Long-term fire frequency not linked to prehistoric occupations in northern Swedish boreal forest

Knowledge of past fire regimes is crucial for understanding the changes in fire frequency that are likely to occur during the coming decades as a result of global warming and land-use change. This is a key issue for the sustainable management of forest biodiversity because fire regimes may be controlled by vegetation, human activities, and/or climate. The present paper aims to reconstruct the pattern of fire frequency over the Holocene at three sites located in the same region in the northern Swedish boreal forest. The fire regime is reconstructed from sedimentary charcoal analysis of small lakes or ponds. This method allows fire events to be characterized, after detrending the charcoal influx series, and allows estimation of the time elapsed between fires. The long-term fire regime, in terms of fire-free intervals, can thus be elucidated. At the three sites, the mean fire-free intervals through the Holocene were long and of similar magnitude (approximately 320 years). This similarity suggests that the ecological processes controlling fire ignition and spread were the same. At the three sites, the intervals were shorter before 8600 cal yr BP (calibrated years before present), between 7500 and 4500 cal yr BP, and after 2500 cal yr BP. Geomorphological and vegetation factors cannot explain the observed change, because the three sites are located in the same large ecological region characterized by Pinus sylvestris-Ericaceae mesic forests, established on morainic deposits at the same elevation. Archaeological chronologies also do not match the fire chronologies. A climatic interpretation is therefore the most likely explanation of the long-term regional pattern of fire. Although recent human activities between the 18th and the 20th centuries have clearly affected the fire regime, the dominant factor controlling it for 10000 years in northern Sweden has probably been climatic.     

Decomposing the mid-Holocene Tsuga decline in eastern North America

The mid-Holocene decline of Tsuga canadensis (hereafter Tsuga) populations across eastern North America is widely perceived as a synchronous event, driven by pests/pathogens, rapid climate change, or both. Pattern identification and causal attribution are hampered by low stratigraphic density of pollen-sampling and radiometric dates at most sites, and by absence of highly resolved, paired pollen and paleoclimate records from single sediment cores, where chronological order of climatic and vegetational changes can be assessed. We present an intensely sampled (contiguous 1-cm intervals) record of pollen and water table depth (inferred from testate amoebae) from a single core spanning the Tsuga decline at Irwin Smith Bog in Lower Michigan, with high-precision chronology. We also present an intensively sampled pollen record from Tower Lake in Upper Michigan. Both sites show high-magnitude fluctuations in Tsuga pollen percentages during the pre-decline maximum. The terminal decline is dated at both sites ca. 5000 cal yr BP, some 400 years later than estimates from other sites and data compilations. The terminal Tsuga decline was evidently heterochronous across its range. A transient decline ca. 5350 cal yr BP at both sites may correspond to the terminal decline at other sites in eastern North America. At Irwin Smith Bog, the terminal Tsuga decline preceded an abrupt and persistent decline in water table depths by approximately 200 years, suggesting the decline was not directly driven by abrupt climate change. The Tsuga decline may best be viewed as comprising at least three phases: a long-duration pre-decline maximum with high-magnitude and high-frequency fluctuations, followed by a terminal decline at individual sites, followed in turn by two millennia of persistently low Tsuga populations. These phases may not be causally linked, and may represent dynamics taking place at multiple temporal and spatial scales. Further progress toward understanding the phenomenon requires an expanded network of high-resolution pollen and paleoclimate chronologies.     

The influence of aridity and fire on holocene prairie communities in the eastern Prairie Peninsula

The role of climate and fire in the development, maintenance, and species composition of prairie in the eastern axis of the tallgrass Prairie Peninsula intrigued early North American ecologists. However, evaluation of the long-standing hypotheses about the region's environmental history has been hampered by the scarcity of paleorecords. We conducted multiproxy analyses on early and middle Holocene sediments from two Illinois, USA, lakes to assess long-term climatic, vegetational, and fire variability in the region. Sediment mineral composition, carbonate delta18O, ostracode assemblages, and diatom assemblages were integrated to infer fluctuations in moisture availability. Pollen and charcoal delta13C were used to reconstruct vegetation composition, and charcoal influx was used to reconstruct fire. Results indicate that fire-sensitive trees (e.g., Ulmus, Ostrya, Fraxinus, and Acer saccharum) declined and prairie taxa expanded with increased aridity from 10,000 yr BP to 8500 yr BP. Between approximately 8500 yr BP and approximately 6200 yr BP, aridity declined, and prairie coexisted with fire-sensitive and fire-tolerant (e.g., Quercus and Carya) trees. After approximately 6200 yr BP, prairie taxa became dominant, although aridity was not more severe than it was around 8500 yr BP. Along with aridity, fire appears to have played an important role in the establishment and maintenance of prairie communities in the eastern Prairie Peninsula, consistent with the speculations of the early ecologists. Comparison of our data with results from elsewhere in the North American midcontinent indicates that spatial heterogeneity is a characteristic feature of climatic and vegetational variations on millennial time scales.     

