Forests contain approximately 80% of aboveground terrestrial carbon. Therefore, minor alterations to carbon stocks or cycling in forests ecosystems may exacerbate global warming by increasing atmospheric carbon dioxide levels. Yet, current climate change is expected to increase the severity and frequency of stand-replacing disturbances such as wildfire and windthrow, which will ultimately decrease ecosystem carbon stocks over large areas for several decades. Previous studies have assessed the effects of individual disturbance on forest carbon storage. However, the consequences of multiple, interacting disturbances are relatively unknown. Using field data and statistical analyses, Bradford et al. (2011) quantified the changes in carbon stocks in ecosystem carbon pools (live biomass, snags, down woody debris, forest floor, and total ecosystem) that resulted from blowdown from windstorms, post-disturbance salvage logging, and wildfire in a forest in Northern Minnesota. Each disturbance was analyzed individually and in combination with one another. The authors found that total live carbon and carbon in live trees were highest in the control plot (no disturbance) and lower in all other treatments. Carbon in understory biomass was highest in the blowdown followed by salvage logging followed by fire plot (BSF). Carbon in snags and dead woody material was highest in the fire treatment while carbon in down woody debris and the florest floor was highest in the blowdown treatment. Total ecosystem carbon was highest in the control treatment, intermediate in the blowdown and fire treatment, and lowest in the blowdown and fire (BF) and BSF plots. Both blowdown and fire disturbances resulted in roughly equal decreases in live carbon and total ecosystem carbon. Fire further decreased carbon in the forest floor and down woody debris after blowdown, resulting in additional total ecosystem carbon losses. Salvage logging and fire after blowdown demonstrated similar results. Overall, these results indicate that increasing disturbance frequencies may challenge land management efforts to sustain and enhance ecosystem carbon stocks.—Megan Smith
Bradford, J.B., Fraver, S., Milo, A.M., D’Amato, A.W., Palik, B. Shinneman, D.J., 2012. Effects of Multiple Interacting Disturbances and Salvage Logging on Forest Carbon Stocks. Forest Ecology and Management (267), pgs. 209 – 214.
The study was conducted in Northeastern Minnesota along the Southern edge of the North American boreal forest ecotone, and within the Superior National Forest. On July 4, 1999, a large derecho (a widespread, straight-line windstorm) affected over 200,000 hectares of the Superior National Forest. After the storm, the US Forest Service began salvage logging operations to reduce fuel loads and mitigate wildfire. Salvage logging occurred between the fall of 1999 and the fall of 2002. Despite these efforts, a section of the region was burned by the Ham Lake Wildfire in 2007. Patchy, overlapping disturbance patterns resulted in five “treatments:” undisturbed control, blowdown only (B), fire only (F), blowdown followed by fire (BF), and blowdown followed by salvage logging followed by fire (BSF). Unburned salvaged areas were unidentifiable. The authors examined mature jack pine (Pinus banksiana) communities. A map displaying the location of the 1999 blowdown, the 2007 wildfire, and the study site was constructed.
Bradford et al. established six study sites in each study plot, creating 30 sites total. Eight of these sites contained previous data from an earlier investigation and were included in the author’s study. The remaining 22 sites were randomly selected. Although the authors lacked pre-disturbance data for most of their sites, they conducted comparisons of treatment stand structures and reconstructed structures for the other treatments (based on deadwood pools) with the plots with pre-existing data. As a result, the authors confirmed that all the sites were comparable regarding pre-disturbance stand structure and successional stage. Randomly selected sites were then ground-truthed for adherence to the expected forest type and disturbance combination. Within each site, 6–10 circular plots (200-m2) were established along a 40 x 40 m grid that originated from a randomly chosen starting point, creating 239 plots total.
