Seedling diversity in actively and passively restored tropical forest understories.

Alternative methods for restoring tropical forests influence the ecological processes that shape recruitment of understory species. In turn, the traits of species recruited will influence the ecological processes the forests provide now and over the long term. We assess the phylogenetic and functional structure of seedlings beneath monoculture plantations, mixed-species plantations (both active restoration) and regenerating selectively logged native forests (passive restoration), considering traits of specific leaf area (SLA, including within-species variation), leaf nitrogen and phosphorus content, life-form, potential plant height, and dispersal type. Monoculture plantations comprised seedlings that were more closely related then would be expected by chance (i.e., phylogenetically clustered), and regenerating forest contained species more distantly related then would be expected by chance (i.e., phylogenetically overdispersed). This suggests that seedlings beneath monocultures assemble through environmental filtering and through the dispersal limitation of predictable functional guilds. However, dispersal limitation is frequently overcome by human-assisted dispersal, increasing trait diversity. Comparing SLA values revealed that regenerating forests recruit seedlings with both high and low mean and variation of SLA, leading to higher overall diversity. Regenerating forest seedlings showed signs of environmental filtering, only based on within-species variation of SLA. Regenerating forest understories appear to favor species that show a high intraspecific variation in SLA values (e.g., Pterocarpus indicus Willd.) and at the same time provided habitat for later successional seedlings that show a lower intraspecific variation in SLA (e.g., Canarium luzonicum (Blume) A.Gray). This trait diversity suggests limiting similarity or competitive exclusion may be reduced because of niche differences, allowing species with different traits to coexist. Phylogenetic and functionally distinct species are restricted in their regeneration capacity, many of which are of conservation significance (under the IUCN Red List). Reforestation projects should maximize desired ecological services (including conservation value) by actively managing for the recruitment of species that are phylogenetically and functionally (including intraspecifically) distinct. This management aim will increase the probability of fulfilling a wider array of niche spaces and potentially increase the diversity of ecosystem services provided.

Tree leaf trade-offs are stronger for sub-canopy trees: leaf traits reveal little about growth rates in canopy trees.

Can morphological plant functional traits predict demographic rates (e.g., growth) within plant communities as diverse as tropical forests? This is one of the most important next-step questions in trait-based ecology and particularly for global reforestation efforts. Due to the diversity of tropical tree species and their longevity, it is difficult to predict their performance prior to reforestation efforts. In this study, we investigate if simple leaf traits are predictors of the more complex ecological process of plant growth in regenerating selectively logged natural forest within the Wet Tropics (WTs) bioregion of Australia. This study used a rich historical data set to quantify tree growth within plots located at Danbulla National Park and State Forest on the Atherton Tableland. Leaf traits were collected from trees that have exhibited fast or slow growth over the last ~50 yr of measurement. Leaf traits were found to be poor predictors of tree growth for trees that have entered the canopy; however, for sub-canopy trees, leaf traits had a stronger association with growth rates. Leaf phosphorus concentrations were the strongest predictor of Periodic Annual Increment (PAI) for trees growing within the sub-canopy, with trees with higher leaf phosphorus levels showing a higher PAI. Sub-canopy tree leaves also exhibited stronger trade-offs between leaf traits and adhere to theoretical predictions more so than for canopy trees. We suggest that, in order for leaf traits to be more applicable to reforestation, size dependence of traits and growth relationships need to be more carefully considered, particularly when reforestation practitioners assign mean trait values to tropical tree species from multiple canopy strata.

Impacts of Myrtle Rust Induced Tree Mortality on Species and Functional Richness within Seedling Communities of a Wet Sclerophyll Forest in Eastern Australia.

Austropuccinia psidii is an introduced plant pathogen known to have caused significant declines in populations of several Australian native Myrtaceae species. However, limited research has focused on the impacts of the pathogen on plant communities in the aftermath of its invasion. This study investigated the relationship between disease impact level, plant species diversity, and functional richness in seedling communities in a wet sclerophyll forest in southeast Queensland. A clear shift was found from early colonizer Myrtaceae species in the mid- and understory to a more diverse non-Myrtaceae seedling community indicative of secondary succession. Comparisons of key Myrtaceae species and the seedling community suggest that there may also be a shift towards species that produce drupes and larger seeds, and overall, a current reduction in fruit availability due to the dramatic loss of previously dominant species. Seedling diversity showed no significant correlation with tree mortality, possibly due to favorable rainfall conditions during the study period. The more subtle changes in forest composition, such as changes in fruit type and availability due to myrtle rust, however, could affect the visitation of local bird species in the short term and certainly reduce the store of early colonizing native shrub and tree species.

