查看更多>>摘要：? 2021 Elsevier B.V.Forest ecosystems are increasingly exposed to the combined pressure of climate change and attacks by pests and pathogens. These stress factors can threaten already vulnerable species triggering dieback and rising defoliation and mortality rates. To characterize abiotic (drought, climate warmings) and biotic (pathogens) risks and their spatiotemporal patterns we quantified the recent loss of vitality for the endangered and relict Abies pinsapo forests from Andalusia, south-eastern Spain. Abies pinsapo is an iconic Mediterranean fir showing a high vulnerability to drought stress and also to several pests (Cryphalus numidicus) and root rot fungi (Armillaria mellea). We analyzed a monitoring network dataset of radial growth, defoliation and mortality from 2001 to 2017 including 1025 trees situated in three major mountain ranges (Sierra de Grazalema, Sierra de las Nieves, and Sierra Bermeja). We fitted several statistical models to determine the main drivers of changes in defoliation, a proxy of tree vigor, and mortality. Defoliation and mortality rates were much higher towards the East of the study area, mirroring the gradient from Atlantic to Mediterranean climatic conditions. In the most affected stands tree defoliation increased in response to a combination of long and severe droughts, with attacks by the beetle C. numidicus. Mortality rates increased in response to a higher defoliation rate, a lower relative radial-growth rate, long and severe droughts and a higher incidence of A. mellea. Our findings illustrate the value of monitoring networks recording changes in forest health to quantify and forecast future vulnerability of threatened tree species.
查看更多>>摘要：? 2021 Elsevier B.V.The boreal forests of the Northern Hemisphere (i.e., covering the USA, Canada and Russia) are the grandest carbon sinks of the world. A significant increase in wildfires could cause disequilibrium in the Northern boreal forest's capacity as a carbon sink and cause significant impacts on wildlife and people worldwide. That is why the ability to forecast wildfires is essential in order to minimize all risks and vulnerabilities. We present a novel methodology utilizing the Bayesian Machine Learning models to identify climatic variations that induce high and low wildfire activity cycles and forecast long-term occurrences of wildfires. The data analyzed are observed records of wildfires, climate change and climate teleconnections, atmospheric, oceanographic, and environmental factors, starting from the first half of the 20th century. Our Bayesian machine learning models show that a new phase of high wildfire activity in the USA, Canada and Russia began in 2020. While USA has a detectable, oscillation of 40±5 years; Russia and Canada have oscillatory patterns of 30±5 and 60±5 years, respectively. Also, our Machine Learning model forecasts peak wildfire activity at around 2022±3,2035±3, and 2045±5 years for USA, Russia, and Canada, respectively. The new high wildfire activity phase will persist in Russia, USA, and Canada, until 2045, 2030, and 2055, respectively.
查看更多>>摘要：? 2021 Elsevier B.V.High grading is, by definition, wealth-destructive, and yet the practice is widespread. Explanations of high grading typically assume landowners are greedy, impatient, or ignorant. None of these explanations are robust. This paper develops an analytical model of landowner decision making that provides a richer explanation for this counterproductive practice. The model centers on buyers’ behavior in markets for heterogeneous-quality forestland. Facing imperfect information, it is rational for buyers to shade their estimated valuation of a prospective property up or down toward the region-wide average. Because high-quality forestland thus sells at a discount, and low-quality forestland for a premium, the so-called strip-and-flip strategy can outperform good, long-term silviculture. A simulation case study for a northern hardwood forest in the Adirondack region of New York illustrates this theoretical model. The simulation incorporates empirical growth models, continuous quality-specific price functions, and integer programming methods to specify the tree-by-tree harvest schedule that maximizes long-term net present value. An alternative simulation conditions cutting decisions on the expected sale price at the end of a ten-year investment period, resulting in a systematic—or, perhaps, “selective”—shift in harvesting patterns favoring removal of high-value trees. In presenting an improved theory of high grading, this study helps direct policy makers’ attention away from dismissive characterizations of landowners as dumb, greedy, or both, and toward closer analysis of the institutional factors that drive deliberate forest degradation.
