The Potential of Tree Rings for the Study of Forest Succession in Southern Mexico

Studies of tropical secondary forest succession face strong limitations due to the slow pace of succession and the time-consuming task of monitoring processes. The occurrence of tree rings in secondary forest trees may help expand our knowledge on succession in these systems and may be useful for fallow dating in chronosequence studies. We examine here the potential of tree rings to study forest succession by sampling 70 species along chronosequences of dry and wet forests in southern Mexico. Based on wood anatomical features, we estimated that about 37 percent of the species presented distinct growth rings useful for ring studies. Overall, maximum number of rings matched well the interview-based fallow ages but, at some sites, trees had consistently higher numbers of rings, probably due to errors in fallow ages derived from interviews. Best fallow age estimations were obtained by examining rings in both pioneer and nonpioneer species. Reconstruction of species' establishment dates revealed that pioneer and nonpioneer species establish early during succession, and that species of both groups continue to recruit after many years. Our study clearly shows that tree ring analysis is a promising tool for studies on secondary forest succession in the tropics.     

Dietary and environmental reconstruction with stable isotope analyses of herbivore tooth enamel from the Miocene locality of Fort Ternan, Kenya

Tooth enamel of nine Middle Miocene mammalian herbivores from Fort Ternan, Kenya, was analyzed for δ¹³C and δ¹⁸O. The δ¹⁸O values of the tooth enamel compared with pedogenic and diagenetic carbonate confirm the use of stable isotope analysis of fossil tooth enamel as a paleoenvironmental indicator. Furthermore, the δ¹⁸O of tooth enamel indicates differences in water sources between some of the mammals. The δ¹³C values of tooth enamel ranged from −8·6–−13·0‰ which is compatible with a pure C₃diet, though the possibility of a small C₄fraction in the diet of a few of the specimens sampled is not precluded. The carbon isotopic data do not support environmental reconstructions of a Serengeti-typed wooded grassland with a significant proportion of C₄grasses. This study does not preclude the presence of C₃grasses at Fort Ternan; it is possible that C₃grasses could have had a wider geographic range if atmospheric CO₂levels were higher than the present values.     

Extinct large colobines from the Plio-Pleistocene of Africa

Three genera and six species of extinct African Plio-Pleistocene Colobinae are discussed. One new genus, Rhinocolobus and three new species, R. turkanaensis, Cercopithecoides kimeui and Paracolobus mutiwa are named. These colobines show a diversity in postcranial and dental morphology not seen among extant species. Rhinocolobus was most similar to extant Colobus in postcranial morphology and had similar high-cusped shearing teeth. Cercopithecoides shows a number of postcranial skeletal features typical of terrestrial cercopithecid species and has lower cusped teeth. Paracolobus, while generally more similar to Rhinocolobus than to Cercopithecoides, is intermediate in some features of its postcranial morphology. The distribution of the various taxa among East and South African sites with different palaeoenvironments is generally consistent with the morphological interpretation. With the exception of Cercopithecoides kimeui, which persisted a little longer, these large colobines disappear from the fossil record about 1.8 million years ago. Their disappearance coincides with an interval of increasing aridity documented at Olduvai Gorge, the Omo Valley, and East Turkana.     

Spatial structure of the abiotic environment and its association with sapling community structure and dynamics in a cloud forest

Analyzing the relationship between the spatial structures of environmental variables and of the associated seedling and sapling communities is crucial to understanding the regeneration processes in forest communities. The degree of spatial structuring (i.e., spatial autocorrelation) of environmental and sapling community variables in the cloud forest of Teipan, S Mexico, were analyzed at a 1-ha scale using geostatistical analysis; after fitting semivariogram models for each set of variables, the association between the two sets was examined through cross-variograms. Kriging maps of the sapling community variables (density, cover, species richness, and mortality and recruitment rates) were obtained through conditional simulation method. Canopy openness, total solar radiation, litter depth, soil temperature and soil moisture were spatially structured, as were sapling density, species richness and sapling mortality rate. Mean range in semivariograms for environmental and sapling community variables were 13.14 ± 3.67 and 12.68 ± 5.71 m (±SE), respectively. The spatial structure of litter depth was negatively associated with the spatial structures of sapling density, species richness, and sapling community cover; in turn, the spatial structure of soil moisture was positively associated with the spatial structure of recruitment rate. These associations of the spatial structures of abiotic and sapling community variables suggest that the regeneration processes in this cloud forest is driven by the existence of different microsites, largely characterized by litter depth variations, across which saplings of tree species encounter a range of opportunities for successful establishment and survival.     

