Spatial Patterns and Predictors of Forest Carbon Stocks in Western Mediterranean

Mediterranean semi-arid forest ecosystems are especially sensitive to external forcing. An understanding of the relationship between forest carbon (C) stock, and environmental conditions and forest structure enable prediction of the impacts of climate change on C stocks and help to define management strategies that maximize the value of forests for C mitigation. Based on the national forest inventory of Spain (1997?C2008 with 70,912 plots), we estimated the forest C stock and spatial variability in Peninsular Spain and, we determined the extent to which the observed patterns of stand C stock can be explained by structural and species richness, climate and disturbances. Spain has an average stand C stock of 45.1?Mg C/ha. Total C stock in living biomass is 621 Tg C (7.8% of the C stock of European forests). The statistical models show that structural richness, which is driven by past land use and life forest history including age, development stage, management activities, and disturbance regime, is the main predictor of stand tree C stock with larger C stocks in structurally richer stands. Richness of broadleaf species has a positive effect on both conifer and broadleaf forests, whereas richness of conifer species shows no significant or even a negative effect on C stock. Climate variables have mainly an indirect effect through structural richness but a smaller direct predictive ability when all predictors are considered. To achieve a greater standing C stock, our results suggest promoting high structural richness by managing for uneven-aged stands and favoring broadleaf over conifer species.     

Reconstruction of Historic Forest Cover Changes Indicates Minor Effects on Carbon Stocks in Swiss Forest Soils

Forest cover in Switzerland and other European countries has gradually increased in the past century. Our knowledge of the impacts of forest expansion and development on soil organic carbon (SOC) storage is, however, limited due to uncertainties in land-use history and lack of historical soil samples. We investigated the effect of forest age on current SOC storage in Switzerland. For 857 sites, we analysed SOC stocks and determined the minimal forest age for all presently forested sites using digitized historical maps, classifying all sites into three categories: young (≤60 years), medium (60–120 years), and old (≥120 years) forests. Grassland was the primary previous use of afforested land. Forest age affected current SOC stocks only moderately, whereas climate, soil chemistry, and tree species exerted a stronger impact. In the organic layer, highest SOC stocks were found in medium sites (3.0 ± 0.3 kg C m^?2). As compared to other age categories, these sites had a 10% higher cover in coniferous forests with higher organic layer C stocks than broadleaf forests. SOC stocks in mineral soils decreased with increasing forest age (12.5 ± 0.9, 11.4 ± 0.5, 10.5 ± 0.3 kg C m^?2). This decrease was primarily related to a 200-m higher average elevation of young sites and higher SOC stocks in a colder and more humid climate. In summary, forest age has only a minor effect on SOC storage in Swiss forest soils. Therefore, ongoing forest expansion in mountainous regions of Europe is unlikely contributing to soil C sequestration.     

From Models to Measurements: Comparing Downed Dead Wood Carbon Stock Estimates in the U.S. Forest Inventory

The inventory and monitoring of coarse woody debris (CWD) carbon (C) stocks is an essential component of any comprehensive National Greenhouse Gas Inventory (NGHGI). Due to the expense and difficulty associated with conducting field inventories of CWD pools, CWD C stocks are often modeled as a function of more commonly measured stand attributes such as live tree C density. In order to assess potential benefits of adopting a field-based inventory of CWD C stocks in lieu of the current model-based approach, a national inventory of downed dead wood C across the U.S. was compared to estimates calculated from models associated with the U.S.’s NGHGI and used in the USDA Forest Service, Forest Inventory and Analysis program. The model-based population estimate of C stocks for CWD (i.e., pieces and slash piles) in the conterminous U.S. was 9 percent (145.1 Tg) greater than the field-based estimate. The relatively small absolute difference was driven by contrasting results for each CWD component. The model-based population estimate of C stocks from CWD pieces was 17 percent (230.3 Tg) greater than the field-based estimate, while the model-based estimate of C stocks from CWD slash piles was 27 percent (85.2 Tg) smaller than the field-based estimate. In general, models overestimated the C density per-unit-area from slash piles early in stand development and underestimated the C density from CWD pieces in young stands. This resulted in significant differences in CWD C stocks by region and ownership. The disparity in estimates across spatial scales illustrates the complexity in estimating CWD C in a NGHGI. Based on the results of this study, it is suggested that the U.S. adopt field-based estimates of CWD C stocks as a component of its NGHGI to both reduce the uncertainty within the inventory and improve the sensitivity to potential management and climate change events.     

