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Grogan J.,Mount Holyoke College | Grogan J.,Instituto Floresta Tropical | Loveless M.D.,College of Wooster
American Journal of Botany | Year: 2013

Premise of the study: Flowering phenology is a crucial determinant of reproductive success and offspring genetic diversity in plants. We measure the flowering phenology of big-leaf mahogany (Swietenia macrophylla, Meliaceae), a widely distributed neotropical tree, and explore how disturbance from logging impacts its reproductive biology. Methods: We use a crown scoring system to estimate the timing and duration of population-level flowering at three forest sites in the Brazilian Amazon over a five-year period. We combine this information with data on population structure and spatial distribution to consider the implications of logging for population flowering patterns and reproductive success. Key results: Mahogany trees as small as 14 cm diam flowered, but only trees > 30 cm diam flowered annually or supra-annually. Mean observed flowering periods by focal trees ranged from 18-34 d, and trees flowered sequentially during 3-4 mo beginning in the dry season. Focal trees demonstrated significant interannual correlation in flowering order. Estimated population-level flowering schedules resembled that of the focal trees, with temporal isolation between early and late flowering trees. At the principal study site, conventional logging practices eliminated 87% of mahogany trees > 30 cm diam and an estimated 94% of annual pre-logging floral effort. Conclusions: Consistent interannual patterns of sequential flowering among trees create incompletely isolated subpopulations, constraining pollen flow. After harvests, surviving subcommercial trees will have fewer, more distant, and smaller potential partners, with probable consequences for post-logging regeneration. These results have important implications for the sustainability of harvesting systems for tropical timber species. © 2013 Botanical Society of America. Source

Free C.M.,Rutgers University | Matthew Landis R.,Middlebury College | Grogan J.,Mount Holyoke College | Grogan J.,Instituto Floresta Tropical | And 4 more authors.
Forest Ecology and Management | Year: 2014

Knowledge of tree age-size relationships is essential towards evaluating the sustainability of harvest regulations that include minimum diameter cutting limits and fixed-length cutting cycles. Although many tropical trees form annual growth rings and can be aged from discs or cores, destructive sampling is not always an option for valuable or threatened species. We used an individual-based population model developed for big-leaf mahogany (Swietenia macrophylla, Meliaceae) in southeast Amazonia, Brazil to simulate stem age-size relationships and examine forest management implications of mortality and diameter increment growth. Growth trajectories of 10,000 simulated mahogany trees were consistent with growth trajectories reconstructed from 32 mahogany discs with annual rings. Trajectories were highly variable and strongly autocorrelated; diameter was a poor predictor of tree age even when accounting for up to 10. years of previous growth history. Commercial-sized trees (60. cm diameter) ranged in age from 33 to 180. years (74. yr median). Only 12.5% of seedlings survived to this size, but survival and time to reach commercial size depended strongly on early growth history (first 10. years of life). A tree grown at the 75th percentile growth rate throughout its lifetime requires 70. years to attain commercial size, but Brazilian forest management regulations imply a rotation length of 60. years. These results demonstrate that individual-based models parameterized with typical census data can incorporate individual variation and growth autocorrelation and realistically simulate tree growth and mortality. In the absence of tree ring data, such models can be used to evaluate the consequences of long-term growth and mortality for sustainable management. In the case of mahogany, our results suggest that non-detrimental harvests cannot be achieved without lengthening cutting cycles, increasing commercial tree retention rates, and regularly applying silvicultural treatments designed to increase stem diameter growth rates. Forest managers can have the greatest effect on the rate of commercial recruitment in the first 10. years after a harvest by implementing treatments ensuring that adequate numbers of new stems establish and recruit to dominant positions in recovering canopy gaps. Regrettably, sustainable mahogany management systems developed based on understanding of the species' ecology will not be as simple as current harvest regulations in Brazil imply. © 2014 Elsevier B.V. Source

Grogan J.,Mount Holyoke College | Grogan J.,Instituto Floresta Tropical | Landis R.M.,Middlebury College | Landis R.M.,ISciences LLC | And 5 more authors.
Journal of Applied Ecology | Year: 2014

Summary: The impacts of selective harvesting in tropical forests on population recovery and future timber yields by high-value species remain largely unknown for lack of demographic data spanning all phases of life history, from seed to senescence. In this study, we use an individual-based model parameterized using 15 years of annual census data to simulate population dynamics of big-leaf mahogany Swietenia macrophylla King in southeast Amazonia in response to multiple harvests and in the absence of harvesting. The model is based on regression equations of stem diameter growth, mortality, and fruit production estimated as a function of stem diameter and prior growth; it includes functions for germinating seeds, growing trees from seedling to adult senescence, producing seeds, and creating disturbances at specified spatial scales and return intervals, including logging. We simulate six harvest scenarios by varying the minimum diameter cutting limit (60 cm, 80 cm) and the retention rate requirement (20%, 40% and 60% commercial population retained). Without logging, simulated populations grew over 100 years by 182% from observed densities, indicating that one or more parameters in the model may overestimate long-term demographic rates on this landscape. However, 100-year densities did not far exceed values reported from forests across this region, and other modelled demographic parameters resembled observed behaviours. Under current harvest regulations for mahogany in Brazil (60 cm minimum diameter cutting limit, 20% commercial-sized tree retention rate, minimum 5 commercial-sized trees 100 ha-1 retained after harvest, 30-year cutting cycle), commercial densities at the study site would decline from 39·7 to 11·3 trees 100 ha-1 before the fourth harvest in year 90, yielding an estimated 16·4% of the initial harvest volume during the fourth harvest. Increasing retention rates caused first-cut harvest volumes to decline but improved population recovery rates between harvests. Under both minimum diameter cutting limit scenarios, increasing retention rates led to more robust population recovery compared with the current 20% rate, and higher subsequent harvest yields relative to initial (first-cut) values. Synthesis and applications. These results indicate that current harvest regulations in Brazil for mahogany and other high-value timber species with similar life histories will lead to commercial depletion after 2-3 cutting cycles. Increasing commercial-sized tree retention rates improved population recovery at the cost of reduced initial harvest volume yields. Sustainable harvests will require, in combination, a moderate increase in the retention rate, investment in artificial regeneration to boost population recovery, and implementation of silvicultural practices designed to increase growth rates by future crop trees. © 2013 British Ecological Society. Source

Baker T.R.,University of Leeds | Pennington R.T.,Royal Botanic Garden Edinburgh | Magallon S.,National Autonomous University of Mexico | Gloor E.,University of Leeds | And 61 more authors.
Ecology Letters | Year: 2014

The Amazon rain forest sustains the world's highest tree diversity, but it remains unclear why some clades of trees are hyperdiverse, whereas others are not. Using dated phylogenies, estimates of current species richness and trait and demographic data from a large network of forest plots, we show that fast demographic traits - short turnover times - are associated with high diversification rates across 51 clades of canopy trees. This relationship is robust to assuming that diversification rates are either constant or decline over time, and occurs in a wide range of Neotropical tree lineages. This finding reveals the crucial role of intrinsic, ecological variation among clades for understanding the origin of the remarkable diversity of Amazonian trees and forests. © 2014 The Authors. Ecology Letters published by John Wiley & Sons Ltd and CNRS. Source

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