Historical Reference Conditions Research Articles

What did it used to look like? A case study from tall, wet mainland Mountain Ash forests prior to British invasion

AbstractThere has been extensive commentary about historical First Nations' land management in Australia, including in tall, wet forests, and therefore their condition at the time of the British invasion in 1788. Popular texts have interpreted records kept by early British invaders to argue that extensive areas of tall, wet forest were kept open through frequent burning by the First Peoples. However, these interpretations conflict with historical and ecological evidence, which is rarely acknowledged in public discourse. Here, we present evidence about what Victorian Mountain Ash (Eucalyptus regnans) forests were like at the time of the British invasion. We show that at the time of the British invasion, most areas of mainland Mountain Ash forests were likely to have been naturally dense and wet, with: (1) overstorey trees spaced relatively widely; and (2) an understorey consisting of a cool temperate rainforest mesic layer. Ecological and physiological evidence suggests that Mountain Ash forests evolved under conditions where high‐severity wildfire was comparatively rare, leading to patterns of landscape‐level cover dominated by relatively mature forests. This is broadly consistent with reports from the First Peoples, early historical accounts, paintings, and photographs. These forests were not open or park‐like, as may have been the case in some other Australian vegetation types. However, these forests were not wilderness, but places of significance to the First Peoples. Understanding forest structure at the time of the British invasion is critically important in establishing historical reference conditions for guiding appropriate restoration programmes, especially the reinstatement of traditional ecological knowledge, after long periods of post‐British invasion disturbance and degradation. Notably, the dense, wet understorey that characterizes Mountain Ash forests should be recognized as an inherent and entirely natural part of the ecological dynamics of this ecosystem, with approaches to thin, burn, or remove it highly likely to be counterproductive and have a range of detrimental environmental effects.

A roadmap for pyrodiversity science

AbstractBackgroundContemporary and projected shifts in global fire regimes highlight the importance of understanding how fire affects ecosystem function and biodiversity across taxa and geographies. Pyrodiversity, or heterogeneity in fire history, is often an important driver of biodiversity, though it has been largely overlooked until relatively recently. In this paper, we synthesise previous research to develop a theoretical framework on pyrodiversity–biodiversity relationships and propose future research and conservation management directions.Theoretical FrameworkPyrodiversity may affect biodiversity by diversifying available ecological niches, stabilising community networks and/or supporting diverse species pools available for post‐fire colonisation. Further, pyrodiversity's effects on biodiversity vary across different spatial, temporal and organismal scales depending on the mobility and other life history traits of the organisms in question and may be mediated by regional eco‐evolutionary factors such as historical fire regimes. Developing a generalisable understanding of pyrodiversity effects on biodiversity has been challenging, in part because pyrodiversity can be quantified in various ways.Applying the Pyrodiversity ConceptExclusion of Indigenous fire stewardship, fire suppression, increased unplanned ignitions and climate change have led to dramatic shifts in fire regimes globally. Such shifts include departures from historic levels of pyrodiversity and add to existing challenges to biodiversity conservation in fire‐prone landscapes. Managers navigating these challenges can be aided by targeted research into observed contemporary pyrodiversity–biodiversity relationships as well as knowledge of historical reference conditions informed by both Indigenous and local ecological knowledge and western science.Future Research DirectionsSeveral promising avenues exist for the advancement of pyrodiversity science to further both theoretical and practical goals. These lines of investigation include but are not limited to (1) testing the increasing variety of pyrodiversity metrics and analytical approaches; (2) assessing the spatial and temporal scale‐dependence of pyrodiversity's influence; (3) reconstructing historical pyrodiversity patterns and developing methods for predicting and/or promoting future pyrodiversity; and (4) expanding the focus of pyrodiversity science beyond biodiversity to better understand its influence on ecosystem function and processes more broadly.

