Casandra Reyes Garcia
Abstract
In an open canopy of a seasonally dry forest, competition between the established bromeliad epiphytes seems unlikely, as the abiotic stress of drought is an attractive candidate for a driving factor determining the species composition. However, in this study I find that 40% of the epiphytes in the seasonally dry forest of Chamela are distributed in only 5% of the trees. The driving factor determining the establishment appears to be the leaf type of the host tree, as the trees with high epiphytic occurrence all have compound leaves that allow a higher amount of scattered light through the canopy. The effect of leaf type relegates the effect of bark type, which constitutes the main factor determining seedling establishment in all previously studied moist forests, to a minor component towards host preference in the bromeliad epiphytes. This difference in the community structure underlines the disparity in the stresses determining survival in dry vs. moist forests. The establishment in a tree that provides high light during the short rainy season allows high photosynthetic activity during the productive season, permitting a greater survival rate during the 8 month drought. The seasonal variation at Chamela, with a short rainy season (July to October), a season without rain but with high frequency of dew events (November-March) and an extended drought (until June), allows the study of water use in the bromeliad species with contrasting life forms that inhabit it.
The community structure of the epiphytic bromeliads of Chamela was found to accommodate 10 species that are confined to the upper (6.5 m) or the lower strata (4.5 m) of the canopy. A high plasticity in light use was found among the bromeliad species of Chamela. All species exhibited a high capacity for photoprotection through NPQ and photorespiration. But the species confined to the lower canopy suffered photodamage when exposed to high light and drought.
The different bromeliad species native to Chamela all exhibit CAM metabolism, linked to high water use efficiency by minimizing transpiration. The life forms represented are: terrestrial species, which have, as all other bromeliad species, a rosette morphology that funnels water towards the roots; tank species, that possess a water impounding tank formed by the base of overlapping leaves, where water and nutrients are stored and absorbed through specialized leaf trichomes; and atmospheric species, which efficiently absorb water through a great density of leaf trichomes distributed throughout the leaf. Physiological differences in the water use have been also described in relation to these life forms, being the atmospheric species the most drought resistant according to previous studies.
The present study found that the tank species of Chamela exhibit traits that have been associated with atmospheric species in the past, like a high trichome cover throughout the leaf that can maximize localized absorption of precipitation, while maintaining the tank as a prolonged source of water between rain events. The tank species of Chamela were characterized as drought avoiders, since the photosynthetic activity ceased when the relative water content (RWC) dropped below 60%. By closing the stomata these plants delay further water loss. Contrastingly, atmospheric species continue photosynthesis even under 40% RWC, when most xerophytic plants have become dormant, and thus are characterized here as drought tolerant. The atmospherics rely on high night time humidity to prevent complete dehydration and show a similar water status when maintained for 22 days under low relative humidity (60%), even if provided with water, than when maintained under complete drought at a higher humidity (75%) for the same period of time. The terrestrial species did not suffer the effect of drought (drought avoiders) as they maintained a high RWC throughout the year, possibly by accessing water through the roots and by mobilizing water to the chlorenchyma from their abundant storage tissue.
All the species responded to fog events after a prolonged drought by recovering from photodamage. Yet differences in the responses to dewfall and trace rain events were found. The atmospheric species had a rapid recovery of RWC (3 d), while the tank species had a more delayed rehydration (10 d). The atmospheric species T. bartramii was capable of resuming carbon assimilation after dew or trace rain events, even if no significant increase in RWC was observed. I propose that pulse events are very important in the forest of Chamela, as even within the months of the rainy season, more than 50% of the days do not exhibit rain events higher than 1 mm and of those rains, 60% measured less than 10 mm. In this respect, and giving the high clumping of individual epiphytes in the suitable trees, I propose that a competition for intercepting small rain events is an important factor towards determining the distribution of the epiphytic species in Chamela. Segregation among different branch sizes according to the plant size, light use and water interception strategies is discussed in the main discussion. The differential use of dew, intercepted rains, throughfall in trunks and dew/rain drip as a main source of water from the different species is suggested.
A high dependence on the delta18O signal of atmospheric vapour was observed in the leaf water delta18O of the epiphytes. This was linked to the high exchange between stomata and the atmospheric vapour at the saturating night time humidity that the epiphytes require. By comparing greenhouse and field responses to water availability in the leaf water delta18O of the different life forms, inferences about microenvironmental conditions at the site in which each species establish were made. The non-steady state model developed by B. Helliker (modified from the Craig-Gordon equation, unpublished data) to describe the patterns enrichment in an epiphytic bromeliad leaf was found to fit both greenhouse and field data. This model was highly sensitive to changes in the delta18O of atmospheric vapour, indicating that predictions on this signal can be made from leaf water delta18O and environmental data in the field. The ability to predict the signal of atmospheric vapour from leaf water simplifies the methodology to obtain this value and can be highly useful for applying to global climate models.