Persistent and pervasive compositional shifts of western boreal forest plots in Canada

Species compositional shifts have important consequences to biodiversity and ecosystem function and services to humanity. In boreal forests, compositional shifts from late‐successional conifers to early‐successional conifers and deciduous broadleaves have been postulated based on increased fire frequency associated with climate change truncating stand age‐dependent succession. However, little is known about how climate change has affected forest composition in the background between successive catastrophic fires in boreal forests. Using 1797 permanent sample plots from western boreal forests of Canada measured from 1958 to 2013, we show that after accounting for stand age‐dependent succession, the relative abundances of early‐successional deciduous broadleaves and early‐successional conifers have increased at the expense of late‐successional conifers with climate change. These background compositional shifts are persistent temporally, consistent across all forest stand ages and pervasive spatially across the region. Rising atmospheric CO₂ promoted early‐successional conifers and deciduous broadleaves, and warming increased early‐successional conifers at the expense of late‐successional conifers, but compositional shifts were not associated with climate moisture index. Our results emphasize the importance of climate change on background compositional shifts in the boreal forest and suggest further compositional shifts as rising CO₂ and warming will continue in the 21st century.     

Climate change effects on net carbon exchange of a boreal aspen–hazelnut forest: estimates from the ecosystem model ecosys

Although boreal forests are currently sinks for atmospheric C, there is some concern that they may not remain so under hypothesized warming of the boreal climate. The ecosystem model ecosys was used to evaluate possible changes in ecosystem C exchange and accumulation under changes in atmospheric CO₂ concentration (C_a) proposed in emissions scenario IS92a, and accompanying changes in air temperature and precipitation proposed by general circulation models running under IS92a. Ecosys was first tested under current climate by comparing modelled rates of C exchange and accumulation with those measured in a mixed aspen–hazelnut stand in central Saskatchewan. The model was then run with daily increments of C_a, temperature and precipitation, and differences in C exchange and accumulation between current and changing climates were evaluated. Model results indicated that over a 120‐y period, a mixed aspen–hazelnut stand currently accumulates about 14 kg C m^?2. Under the hypothesized changes in climate this stand would accumulate an additional 8.5 kg C m^?2, largely through higher rates of CO₂ fixation and longer growing seasons under higher C_a and temperature. This additional accumulation would be entirely as aspen wood, while soil organic matter would change little. This accumulation would therefore be vulnerable to losses from fire and insects.     

Climate change affects populations of northern birds in boreal protected areas

Human land-use effects on species populations are minimized in protected areas and population changes can thus be more directly linked with changes in climate. In this study, bird population changes in 96 protected areas in Finland were compared using quantitative bird census data, between two time slices, 1981-1999 and 2000-2009, with the mean time span being 14 years. Bird species were categorized by distribution pattern and migratory strategy. Our results showed that northern bird species had declined by 21 per cent and southern species increased by 29 per cent in boreal protected areas during the study period, alongside a clear rise (0.7-0.8 °C) in mean temperatures. Distribution pattern was the main factor, with migratory strategy interacting in explaining population changes in boreal birds. Migration strategy interacted with distribution pattern so that, among northern birds, densities of both migratory and resident species declined, whereas among southern birds they both increased. The observed decline of northern species and increase in southern species are in line with the predictions of range shifts of these species groups under a warming climate, and suggest that the population dynamics of birds are already changing in natural boreal habitats in association with changing climate.     

The impact of global climate change on tropical forest biodiversity in Amazonia

Aim To model long‐term trends in plant species distributions in response to predicted changes in global climate. Location Amazonia. Methods The impacts of expected global climate change on the potential and realized distributions of a representative sample of 69 individual Angiosperm species in Amazonia were simulated from 1990 to 2095. The climate trend followed the HADCM2GSa1 scenario, which assumes an annual 1% increase of atmospheric CO₂ content with effects mitigated by sulphate forcing. Potential distributions of species in one‐degree grid cells were modelled using a suitability index and rectilinear envelope based on bioclimate variables. Realized distributions were additionally limited by spatial contiguity with, and proximity to, known record sites. A size‐structured population model was simulated for each cell in the realized distributions to allow for lags in response to climate change, but dispersal was not included. Results In the resulting simulations, 43% of all species became non‐viable by 2095 because their potential distributions had changed drastically, but there was little change in the realized distributions of most species, owing to delays in population responses. Widely distributed species with high tolerance to environmental variation exhibited the least response to climate change, and species with narrow ranges and short generation times the greatest. Climate changed most in north‐east Amazonia while the best remaining conditions for lowland moist forest species were in western Amazonia. Main conclusions To maintain the greatest resilience of Amazonian biodiversity to climate change as modelled by HADCM2GSa1, highest priority should be given to strengthening and extending protected areas in western Amazonia that encompass lowland and montane forests.     

