Saara J. DeWalt

Biomass resilience of Neotropical secondary forests

Land-use change occurs nowhere more rapidly than in the tropics, where the imbalance between deforestation and forest regrowth has large consequences for the global carbon cycle. However, considerable uncertainty remains about the rate of biomass recovery in secondary forests, and how these rates are influenced by climate, landscape, and prior land use. Here we analyse aboveground biomass recovery during secondary succession in 45 forest sites and about 1,500 forest plots covering the major environmental gradients in the Neotropics. The studied secondary forests are highly productive and resilient. Aboveground biomass recovery after 20 years was on average 122 megagrams per hectare (Mg ha−1), corresponding to a net carbon uptake of 3.05 Mg C ha−1 yr−1, 11 times the uptake rate of old-growth forests. Aboveground biomass stocks took a median time of 66 years to recover to 90% of old-growth values. Aboveground biomass recovery after 20 years varied 11.3-fold (from 20 to 225 Mg ha−1) across sites, and this recovery increased with water availability (higher local rainfall and lower climatic water deficit). We present a biomass recovery map of Latin America, which illustrates geographical and climatic variation in carbon sequestration potential during forest regrowth. The map will support policies to minimize forest loss in areas where biomass resilience is naturally low (such as seasonally dry forest regions) and promote forest regeneration and restoration in humid tropical lowland areas with high biomass resilience.

Rapid adaptation of insect herbivores to an invasive plant

Introduced plant success often is attributed to release from natural enemies in their new ranges. However, herbivores may accumulate over time and reduce invasiveness but evidence for this process to date is weak. We report here that enemy release is indeed limited to the early stages of introduction of the Chinese tallow tree (Sapium sebiferum). In bioassays and gardens along a geographical gradient of time since tallow tree introduction, herbivory was highest and tree performance was poorest where tallow tree has been present longer (i.e. introduced earlier). Additionally, Asian ecotypes (grown from seeds collected in Asia) had lower survival than North American ecotypes (seeds collected in North America), which is consistent with genetic responses to low herbivory in the introduced range (EICA Hypothesis). Release from insect herbivores appears to contribute to early success of the tallow tree, but accumulation of insect herbivores has apparently reduced this benefit over time.

Carbon sequestration potential of second-growth forest regeneration in the Latin American tropics

Models reveal the high carbon mitigation potential of tropical forest regeneration. Regrowth of tropical secondary forests following complete or nearly complete removal of forest vegetation actively stores carbon in aboveground biomass, partially counterbalancing carbon emissions from deforestation, forest degradation, burning of fossil fuels, and other anthropogenic sources. We estimate the age and spatial extent of lowland second-growth forests in the Latin American tropics and model their potential aboveground carbon accumulation over four decades. Our model shows that, in 2008, second-growth forests (1 to 60 years old) covered 2.4 million km2 of land (28.1% of the total study area). Over 40 years, these lands can potentially accumulate a total aboveground carbon stock of 8.48 Pg C (petagrams of carbon) in aboveground biomass via low-cost natural regeneration or assisted regeneration, corresponding to a total CO2 sequestration of 31.09 Pg CO2. This total is equivalent to carbon emissions from fossil fuel use and industrial processes in all of Latin America and the Caribbean from 1993 to 2014. Ten countries account for 95% of this carbon storage potential, led by Brazil, Colombia, Mexico, and Venezuela. We model future land-use scenarios to guide national carbon mitigation policies. Permitting natural regeneration on 40% of lowland pastures potentially stores an additional 2.0 Pg C over 40 years. Our study provides information and maps to guide national-level forest-based carbon mitigation plans on the basis of estimated rates of natural regeneration and pasture abandonment. Coupled with avoided deforestation and sustainable forest management, natural regeneration of second-growth forests provides a low-cost mechanism that yields a high carbon sequestration potential with multiple benefits for biodiversity and ecosystem services.

Geographic distribution of genetic variation among native and introduced populations of Chinese tallow tree, Triadica sebifera (Euphorbiaceae)

PREMISE OF THE STUDY Invasive plants often display genetically determined variation in patterns of growth and resource allocation between native and introduced genotypes, as well as among genotypes within different regions of the introduced range. We examined patterns of genetic variation within and among native and introduced populations of the tetraploid Chinese tallow tree (Triadica sebifera, Euphorbiaceae) to determine whether nonselective evolutionary processes or the introduction history could contribute to previously observed phenotypic differences between native and introduced populations as well as among introduced populations. METHODS We used six microsatellite markers to study 12 native populations in China, 51 introduced populations in the southeastern USA, and one introduced population in Australia. KEY RESULTS Genetic diversity was greater within and among native populations than introduced populations. Within the southeastern USA, populations in Georgia and South Carolina differed substantially in their genetic composition and had greater genetic diversity than the rest of the southeastern USA. Greater genetic similarity between some populations in the native range and introduced range indicate a common provenance for Georgia and South Carolina populations that could have come from any of several western or southern Chinese populations and a different provenance for other southeastern USA populations and the Australian population, which were most similar to more northeastern Chinese populations. CONCLUSIONS Differences among introduced populations in potentially adaptive traits (e.g., herbivore tolerance, herbivore resistance, growth rates) may result in part from the introduction history, in particular from differences present among source populations in the native range.

