Rebecca J. Cole

Landscape-Scale Controls on Aboveground Forest Carbon Stocks on the Osa Peninsula, Costa Rica

Tropical forests store large amounts of carbon in tree biomass, although the environmental controls on forest carbon stocks remain poorly resolved. Emerging airborne remote sensing techniques offer a powerful approach to understand how aboveground carbon density (ACD) varies across tropical landscapes. In this study, we evaluate the accuracy of the Carnegie Airborne Observatory (CAO) Light Detection and Ranging (LiDAR) system to detect top-of-canopy tree height (TCH) and ACD across the Osa Peninsula, Costa Rica. LiDAR and field-estimated TCH and ACD were highly correlated across a wide range of forest ages and types. Top-of-canopy height (TCH) reached 67 m, and ACD surpassed 225 Mg C ha-1, indicating both that airborne CAO LiDAR-based estimates of ACD are accurate in tall, high-biomass forests and that the Osa Peninsula harbors some of the most carbon-rich forests in the Neotropics. We also examined the relative influence of lithologic, topoedaphic and climatic factors on regional patterns in ACD, which are known to influence ACD by regulating forest productivity and turnover. Analyses revealed a spatially nested set of factors controlling ACD patterns, with geologic variation explaining up to 16% of the mapped ACD variation at the regional scale, while local variation in topographic slope explained an additional 18%. Lithologic and topoedaphic factors also explained more ACD variation at 30-m than at 100-m spatial resolution, suggesting that environmental filtering depends on the spatial scale of terrain variation. Our result indicate that patterns in ACD are partially controlled by spatial variation in geologic history and geomorphic processes underpinning topographic diversity across landscapes. ACD also exhibited spatial autocorrelation, which may reflect biological processes that influence ACD, such as the assembly of species or phenotypes across the landscape, but additional research is needed to resolve how abiotic and biotic factors contribute to ACD variation across high biomass, high diversity tropical landscapes.

Migratory bird species in young tropical forest restoration sites: effects of vegetation height, planting design, and season

Summary Tropical land cover change has negatively affected numerous migratory bird populations. Forest restoration can augment migrant wintering habitat. However, almost no information exists about factors that influence migrant use of tropical restoration sites. We sampled migrant birds in young restoration sites in Costa Rica from February 2006 to April 2008 to determine how vegetation height, planting design, season, and landscape forest cover influenced capture rates of four declining species. We also documented total numbers of migratory species and individuals captured in each planting design treatment; each site had a control treatment where seedlings were not planted, an island treatment where seedlings were planted in patches, and a plantation treatment where seedlings were planted to cover the entire area. Sites varied in landscape forest cover within 500 m buffers. Three out of four focal species were captured significantly more often in plantation treatments than island or control treatments. Two of the four species showed seasonal patterns and one species was captured more often in high-vegetation sites. Greater numbers of species and individuals were captured in plantation treatments compared to island and control treatments. The plantation planting design increased migrant use more quickly than the island planting design. When resources are available, we recommend planting plantation-style to rapidly increase the value of restoration sites to a range of species, particularly those that use woody vegetation. When resources are more limited, planting islands may be a cost-effective, although not as ecologically effective, alternative that supports a diversity of migrant species compared to unplanted controls.

Leaf litter arthropod responses to tropical forest restoration

Abstract Soil and litter arthropods represent a large proportion of tropical biodiversity and perform important ecosystem functions, but little is known about the efficacy of different tropical forest restoration strategies in facilitating their recovery in degraded habitats. We sampled arthropods in four 7‐ to 8‐year‐old restoration treatments and in nearby reference forests. Sampling was conducted during the wet and dry seasons using extractions from litter and pitfall samples. Restoration treatments were replicated in 50 × 50‐m plots in four former pasture sites in southern Costa Rica: plantation – trees planted throughout the plot; applied nucleation/islands – trees planted in patches of different sizes; and natural regeneration – no tree planting. Arthropod abundance, measures of richness and diversity, and a number of functional groups were greater in the island treatment than in natural regeneration or plantation treatments and, in many cases, were similar to reference forest. Litter and pitfall morphospecies and functional group composition in all three restoration treatments were significantly different than reference sites, but island and plantation treatments showed more recovery than natural regeneration. Abundance and functional group diversity showed a much greater degree of recovery than community composition. Synthesis and applications: The less resource‐intensive restoration strategy of planting tree islands was more effective than tree plantations in restoring arthropod abundance, richness, and functional diversity. None of the restoration strategies, however, resulted in similar community composition as reference forest after 8 years of recovery, highlighting the slow rate of recovery of arthropod communities after disturbance, and underscoring the importance of conservation of remnant forests in fragmented landscapes.

Topographic distributions of emergent trees in tropical forests of the Osa Peninsula, Costa Rica

Tropical rainforests are reservoirs of terrestrial carbon and biodiversity. Large and often emergent trees store disproportionately large amounts of aboveground carbon and greatly influence the structure and functioning of tropical rainforests. Despite their importance, controls on the abundance and distribution of emergent trees are largely unknown across tropical landscapes. Conventional field approaches are limited in their ability to characterize patterns in emergent trees across vast landscapes with varying environmental conditions and floristic composition. Here, we used a high-resolution light detection and ranging (LiDAR) sensor aboard the Carnegie Airborne Observatory Airborne Taxonomic Mapping System (CAO-AToMS) to examine the abundance and distribution of tall emergent tree crowns (ETC) relative to surrounding tree crowns (STC) across the Osa Peninsula, a geologically and topographically diverse region of Costa Rica. The abundance of ETC was clearly influenced by fine-scale topographic variation, with distribution patterns that held across a variety of geologic substrates. Specifically, the density of ETC was much greater on lower slopes and in valleys, compared to upper slopes and ridges. Furthermore, using the CAO high-fidelity imaging spectrometer, ETC had a different spectral signature than that of STC. Most notably, ETC had lower remotely sensed foliar nitrogen than STC, which was verified with an independent field survey of canopy leaf chemistry. The underlying mechanisms to explain the topographic-dependence of ETCs and linkages to canopy N are unknown, and remain an important area of research.

Legume abundance along successional and rainfall gradients in Neotropical forests

The nutrient demands of regrowing tropical forests are partly satisfied by nitrogen-fixing legume trees, but our understanding of the abundance of those species is biased towards wet tropical regions. Here we show how the abundance of Leguminosae is affected by both recovery from disturbance and large-scale rainfall gradients through a synthesis of forest inventory plots from a network of 42 Neotropical forest chronosequences. During the first three decades of natural forest regeneration, legume basal area is twice as high in dry compared with wet secondary forests. The tremendous ecological success of legumes in recently disturbed, water-limited forests is likely to be related to both their reduced leaflet size and ability to fix N2, which together enhance legume drought tolerance and water-use efficiency. Earth system models should incorporate these large-scale successional and climatic patterns of legume dominance to provide more accurate estimates of the maximum potential for natural nitrogen fixation across tropical forests.Data from 42 chronosequence sites show a geater abundance of legumes in seasonally dry forests than in wet forests, particularly during early secondary succession, probably owing to legumes’ nitrogen-fixing ability and reduced leaflet size.