeder shrubs are greater than these resprouters. Populations of these resprouters may be more able to persist than obligate seeders when the fire frequency is either very high (&lt;6 years) or low (&gt;50years), though the density of resprouter populations may slowly decline under such fire regimes.