Differences in cold hardiness between introduced populations of an invasive tree

The potential for populations of exotic invasive plants to differ in their response to stressful environmental conditions is an underexplored issue in determining invasive species’ range limits. Introduced genotypes may differ in response to climatic, edaphic, or biotic factors within their introduced range leading to differences in potential ranges among populations. We examined differences in cold hardiness (resistance and tolerance to winter conditions and freeze events) among Chinese tallow tree (Triadica sebifera (L.) Small) seeds and seedlings from two genetically distinct populations in the northernmost portion of its introduced U.S. range (North Carolina and South Carolina). Seed germination from these two sources was compared between fall plantings (mimicking natural dispersal timing) and spring plantings (occurring post-frost) as well as among areas within and inland of Chinese tallow’s core coastal distribution in South Carolina. Overwinter seedling survival and damage were also assessed among seedlings planted in the piedmont of South Carolina and following artificial freeze events in the lab. Seeds and seedlings from South Carolina sources showed greater reductions in germination success by inland winters, greater winter damage in field plantings in the piedmont, and lower survival after prolonged freezes than those from North Carolina. These results indicate that differences in cold hardiness exist among introduced populations of Chinese tallow and suggest that genotypes from North Carolina possess greater potential for expansion into areas with more severe winters. Differences among introduced populations should be considered when evaluating the potential range expansion of Chinese tallow and other invasive species.

An experimental test of the EICA hypothesis in multiple ranges: invasive populations outperform those from the native range independent of insect herbivore suppression

This paper reports the results of a multi-year manipulative common garden experiment that investigates the roles of variation in plant defense and growth in the competitive ability of an invasive tree in the native range and two introduced ranges. We found that plants from invasive populations had more rapid aboveground growth rates that contributed to their success in both introduced and native ranges independent of aboveground herbivory. However, strong variation among sites indicated that plants from invasive populations only have a strong advantage in a subset of sites in the introduced range.

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.

Fertilization influences the nutrient acquisition strategy of a nomadic vine in a lowland tropical forest understory

AimsTropical tree and lianas in the understory are limited by soil nutrients despite growing in extremely low light. It is not known if nomadic vines are also limited by nutrients in low light conditions.MethodsWe measured differences in root architecture and mycorrhizal colonization, and leaf nutrients of a nomadic vine, Philodendron fragrantissimum (Araceae), in nitrogen (N) and phosphorus (P) fertilization plots in a lowland tropical moist forest in central Panama to measure potential nutrient limitation.ResultsRelative to plants in control plots, leaf P concentration was 54% higher and leaf N concentration was 10% higher for plants in the P- and N-addition treatments, respectively. The N:P of leaves suggested P-limitation in the N-addition treatment and the control but not in the P-addition treatment. Root branching was highest in the P-addition treatment, and P-addition reduced mycorrhizal colonization.ConclusionsThe large effect of P fertilization suggests that, like many tropical plants, P. fragrantissimum has the potential to be P-limited. Although further study is needed, we suggest that nomadic vines be added to the growth forms that respond to nutrient addition in the forest understory and conclude that nutrient-limitation seems like the rule rather than the exception in the light-limited understory.

Seed Removal by an Introduced Scatter-Hoarder on a Caribbean Island

Abstract The introduction of non-native seed dispersers has the potential to significantly alter distributions and relative abundances of native plants. Although effects of introduced seed predators have been documented, little is known about how introduced dispersers influence seed movement patterns. We investigated seed removal of seven rainforest species on the island of Dominica in the Lesser Antilles by the entire seed-remover community and specifically by the Red-rumped Agouti, Dasyprocta leporina, a scatter-hoarding rodent introduced to the island approximately 2500 years ago. We recorded removal rates in three regions of Dominica from 168 experimentally placed seed groups containing a total of 1356 seeds. Seed groups were either accessible to the entire seed-remover community or placed within exclosures designed to exclude agoutis. Within 13 days, 47 percent and 28 percent of seeds had been removed from control groups and agouti exclosure groups, respectively, leading to 19 percent of seed removal being attributed to agoutis. Species with smaller seeds were preferentially taken by seed removers other than agoutis, whereas agoutis were responsible for the majority of the removal of larger-seeded species. Seed removal was greater in areas with higher regional conspecific adult densities regardless of treatment, but agoutis had a greater impact relative to other seed removers on the seed removal of the studys rarest species. The results of this study highlight the potential impacts that introduced dispersers may have on native plant communities and call for further study of disperser introductions worldwide.