Assessing the effectiveness of landscape-scale forest adaptation actions to improve resilience under projected climate change

Climate change will increase disturbance pressures on forested ecosystems worldwide. In many areas, longer, hotter summers will lead to more wildfire and more insect activity which will substantially increase overall forest mortality. Forest treatments reduce tree density and fuel loads, which in turn reduces fire and insect severity, but implementation has been limited compared to the area needing treatment. Ensuring that forests remain near their reference conditions will require a significant increase in the pace and scale of forest management. In order to assess what pace and scale may be required for a landscape at risk, we simulated forest and disturbance dynamics for the central Sierra Nevada, USA. Our modeling framework included forest growth and succession, wildfire, insect mortality and locally relevant management actions. Our simulations accounted for climate change (five unique global change models on a business-as-usual emissions pathway) and a wide range of plausible forest management scenarios (six total, ranging from less than 1% of area receiving management treatments per year to 6% per year). The climate projections we considered all led to an increasing climatic water deficit, which in turn led to widespread insect caused mortality across the landscape. The level of insect mortality limited the amount of carbon stored and sequestered while leading to significant composition changes, however, only one climate change projection resulted in increased fire over contemporary conditions. While increased pace and scale of treatments led to offsets in fire related tree mortality, managing toward historic reference conditions was not sufficient to reduce insect-caused forest mortality. As such, new management intensities and other adaptation actions may be necessary to maintain forest resilience under an uncertain future climate.

Simulated treatment effects on bird communities inform landscape-scale dry conifer forest management.

Human land use and climate change have increased forest density and wildfire risk in dry conifer forests of western North America, threatening various ecosystem services, including habitat for wildlife. Government policy supports active management to restore historical structure and ecological function. Information on potential contributions of restoration to wildlife habitat can allow assessment of tradeoffs with other ecological benefits when prioritizing treatments. We predicted avian responses to simulated treatments representing alternative scenarios to inform landscape‐scale forest management planning along the Colorado Front Range. We used data from the Integrated Monitoring in Bird Conservation Regions program to inform a hierarchical multispecies occupancy model relating species occupancy and richness with canopy cover at two spatial scales. We then simulated changes in canopy cover (remotely sensed in 2018) under three alternative scenarios, (1) a “fuels reduction” scenario representing landscape‐wide 30% reduction in canopy cover, (2) a “restoration” scenario representing more nuanced, spatially variable treatments targeting historical conditions, and (3) a reference, no‐change scenario. Model predictions showed areas of potential gains and losses for species richness, richness of ponderosa pine forest habitat specialists, and the ratio of specialists to generalists at two (1 km2 and 250 m2) spatial scales. Under both fuels reduction and restoration scenarios, we projected greater gains than losses for species richness. Surprisingly, despite restoration more explicitly targeting ecologically relevant historical conditions, fuels reduction benefited bird species richness over a greater spatial extent than restoration, particularly in the lower montane life zone. These benefits reflected generally positive species associations with moderate canopy cover promoted more consistently under the fuels reduction scenario. In practice, contemporary forest management is likely to lie somewhere between the fuels reduction and restoration scenarios represented here. Therefore, our results inform where and how active forest management can best support avian diversity. Although our study raises questions regarding the value of including landscape‐scale heterogeneity as a management objective, we do not question the value of targeting finer scale heterogeneity (i.e., stand and treatment level). Rather, our results combined with those from previous work clarify the scale at which targeting structural heterogeneity and historical reference conditions can promote particular ecosystem services.

Detecting historic tar kilns and tar production sites using high-resolution, aerial LiDAR-derived digital elevation models: Introducing the Tar Kiln Feature Detection workflow (TKFD) using open-access R and FIJI software

Significant declines in longleaf pine ecosystems during the last 150 years have motivated research and conservation programs focused on their restoration to perceived historical reference conditions. However, the ecological impacts of the historical naval stores industry on forest structure, fire regimes, and fuel conditions are not currently considered in reference conditions constructed from 18th, 19th, and 20th century sources. We present the Tar Kiln Feature Detection workflow (TKFD), an open-access, scripted, and replicable process developed in R and FIJI to identify archaeological tar kilns within high-resolution digital elevation models derived from aerial LiDAR datasets. The workflow is developed and validated on the entirety of the Francis Marion National Forest in coastal South Carolina. The TKFD has identified and measured over 2,700 tar kilns within our 420,000-acre study area and validation studies demonstrate a balanced identification accuracy of 90.6%. This is the most comprehensive dataset of tar production sites in North America and has implications for understanding the historical distribution of longleaf pine stands, anthropogenic impacts on fire and fuels, and the nature of these unique archaeological sites.

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.