查看更多>>摘要：? 2021 Elsevier B.V.A study was conducted on three species (Khaya senegalensis, Corymbia citriodora and Casuarina equisetifolia) planted in desert lands of Egypt and irrigated with basic treated municipal wastewater. Every single tree is permanently drip irrigated and at the same time fertilised by the nutrient-rich wastewater. Trees of different classes of the diameter at breast height (eight for each species) were felled to determine the form factor and stem volume, as well as the biomass and carbon sequestration of stem, crown and root of the individual trees. In addition, a scenario for the development of un-thinned stands from ages 5 to 35 years was constructed. The age of the sample trees at felling was: 128, 66 and 82 months for K. senegalensis, C. citriodora and C. equisetifolia, respectively. The vigorous sampled trees had a stem volume of 0.289, 0.162 and 0.229 m3 and an aboveground and belowground biomass of 451, 161 and 264 kg for K. senegalensis, C. citriodora and C. equisetifolia, respectively. Results of the scenario showed an unexpectedly high growth rate of afforestation in desert lands using municipal wastewater. At the age of 25, the stem volume for all three species exceeds 398 m3/ha. At the same age of 25 years, K. senegalensis, C. citriodora and C. equisetifolia sequester 1131, 1068 and 860 CO2 t/ha, respectively. Afforestation in desert areas presents a unique situation in forestry, as growth is stimulated by sufficient sunlight, water and nutrients. Competition for water and nutrients is strongly diminished. Self-thinning of stands under such conditions is expected to be much slower than in rain-fed plantation forests. On the other hand, tree maturity span is curtailed. This opens new possibilities for different forest management and practices towards resilient forest-based mitigation of climate change. For its unexpected role as an effective mitigation measure, afforestation in desert lands should be encouraged and further research conducted.
查看更多>>摘要：? 2021 Elsevier B.V.Human land use often leads to distinct modifications of soils within small, clearly delimited areas. Legacy effects of past land use are also abundant in recent forest areas, but are hardly considered in ecosystem assessment and soil mapping. The soils on relict charcoal hearths (RCHs) are a widespread example for such land use legacy soils in forests. Soils on RCHs differ clearly from surrounding forest soils in their stratigraphy and properties, and are most prominently characterized by a technogenic substrate layer with high contents of charcoal and thus soil organic matter (SOM). Although RCH soils can clearly contribute to the SOM stocks of forest soils, their relevance on the landscape scale has hardly been quantified. In this paper we analyze and discuss the distribution and effects of RCH soils across scales for forests in the state of Brandenburg, Germany, with a focus on SOM stocks. Our analysis is based on a large-scale mapping of RCHs from digital elevation models (DEMs), combined with a linear regression model of SOM stocks in RCH soils. The spatial distribution of RCH soils in the study region has a scale dependent heterogeneity. The large-scale variation in site densities is related to the concentration of charcoal production to specific forest areas while the small-scale variation is related to the irregular distribution of single RCHs within the charcoal production fields. Up to 1.5 % of the surface are covered by RCH soils in the forest areas within the study region. The results also show that RCHs can significantly contribute to the SOM stocks of forests, even where they cover only a small fraction of the soil surface, with up to 18 % additional SOM for forest areas. Because of the heterogeneous distribution of RCHs, the SOM stock additions depend on the scale of observation. Our results suggest that RCH SOM additions can be overestimated when sites in small areas (< 1 km2) are analyzed; and that characteristic spatial concentration patterns of RCHs are even observable when values are aggregated to large areas (25 km2 and more). Aggregating effects to irregularly-sized forest administrative units resulted in maps that well represent the spatial distribution of RCH soils and SOM. The study highlights that considering land use legacy effects can be relevant for the results of soil mapping and inventories; and that prospecting and mapping land use legacies from DEMs can contribute to improving such approaches.
