Local forest structure variability increases resilience to wildfire in dry western U.S. coniferous forests.

A 'resilient' forest endures disturbance and is likely to persist. Resilience to wildfire may arise from feedback between fire behaviour and forest structure in dry forest systems. Frequent fire creates fine-scale variability in forest structure, which may then interrupt fuel continuity and prevent future fires from killing overstorey trees. Testing the generality and scale of this phenomenon is challenging for vast, long-lived forest ecosystems. We quantify forest structural variability and fire severity across >30 years and >1000 wildfires in California's Sierra Nevada. We find that greater variability in forest structure increases resilience by reducing rates of fire-induced tree mortality and that the scale of this effect is local, manifesting at the smallest spatial extent of forest structure tested (90 × 90 m). Resilience of these forests is likely compromised by structural homogenisation from a century of fire suppression, but could be restored with management that increases forest structural variability

Forest floor vegetation in Sweden

In boreal forests, dwarf-shrubs (Vaccinium spp.) often dominate the forest floor and are key-stone species in ecosystems due to their importance for nutrient cycling and as a major food source for herbivores. Forestry affects the vegetation both directly through management and indirectly by altering the forest structure. Forest fertilization with N at the end of the rotation period is a common practice in Swedish boreal forests. Even higher timber production can be achieved if fertilization with multi-nutrient fertilizer is applied early in the rotation period, but the effects on forest floor vegetation have not been studied. The objectives of this thesis were to increase knowledge regarding how 1) intensive fertilization in young forest affects forest floor vegetation; 2) background deposition of N influences the effects of N addition; and 3) to relate observed changes in common species abundances to changes in forest structure. Fertilization decreased the abundance of many common forest plant species while only few species increased (I). Surprisingly, also species known as nitrophilous decreased in abundance. Paper I shows that the decrease in availability of light induced by fertilization is a crucial factor behind this change. Consequently, fertilization reduced both species richness, species diversity and the between site (β) diversity (II). In areas where the background N deposition was low (4 kg ha-1 yr-1), the effects of N addition were larger than in areas with intermediate (16 kg ha-1 yr-1) deposition (III). Key-stone species among the forest floor vegetation of boreal Sweden (e.g. Vaccinium myrtillus) were found to decrease in abundance (IV). These species are strongly dependent on aspects of forest structure, such as forest density and age, and likewise, temporal changes in species abundance coincided with corresponding changes in forest structure (IV). In conclusion, in large parts of Sweden the prevailing forest management is incompatible with a productive forest floor vegetation possessing a high diversity of plant species, and this situation will only be exacerbated by more intensive use of fertilization regimes. To avoid associated cascading effects from the decreased abundance of key-stone species, forestry intensity needs to be relaxed on the landscape level which would likely result in a considerable loss of timber production. Compensation for this loss through intensified forestry on other areas would indicate the need for altered forest zoning

Markedly Divergent Tree Assemblage Responses to Tropical Forest Loss and Fragmentation across a Strong Seasonality Gradient

We examine the effects of forest fragmentation on the structure and composition of tree assemblages within three seasonal and aseasonal forest types of southern Brazil, including evergreen, Araucaria, and deciduous forests. We sampled three southernmost Atlantic Forest landscapes, including the largest continuous forest protected areas within each forest type. Tree assemblages in each forest type were sampled within 10 plots of 0.1 ha in both continuous forests and 10 adjacent forest fragments. All trees within each plot were assigned to trait categories describing their regeneration strategy, vertical stratification, seed-dispersal mode, seed size, and wood density. We detected differences among both forest types and landscape contexts in terms of overall tree species richness, and the density and species richness of different functional groups in terms of regeneration strategy, seed dispersal mode and woody density. Overall, evergreen forest fragments exhibited the largest deviations from continuous forest plots in assemblage structure. Evergreen, Araucaria and deciduous forests diverge in the functional composition of tree floras, particularly in relation to regeneration strategy and stress tolerance. By supporting a more diversified light-demanding and stress-tolerant flora with reduced richness and abundance of shade-tolerant, old-growth species, both deciduous and Araucaria forest tree assemblages are more intrinsically resilient to contemporary human-disturbances, including fragmentation-induced edge effects, in terms of species erosion and functional shifts. We suggest that these intrinsic differences in the direction and magnitude of responses to changes in landscape structure between forest types should guide a wide range of conservation strategies in restoring fragmented tropical forest landscapes worldwide

MODERATE SEVERITY DISTURBANCE HAS SIMILAR EFFECTS ON THE PRODUCTION OF THREE FORESTS NESTED WITHIN THE UPPER GREAT LAKES LANDSCAPE