Long-Term Increase in Aboveground Carbon Stocks Following Exclusion of Grazers and Forest Establishment in an Alpine Ecosystem

Ecosystem stores of carbon are a key component in the global carbon cycle. Many studies have examined the impact of climate change on ecosystem carbon storage, but few have investigated the impact of land-use change and herbivory. However, land-use change is a major aspect of environmental change, and livestock grazing is the most extensive land use globally. In this study, we combine a grazing exclosure experiment and a natural experiment to test the impact of grazer exclusion on vegetation dynamics and ecosystem carbon stores in the short term (12-year exclosures), and the long term (islands inaccessible to livestock), in a heavily grazed mountain region in Norway. Following long-term absence of sheep, birch forest was present. The grazing-resistant grass Nardus stricta, dominated under long-term grazing, whilst the selected grass Deschampsia flexuosa and herb species dominated the vegetation layer in the long-term absence of sheep. The established birch forest led to vegetation carbon stocks being higher on the islands (0.56 kg C m^?2 on the islands compared to 0.18 kg C m^?2 where grazed) and no difference in soil carbon stocks. In the short-term exclusion of sheep, there were minor differences in carbon stocks reflecting the longer term changes. These results show that aboveground carbon stocks are higher in the long-term absence of sheep than in the continual presence of high sheep densities, associated with a vegetation state change between tundra and forest. The reduction of herbivore populations can facilitate forest establishment and increase aboveground carbon stocks, however, the sequestration rate is low.     

20th Century Carbon Budget of Forest Soils in the Alps

Dendrochronological studies and forest inventory surveys have reported increased growth and biospheric carbon (C) sequestration for European forests in the recent past. The potential of concomitant changes in forest soil C stocks are not accounted for in the IPCC guidelines for national greenhouse gas inventories. We developed a model-based approach to address this problem and assess the role of soils in forest C balance in the European Alps. The decomposition model FORCLIM-D was driven by long-term (that is, 1900–1985 AD) litter input scenarios constructed from forest inventory data, region-specific dendrochronological basal area indices, and time series of anthropogenic litter removal. The effect of spatial climate variability on organic matter decomposition across the case study region (Switzerland) was explicitly accounted for by constant long-term annual means of actual evapotranspiration and temperature. Uncertainties in forest development, litter removal, fine root litter input, and dynamics of forest soil C were studied by an explorative factorial sensitivity analysis. We found that forest soils contribute substantially to the biospheric C sequestration for Switzerland: Our “best estimate” yielded an increase of 0.35 Mt C/y or 0.33 t C/(ha y) in forest soils for 1985, that is, 27% of the C sequestered by forest trees (BUWAL 1994). Uncertainties regarding C accumulation in forest soils were substantial (0.11–0.58 Mt C/y) but could be reduced by estimating forest soil C stocks in the future. Whereas soils can be important for the C balance in naturally regrowing forests, their C sequestration is negligible (less than 5%) relative to anthropogenic CO₂ emissions in Western Europe at present. Received 25 August 1998; accepted 17 March 1999.     

Estimating soil carbon fluxes following land-cover change: a test of some critical assumptions for a region in Costa Rica

Changes in soil carbon storage that accompany land‐cover change may have significant effects on the global carbon cycle. The objective of this work was to examine how assumptions about preconversion soil C storage and the effects of land‐cover change influence estimates of regional soil C storage. We applied three models of land‐cover change effects to two maps of preconversion soil C in a 140 000 ha area of northeastern Costa Rica. One preconversion soil C map was generated using values assigned to tropical wet forest from the literature, the second used values obtained from extensive field sampling. The first model of land‐cover change effects used values that are typically applied in global assessments, the second and third models used field data but differed in how the data were aggregated (one was based on land‐cover transitions and one was based on terrain attributes). Changes in regional soil C storage were estimated for each combination of model and preconversion soil C for three time periods defined by geo‐referenced land‐cover maps. The estimated regional soil C under forest vegetation (to 0.3 m) was higher in the map based on field data (10.03 Tg C) than in the map based on literature data (8.90 Tg C), although the range of values derived from propagating estimation errors was large (7.67–12.40 Tg C). Regional soil C storage declined through time due to forest clearing for pasture and crops. Estimated CO₂ fluxes depended more on the model of land‐cover change effects than on preconversion soil C. Cumulative soil C losses (1950–1996) under the literature model of land‐cover effects exceeded estimates based on field data by factors of 3.8–8.0. In order to better constrain regional and global‐scale assessments of carbon fluxes from soils in the tropics, future research should focus on methods for extrapolating regional‐scale constraints on soil C dynamics to larger spatial and temporal scales.     