查看更多>>摘要：? 2021 Elsevier B.V.Accurate quantification of fine fuel loads (e.g. foliage and twigs) in forests is required for many fire behaviour models, and for assessing post-fire changes in carbon stocks and modelling smoke emissions. Fine fuels burn readily and are thus often targeted for fuel load assessments. Estimates of fine live fuel loads often rely on visual assessments or utilise allometric equations that relate stem diameter of plants to total above-ground biomass. Here, we develop allometric equations for shrubs that relate stem diameter to the portion of above-ground biomass comprised of fine fuel. Our study area is within the temperate eucalypt forests of south-eastern Australia. We present equations for (i) foliage; (ii) all biomass < 3 mm diameter; (iii) all biomass < 6 mm diameter; and (iv) all above-ground biomass. Simple power-law models were developed for five shrub species and saplings of two tree species. Models combining all species (RMSE = 0.03–0.0.06) worked similarly well to species-specific models (RMSE = 0.01–0.08). We then applied these all-species combined models to field observations of shrub stem diameters, measured before and after planned burns. In unburnt forest, the proportion of shrub biomass comprised of fine fuel varied considerably (from 6 to 58%). Fine fuel loads were positively related to total above-ground biomass (R2 = 0.75) and basal area of shrubs (R2 = 0.79). There was considerable variation in consumption of fine fuel. The median reduction in fine fuel load was 22.4%, whereas the median reduction in total above-ground biomass was only 2.3%. Our models of shrub fine fuels can be readily applied to field-based assessments or combined with existing models or remotely sensed estimates of above-ground biomass to model fine fuel loads over large heterogeneous study areas.
查看更多>>摘要：? 2021 Elsevier B.V.In addition to long-standing concerns about sustaining forest productivity, maintaining forest ecosystems under changing conditions and emerging threats has become increasingly important when planning forest management. With the aim of understanding effects of management on both productivity and recovery, we quantified the 25-year impact of varying degrees of organic matter (OM) removal and soil compaction on above-ground biomass, soil carbon and nutrients, soil bulk density, and stand development in aspen-dominated forests in the upper Lake States region of the US. Treatment impacts were assessed at three different sites with comparable overstory composition, but with varying soil texture, site quality, and climate. Across all sites, soil C and N generally decreased with increasing OM removal, and bulk density increased with increasing compaction; 25-year observations indicate recovery of bulk density at the surface (0–10 cm) but not at deeper portions of the soil profile. At the most productive site (loamy soils) with favorable initial soil porosity, severe compaction decreased mean aboveground biomass (-46%), particularly of trees (-73%). Biomass at 25 years did not differ among organic matter removal treatments (e.g. stem-only harvest), but a greater increase in soil C occurred with stem-only harvest relative to whole-tree harvest plus forest floor removal. In contrast, at a less productive site with sandy soils poorly buffered to nutrient and C removals, whole-tree harvest reduced biomass by 25% (tree biomass declined 35%) relative to stem-only harvest while compaction treatments did not differ in effects on biomass production, soil C or soil N. On clay soils, compaction treatments did not significantly impact biomass production, but whole-tree harvest plus forest floor removal reduced tree biomass by 47% relative to whole-tree harvest alone. Assessment of mean relative density indicates canopy closure has not yet occurred at the least productive site (clay soils) or the more severely disturbed stands at the intermediate site (sandy soils), suggesting the possibility for treatment impacts not yet discernible to become more pronounced as stands develop and nutrient uptake continues in the future. Our results align with concepts of soil quality and texture-specific limitations to growth, underlying a need to understand key soil limitations when considering forest management impacts to aboveground structure and productivity.
查看更多>>摘要：? 2021Removal of riparian trees can alter aquatic ecosystem structure and function by influencing factors such as light availability, sediment input, and stream temperature. Contemporary forest practices such as variable retention harvest are used to mitigate the effects of clear-cut forest harvest on stream communities, but few studies have examined the effects of these techniques on aquatic ecosystems. We examined the effects of variable retention harvest on light, temperature, biofilm biomass and macroinvertebrate consumers in three coastal headwater streams in British Columbia, Canada and compared them to three nearby reference streams with unlogged riparian stands. Variable retention harvest increased light and stream temperature variability. Harvested catchments had higher stream biofilm biomass in all seasons except winter and higher invertebrate abundance in summer. Variable retention harvest altered invertebrate community composition, largely driven by increasing Chironomidae abundance and decreasing Simuliidae abundance. In conclusion, we found that variable retention harvest modified stream benthic communities, but responses varied seasonally and among taxa. This is one of few studies to investigate the impacts of variable retention harvest on multiple trophic levels over multiple seasons and years. Understanding the cascading effects of forest harvest over multi-year time scales is important for management decisions.
