Integrated global assessment of the natural forest carbon potential

Forests are a substantial terrestrial carbon sink, but anthropogenic changes in land use and climate have considerably reduced the scale of this system1. Remote-sensing estimates to quantify carbon losses from global forests2-5 are characterized by considerable uncertainty and we lack a comprehensive ground-sourced evaluation to benchmark these estimates. Here we combine several ground-sourced6 and satellite-derived approaches2,7,8 to evaluate the scale of the global forest carbon potential outside agricultural and urban lands. Despite regional variation, the predictions demonstrated remarkable consistency at a global scale, with only a 12% difference between the ground-sourced and satellite-derived estimates. At present, global forest carbon storage is markedly under the natural potential, with a total deficit of 226 Gt (model range = 151-363 Gt) in areas with low human footprint. Most (61%, 139 Gt C) of this potential is in areas with existing forests, in which ecosystem protection can allow forests to recover to maturity. The remaining 39% (87 Gt C) of potential lies in regions in which forests have been removed or fragmented. Although forests cannot be a substitute for emissions reductions, our results support the idea2,3,9 that the conservation, restoration and sustainable management of diverse forests offer valuable contributions to meeting global climate and biodiversity targets.

The global biogeography of tree leaf form and habit

Understanding what controls global leaf type variation in trees is crucial for comprehending their role in terrestrial ecosystems, including carbon, water and nutrient dynamics. Yet our understanding of the factors influencing forest leaf types remains incomplete, leaving us uncertain about the global proportions of needle-leaved, broadleaved, evergreen and deciduous trees. To address these gaps, we conducted a global, ground-sourced assessment of forest leaf-type variation by integrating forest inventory data with comprehensive leaf form (broadleaf vs needle-leaf) and habit (evergreen vs deciduous) records. We found that global variation in leaf habit is primarily driven by isothermality and soil characteristics, while leaf form is predominantly driven by temperature. Given these relationships, we estimate that 38% of global tree individuals are needle-leaved evergreen, 29% are broadleaved evergreen, 27% are broadleaved deciduous and 5% are needle-leaved deciduous. The aboveground biomass distribution among these tree types is approximately 21% (126.4 Gt), 54% (335.7 Gt), 22% (136.2 Gt) and 3% (18.7 Gt), respectively. We further project that, depending on future emissions pathways, 17-34% of forested areas will experience climate conditions by the end of the century that currently support a different forest type, highlighting the intensification of climatic stress on existing forests. By quantifying the distribution of tree leaf types and their corresponding biomass, and identifying regions where climate change will exert greatest pressure on current leaf types, our results can help improve predictions of future terrestrial ecosystem functioning and carbon cycling.

Native diversity buffers against severity of non-native tree invasions

Determining the drivers of non-native plant invasions is critical for managing native ecosystems and limiting the spread of invasive species1,2. Tree invasions in particular have been relatively overlooked, even though they have the potential to transform ecosystems and economies3,4. Here, leveraging global tree databases5-7, we explore how the phylogenetic and functional diversity of native tree communities, human pressure and the environment influence the establishment of non-native tree species and the subsequent invasion severity. We find that anthropogenic factors are key to predicting whether a location is invaded, but that invasion severity is underpinned by native diversity, with higher diversity predicting lower invasion severity. Temperature and precipitation emerge as strong predictors of invasion strategy, with non-native species invading successfully when they are similar to the native community in cold or dry extremes. Yet, despite the influence of these ecological forces in determining invasion strategy, we find evidence that these patterns can be obscured by human activity, with lower ecological signal in areas with higher proximity to shipping ports. Our global perspective of non-native tree invasion highlights that human drivers influence non-native tree presence, and that native phylogenetic and functional diversity have a critical role in the establishment and spread of subsequent invasions.

Co-limitation towards lower latitudes shapes global forest diversity gradients

The latitudinal diversity gradient (LDG) is one of the most recognized global patterns of species richness exhibited across a wide range of taxa. Numerous hypotheses have been proposed in the past two centuries to explain LDG, but rigorous tests of the drivers of LDGs have been limited by a lack of high-quality global species richness data. Here we produce a high-resolution (0.025° × 0.025°) map of local tree species richness using a global forest inventory database with individual tree information and local biophysical characteristics from ~1.3 million sample plots. We then quantify drivers of local tree species richness patterns across latitudes. Generally, annual mean temperature was a dominant predictor of tree species richness, which is most consistent with the metabolic theory of biodiversity (MTB). However, MTB underestimated LDG in the tropics, where high species richness was also moderated by topographic, soil and anthropogenic factors operating at local scales. Given that local landscape variables operate synergistically with bioclimatic factors in shaping the global LDG pattern, we suggest that MTB be extended to account for co-limitation by subordinate drivers.

