Jorge A. Meave

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.

Floristic composition and structure of vegetation under isolated trees in neotropical pastures

. Large isolated trees are a common feature of the agricultural landscape in humid tropical regions originally covered by rain forest. These isolated trees are primarily used as a source of shade for cattle and people. 13 pastures (totalling ca. 80 ha) currently used as cattle pasture were studied. In them, we registered 265 isolated trees belonging to 57 species. 50 trees of the most frequent species (Ficus spp. n = 30 and Nectandra ambigens n = 20) were selected to examine the influence of isolated trees on floristic composition and vegetation structure in the pastures. At each tree, three 4–m2 quadrats were sampled: under the canopy, directly under the canopy perimeter, and beyond the canopy in the open pasture. Under-canopy vegetation was structurally and floristically different from the other two sampling sites. Mean species richness per quadrat was significantly higher under the canopy (17.8 ± 4.3 SD) than at the canopy perimeter (11.2 ± 3.4) and in the open pasture (10.6 ± 3.6) sites. Stem density was higher at under-canopy sites, where greater proportions of endozoochorous and rain-forest species were found. Isolated trees function as nursery plants for rain-forest species by facilitating the establishment of zoochorous species whose seeds are deposited under the tree canopies by frugivorous birds or bats. Our results imply that isolated trees may play a major role in seed dispersal and establishment of native species, which is of consequence for the preservation of rain-forest species in these fragmented landscapes.

Successional dynamics in Neotropical forests are as uncertain as they are predictable

Significance Although forest succession has been approached as a predictable process, successional trajectories vary widely, even among nearby stands with similar environmental conditions and disturbance histories. We quantified predictability and uncertainty during tropical forest succession using dynamical models describing the interactions among stem density, basal area, and species density over time. We showed that the trajectories of these forest attributes were poorly predicted by stand age and varied significantly within and among sites. Our models reproduced the general successional trends observed, but high levels of noise were needed to increase model predictability. These levels of uncertainty call into question the premise that successional processes are consistent over space and time, and challenge the way ecologists view tropical forest regeneration. Although forest succession has traditionally been approached as a deterministic process, successional trajectories of vegetation change vary widely, even among nearby stands with similar environmental conditions and disturbance histories. Here, we provide the first attempt, to our knowledge, to quantify predictability and uncertainty during succession based on the most extensive long-term datasets ever assembled for Neotropical forests. We develop a novel approach that integrates deterministic and stochastic components into different candidate models describing the dynamical interactions among three widely used and interrelated forest attributes—stem density, basal area, and species density. Within each of the seven study sites, successional trajectories were highly idiosyncratic, even when controlling for prior land use, environment, and initial conditions in these attributes. Plot factors were far more important than stand age in explaining successional trajectories. For each site, the best-fit model was able to capture the complete set of time series in certain attributes only when both the deterministic and stochastic components were set to similar magnitudes. Surprisingly, predictability of stem density, basal area, and species density did not show consistent trends across attributes, study sites, or land use history, and was independent of plot size and time series length. The model developed here represents the best approach, to date, for characterizing autogenic successional dynamics and demonstrates the low predictability of successional trajectories. These high levels of uncertainty suggest that the impacts of allogenic factors on rates of change during tropical forest succession are far more pervasive than previously thought, challenging the way ecologists view and investigate forest regeneration.

Successional changes in functional composition contrast for dry and wet tropical forest

We tested whether and how functional composition changes with succession in dry deciduous and wet evergreen forests of Mexico. We hypothesized that compositional changes during succession in dry forest were mainly determined by increasing water availability leading to community functional changes from conservative to acquisitive strategies, and in wet forest by decreasing light availability leading to changes from acquisitive to conservative strategies. Research was carried out in 15 dry secondary forest plots (5-63 years after abandonment) and 17 wet secondary forest plots (< 1-25 years after abandonment). Community-level functional traits were represented by community-weighted means based on 11 functional traits measured on 132 species. Successional changes in functional composition are more marked in dry forest than in wet forest and largely characterized by different traits. During dry forest succession, conservative traits related to drought tolerance and drought avoidance decreased, as predicted. Unexpectedly acquisitive leaf traits also decreased, whereas seed size and dependence on biotic dispersal increased. In wet forest succession, functional composition changed from acquisitive to conservative leaf traits, suggesting light availability as the main driver of changes. Distinct suites of traits shape functional composition changes in dry and wet forest succession, responding to different environmental filters.

