Global changes and tropical canopies: Increasing temperatures and the ability of the epiphytes to migrate within the same tree

Background/Question/Methods

Microclimatic conditions at forest interior in northern Andean are changing as a double effect of forest fragmentation and climatic change. Epiphytes are distributed along the tree but mainly in the crown. What are the potential losses or gains in epiphytic biomass with raising temperatures in the Andes? Are epiphytes able to migrate lower in the trunk? Epiphytes were studied in 10 sites spanning 2000 km², with changes in elevation from 50 to 3000 m. Sites are fully inventoried 1 ha plots where random trees were selected until reaching 35 trees hosting epiphytes. Epiphytes were divided in four contrasting growth forms: bromeliads, true-epiphytes, nomads, treelets. Biomass was calculated from allometric equations specific for each growth form from 30 randomly selected individuals in each class. Biomass was estimated per tree and extrapolated to one hectare. Host trees were divided in three height classes: base, upper trunk and canopy. Instrumentation for temp and RH was placed in selected sites in the three height zones. Simulations of within the forest temp, radiation and RH gradients were run to estimate the potential effects of climate change in the inner forest. We use LGMs including growth forms, future climates were modeled after simulations developed for inter Andean tropical valleys. 

Results/Conclusions

Our results showed that forests above 2.000 m had the largest biomass with up to 5 ton/ha; tropical rain forest below 1.000 m had 129.7 kg/ha in average. Host trees had an average of 114 g per tree in lowland forest up to 3.700 g in high elevation forests. Growth forms were restricted to particular height zones in the tree and we found a higher diversity of growth forms at higher elevations. Climatic models indicated that epiphytes from the outer canopy are going to be overexposed to excessive heat in the next 30 years and milder conditions will be found in lower strata of the tree.

Increasing temperatures will force epiphytes from the canopy to lower strata. We predict that the biomass from growth forms exclusively found in the canopy will be unable to migrate within the tree. Uphill migration will be limited because the closeness of the treeline. In conclusion rosette species will be facing increasing risk of extinction since they are only found in the outher canopy. Lowland species are suffering from attrition and only a limited set of growth forms with scarce biomass is expected to survive the increasing temperatures of the lowlands.