Terrestrial carbon stocks across a gradient of urbanization: a study of the Seattle,WA region

Most of our global population and its CO₂ emissions can be attributed to urban areas. The process of urbanization changes terrestrial carbon stocks and fluxes, which, in turn, impact ecosystem functions and atmospheric CO₂ concentrations. Using the Seattle, WA, region as a case study, this paper explores the relationships between aboveground carbon stocks and land cover within an urbanizing area. The major objectives were to estimate aboveground live and dead terrestrial carbon stocks across multiple land cover classes and quantify the relationships between urban cover and vegetation across a gradient of urbanization. We established 154 sample plots in the Seattle region to assess carbon stocks as a function of distance from the urban core and land cover [urban (heavy, medium, and low), mixed forest, and conifer forest land covers]. The mean (and 95% CI) aboveground live biomass for the region was 89±22 Mg C ha^?1 with an additional 11.8±4 Mg C ha^?1 of coarse woody debris biomass. The average live biomass stored within forested and urban land covers was 140±40 and 18±14 Mg C ha^?1, respectively, with a 57% mean vegetated canopy cover regionally. Both the total carbon stocks and mean vegetated canopy cover were surprisingly high, even within the heavily urbanized areas, well exceeding observations within other urbanizing areas and the average US forested carbon stocks. As urban land covers and populations continue to rapidly increase across the globe, these results highlight the importance of considering vegetation in urbanizing areas within the terrestrial carbon cycle.     

中国天然林保护工程区森林覆盖遥感监测

天然林资源保护工程（天保工程）自1998年开始实施至2020年底结束,标志着我国林业由以木材生产为主向以生态建设为主进行转变,对天然林资源开展定量监测与评估,有助于全面、及时、科学地评价天保工程对森林资源的保护成效。基于公开发布的土地覆盖产品,利用新的遥感数据合成算法构建了森林类型综合提升方法,生产了一套我国天保工程区的森林覆盖产品,对天保工程区1997年至2020年间的森林覆盖情况开展监测与评估。利用中国森林生态系统定位研究网络数据（CFERN）、森林资源规划设计调查数据和地面调查数据进行精度检验,森林类型的分类精度优于90%。分析结果显示,自天保工程实施以来,工程区的森林覆盖度总体呈恢复性增长趋势,森林覆盖从1997年的30.15%增长到2020年的31.74%,净增长1.59%,其中,长江上游地区森林增长量最高,占全工程区森林增长面积的50.97%。研究结果表明我国天保工程区内森林资源得到有效保护和恢复,天保工程实施效果显著。     

Land Cover Change in Colombia: Surprising Forest Recovery Trends between 2001 and 2010

Background

Monitoring land change at multiple spatial scales is essential for identifying hotspots of change, and for developing and implementing policies for conserving biodiversity and habitats. In the high diversity country of Colombia, these types of analyses are difficult because there is no consistent wall-to-wall, multi-temporal dataset for land-use and land-cover change.

Methodology/Principal Findings

To address this problem, we mapped annual land-use and land-cover from 2001 to 2010 in Colombia using MODIS (250 m) products coupled with reference data from high spatial resolution imagery (QuickBird) in Google Earth. We used QuickBird imagery to visually interpret percent cover of eight land cover classes used for classifier training and accuracy assessment. Based on these maps we evaluated land cover change at four spatial scales country, biome, ecoregion, and municipality. Of the 1,117 municipalities, 820 had a net gain in woody vegetation (28,092 km²) while 264 had a net loss (11,129 km²), which resulted in a net gain of 16,963 km² in woody vegetation at the national scale. Woody regrowth mainly occurred in areas previously classified as mixed woody/plantation rather than agriculture/herbaceous. The majority of this gain occurred in the Moist Forest biome, within the montane forest ecoregions, while the greatest loss of woody vegetation occurred in the Llanos and Apure-Villavicencio ecoregions.