Within each circular plot, standing live and dead trees (diameters greater than 10 cm at breast height), and saplings (stems sizes greater than 2.5 cm and diameters less than 10 cm at breast height) were recorded by species and diameter. Stems of shrubs and tree seedlings (stems smaller than sapling class) were tallied by species within a 10-m2 circular plot centered within each 200-m2 plot. Additional seedling data were collected in in 10-m2 plots located equidistant between each 200-m2 plot, resulting in 14 – 20 seedling plots at each site. Downed woody debris (DWD) on each plot was inventoried. Diameters of all DWD with measurements greater than 7.6 cm were recorded along a 32-m transect that passed through the middle of each 200-m2. The authors also collected samples of herbaceous vascular plant material, forest floor material, and soils at a set location within each 200-m2 plot.
The biomass of living and intact standing dead trees was calculated for all woody stems greater than 2.5 cm in diameter and breast height. The authors used species-specific allometric equations that were regionally derived. The biomass of broken standing dead trees was estimated using taper functions to determine large and small end diameters. Then, a conic-paraboloid formula was applied to determine the volume of the intact portion of the tree. Volume was converted to biomass using species-specific density values for decay class I taken from another author’s study. For unidentifiable standing dead trees, the authors used the average decay class II densities from all species present. Shrub and tree seedling biomass was calculated using species-specific allometric equations. Biomass calculations for these components included stems, roots, branches, as well as foliage, if it was alive. Dead woody debris biomass was calculated using Van Wagner’s formula. This formula was applied to the planar intercept data, adjusting for species and decay class-specific densities. The volume of coarse woody debris in decay class IV or V was adjusted for collapse using collapse ratios of 0.82 and 0.42. Carbon content was calculated from total biomass using species-specific values. Additionally, carbon was assumed to compose 50% of biomass for shrub species and unidentified down woody debris.
Separate mixed-model analyses of variance (ANOVA) were used to assess differences in carbon pool sizes among treatments. Disturbance combination was treated as a fixed effect and site was treated as a random effect. Analyses of variance was also calculated for carbon stored in live trees, understory (herbaceous plants, shrubs, and seedlings), live biomass (sum of live trees and understory), down woody debris (DWD), snags, dead woody material (snags plus DWD), forest floor, soil, and total ecosystem carbon (sum of live biomass, deadwood, forest floor, and soil). When the authors detected significant disturbance effects, post hoc Tukey’s honest significant difference tests were used for pairwise comparisons between disturbance types. All data were checked for normality and transformed before statistical analyses were undertaken.
Brandon et al. found that carbon in live biomass was stored primarily in live trees. Total live carbon and carbon in live trees were highest in the control plot (104 MgC ha-1) and lower in all other treatments (from 1 MgC ha-1 in the BSF plot to 21 MgC ha-1 in the blowdown treatment). Carbon in understory biomass was higher in the BSF treatment (1.3 MgC ha-1) than the control, blowdown, or fire treatments (0.2 to 0.6 MgC ha-1). Carbon in snags was much higher in the fire treatment than other treatments (53 MgC ha-1), and the BF treatment (19 MgC ha-1) was higher than the BSF treatment (1.7 MgC ha-1).
Carbon in DWD was highest in the blowdown stands (35 MgC ha-1), intermediate in the BF treatment (21 MgC ha-1), and lowest in all other treatments (9–10 MgC ha-1). Carbon in all dead woody material (snags and DWD) was highest in the fire treatment (62 MgC ha-1) and lowest in the BSF treatment (11 MgC ha-1). Carbon stored in the forest floor was highest in the blowdown treatment (35 MgC ha-1) and lowest in the BSF treatment (9 MgC ha-1). Furthermore, soil carbon means were not significantly different among treatments. Total ecosystem carbon was highest in the control treatment (177 MgC ha-1), intermediate in the blowdown and fire treatments (122 and 106 MgC ha-1), and lowest in the BF and BSF treatments (66 and 40 MgC ha-1).
