Miguel Martínez-Ramos

Are functional traits good predictors of demographic rates? Evidence from five neotropical forests

A central goal of comparative plant ecology is to understand how functional traits vary among species and to what extent this variation has adaptive value. Here we evaluate relationships between four functional traits (seed volume, specific leaf area, wood density, and adult stature) and two demographic attributes (diameter growth and tree mortality) for large trees of 240 tree species from five Neotropical forests. We evaluate how these key functional traits are related to survival and growth and whether similar relationships between traits and demography hold across different tropical forests. There was a tendency for a trade-off between growth and survival across rain forest tree species. Wood density, seed volume, and adult stature were significant predictors of growth and/or mortality. Both growth and mortality rates declined with an increase in wood density. This is consistent with greater construction costs and greater resistance to stem damage for denser wood. Growth and mortality rates also declined as seed volume increased. This is consistent with an adaptive syndrome in which species tolerant of low resource availability (in this case shade-tolerant species) have large seeds to establish successfully and low inherent growth and mortality rates. Growth increased and mortality decreased with an increase in adult stature, because taller species have a greater access to light and longer life spans. Specific leaf area was, surprisingly, only modestly informative for the performance of large trees and had ambiguous relationships with growth and survival. Single traits accounted for 9-55% of the interspecific variation in growth and mortality rates at individual sites. Significant correlations with demographic rates tended to be similar across forests and for phylogenetically independent contrasts as well as for cross-species analyses that treated each species as an independent observation. In combination, the morphological traits explained 41% of the variation in growth rate and 54% of the variation in mortality rate, with wood density being the best predictor of growth and mortality. Relationships between functional traits and demographic rates were statistically similar across a wide range of Neotropical forests. The consistency of these results strongly suggests that tropical rain forest species face similar trade-offs in different sites and converge on similar sets of solutions.

Rates of change in tree communities of secondary Neotropical forests following major disturbances

Rates of change in tree communities following major disturbances are determined by a complex set of interactions between local site factors, landscape history and structure, regional species pools and species life histories. Our analysis focuses on vegetation change following abandonment of agricultural fields or pastures, as this is the most extensive form of major disturbance in Neotropical forests. We consider five tree community attributes: stem density, basal area, species density, species richness and species composition. We describe two case studies, in northeastern Costa Rica and Chiapas, Mexico, where both chronosequence and annual tree dynamics studies are being applied. These case studies show that the rates of change in tree communities often deviate from chronosequence trends. With respect to tree species composition, sites of different ages differ more than a single site followed over time through the same age range. Dynamic changes in basal area within stands, on the other hand, generally followed chronosequence trends. Basal area accumulation was more linked with tree growth rates than with net changes in tree density due to recruitment and mortality. Stem turnover rates were poor predictors of species turnover rates, particularly at longer time-intervals. Effects of the surrounding landscape on tree community dynamics within individual plots are poorly understood, but are likely to be important determinants of species accumulation rates and relative abundance patterns.

Integrating agricultural landscapes with biodiversity conservation in the Mesoamerican hotspot.

CELIA A. HARVEY,∗‡‡‡ OLIVER KOMAR,† ROBIN CHAZDON,‡ BRUCE G. FERGUSON,§ BRYAN FINEGAN,∗∗ DANIEL M. GRIFFITH,†† MIGUEL MARTINEZ-RAMOS,‡‡ HELDA MORALES,§ RONALD NIGH,§§ LORENA SOTO-PINTO,§ MICHIEL VAN BREUGEL,∗∗∗ AND MARK WISHNIE††† ∗Department of Agriculture and Agroforestry, CATIE, Apdo 7170, Turrialba, Costa Rica †Programa de Ciencias para la Conservacion, SalvaNATURA, Colonia Flor Blanca, 33 Avenida Sur #640, San Salvador, El Salvador ‡Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06268-3043, U.S.A. §Departamento de Agroecoloǵia, El Colegio de la Frontera Sur, Carretera Panamericana y Periferico Sur s-n, San Cristobal de Las Casas, Chiapas, Mexico ∗∗Department of Natural Resources and Environment, CATIE, Apdo 7170, Turrialba, Costa Rica ††Biodiversity of BOSAWAS Biosphere Reserve, Saint Louis Zoo, Managua, Nicaragua ‡‡Centro de Investigaciones en Ecosistemas, UNAM, AP 27-3 Santa Maŕia de Guido, CP 58089, Morelia, Michoacan, Mexico §§Centro de Investigaciones y Estudios Superiores en Antropoloǵia Social, San Cristobal de las Casas, Chiapas, Mexico ∗∗∗Centre for Ecosystem Studies, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands †††Equator Environmental, LLC, 250 Park Avenue South, New York, NY 10003, U.S.A.

