Historical Range Of Variability Research Articles

Short-Interval, High-Severity Wildfire Depletes Diversity of Both Extant Vegetation and Soil Seed Banks in Fire-Tolerant Eucalypt Forests

Many plant species are well-adapted to historical fire regimes. An increase in the severity, frequency, and extent of wildfires could compromise the regenerative capacity of species, resulting in permanent shifts in plant diversity. We surveyed extant vegetation and soil seed banks across two forest types with contrasting historical fire regimes—Shrubby Dry Forest (fire return interval: 10–20 years) and Sub-Alpine Woodland (50–100 years). Over the past 20 years, both forests have been subject to repeated, high-severity wildfires at intervals significantly shorter than their historical return intervals. We examined the soil seed bank response to fire-cued germination, and whether the plant diversity in soil seed banks and extant vegetation demonstrated similar responses to short-interval, high-severity wildfires. The soil seed bank demonstrated a positive response to heat in combination with smoke, and for the Sub-Alpine Woodland, this was limited to sites more frequently burnt by fire. With an increase in fire frequency, there was a decline in species richness and Shannon’s Diversity and a shift in species composition in both extant vegetation and the soil seed bank. The fire frequency effects on the relative richness of trait associations were restricted to the Shrubby Dry Forest, and included an increase in short-lived obligate seeders, wind-dispersed species, and ant-dispersed shrubs in burnt relative to long unburnt sites in both extant vegetation and the soil seed bank. Graminoids were the most abundant component of the soil seed banks of Sub-Alpine Woodlands, and this increased with more frequent fire, with a similar trend (p = 0.06) in extant vegetation. Clear shifts in plant diversity in both soil seed banks and extant vegetation in forest types with contrasting historical fire regimes suggest that emerging fire regimes are pushing ecosystems beyond their historical range of variability, including potentially more flammable states and a decline in the buffering capacity of soil seed banks.

Fire severity drives understory community dynamics and the recovery of culturally significant plants

AbstractAnthropogenic influences are altering fire regimes worldwide, resulting in an increase in the size and severity of wildfires. Simultaneously, throughout western North America, there is increasing recognition of the important role of Indigenous fire stewardship in shaping historical fire regimes and fire‐adapted ecosystems. However, there is limited understanding of how ecosystems are affected by or recover from contemporary “megafires,” particularly in terms of understory plant communities that are critical to both biodiversity and Indigenous cultures. To address this gap, our collaborative study, in partnership with Secwépemc First Nations, examined understory community recovery following a large, mixed‐severity wildfire that burned in the dry and mesic conifer forests of British Columbia, Canada, with a focus on plants of high cultural significance to Secwépemc communities. To measure the effect of a continuous gradient of fire severity across forest types, we conducted field assessments of fire severity and sampled understory plants 4 years postfire. We found that native species richness and richness of species of high cultural significance were lowest in areas that burned at high severity, with distinct compositional differences between unburned areas and those that burned at high severity. These findings were consistent across forest types characterized by distinct historical fire regimes. In contrast, richness of exotic species increased with increasing fire severity in the dominant montane interior Douglas‐fir forests, with exotic species closely associated with areas that burned at high severity. Our study indicates that recent megafires may be pushing ecosystems outside their historical range of variability, with negative implications for ecosystem recovery and cultural use across these fire‐affected landscapes. We also found consistently higher plant diversity, and both native and cultural species richness, in subalpine forests. Collectively, our results provide strong evidence of the ecological and cultural significance of low‐ to moderate‐severity fire and subalpine forests, and the longstanding and ongoing role of Indigenous peoples in shaping these landscapes. As wildfires continue to impact ecosystems and human communities, this study offers novel insights into the recovery of important ecological and cultural values, while highlighting the need to support ethical research collaborations with Indigenous communities and Indigenous‐led revitalization of fire and plant stewardship.