Multidecadal climate variability and climate interactions affect subalpine fire occurrence, western Colorado (USA)

This study investigates the influence of climatic variability on subalpine forest fire occurrence in western Colorado during the AD 1600-2003 period. Interannual and multidecadal relationships between fire occurrence and the El Ni?o Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) were examined, in addition to the effects of phase interactions among these oscillations. Fires occurred during short-term periods of significant drought and extreme cool (negative) phases of ENSO and PDO and during positive departures from mean AMO index. At longer time scales, fires exhibited 20-year periods of synchrony with the cool phase of the PDO, and 80-year periods of synchrony with extreme warm (positive) phases of the AMO. Years of combined positive AMO and negative ENSO and PDO phases represent "triple whammies" that significantly increased the occurrence of drought-induced fires. Fires were synchronous with this phase combination over 0-30 year periods and distinctly asynchronous with the opposite phase combination. Overall, because fires are synchronous at supra-annual to multidecadal time scales with warm AMO events, particularly when combined with cool ENSO and PDO phases, this suggests that we may be entering a qualitatively different fire regime in the next few decades due to the recent shift in 1998 to a likely long-term warm AMO phase. Although uncertainty remains regarding the effects of CO2-induced warming at regional scales, given the multidecadal persistence of the AMO there is mounting evidence that the recent shift to the positive phase of the AMO will promote higher fire frequencies in the region.     

Testing a pyroclimatic hypothesis on the Mexico-United States border

The "pyroclimatic hypothesis" proposed by F. Biondi and colleagues provides a basis for testable expectations about climatic and other controls of fire regimes. This hypothesis asserts an a priori relationship between the occurrence of widespread fire and values of a relevant climatic index. Such a hypothesis provides the basis for predicting spatial and temporal patterns of fire occurrence based on climatic control. Forests near the Mexico-United States border offer a place to test the relative influence of climatic and other controls in mountain ranges that are ecologically similar and subject to broadly similar top-down climatic influence, but with differing cultural influences. We tested the pyroclimatic hypothesis by comparing fire history information from the Mesa de las Guacamayas, a mountain range in northwestern Chihuahua, with previously published fire data from the Chiricahua Mountains, in southeastern Arizona, approximately 150 km away. We developed a priori hypothetical models of fire occurrence and compared their performance to empirical climate-based models. Fires were frequent at all Mesa de las Guacamayas study sites through the mid-20th century and continued uninterrupted to the present at one site, in contrast to nearly complete fire exclusion after 1892 at sites in the Chiricahua Mountains. The empirical regression models explained a higher proportion of the variability in fire regime associated with climate than did the a priori models. Actual climate-fire relationships diverged in each country after 1892. The a priori models predicted continuing fires at the same rate per century as prior to 1892; fires did in fact continue in Mexico, albeit with some alteration of fire regimes, but ceased in the United States, most likely due to changes in land use. The cross-border comparison confirms that a frequent-fire regime could cease without a climatic cause, supporting previous arguments that bottom-up factors such as livestock grazing can rapidly and drastically alter surface fire regimes. Understanding the historical patterns of climate controls on fire could inform the use of historical data as ecological reference conditions and for future sustainability.     