Elevational Variation of Leaf Traits in Montane Rain Forest Tree Species at La Chinantla, Southern México1

Variation in leaf traits of dominant tree species in six montane rain forest communities was analyzed along an elevational gradient ranging from 1220 to 2560 m within a single basin at La Chinantla, Oaxaca, México. Three groups of characters were used: morphological (leaf shape, margin, blade configuration, and phyllotaxy), morphometric (leaf area, leaf mass per area, stomatal density, and blade length/width ratio), and anatomical (thicknesses of blade, palisade [PP], and spongy [SP] parenchymae, PP/SP ratio, and epidermis and cuticle thicknesses). The variation of morphological characteristics was only evident at the highest elevations; in contrast, thickness of leaf blade, PP, SP, as well as leaf mass per area clearly increased along the gradient, whereas leaf area was the only variable that significantly decreased with elevation. Thicknesses of epidermis and of the two cuticles were not significantly correlated with elevation. A classification analysis based on a leaf trait matrix led to the distinction between low and high elevation communities, with an approximate limit between them at ca 2300 to 2400 m. The results are discussed in light of environmental changes occurring along elevational gradients. Leaf characteristics of montane rain forest plants offer important insights about the complex roles of abiotic factors operating in these environments and supplement the traditional physiognomic classification schemes for these communities.     

Successional theories

Succession is a fundamental concept in ecology because it indicates how species populations, communities, and ecosystems change over time on new substrate or after a disturbance. A mechanistic understanding of succession is needed to predict how ecosystems will respond to land-use change and to design effective ecosystem restoration strategies. Yet, despite a century of conceptual advances a comprehensive successional theory is lacking. Here we provide an overview of 19 successional theories (‘models’) and their key points, group them based on conceptual similarity, explain conceptual development in successional ideas and provide suggestions how to move forward. Four groups of models can be recognised. The first group (patch & plants) focuses on plants at the patch level and consists of three subgroups that originated in the early 20th century. One subgroup focuses on the processes (dispersal, establishment, and performance) that operate sequentially during succession. Another subgroup emphasises individualistic species responses during succession, and how this is driven by species traits. A last subgroup focuses on how vegetation structure and underlying demographic processes change during succession. A second group of models (ecosystems) provides a more holistic view of succession by considering the ecosystem, its biota, interactions, diversity, and ecosystem structure and processes. The third group (landscape) considers a larger spatial scale and includes the effect of the surrounding landscape matrix on succession as the distance to neighbouring vegetation patches determines the potential for seed dispersal, and the quality of the neighbouring patches determines the abundance and composition of seed sources and biotic dispersal vectors. A fourth group (socio-ecological systems) includes the human component by focusing on socio-ecological systems where management practices have long-lasting legacies on successional pathways and where regrowing vegetations deliver a range of ecosystem services to local and global stakeholders. The four groups of models differ in spatial scale (patch, landscape) or organisational level (plant species, ecosystem, socio-ecological system), increase in scale and scope, and reflect the increasingly broader perspective on succession over time. They coincide approximately with four periods that reflect the prevailing view of succession of that time, although all views still coexist. The four successional views are: succession of plants (from 1910 onwards) where succession was seen through the lens of species replacement; succession of communities and ecosystems (from 1965 onwards) when there was a more holistic view of succession; succession in landscapes (from 2000 onwards) when it was realised that the structure and composition of landscapes strongly impact successional pathways, and increased remote-sensing technology allowed for a better quantification of the landscape context; and succession with people (from 2015 onwards) when it was realised that people and societal drivers have strong effects on successional pathways, that ecosystem processes and services are important for human well-being, and that restoration is most successful when it is done by and for local people. Our review suggests that the hierarchical successional framework of Pickett is the best starting point to move forward as this framework already includes several factors, and because it is flexible, enabling application to different systems. The framework focuses mainly on species replacement and could be improved by focusing on succession occurring at different hierarchical scales (population, community, ecosystem, socio-ecological system), and by integrating it with more recent developments and other successional models: by considering different spatial scales (landscape, region), temporal scales (ecosystem processes occurring over centuries, and evolution), and by taking the effects of the surrounding landscape (landscape integrity and composition, the disperser community) and societal factors (previous and current land-use intensity) into account. Such a new, comprehensive framework could be tested using a combination of empirical research, experiments, process-based modelling and novel tools. Applying the framework to seres across broadscale environmental and disturbance gradients allows a better insight into what successional processes matter and under what conditions.     