Resource objective wildfire leveraged to restore old growth forest structure while stabilizing carbon stocks in the southwestern United States

Wildfire futures and aboveground carbon (C) dynamics associated with forest restoration programs that integrate resource objective wildfire as part of a larger treatment strategy are not well understood. Using simulation modeling, we examined alternative forest and fuel management strategies on a 237,218-ha study area within a 778,000-ha landscape that is a high priority target for federal restoration programs. We simulated two wildfire management scenarios combined with three levels of conventional forest restoration treatments over 64 years using a detailed landscape disturbance and succession model developed in prior work. We found accelerated forest restoration used in concert with resource objective wildfire was the most effective at returning old growth forest structure, while stabilizing aboveground C stocks and restoring the fire return interval to its historic range of variation. In scenarios without forest restoration, the continued practice of resource objective wildfires during shoulder fire seasons reduced summer emissions in a negative feedback loop. In the short term, scenarios without forest restoration increased live tree C, but also increased the likelihood of C loss during wildfire activity driven by extreme fire weather. We found scenarios most effective at restoring fire-excluded pine forests to their historical old growth conditions came at a short-term cost of lost C, but with the long-term benefit of substantially increasing fire-resistant live tree C. Our results inform how local decision making can best balance competing goals of sequestering C, and stabilizing C stocks in frequent-fire pine forests using the principles of local fire ecology to restore and maintain old growth forest structure.

Response of pollinator taxa to fire is consistent with historic fire regimes in the Sierra Nevada and mediated through floral richness†.

Many fire-prone forests are experiencing wildfires that burn outside the historical range of variation in extent and severity. These fires impact pollinators and the ecosystem services they provide, but how the effects of fire are mediated by burn severity in different habitats is not well understood. We used generalized linear mixed models in a Bayesian framework to model the abundance of pollinators as a function of burn severity, habitat, and floral resources in post-fire, mid-elevation, conifer forest, and meadow in the Sierra Nevada, California. Although most species-level effects were not significant, we found highly consistent negative impacts of burn severity in meadows where pollinators were most abundant, with only hummingbirds and some butterfly families responding positively to burn severity in meadows. Moderate-severity fire tended to increase the abundance of most pollinator taxa in upland forest habitat, indicating that even in large fires that burn primarily at high- and moderate-severity patches may be associated with improved habitat conditions for pollinator species in upland forest. Nearly all pollinator taxa responded positively to floral richness but not necessarily to floral abundance. Given that much of the Sierra Nevada is predicted to burn at high severity, limiting high-severity effects in meadow and upland habitats may help conserve pollinator communities whereas low- to moderate-severity fire may be needed in both systems.

Exceptional variability in historical fire regimes across a western Cascades landscape, Oregon, USA

AbstractDetailed information about the historical range of variability in wildfire activity informs adaptation to future climate and disturbance regimes. Here, we describe one of the first annually resolved reconstructions of historical (1500–1900 ce) fire occurrence in coast Douglas‐fir dominated forests of the west slope of the Cascade Range in western Oregon. Mean fire return intervals (MFRIs) across 16 sites within our study area ranged from 6 to 165 years. Variability in MFRIs was strongly associated with average maximum summer vapor pressure deficit. Fire occurred infrequently in Douglas‐fir forest stands seral to mountain hemlock or silver fir, but fire frequency was much shorter than predicted by theory in other forest types. MFRIs within Douglas‐fir stands seral to western hemlock or grand fir ranged from 19 to 45 years, and MFRIs in stands seral to Douglas‐fir ranged from 6 to 11 years. There was little synchrony in fire occurrence or tree establishment across 16 sites separated by 4 km. The lack of synchrony in fire suggests that large, wind‐driven fire events that are often considered to be characteristic of coast Douglas‐fir forests were not an important driver of succession in our study area during the last ~400–500 years. Climate was more arid than normal during fire years in most forest types, but historical fire in stands seral to Douglas‐fir was strongly associated with antecedent moisture and less strongly associated with drought. We interpret the extraordinary tempo of fire we observed in stands seral to Douglas‐fir and the unique climate pattern associated with fire in these stands to be indicative of Indigenous fire stewardship. This study provides evidence of far more frequent historical fire in coast Douglas‐fir forests than assumed by managers or scientists—including some of the most frequent fire return intervals documented in the Pacific Northwest. We recommend additional research across the western Cascades to create a comprehensive account of historical fire in highly productive forests with significant cultural, economic, and ecological importance.

