Findings Phase I
Successional changes in composition of tree functional groups show similar trends within and across secondary forest plots in Costa Rica, with tree stature and growth rates capturing much of the functional group variation that drives successional dynamics (Chazdon et al. 2010). Fast-growing midstory and subcanopy trees reach peak densities early in succession, within and across plots. Density of fast-growing canopy and emergent trees increases during the first 20 yr of succession, whereas their basal area continues to increase beyond 40 yr. Over time, slow-growing subcanopy and canopy tree species increase steadily in density and basal area within and across successional plots (Chazdon et al. 2010).
In Mexican sites, stand structure and environmental conditions change directionally with fallow age, along with directional variation in plant functional traits (Lohbeck 2010, Lohbeck et al 2012). Trait presence is predominantly explained by the amount of light penetrating to the understory (Lohbeck 2010). Likewise in Central Amazon sites, understory light limits seedling growth in Cecropia-dominated stands, but not in the arrested Vismia-dominated stands (Jakovac et al.2012). Variation in functional attributes among tree species is an important driver of successional change. We predicted a shift over time in abundance and basal area from species with traits that enable fast returns on tissue investment early in succession to species with traits that lead to slow economic returns later in succession. Based on wet secondary forest sites in Chiapas, Mexico and Costa Rica, we demonstrated a shift from “fast” traits early in succession towards “slow” traits later in succession (Lohbeck et al. 2012). Functional characteristics of the plant community in Chiapas sites were described with the community-weighted mean calculated based on 12 functional traits. Consistent with predictions, leaf tissue density and dry matter content increased during succession, whereas specific leaf area decreased. Community weighted average functional traits change over time in eight Costa Rican plots in patterns that are consistent with these chronosequence trends (Vanessa Boukili, unpublished data). Successional changes in species and functional group composition are far more pronounced in the lower canopy than upper canopy in second-growth forests in Mexico and Costa Rica, leading to distinct successional trends in upper vs. lower canopy layers. Long-lived pioneer tree species in second-growth forests create a suite of leaf traits in the upper canopy that differ from traits of more recently recruited shade-tolerant tree species in the lower canopy (Chazdon and Boukili, in preparation).
Successional changes in functional trait composition within and among assemblages are likely to have a strong phylogenetic basis. We are pioneering analyses of phylogenetic community structure during succession, comparing temporal changes in phylogenetic structure with chronosequence patterns (Letcher 2010, Norden et al. 2012, Letcher et al. 2012). Trees in young second-growth forests in Costa Rica are significantly more closely related, whereas trees in older second-growth and old-growth forests are significantly more evenly distributed across phylogeny compared to randomly selected species from the regional pool (Letcher 2009). Phylogenetic community structure across chronosequences shows similar trends across all three regions (Letcher et al. 2012). Mortality and recruitment processes drive parallel temporal changes in species composition and phylogenetic community structure during succession in Costa Rican sites (Norden et al. 2012). During succession, closely related species show higher mortality rates, whereas new recruits are more distantly related. These demographic processes increase the representation of different evolutionary groups in later stages of succession, reflecting biotic filtering processes (Norden et al. 2012). These findings imply that mechanisms of community assembly operate in a similar fashion within and across successional plots in different regions.
Our studies are generating new insights into factors affecting resilience of secondary forests. Young and intermediate secondary forests with low intensities of prior land use that are close to mature forest remnants in NE Costa Rica show convergence of species composition of woody seedlings and saplings with mature forests over time (Norden et al. 2009). In contrast,
vegetation succession in the Central Amazon exhibits alternative pathways according to prior land use. Cecropia species dominate after clearcuts of mature forest, whereas Vismia species dominate after burning of abandoned pastures. Basal area in Cecropia stands increased quickly in the first decade, outpacing accumulation in Vismia stands, but both stand types converged in basal area after 25 years. Vismia stands were much more variable than Cecropia stands in stem density and basal area. Species density increased at a much faster rate in Cecropia than Vismia stands, creating a large divergence after 25 years. Cecropia sites became less similar in species composition during the first decade of succession, whereas Vismia sites maintained a high similarity (Norden et al. 2011).
Based on data from Brazil sites, Bentos et al. (2013) recast the traditional seed size/seed production tradeoff into a new formulation: the economics of seed packaging. The seed mass-seed production tradeoff in plants, generally interpreted as a tradeoff between dispersal and establishment, is viewed as only one component of the economics of seed packaging—i.e., how biomass is allocated among seed and fruit traits across plant species. Seedling and adult densities of 12 pioneer species were better explained by fruit traits than seed traits. Fruit variables may be more important generally, or perhaps for specific guilds, such as pioneers, where dispersal is relatively more limiting than establishment. Research in neoSelvas LTREB plots has also examined flowering and fruiting phenology of trees in secondary forests (Bentos et al. 2008, Vilchez et al. 2009) and the commercial and conservation value of secondary forests (Vilchez et al. 2008, Chazdon et al. 2009).
We have developed a new statistical classification method to distinguish successional generalists and specialists based on species relative abundance across landscapes (Chazdon et al. 2011). These species classifications provide a tool for comparing the ecological composition of species pools in different tropical regions and for evaluating species differences in functional traits and vital rates.