Differences in carbon pools suggest that individual natural disturbances (blowdown and fire) resulted in similar decreases in live carbon (83 and 91 MgC ha-1) and total ecosystem carbon (55 and 71 MgC ha-1). Blowdown increased DWD carbon by 25 MgC ha-1 and forest floor carbon by16 MgC ha-1 and had no significant effect on sang carbon. However, fire increased snag carbon by 40 MgC ha-1. Compared to blowdown alone, fire following blowdown decreased both DWD and forest floor carbon by another 14 and 22 MgC ha-1, and resulted in additional total ecosystem carbon losses of 56 MgC ha-1. Similarly, salvage logging and fire after blowdown decreased carbon in both DWD and forest floor by another 26 and 39 MgC ha-1, and caused additional ecosystem carbon loss of 83 MgC ha-1 when compared to blowdown alone. A figure displaying carbon stocks for individual carbon pools and total ecosystem carbon for forest stands in each treatment was constructed, as was a figure demonstrating the consequences of windthrow, salvage logging, and wildfire on ecosystem carbon stocks.
These findings indicate that all combinations of disturbances resulted in lower total ecosystem carbon than the undisturbed control. Additionally, the results demonstrate that individual disturbances (wind and fire) resulted in similar shifts of carbon from live biomass to detrital pools, as well as similar losses of total ecosystem carbon. However, blowdown shifted live tree carbon into downed woody debris and forest floor pools while fire shifted this carbon into snags, which have slower decomposition rates than downed wood or forest floor. Total ecosystem carbon loss was attributed to the amount of carbon immediately lost from trees.
Overall, the authors’ results demonstrate that secondary major disturbances can cause substantial additional decreases in ecosystem carbon, though the magnitude may vary depending on the time between successive disturbances and disturbance types. Furthermore, the original blowdown disturbance increased carbon stored in DWD and forest floor, providing larger surface fuel loads that increased burn severity and carbon loss. Although forest floor and DWD carbon pools were elevated after the blowdown, the fire released all the carbon added to the litter in the forest floor and more than half of the carbon added to DWD. Salvage logging also modestly enhanced carbon losses. Post-blowdown salvage logging prior to fire decreased carbon in DWD to control levels, removing all of the carbon added to DWD in the blowdown. It also resulted in less carbon stored in snags. Yet, salvage logging did not significantly affect total ecosystem carbon despite its effects on individual carbon pools.
In conclusion, forest managers must address the issue that increasing disturbance frequencies may counteract efforts to sustain and enhance ecosystem carbon stocks, thereby exacerbating climate change by releasing more carbon into the atmosphere.
Mountainous areas in southern Europe act as wet-cool microclimate refugia in a warm-dry region for genetically unique and endemic species. However, recent predictions suggest that species rich regions within the Mediterranean mountains are at great risk to climate change because changes in atmospheric dynamics will cause these areas to become warmer and drier. Therefore, evaluating the effects of future climate change on the diversity, composition, and distribution of Mediterranean mountainous species is essential to the persistence of their genetic heritage. By using high spatial resolution data, generalized linear models, and machine learning models, Ruiz-Labourdette et al. (2011) modeled the current and future distributions of 15 tree species in two connected mountain ranges in the Iberian Peninsula across the elevational gradient. The distributions were modeled as functions of climate, lithology, and soil water availability. Additionally, Ruiz-Labourdette et al. analyzed potential changes in the composition of tree communities. The authors’ models predicted an upward migration of Mediterranean tree communities to higher elevations, an expansion of drought and high temperature tolerant tree species’ ranges, and a decline in temperate, cold-adapted tree communities with moderate water requirements. Furthermore, the mountain lowlands showed the largest projected changes in species distribution. Ultimately, these findings indicate that climate change may result in a loss of tree species diversity within these mountain ranges.—Megan Smith
Ruiz-Labourdette, D., Dogues-Bravo, D., Ollero, H.S., Schmitz, M.F., Pineda, F.D., 2011. Forest Composition in Mediterranean Mountains is Projected to Shift Along the Entire Elevational Gradient Under Climate Change. Journal of Biogeography 39:1. DOI: 10.1111/j.1365-2699.2011.02592.x
The varied topography within the Mediterranean Mountains created a variety of microclimates that acted as refugia for species that thrived in warm environments during periods of adverse climatic conditions. Additionally, the mountains protected other species that grew in cold and wet environments from post-glacial warming period. As a result, mountain species’ isolation in refugia resulted in one of the highest rates of endemism in Europe.