Wildfire activity in northern Rocky Mountain subalpine forests still within millennial-scale range of variability

Increasing area burned across western North America raises questions about the precedence and magnitude of changes in fire activity, relative to the historical range of variability (HRV) that ecosystems experienced over recent centuries and millennia. Paleoecological records of past fire occurrence provide context for contemporary changes in ecosystems characterized by infrequent, high-severity fire regimes. Here we present a network of 12 fire-history records derived from macroscopic charcoal preserved in sediments of small subalpine lakes within a c. 10 000 km2 landscape in the U.S. northern Rocky Mountains (Northern Rockies). We used this network to characterize landscape-scale burning over the past 2500 yr, and to evaluate the precedence of widespread regional burning experienced in the early 20th and 21st centuries. We further compare the Northern Rockies fire history to a previously published network of fire-history records in the Southern Rockies. In Northern Rockies subalpine forests, widespread fire activity was strongly linked to seasonal climate conditions, in contemporary, historical, and paleo records. The average estimated fire rotation period (FRP) over the past 2500 yr was 164 yr (HRV: 127–225 yr), while the contemporary FRP from 1900 to 2021 CE was 215 yr. Thus, extensive regional burning in the early 20th century (e.g. 1910 CE) and in recent decades remains within the HRV of recent millennia. Results from the Northern Rockies contrast with the Southern Rockies, which burned with less frequency on average over the past 2500 yr, and where 21st-century burning has exceeded the HRV. Our results support expectations that Northern Rockies fire activity will continue to increase with climatic warming, surpassing historical burning if more than one exceptional fire year akin to 1910 occurs within the next several decades. The ecological consequences of climatic warming in subalpine forests will depend, in large part, on the magnitude of fire-regime changes relative to the past.

Paleoecology provides context for conserving culturally and ecologically important pine forest and barrens communities.

In fire-prone ecosystems, knowledge of vegetation-fire-climate relationships and history of fire suppression and Indigenous cultural burning can inform discussions of how to use fire as a management tool, particularly as climate continues to change rapidly. On Wiisaakodewan-minis/Stockton Island in the Apostle Islands National Lakeshore of Wisconsin, USA, structural changes in a pine-dominated natural area containing a globally rare barrens community occurred after cessation of cultural burning by Indigenous Ojibwe people and the imposition of fire-suppression policies, leading to questions about the historical role of fire in this culturally and ecologically important area. To help better understand the ecological context needed to steward these pine forest and barrens communities, we developed paleoecological records of vegetation, fire, and hydrological change using pollen, charcoal, and testate amoebae preserved in peat and sediment cores collected from bog and lagoon sediments within the pine-dominated landscape. Results indicated that fire has been an integral part of Stockton Island ecology for at least 6000 years. Logging in the early-1900s led to persistent changes in island vegetation, and post-logging fires of the 1920s and 1930s were anomalous in the context of the past millennium, likely reflecting more severe and/or extensive burning than in the past. Before that, the composition and structure of pine forest and barrens changed little, perhaps due to regular low-severity surface fires, which may have occurred with a frequency consistent with Indigenous oral histories (~4-8 years). Higher-severity fire episodes, indicated by large charcoal peaks above background levels in the records, occurred predominantly during droughts, suggesting that more frequent or more intense droughts in the future may increase fire frequency and severity. Persistence of pine forest and barrens vegetation through past periods of climatic change indicates considerable ecological resistance and resilience. Future persistence in the face of climate changes outside this historical range of variability may depend in part on returning fire to these systems. This article is protected by copyright. All rights reserved.