Broader Impacts Phase I
Research activity of neoSelvas investigators has led to the publication of over 45 papers in international peer-reviewed journals and 5 book chapters since 2007, all focusing on Neotropical secondary forest regeneration or based on data emerging from neoSelvas plots. In addition, we published 6 papers comparing data from two or more study areas (Chazdon et al. 2007, Chazdon et al. 2010, Chazdon et al. 2011, Letcher et al. 2012, Norden et al. 2012).
Training and education are a major focus of the neoSelvas LTREB. Since our joint project began in 2007, we have mentored 14 undergraduate and masters students, 16 doctoral students and 1 postdoctoral associate, Natalia Norden. Doctoral students who conducted their research projects in the LTREB-funded sites are from the US, Costa Rica, Mexico, and the Netherlands. We have trained 10 local field technicians (“paraforesters”) to establish plots, identify tree species, map stems, measure tree DBH, and to enter data. This training has improved their prospects for professional advancement. We have also published papers in Spanish or Portuguese in specialized journals in Mexico, Costa Rica, and Brazil (Martínez-Ramos and Garcia-Orth 2007, Vilchez et al. 2009, Martínez-Ramos 2010, Chazdon et al. 2012). LTREB investigators organized and participated in an international workshop on Neotropical secondary forest regeneration in October 2009 in Morelia, Mexico, funded jointly by NSF and UNAM (Mexico). Over 60 participants from 7 countries attended the workshop, including 7 graduate students, 7 postdoctoral fellows, and junior and senior scientists. This outreach activity led to the formation of the neoSelvas Network to promote a deeper socio-ecological understanding of secondary forest regeneration and restoration across tropical regions. We sponsored a neoSelvas Symposium at the Association for Tropical Biology and Conservation Meeting in Bali, Indonesia (July 2010) and sponsored a second Symposium at the Society for Ecological Restoration International Meeting in Merida, Mexico (August 2011). Martínez-Ramos coordinated a symposium during the Second Mexican Ecological Congress November 2008 in Merida, Mexico featuring presentations by Chazdon, Martínez-Ramos and Bongers.
Robin Chazdon and Miguel Martínez-Ramos co-organized a symposium: “Understanding successional processes in tropical forests using long-term data: from individuals to ecosystems”, Association for Tropical Biology and Conservation, Bonito, MS, Brazil, June 20, 2012. The symposium featured presentations from all project investigators:
(1) Madelon Lohbeck, Eva van den Elzen, Alejandra Tauro, Jorge Meave, Edwin Lebrija-Trejos, Eunice Romero, Lourens Poorter, Miguel Martínez-Ramos, Horacio Paz, Frans Bongers. Plant functional traits and the slow-fast continuum along successional gradients of Mexico: Do dry and wet tropical forests show parallel trait continua?
(2) Vanessa Boukili and Robin Chazdon. Successional convergence in functional traits and species composition of trees in Costa Rican wet forests
(3) Bryan Finegan and Robin Chazdon. Long-term tree population change in secondary rain forests and the effects of stand and population density, functional diversity and phylogenetic relatedness
(4) Bruce Williamson, Rita Mesquita, Benjamin Longworth, and Tony Vizcarra Bentos. Convergence and divergence in alternative successional pathways in Central Amazonia
(5) Natalia Norden, Robin Chazdon, Frans Bongers, Bryan Finegan, Miguel Martínez-Ramos, Rita Mesquita & Bruce Williamson. Beyond chronosequences: spatio-temporal models of successional vegetation change in three Neotropical forests.
(6) Patricia Balvanera, Felipe Arreola, Luis Daniel Avila, Felipe Barragan, Karina Boege, Frans Bongers, Monica Flores-Hidalgo, Mayra Gavito, Eduardo Garcia-Frapolli, Maria del Carmen Godinez, Omar Hernández, Madelon Lohbeck, Miguel Martínez-Ramos, Angelina Martinez-Yrizar, Francisco Mora, Horacio Paz, Erika de la Peña, Diego Perez, Jorge Schondube, Kathryn Stoner, Ireri Suazo, Ilyas Siddique, Ek del Val. Successional changes in ecosystem services in wet and dry tropical forests.
(7) Ana Catarina Jakovac, Tony Bentos, Rita Mesquita, Bruce Williamson. Successional age and light effects on seedling growth in alternative successional sequences in the Central Amazon.
(8) Rita Mesquita, Tony Bentos, Ana Catarina Jakovac, Paulo Massoca, Bruce Williamson. Amazonian secondary forests respond to climatic variation: Biomass dynamics.
(9) Tony Bentos, Henrique Nascimento, and Bruce Williamson. Recruitment Success of Pioneer Tree Species: Importance of topography and gap micro-sites conditions
(Poster presentation)
Click here to see the abstracts from this symposium.
Our project has stimulated outreach activities with local communities. The alternative successional pathways resulting from different land-use histories in the Central Amazon sites prompted a set of enrichment planting experiments to determine impediments to succession (Jakovac et al. 2012). This project was initiated with the cooperation of small landholders near our LTREB permanent plots. The link to rural stakeholders has led to cooperative efforts in our science and in a series of seminars on agroforestry, apiculture and aquaculture for the local producers, as well as environmental education projects in local schools.