Combining geostatistical models and remotely sensed data to improve tropical tree richness mapping

Information on the spatial distribution and composition of biological communities is essential in designing effective strategies for biodiversity conservation and management. Reliable maps of species richness across the landscape can be useful tools for these purposes. Acquiring such information through traditional survey techniques is costly and logistically difficult. The kriging interpolation method has been widely used as an alternative to predict spatial distributions of species richness, as long as the data are spatially dependent. However, even when this requirement is met, researchers often have few sampled sites in relation to the area to be mapped. Remote sensing provides an inexpensive means to derive complete spatial coverage for large areas and can be extremely useful for estimating biodiversity. The aim of this study was to combine remotely sensed data with kriging estimates (hybrid procedures) to evaluate the possibility of improving the accuracy of tree species richness maps. We did this through the comparison of the predictive performance of three hybrid geostatistical procedures, based on tree species density recorded in 141 sampling quadrats: co-kriging (COK), kriging with external drift (KED), and regression kriging (RK). Reflectance values of spectral bands, computed NDVI and texture measurements of Landsat 7 TM imagery were used as ancillary variables in all methods. The R² values of the models increased from 0.35 for ordinary kriging to 0.41 for COK, and from 0.39 for simple regression estimates to 0.52 and 0.53 when using simple KED and RK, respectively. The R² values of the models also increased from 0.60 for multiple regression estimates to 0.62 and 0.66 when using multiple KED and RK, respectively. Overall, our results demonstrate that these procedures are capable of greatly improving estimation accuracy, with multivariate RK being clearly superior, because it produces the most accurate predictions, and because of its flexibility in modeling multivariate relationships between tree richness and remotely sensed data. We conclude that this is a valuable tool for guiding future efforts aimed at conservation and management of highly diverse tropical forests.     

Successional dynamics of the bee community in a tropical dry forest: Insights from taxonomy and functional ecology

Despite the recent rapid growth of tropical dry forest succession ecology, most studies on this topic have focused on plant community attribute recovery, whereas animal community successional dynamics has been largely overlooked, and the few existing studies have used taxonomic approaches. Here, we analyze the successional changes in the bee community in a Mexican tropical dry forest, by integrating taxonomic (species, genus, and family diversity) and functional (sociability, nesting strategy, and body size) information for bees. Over one year, in a successional chronosequence (2–67 years after abandonment) we collected 469 individual bees, representing five families, 36 genera, and 69 species. Linear modeling showed decreases in taxonomic diversity with succession, more strongly so for species. Bee species turnover along succession ranged from moderate to high, decreasing slightly at intermediate stages. An RLQ analysis (ordination method that allows relating environmental variables with functional attributes) revealed clear relations between bee functional traits and the plant community. RLQ axis 1 was positively related to vegetation structural and diversity variables, and to eusociality, while solitary, parasociality, and ground nesting was negatively associated with it. Early successional fallows attract mostly solitary and parasocial bees; older fallows tend to attract eusocial bees with aerial nesting. The continuous taxonomic turnover observed by us and the functional analysis suggest that the disappearance of old fallows from agricultural landscapes would likely result in significant reductions and even local extinctions of particular bee guilds. Considering the low viability of preserving large mature tropical dry forest tracts, the conservation of older successional stands emerges as a crucial component of landscape management.Abstract in Spanish is available with online material.     

Demographic Drivers of Aboveground Biomass Dynamics During Secondary Succession in Neotropical Dry and Wet Forests

The magnitude of the carbon sink in second-growth forests is expected to vary with successional biomass dynamics resulting from tree growth, recruitment, and mortality, and with the effects of climate on these dynamics. We compare aboveground biomass dynamics of dry and wet Neotropical forests, based on monitoring data gathered over 3–16 years in forests covering the first 25 years of succession. We estimated standing biomass, annual biomass change, and contributions of tree growth, recruitment, and mortality. We also evaluated tree species’ contributions to biomass dynamics. Absolute rates of biomass change were lower in dry forests, 2.3 and 1.9 Mg ha^?1 y^?1, after 5–15 and 15–25 years after abandonment, respectively, than in wet forests, with 4.7 and 6.1 Mg ha^?1 y^?1, in the same age classes. Biomass change was largely driven by tree growth, accounting for at least 48% of biomass change across forest types and age classes. Mortality also contributed strongly to biomass change in wet forests of 5–15 years, whereas its contribution became important later in succession in dry forests. Biomass dynamics tended to be dominated by fewer species in early-successional dry than wet forests, but dominance was strong in both forest types. Overall, our results indicate that biomass dynamics during succession are faster in Neotropical wet than dry forests, with high tree mortality earlier in succession in the wet forests. Long-term monitoring of second-growth tropical forest plots is crucial for improving estimates of annual biomass change, and for enhancing understanding of the underlying mechanisms and demographic drivers.