Evaluating Restoration Treatment Effectiveness through a Comparison of Residual Composition, Structure, and Spatial Pattern with Historical Reference Sites

AbstractForest-restoration efforts are increasing in the western United States in response to realized and expected changes in climate and disturbance regimes. Managers are challenged to find practical and defensible targets to shift forest composition, structure, and spatial pattern to a more resistant and resilient state. The Northeast Washington Forest Vision 2020 project on the Colville National Forest presented an opportunity to map and use previously uncaptured mesic stand-level historical reference conditions to a large restoration project. We reconstructed historical forest conditions in 12 plots across a range of plant-association groups and mapped five restoration treatment units after implementation. We evaluated treatment effectiveness both in terms of meeting the prescriptions’ stated objectives and by similarity to observed reference conditions using metrics of density, species composition, clump-size patterns, and open-space patterns. We found that dry plant associations were historically dominated by distributed clumps of large shade-intolerant trees, whereas cold mesic plant associations were structured as a gap-matrix spatial pattern. Treatments were effective at meeting prescribed density and species-composition targets, but generally resulted in stands that were overly uniform or clumped compared to historical reference conditions.

A taxonomic, phylogenetic, and functional comparison of restoration seed mixes and historical plant communities in Midwestern oak savannas

Ecological restoration offers an effective way to promote recovery of natural communities and is particularly important for ecosystems that have experienced extensive habitat loss, such as oak savannas in the Midwestern USA. Seeds are commonly added to restoration sites to introduce desired species, but the identity and composition of which species are included in seed mixes varies greatly and may be influenced more by seed availability and cost rather than historic prevalence or ecological function. Therefore, a key question is how the composition and function of seeded species differs from what was historically present. Here, we compare the taxonomy, phylogeny, and functional traits of savanna seed mixes that are commercially available in the Midwest with historical vegetation surveys of savannas in southern Wisconsin. Species composition differs greatly between historical savannas and seed mixes; most species are unique to either seed mixes or historical savannas rather than shared between them. The functional overlap of species (measured via functional traits: seed mass, SLA, and plant height), however, is remarkably similar between seed mixes and historical savannas. The phylogenetic representation between the two groups is also similar, however seed mixes tend to not include certain plant families and species are more clustered across the phylogeny in seed mixes. Together, this suggests that restoration efforts using commercially available seed mixes may only partially recover the species composition of historical oak savannas, but that the functional capacity of restored sites is similar to the past. Determining how restoration seeding efforts compare to historical reference conditions is an important aspect of assessing whether restoration goals related to species composition and ecosystem function are being achieved and may identify gaps that can be filled when designing seed mixes.

Fire regimes and structural changes in oak-pine forests of the Mogollon Highlands ecoregion: Implications for ecological restoration

Ponderosa pine (Pinus ponderosa) forests occur at their warmer, drier environmental limits in the Mogollon Highlands ecoregion (MHE) of the Southwestern United States, and are commonly found in stringers or discrete stands that form ecotones with interior chaparral. These “rear edge” forests are likely to be highly vulnerable to rapid changes in structure and composition with climate warming, drought, and wildfire. There is increasing interest in understanding historical conditions, ecosystem changes, and restoration needs for MHE forests. However, comprehensive reconstruction analysis of fire regimes and stand structure has not been done for these systems, which differ from many montane ponderosa pine forests by having an abundance of understory shrubs. In this study we used demographic data from field plots, fire scar samples, and dendroecology to reconstruct historical fire regimes and landscape structure at ponderosa pine-dominated sites that spanned a range of environmental conditions on the Prescott and Tonto National Forests. We found strong evidence of historical surface fire regimes with mean fire intervals ranging 1.3–15.6 years across the five MHE sites during the period 1700–1879. We found very little evidence of historical high-severity fire at any study site. Historical forest structure was open with tree densities ranging 84.7–136.4 trees ha−1 and stand basal area (BA) ranging 4.5–8.4 m2 ha−1. Historical composition showed codominance of ponderosa pine, Arizona white oak (Quercus arizonica), Emory oak (Q. emoryi), and Gambel oak (Q. gambelii). Thus, oak species and likely other hardwoods were important historical components of these ecosystems. Contemporary forests are greater in stand density and BA by 359–703% and 285–502%, respectively, compared to historical estimates. In addition, we observed contemporary shifts in species composition. Changes related to disruption of historical fire regimes have increased susceptibility of ponderosa pine forests in the MHE to rapid shifts in structure and composition that may come about with climate change and high-intensity wildfire. Meeting fuels reduction and ecological restoration goals will be challenging for land managers due to vigorous regeneration responses of shrubs to tree thinning, prescribed burning, or other management activities. Managers will be required to balance attention to historical reference conditions, conservation of biological diversity, and needs for fuels management.

