Landscape context mediates avian habitat choice in tropical forest restoration

Temporal dynamics in the composition of bird communities along a gradient of farmland restoration

Revegetation plantings are a key activity in farmland restoration and are commonly assumed to support biotic communities that, with time, replicate those of reference habitats. Restoration outcomes, however, can be highly variable and difficult to predict; hence there is value in quantifying restoration success to improve future efforts. We test the expectation that, over time, revegetation will restore bird communities to match those in reference habitats; and assess whether specific planting attributes enhance restoration success. We surveyed birds in 255 sites in south-east Australia, arranged along a restoration gradient encompassing three habitat types: unrestored farmland (paddocks), revegetation plantings (comprising a chronosequence up to 52 years old) and reference habitats (remnant native vegetation). Surveys were undertaken in 2006/2007 and again in 2019, with data used to compare bird assemblages between habitat types. We also determined whether, in the intervening 12 years, bird communities in revegetation had shifted toward reference habitats on the restoration gradient. Our results showed that each habitat contained a unique bird community and that, over time, assemblages in revegetation diverged away from those in unrestored farmland and converged toward those in reference habitats. Two planting attributes influenced this transition: the bird assemblages of revegetation were more likely to have diverged away from those of unrestored farmland (with scattered mature trees) 12 years later if they were located in areas with more surrounding tree cover, and were mostly ungrazed by livestock (compared with grazed plantings). Our results highlight three key ways in which revegetation contributes to farmland restoration: (1) by supporting richer and more diverse bird assemblages than unrestored farmland, (2) by enhancing beta diversity in rural landscapes through the addition of a unique bird community, and (3) by shifting bird assemblages toward those found in reference habitats over time. However, revegetation plantings did not replicate reference habitats by ~40-50 years in our region, and complete convergence may take centuries. These findings have implications for environmental offset programs and mean that effective conservation in farmland environments depends on the retention and protection of natural and seminatural habitats as a parallel management strategy to complement restoration.

Effects of landscape structure on restoration success in tropical premontane forest

Reversing large-scale habitat degradation and deforestation goes beyond what can be achieved by site-level ecological restoration and a landscape ecology perspective is fundamental. Here we assess the relative importance of tree cover and its configuration on forest-dependent birds and late-successional tree seedlings in restoration sites in southern Costa Rica. The abundance and species richness of birds increased in landscapes with more corridors, higher tree cover, and lower levels of fragmentation, highlighting the importance of riparian corridors for connectivity, and continuous tree cover as suitable habitat. Landscape variables affected abundance and species richness of seedlings similarly, but effects were weaker, possibly because seedlings face establishment limitation in addition to dispersal limitation. Moreover, the scale of landscape effects on seedlings was small, likely because proximal individual trees can significantly influence recruitment in restoration plots. Results underscore the importance of incorporating landscape-level metrics to restoration projects, as knowing the extent, and how the landscape may affect restoration outcomes can help to infer what kind of species will arrive to restoration plots.

Effects of dispersal- and niche-based factors on tree recruitment in tropical wet forest restoration

Both dispersal- and niche-based factors can impose major barriers on tree establishment. Our understanding of how these factors interact to determine recruitment rates is based primarily on findings from mature tropical forests, despite the fact that a majority of tropical forests are now secondary. Consequently, factors influencing seed limitation and the seed-to-seedling transition (STS) in disturbed landscapes, and how those factors shift during succession, are not well understood. We used a 3.5-yr record of seed rain and seedling establishment to investigate factors influencing tree recruitment after a decade of recovery in a tropical wet forest restoration experiment in southern Costa Rica. We asked (1) how do a range of restoration treatments (natural regeneration, applied nucleation, plantation), canopy cover, and life-history traits influence the STS and (2) how do seed and establishment limitation (lack of seed arrival or lack of seedling recruitment, respectively) influence vegetation recovery within restoration treatments as compared to remnant forest? We did not observe any differences in STS rates across restoration treatments. However, STS rates were lowest in adjacent later successional remnant forests, where seed source availability did not highly limit seed arrival, underscoring that niche-based processes may increasingly limit recruitment as succession unfolds. Additionally, larger-seeded species had consistently higher STS rates across treatments and remnant forests, though establishment limitation for these species was lowest in the remnant forests. Species were generally seed limited and almost all were establishment limited; these patterns were consistent across treatments. However, our results suggest that differences in recruitment rates could be driven by differential dispersal to treatments with higher canopy cover. We found evidence that barriers to recruitment shift during succession, with the influence of seed limitation, mediated by species-level seed deposition rates, giving way to niche-based processes. However, establishment limitation was lowest in the remnant forests for large-seeded and late successional species, highlighting the importance of habitat specialization and life-history traits in dictating recruitment dynamics. Overall, results demonstrate that active restoration approaches such as tree planting catalyze forest recovery, not only by decreasing components of seed limitation, but also by developing canopy cover that increases establishment rates of larger-seeded species.