Conclusions

The unexpected forest recovery trend, particularly in the Andes, provides an opportunity to expand current protected areas and to promote habitat connectivity. Furthermore, ecoregions with intense land conversion (e.g. Northern Andean Páramo) and ecoregions under-represented in the protected area network (e.g. Llanos, Apure-Villavicencio Dry forest, and Magdalena-Urabá Moist forest ecoregions) should be considered for new protected areas.     

Effect of Native Vegetation Loss on Stream Ecosystem Processes: Dissolved Organic Matter Composition and Export in Agricultural Landscapes

Stream and river ecosystems are dependent on energetic inputs from their watersheds and thus shifts in land use from forest cover to agriculture will affect stream community composition and function. The disruption of forest-aquatic linkages alters the organic matter resources in agricultural streams. Dissolved organic matter (DOM) is the dominant form of organic matter in aquatic ecosystems, and a microbial energy source that is important for stream respiration. The concentrations and characteristics of DOM are regulated by both terrestrial (for example, terrestrial organic matter supply) and in-stream processes (for example, microbial respiration and periphyton production) that are influenced by land management. The effects of watershed land use and topographic, soil and climatic variables on DOM quantity (dissolved organic carbon concentration and load), source (terrestrial or in-stream) and quality (composition and lability) were measured in 14 streams across an agricultural land-use gradient. DOC concentration was positively correlated with watershed pasture cover and negatively correlated with watershed relief. No watershed variables were important correlates of DOC load. Stream DOM was primarily of terrestrial origin, but DOM in agricultural streams had a greater proportion of sources from in-stream sources. This may be due to reduced connection with riparian vegetation and increased in-stream primary production. We suggest that maintaining watershed tree cover greater than 52% and ensuring less than 10% of the length of riparian corridor is cleared for pasture could minimize changes to DOM composition. This is important to avoid flow-on effects for stream ecosystem processes that are mediated by DOM. Long-term DOM monitoring will be valuable for assessing the functional impacts of land-use change.     

Consequences of the Armed Conflict, Forced Human Displacement, and Land Abandonment on Forest Cover Change in Colombia: A Multi-scaled Analysis

Most studies of land change have focused on patterns, rates, and drivers of deforestation, but much less is known about the dynamics associated with agricultural abandonment and ecosystem recovery. Furthermore, most studies are conducted at a single spatial scale, and few have included variables related with internal socio-political conflicts. Here we evaluated the effect of environmental, demographic, and socio-economic variables on woody cover change in Colombia between 2001 and 2010 at the country, biome, and ecoregion scales. We also incorporated factors that reflect the unique history of Colombia such as the presence of illegal-armed groups and forced human displacement. Environmental variables explained the patterns of deforestation and forest regrowth at all scales because they can restrict or encourage different land uses across multiple spatial scales. Demographic variables were important at the biome and ecoregion scales and appear to be a consequence of the armed conflict, particularly through forced human displacement (for example, rural–urban migration), which in some areas has resulted in forest regrowth. In other areas, the impact of illegal armed groups has reduced forest cover, particularly in areas rich in gold and lands appropriate for cattle grazing. This multi-scale and multivariate approach provides a new insight into the complex relationship between woody cover change and land abandonment triggered mainly by armed conflict.     