Spatial and temporal variations in boreal forest fire frequency in northern Alberta

Abstract. Spatial and temporal variations in fire frequency in the boreal forest of Wood Buffalo National Park (WBNP) were assessed using forest stand age, fire scar and historical data. I test the hypotheses that (1) fire frequency is higher in jack pine forests and aspen forests than in black spruce forests and white spruce forests, (2) these variations in fire frequency can be related to the mean waterbreak distance (MWD) around a site and (3) fire frequency has changed over the past 300 years. The fire cycles (the time required to burn an area equal in size to the entire study area) in jack pine forests (39 years) and in aspen forests (39 years) were significantly shorter than those in black spruce forests (78 years) and in white spruce forests (96 years). The length of the fire cycle varies inversely with the MWD around a site, and the MWD was significantly higher in jack pine and aspen forests than in black or white spruce forests. It is suggested that covariations between soil type and the MWD influence, respectively, variations in forest dominant and fire frequency. A change in fire frequency at 1860 was apparent in the fire history for all of WBNP, the black spruce dominated stands, and the near and medium MWD classes. The fire cycle estimates for these classes were all significantly shorter during the period 1750 to 1859 (fire cycles = 25–49 years) than they were in the period 1860 to 1989 (fire cycles = 59–89 years). The possible roles of changes in climate and aboriginal burning practices in causing the temporal change in fire frequency are discussed.     

Effects of forest fire and drought on acidity of a base-poor boreal forest stream: similarities between climatic warming and acidic precipitation

In a boreal forest catchment in the Experimental Lakes Area in northwestern Ontario, wildfire caused an increase in the concentrations of strong acid anions and base cations of the stream. In the naturally base-poor Northwest (NW) Subbasin, a 1980 wildfire caused exports of strong acid anions to increase more than export of base cations, causing a 2.5 fold increase in the acidity of the stream. Mean annual stream pH declined from 5.15 prior to fire to 4.76 two years after fire. Acid-neutralizing capacity (ANC), calculated as the difference between total base cations and strong acid anions, decreased to 20% of pre-fire values. Sulfate and chloride were the strong acid anions responsible for the decline in ANC, increasing four-fold. While nitrate increased eleven-fold, concentrations were too low to significantly affect ANC. There was a significant correlation between weekly sulfate concentration and base cation concentration (r ² = 0.83) in the two years after fire. Recovery of ANC was caused by the more rapid decline in concentration of sulfate than by changes in base cations. Drought produced a similar but weaker response than fire, with increased sulfate concentrations and decreased stream pH. Climatic warming that increases drought and fire frequency would have effects that mimic the impacts of acidic precipitation (i.e. higher sulfate concentrations and acidic stream waters). Areas which have higher concentrations of stored S from past acid precipitation or have large areas of peatlands in the watershed may have aggravated losses of S and H⁺ after drought and fire.     

Radial growth response of four dominant boreal tree species to climate along a latitudinal gradient in the eastern Canadian boreal forest

To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species –Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana– to climatic variations along a broad latitudinal gradient from 46 to 54°N in the eastern Canadian boreal forest. Tree‐ring chronologies from 34 forested stands distributed at a 1° interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950–2003 to identify climate factors limiting the radial growth and the detailed radial growth–climate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March–April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site‐specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current‐year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 49°N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 47°N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.     

Disturbance legacies and climate jointly drive tree growth and mortality in an intensively studied boreal forest