Water availability drives aboveground biomass and bird richness in forest restoration plantings to achieve carbon and biodiversity cobenefits

To combat global warming and biodiversity loss, we require effective forest restoration that encourages recovery of species diversity and ecosystem function to deliver essential ecosystem services, such as biomass accumulation. Further, understanding how and where to undertake restoration to achieve carbon sequestration and biodiversity conservation would provide an opportunity to finance ecosystem restoration under carbon markets. We surveyed 30 native mixed-species plantings in subtropical forests and woodlands in Australia and used structural equation modeling to determine vegetation, soil, and climate variables most likely driving aboveground biomass accrual and bird richness and investigate the relationships between plant diversity, aboveground biomass accrual, and bird diversity. We focussed on woodland and forest-dependent birds, and functional groups at risk of decline (insectivorous, understorey-nesting, and small-bodied birds). We found that mean moisture availability strongly limits aboveground biomass accrual and bird richness in restoration plantings, indicating potential synergies in choosing sites for carbon and biodiversity purposes. Counter to theory, woody plant richness was a poor direct predictor of aboveground biomass accrual, but was indirectly related via significant, positive effects of stand density. We also found no direct relationship between aboveground biomass accrual and bird richness, likely because of the strong effects of moisture availability on both variables. Instead, moisture availability and patch size strongly and positively influenced the richness of woodland and forest-dependent birds. For understorey-nesting birds, however, shrub cover and patch size predicted richness. Stand age or area of native vegetation surrounding the patch did not influence bird richness. Our results suggest that in subtropical biomes, planting larger patches to higher densities, ideally using a diversity of trees and shrubs (characteristics of ecological plantings) in more mesic locations will enhance the provision of carbon and biodiversity cobenefits. Further, ecological plantings will aid the rapid recovery of woodland and forest bird richness, with comparable aboveground biomass accrual to less diverse forestry plantations.

Species-specific responses to habitat conversion across scales synergistically restructure Neotropical bird communities

Ecologists are increasingly exploring methods for preserving biodiversity in agricultural landscapes. Yet because species vary in how they respond to habitat conversion, ecological communities in agriculture and more natural habitats are often distinct. Unpacking the heterogeneity in species responses to habitat conversion will be essential for predicting and mitigating community shifts. Here, we analyze two years of bird censuses at 150 sites across gradients of local land cover, landscape forest amount and configuration, and regional precipitation in Costa Rica to holistically characterize species responses to habitat conversion. Specifically, we used Poisson-binomial mixture models to (1) delineate groups of species that respond similarly to environmental gradients, (2) explore the relative importance of local vs. landscape-level habitat conversion, and (3) determine how landscape context influences species' local habitat preferences. We found that species fell into six groups: habitat generalists, abundant and rare forest specialists, and three groups of agricultural specialists that differed in their responses to landscape forest cover, fragmentation, and regional precipitation. Birds were most sensitive to local forest cover, but responses were contingent on landscape context. Specifically, forest specialists benefitted most when local forest cover increased in forested landscapes, while habitat generalists exhibited compensatory dynamics, peaking at sites with either local or landscape-level forest, but not both. Our study demonstrates that species responses to habitat conversion are complex but predictable. Characterizing species-level responses to environmental gradients represents a viable approach for forecasting the winners and losers of global change and designing interventions to minimize the ongoing restructuring of Earth's biota.

Remote sensing-based landscape indicators for the evaluation of threatened-bird habitats in a tropical forest

Avian species persistence in a forest patch is strongly related to the degree of isolation and size of a forest patch and the vegetation structure within a patch and its matrix are important predictors of bird habitat suitability. A combination of space‐borne optical (Landsat), ALOS‐PALSAR (radar), and airborne Light Detection and Ranging (LiDAR) data was used for assessing variation in forest structure across forest patches that had undergone different levels of forest degradation in a logged forest—agricultural landscape in Southern Laos. The efficacy of different remote sensing (RS) data sources in distinguishing forest patches that had different seizes, configurations, and vegetation structure was examined. These data were found to be sensitive to the varying levels of degradation of the different patch categories. Additionally, the role of local scale forest structure variables (characterized using the different RS data and patch area) and landscape variables (characterized by distance from different forest patches) in influencing habitat preferences of International Union for Conservation of Nature (IUCN) Red listed birds found in the study area was examined. A machine learning algorithm, MaxEnt, was used in conjunction with these data and field collected geographical locations of the avian species to identify the factors influencing habitat preference of the different bird species and their suitable habitats. Results show that distance from different forest patches played a more important role in influencing habitat suitability for the different avian species than local scale factors related to vegetation structure and health. In addition to distance from forest patches, LiDAR‐derived forest structure and Landsat‐derived spectral variables were important determinants of avian habitat preference. The models derived using MaxEnt were used to create an overall habitat suitability map (HSM) which mapped the most suitable habitat patches for sustaining all the avian species. This work also provides insight that retention of forest patches, including degraded and isolated forest patches in addition to large contiguous forest patches, can facilitate bird species retention within tropical agricultural landscapes. It also demonstrates the effective use of RS data in distinguishing between forests that have undergone varying levels of degradation and identifying the habitat preferences of different bird species. Practical conservation management planning endeavors can use such data for both landscape scale monitoring and habitat mapping.