Recent weakening of seasonal temperature difference in East Asia beyond the historical range of variability since the 14th century

Recent weakening of seasonal temperature difference in East Asia beyond the historical range of variability since the 14th century

Countering Omitted Evidence of Variable Historical Forests and Fire Regime in Western USA Dry Forests: The Low-Severity-Fire Model Rejected

The structure and fire regime of pre-industrial (historical) dry forests over ~26 million ha of the western USA is of growing importance because wildfires are increasing and spilling over into communities. Management is guided by current conditions relative to the historical range of variability (HRV). Two models of HRV, with different implications, have been debated since the 1990s in a complex series of papers, replies, and rebuttals. The “low-severity” model is that dry forests were relatively uniform, low in tree density, and dominated by low- to moderate-severity fires; the “mixed-severity” model is that dry forests were heterogeneous, with both low and high tree densities and a mixture of fire severities. Here, we simply rebut evidence in the low-severity model’s latest review, including its 37 critiques of the mixed-severity model. A central finding of high-severity fire recently exceeding its historical rates was not supported by evidence in the review itself. A large body of published evidence supporting the mixed-severity model was omitted. These included numerous direct observations by early scientists, early forest atlases, early newspaper accounts, early oblique and aerial photographs, seven paleo-charcoal reconstructions, ≥18 tree-ring reconstructions, 15 land survey reconstructions, and analysis of forest inventory data. Our rebuttal shows that evidence omitted in the review left a falsification of the scientific record, with significant land management implications. The low-severity model is rejected and mixed-severity model is supported by the corrected body of scientific evidence.

Old forest structural development drives complexity of nest webs in a naturally disturbed boreal mixedwood forest landscape

Structural complexity generated by forest development processes and tree species compositional changes provide key habitat features for vertebrate communities that rely upon tree size and decay processes for foraging, denning or nesting. Complexity of forest structure in old stands could not only be key for harboring increased taxonomic species diversity but also greater functional diversity through more complexity in networks of tree cavity dependent species. Using a nest web approach that hierarchically links cavity-bearing trees with cavity formation agents (natural decay processes and avian excavators) and cavity users (non-excavator species), we compared network characteristics of nest webs along a time since fire gradient in a naturally disturbed boreal mixedwood forest landscape in eastern North America. Since 2003, twelve 24 to 40 ha plots ranging from 61 to more than 245 years after fire were surveyed at the Lake Duparquet Research and Teaching Forest in Abitibi, Quebec, Canada to detect active nesting, and denning cavities. We found that network complexity both in terms of number of vertebrate species and number of interactions among species, increased along the age gradient and was significantly higher in the older stands than predicted by chance. Whereas cavity-nesting communities in old forests used a higher diversity of tree species over a wide range of decay stages, trembling aspen remained a key cavity-bearing tree throughout the age gradient. Woodpeckers were the main cavity formation agents whereas less than 1% of cavities originated from natural decay. The structural development of older forests is thus a driver for functional diversity in cavity-using vertebrate communities through higher interaction richness in nest webs, among cavity-bearing trees, excavators and non-excavating users. The pivotal contribution of the entire gradient of old forest cover types to the overall complexity of nest webs in the boreal mixedwood zone is also a key for the resilience of the cavity-using vertebrate community to natural disturbances. We discuss how such resilience may be compromised by even-aged industrial timber harvesting with short rotations that shifts the age structure of boreal landscapes toward regenerating and young pole forests whereas old forest cover types become below their historical range of variability.