Most North American forests are at some stage of post‐disturbance regrowth, subject to a changing climate, and exhibit growth and mortality patterns that may not be closely coupled to annual environmental conditions. Distinguishing the possibly interacting effects of these processes is necessary to put short‐term studies in a longer term context, and particularly important for the carbon‐dense, fire‐prone boreal forest. The goals of this study were to combine dendrochronological sampling, inventory records, and machine‐learning algorithms to understand how tree growth and death have changed at one highly studied site (Northern Old Black Spruce, NOBS) in the central Canadian boreal forest. Over the 1999–2012 inventory period, mean tree diameter increased even as stand density and basal area declined significantly. Tree mortality averaged 1.4 ± 0.6% yr^?1, with most mortality occurring in medium‐sized trees; new recruitment was minimal. There have been at least two, and probably three, significant influxes of new trees since stand initiation, but none in recent decades. A combined tree ring chronology constructed from sampling in 2001, 2004, and 2012 showed several periods of extreme growth depression, with increased mortality lagging depressed growth by ~5 years. Higher minimum and maximum air temperatures exerted a negative influence on tree growth, while precipitation and climate moisture index had a positive effect; both current‐ and previous‐year data exerted significant effects. Models based on these variables explained 23–44% of the ring‐width variability. We suggest that past climate extremes led to significant mortality still visible in the current forest structure, with decadal dynamics superimposed on slower patterns of fire and succession. These results have significant implications for our understanding of previous work at NOBS, the carbon sequestration capability of old‐growth stands in a disturbance‐prone landscape, and the sustainable management of regional forests in a changing climate.     

Thermokarst rates intensify due to climate change and forest fragmentation in an Alaskan boreal forest lowland

Lowland boreal forest ecosystems in Alaska are dominated by wetlands comprised of a complex mosaic of fens, collapse‐scar bogs, low shrub/scrub, and forests growing on elevated ice‐rich permafrost soils. Thermokarst has affected the lowlands of the Tanana Flats in central Alaska for centuries, as thawing permafrost collapses forests that transition to wetlands. Located within the discontinuous permafrost zone, this region has significantly warmed over the past half‐century, and much of these carbon‐rich permafrost soils are now within ~0.5 °C of thawing. Increased permafrost thaw in lowland boreal forests in response to warming may have consequences for the climate system. This study evaluates the trajectories and potential drivers of 60 years of forest change in a landscape subjected to permafrost thaw in unburned dominant forest types (paper birch and black spruce) associated with location on elevated permafrost plateau and across multiple time periods (1949, 1978, 1986, 1998, and 2009) using historical and contemporary aerial and satellite images for change detection. We developed (i) a deterministic statistical model to evaluate the potential climatic controls on forest change using gradient boosting and regression tree analysis, and (ii) a 30 × 30 m land cover map of the Tanana Flats to estimate the potential landscape‐level losses of forest area due to thermokarst from 1949 to 2009. Over the 60‐year period, we observed a nonlinear loss of birch forests and a relatively continuous gain of spruce forest associated with thermokarst and forest succession, while gradient boosting/regression tree models identify precipitation and forest fragmentation as the primary factors controlling birch and spruce forest change, respectively. Between 1950 and 2009, landscape‐level analysis estimates a transition of ~15 km² or ~7% of birch forests to wetlands, where the greatest change followed warm periods. This work highlights that the vulnerability and resilience of lowland ice‐rich permafrost ecosystems to climate changes depend on forest type.     

Future climate change effects on US forest composition may offset benefits of reduced atmospheric deposition of N and S

Climate change and atmospheric deposition of nitrogen (N) and sulfur (S) are important drivers of forest demography. Here we apply previously derived growth and survival responses for 94 tree species, representing >90% of the contiguous US forest basal area, to project how changes in mean annual temperature, precipitation, and N and S deposition from 20 different future scenarios may affect forest composition to 2100. We find that under the low climate change scenario (RCP 4.5), reductions in aboveground tree biomass from higher temperatures are roughly offset by increases in aboveground tree biomass from reductions in N and S deposition. However, under the higher climate change scenario (RCP 8.5) the decreases from climate change overwhelm increases from reductions in N and S deposition. These broad trends underlie wide variation among species. We found averaged across temperature scenarios the relative abundance of 60 species were projected to decrease more than 5% and 20 species were projected to increase more than 5%; and reductions of N and S deposition led to a decrease for 13 species and an increase for 40 species. This suggests large shifts in the composition of US forests in the future. Negative climate effects were mostly from elevated temperature and were not offset by scenarios with wetter conditions. We found that by 2100 an estimated 1 billion trees under the RCP 4.5 scenario and 20 billion trees under the RCP 8.5 scenario may be pushed outside the temperature record upon which these relationships were derived. These results may not fully capture future changes in forest composition as several other factors were not included. Overall efforts to reduce atmospheric deposition of N and S will likely be insufficient to overcome climate change impacts on forest demography across much of the United States unless we adhere to the low climate change scenario.     

