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.
Following perturbation, different assemblages that originate under the same abiotic conditions initiate successional pathways that may continue to diverge or converge toward an eventual climax. ...Forest regeneration in the Central Amazon begins with alternative successional pathways associated with prior land use. In a 12‐yr study of secondary forests, initially ranging between 2 and 19 yrs after abandonment, we compared species compositions through time along two pathways, abandoned clear‐cuts dominated by Cecropia and abandoned pastures dominated by Vismia; prior results at these sites have not directly evaluated species composition. At all ages, the Chao–Jaccard similarity index of species composition was highest comparing pasture transects to each other, lowest comparing pastures transects to clear‐cut transects, and intermediate comparing clear‐cut transects to each other. Through time, clear‐cut transects became less similar to each other, as did pasture transects. Changes in similarity reflected declining dominance along both pathways, but Cecropia dominance of clear‐cut transects declined more rapidly than Vismia dominance of pasture transects. A rich association of species replaced Cecropia in clear‐cut transects, resulting in decreased similarity among them. In pasture transects one genus, Bellucia, replaced the lost Vismia, so similarity of Vismia transects was maintained despite some turnover in dominance. Overall, even with turnover of individuals and decline of the dominant pioneers, the alternative pathways exhibited strikingly different species assemblies after two decades of succession, suggesting that the effect of land use persists well beyond initial floristic composition.
Based on eight years of annual censuses in secondary forests in central Amazonia, we compared successional dynamics in areas presenting alternative states due to different land use histories. Sites ...that had been clearcut without subsequent use are dominated by the pioneer genus Cecropia, but their understory is characterized by a diverse species assemblage. In contrast, areas clearcut and then used for pasture are dominated by the genus Vismia, forming nearly monogeneric stands. We evaluated whether such patterns were the outcome of differences in community compensatory trends, leading to a dynamic system of sequential replacement of species in Cecropia stands, and to a persistent stage of succession in Vismia stands. Floristic turnover in Cecropia stands showed strong and consistent negative frequency dependence. In contrast, Vismia stands exhibited little or no frequency dependence, likely due to local competitive interactions or priority effects. In these stands, species of the genera Vismia and Bellucia remained dominant throughout the monitoring period, whereas species initially of low abundance and frequency remained so. Differences in recruitment were the major driver of these alternative states. As species colonization proceeds, we expect dominance in the Vismia stands to diminish, albeit slowly. Our approach proved to be a useful tool for comparing species turnover in systems presenting alternative states.
Background: Plant succession and community assembly following different land-use histories in the Amazon Basin are poorly understood. Aims: Changes in woody vegetation were monitored across ...chronosequences of abandoned pastures and abandoned clearcuts in order to compare their successional patterns. Methods: In chronosequences, initially 5–19 years old in abandoned clearcuts and 2–11 years old in abandoned pastures, trees (≥ 3 cm dbh) were tagged and recruitment and mortality recorded annually for 12 years. Results: Stem densities exhibited no significant trend during the first 25 years of succession regardless of land-use history. Basal area in abandoned clearcuts increased rapidly in the first decade, outpacing accumulation in abandoned pastures, although basal area on the two pathways converged at 25 years post-abandonment. Transects in abandoned pastures were much more variable in stem density and basal area than those in abandoned clearcuts, reflecting cohort growth and thinning by the dominant genus Vismia in the pastures. Species density, initially similar in the young stands, increased at a much faster rate in abandoned clearcuts than in abandoned pastures, resulting in a large divergence after 25 years. Conclusions: Succession following deforestation in the Amazon exhibits alternative pathways that correspond to prior land use – abandoned clearcuts of primary forest or clearcuts converted to pastures through prescribed burns and later abandoned. The most important divergence in the two successions was the extremely slow accumulation of species over 25 years in abandoned pastures.
Background : In central Amazonia, previous low intensity land use engenders succession dominated by Cecropia spp. which proceeds at high rates; however, at higher intensity of use succession is ...arrested and dominated by Vismia spp. over the long-term. Factors driving these two successional pathways are unknown. Aims : We aimed to elucidate seedling growth under the two alternative successional pathways. Methods : We experimentally determined the effects of successional age and photosynthetically active radiation (PAR) on relative height growth (RHG) of nine species of shade-tolerant tree seedlings in secondary forests dominated by Cecropia and Vismia , varying in age from 1–20 years. Results : In Cecropia -dominated successions, seedling RHG decreased with increasing successional age and with associated decreasing PAR. In Vismia -dominated successions, RHG was independent of successional age and PAR, and PAR did not change with successional age, being always higher than in Cecropia stands. The RHG of seedlings was lower in Vismia - than in Cecropia -dominated stands for similar PAR levels . Conclusions : Successional age and light availability affect seedlings growth differently in the two successional pathways. Unlike in Cecropia -dominated successions, in Vismia -dominated secondary forests seedling growth is limited by factors other than light. In a scenario of increasing land use intensity, constraints to seedling development in secondary forests can reduce species diversity in human-altered landscapes.