The community capacity curve applied to reforestation: a framework to support success.

Community involvement is critical for the success of many interventions designed to promote reforestation. To secure this involvement, it helps to recognize that communities are heterogenous both within and among themselves and possess diverse mixes of livelihood assets required to implement reforestation. We explore the relationship between livelihood assets and reforestation success and outline a conceptual model that we call the community capacity curve (CCC) applied to reforestation. We argue that the shape of the CCC is sigmoidal. Importantly, communities at the lower end of the CCC have limited capacity to implement reforestation projects without substantial and ongoing capacity building and other sorts of support, including through livelihood projects that improve food security and provide cash benefits. Communities at the higher part of the CCC have greater capacity to implement reforestation projects, especially projects focused on biodiversity and environmental services. The CCC can help design, implement, monitor and assess reforestation projects, select appropriate livelihood activities and types of reforestation, select communities suited to a reforestation project, guide implementation and understand projects' successes and failure. The CCC also provides a framework to engage with policy makers and funding bodies to explore the types of support for communities to reforest successfully. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Fifteen essential science advances needed for effective restoration of the world's forest landscapes.

There has never been a more pressing and opportune time for science and practice to collaborate towards restoration of the world's forests. Multiple uncertainties remain for achieving successful, long-term forest landscape restoration (FLR). In this article, we use expert knowledge and literature review to identify knowledge gaps that need closing to advance restoration practice, as an introduction to a landmark theme issue on FLR and the UN Decade on Ecosystem Restoration. Aligned with an Adaptive Management Cycle for FLR, we identify 15 essential science advances required to facilitate FLR success for nature and people. They highlight that the greatest science challenges lie in the conceptualization, planning and assessment stages of restoration, which require an evidence base for why, where and how to restore, at realistic scales. FLR and underlying sciences are complex, requiring spatially explicit approaches across disciplines and sectors, considering multiple objectives, drivers and trade-offs critical for decision-making and financing. The developing tropics are a priority region, where scientists must work with stakeholders across the Adaptive Management Cycle. Clearly communicated scientific evidence for action at the outset of restoration planning will enable donors, decision makers and implementers to develop informed objectives, realistic targets and processes for accountability. This article paves the way for 19 further articles in this theme issue, with author contributions from across the world. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Isotopic fractionation from deep roots to tall shoots: A forensic analysis of xylem water isotope composition in mature tropical savanna trees.

Studies of plant water sources generally assume that xylem water integrates the isotopic composition (δ2H and δ18O) of water sources and does not fractionate during uptake or transport along the transpiration pathway. However, woody xerophytes, halophytes, and trees in mesic environments can show isotopic fractionation from source waters. Isotopic fractionation and variation in isotope composition can affect the interpretation of tree water sources, but most studies to date have been greenhouse experiments. Here we present a field-based forensic analysis of xylem water isotope composition for 12 Eucalyptus tetrodonta and Corymbia nesophila trees. We used a 25-tonne excavator to access materials from the trees' maximum rooting depth of 3 m to their highest canopies at 38 m. Substantial within-tree variation occurred in δ2H (-91.1‰ to -35.7‰ E. tetrodonta; -88.8‰ to -24.5‰ C. nesophila) and δ18O (-12.3‰ to -5.0‰ E. tetrodonta; -10.9‰ to -0.3‰ C. nesophila), with different root-to-branch isotope patterns in each species. Soil water δ2H and δ18O dual isotope slopes (7.26 E. tetrodonta, 6.66 C. nesophila) were closest to the Local Meteoric Water Line (8.4). The dual isotope slopes of the trees decreased progressively from roots (6.45 E. tetrodonta, 6.07 C. nesophila), to stems (4.61 E. tetrodonta, 5.97 C. nesophila) and branches (4.68 E. tetrodonta, 5.67 C. nesophila), indicative of fractionation along the xylem stream. Roots of both species were more enriched in 2H and 18O than soil water at all sampled depths. Bayesian mixing model analysis showed that estimated proportions of water sourced from different depths reflected the contrasting root systems of these species. Our study adds evidence of isotopic fractionation from water uptake and along the transpiration stream in mature trees in monsoonal environments, affecting the interpretation of water sources. We discuss the findings with view of interpreting aboveground xylem water isotopic composition, incorporating knowledge of root systems.