Fruit-eating birds in experimental plantings in southern Mexico

Maintenance of biodiversity in tropical agrarian landscapes is challenging in the face of anthropomorphic simplification of habitats. As part of an experiment testing influences of planting treatment on tree recruitment in southern Mexico, counts of bird species were made over 10 years in twenty-four 30 × 30-m fenced plots in over-grazed pasture. Plots were planted with native tree species or left as unplanted controls in 2006. Annual censuses of birds in the plots from 2007–2016 indicated statistically significant increases in the number of fruit-eating species and individuals as vegetation matured, but increases in non-frugivorous species and individuals over the decade were not significant. Among four species of planted animal-dispersed trees that bore fruit during this time,Cecropia obtusifoliaconsistently produced substantial crops after 2009. In 2015, all 53 planted or passively recruited female trees of mature size of this species bore fruit. The summed body masses of fruit-eating birds in each of 24 plots were significantly correlated with rank order of available fruit per plot. Differential use of habitat patches in an agrarian landscape suggested substantial value to frugivores, but less to non-frugivorous birds than expected.

When does seed limitation matter for scaling up reforestation from patches to landscapes?

Restoring forest to hundreds of millions of hectares of degraded land has become a centerpiece of international plans to sequester carbon and conserve biodiversity. Forest landscape restoration will require scaling up ecological knowledge of secondary succession from small-scale field studies to predict forest recovery rates in heterogeneous landscapes. However, ecological field studies reveal widely divergent times to forest recovery, in part due to landscape features that are difficult to replicate in empirical studies. Seed rain can determine reforestation rate and depends on landscape features that are beyond the scale of most field studies. We develop mathematical models to quantify how landscape configuration affects seed rain and forest regrowth in degraded patches. The models show how landscape features can alter the successional trajectories of otherwise identical patches, thus providing insight into why some empirical studies reveal a strong effect of seed rain on secondary succession, while others do not. We show that seed rain will strongly limit reforestation rate when patches are near a threshold for arrested succession, when positive feedbacks between tree canopy cover and seed rain occur during early succession, and when directed dispersal leads to between-patch interactions. In contrast, seed rain has weak effects on reforestation rate over a wide range of conditions, including when landscape-scale seed availability is either very high or very low. Our modeling framework incorporates growth and survival parameters that are commonly estimated in field studies of reforestation. We demonstrate how mathematical models can inform forest landscape restoration by allowing land managers to predict where natural regeneration will be sufficient to restore tree cover. Translating quantitative forecasts into spatially targeted interventions for forest landscape restoration could support target goals of restoring millions of hectares of degraded land and help mitigate global climate change.

Nest sites selection by sympatric cavity-nesting birds in miombo woodlands

Deforestation and habitat fragmentation have long been known as drivers of wildlife depletion but information on their specific impacts on cavity-nesting birds in the miombo woodlands has been lacking. A comparative study of disturbed and undisturbed sites was conducted in miombo woodlands of Zambia to assess impacts of environmental stressors on birds. Foot patrols were employed to locate, identify and count host trees and cavities for cavity-nesting birds on twenty 200 m × 200 m sample plots. Undisturbed forests had three times more cavities (the nesting sites for birds), while there were 24.6% fewer abandoned cavities in undisturbed forests than in disturbed forests. The rate of cavity abandonment was about twice as high in human-dominated forests compared to undisturbed forests (61.3% c.f. 31.9%). Cavity-nesting birds preferred larger (> 36.0 cm diameter at breast height) and taller (> 5.0 m) trees for nest placement, especially in human-dominated forests. A number of cavity-nesting birds preferred Brachystegia spiciformis (zebrawood), Julbernadia paniculata (munsa), Parinari curatellifolia (mobola-plum) and Uapaca kirkiana (mahobohobo) as host trees to 14 other miombo tree species. Arnot’s Chat (Myrmecocichla arnoti) had a wider selection of host trees for cavity-nesting than the other 40 cavity-nesting birds in the study areas. Anthropogenic activities such as uncontrolled firewood collection, wild fires, logging, and land clearing for agriculture negatively influenced wood abundance and diversity, with potential implications for persistence of cavity-nesting birds. The negative impacts of anthropogenic activities could be counteracted by conservation strategies such as implementation of sound forest policies, integrative land use practices, sustainable livelihood security and stakeholders’ awareness of the need to safeguard forest-dependent avifauna.Conservation implications: This comparative study unravels specific anthropogenic impacts on the cavity-nesting birds in the miombo woodlands, which would be relevant for designing and implementing targeted biodiversity conservation interventions against negative local environmental values and attitudes that support rural development on the expense of conservation of biodiversity such as birds.