Succession in the Brazilian Amazon depends on prior land-use history. Abandoned clearcuts become dominated by Cecropia trees and exhibit species replacements characteristic of natural succession in ...forest lightgaps. In contrast, abandoned pastures are dominated by Vismia trees that inhibit natural succession for a decade or more. Here we explore how advance regeneration and limited seed dispersal may contribute to the arrested succession in Vismia-dominated stands. Vegetation surveys showed that every Vismia stem in 3–8 year old Vismia stands originated as a re-sprout. In Cecropia stands, all tree species, including Vismia, originated mostly from seeds, after deforestation and abandonment. The 100% re-sprouts of Vismia in the abandoned pastures confirms that Vismia dominance results from re-sprouting following pasture fires. Seed rain in both Vismia and Cecropia dominated stands was limited almost exclusively to second growth species already reproducing in those stands, suggesting that the bats and birds foraging there were not bringing mature forest seeds into the second growth, but simply feeding and depositing local second growth species. As dispersal was similar in both stand types, dispersal differences cannot account for the ongoing dominance of Vismia relative to the ongoing successional transitions in Cecropia stands. Overall, advance regeneration in the form of Vismia re-sprouts is much more likely to be the driver of Vismia dominated succession than differential dispersal of mature forest seeds. In order to avoid extensive forest conversion into unproductive Vismia wastelands in the Amazon Basin, forestry permits for harvesting timber should include restrictions on subsequent anthropogenic degradation, such as conversion to pasture and prescribed burning.
This study characterizes the flowering and fruiting phenology of the 13 most common pioneer tree species in early successional forests of the Central Amazon. For each species, 30 individuals, 10 each ...in three secondary forests, were monitored monthly for four years at the Biological Dynamics of Forest Fragments Project, north of Manaus. Five species showed nearly continuous flowering and fruiting throughout the study, indicating that resources were available to pollinators and dispersers on a regular basis. The other eight species showed stronger seasonality in reproduction, seven of them annually, and one supra-annually. Overall, flowering was concentrated in the transition from the dry to the rainy season and fruiting was concentrated in the rainy season. There was no relationship between reproductive phenology and tree pollinator type or dispersal mode. Reproductive phenology was remarkably consistent year to year. The pioneer community showed a variety of phenological patterns but as a whole tended to be characterized by annual flowering and fruiting, either continuously or seasonally, thereby fitting generalizations of pioneer species relative to mature forest species.
Background: The tradeoff between seed mass and seed number per plant is widely established for different taxa, guilds, and communities. Relative to primary forest species, pioneer species generally ...produce large numbers of small seeds. Aims: We tested if the relationship between seed mass and seed number was connected to the fruit variables – namely, fruit mass and fruit number per tree – in order to evaluate tradeoffs in seed packaging. Methods: Seed mass and seed number per tree as well as fruit mass and fruit number per tree were measured for 12 pioneer species common to secondary forests in the central Amazon. Results: Seed mass, seed number, fruit mass, and fruit number varied by several orders of magnitude among species. Seed number was explained only partially by seed mass alone (R ² = 0.55), but nearly completely by the combination of seed mass, fruit mass and fruit number (R ² = 0.94). The number of seeds per fruit was positively correlated with fruit mass and total seed number per tree and negatively with seed mass and fruit number. Seedling and adult abundances were most dependent on fruit number and fruit mass, not seed number and seed mass. Conclusions: Biomass tradeoffs between seed mass and seed number are partially dependent on seed packaging, specifically seeds per fruit, fruit mass and fruit number per tree for pioneer trees in the central Amazon.
1. Successional gradients are ubiquitous in nature, yet few studies have systematically examined the evolutionary origins of taxa that specialize at different successional stages. Here we quantify ...successional habitat specialization in Neotropical forest trees and evaluate its evolutionary lability along a precipitation gradient. Theoretically, successional habitat specialization should be more evolutionarily conserved in wet forests than in dry forests due to more extreme microenvironmental differentiation between early and late-successional stages in wet forest. 2. We applied a robust multinomial classification model to samples of primary and secondary forest trees from 14 Neotropical lowland forest sites spanning a precipitation gradient from 788 to 4000 mm annual rainfall, identifying species that are old-growth specialists and secondary forest specialists in each site. We constructed phylogenies for the classified taxa at each site and for the entire set of classified taxa and tested whether successional habitat specialization is phylogenetically conserved. We further investigated differences in the functional traits of species specializing in secondary vs. old-growth forest along the precipitation gradient, expecting different trait associations with secondary forest specialists in wet vs. dry forests since water availability is more limiting in dry forests and light availability more limiting in wet forests. 3. Successional habitat specialization is non-randomly distributed in the angiosperm phylogeny, with a tendency towards phylogenetic conservatism overall and a trend towards stronger conservatism in wet forests than in dry forests. However, the specialists come from all the major branches of the angiosperm phylogeny, and very few functional traits showed any consistent relationships with successional habitat specialization in either wet or dry forests. 4. Synthesis. The niche conservatism evident in the habitat specialization of Neotropical trees suggests a role for radiation into different successional habitats in the evolution of species-rich genera, though the diversity of functional traits that lead to success in different successional habitats complicates analyses at the community scale. Examining the distribution of particular lineages with respect to successional gradients may provide more insight into the role of successional habitat specialization in the evolution of species-rich taxa.
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