Rapid recovery of tropical forest diversity and structure after shifting cultivation in the Philippines uplands.

Shifting cultivation is a widespread land-use in the tropics that is considered a major threat to rainforest diversity and structure. In the Philippines, a country with rich biodiversity and high rates of species endemism, shifting cultivation, locally termed as kaingin, is a major land-use and has been for centuries. Despite the potential impact of shifting cultivation on forests and its importance to many people, it is not clear how biodiversity and forest structure recover after kaingin abandonment in the country, and how well these post-kaingin secondary forests can complement the old-growth forests. We investigated parameters of forest diversity and structure along a fallow age gradient in secondary forests regenerating after kaingin abandonment in Leyte Island, the Philippines (elevation range: 445-650 m asl). We first measured the tree diversity and forest structure indices in regenerating secondary forests and old-growth forest. We then measured the recovery of tree diversity and forest structure parameters in relation to the old-growth forest. Finally, using linear mixed effect models (LMM), we assessed the effect of different environmental variables on the recovery of forest diversity and structure. We found significantly higher species density in the oldest fallow sites, while Shannon's index, species evenness, stem number, basal area, and leaf area index were higher in the old-growth forest. A homogeneous species composition was found across the sites of older fallow age. Multivariate analysis revealed patch size as a strong predictor of tree diversity and forest structure recovery after shifting cultivation. Our study suggests that, secondary forests regenerating after shifting cultivation abandonment can recover rapidly. Although recovery of forest structure was not as rapid as the tree diversity, our older fallow sites contained a similar number of species as the old-growth forest. Many of these species are also endemic to the Philippines. Novel and emerging ecosystems like tropical secondary forests are of high conservation importance and can act as a refuge for dwindling tropical forest biodiversity.

Above-ground biomass recovery following logging and thinning over 46 years in an Australian tropical forest.

Managed tropical forests are a globally important carbon pool, but the effects of logging and thinning intensities on long-term biomass dynamics are poorly known. We investigated the demographic mechanisms of above-ground biomass recovery over 48 years in an Australian tropical forest following four silvicultural treatments: selective logging only as a control and selective logging followed by low-, medium- and high-intensity thinning. Initial biomass recovery rates following thinning were poor predictors of the long-term changes. Initial biomass recovery from 1969 to 1973 was slow and was largely concentrated on an increase in the biomass of residual stems. From 1973 to 1997, above ground biomass (AGB) increased almost linearly, with a similar slope for all sites. From 1997 to 2015, the rate of biomass accumulation slowed, especially for the L treatment. All thinning treatments stimulated more recruitment and regrowth of non-harvested remaining trees compared to the untreated control. Biomass at both the low and medium intensity treatments has almost fully recovered to 98% and 97% of pre-logging biomass levels respectively. The predicted times of complete above-ground biomass recovery for the logging only and high intensity treatments are 55 and 77 years respectively. The slower biomass recovery at the logging only site was largely due to increased mortality in the last measurement period. The slower recovery of the high intensity site was due to a combination of a higher initial reduction in biomass from thinning and the increased mortality in the last measurement period. The high mortality rates in the most recent measurement period are likely due to the impacts of two cyclones that impacted the study site. Our results suggest that it will take at least around 50 years for this site to recover to its pre-harvest biomass, much longer than many of the cutting cycles currently used in tropical forest management.

Without management interventions, endemic wet-sclerophyll forest is transitioning to rainforest in World Heritage listed K'gari (Fraser Island), Australia.