Spatial patterns and drivers of fire occurrence and its future trend under climate change in a boreal forest of Northeast China

Understanding the spatial patterns of fire occurrence and its response to climate change is vital to fire risk mitigation and vegetation management. Focusing on boreal forests in Northeast China, we used spatial point pattern analysis to model fire occurrence reported from 1965 to 2009. Our objectives were to quantitate the relative importance of biotic, abiotic, and human influences on patterns of fire occurrence and to map the spatial distribution of fire occurrence density (number of fires occurring over a given area and time period) under current and future climate conditions. Our results showed human‐caused fires were strongly related to human activities (e.g. landscape accessibility), including proximity to settlements and roads. In contrast, fuel moisture and vegetation type were the most important controlling factors on the spatial pattern of lightning fires. Both current and future projected spatial distributions of the overall (human‐ + lightning‐caused) fire occurrence density were strongly clustered along linear components of human infrastructure. Our results demonstrated that the predicted change in overall fire occurrence density is positively related to the degree of temperature and precipitation change, although the spatial pattern of change is expected to vary spatially according to proximity to human ignition sources, and in a manner inconsistent with predicted climate change. Compared to the current overall fire occurrence density (median value: 0.36 fires per 1000 km² per year), the overall fire occurrence density is projected to increase by 30% under the CGCM3 B1 scenario and by 230% under HadCM3 A2 scenario in 2081–2100, respectively. Our results suggest that climate change effects may not outweigh the effects of human influence on overall fire occurrence over the next century in this cultural landscape. Accurate forecasts of future fire‐climate relationships should account for anthropogenic influences on fire ignition density, such as roads and proximity to settlements.     

Net aboveground biomass declines of four major forest types with forest ageing and climate change in western Canada's boreal forests

Biomass change of the world's forests is critical to the global carbon cycle. Despite storing nearly half of global forest carbon, the boreal biome of diverse forest types and ages is a poorly understood component of the carbon cycle. Using data from 871 permanent plots in the western boreal forest of Canada, we examined net annual aboveground biomass change (ΔAGB) of four major forest types between 1958 and 2011. We found that ΔAGB was higher for deciduous broadleaf (DEC) (1.44 Mg ha^?1 year^?1, 95% Bayesian confidence interval (CI), 1.22–1.68) and early‐successional coniferous forests (ESC) (1.42, CI, 1.30–1.56) than mixed forests (MIX) (0.80, CI, 0.50–1.11) and late‐successional coniferous (LSC) forests (0.62, CI, 0.39–0.88). ΔAGB declined with forest age as well as calendar year. After accounting for the effects of forest age, ΔAGB declined by 0.035, 0.021, 0.032 and 0.069 Mg ha^?1 year^?1 per calendar year in DEC, ESC, MIX and LSC forests, respectively. The ΔAGB declines resulted from increased tree mortality and reduced growth in all forest types except DEC, in which a large biomass loss from mortality was accompanied with a small increase in growth. With every degree of annual temperature increase, ΔAGB decreased by 1.00, 0.20, 0.55 and 1.07 Mg ha^?1 year^?1 in DEC, ESC, MIX and LSC forests, respectively. With every cm decrease of annual climatic moisture availability, ΔAGB decreased 0.030, 0.045 and 0.17 Mg ha^?1 year^?1 in ESC, MIX and LSC forests, but changed little in DEC forests. Our results suggest that persistent warming and decreasing water availability have profound negative effects on forest biomass in the boreal forests of western Canada. Furthermore, our results indicate that forest responses to climate change are strongly dependent on forest composition with late‐successional coniferous forests being most vulnerable to climate changes in terms of aboveground biomass.     

Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics

Carbon exchange by the terrestrial biosphere is thought to have changed since pre-industrial times in response to increasing concentrations of atmospheric CO₂ and variations (anomalies) in inter-annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process-based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (R_h) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter-annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO₂ concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO₂ concentration but to decrease with warming. As CO₂ concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased R_h, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO₂ and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year^?1 for boreal forests and 0–23% year^?1 in the tundra between 1960 and 1990.     