Sixty-Seven Years of Land-Use Change in Southern Costa Rica

Habitat loss and fragmentation of forests are among the biggest threats to biodiversity and associated ecosystem services in tropical landscapes. We use the vicinity of the Las Cruces Biological Station in southern Costa Rica as a regional case study to document seven decades of land-use change in one of the most intensively studied sites in the Neotropics. Though the premontane wet forest was largely intact in 1947, a wave of immigration in 1952 initiated rapid changes over a short period. Overall forest cover was reduced during each time interval analyzed (1947-1960, 1960-1980, 1980-1997, 1997-2014), although the vast majority of forest loss (>90%) occurred during the first two time intervals (1947-1960, 1960-1980) with an annual deforestation rate of 2.14% and 3.86%, respectively. The rate dropped to <2% thereafter and has been offset by forest recovery in fallow areas more recently, but overall forest cover has continued to decline. Approximately 27.9% of the study area is forested currently. Concomitantly, the region shifted from a single contiguous forest to a series of progressively smaller forest fragments with each successive survey. A strong reduction in the amount of core habitat was paralleled by an increased proportion of edge habitat, due to the irregular shape of many forest fragments. Structural connectivity, however, remains high, with an expansive network of >100 km of linear strips of vegetation within a 3 km radius of the station, which may facilitate landscape-level movement for some species. Despite the extent of forest loss, a substantial number of regional landscape-level studies over the past two decades have demonstrated the persistence of many groups of organisms such as birds and mammals. Nonetheless, the continued decline in the quantity and quality of remaining habitat (~30% of remaining forest is secondary), as well as the threat of an extinction debt (or time lag in species loss), may result in the extirpation of additional species if more proactive conservation measures are not taken to reverse current trends-a pattern that reflects many other tropical regions the world over.

Seed dispersal limitations shift over time in tropical forest restoration

Past studies have shown that tropical forest regeneration on degraded farmlands is initially limited by lack of seed dispersal, but few studies have tracked changes in abundance and composition of seed rain past the first few years after land abandonment. We measured seed rain for 12 months in 10 6-9-year-old restoration sites and five mature, reference forests in southern Costa Rica in order to learn (1) if seed rain limitation persists past the first few years of regeneration; (2) how restoration treatments influence seed community structure and composition; and (3) whether seed rain limitation is contingent on landscape context. Each restoration site contained three 0.25-ha treatment plots: (1) a naturally regenerating control, (2) tree islands, and (3) a mixed-species tree plantation. Sites spanned a deforestation gradient with 9-89% forest area within 500 m around the treatment plots. Contrary to previous studies, we found that tree seeds were abundant and ubiquitous across all treatment plots (585.1 ± 142.0 seeds · m(-2) · yr(-1) [mean ± SE]), indicating that lack of seed rain ceased to limit forest regeneration within the first decade of recovery. Pioneer trees and shrubs comprised the vast majority of seeds, but compositional differences between restoration sites and reference forests were driven by rarer, large-seeded species. Large, animal-dispersed tree seeds were more abundant in tree islands (4.6 ± 2.9 seeds · m(-2) · yr(-1)) and plantations (5.8 ± 3.0 seeds · m(-2) · yr(-1)) than control plots (0.2 ± 0.1 seeds · m(-2) · yr(-1)), contributing to greater tree species richness in actively restored plots. Planted tree species accounted for < 1% of seeds. We found little evidence for landscape forest cover effects on seed rain, consistent with previous studies. We conclude that seed rain limitation shifted from an initial, complete lack of tree seeds to a specific limitation on large-seeded, mature forest species over the first decade. Although total seed abundance was equal among restoration treatments, tree plantations and tree islands continued to diversify seed rain communities compared to naturally regenerating controls. Compositional differences between regenerating plots and mature forests suggest that large-seeded tree species are appropriate candidates for enrichment planting.