The study area encompassed a mountainous region of 71,700 km2 in southern Europe and included two large mountain ranges: the Central Mountain Range, located in the central west part of the Iberian Peninsula, and the Iberian Mountain Range, located in the central east part of the peninsula. The Iberian Mountains are primarily composed of limestones, calcarenites, marls, evaporates, dolomites, and sandstones. The Central Mountains consist of granite, metamorphic materials, a homogenous siliceous substratum, and limestone. A map showing the location of the Central Iberian Mountain Ranges, as well as the elevations of the highest summits in these mountain ranges, was displayed.
Forest composition varies between the two mountain ranges. The wetter, northern areas contain forests of temperate and broadleaf species. Mountainous regions with a more continental climate consist of pine forests, while the lower mountainous regions contain sub-Mediterranean deciduous forests. Additionally, wet-warm Mediterranean forests are found to the west of the study area, while warmer and drier piedmont environments and the shaded low-elevation mountainous areas contain tree species resilient to summer droughts.
Spatial distribution models (SDMs) are used to project climate changes in areas that have a suitable climate for the species in question. However, most SDMs utilize low-resolution biological and climatic datasets that cannot identify small patches with suitable conditions. These small patches might reduce the negative effects of climate change by providing microrefugia for communities. Therefore, Ruiz-Labourdette et al. utilized high-resolution data in their study to examine the topographic complexities and microrefugia present in the Iberian mountain ranges. Ultimately, this method achieved a more accurate representation of species distribution and community composition.
The authors modeled the current and future distributions of 15 tree species in Iberian Peninsula mountain ranges as functions of climate, lithology, and soil-water availability using generalized linear models and data mining models. Ruiz-Labourdette et al. also mapped the variation in the composition of current and potential forest communities in the study region using a multivariate ordination of a matrix of presence/absence of tree species. Two IPCC Special Report on Emission Scenarios (SRES) (A2 and B2) between the years of 2041–70 and 2071–2100 were used to model the study area’s future forest composition.
Forest species data were collected from a forest map of Spain at a scale of 1:50,000. The map was rasterized to a 500 x 500 m grid to create a data matrix of the current presence/absence of tree species in 286,688 cells. Tree species that occurred in more than 500 cells were incorporated in the study. Lithological data were collected from a geological map of the Iberian Peninsula and were classified into six lithological groups. Additionally, the authors incorporated soil hydromorphology into their study by calculating the topographic ratio (TR), which assesses the effects of topography on water drainage.
Ten climatic variables were used to model the potential distribution of tree species. These variables were obtained from monthly measurements recorded at 752 rainfall stations and 197 temperature stations that belong to the observation network of the Spanish State Meteorology Agency. Current climate conditions were recorded for the period 1961–90. For the periods 2041–70 and 2071–2100, the IPCC SRES scenarios A2 and B2 were used. The scenarios differ in the amount of carbon emitted from energy and industrial sources by 2100, with A2 being the most severe. The global models were downscaled to a regional scale to make the information on current and future climatic conditions available at the level of individual weather stations. These data were used to develop climatic maps. To construct climatic maps in a 500 x 500 m grid format, current and future climate data were inserted over the study area using multivariate stepwise regressions that incorporated quantitative geographic data (elevation, distance from coast, latitude, longitude, and slope) and qualitative data (drainage basins and aspect). Hydromorphology of the soils in the future scenarios was calculated by applying the expected increase or decrease in yearly rainfall volume to the current hydromorphology map.
GLM and data mining (gradient boosting) species distribution models were used to create alternative species spatial projections. The models were validated by splitting the data into two groups. Two-thirds of the data were used for calibration and one-third was used to evaluate and validate the models. Two probability maps were constructed for each species, one using GLM and the other using the gradient boosting technique. Binary presence/absence maps were created from probability maps using a threshold to maximize the kappa statistic, which defines the similarity between the binary map and the available biological evidence. Ruiz-Labourdette et al. chose between the GLM and gradient boosting models based on the kappa value obtained with the validation subset. They selected the model that best explained the current species distribution. Species were only modeled if they achieved a kappa value > 0.4 in one of two models. The model that was selected and validated for each species was applied to the future climate scenarios. A table displaying the kappa values obtained for each tree species in the GLM and gradient-boosting models was constructed.