查看更多>>摘要：? 2021 Elsevier B.V.We investigated the relative importance of daily fire weather, landscape position, climate, recent forest and fuels management, and fire history to explaining patterns of remotely-sensed burn severity – as measured by the Relativized Burn Ratio – in 150 fires occurring from 2001 to 2019, which burned conifer forests of northeastern Washington State, USA. Daily fire weather, annual precipitation anomalies, and species’ fire resistance traits were important predictors of wildfire burn severity. In areas burned within the past two to three decades, prior fire decreased the severity of subsequent burns, particularly for the first 16 postfire years. In areas managed before a wildfire, thinning and prescribed burning treatments lowered burn severity relative to untreated controls. Prescribed burning was the most effective treatment at lowering subsequent burn severity, and prescribed burned areas were usually unburned or burned at low severity in subsequent wildfires. Patches that were harvested and planted <10 years before a wildfire burned with slightly higher severity. In areas managed within 5 years after an initial fire, postfire harvest and planting reduced prevalence of stand-replacing fire in reburns. However, overall, postfire management actions after a first wildfire only weakly influenced the severity of subsequent fires. The importance of fire-fire interactions to moderating burn severity establishes the importance of stabilizing feedbacks in active fire regimes, and our results demonstrate how silvicultural treatments can be combined with prescribed fire and wildfires to maintain resilient landscapes.
查看更多>>摘要：? 2021 Elsevier B.V.There are many studies on the applicability of natural regeneration for recovery of degraded forests. Studies of the potential of natural regeneration and succession to restore degraded forest areas under invasive exotic plants are few in comparison. This study examined the potential of these processes to bring about the recovery of large areas under the exotic paper mulberry Broussonetia papyrifera in Mabira Forest, Uganda. The areas of focus were degraded by farming but abandoned and invaded by mulberry 30 years ago. Sampling was conducted in three mulberry-dominant stands (Int 1, Int 2, and Hv) that were differentiated by the degree of mulberry cutting by local communities and other anthropogenic disturbances. Primary forest (Nt) was a fourth stand used as a reference. Data were collected using Modified-Whittaker Plots, transects and other plot types adapted to record impact of disturbance. Analyses of stem density showed that both mulberry and native trees occurred in all the stands in varying densities and age classes from seedling to tree class size. Non-metric multidimensional scaling showed Int 1 (a relatively undisturbed stand with fully grown mulberry) to be closest to Nt floristically, followed by Int 2 (a disturbed stand with fully grown mulberry). Hv (a disturbed stand with low stature mulberry) was least similar to Nt. Overall, Int 1 was closer to Nt than to any of the other stands; while Int 2 was closer to Hv than to Int 1 or to Nt. Most analyses of seedling and sapling densities and species diversity of native trees showed a pattern whereby Hv < Int 2 < Int 1 < Nt. Numbers of forest-typical species varied between stands but not significantly. Inter-stand differences in the levels of disturbance were pronounced with Hv and Int 2 having significantly higher incidences of illegal indigenous tree cutting and charcoal kilning than Int 1. Regressions of percent cover values on seedling densities, sapling densities and species richness showed negative relationships between mulberry cover and native tree seedling density and species richness but not sapling density. Indigenous tree sapling densities were positively correlated with indigenous tree species cover. These findings suggest that anthropogenic disturbance and to a smaller extent mulberry factors; and not differences in rates of species colonization and degrees of species performance explain the current state of recovery of the stands. Natural regeneration and succession therefore have potential to recover the degraded sites and reverse the dominance of mulberry if the ongoing human disturbance is stopped.
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