Demography and allometry of Cecropia obtusifolia, a neotropical pioneer tree - an evaluation of the climax-pioneer paradigm for tropical rain forests

Summary 1. Demography and allometry of the dioecious tree Cecropia obtusifolia were studied in a 5-ha permanent plot at Los Tuxtlas rain forest, Mexico. Treefall gaps were aged and a significant positive correlation was found between gap age and tree size. Active regeneration of the species was suggested by the high proportion (>35%) of young individuals. The population was highly clumped among young juvenile trees and became less aggregated as density within-gaps decreased among older trees. New and successful recruitment was restricted to recently formed ( 100-M2) gaps. Oldest individuals (35 m high) were growing in 37-year-old gaps. 2. Tree height and stem diameter increased allometrically with a slope 0-3 m tall. The margin between the actual tree diameter (d) and the theoretical minimum buckling diameter (dmin.) declined exponentially from c. 20 in juvenile trees to almost 1 in the oldest trees (>30 years old). Whilst the number of leaves remains fairly constant before branching, it increases linearly as the number of branches increase. Changes in area per leaf during the life cycle yield an overall allometric pattern between tree height and total leaf area. 3. Seed and seedling mortality was >99%. Reduced light, falling debris, movement of rotting logs and soil dryness were important mortality factors in pre-reproductive stages. Oldest adults died standing, apparently due to senescence. Maximum life expectancy is reached when trees attain a non-suppressed canopy position (c. 20 m high). Individuals showed high growth rates (a stem-diameter increment of up to 2 cm year-1). Growth-rate variation in juvenile stages seem to be related to high environmental heterogeneity. In adult stages the number of conspecifics growing in the same gap seems to affect diameter growth rate negatively. First reproduction occured at about 3 years of age (when 10 cm in diameter) and increased with age. Annual fecundity rates of individuals ranged from 1-4 x 104 to 1-4 x 107 seeds and increased allometrically with stem diameter and number of branches. 4. The most important environmental filter in the life-cycle of C. obtusifolia operates from the seedling to the juvenile stage. This finding does not support the suggestion that pioneer limitation to gap regeneration is determined during germination. The allometric and demographic traits of C. obtusifolia correspond to those expected for a shade-intolerant species that colonizes ephemeral sites, and they contrast with those of purported shade-tolerant tree species reported in the literature. However, rather than finding two clearly distinguishable types of lifehistories, a continuum of demographic life-cycles was found. Therefore, the dichotomous pioneer-climax framework should be viewed as a means of identifying the extremes of a continuum of tree life histories.

On the hope for biodiversity-friendly tropical landscapes

With the decreasing affordability of protecting large blocks of pristine tropical forests, ecologists have staked their hopes on the management of human-modified landscapes (HMLs) to conserve tropical biodiversity. Here, we examine key forces affecting the dynamics of HMLs, and propose a framework connecting human disturbances, land use, and prospects for both tropical biodiversity and ecosystem services. We question the forest transition as a worldwide source of new secondary forest; the role played by regenerating (secondary) forest for biodiversity conservation, and the resilience of HMLs. We then offer a conceptual model describing potential successional trajectories among four major landscape types (natural, conservation, functional, and degraded) and highlight the potential implications of our model in terms of research agendas and conservation planning.

Seed bank versus seed rain in the regeneration of a tropical pioneer tree

SummaryWe used the tropical pioneer tree, Cecropia obtusifolia to evaluate the relative importance of different sources of seeds in the regeneration of species that depend on ephemeral sites. We studied seed production in a population established in a 5 ha plot, and dispersal, dormancy and seed predation in two recent treefall gaps (<1 year-old), two building or successional forest patches (10–15 since disturbed), and two mature forest patches (>35 years since disturbed) for a one year period at Los Tuxtlas (Mexico). Flowers and fruits were counted at monthly intervals. Annual fecundity per tree ranged from 1.4×104 to 1.4×107 seeds. Seeds were continuously available on the trees and on the ground. Average annual seed rain per m2 (as measured by 0.5×0.5 m seed traps) varied from 184 to 1925 among the six sites. Distance to nearest seed source and patch type explained more than 60% of the seed rain variation among sites. Soil seed density, estimated by counting seeds from ten samples (78.5 cm2×10 cm deep) collected from each site in October and January, ranged among the six sites from 269 to 4485 seeds per m2 in January and from 204 to 5073 in October. Soil seed viabilities were much lower (17.1% in October and 5.1% in January) than those of rain seeds (48.26%). Annual survivorships of 2.2% were estimated for seeds artificially sown on the soil surface of a gap and a mature patch, and 3.75% in a building patch. In two other experiments seed removal rates ranged from 27% to 98% in 4 days. Removal rates were significantly higher in gap and mature patches than in building patches. Ants (Paratrechina vividula) and grasshopper nymphs (Hygronemobius. sp.) were the main predators. We draw three main conclusions from our data: (1) Pathogens and predators determine low survivorship of C. obtusifolias seeds in the soil and a rapid turnover rate (1.07 to 1.02 years) of its seed bank; (2) a continuous and copious seed production and an abundant and extensive seed rain replenish the soil seed pool in patches with different disturbance ages at least up to 86 m from nearest source; (3) more than 90% of the seeds contributing to C. obtusifolia seedling recruitment in gaps are less than one year-old. We discuss our results in the context of previous similar studies for tropical forests.