Variation at local scales need not impede tests for broader scale patterns

Ecologists are not always mindful of the constraints imposed by their scale of observation and sometimes prematurely attempt broad generalisations or become mesmerised by local details depending on their predilections. We specifically chose a character that is known for its local and unpredictable variation (morphology of kelp) to test the effect of scale on our ability to determine spatial patterns. We compared the morphology of Ecklonia radiata between monospecific and mixed stands of canopy-forming algae across temperate Australia (>5,100 km coastal distance) within a hierarchy of several spatial scales. While E. radiata specimens were generally larger in monospecific than in mixed stands, we failed to observe differences in morphology between stands at many sites and locations. Despite substantial local variation, differences between stands became increasingly clear at broader scales. The frequency of inconsistent differences between stands was greatest at local scales (sites separated by kms), intermediate at intermediate scales (locations separated by 100s of kms) and least at regional scales (regions separated by 1,000s of kms). These observations support the idea that large scale patterns can emerge from apparent stochasticity at small scales, and that unaccountable variation at local scales need not impede tests for similar patterns at broader scales. Most ecologists work at scales where complexity tends to be greatest (i.e. local) and is likely to be explained by special and unique events. It is encouraging, therefore, to observe that patterns can emerge from complexity at local scales to provide new opportunities to answer some of the more interesting questions about the relative importance of processes across the vast parts of the worlds coast.Communicated by M.S. Johnson, Crawley     

A Hierarchical Bayesian model of wildfire in a Mediterranean biodiversity hotspot: Implications of weather variability and global circulation

In this study we combined an extensive database of observed wildfires with high-resolution meteorological data to build a novel spatially and temporally varying survival model to analyze fire regimes in the Mediterranean ecosystem in the Cape Floristic Region (CFR) of South Africa during the period 1980-2000. The model revealed an important influence of seasonally anomalous weather on fire probability, with increased probability of fire in seasons that are warmer and drier than average. In addition to these local-scale influences, the Antarctic Ocean Oscillation (AAO) was identified as an important large-scale influence or teleconnection to global circulation patterns. Fire probability increased in seasons during positive AAO phases, when the subtropical jet moves northward and low level moisture transport decreases. These results confirm that fire occurrence in the CFR is strongly affected by climatic variability at both local and global scales, and thus likely to respond sensitively to future climate change. Comparison of the modelled fire probability between two periods (1951-1975 and 1976-2000) revealed a 4-year decrease in an average fire return time. If, as currently forecasted, climate change in the region continues to produce higher temperatures, more frequent heat waves, and/or lower rainfall, our model thus indicates that fire frequency is likely to increase substantially. The regional implications of shorter fire return times include shifting community structure and composition, favoring species that tolerate more frequent fires.     

A 700-year paleoecological record of boreal ecosystem responses to climatic variation from Alaska

Recent observations and model simulations have highlighted the sensitivity of the forest-tundra ecotone to climatic forcing. In contrast, paleoecological studies have not provided evidence of tree-line fluctuations in response to Holocene climatic changes in Alaska, suggesting that the forest-tundra boundary in certain areas may be relatively stable at multicentennial to millennial time scales. We conducted a multiproxy study of sediment cores from an Alaskan lake near the altitudinal limits of key boreal-forest species. Paleoecological data were compared with independent climatic reconstructions to assess ecosystem responses of the forest tundra boundary to Little Ice Age (LIA) climatic fluctuations. Pollen, diatom, charcoal, macrofossil, and magnetic analyses provide the first continuous record of vegetation fire-climate interactions at decadal to centennial time scales during the past 700 years from southern Alaska. Boreal-forest diebacks characterized by declines of Picea mariana, P. glauca, and tree Betula occurred during the LIA (AD 1500-1800), whereas shrubs (Alnus viridis, Betula glandulosa/nana) and herbaceous taxa (Epilobium, Aconitum) expanded. Marked increases in charcoal abundance and changes in magnetic properties suggest increases in fire importance and soil erosion during the same period. In addition, the conspicuous reduction or disappearance of certain aquatic (e.g., Isoetes, Nuphar, Pediastrum) and wetland (Sphagnum) plants and major shifts in diatom assemblages suggest pronounced lake-level fluctuations and rapid ecosystem reorganization in response to LIA climatic deterioration. Our results imply that temperature shifts of 1-2 degrees C, when accompanied by major changes in moisture balance, can greatly alter high-altitudinal terrestrial, wetland, and aquatic ecosystems, including conversion between boreal-forest tree line and tundra. The climatic and ecosystem variations in our study area appear to be coherent with changes in solar irradiance, suggesting that changes in solar activity contributed to the environmental instability of the past 700 years.     

