PHYSIOLOGICAL ECOLOGY AND DEMOGRAPHY OF THREE SPECIES OF NEOTROPICAL HERBACEOUS BAMBOO (GRAMINEAE) (PHOTOSYNTHESIS, CONDUCTANCE, OSMOTIC POTENTIAL, STOMATE DENSITY, LEAF SIZE)

Physiological and demographic responses of three species of understory herbaceous bamboo to treefall gap formation were studied in Central Panama. These closely-related sympatric species have similar growth forms, but differ in leaf size and ability to grow in gaps and shaded understory. Photosynthesis, growth and reproduction were examined in contrasting light environments, and stomatal conductance and water relations parameters were measured during progressive drought.;Water relations parameters were estimated from nonlinear regression analysis of pressure-volume isoclines derived from leaves of the three species during three sequential periods of drying. No acclimation in isocline parameters was observed. Species differences in drought resistance corresponded to differences in osmotic potentials at saturation. Drought tolerance correlates with the regional distribution of these species in wet and dry forest. Conductance decreased as leaf water potentials declined. While leaf water potentials remained high, P. latifolius and S. sodiroana showed marked stomatal closure after noon, whereas S. spicata was able to maintain gas exchange throughout the day.;P. latifolius allocated a higher percentage of ramet biomass to fruit production under treatments of high light and nutrient availability, and produced larger leaves in gaps. S. spicata showed a direct relationship between light and nutrient availability, and fruit number and total biomass, and did not vary leaf size between gaps and shade. S. sodiroana was unable to use high levels of nutrients or light for growth and reproduction. A large increase in photosynthetic surface area and maximum photosynthetic capacity allows P. latifolius to grow and reproduce in gaps by maximizing carbon gain when environmental conditions permit high rates of gas exchange. Relatively continuous diurnal gas exchange permits S. spicata to grow and reproduce in gaps without the benefit of photosynthetic acclimation. Although P. latifolius and S. spicata have similar demographies, they employ contrasting mechanisms to maintain reproduction subsequent to environmental change.;Pharus latifolius and Streptochaeta spicata showed a large increase in ramet production and sexual reproduction in the sun treatment, but only P. latifolius developed greater maximum photosynthetic capacity under high irradiance. Streptochaeta sodiroana showed high mortality when grown in the sun, and developed considerable photosynthetic efficiency when grown in shade.