Wet-sclerophyll forests are unique ecosystems that can transition to dry-sclerophyll forests or to rainforests. Understanding of the dynamics of these forests for conservation is limited. We evaluated the long-term succession of wet-sclerophyll forest on World Heritage listed K'gari (Fraser Island)-the world's largest sand island. We recorded the presence and growth of tree species in three 0.4 hectare plots that had been subjected to selective logging, fire, and cyclone disturbance over 65 years, from 1952 to 2017. Irrespective of disturbance regimes, which varied between plots, rainforest trees recruited at much faster rates than the dominant wet-sclerophyll forest trees, narrowly endemic species Syncarpia hillii and more common Lophostemon confertus. Syncarpia hillii did not recruit at the plot with the least disturbance and recruited only in low numbers at plots with more prominent disturbance regimes in the ≥10 cm at breast height size. Lophostemon confertus recruited at all plots but in much lower numbers than rainforest trees. Only five L. confertus were detected in the smallest size class (<10 cm diameter) in the 2017 survey. Overall, we find evidence that more pronounced disturbance regimes than those that have occurred over the past 65 years may be required to conserve this wet-sclerophyll forest, as without intervention, transition to rainforest is a likely trajectory. Fire and other management tools should therefore be explored, in collaboration with Indigenous landowners, to ensure conservation of this wet-sclerophyll forest.

Combined effects of climate change and sea-level rise project dramatic habitat loss of the globally endangered Bengal tiger in the Bangladesh Sundarbans.

The Sundarbans, in southern coastal Bangladesh, is the world's largest surviving mangrove habitat and the last stronghold of tiger adapted to living in a mangrove ecosystem. Using MaxEnt (maximum entropy modeling), current distribution data, land-use/land cover and bioclimatic variables, we modeled the likely future distribution of the globally endangered Bengal tiger (Panthera tigris tigris) in the Bangladesh Sundarbans. We used two climatic scenarios (i.e., RCP6.0 and RCP8.5) developed by the Intergovernmental Panel on Climate Change (IPCC) to provide projections of suitable habitats of Bengal tigers in 2050 and 2070. We also combined projected sea-level rise for the area in our models of future species distributions. Our results suggest that there will be a dramatic decline in suitable Bengal tiger habitats in the Bangladesh Sundarbans. Other than various aspects of local climate, sea-level rise is projected to have a substantial negative impact on Bengal tiger habitats in this low-lying area. Our model predicts that due to the combined effect of climate change and sea-level rise, there will be no suitable Bengal tiger habitat remaining in the Sundarbans by 2070. Enhancing terrestrial protected area coverage, regular monitoring, law enforcement, awareness-building among local residents among the key strategies needed to ensure long-term survival and conservation of the Bengal tiger in the Bangladesh Sundarbans.

Drivers of Tree Growth, Mortality and Harvest Preferences in Species-Rich Plantations for Smallholders and Communities in the Tropics.

There is growing interest in multi-species tropical plantations but little information exists to guide their design and silviculture. The Rainforestation Farming system is the oldest tropical polyculture planting system in the Philippines and provides a unique opportunity to understand the underlying processes affecting tree performance within diverse plantings. Data collected from 85 plots distributed across the 18 mixed-species plantations in the Philippines was used to identify the factors influencing growth, probability of harvest, and death of trees in these complex plantings. The 18 sites (aged from 6 to 11 years at time of first measurement) were measured on three occasions over a 6-year period. We used data from the first period of data collection to develop models predicting harvesting probability and growth of trees in the second period. We found little evidence that tree species diversity had an effect on tree growth and tree loss at the community level, although a negative effect was found on tree growth of specific species such as Parashorea plicata and Swietenia macrophylla. While tree density of stands at age 10+ years (more than 1000 trees/ha with diameter > 5cm) did not have an impact on growth, growth rates were decreasing in stands with a high basal area. Tree size in the first period of measure was a good predictor for both tree growth and tree status in the next period, with larger trees tending to grow faster and having a greater chance of being harvested, and a lower possibility of mortality than smaller trees. Shade-intolerant trees were both more likely to be harvested, and had a higher probability of death, than shade-tolerant individuals. Native species and exotic species were equally likely to have been lost from the plots between measurement periods. However, shade-tolerant native trees were likely to grow faster than the others at age 10+ years. Our findings suggest that species traits (e.g. shade tolerance) could play an important role in optimizing species composition for this type of plantation. Shade-intolerant species with rapid early growth could contribute early income for farmers in mixed plantings where some products may take years to realize. We also suggest selective harvesting or thinning (for small shade-intolerant trees) applied at age 10+ years could reduce the competition for resources between individuals.