Trends in forest structure restoration need over three decades with increasing wildfire activity in the interior Pacific Northwest US

Wildfire is a keystone ecological process in many forests worldwide, but fire exclusion and suppression have driven profound shifts in forest structure (e.g., increased density, canopy cover, biomass) that have contributed to increases in large, high-severity fire in many seasonally dry forests and woodlands of the western United States. Comparisons between contemporary and historic range of variability (HRV) in forest structure can quantify the amount and types of restoration that shift landscapes toward structural conditions that have historically fostered resilience to fire. However, landscapes are dynamic over time and conditions reflect the net effects of planned actions (e.g., fuel reduction treatments) and unplanned actions (e.g., wildfire). How wildfire activity may shift landscapes toward or away from the HRV and correspondingly affect the need for restoration, has not been widely tested. Here, we quantify long-term (1986–2017), and continuous (annual resolution) trends of forest restoration need and ask how wildfire activity during this period has affected restoration need trends at three nested spatial extents: across the eastern Washington (USA) ecoregion, among fire regimes within this ecoregion, and between watersheds of a frequent/low-severity fire regime that experienced contrasting amounts of wildfire. At the broadest scale, restoration need did not change substantially during the study period, with approximately 35 % of forest area in need of disturbance restoration—despite 16.6 % of the total forested area (593,000 ha) burning from 1986 to 2016. At intermediate spatial extents (among fire regimes), forests characterized by historically frequent/low-severity fire experienced the greatest decrease in disturbance restoration need following recent fire activity. Although > 50 % of forests within this fire-regime remained in need of disturbance restoration at the end of the study period, we found a strong correspondence between forested area burned and decreased restoration need in this fire regime; relationships were equivocal or non-existent in other fire regimes. At the finest spatial scale (watersheds dominated by historically frequent/low-severity fire-regime forests), we found sharp contrasts between areas that experienced high fire activity in recent years and those that did not. At this scale, recent large fires have decreased disturbance restoration need by > 25 %. Our findings suggest that recent large wildfires have reduced the amount of forest in need of restoration, but have done so modestly and primarily at local or sub-regional extents. Overall, our approach can be applied to understanding how wildfires or other disturbances contribute to affecting forest structure and management targets in other ecosystems through time and space.

Vegetation responses to past volcanic disturbances at the Araucaria araucana forest-steppe ecotone in northern Patagonia.

Volcanic eruptions play an important role in vegetation dynamics and its historical range of variability. However, large events are infrequent and eruptions with a significant imprint in today's vegetation occurred far in the past, limiting our understanding of ecological processes. Volcanoes in southern Andes have been active during the last 10 ka and support unique ecosystems such as the Araucaria–Nothofagus forest. Araucaria is an endangered species, with a fragmented distribution and well‐adapted to fire and volcanic disturbances. Yet, it was suggested that volcanism might have increased the fragmentation. Through the use of pollen and tephra analysis from a sedimentary record, this paleoecological study aims to provide an insight into the vegetation responses to past volcanic disturbances, to assess the role of volcanic disturbance on the vegetation dynamics and to determine if the current fragmentation has been caused by volcanism. Results show that during the last 9 kyr, 39 tephra falls buried the vegetation around Lake Relem, more frequently between 4 and 2 ka. The pollen percentage indicates that the vegetation changed after small tephra fall but seldom caused significant changes. However, the large eruption of Sollipulli volcano (~3 ka) changed the environmental conditions affecting severely the vegetation. Ephedra dominated the early successional stage, perhaps facilitating Nothofagus recovering after ~500 years. Slight increase of Araucaria and Nothofagus obliqua‐type pollen percentages suggests that forest resisted without permanent changes and recovered relatively fast after the large eruption, perhaps because of sparse biological legacies distributed in the landscape. In the study area, the relative stability of Araucaria pollen after several tephra fall suggests no change in its past distribution at the current forest‐steppe ecotone, thus not affecting its current conservation status. Perhaps, random factors, the colonization patterns of the high elevations in the Andes after deglaciation and topography might play a more important role than previously thought.

Is Climate Change Restoring Historical Fire Regimes across Temperate Landscapes of the San Juan Mountains, Colorado, USA?