Historical range of variability for restoration and management in Wisconsin

In Wisconsin, as in other states, management goals sometimes include restoration of historical forest conditions, which may prepare forests to be more compatible with future climates, disturbances such as drought and fire, and forest health threats. We quantified historical (1830–1866) composition and structure to develop historical reference conditions for restoration and documented changes based on current (2005–2009) forest surveys in Wisconsin. We provided structural metrics, functional group composition, and forest types for 186 ecological land types, and we also summarized trends in composition and structure. Wisconsin forests historically were comprised of 46% oak or pine savanna or woodland, 6% pine forest, and 48% forests primarily consisting of late-successional eastern broadleaf forest species and early-successional northern mixed forest species; densities of these forest types ranged from 60 to 460 trees/ha. In the Eastern Broadleaf Forest ecological division, increased composition of the early-successional and mid-successional eastern broadleaf forest groups (from 10 to 40%) and (planted) pine group (8–23%) occurred along with decreased fire-tolerant oak composition (from 65 to 23%). Density increased in current forests compared to historical forests by a factor of 2.2; despite increased density, basal area increased only slightly due to the presence of larger diameter trees in historical tree surveys. In the Northern Mixed Forest ecological division, increased composition of the mid-successional eastern broadleaf forest group (from 12 to 24%) and late-successional northern mixed forest group (from 10 to 17%) occurred due to decreased composition of the fire-tolerant pine group (from 17 to 9%) and late-successional eastern broadleaf forest group (from 30 to 20%). Density remained similar in current forests compared to historical forests but current basal area was 50% of historical basal area. The transition from open fire-tolerant oak and pine forests, with rarity of early-successional tree species, to closed forests composed of a variety of early- and mid-successional tree species parallels results from other research. Replacement of open oak or pine forest ecosystems by dense forests has moved Wisconsin outside of the historical range of variability, likely reducing plant and wildlife species associated with open oak and pine ecosystems.

Collaborative restoration effects on forest structure in ponderosa pine-dominated forests of Colorado

In response to large, severe wildfires in historically fire-adapted forests in the western US, policy initiatives, such as the USDA Forest Service’s Collaborative Forest Landscape Restoration Program (CFLRP), seek to increase the pace and scale of ecological restoration. One required component of this program is collaborative adaptive management, in which monitoring data are used to iteratively evaluate and improve future management actions. Here, we assess the success of seven CFLRP treatments, implemented on 2,300 ha during the first three years of the Colorado Front Range Landscape Restoration Initiative (LRI) at achieving desired forest structure by comparing pre- and post-treatment conditions. We also compare post-treatment conditions with reconstructions of historical (ca. 1860) forest conditions to contextualize the magnitude of treatment effects. Restoration projects moved stands toward desired conditions by reducing basal area, tree density, and canopy cover and increasing average tree diameter, large gap cover, and abundance of small- to medium-sized tree groups. Post-treatment stands were similar to historical stands with respect to basal area of ponderosa pine; however, they had higher total tree density and fewer gaps than historical reference conditions, suggesting that restoration prescriptions may be improved with increased flexibility for density reduction of Douglas-fir and increased gap creation. This examination of early CFLRP treatment outcomes as they relate to desired conditions informs potential areas of adjustments to future treatments and provides baseline data to evaluate the evolution of treatments over the program’s lifespan. We also identify and discuss several scientific, social, and logistical constraints to large-scale restoration success and make several recommendations to improve restoration outcomes.

Assessment of the ecological health of heavily utilized, large lowland rivers: example of the lower Yellow River, China

We describe an approach to assessing biological health in a heavily utilized, large lowland river setting, using the example of the lower Yellow River, China. In this study, the river was divided into four reaches, and a field survey of fish, benthic macroinvertebrates, and riparian plants was undertaken. Data from historical records were used for reference. The scoring system used weighting to adjust for differences in the importance of the chosen indicators and the relative efficiency of the sampling effort. The results indicated that the biological health of the lower Yellow River is distant from historical reference conditions, with its condition generally declining in the downstream direction. This result is consistent with the river’s history of impairment of hydrology, water quality, wetland area and character, and physical form. We conclude that the reference river state based on historical conditions is unlikely to equate to the best attainable river health because certain changes have occurred in the river that may prohibit a return to the previous state of ecological health. On the basis of the results of this assessment as well as a review of the literature, we propose field data collection methodologies and indicators that can be applied in future assessments.