Paleobiology, community ecology, and scales of ecological pattern

The fossil record provides a wealth of data on the role of regional processes and historical events in shaping biological communities over a variety of time scales. The Quaternary record with its evidence of repeated climatic change shows that both terrestrial and marine species shifted independently rather than as cohesive assemblages over scales of thousands of years. Larger scale patterns also show a strong individualistic component to taxon dynamics; assemblage stability, when it occurs, is difficult to separate from shared responses to low rates of environmental change. Nevertheless, the fossil record does suggest that some biotic interactions influence large-scale ecological and evolutionary patterns, albeit in more diffuse and protracted fashions than those generally studied by community ecologists. These include: (1) the resistance by incumbents to the establishment of new or invading taxa, with episodes of explosive diversification often appearing contingent on the removal of incumbents at extinction events; (2) steady states of within-habitat and global diversity at longer time scales (10(7)-l0(8) yr), despite enormous turnover of taxa; and (3) morphological and biogeographic responses to increased intensities of predation and substratum disturbance over similarly long time scales. The behavior of species and communities over the array of temporal and spatial scales in the fossil record takes on additional significance for framing conservation strategies, and for understanding recovery of species, lineages, and communities from environmental changes.     

Spatial synchrony in coral reef fish populations and the influence of climate

We investigated spatial patterns of synchrony among coral reef fish populations and environmental variables over an eight-year period on the Great Barrier Reef, Australia. Our aims were to determine the spatial scale of intra- and interspecific synchrony of fluctuations in abundance of nine damselfish species (genus Pomacentrus) and assess whether environmental factors could have influenced population synchrony. All species showed intraspecific synchrony among populations on reefs separated by < or =100 km, and interspecific synchrony was also common at this scale. At greater spatial scales, only four species showed intraspecific synchrony, over distances ranging from 100-300 km to 500-800 km, and no cases of interspecific synchrony were recorded. The two mechanisms most likely to cause population synchrony are dispersal and environmental forcing through regionally correlated climate (the Moran effect). Dispersal may have influenced population synchrony over distances up to 100 km as this is the expected spatial range for ecologically significant reef fish dispersal. Environmental factors are also likely to have synchronized population fluctuations via the Moran effect for three reasons: (1) dispersal could not have caused interspecific synchrony that was common over distances < or =100 km because dispersal cannot link populations of different species, (2) variations in both sea surface temperature and wind speed were synchronized over greater spatial scales (>800 km) than fluctuations in damselfish abundance (< or =800 km) and were correlated with an index of global climate variability, the El Ni?o-Southern Oscillation (ENSO), and (3) synchronous population fluctuations of most damselfish species were correlated with ENSO; large population increases often followed ENSO events. We recorded regional variations in the strength of population synchrony that we suspect are due to spatial differences in geophysical, oceanographic, and population characteristics, which act to dilute or enhance the effects of synchronizing mechanisms. We conclude that synchrony is common among Pomacentrus populations separated by tens of kilometers but less prevalent at greater spatial scales, and that environmental variation linked to global climate is likely to be a driving force behind damselfish population synchrony at all spatial scales on the Great Barrier Reef.     

Potential shifts in dominant forest cover in interior Alaska driven by variations in fire severity

Large fire years in which >1% of the landscape burns are becoming more frequent in the Alaskan (USA) interior, with four large fire years in the past 10 years, and 79 000 km2 (17% of the region) burned since 2000. We modeled fire severity conditions for the entire area burned in large fires during a large fire year (2004) to determine the factors that are most important in estimating severity and to identify areas affected by deep-burning fires. In addition to standard methods of assessing severity using spectral information, we incorporated information regarding topography, spatial pattern of burning, and instantaneous characteristics such as fire weather and fire radiative power. Ensemble techniques using regression trees as a base learner were able to determine fire severity successfully using spectral data in concert with other relevant geospatial data. This method was successful in estimating average conditions, but it underestimated the range of severity. This new approach was used to identify black spruce stands that experienced intermediate- to high-severity fires in 2004 and are therefore susceptible to a shift in regrowth toward deciduous dominance or mixed dominance. Based on the output of the severity model, we estimate that 39% (approximately 4000 km2) of all burned black spruce stands in 2004 had <10 cm of residual organic layer and may be susceptible a postfire shift in plant functional type dominance, as well as permafrost loss. If the fraction of area susceptible to deciduous regeneration is constant for large fire years, the effect of such years in the most recent decade has been to reduce black spruce stands by 4.2% and to increase areas dominated or co-dominated by deciduous forest stands by 20%. Such disturbance-driven modifications have the potential to affect the carbon cycle and climate system at regional to global scales.     