Moderate severity disturbances, which only kill a subset of canopy trees (e.g., via insects, pathogens, and windthrow), are increasingly widespread, and can alter forest structure and production. Whether moderate severity disturbance similarly affects the net primary production (NPP) of different forest stands within inherently heterogeneous landscapes, however, is unknown. We experimentally disturbed three, 2-ha stands varying in forest structure and primary production, reducing stand basal area 38 to 66 % by stem girdling all mature early successional aspen (Populus) and birch (Betula). For nearly a decade, we examined how the forest stands restructured and recovered, and linked post-recovery physical and biological structure with light absorption and wood NPP. Disturbance significantly altered the structure of all stands and prompted a similar decade-long pattern of primary production decline and recovery. All stands exhibited an initial reduction in wood NPP, recovering to, or exceeded pre-disturbance levels within eight years. Following the recovery of wood NPP, more biologically diverse forest canopies with higher leaf area indexes captured more light, and, subsequently, had higher rates of wood NPP. We provide limited support that disturbance may enhance long-term primary production through its effects on canopy structural reorganization. We conclude that, while the forests examined responded similarly to disturbance, improved understanding of different forest ecosystems’ response to disturbance remains critical to informing carbon management decisions across diverse landscape mosaics

Ice storm effects on the canopy structure of a northern hardwood forest after 8 years

Ice storms can cause severe damage to forest canopies, resulting in differential mortality among tree species and size classes and leading to long-lasting changes in the vertical structure and composition of the forest. An intense ice storm in 1998 damaged large areas of the northern hardwood forest, including much of the Hubbard Brook Experimental Forest, New Hampshire (USA). Following up on detailed poststorm assessments, we measured changes in the vertical structure of the forest canopy 8 years poststorm. We focused on how the presence of disease-induced advance regeneration of American beech (Fagus grandifolia Ehrh.) has affected canopy structure in the recovering forest. We measured foliage-height profiles using a point-quadrat approach and a pole-mounted leaf area index (LAI) sensor. Although the total LAIs of damaged and undamaged areas were similar, areas damaged in 1998 showed an increased proportion of total leaf area between 6 and 10 m above the ground. The foliage at this height is largely (54%) beech. To the extent that this heavily beech-dominated understory layer suppresses regeneration of other species, these findings suggest that rare disturbances of mature northern hardwood forests affected by beech bark disease will increase the importance of damage-prone and economically marginal beech

Does degradation from selective logging and illegal activities differently impact forest resources? A case study in Ghana

Degradation, a reduction of the ecosystem’s capacity to supply goods and services, is widespread in tropical forests and mainly caused by human disturbance. To maintain the full range of forest ecosystem services and support the development of effective conservation policies, we must understand the overall impact of degradation on different forest resources. This research investigates the response to disturbance of forest structure using several indicators: soil carbon content, arboreal richness and biodiversity, functional composition (guild and wood density), and productivity. We drew upon large field and remote sensing datasets from different forest types in Ghana, characterized by varied protection status, to investigate impacts of selective logging, and of illegal land use and resources extraction, which are the main disturbance causes in West Africa. Results indicate that functional composition and the overall number of species are less affected by degradation, while forest structure, soil carbon content and species abundance are seriously impacted, with resources distribution reflecting the protection level of the areas. Remote sensing analysis showed an increase in productivity in the last three decades, with higher resiliency to change in drier forest types, and stronger productivity correlation with solar radiation in the short dry season. The study region is affected by growing anthropogenic pressure on natural resources and by an increased climate variability: possible interactions of disturbance with climate are also discussed, together with the urgency to reduce degradation in order to preserve the full range of ecosystem functions

Coupling Avicennia tree architecture to radar backscatter models for estimating mangrove forest biomass

The understanding of mangrove ecosystems functioning requires techniques allowing their forest structure and forest biomass to be assessed. Among them, radar measurements have demonstrated powerful capabilities that, however, must be validated extensively, both spatially and physically. Beyond their intrinsic potential to provide basic information on tree growth and forest functioning, 3D tree architecture studies could help the interpretation of radar signal scattering within forest canopies to be performed and thus, improve the accuracy of radar inversion algorithms for estimating forest parameters such as total above-ground biomass and component biomass. Topological measurements have been conducted at different growth stages on Avicennia germinans trees with height less than 5 meters. For each tree measured, leaf size, nodes length and diameter are coded using the AMAPmod terminology. Thanks to this software and assuming stands composed of identical trees, input parameters for the radar backscatter models of forests are then derived from architectural analysis. The simulated radar responses are compared to radar measurements when available. The effect of tree architecture and the structure of associated stands are discussed with the objectives to estimate forest parameters using radar data. This work comes within the international project ALOS Kyoto and Carbon Initiative aiming to monitor forest dynamics, particularly mangrove forests. (Texte intégral

Canopy structural modeling using object-oriented image classification and laser scanning

A terrestrial laser scanning (TLS) experiment was carried out in the EAGLE 2006 campaign to characterize and model the canopy structure of the Speulderbos forest. Semi-variogram analysis was used to describe spatial variability of the surface. The dependence of the spatial variability on the applied grid size showed, that in this forest spatial details of the digital surface model are lost in the case of larger than 0.3-0.4 m grid size. Voxel statistics was used for describing the density of the canopy structure. Five zones of the canopy were identified according to their density distribution. Basic geometric structures were tested for modeling the forest at the individual tree level. The results create a firm basis for modeling physical processes in the canopy