Casandra Reyes-García

Climatic and Structural Factors Influencing Epiphytic Bromeliad Community Assemblage along a Gradient of Water-Limited Environments in the Yucatan Peninsula, Mexico

There is a lack of knowledge on the factors driving epiphytic community assemblage along water-limited environments. Epiphytic bromeliad and host communities were analyzed in a range of vegetation types, following a precipitation gradient from 500 to 1500 mm y−1. Plots were set up in scrub mangrove, coastal sand dune scrub, deciduous, semi-deciduous and sub-perennial forests within the Yucatan Peninsula in Mexico. Identity and abundance of hosts and epiphytes, as well as host height and seasonal microenvironmental variables, were recorded at each vegetation type. The study found epiphytic bromeliads to be mainly located in the lower canopy strata of all but the wettest site (sub-peren nial forest). Total epiphyte richness (estimated using species accumulation curves) increased with annual precipitation. Bromeliad species density decreased with vapor pressure deficit and increased with host species density. Semi-variograms and kriging analysis showed a high spatial correlation in bromeliad and host species density. The species composition, however, was unrelated to space, according to a Mantel test, but related to host species composition. The current study shows that diversity and structural integrity of the canopy may be as important as climate in the conservation of epiphytic composition in water-limited environments, where epiphytes are found in sheltered, lower canopy strata.

Water table depth may influence the asymmetric arrangement of epiphytic bromeliads in a tropical dry forest

Studies of metapopulations of epiphytic bromeliads have been useful to test dispersal limitations and niche effects on the distribution of species, but most studies have been done in wet forests. To advance understanding of these processes in dry environments and using additional microclimatic data, we examined the horizontal distribution, vertical stratification and abundance of epiphytic bromeliads in trees from a dry forest and tested whether water table depth (indicated by the distance to an exposed underground lake, cenote) influenced the microenvironment or structure of the vegetation. Seasonal microenvironmental variables (relative humidity, temperature and light) were characterized among sites, two tree species and three vertical canopy strata. Epiphytes and trees were surveyed in plots, and various tree traits were recorded. Epiphyte density decreased with distance from the cenote (water hole) as the water table became farther from the ground surface and thus unavailable to trees. Sites close to the cenote had higher nighttime humidity, irrespective of the tree species identity. Within the same tree species, the base of the canopy was lower at sites close to the cenote, denoting that the cenote influenced the structure of the canopy. These factors may determine the distribution of the epiphytes by positively influencing the water status of these nocturnally transpiring plants with Crassulacean acid metabolism. Epiphytes were distributed randomly among all the species of trees close to the cenote, but they exhibited a species-specific vertical stratification, which may reflect physiological differences among the epiphyte species.

Physiological Ecology of Vascular Plants

The Yucatan Peninsula has opposing gradients of precipitation and evaporation from north to south: as precipitation increases, evaporation rates decrease. Also, due to bedrock composed primarily of limestone, the area presents high porosity and rainfall infiltrates quickly, resulting in no superficial runoff. Natural disturbances such as hurricanes and fires are also common. The interaction of these factors has created a mosaic of environmental conditions that has given rise to a series of physiological adaptations in the plant species of the area. This chapter focuses mainly on the morphophysiological responses and adaptations of native plant species of the Yucatan to natural conditions, and on how plants respond to environmental factors at the level of the individual, species, population and/or functional groups. In this chapter, species were grouped according to their metabolic pathway (C3 species or crassulacean acid metabolism species (CAM)), and discussed based on adaptations to limiting resources: water, nutrients, temperature, and light. For C3 species only trees were included, and studies incorporated in this section spanned from plant establishment, growth, water use and water relations, alternative water sources, and carbon and nutrient flow. For the section on CAM the focus was on the factors that affect CAM plants in the microenvironments in which they occur. Other issues analyzed include plant physiological responses to natural disturbances, the potential impact of climate change on plant populations, and gaps in information as well as additional perspectives of study.

Morphophysiological Plasticity in Epiphytic Bromeliads Across a Precipitation Gradient in the Yucatan Peninsula, Mexico

Plasticity may be a key factor to determine plant survival under a changing environment as a result of climate change or land use modification. Plasticity in physiological and morphological traits was evaluated in seven epiphytic Tillandsia species (Bromeliaceae) from six vegetation communities along a precipitation gradient in the Yucatan Peninsula, Mexico. Microenvironmental conditions (air temperature and humidity, light, and vapor pressure deficit), as well as Δ titratable acidity, osmotic potential, relative water content, and succulence were characterized during wet, early dry, and dry seasons. We calculated the relative distances plasticity index using physiological data from the wet and dry seasons; morphological plasticity was also calculated for foliar trichome and stomatal traits from previously published data. We found high variation in microenvironmental conditions between seasons, particularly for the tropical dry deciduous forest. The dry season had a negative effect in all physiological variables (decrease from 40% to 59% for Δ titratable acidity and 10% to 38% for relative water content). The highest plasticity was registered for T. balbisiana (physiological: 0.29, anatomical: 0.18) and the lowest for T. fasciculata and T. yucatana. Nonmetric multidimensional scaling analysis separated individuals distributed in the wettest vegetation types from those distributed in the driest vegetation types, irrespective of the species, showing convergent physiological strategies to confront environmental variation. We found higher plasticity in water use traits in atmospheric species, compared to tanks and higher plasticity in general in species with wide distribution compared to those with small distribution ranges.

High resilience to extreme climatic changes in the CAM epiphyte Tillandsia utriculata L. (Bromeliaceae)

Climate change is expected to increase the frequency of extreme climatic events, yet few studies have addressed the capacity of plant species to deal with such events. Species that are widespread are predicted to be highly plastic and able to acclimate to highly changing conditions. To study the plasticity in physiological responses of the widely distributed epiphyte Tillandsia utriculata, we transplanted individuals from a coastal scrub and broadleaf evergreen forest to a similar coastal scrub site and forest. After a 45-day acclimation, the plants were moved to a semi-controlled greenhouse at each site, and then subjected to a 20-day drought. Physiological variables were measured during the acclimation and the drought. The individuals of scrub and forest populations had similar relative water content and carbon assimilation in the contrasting conditions of the two transplantation sites despite the high discrepancy between the environments at their original site. Electron transport rates were higher in individuals from the scrub population. Electron transport rates were also higher than estimated from carbon assimilation, suggesting that photorespiration was present. The individuals of the coastal scrub population had a higher capacity to dissipate excess energy this way. The relative distance index of plasticity was high overall, indicating that some traits are highly plastic (titratable acidity, carbon assimilation) in order to maintain the stability of others (maximum quantum yield Fv /Fm and relative water content). We conclude that T. utriculata is a highly plastic species with a high capacity to tolerate extreme environmental changes over a short time.