Using trait‐based ecology to restore resilient ecosystems: historical conditions and the future of montane forests in western North America

AbstractHistorical reference conditions have provided empirical benchmarks for guiding ecological restoration but the relevance of historical conditions has been questioned in this era of rapid global change. Trait‐based ecology offers an alternative approach for setting restoration objectives. If we understand which phenotypic traits confer high survival in a given environment, then we can restore assemblages of species with favorable trait combinations to reduce mortality risk, achieve functional outcomes, and enhance restoration success. Our objective was to compare restoration prescriptions based on historical reference conditions versus trait‐based objectives in southwestern United States mixed conifer forests. To optimize survival and enhance resilience under projected increasing frequencies of fire and drought, we constructed and evaluated models based on combinations of three traits: thick bark, dense wood, and moderate leaf nitrogen concentration. Models with multiple traits accurately derived historical species abundance distributions, which is a necessary condition for the application of trait‐based models under less certain future conditions. Model results indicated that trait‐based restoration objectives could be achieved in two ways: by manipulating abundances of species that already coexist at a site or by adding native species from warmer climates to the local species pool. The latter approach may create no‐analog communities of native species. Restoration goals based on either historical reference conditions or future projections are special cases of a more general class of desired future conditions that are derived from trait‐based objectives. Functional ecology provides a general, flexible, and theory‐based approach to restoring resilient ecosystems at a time of rapid environmental change.

Building an eco-hydrological framework for the management of large river systems

1.Rehabilitating degraded riverine landscapes in order to restore their ecosystem services require a scientific approach. The tasks include the development of methods for the assessment of ecological conditions and trends, for benchmarking of management alternatives and for the definition of ecological goals.2.The central challenge is to develop a coherent eco-hydrological research protocol analyzing the causal effects of hydrology on geomorphic processes and ecology.3.A major aspect is the formulation of ecological targets: historical reference conditions can be used to illustrate ecological deficiencies. However, at the planning and operational levels, they have to be substituted by a set of models which predict the responses towards management measures.4.Large scale restoration programmes should be accompanied by hypothesis-based research in order to analyze the effects of engineering measures and advance our understanding of river-floodplain ecology.5.A focus has to be the promotion of comprehensive management concepts, taking into account all concerned parties and assuring their active involvement. It is important to take advantage of management initiatives of various stakeholders, because governmental water policies rarely promote ecological improvement as a goal in itself.6.Institutional frameworks should be established with the mandate to develop large-scale and comprehensive rehabilitation concepts and guaranteeing transparent planning and decision processes. It is imperative that expert panels comprising of scientists, river-engineers, planners and managers are installed, with long-term mandates, clearly defined responsibilities and authority for decision making.

Five-year post-restoration conditions and simulated climate-change trajectories in a warm/dry mixed-conifer forest, southwestern Colorado, USA

Some warm/dry mixed-conifer forests are at increasing risk of uncharacteristically large, high-severity fires. As a result, managers have begun ecological restoration efforts using treatments such as mechanical thinning and prescribed fire. Empirical information on the long-term impacts of these treatments is limited, especially in light of potential climate change. We assessed changes in forest structure and composition five-years following three alternative restoration treatments in a warm/dry mixed-conifer forest: (1) thin/burn, (2) prescribe burn, and (3) control. We used the Climate-Forest Vegetation Simulator (Climate-FVS) model to quantify potential forest trajectories under alternative climate scenarios. Five years following treatments, changes in forest structure were similar to initial post-treatment conditions, with thin/burn being the only treatment to shift and maintain forest structure and composition within historical reference conditions. By 2013, the thin/burn had reduced basal area (11.3m2ha−1) and tree density (117.2treeha−1) by 56% and 79% respectively, compared to pre-treatment values. In the burn, basal area (20.5m2ha−1) and tree density (316.6treeha−1) was reduced by 20% and 35% respectively, from 2002 to 2013. Mortality of large ponderosa pine trees (the most fire-resistant species) throughout the duration of the experiment, averaged 6% in the burn compared to 16% in the thin/burn treatment. Changes five years following treatments were largely due to increases in sprouting species. Shrub and sapling densities were approximately two to three times higher (respectively) in the thin/burn compared to burn and control and dominated by sprouting oak and aspen. Under climate simulations, the thin/burn was more resilient in maintaining forest conditions compared to burn and control which approached meager forest conditions (3–4m2ha−1). These results indicate that restoration treatment that include both thinning and burning can maintain forest integrity over the next few decades.