Multiple successional pathways in human-modified tropical landscapes : New insights from forest succession, forest fragmentation and landscape ecology research

Old‐growth tropical forests are being extensively deforested and fragmented worldwide. Yet forest recovery through succession has led to an expansion of secondary forests in human‐modified tropical landscapes (HMTLs). Secondary forests thus emerge as a potential repository for tropical biodiversity, and also as a source of essential ecosystem functions and services in HMTLs. Such critical roles are controversial, however, as they depend on successional, landscape and socio‐economic dynamics, which can vary widely within and across landscapes and regions. Understanding the main drivers of successional pathways of disturbed tropical forests is critically needed for improving management, conservation, and restoration strategies. Here, we combine emerging knowledge from tropical forest succession, forest fragmentation and landscape ecology research to identify the main driving forces shaping successional pathways at different spatial scales. We also explore causal connections between land‐use dynamics and the level of predictability of successional pathways, and examine potential implications of such connections to determine the importance of secondary forests for biodiversity conservation in HMTLs. We show that secondary succession (SS) in tropical landscapes is a multifactorial phenomenon affected by a myriad of forces operating at multiple spatio‐temporal scales. SS is relatively fast and more predictable in recently modified landscapes and where well‐preserved biodiversity‐rich native forests are still present in the landscape. Yet the increasing variation in landscape spatial configuration and matrix heterogeneity in landscapes with intermediate levels of disturbance increases the uncertainty of successional pathways. In landscapes that have suffered extensive and intensive human disturbances, however, succession can be slow or arrested, with impoverished assemblages and reduced potential to deliver ecosystem functions and services. We conclude that: (i) succession must be examined using more comprehensive explanatory models, providing information about the forces affecting not only the presence but also the persistence of species and ecological groups, particularly of those taxa expected to be extirpated from HMTLs; (ii) SS research should integrate new aspects from forest fragmentation and landscape ecology research to address accurately the potential of secondary forests to serve as biodiversity repositories; and (iii) secondary forest stands, as a dynamic component of HMTLs, must be incorporated as key elements of conservation planning; i.e. secondary forest stands must be actively managed (e.g. using assisted forest restoration) according to conservation goals at broad spatial scales.

Environmental changes during secondary succession in a tropical dry forest in Mexico

Vegetation and environment change mutually during secondary succession, yet the idiosyncrasies of the vegetationeffectontheunderstoreyenvironmentarepoorlyunderstood.Totestwhetherthesuccessionalunderstorey environment changes predictably and is shaped by the structure and seasonality of tropical dry forests, we estimated basal area and vegetation cover, and measured understorey temperature, light and moisture conditions, in 17 plots forming a 60-y chronosequence and a mature forest. Light and air and soil temperature decreased with time (75−15% of open-sky radiation, 31.7−29.3 ◦ C, and +2.5 ◦ Ct o−0.5 ◦ C relative to ambient, respectively), whereas relativehumidityincreased(67−74%).Soilwateravailabilityincreasedwithearly-successionaldevelopment(−45to −1kPa)butdecreasedafterwards(to −18kPa).ThefirstaxisofaPCAoftherainy-seasonenvironmentexplained60% ofthevariationandwasstronglyrelatedtoairtemperatureandrelativehumidity.Duringtropicaldry-forestsuccession, such factors may be more important than light, the reduction in which is not extreme compared with taller and more verticallystratifiedwetforests.Seasonalitysignificantlyaffectedthesuccessionalenvironmentalgradients,whichwere marked mainly during the wet season. Environmental heterogeneity was higher in the wet than in the dry season, and larger for resources (light and water) than for conditions (temperature and humidity). The wet-season increase inenvironmentalheterogeneitypotentiallycreatesdifferentialgrowingscenarios;theenvironmentalharshnessofthe dry season would mostly challenge seedling survival.