Measure it to better manage it: a biodiversity monitoring framework for the Australian rangelands

The need for broad-scale, long-term biodiversity monitoring to support evidence-based policy and management in the Australian rangelands is clear and pressing but, despite protracted discussion of this need, there has been little progress towards implementation. To prompt real progress, we propose a framework of spatially hierarchical and complementary components that together use a combination of direct and indirect measures of biodiversity and drivers: • Targeted monitoring; involving localised field-based monitoring of target species, addressing specific management questions. • Surveillance monitoring; involving broad-scale, field-based sampling of multi-taxa and a set of habitat and condition attributes. • Landscape-scale monitoring; providing regional to national-scale intelligence on habitat quality and trends in threats to or drivers of biodiversity, with data obtained using systematic ground-based and remote methods. The framework aims to provide information on the response of biodiversity to rangeland management that is relevant to regional, state and national jurisdictions. We believe the characteristics of the proposed framework address many of the pitfalls that often stall biodiversity monitoring in Australia. These characteristics include: clarification of the desired outcomes and management requirements; a strong collaborative partnership that oversees the administration of the framework and ensures long-term commitment; a conceptual model that guides clear and relevant question-setting; careful design and analysis aimed at addressing the set questions; timely and relevant communication and reporting; and, regular data analysis and review, providing an adaptive mechanism for the framework to evolve and remain relevant. The proposed framework can be incrementally implemented at a moderate cost, relative to current total expenditure in natural resource management in the Australian rangelands.

Buffel grass in Queensland's semi-arid woodlands: response to local and landscape scale variables, and relationship with grass, forb and reptile species

Buffel grass [Pennisetum ciliare (L.) Link] has been widely introduced in the Australian rangelands as a consequence of its value for productive grazing, but tends to competitively establish in non-target areas such as remnant vegetation. In this study, we examined the influence landscape-scale and local-scale variables had upon the distribution of buffel grass in remnant poplar box (Eucalyptus populnea F.Muell.) dominant woodland fragments in the Brigalow Bioregion, Queensland. Buffel grass and variables thought to influence its distribution in the region were measured at 60 sites, which were selected based on the amount of native woodland retained in the landscape and patch size. An information-theoretic modelling approach and hierarchical partitioning revealed that the most influential variable was the percent of retained vegetation within a 1-km spatial extent. From this, we identified a critical threshold of ~30% retained vegetation in the landscape, above which the model predicted buffel grass was not likely to occur in a woodland fragment. Other explanatory variables in the model were site based, and included litter cover and long-term rainfall. Given the paucity of information on the effect of buffel grass upon biodiversity values, we undertook exploratory analyses to determine whether buffel grass cover influenced the distribution of grass, forb and reptile species. We detected some trends; hierarchical partitioning revealed that buffel grass cover was the most important explanatory variable describing habitat preferences of four reptile species. However, establishing causal links – particularly between native grass and forb species and buffel grass – was problematic owing to possible confounding with grazing pressure. We conclude with a set of management recommendations aimed at reducing the spread of buffel grass into remnant woodlands.

Characteristics of sap trees used by yellow-bellied gliders in southern Queensland

An extensive survey was conducted to locate sap trees used by the yellow-bellied glider (Petaurus australis) throughout the forests and woodlands of southern Queensland, across an area of 43.7 million hectares. We recorded the characteristics of 478 sap trees located at 109 of 297 sites surveyed. Only 13 tree species were selected by gliders for sap feeding throughout the study region, with the grey gum species Eucalyptus longirostrata and E. biturbinata most likely to be incised for sap. Of the tree species used for sap feeding by gliders, trees >40 cm in diameter at breast height (dbh) were used more than would be expected on the basis of their abundance in the forest. The number of sap trees with active and recently active feed scars at a site ranged from 0 to 12. Factors that appear to influence the abundance of active and recently active sap trees at a site include intensity of disturbance (basal area of cut stumps and dead trees), the number of stems in the 41–60 and 61–80 cm dbh classes, and number of Myrtaceae species. The response to these variables could be indicative of habitat quality, and the availability of alternative foraging substrates such as flower cover and decorticating bark. Intensification of selective logging in south-east Queensland, as an outcome of the Southeast Queensland Regional Forest Agreement, could potentially marginalise glider habitat. This would necessitate the retention of potential, or ‘recruit’, sap trees to maintain yellow-bellied glider habitat in these areas.

