S. Joseph Wright

PLASTIC PHENOTYPIC RESPONSE TO LIGHT OF 16 CONGENERIC SHRUBS FROM A PANAMANIAN RAINFOREST

The comparative phenotypic plasticity of 16 species of tropical rainforest shrubs (genus Psychotria, Rubiaceae) was investigated by growing plants in three light environments on Barro Colorado Island (BCI, Panama). The three light environments gave daily photon flux densities (PPFD) similar to the natural light gradient from shaded forest understory to small and large canopy gaps. Six of the species are principally found in gaps or forest edge environments, whereas the other ten species are principally found in shaded understories. Interactions between light treatment and species resulted in unpredictable mean phenotypic expression across light treatments. Shoot relative growth rates (RGR) were similar for understory and gap species in the low light treatment. Gap species had significantly greater shoot RGR in the intermediate light treatment than in the high light treatment. Mean plasticity was significantly lower for morphological variables when com- pared to physiological variables, while variation in plasticity was significantly greater for structural variables. Significant differences between gap and understory species were found in the plasticity of six out of the seven variables. The mean phenotypic plasticity of the seven variables was significantly greater for gap than for understory species. The high plasticity of gap species was consistent with the hypothesis that specialization in a more favorable environment increases plasticity. The species exhibited a wide range of leaf longevities, from four to 29 months, with gap species having, on average, shorter leaf life- span than understory species. Mean phenotypic plasticity decreased with increasing leaf longevity. Selection for greater plasticity may be stronger in the gap species because gaps exhibit a relatively predictable decrease in PPFD for which plasticity could be adaptive. While we have found a significant correlation between phenotypic plasticity and habitat affiliation, phylogeny (subgenus ascription) was not correlated with plasticity or with plant performance in any given PPFD treatment, reinforcing the hypothesis that phenotypic plas- ticity has evolved through natural selection in this diverse genus.

Functional traits and the growth?mortality trade-off in tropical trees

A trade-off between growth and mortality rates characterizes tree species in closed canopy forests. This trade-off is maintained by inherent differences among species and spatial variation in light availability caused by canopy-opening disturbances. We evaluated conditions under which the trade-off is expressed and relationships with four key functional traits for 103 tree species from Barro Colorado Island, Panama. The trade-off is strongest for saplings for growth rates of the fastest growing individuals and mortality rates of the slowest growing individuals (r2 = 0.69), intermediate for saplings for average growth rates and overall mortality rates (r2 = 0.46), and much weaker for large trees (r2 < or = 0.10). This parallels likely levels of spatial variation in light availability, which is greatest for fast- vs. slow-growing saplings and least for large trees with foliage in the forest canopy. Inherent attributes of species contributing to the trade-off include abilities to disperse, acquire resources, grow rapidly, and tolerate shade and other stresses. There is growing interest in the possibility that functional traits might provide insight into such ecological differences and a growing consensus that seed mass (SM), leaf mass per area (LMA), wood density (WD), and maximum height (H(max)) are key traits among forest trees. Seed mass, LMA, WD, and H(max) are predicted to be small for light-demanding species with rapid growth and mortality and large for shade-tolerant species with slow growth and mortality. Six of these trait-demographic rate predictions were realized for saplings; however, with the exception of WD, the relationships were weak (r2 < 0.1 for three and r2 < 0.2 for five of the six remaining relationships). The four traits together explained 43-44% of interspecific variation in species positions on the growth-mortality trade-off; however, WD alone accounted for > 80% of the explained variation and, after WD was included, LMA and H(max) made insignificant contributions. Virtually the full range of values of SM, LMA, and H(max) occurred at all positions on the growth-mortality trade-off. Although WD provides a promising start, a successful trait-based ecology of tropical forest trees will require consideration of additional traits.