Beyond Kyoto: Forest Management in a Time of Rapid Climate Change

Abstract: Policies to reduce global warming by offering credits for carbon sequestration have neglected the effects of forest management on biodiversity. I review properties of forest ecosystems and management options for enhancing the resistance and resilience of forests to climate change. Although forests, as a class, have proved resilient to past changes in climate, today's fragmented and degraded forests are more vulnerable. Adaptation of species to climate change can occur through phenotypic plasticity, evolution, or migration to suitable sites, with the latter probably the most common response in the past. Among the land-use and management practices likely to maintain forest biodiversity and ecological functions during climate change are (1) representing forest types across environmental gradients in reserves; (2) protecting climatic refugia at multiple scales; (3) protecting primary forests; (4) avoiding fragmentation and providing connectivity, especially parallel to climatic gradients; (5) providing buffer zones for adjustment of reserve boundaries; (6) practicing low-intensity forestry and preventing conversion of natural forests to plantations; ( 7) maintaining natural fire regimes; (8) maintaining diverse gene pools; and (9) identifying and protecting functional groups and keystone species. Good forest management in a time of rapidly changing climate differs little from good forest management under more static conditions, but there is increased emphasis on protecting climatic refugia and providing connectivity.     

Century-long synchrony of fossil algae in a chain of Canadian prairie lakes

Synchronous fluctuations in limnological variables among lakes may signal that large-scale environmental factors regulate lake ecosystem structure, yet most estimates of temporal coherence are based on short (<25 yr) time series, and little is known of how synchrony varies among biological taxa or of the causes of temporal coherence. Here we used time series of 13 fossil pigments from diverse algal groups in seven lakes of the climatically sensitive Northern Great Plains to demonstrate that algal synchrony (S) during the 20th century arose mainly from interdecadal increases in algal abundance rather than from interannual coherence. Synchrony of time series differed greatly among algal taxonomic groups (S = 0.0-0.75) and was not usually spatially structured, but decreased 83% following removal of long-term trends using exponential models or first-difference calculations. Overall, coherence was greatest for labile compounds chl a (S = 0.53) and fucoxanthin (S = 0.75), possibly reflecting the influence of postdepositional degradation processes on fossil time series. However, analysis of chemically stable pigments also indicated that synchrony was great for cryptophytes (as alloxanthin, S = 0.42) and diatoms (diatoxanthin, S = 0.37), taxa that bloom in spring. In contrast, synchrony of total algal abundance was low (beta-carotene, pheophytin a, S < 0.10), reflecting low interannual coherence of summer taxa including colonial cyanobacteria and chlorophytes. Unexpectedly, past variations in climate, resource use, and urbanization explained >85% of algal variation in individual lakes, but only 35% of synchronous algal fluctuations, suggesting that the factors controlling algal synchrony differ from those regulating algal abundance.     

A model of surface fire,climate and forest pattern in the Sierra Nevada,California

A spatially explicit forest gap model was developed for the Sierra Nevada, California, and is the first of its kind because it integrates climate, fire and forest pattern. The model simulates a forest stand as a grid of 15×15 m forest plots and simulates the growth of individual trees within each plot. Fuel inputs are generated from each individual tree according to tree size and species. Fuel moisture varies both temporally and spatially with the local site water balance and forest condition, thus linking climate with the fire regime. Fires occur as a function of the simulated fuel loads and fuel moisture, and the burnable area is simulated as a result of the spatially heterogeneous fuel bed conditions. We demonstrate the model’s ability to couple the fire regime to both climate and forest pattern. In addition, we use the model to investigate the importance of climate and forest pattern as controls on the fire regime. Comparison of model results with independent data indicate that the model performs well in several areas. Patterns of fuel accumulation, climatic control of fire frequency and the influence of fuel loads on the spatial extent of fires in the model are particularly well-supported by data. This model can be used to examine the complex interactions among climate, fire and forest pattern across a wide range of environmental conditions and vegetation types. Our results suggest that, in the Sierra Nevada, fuel moisture can exert an important control on fire frequency and this control is especially pronounced at sites where most of the annual precipitation is in the form of snow. Fuel loads, on the other hand, may limit the spatial extent of fire, especially at elevations below 1500 m. Above this elevation, fuel moisture may play an increasingly important role in limiting the area burned.     