Wildfires are increasing with human-induced climate change, but could this be ecologically beneficial in landscapes where recent fire is deficient relative to historical? I compiled 1980–2020 fire data for the San Juan Mountains, Colorado. I analyzed fire sizes and trends in area burned and fire severity, and compared fire density and rotations between 1980–2010 and 2011–2020 among ecosystem types and watersheds. I compared historical (pre-industrial) evidence from tree-ring, charcoal, and land-survey reconstructions to evaluate whether recent fire is outside the historical range of variability (HRV). Nearly all burned area was in the southwestern San Juans in 5 of 41 years and 35 of 4716 wildfires. Between 1980–2010 and 2011–2020, fire densities increased ∼200% and rotations shortened to ∼25%, similarly among ecosystems and watersheds, consistent with climatic effects. Fire rotations in 2011–2020 were within HRV for three ecosystems and deficient for four. Fire sizes and severities were within HRV. Moderate- and high-severity fire had no significant trend. Thus, reducing fire size or severity is currently ecologically unnecessary. Instead, incorporating fire from climate change, via wildland fire use, supplemented by prescribed burning, could feasibly restore historical fire regimes in most San Juan landscapes by 2050, the target of the Paris 1.5 °C goal.

Climate change impacts on maize and soybean yields in Zambia

AbstractProjected changes in temperature and rainfall directly threaten the overall agricultural crop production system in Zambia where 75% of agricultural production is rainfed. This study investigates the effects of future climate on maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields using relatively high spatial resolution downscaled climate data in Central and Eastern Zambia. The Agrometshell model was applied to simulate historical and future crop yields. To account for the effects of climate change on crop yields, global climate models (GCMs) and Representative Concentration Pathways (RCPs) were used. Most yield functions were significant with R2 values >.7 with p < .05, indicating the model's ability to capture variations in observed data. Mean maize yield change decreased across RCPs and GCMs in Central Province and the decrease amplified in several areas in the far future, suggesting worsening effects into the future. But the projected mean yield changes in Eastern Province remain mostly within the historical range of variability, suggesting that maize yield in this province is less sensitive to climate change. Projected mean soybean yield changes in Central Province consist of a combination of reductions and improvements. Soybean yields in Eastern Province were projected to increase in the near future, shifting to large increases in the far future. Drastic reductions in maize yields are likely to cause major negative effects in the agricultural system because maize is a staple crop in the country. In contrast, soybean showed less sensitivity to future climate and demonstrates the value of diversifying agricultural systems and increasing the sustainability of food production systems.

Increase in Arctic coastal erosion and its sensitivity to warming in the twenty-first century

Arctic coastal erosion damages infrastructure, threatens coastal communities and releases organic carbon from permafrost. However, the magnitude, timing and sensitivity of coastal erosion increase to global warming remain unknown. Here we project the Arctic-mean erosion rate to increase and very likely exceed its historical range of variability before the end of the century in a wide range of emission scenarios. The sensitivity of erosion to warming roughly doubles, reaching 0.4–0.8 m yr−1 °C−1 and 2.3–4.2 TgC yr−1 °C−1 by the end of the century. We develop a simplified semi-empirical model to produce twenty-first-century pan-Arctic coastal erosion rate projections. Our results will inform policymakers on coastal conservation and socioeconomic planning, and organic carbon flux projections lay out the path for future work to investigate the impact of Arctic coastal erosion on the changing Arctic Ocean, its role as a global carbon sink, and the permafrost–carbon feedback.

Frequency of disturbance mitigates high-severity fire in the Lake Tahoe Basin, California and Nevada

Because of past land use changes and changing climate, forests are moving outside of their historical range of variation. As fires become more severe, forest managers are searching for strategies that can restore forest health and reduce fire risk. However, management activities are only one part of a suite of disturbance vectors that shape forest conditions. To account for the range of disturbance intensities and disturbance types (wildfire, bark beetles, and management), we developed a disturbance return interval (DRI) that represents the average return period for any disturbance, human or natural. We applied the DRI to examine forest change in the Lake Tahoe Basin of California and Nevada. We specifically investigated the consequences of DRI on the proportion of high-severity fire and the net sequestration of carbon. In order to test the management component of the DRI, we developed management scenarios with forest managers and stakeholders in the region; these scenarios were integrated into a mechanistic forest landscape model that also accounted for climate change, as well as natural disturbances of wildfire and insect outbreaks. Our results suggest increasing the frequency of disturbances (a lower DRI) would reduce the percentage of high-severity fire on landscape but not the total amount of wildfire in general. However, a higher DRI reduced carbon storage and sequestration, particularly in management strategies that emphasized prescribed fire over hand or mechanical fuel treatments.