Comparing historical and current wildfire regimes in the Northern Rocky Mountains using a landscape succession model

Wildland fire is a key ecosystem process that shapes the landscape of Western United States. Changes in fire regimes can therefore have profound impact on ecosystem functions and services, including carbon cycling, habitat conditions, and biodiversity. This study examined fire regime changes in the Northern Rocky Mountain region using a landscape succession model (LANDSUMv4). The objective is to report a new method to develop fire regime condition class (FRCC) maps by comparing historical reference conditions of fire regimes – simulated using historical fire record and the LANDSUMv4 model – with current fire regimes developed using 27years remote sensing fire product and LANDSUMv4. Results of this study indicate that most forest ecosystems in the study region experienced less frequent but more severe fires during the contemporary time period compared to the historical conditions. Fire regimes have changed the most for montane forests which historically were dominated by frequent and nonlethal fires. Forest ecosystems characterized by infrequent stand replacement fires also experienced moderate departure from historical fire conditions. FRCC assessment method proposed in this study can be applied to other areas of U.S. The findings of this study will help reveal contemporary fire dynamics in this region and serve for future fire and fuel studies and other forest management applications.

Quantifying habitat loss: Assessing tree encroachment into a serpentine savanna using dendroecology and remote sensing

For over a century, increases in the abundance of woody plants in savannas have been occurring worldwide in a process known as encroachment. Encroachment into savannas is a significant management concern because it affects the unique values associated with savanna systems, including high levels of both taxonomic and landscape diversity. Improving methods for reconstructing encroachment patterns should aid savanna management, especially if the methods provide a baseline from which to assess and project ecological change. We reconstructed the encroachment history of a small serpentine Jeffrey pine savanna and forecasted future landscape change using two distinct approaches. First, we used dendroecology to determine encroachment rates, establish historical site reference conditions, and project tree-growth trends. Second, we used historical aerial photographs to construct a spatial model of past tree encroachment and to predict near future encroachment. We found encroachment began ∼1850 and was not related to differences in topography across the landscape. Trees greater than 30cm in diameter have increased from a mean stem density of ∼1.6 trees per ha in 1890 to a mean stem density of ∼13.8 trees per ha in 2009. Concomitant with the increase in tree density and average tree size was the contraction of the grass-dominated areas of the savanna, which represented ∼50% of Little Bald Hills in 1942, but less than 10% in 2009. If current encroachment rates continue, our models suggest that less than 5% of Little Bald Hills will be grassland in 50years. This is not the first study to utilize both historical photo analysis and dendroecology, but it is the first to use these tools to identify explicit locations where encroachment is likely to occur in the near future.

Forest structure and fuels dynamics following ponderosa pine restoration treatments, White Mountains, Arizona, USA

