Stephen P. Hubbell

Assessing the response of plant functional types to climatic change in tropical forests

. We propose and test a classification of plant functional types for tropical trees based on demography, growth form, phenology, and moisture requirements, using data from a 50-ha forest dynamics plot in Panama. Correlations among demographic variables for individual species - mortality, growth, and the tendency to colonize light gaps - were strong, and a single principal component (PC) accounted for a large fraction of the demographic variability. Most species - shade-tolerants - were clustered at the low end of the PC axis (low growth, low mortality), while the rest were continuously distributed over a wide range. Three demographic guilds could be defined from scores on this axis: we call these pioneer, building phase, and shade-tolerant trees, following earlier terminology. n n n nLeaf lifetime correlated negatively with the demographic axis, and there was a weak relationship between demography and moisture-preference: no species with high demographic scores also had high moisture requirements. There was no significant relationship between deciduousness and the demographic axis, but deciduousness was negatively correlated with leaf lifetime and moisture index. Altogether, 11 different combinations of demographic variables, deciduousness, moisture needs, and growth form (canopy vs. understory species) were identified. n n n nWe evaluated how these functional types changed in abundance between 1982 and 1995. Because of a recent run of dry years and long dry seasons, we predicted that deciduous species, canopy species, pioneers, and drought-tolerant species would be increasing at the expense of their counterparts. Only one aspect of this prediction was borne out: moisture- demanding species declined sharply in abundance relative to drought-tolerant species. Neither deciduousness nor growth form was associated with population change, and pioneer species declined in abundance more often than shade-tolerants. The overall structure of the forest - the density of deciduous, pioneer, and understory species - did not change much, but the decline of the moisture-demanding guild indicates that a change in composition is preceding a structural change.

Identifying fast-growing native trees from the Neotropics using data from a large, permanent census plot

Abstract To screen for rapidly growing trees, lifetime growth histories of 160 species were estimated from data collected in a permanent 50 ha census plot in tropical moist forest in Panama. Most of the 160 species had never been studied before in detail, and newly encountered species with rapid growth might provide better techniques for reforesting degraded soils in Central America. To estimate lifetime growth, polynomial regressions were fitted to instantaneous growth rates expressed as a function of log-transformed diameter at breast height (dbh). These functions represent a differential equation in dbh, and explicit solutions for the equations provided dbh trajectories as a function of age (starting at 1 cm dbh, which was the smallest size included in the census). Dbh trajectories were calculated for 160 species, and full growth data are presented for the 28 species that ranked among the fastest 15 to reach a dbh of 10, 30, or 60 cm. Dbh trajectories based on growth of one standard deviation above the mean were also estimated for these species by fitting a polynomial regression to the residuals around the original regression. The fastest-growing tree in the 50 ha plot was the balsa, Ochroma pyramidale , which reached 10 cm in 5 years and 30 cm in 10. Cavanillesia platanifolia, Trema micrantha, Zanthoxylum belizense , and Vochysia ferruginea were the other top-ranking species. At mean growth, the top 15 ranking species required 5–25 years to reach 10 cm, 10–67 years to reach 30 cm, and 32–111 years to reach 60 cm, starting at 1 cm. At growth one standard deviation above the mean, the same species requried 4–18 years to reach 10 cm, 8–35 years to reach 30 cm, and 19–69 years to reach 60 cm dbh. We recommend that the little-known species from this list be further tested in reforestation trials.

Mortality and growth of a commercial hardwood 'el cativo', Prioria copaifera, in Panama

Abstract The demography of a valuable timber tree, Prioria copaifera , was studied in undisturbed forest on Barro Colorado Island (BCI) in central Panama using data from a permanent, 50 ha census plot. All individuals above 20 mm diameter at breast height (dbh) in the plot were mapped and dbh measures taken in 1982, 1985, and 1990. Although Prioria is mainly known from swamp forest, it was abundant in the upland forest of BCI, with a mean density of 27–29 stems ha −1 of 10 mm dbh or more. Mortality rates of Prioria were 0.5 and 0.6% year −1 during the two census intervals and did not depend on dbh. Mean growth of saplings was slow, less than 1 mm year −1 , but larger trees grew rapidly, at 8–16 mm year −1 . Growth rates of medium and large trees were higher during 1985–1990, probably because a severe drought in 1983 increased forest-wide mortality rates and opened the forest canopy, allowing more light to penetrate the forest. During both census intervals, growth was extremely variable, ranging from 0 to 40 mm year −1 in different individuals. The range of variability changed little with size: across dbh values from 100 to 1000 mm, maximum growth rates were 20–40 mm year −1 . Unlike absolute growth rates, however, relative growth declined with size, from about 5% year −1 in saplings to 1% year −1 in large trees, and was much more variable among small stems than among larger. Information about variation in growth between individuals and between census periods will be important in forest management, as it indicates how and to what extent silvicultural treatment might enhance timber production in Prioria . In addition, rapid growth of some stems suggests that plantation forestry might be productive in Prioria . Our finding that population density and performance were good in upland soils suggests that plantations of this species could be attempted in drier soils than previously thought necessary.

Biotic Interactions in the Tropics: Neighbourhood effects on sapling growth and survival in a neotropical forest and the ecological-equivalence hypothesis

Introduction In 1980 S. P. Hubbell and R. B. Foster began a long-term, large-scale study of tropical forest dynamics on Barro Colorado Island (BCI), Panama. The objective of the study was to test competing hypotheses about the maintenance of high tree species richness in the BCI forest, and in tropical moist forests more generally. Hubbell and Foster established a 50-ha permanent plot on the summit plateau of BCI, within which all free-standing woody plants with a stem diameter at breast height (DBH) of a centimetre or larger were tagged, measured, mapped and identified by 1982. Subsequent complete censuses of the BCI plot have been conducted from 1985 to 2000 at 5-year intervals. In setting up the BCI plot, Hubbell and Foster (1983) reasoned that whatever diversity-maintaining mechanisms were important, they would have to operate in a spatially dependent manner in communities of sessile plants such as the BCI tree community, which meant that the trees had to be mapped. A decade earlier, Janzen (1970) and Connell (1971) had independently proposed a spatially explicit ‘enemies hypothesis’, now known as the Janzen–Connell hypothesis. They hypothesized that host-specific seed and seedling predators were responsible for maintaining tropical tree diversity by causing dependence on density and frequency (rare species advantage), through an interaction between seed dispersal and densitydependent seed predation. In 1980, there were essentially just two principal tropical forest diversity theories to test: the enemies hypothesis and its variants, and the ‘intermediate disturbance’ hypothesis (Connell 1977) and its variants that invoked a role for disturbances associated with opening, growth and closure of light gaps (e.g.

Towards the National Institutes for the Environment

Abstract This report describes ongoing efforts to organize a new US environmental institution that promises to become a model for global change research and policy initiatives. The Committee for National Institutes for the Environment came into being in December 1989 since when it has established a network of participants throughout the USA, given testimony in the US Senate and House of Representatives, called for an evaluation of the state of all environmental sciences and testified in various Environmental Protection Agency panels. Problems encountered and plans for the future are outlined.