Ecological restoration guided by historical reference conditions can increase resilience to climate change of southwestern U.S. Ponderosa pine forests

Broad-scale forest restoration projects are implemented across the western United States to restore seasonally dry, frequent-fire-adapted ecosystems to improve ecological function and enhance resilience by increasing resistance to crown fire and climatic stressors. Despite the widespread use of restoration treatments that center on tree thinning and application of prescribed fire, the longevity of beneficial effects and the robustness of outcomes under future climate change predictions remains unclear. In this study, we remeasured a set of experimental areas established a minimum of 20 years ago that comprise a network of ponderosa pine (Pinus ponderosa) forest restoration study sites in northern Arizona. We analyzed ecological resiliency by evaluating forest conditions in terms of resistance to climatic stressors and potential crown fire in units that were thinned following evidence-based restoration guidelines (ERG), then burned with prescribed fire at multiple intervals, compared against paired untreated controls. Resilience indicators included forest structure, tree mortality, tree growth, regeneration, canopy fuels, and crowning index. We also simulated future forest conditions under a warming climate scenario (RCP 4.5) with a range of prescribed fire return intervals. Results indicated that experimental areas where restoration treatments were implemented remained more resilient to climate stressors compared to controls after 20 years. Treated areas had significantly lower tree mortality and greater average diameter growth compared to controls. Furthermore, forest structure generally remained similar to historical reference conditions in treated units with the exception of increases in ingrowth of sprouting species at the drier sites. Canopy fuel load and crown fire hazard in treated units remained significantly lower than controls, indicating that treatments remained effective in reducing crown fire potential over the 20-year study period without the need for additional tree thinning. Modeling basal area, crowning index, and the proportion of basal area in large trees under a future warming scenario suggested that the treated units underwent less changed than untreated areas. Under climate change, management of fire regimes even at longer-than-historical intervals (historical ≈ 5 yr, tested 5, 10, and 20 yr) would maintain basal area within our historical range of variability and maintain fire resistant forest over the next several decades. However, decline by the end of the century is concerning. Our results suggest that forest restoration treatments, guided by historical reference conditions, promote ecological resilience in the long-term and continued maintenance burning into the future is likely warranted even with continued drought and warming.

Synergistic impacts of anthropogenic fires and aridity on plant diversity in the Western Ghats: Implications for management of ancient social-ecological systems

Identifying the impacts of anthropogenic fires on biodiversity is imperative for human-influenced tropical rainforests because: i) these ecosystems have been transformed by human-induced fires for millennia; and ii) their effective management is essential for protecting the world's terrestrial biodiversity in the face of global environmental change. While several short-term studies elucidate the impacts of fires on local plant diversity, how plant diversity responds to fire regimes over long timescales (>100 years) is a significant knowledge gap, posing substantial impediment to evidence-based management of tropical social-ecological systems. Using wet evergreen forests of the Western Ghats of India as a model system, we discuss the synergistic effects of anthropogenic fires and enhanced aridity on tropical plant diversity over the past 4000 years by examining fossil pollen-based diversity indices (e.g., pollen richness and evenness, and temporal β-diversity), past fire management, the intervals of enhanced aridity due to reduced monsoon rainfall and land use history. By developing a historical perspective, our aim is to provide region-specific management information for biodiversity conservation in the Western Ghats. We observe that the agroforestry landscape switches between periods of no fires (4000-1800 yr BP, and 1400-400 yr BP) and fires (1800-1400 yr BP, and 400-0 yr BP), with both fire periods concomitant with intervals of enhanced aridity. We find synergistic impacts of anthropogenic fires and aridity on plant diversity uneven across time, pointing towards varied land management strategies implemented by the contemporary societies. For example, during 1800–1400 yr BP, diversity reduced in conjunction with a significant decrease in the canopy cover related to sustained use of fires, possibly linked to large-scale intensification of agriculture. On the contrary, the substantially reduced fires during 400–0 yr BP may be associated with the emergence of sacred forest groves, a cultural practice supporting the maintenance of plant diversity. Overall, notwithstanding apparent changes in fires, aridity, and land use over the past 4000 years, present-day plant diversity in the Western Ghats agroforestry landscape falls within the range of historical variability. Importantly, we find a strong correlation between plant diversity and canopy cover, emphasising the crucial role of maintenance of trees in the landscape for biodiversity conservation. Systematic tree management in tropical social-ecological systems is vital for livelihoods of billions of people, who depend on forested landscapes. In this context, we argue that agroforestry landscapes can deliver win-win solutions for biodiversity as well as people in the Western Ghats and wet tropics at large.