Southwestern ponderosa pine forest ecosystems have become uncharacteristically dense as a result of intensive livestock grazing, logging, and fire exclusion, which have contributed to a buildup of fuels and increased vulnerability to high-severity, landscape-scale crown fires. In 2002, we implemented a replicated ecological restoration experiment to (1) quantify site-specific reference conditions, (2) analyze effects of elevation on forest structure, surface fuels, and canopy fuels, (3) test responses to three treatments: control, full treatment, and burn-only, and (4) evaluate effectiveness of treatments for restoring attributes to near historical reference conditions. Reconstructed reference conditions showed that basal area (BA) averaged 9.2m2ha−1 and tree density averaged 86.2treesha−1 across the site in 1880 prior to widespread fire exclusion, with the highest tree density values occurring at the two upper elevation sites. In 2002, prior to treatment, BA averaged 28.9m2ha−1 and density averaged 927.9treesha−1, representing three- and ten-fold increases, respectively, compared to 1880 values with a higher proportion of the increase coming from mesic species at higher elevations. By 2013 (5-years post-treatment), the full treatment showed BA was reduced by 52% compared to pre-treatment values from 31m2ha−1 to 15m2ha−1 and density was reduced by 85% from 998treesha−1 to 153treesha−1. In the burn-only treatment, BA increased by 6% from 27.5m2ha−1 to 29.1m2ha−1 although density was reduced by 25% from 837treesha−1 to 625treesha−1 between 2002 and 2013. Canopy fuels dynamics were similar to forest structure responses: low values prior to fire exclusion, a marked increase by the pre-treatment measurement, substantial reductions following thinning plus burning, and minor reductions following burn-only treatments. Post-treatment diameter distributions in the full treatment closely resembled reference conditions; minor reductions occurred in the lower diameter classes in the burn-only. Mortality of trees established before European settlement circa 1880 varied by treatment: 25% of presettlement trees alive in 2002 died by 2013 in the full treatment, 31% died in the burn-only treatment, and 15% died in the control. Of the treatments tested, the full treatment was the only one that rapidly shifted forest structure, diameter distributions, and canopy fuels values to levels near or within the historical reference conditions. Results from this study showed that managers will have to consider tradeoffs between alternative treatments in the context of climate-induced, landscape-scale forest fires.

Reconstructing Vegetation Past: Pre-Euro-American Vegetation for the Midwest Driftless Area, USA

Historical reference conditions provide important context for creating ecological restoration and management plans. The U.S. 19th Century Public Land Survey (PLS) records provide extensive ecological information for constructing such reference conditions. We used PLS records to reconstruct pre-Euro-American tree species cover class and vegetation structure types for the Midwest Driftless Area, a 55,000 km2 region currently experiencing multiple conservation threats. We related cover classes to soil texture, topographic roughness, and distance from waterway. Our analyses revealed that the landscape of the Driftless Area was mostly composed of savanna, with two large patches of closed forest and smaller, scattered patches of closed forest, open woodland, and prairie. The Driftless Area was heavily dominated by a variety of oak communities, with bur ( Quercus macrocarpa ), white ( Q. alba ), and black ( Q. velutina ) oak by far the most dominant species across the region. A variety of non-oak communities occurred within the closed forest patches, along rivers, or in smaller areas near the periphery of the region. The prevalence of savanna and oak communities indicates that fire played a key role in mediating historical landscape patterns and ecosystem processes in the region. Variation in soil texture, topographic roughness, and distance from waterways additionally contributed to the diversity of cover classes present prior to Euro-American settlement. Restoration practitioners can use our reconstructions to inform regional and site-specific restoration planning. Because oaks tend to be foundational species within ecosystems and are currently in decline throughout the Driftless Area, restoration activities that encourage these species are urgently needed.

Establishing water quality reference conditions for nutrients, chlorophyll a and Secchi depth for 7 typical lakes in arid and semiarid ecoregion, China

The increasingly serious lake eutrophication triggers the need for pre-disturbance conditions or historical reference conditions. Establishing nutrient reference condition of lakes in ecoregion is a critical consideration in the development of scientifically defensible aquatic nutrient criteria. Total dissolved solid (TDS) has a significant influence on the growth of algae in arid and semiarid ecoregion (Xinjiang ecoregion) lakes, China. Three methods including lake population distribution, trisection method and multiple regression models were applied to determine reference conditions for seven typical lakes (based on TDS) with respect to total phosphorus (TP), total nitrogen (TN), planktonic chlorophyll a (Chl a) and Secchi depth (SD). Results show that the first two methods have no significant differences in reference values, and the third method can be used as the validation of the first two methods. The values or ranges considered as nutrients reference conditions are given as follows: TP 0.005 mg/L, TN 0.11 mg/L, Chl a 0.86–0.96 μg/L, SD 2.69–3.50 m for freshwater lakes; TP 0.005 mg/L, TN 0.58–0.69 mg/L, Chl a 0.64–0.86 μg/L, SD 2.64–3.00 m for saltwater lakes; TP 0.010 mg/L, TN 1.13–1.22 mg/L, Chl a 7.00–7.60 μg/L, SD 0.43–0.45 m for brine lakes. Overall, the obtained reference conditions demonstrate that multiple methods can be used to determine reference condition, and reference conditions for various lake types correspond to oligotrophic, oligotrophic, and mesotrophic status, respectively.