Modeling and empirical validation of long‐term carbon sequestration in forests (France, 1850–2015)

The development of appropriate tools to quantify long‐term carbon (C) budgets following forest transitions, that is, shifts from deforestation to afforestation, and to identify their drivers are key issues for forging sustainable land‐based climate‐change mitigation strategies. Here, we develop a new modeling approach, CRAFT (CaRbon Accumulation in ForesTs) based on widely available input data to study the C dynamics in French forests at the regional scale from 1850 to 2015. The model is composed of two interconnected modules which integrate biomass stocks and flows (Module 1) with litter and soil organic C (Module 2) and build upon previously established coupled climate‐vegetation models. Our model allows to develop a comprehensive understanding of forest C dynamics by systematically depicting the integrated impact of environmental changes and land use. Model outputs were compared to empirical data of C stocks in forest biomass and soils, available for recent decades from inventories, and to a long‐term simulation using a bookkeeping model. The CRAFT model reliably simulates the C dynamics during France's forest transition and reproduces C‐fluxes and stocks reported in the forest and soil inventories, in contrast to a widely used bookkeeping model which strictly only depicts C‐fluxes due to wood extraction. Model results show that like in several other industrialized countries, a sharp increase in forest biomass and SOC stocks resulted from forest area expansion and, especially after 1960, from tree growth resulting in vegetation thickening (on average 7.8 Mt C/year over the whole period). The difference between the bookkeeping model, 0.3 Mt C/year in 1850 and 21 Mt C/year in 2015, can be attributed to environmental and land management changes. The CRAFT model opens new grounds for better quantifying long‐term forest C dynamics and investigating the relative effects of land use, land management, and environmental change.     

Land-Use History as Long-Term Broad-Scale Disturbance: Regional Forest Dynamics in Central New England

Human land-use activities differ from natural disturbance processes and may elicit novel biotic responses and disrupt existing biotic-environmental relationships. The widespread prevalence of land use requires that human activity be addressed as a fundamental ecological process and that lessons from investigations of land-use history be applied to landscape conservation and management. Changes in the intensity of land use and extent of forest cover in New England over the past 3 centuries provide the opportunity to evaluate the nature of forest response and reorganization to such broad-scale disturbance. Using a range of archival data and modern studies, we assessed historical changes in forest vegetation and land use from the Colonial period (early 17th century) to the present across a 5000 km² area in central Massachusetts in order to evaluate the effects of this novel disturbance regime on the structure, composition, and pattern of vegetation and its relationship to regional climatic gradients. At the time of European settlement, the distribution of tree taxa and forest assemblages showed pronounced regional variation and corresponded strongly to climate gradients, especially variation in growing degree days. The dominance of hemlock and northern hardwoods (maple, beech, and birch) in the cooler Central Uplands and oak and hickory at lower elevations in the Connecticut Valley and Eastern Lowlands is consistent with the regional distribution of these taxa and suggests a strong climatic control over broad-scale vegetation patterns. We infer from historical and paleoecological data that intensive natural or aboriginal disturbance was minimal in the Uplands, whereas infrequent surface fires in the Lowlands may have helped to maintain the abundance of central hardwoods and to restrict the abundance of hemlock, beech, and sugar maple in these areas. The modern vegetation is compositionally distinct from Colonial vegetation, exhibits less regional variation in the distribution of tree taxa or forest assemblages defined by tree taxa, and shows little relationship to broad climatic gradients. The homogenization of the vegetation, disruption of vegetation-environment relationships, and formation of new assemblages appear to be the result of (a) a massive, novel disturbance regime; (b) ongoing low-intensity human and natural disturbance throughout the reforestation period to the present; (c) permanent changes in some aspects of the biotic and abiotic environment; and (d) a relatively short period for forest recovery (100–150 years). These factors have maintained the regional abundance of shade intolerant and moderately tolerant taxa (for example, birch, red maple, oak, and pine) and restricted the spread and increase of shade-tolerant, long-lived taxa such as hemlock and beech. These results raise the possibility that historical land use has similarly altered vegetation-environment relationships across broader geographic regions and should be considered in all contemporary studies of global change. Received 5 May 1997; accepted 5 August 1997.     