Secondary invasion? Emerald ash borer (Agrilus planipennis) induced ash (Fraxinus spp.) mortality interacts with ecological integrity to facilitate European buckthorn (Rhamnus cathartica)

Invasive insects facilitate secondary invasive species by altering forest structure and function. Specifically, invasive insect herbivores may promote the establishment and growth of invasive plants by creating canopy gaps. Such secondary invasions may be influenced by ecological integrity — the degree to which ecosystem composition, structure, and function deviate from their natural or historical range of variation. Here we investigate (i) whether emerald ash borer (Agrilus planipennis; EAB) induced ash (Fraxinus spp.) mortality facilitates European buckthorn (Rhamnus cathartica) — an invasive, shade-tolerant shrub, and (ii) the role of ecological integrity in this relationship. We use a principal component analysis (PCA) to calculate an index of ecological integrity and a zero-altered negative binomial generalized linear mixed model (GLMM) to describe European buckthorn occurrence and abundance. European buckthorn occurrence is influenced by canopy gaps, independent of EAB-induced gap formation. Ecological integrity and EAB-induced ash mortality interact to control European buckthorn abundance, with high ecological integrity limiting EAB-facilitated buckthorn invasion. This is the first evidence for EAB-facilitated buckthorn invasion and for an interaction between a secondary invasion and ecological integrity. Thus, ecological integrity plays an important role in the EAB–buckthorn system and may be used to manage the impacts of secondary invasions.

Defining variation in pre-human ecosystems can guide conservation: An example from a Caribbean coral reef

Many Caribbean coral reefs are heavily degraded, yet their pre-human, natural states are often assumed or estimated using space-for-time substitution approaches. Here we use an 11-hectare suite of fossilised mid-Holocene (7.2–5.6 ka) fringing reefs in Caribbean Panama to define natural variation in hard coral community structure before human-impact to provide context to the states of the same reefs today. We collected bulk samples from four trenches dug into the mid-Holocene fossil reef and surficial bulk samples from 2–10 m depths on five adjacent modern reefs extending over 5 km. Analysis of the abundances of coral taxa in fossil bulk samples define the Historical Range of Variation (HRV) in community structure of the reefs. When compared to the community structure of adjacent modern reefs, we find that most coral communities today fall outside the HRV, identifying them as novel ecosystems and corroborating the well-documented transition from acroporid-dominated Caribbean reefs to reefs dominated by stress-tolerant taxa (Porites and Agaricia). We find one modern reef, however, whose community composition remains within the HRV showing that it has not transitioned to a novel state. Reef-matrix cores extracted from this reef reveal that the coral community has remained in this state for over 800 years, suggesting long-term stability and resistance to the region-wide shift to novel states. Without these data to provide historical context, this potentially robust and stable reef would be overlooked since it does not fulfil expectations of what a Caribbean coral reef should look like in the absence of humans. This example illustrates how defining past variation using the fossil record can improve our understanding of modern degradation and guide conservation.