Tropical secondary forests regenerating after shifting cultivation in the Philippines uplands are important carbon sinks.

In the tropics, shifting cultivation has long been attributed to large scale forest degradation, and remains a major source of uncertainty in forest carbon accounting. In the Philippines, shifting cultivation, locally known as kaingin, is a major land-use in upland areas. We measured the distribution and recovery of aboveground biomass carbon along a fallow gradient in post-kaingin secondary forests in an upland area in the Philippines. We found significantly higher carbon in the aboveground total biomass and living woody biomass in old-growth forest, while coarse dead wood biomass carbon was higher in the new fallow sites. For young through to the oldest fallow secondary forests, there was a progressive recovery of biomass carbon evident. Multivariate analysis indicates patch size as an influential factor in explaining the variation in biomass carbon recovery in secondary forests after shifting cultivation. Our study indicates secondary forests after shifting cultivation are substantial carbon sinks and that this capacity to store carbon increases with abandonment age. Large trees contribute most to aboveground biomass. A better understanding of the relative contribution of different biomass sources in aboveground total forest biomass, however, is necessary to fully capture the value of such landscapes from forest management, restoration and conservation perspectives.

Designing mixed species tree plantations for the tropics: balancing ecological attributes of species with landholder preferences in the Philippines.

A mixed species reforestation program known as the Rainforestation Farming system was undertaken in the Philippines to develop forms of farm forestry more suitable for smallholders than the simple monocultural plantations commonly used then. In this study, we describe the subsequent changes in stand structure and floristic composition of these plantations in order to learn from the experience and develop improved prescriptions for reforestation systems likely to be attractive to smallholders. We investigated stands aged from 6 to 11 years old on three successive occasions over a 6 year period. We found the number of species originally present in the plots as trees >5 cm dbh decreased from an initial total of 76 species to 65 species at the end of study period. But, at the same time, some new species reached the size class threshold and were recruited into the canopy layer. There was a substantial decline in tree density from an estimated stocking of about 5000 trees per ha at the time of planting to 1380 trees per ha at the time of the first measurement; the density declined by a further 4.9% per year. Changes in composition and stand structure were indicated by a marked shift in the Importance Value Index of species. Over six years, shade-intolerant species became less important and the native shade-tolerant species (often Dipterocarps) increased in importance. Based on how the Rainforestation Farming plantations developed in these early years, we suggest that mixed-species plantations elsewhere in the humid tropics should be around 1000 trees per ha or less, that the proportion of fast growing (and hence early maturing) trees should be about 30-40% of this initial density and that any fruit tree component should only be planted on the plantation margin where more light and space are available for crowns to develop.

Landholder profiling and typologies for natural resource-management policy and program support: potential and constraints.

The use of landholder typologies to aid the development, implementation, and monitoring natural-resource management (NRM) policies and programs has increased considerably during the past decade. This article explores the potential for using such typologies for a variety of NRM and rural and regional development applications. Review of typology use further suggests that there is potential to refine the way that typologies are developed and applied to better aid NRM, farming systems analyses, and rural and regional development. Before typologies will be adopted more widely, a number of theoretical and methodologic issues must be addressed. These include the following questions: (1) Which criteria and methods should or can be used to classify landholders? (2) How should studies across spatial and temporal scales be integrated? (3) How should multiple and single industry studies be integrated to gain the most value from research? We argue that quantitative research techniques are well suited to provide an underlying structure for landholder typologies, and qualitative research techniques are useful for developing understanding of the nature of variation within and between landholder types. We argued further that because of the potential utility and breadth for the application of landholder typologies, a nested set of landholder typologies could be developed that are coordinated at the national, regional, and local geographic levels, with repeated measures used to track the evolution with time of landholder practices, management values, and socioeconomic characteristics.