Forest transitions in Eastern Europe and their effects on carbon budgets

Forests often rebound from deforestation following industrialization and urbanization, but for many regions our understanding of where and when forest transitions happened, and how they affected carbon budgets remains poor. One such region is Eastern Europe, where political and socio‐economic conditions changed drastically over the last three centuries, but forest trends have not yet been analyzed in detail. We present a new assessment of historical forest change in the European part of the former Soviet Union and the legacies of these changes on contemporary carbon stocks. To reconstruct forest area, we homogenized statistics at the provincial level for ad 1700–2010 to identify forest transition years and forest trends. We contrast our reconstruction with the KK11 and HYDE 3.1 land change scenarios, and use all three datasets to drive the LPJ dynamic global vegetation model to calculate carbon stock dynamics. Our results revealed that forest transitions in Eastern Europe occurred predominantly in the early 20th century, substantially later than in Western Europe. We also found marked geographic variation in forest transitions, with some areas characterized by relatively stable or continuously declining forest area. Our data suggest extensive deforestation in European Russia already prior to ad 1700, and even greater deforestation in the 18th and 19th centuries than in the KK11 and HYDE scenarios. Based on our reconstruction, cumulative carbon emissions from deforestation were greater before 1700 (60 Pg C) than thereafter (29 Pg C). Summed over our entire study area, forest transitions led to a modest uptake in carbon over recent decades, with our dataset showing the smallest effect (<5.5 Pg C) and a more heterogeneous pattern of source and sink regions. This suggests substantial sequestration potential in regrowing forests of the region, a trend that may be amplified through ongoing land abandonment, climate change, and CO₂ fertilization.     

Cervid Exclusion Alters Boreal Forest Properties with Little Cascading Impacts on Soils

Large herbivores are capable of modifying entire ecosystems with a combination of direct (for example browsing/grazing, trampling, defecation) and indirect (for example affecting plant species composition that then alters soil properties) effects. With many ungulate populations increasing across the northern hemisphere it is important to develop a general theory for how these animals can be expected to impact their habitats. Here we present the results of an 8-year experimental exclusion of moose (Alces alces) from 15 recent boreal forest clear-cut sites in Central Norway. We used standard univariate techniques to describe the treatment effect on multiple forest and soil properties and combined this with a multivariate Bayesian network structure learning approach to objectively assess the potential mechanistic pathways for indirect effects on soils and soil fertility. We found that excluding moose had predictable direct effects, such as increasing the ratio of deciduous to coniferous tree biomass and the canopy cover and decreasing soil bulk density and temperature. However, we found no treatment effects on any measures of soil processes or quality (decomposition, nitrogen availability, C/N ratio, pH, nutrient stocks), and furthermore, we found only limited evidence that the direct effects had cascading (indirect) effects on soils. These findings oppose the commonly held belief that moose exclusion will increase soil fertility, but still highlights the strong ability of moose to directly modify forested ecosystems.     

A multi-temporal approach in MaxEnt modelling: A new frontier for land use/land cover change detection

Land-cover change, a major driver of the distribution and functioning of ecosystems, is characterized by a high diversity of patterns of change across space and time. Thus, a large amount of information is necessary to analyse change and develop plans for proper management of natural resources. In this work we tested MaxEnt algorithm in a completely remote land-cover classification and change analysis. In order to provide an empirical example, we selected south-eastern Italian Alps, manly Trentino-South Tyrol, as test region. We classified two Landsat images (1976 and 2001) in order to forecast probability of occurrence for unsampled locations and to determine the best subset of predictors (spectral bands). A difference map for each land cover class, representing the difference between 1976 and 2001 probability of occurrence values, was built. In order to better address the patterns of change analysis, we put together difference maps and topographic variables. The latter are considered, at least in the study area, as the main environmental drivers of land-use change, in connection with climate change. Our results indicate that the selected algorithm, applied to land cover classes, can provide reliable data, especially when referring to classes with homogeneous texture properties and surface reflectance. The performed models had satisfactory predictive performance, showing relatively clear patterns of difference between the two considered time steps. The development of a methodology that, in the absence of field data, allow to obtain data on land use change dynamics, is of extreme importance for land planning and management.     

Forest carbon budgets in Southeast Asia following harvesting and land cover change

Terrestrial ecosystems play an important role in the global carbon (C) cycle. Tropicalforests in Southeast Asia are constantly changing as a result of harvesting and conversion to otherland cover. As a result of these changes, research on C budgets of forest ecosystems has intensi-fied in the region over the last few years. This paper reviews and synthesizes the available infor-mation. Natural forests in SE Asia typically contain a high C density (up to 500 Mg/ha). Logging activities are responsible for at least 50% decline in forest C density. Complete deforestation (conversion from forest to grassland or annual crops) results in C density of less than 40 Mg/ha. Conversion to tree plantations and other woody perennial crops also reduces C density to lessthan 50% of the original C forest stocks. While much information has been generated recently, there are still large gaps of information on C budgets of tropical forests and its conversion to otherland uses in SE Asia. There is therefore a need to intensify research in this area.     