Climate determines upper, but not lower, altitudinal range limits of Pacific Northwest conifers

Does climate determine species' ranges? Rapid rates of anthropogenic warming make this classic ecological question especially relevant. We ask whether climate controls range limits by quantifying relationships between climatic variables (precipitation, temperature) and tree growth across the altitudinal ranges of six Pacific Northwestern conifers on Mt. Rainier, Washington, USA. Results for three species (Abies amabilis, Callitropsis nootkatensis, Tsuga mertensiana) whose upper limits occur at treeline (> 1600 m) imply climatic controls on upper range limits, with low growth in cold and high snowpack years. Annual growth was synchronized among individuals at upper limits for these high-elevation species, further suggesting that stand-level effects such as climate constrain growth more strongly than local processes. By contrast, at lower limits climatic effects on growth were weak for these high-elevation species. Growth-climate relationships for three low-elevation species (Pseudotsuga menziesii, Thuja plicata, Tsuga heterophylla) were not consistent with expectations of climatic controls on upper limits, which are located within closed-canopy forest (< 1200 m). Annual growth of these species was poorly synchronized among individuals. Our results suggest that climate controls altitudinal range limits at treeline, while local drivers (perhaps biotic interactions) influence growth in closed-canopy forests. Climate-change-induced range shifts in closed-canopy forests will therefore be difficult to predict accurately.     

A spatially explicit model to predict future landscape composition of aspen woodlands under various management scenarios

Quaking aspen (Populus tremuloides) is declining across the western United States. Aspen habitats are among the most diverse plant communities in this region and loss of these habitats can result in shifts in biodiversity, productivity, and hydrology across a range of spatial scales. Western aspen occurs on the majority of sites seral to conifer species, and long-term maintenance of these aspen woodlands requires periodic fire. Over the past century, fire intervals, extents, and intensities have been insufficient to regenerate aspen stands at historic rates; however the effects of various fire regimes and management scenarios on aspen vegetation dynamics at broad spatial and temporal scales are unexplored. Here we use field data, remotely sensed data, and fire atlas information to develop a spatially explicit landscape simulation model to assess the effects of current and historic wildfire regimes and prescribed burning programs on landscape vegetation composition across two mountain ranges in the Owyhee Plateau, Idaho. Model outputs depict the future structural makeup and species composition of the landscape at selected time steps under simulated management scenarios. We found that under current fire regimes and in the absence of management activities, loss of seral aspen stands will continue to occur over the next two centuries. However, a return to historic fire regimes (burning 12–14% of the modeled landscape per decade) would maintain the majority of aspen stands in early and mid seral woodland stages and minimizes the loss of aspen. A fire rotation of 70–80 years was estimated for the historic fire regime while the current fire regime resulted in a fire rotation of 340–450 years, underscoring the fact that fire is currently lacking in the system. Implementation of prescribed burning programs, treating aspen and young conifer woodlands according to historic fire occurrence probabilities, are predicted to prevent conifer dominance and loss of aspen stands.     

Climate variability and spruce beetle (Dendroctonus rufipennis) outbreaks in south-central and southwest Alaska

We used tree ring data (AD 1601-2007) to examine the occurrence of and climatic influences on spruce beetle (Dendroctonus rufipennis) outbreaks in south-central and southwest Alaska and found evidence of regional-scale outbreaks dating from the mid-1700s, related to climate variability at multiple temporal scales. Over interannual time scales (approximately 1-3 years), El Ni?o years, combined with severe late-summer drought, appeared to contribute significantly to spruce beetle outbreaks in the study area. Over multidecadal time scales (up to approximately 40 years), cool-phase Pacific Decadal Oscillation (PDO) conditions tended to precede beetle outbreaks, regardless of the phase of El Ni?o-Southern Oscillation (ENSO). All sites showed low-severity disturbances attributed to spruce beetle damage, most notably during the 1810s. During other major periods of disturbance (i.e., 1870s, 1910s, 1970s), the effects of spruce beetle outbreaks were of moderate or higher severity. The highly synchronized timing of spruce beetle outbreaks at interannual to multidecadal scales, and particularly the association between cool-phase PDO conditions and beetle disturbance, suggests that climate (i.e., temperature, precipitation) is a primary driver of outbreaks in the study area. Our disturbance chronologies (mid-1700s to present) suggest that recent irruptions (1990s to present) in south-central and southwest Alaska are within the historical geographic range, but that outbreaks since the 1990s show greater spatiotemporal synchrony (i.e., more sites record high-severity infestations) than at any other time in the past approximatly 250 years.     