Bioenergy production and forest landscape change in the southeastern United States

Production of woody biomass for bioenergy, whether wood pellets or liquid biofuels, has the potential to cause substantial landscape change and concomitant effects on forest ecosystems, but the landscape effects of alternative production scenarios have not been fully assessed. We simulated landscape change from 2010 to 2050 under five scenarios of woody biomass production for wood pellets and liquid biofuels in North Carolina, in the southeastern United States, a region that is a substantial producer of wood biomass for bioenergy and contains high biodiversity. Modeled scenarios varied biomass feedstocks, incorporating harvest of ‘conventional’ forests, which include naturally regenerating as well as planted forests that exist on the landscape even without bioenergy production, as well as purpose‐grown woody crops grown on marginal lands. Results reveal trade‐offs among scenarios in terms of overall forest area and the characteristics of the remaining forest in 2050. Meeting demand for biomass from conventional forests resulted in more total forest land compared with a baseline, business‐as‐usual scenario. However, the remaining forest was composed of more intensively managed forest and less of the bottomland hardwood and longleaf pine habitats that support biodiversity. Converting marginal forest to purpose‐grown crops reduced forest area, but the remaining forest contained more of the critical habitats for biodiversity. Conversion of marginal agricultural lands to purpose‐grown crops resulted in smaller differences from the baseline scenario in terms of forest area and the characteristics of remaining forest habitats. Each scenario affected the dominant type of land‐use change in some regions, especially in the coastal plain that harbors high levels of biodiversity. Our results demonstrate the complex landscape effects of alternative bioenergy scenarios, highlight that the regions most likely to be affected by bioenergy production are also critical for biodiversity, and point to the challenges associated with evaluating bioenergy sustainability.     

Effects of fire on regional evapotranspiration in the central Canadian boreal forest

Changes in fire regimes are driving the carbon balance of much of the North American boreal forest, but few studies have examined fire‐driven changes in evapotranspiration (ET) at a regional scale. This study used a version of the Biome‐BGC process model with dynamic and competing vegetation types, and explicit spatial representation of a large (10⁶ km²) region, to simulate the effects of wildfire on ET and its components from 1948 to 2005 by comparing the fire dynamics of the 1948–1967 period with those of 1968–2005. Simulated ET averaged, over the entire temporal and spatial modeling domain, 323 mm yr⁻¹; simulation results indicated that changes in fire in recent decades decreased regional ET by 1.4% over the entire simulation, and by 3.9% in the last 10 years (1996–2005). Conifers dominated the transpiration (E_C) flux (120 mm yr⁻¹) but decreased by 18% relative to deciduous broadleaf trees in the last part of the 20th century, when increased fire resulted in increased soil evaporation, lower canopy evaporation, lower E_C, and a younger and more deciduous forest. Well‐ and poorly drained areas had similar rates of evaporation from the canopy and soil, but E_C was twice as high in the well‐drained areas. Mosses comprised a significant part of the evaporative flux to the atmosphere (22 mm yr⁻¹). Modeled annual ET was correlated with net primary production, but not with temperature or precipitation; ET and its components were consistent with previous field and modeling studies. Wildfire is driving significant changes in hydrological processes by affecting mean stand age, forest species, and energy balance. These changes, particularly in poorly drained areas, may control the future carbon balance of the boreal forest.     

Sugar maple (Acer saccharum Marsh.) establishment in boreal forest: results of a transplantation experiment

Aim To evaluate whether seedlings of sugar maple (Acer saccharum Marsh.) can establish beyond the species northern range limit in adjacent boreal forest. Location The hardwood–boreal forest transition zone on the north‐east shore of Lake Superior, Ontario, Canada. Methods Seed fall of sugar maple was monitored for 5 years in a stand of this species at its northern range limit, and seed from this stand was transplanted to five micro‐habitat types in an adjacent boreal forest. The establishment and survival of sugar maple seedlings there, and in the seed‐source stand, was monitored for the following 7–11 years. Soil‐surface light levels were measured in both forest types. Results Most seed fell in the final year of monitoring, when c. 250 seeds m^?2 were recorded. First‐year seedling establishment rates in the maple stand, deriving from this mast seed year, was approximately double that deriving from seed transplanted to the boreal forest sites; this is tentatively attributed to seed predator satiation in the maple stand. However, subsequent seedling survivorship in the boreal forest was greater than that in the maple stand, resulting in comparable seedling densities by the end of 6 years. This difference is tentatively attributed to better illumination in the boreal forest sites, and canopy‐opening disturbances appear to be especially facilitative of seedling survival. Main conclusions There is no fundamental impediment to sugar maple seedlings establishing in boreal forest communities if climate warming occurs and seed is available. If management intervention is needed to accelerate seed availability in a rapidly warming boreal forest, then diffuse seed application to disturbed boreal forest sites during mast years of local boreal tree species is recommended as the most effective way of avoiding seed predation and increasing seedling survival.