Forest carbon budgets in Southeast Asia following harvesting and land cover change

Terrestrial ecosystems play an important role in the global carbon (C)cycle. Tropical forests in Southeast Asia are constantly changing as a result of harvesting and conversion to other land cover. As a result of these changes, research on C budgets of forest ecosystems has intensified in the region over thelast few years. This paper reviews and synthesizes the available information. Natural forests in SE Asia typically contain a high C density (up to 500 Mg/ha). Logging activities are responsible for at least 50% decline in forest C density.Complete deforestation (conversion from forest to grassland or annual crops) results in C density of less than 40 Mg/ha. Conversion to tree plantations and other woody perennial crops also reduces C density to less than 50% of the originalC forest stocks. While much information has been generated recently, there are still large gaps of information on C budgets of tropical forests and its conversion to other land uses in SE Asia. There is therefore a need to intensify research in this area.     

Modeling Forest Mortality Caused by Drought Stress: Implications for Climate Change

Climate change is expected to affect forest landscape dynamics in many ways, but it is possible that the most important direct impact of climate change will be drought stress. We combined data from weather stations and forest inventory plots (FIA) across the upper Great Lakes region (USA) to study the relationship between measures of drought stress and mortality for four drought sensitivity species groups using a weight-of-evidence approach. For all groups, the model that predicted mortality as a function of mean drought length had the greatest plausibility. Model tests confirmed that the models for all groups except the most drought tolerant had predictive value. We assumed that no relationship exists between drought and mortality for the drought-tolerant group. We used these empirical models to develop a drought extension for the forest landscape disturbance and succession model LANDIS-II, and applied the model in Oconto county, Wisconsin (USA) to assess the influence of drought on forest dynamics relative to other factors such as stand-replacing disturbance and site characteristics. The simulations showed that drought stress does affect species composition and total biomass, but effects on age classes, spatial pattern, and productivity were insignificant. We conclude that (for the upper Midwest) (1) a drought-induced tree mortality signal can be detected using FIA data, (2) tree species respond primarily to the length of drought events rather than their severity, (3) the differences in drought tolerance of tree species can be quantified, (4) future increases in drought can potentially change forest composition, and (5) drought is a potentially important factor to include in forest dynamics simulations because it affects forest composition and carbon storage.     

Forest Continuity as a Key Determinant of Soil Carbon and Nutrient Storage in Beech Forests on Sandy Soils in Northern Germany

Forest (or tree) age has been identified as an important determinant of the carbon (C) storage potential of forest soils. A large part of Central Europe’s current forested area was affected by land use change with long periods of cultivation in past centuries suggesting that the organic C stocks in the soil (SOC) under recent forest may partly be legacies of the past and that stand age effects have to be distinguished from forest continuity effects (that is, the time since re-afforestation). We examined the influence of mean tree age and forest continuity on the SOC pool and the stores of total N and available P, Ca, Mg, and K in the soil (mineral soil and organic layer) across a sample of 14 beech (Fagus sylvatica) forests on sandy soil with variable tree age (23–189 years) and forest continuity (50-year-old afforestation to ancient (‘permanent’) forest, that is, >230 years of proven continuity). Ancient beech forests (>230 years of continuity) stored on average 47 and 44% more organic C and total N in the soil than recent beech afforestation (50–128 years of continuity). Contrary to expectation, we found large and significant C and N pool differences between the forest categories in the mineral soil but not in the organic layer indicating that decade- or century-long cultivation has reduced the subsoil C and nutrient stores while the organic layer element pools have approached a new equilibrium after only 50–128 years. PCA and correlation analyses suggest that forest continuity cannot be ignored when trying to understand the variation in soil C stocks between different stands. Forest clearing, subsequent cultivation, and eventual re-afforestation with beech resulted in similar relative stock reductions of C and N and, thus, no change in soil C/N ratio. We conclude that the continuity of forest cover, which may or may not be related to tree age, is a key determinant of the soil C and nutrient stores of beech forests in the old cultural landscape of Central Europe.