Treefall age determination and gap dynamics in a tropical forest

(1) Most individuals of Astrocaryum mexicanum, a monopodial neotropical understorey palm, endure treefalls that form gaps in the forest, by bending under falling trees and limbs. After one year, a bent palm recovers vertical growth at its terminal meristem and forms a permanent kink in its stem. Previous detailed demographic studies have allowed us to determine with accuracy the passage of time, based on the age-constant rate of stem elongation of the palm. (2) Using this morphological feature of the palm, together with its high density (3001230 mature individuals ha-1) in a 5-ha tract of rainforest at Los Tuxtlas, Veracruz, Mx more than 50% of the quadrats suffered disturbance in the last thirty years and 28% suffered more than one disturbance in the last seventy years. These results provide evidence that canopy disturbances capable of promoting the release of suppressed seedlings and saplings of forest trees may occur at the small scale of some tens of square metres. (4) The long-term treefall patterns analysed (up to seventy years) show that canopy disturbances are a permanent ecological factor in the rainforest environment. Annual rainfall explains more than 50% of the annual variation of the proportion of forest opened to gaps per year. (5) The yearly disturbance does not indicate the gap availability for species regeneration: for example, a year of high disturbance (e.g. 6 1 % of the forest opened to gaps) may have a similar number of sizeable gaps suitable for pioneer regeneration as a year of low disturbance (e.g. 1 5% of the forest opened to gaps). (6) The results obtained in this study show that the gap-formation process operating at Los Tuxtlas forest promotes a strong temporally and spatially random variation in the physical environment of plants. This heterogeneity may be one of the factors involved in determining the high biological diversity found in most tropical rainforests.

Pioneer species distribution in treefall gaps in Neotropical rain forest; a gap definition and its consequences

An attempt was made to evaluate the consequences of applying a strict definition (Brokaw 1982a) to the delimitation of forest gaps in the field. The northernmost Neotropical rain forest, at Los Tuxtlas, Mexico, was searched for young (1–2 years old) single-event gaps that would meet the criteria of the definition. In 60 ha of rain forest, only 12 such gaps containing pioneer species could be found. Thirty-three pioneer species (shrubs and trees) were used as indicator species for gap conditions. Gap size, measured as projected canopy opening ( sensu Brokaw 1982a), underestimated from 44 to 515% the size of the area colonized by pioneer species. On average the size of the colonized area was 3.4 times larger than the size of the projected canopy opening. The majority of the pioneer species showed a relative preference for gap borders, an area generally not included in the projected canopy opening. Pioneer plant abundance and density, and species richness and density, did not differ significantly between gap centres and gap borders. Floristical variation was not related to gap size or location in the gap. These findings can be explained if gap environment (to which pioneer plants respond) is seen as the result of many interacting factors, of which size of the canopy opening is only one. It is concluded that the definition for delimiting gaps in the field as proposed by Brokaw (1982a) cannot be regarded a generally applicable definition, and that its value as a comparative standard is doubtful.

SEED MASS AND SEEDLING PERFORMANCE WITHIN EIGHT SPECIES OF PSYCHOTRIA (RUBIACEAE)

Large seeds have been hypothesized to confer survival advantages in the shaded forest, and small seeds to confer growth advantages in open habitats. We explored these hypotheses using experimental studies in both field and controlled conditions. Our experiments examined intraspecific effects of seed mass on seedling demography in eight sympatric woody species of Psychotria. We compared the effects of seed mass on seedling size (biomass), maximum relative growth rate (RGR), the proportion of emerged seedlings that survived until one year of age, and the proportion of sown seeds reaching the stage of one-year-old seedlings in gaps compared with shaded rain forest habitats, as well as in contrasting greenhouse light conditions. For each species, seeds were classified into four seed mass categories and introduced to three pairs of shaded forest gap sites. In the greenhouse, emerged seedlings from different seed mass categories were grown in contrasting light environments. A positive effect of seed mass on see...

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.