The global spectrum of plant form and function

Earth is home to a remarkable diversity of plant forms and life histories, yet comparatively few essential trait combinations have proved evolutionarily viable in today’s terrestrial biosphere. By analysing worldwide variation in six major traits critical to growth, survival and reproduction within the largest sample of vascular plant species ever compiled, we found that occupancy of six-dimensional trait space is strongly concentrated, indicating coordination and trade-offs. Three-quarters of trait variation is captured in a two-dimensional global spectrum of plant form and function. One major dimension within this plane reflects the size of whole plants and their parts; the other represents the leaf economics spectrum, which balances leaf construction costs against growth potential. The global plant trait spectrum provides a backdrop for elucidating constraints on evolution, for functionally qualifying species and ecosystems, and for improving models that predict future vegetation based on continuous variation in plant form and function.

Potassium, phosphorus, or nitrogen limit root allocation, tree growth, or litter production in a lowland tropical forest

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 x 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.

THE EL NIÑO SOUTHERN OSCILLATION, VARIABLE FRUIT PRODUCTION, AND FAMINE IN A TROPICAL FOREST

We tested the hypothesis that the El Nino Southern Oscillation influences forest-wide fruit production, which, in turn, limits frugivorous and granivorous mammals on Barro Colorado Island (BCI), Panama. Observations of BCI mammals have been compiled for 49 years. Frugivorous mammals experienced famine between September and January in 1931–1932, 1958–1959, 1970–1971, and 1993–1994. The most recent famine is evident from an 11-yr record of natural deaths of mammals and a 2-yr record of population densities. Famine occurred every time a mild dry season followed an El Nino event in the 49-yr record. This coincidence is statistically improbable, as demonstrated by a randomization test. A 2-yr cycle of high, then low community-level fruit production has been observed twice for BCI when a mild dry season followed an El Nino event. We used 260 litter traps to monitor community- and species-level fruit production from 1 January 1987 through 30 June 1996. Community-level fruit production was greatest during the 199...

GAP‐DEPENDENT RECRUITMENT, REALIZED VITAL RATES, AND SIZE DISTRIBUTIONS OF TROPICAL TREES

In closed-canopy forests, plant morphology and physiology determine shade tolerance and potential growth and mortality rates; potential vital rates and ongoing gap dependence determine realized vital rates; and realized vital rates determine individual size distributions. This hypothesis was evaluated for the 73 most abundant canopy tree species from Barro Colorado Island, Panama. The percentage of recruits located in tree-fall gaps (P), sapling growth (G), and mortality (M) rates, and the coefficient of skewness of size distributions (g1) were determined from censuses of all individuals .10 mm dbh in a 50- ha plot. Seven qualitative, bivariate predictions relating g1, G, M, P, and wood density (W ) were evaluated. Six of the seven predictions were substantiated in pairwise analyses. A path analysis integrated all seven predictions and explained 51% of the interspecific var- iation in g1. Size distributions with many large individuals and a long tail of relatively rare, small individuals (g1 , 0) characterized gap-dependent species with large fecundity, seed mortality, seedling mortality, G, M, and P. Size distributions with many small individuals and a long tail of relatively rare, large individuals ( g1 . 0) characterized shade-tolerant species with the opposite traits. The percentage of tropical tree species that require tree- fall gaps to regenerate has been estimated to range from ,20% to .70% for old-growth forests. Our analyses suggest that there are not large numbers of functionally equivalent species at either extreme of the regeneration continuum. Rather, there are very few extremely shade-tolerant and extremely light-demanding species. Most species have intermediate light requirements and lifestyles.

Cloud cover limits net CO2 uptake and growth of a rainforest tree during tropical rainy seasons

Recent global-scale analyses indicate that climate variability affects net carbon storage but regard temperature and precipitation to be the main contributors. Seasonal and interannual variation in light availability may also limit CO2 uptake. As an experimental test of light limitation by cloud cover during tropical rainy seasons and by the unusually heavy cloud cover associated with La Niña, we installed high-intensity lamps above the forest canopy to augment light for Luehea seemannii, a tropical canopy tree species, during cloudy periods of 1999–2000. Light augmentation only partially compensated for the reduction in photosynthetic photon flux density caused by clouds. Nonetheless, leaves acclimated to the augmented irradiance, and photosynthesis, vegetative growth, and reproduction increased significantly. Light, rather than water, temperature, or leaf nitrogen, was the primary factor limiting CO2 uptake during the rainy season.