Using the results of the models, the authors created a presence/absence matrix for all the tree species in the five scenarios examined (current; B2 for 2041–70; A2 for 2041–70; B2 for 2071–2100; and A2 for 2071–2100). This matrix was analyzed by multiple correspondence analyses (MCA) to identify trends in the variation of the current and future compositions of species. A table displaying the contributions of the tree species to the first axis of the MCA analysis was constructed.
Ruiz-Labourdette et al. incorporated 12 of the 15 species in their study that achieved acceptable kappa values. Gradient-boosting models were selected for the majority of the species (8 species).
The authors observed the largest changes in distribution for Mediterranean species such as xeric conifers and sclerophyllous evergreen species that are tolerant of high temperatures and summertime drought. These species ranges were expected to increase by 350% under the most severe climate change scenario (A2). Interestingly, their future range only occurred in regions in which the elevation did not differ from the elevation of their original range. This suggests that these species may spread from their present range and colonize flat piedmont and low-elevation mountainous areas. In contrast, Ruiz-Labourdette et al. found that Eurosiberian coniferous and broad-leaved species (species that prefer cold and wet environments) would experience a significant decrease in range. This decrease ranged from 80–99%, depending on the climate scenario. The Eurosiberian species are also expected to undergo the largest elevational displacement: upward between 200 and 550 m. As a result, the tree line would rise, and these tree species would colonize treeless areas that are now occupied by high-mountain grasslands. Unfortunately, it is likely that areas with climatic conditions suitable for their survival would disappear from the central and southern massifs.
The models also predicted that the ranges of sub-Mediterranean species (semi-deciduous oak trees, sub-Mediterranean gymnosperms, and sub-Mediterranean phreatophytes) would remain constant or decrease slightly under different climate scenarios (between 5% and 70%). These species optimal growing conditions may also occur at slightly higher elevations, compensating for the warming and water deficits. A figure displaying the current and projected distributions of three tree species in the Central and Iberian Mountain Ranges under the A2 climate scenario was constructed. Additionally, a graph displaying the potential change in the distribution of all the tree species in each three groups was constructed.
The authors’ models revealed that shifts in tree communities would occur as a result of an increase in the area where the climate is Mediterranean. The proportion of Mediterranean forests, especially the perennial sclerophyllous species, will increase, as will the range of xerophyllous vegetation. This species currently occupies marginal areas in warm, dry, sheltered piedmont enclaves. However, in the future, they may become the dominant vegetation within these regions. Semi-deciduous and deciduous species now found in flat piedmont and low elevation areas that require a moderate amount of water are predicted to decline. Furthermore, coniferous forests that grow in cold regions, as well as the Eurosiberian broadleaf forests that grow in wet and cold conditions, will experience a latitudinal displacement towards the north and a reduction in range. They will disappear entirely from the westernmost mountains. Instead, oak forests will become the dominant forests within these regions. A figure displaying the changes in tree communities projected for the Central and Iberian Mountain Ranges in the Iberian Peninsula was constructed.
Overall, Ruiz-Labourdette et al.’s results indicate that climate warming and a decrease in the availability of water will alter the abundance and diversity of mountain tree biota. Tree species resilient to high temperatures and drought that occupy lower elevations could increase in range and elevation, while species that persist in cold and wet environments may decline as a result of water stress. However, changes in the spatial extent of species ranges and in community composition will be greatest at lower, rather than at higher, elevations. Therefore, changes in tree species’ distributions and forest communities will occur across the entire range of elevation among these mountainous regions.
Overall, the study indicates that all amphibian species will experience range modifications under the future climatic scenarios and that these modifications will determine the extent to which these species will be represented in the Italian protected area network. The current reserve network does not represent the entirety of amphibian diversity or its geographic pattern, decreasing the species’ future representations within the reserve system. The reserve system would be improved as a whole if the study’s suggested irreplaceable areas (Sicily, Sardinia, and Northeastern Italy) were included within the network.