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.

Maintenance of rain forest diversity in riparian forests of tropical savannas: implications for species conservation during Pleistocene drought

Structural and floristic attributes of riparian forest patches in a Neotropical savanna were studied in order to evaluate their potential to maintain rain forest plant diversity during periods of drought. Tree density was significantly greater, and basal area significantly lower, than pantropical means for these variables, thus these riparian forests were characterized as low biomass, many-stemmed communities. A total of 292 species ? 0.5 m high were found in an aggregate vegetation sample of 1.6 ha. Most species were typical rain forest taxa, indicating that this flora is not specialized to the riparian environment. Isolation of these patches was found to have some negative effects on species composition, since dioecious and mammal dispersed species were more poorly represented in riparian than in continuous forests. While this riparian system is not floristically depauperate, species densi- ties in individual plots indicate that its richness is comparable to that of continuous forests in Central America. These results suggest that tropical riparian forest fragments have the poten- tial, albeit limited, to maintain large numbers of rain forest species and, thus, that analogous forest fragments may have played an important role as safe sites for tropical rain forest floras during periods of drought in the Pleistocene.

Tropical montane cloud forests: current threats and opportunities for their conservation and sustainable management in Mexico.

Tropical montane cloud forests (TMCF) are among the most threatened ecosystems globally in spite of their high strategic value for sustainable development due to the key role played by these forests in hydrological cycle maintenance and as reservoirs of endemic biodiversity. Resources for effective conservation and management programs are rarely sufficient, and criteria must be applied to prioritize TMCF for conservation action. This paper reports a priority analysis of the 13 main regions of TMCF distribution in Mexico, based on four criteria: (1) forest quality, (2) threats to forest permanence, (3) threats to forest integrity, and (4) opportunities for conservation. Due to the diverse socio-environmental conditions of the local communities living in Mexican TMCF regions, their associated social characteristics were also evaluated to provide a background for the planning of conservation actions. A set of indicators was defined for the measurement of each criterion. To assign priority values for subregions within each main region, an international team of 40 participants evaluated all the indicators using multicriteria decision-making analysis. This procedure enabled the identification of 15 subregions of critical priority, 17 of high priority, and 10 of medium priority; three more were not analysed due to lack of information. The evaluation revealed a number of subjects that had hitherto been undetected and that may prove useful for prioritization efforts in other regions where TMCF is similarly documented and faces equally severe threats. Based on this analysis, key recommendations are outlined to advance conservation objectives in those TMCF areas that are subjected to high pressure on forest resources.

Effect of forest fragmentation on the woody flora of the highlands of Chiapas, Mexico

This study was conducted in the Chiapas Highlands, a tropical mountain region where traditional agricultural practices have resulted in a mosaic landscape of forest fragments embedded in a matrix of secondary vegetation and crop fields. The question addressed was how may woody species richness be affected by forest fragment attributes derived from traditional land-use patterns. Species inventories of total woody species, canopy and understorey trees, and shrubs were obtained in 22 forest fragments (≥5 ha). Multiple regression analyses were applied to examine the effects of size, matrix, isolation and shape of the forest fragments on richness of these species guilds. Fragment size was correlated with shape (r = 0.75) and isolation (r = −0.69), and isolation was correlated with shape (r = −0.75). Total species richness, and number of shrubs and understorey trees in fragments were related to isolation; moreover, additive effects of fragment shape were found for shrubs. The number of canopy species was not related to any fragment variable. Matrix did not help to explain species richness, possibly due to the landscape structure created by the traditional land-use patterns. In addition to size and isolation, we point out the need of considering shape and matrix as additional fragmentation attributes, along with social and economic factors, if we are ever going to be successful in our management and conservation actions.