DOES MAMMAL COMMUNITY COMPOSITION CONTROL RECRUITMENT IN NEOTROPICAL FORESTS? EVIDENCE FROM PANAMA

Patterns of seed predation, germination, and seedling herbivory were investigated in Panamanian forests. We hypothesized that seed and seedling survival would vary with differences in mammal community composition. We tested this hypothesis at five sites in mainland forests adjacent to Gatun Lake, full terrestrial mammalian granivore/herbivore communities with top predators; at five sites on Barro Colorado Island (BCI), also a full mammalian granivore/herbivore community but without the two largest cats; at one site each on five medium-sized islands, with rats, agouti, rabbit, and paca present; and on five small islands that support rats only. Experiments were replicated for Dipteryx panamensis, Gustavia superba, and Virola nobilis, all of which have large seeds. To assess seed removal, seeds were placed in wire exclosure cages and nearby outside the cages. There was no difference in removal rates between forest types, with almost all unprotected seeds removed at all sites. To assess post-removal seed fate, seeds of Gustavia and Virola were attached to threads and placed on the forest floor. All threaded seeds were victims of predation on small islands, whereas 34, 43, and 77% of threaded seeds were dispersed and buried on BCI, medium islands, and the mainland, respectively. To assess seedling herbivory, half of the wire exclosure cages were removed after germination, and seedling survival was assessed after 13–14 mo. Protection from mammals increased seedling survivorship by more than sixfold on the smallest islands, by threefold on the medium islands, by twofold on the mainland, and by less than twofold on BCI. The absence of the two largest cats and the exclusion of poachers from BCI was associated with lower seedling herbivory and higher seed predation than observed on the mainland. In contrast, extreme mammal defaunation on the small and medium islands had large and consistent effects on seedling recruitment, including increased seed predation and increased seedling herbivory relative to sites with more intact mammal communities.

Global patterns of leaf mechanical properties

Leaf mechanical properties strongly influence leaf lifespan, plant-herbivore interactions, litter decomposition and nutrient cycling, but global patterns in their interspecific variation and underlying mechanisms remain poorly understood. We synthesize data across the three major measurement methods, permitting the first global analyses of leaf mechanics and associated traits, for 2819 species from 90 sites worldwide. Key measures of leaf mechanical resistance varied c. 500-800-fold among species. Contrary to a long-standing hypothesis, tropical leaves were not mechanically more resistant than temperate leaves. Leaf mechanical resistance was modestly related to rainfall and local light environment. By partitioning leaf mechanical resistance into three different components we discovered that toughness per density contributed a surprisingly large fraction to variation in mechanical resistance, larger than the fractions contributed by lamina thickness and tissue density. Higher toughness per density was associated with long leaf lifespan especially in forest understory. Seldom appreciated in the past, toughness per density is a key factor in leaf mechanical resistance, which itself influences plant-animal interactions and ecosystem functions across the globe.

Seasonal Drought and Leaf Fall in a Tropical Forest

Peak rates of leaf fall almost always occur during dry seasons in low-latitude, low-elevation tropical forests. The hypothesis that plant water stress is the proximal cue for leaf fall was tested by augmenting water supplies during the 4-mo dry season over two 2.25-ha plots of tropical moist forest on Barro Colorado Island (BCI), Panama. The ma- nipulation maintained soil water potentials at or above field capacity throughout the dry season but did not affect atmospheric conditions in the canopy (i.e., relative humidity, temperature, windspeed, incident radiation). The manipulation ameliorated plant water status; for most species, dry-season leaf water potentials in the manipulated plots were similar to wet-season values and both were consistently greater than dry-season values in the control plots. The manipulation delayed leaf fall for 2 of 9 species of trees for which qualitative data are available and possibly for 2 of 20 species of trees and lianas for which quantitative data are available. The timing of leaf fall was indistinguishable in manipulated and control plots for the remaining 25 species. We conclude that plant water status is rarely the proximal cue for leaf fall on BCI. Atmospheric conditions may be important for some species, but there is no reason to presuppose that a majority of tropical plants are responsive to any single cue.