Hollow-bearing trees in large glider habitat in south-east Queensland, Australia: Abundance, spatial distribution and management

This study examined factors influencing the distribution of live and dead trees with large diameter hollows (>10 cm) in a productive coastal lowland forest of south-east Queensland. Forest age and type, historic logging rules and topographic position influenced the distribution of live hollow-bearing trees across the landscape. Also, some tree species (Eucalyptus acmenoides, Corymbia intermedia and C. trachyphloia) contained hollows at smaller diameters than others (C. citriodora, E. siderophloia and E. fibrosa), suggesting variation in rates of hollow formation among species. The average number of live hollow-bearing trees throughout the forest was 3.4 � 0.4 per ha (mean � s.e.), which is lower than the number of hollow-bearing trees to be retained during logging operations as specified by the Queensland Code of Practice for Native Forest Timber Production. The dead hollow-bearing tree resource is therefore important, and made up 42.3% of the total hollow-bearing tree resource. Dead hollow-bearing trees were available predominantly due to intensive silvicultural treatment conducted throughout the forest >50 years ago. However, the abundance of dead hollow-bearing trees appears to be influenced by fire management. If current management practices persist, it is predicted that in 50 years the dead hollow-bearing tree resource will be depleted. If so, the hollow-bearing tree resource for the Yellow-bellied Glider Petaurus australis and in particular, the Greater Glider Petauroides vofans, will be critically limited in the study area.

Use of sap trees by the yellow-bellied glider near Maryborough in south-east Queensland

Identifying the tree species used in sap feeding by the yellow-bellied glider (Petaurus australis) and describing the pattern of use of these trees is fundamental to forest management in eastern and southern Australia. We conducted surveys at 74 sites across 17�000 ha in St Mary State Forest near Maryborough in south-east Queensland, during five visits over a 2-year period. We identified sap-feeding incisions of gliders on 194 trees (2% of all trees) at 47 of our survey sites, including five trees recruited as sap trees during our monitoring period. Five tree species were involved: Corymbia citriodora (100 trees at 26 sites), Eucalyptus longirostrata (73 trees at 19 sites), Eucalyptus moluccana (17 trees at 6 sites), Eucalyptus tereticornis (3 trees at 3 sites), and Angophora leiocarpa (1 tree). Of these trees, E. longirostrata was used much more than expected on the basis of its abundance in the forest. Active glider incisions were observed on 95 of the sap trees (49% of total) during the monitoring period. During our first visit, 66 sap trees showed no evidence of current or recent use (within the previous month). By the fifth visit, there were still 44 sap trees (23% of all sap trees) that showed no evidence of recent use during any visit. Approximately 40% of sap tree observations were of trees with fewer than five recently active incisions, which is currently the threshold number of incisions for tree retention during logging operations in Queensland. We used Poisson regression to assess the influence of climatic and habitat variables on the abundance of active incisions at sites with sap trees. Three variables had a significant influence on the abundance of active incisions at a site: flowering, site index (a measure of productivity) and density of ringbarked trees. Our results confirm the importance of a small number of forest trees in sap feeding and recommendations are provided to enhance prescriptions for effective management of sap food resources of yellow-bellied gliders in south-east Queensland.

Habitat selection by the rare golden-tipped bat Kerivoula papuensis

Habitat selection by the golden-tipped bat Kerivoula papuensis was described using presence/absence data from 144 harp trapping sites between Cooktown in north-eastern Queensland and Bulahdelah in central-eastern New South Wales. Logistic regression was used to identify structural, topographic and other attributes of sites associated with the presence of K. papuensis. Occupied habitat was characterised by an abundance of vine drapes, the presence of the yellow-throated scrubwren Sericornis citreogularis, close spacing between stream channels, multiple tree layers, high relief and slope characteristics, westerly aspect, higher elevations, low abundance of large tree epiphytes and proximity to the nearest forest ecotone. However, aspects of habitat selection are poorly known, such as maternity roost selection and foraging habitat by the species. Further study is required to fully understand the habitat requirements of this species.