Climate effects on fire regimes and tree recruitment in Black Hills ponderosa pine forests

Climate influences forest structure through effects on both species demography (recruitment and mortality) and disturbance regimes. Here, I compare multi-century chronologies of regional fire years and tree recruitment from ponderosa pine forests in the Black Hills of southwestern South Dakota and northeastern Wyoming to reconstructions of precipitation and global circulation indices. Regional fire years were affected by droughts and variations in both Pacific and Atlantic sea surface temperatures. Fires were synchronous with La Ni?as, cool phases of the Pacific Decadal Oscillation (PDO), and warm phases of the Atlantic Multidecadal Oscillation (AMO). These quasi-periodic circulation features are associated with drought conditions over much of the western United States. The opposite pattern (El Ni?o, warm PDO, cool AMO) was associated with fewer fires than expected. Regional tree recruitment largely occurred during wet periods in precipitation reconstructions, with the most abundant recruitment coeval with an extended pluvial from the late 1700s to early 1800s. Widespread even-aged cohorts likely were not the result of large crown fires causing overstory mortality, but rather were caused by optimal climate conditions that contributed to synchronous regional recruitment and longer intervals between surface fires. Synchronous recruitment driven by climate is an example of the Moran effect. The presence of abundant fire-scarred trees in multi-aged stands supports a prevailing historical model for ponderosa pine forests in which recurrent surface fires affected heterogenous forest structure, although the Black Hills apparently had a greater range of fire behavior and resulting forest structure over multi-decadal time scales than ponderosa pine forests of the Southwest that burned more often.     

Matching the Multiple Scales of Conservation with the Multiple Scales of Climate Change

Abstract:  To anticipate the rapidly changing world resulting from global climate change, the projections of climate models must be incorporated into conservation. This requires that the scales of conservation be aligned with the scales of climate-change projections. We considered how conservation has incorporated spatial scale into protecting biodiversity, how the projections of climate-change models vary with scale, and how the two do or do not align. Conservation planners use information about past and current ecological conditions at multiple scales to identify conservation targets and threats and guide conservation actions. Projections of climate change are also made at multiple scales, from global and regional circulation models to projections downscaled to local scales. These downscaled projections carry with them the uncertainties associated with the broad-scale models from which they are derived; thus, their high resolution may be more apparent than real. Conservation at regional or global scales is about establishing priorities and influencing policy. At these scales, the coarseness and uncertainties of global and regional climate models may be less important than what they reveal about possible futures. At the ecoregional scale, the uncertainties associated with downscaling climate models become more critical because the distributions of conservation targets on which plans are founded may shift under future climates. At a local scale, variations in topography and land cover influence local climate, often overriding the projections of broad-scale climate models and increasing uncertainty. Despite the uncertainties, ecologists and conservationists must work with climate-change modelers to focus on the most likely projections. The future will be different from the past and full of surprises; judicious use of model projections at appropriate scales may help us prepare.     

Using a biogeochemistry model in simulating forests productivity responses to climatic change and [CO2] increase: example of Pinus halepensis in Provence (south-east France)

Tree-ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate the impacts of climatic change and [CO₂] increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east France). Changes in productivity are simulated using the global biogeochemistry model BIOME3, that we have adapted to run with chronological data. Tree-ring data (width and density) were used to estimate, for each stand, an observed series of changes in productivity. Simulated and observed productivity changes are then compared to validate the chronological biogeochemistry model BIOME3C. Variations in productivity were well reconstructed at 15 sites. After this validation, BIOME3C was used to simulate forest productivity changes for a 2×CO₂ scenario. The 2×CO₂ climate used as input was obtained using results from Météo-France’s ARPEGE atmospheric general circulation model (AGCM), downscaled to local meteorological stations. Productivity increases moderately for all stands (from 17 to 24%) when climatic changes alone were taken into account. The main factor responsible for this increase is a reduction in summer drought severity. Productivity increases highly for all stands (from 72 to 86%) when the physiological fertilising effect of the [CO₂] increase is considered separately. When both climatic changes and the [CO₂] increase were taken into account, productivity increases highly, from 107% (for Moustier) to 141% (for La Ciotat). The direct fertilising effect of [CO₂] increase has a greater influence on the forest stands productivity than the indirect climatic changes effect. These results also exhibit the importance of the synergy between the effects of climate change and [CO₂] increase, as the increase in productivity resulting from the combined effects are more than the sum of the individual CO₂ and climate effects. Although the detected effects of global change during the 20th century were slight, acceleration of these changes is likely to lead to great changes in the future productivity of P. halepensis forests.