The Impact of a Severe El Niño Event on Vascular Epiphytes in Lowland Panama

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Abstract

As climate change leads to increasing temperatures, tropical dry seasons are expected to become more severe. An overall intensification of drought events may strongly affect vascular epiphytes. Especially at the community level, the response of epiphytes to intense drought events is still poorly understood. Therefore, the severe El Niño event of 2015/16 was used to assess the impact of prolonged drought on an epiphyte community on Annona glabra host trees, around Barro Colorado Island. Prior census data from 2002 and 2015 served as a reference for background community dynamics. Net species changes and net population changes at the species level were determined for both periods. While the total abundance of the community almost doubled during the 13 years of the reference period, individual numbers decreased by c. 17% within the year of the El Niño event. Overall, the El Niño event strongly affected the epiphyte community and led to a strong decrease in epiphyte numbers and species. These findings contrast with most previous studies in tropical lowlands that found epiphyte populations to be rather resistant to similarly severe drought events.

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Mortality of large trees and lianas following experimental drought in an Amazon forest.

Daniel Nepstad, Ingrid Marisa Tohver, David Ray … (2007)

Severe drought episodes such as those associated with El Niño Southern Oscillation (ENSO) events influence large areas of tropical forest and may become more frequent in the future. One of the most important forest responses to severe drought is tree mortality, which alters forest structure, composition, carbon content, and flammability, and which varies widely. This study tests the hypothesis that tree mortality increases abruptly during drought episodes when plant-available soil water (PAW) declines below a critical minimum threshold. It also examines the effect of tree size, plant life form (palm, liana, tree) and potential canopy position (understory, midcanopy, overstory) on drought-induced plant mortality. A severe, four-year drought episode was simulated by excluding 60% of incoming throughfall during each wet season using plastic panels installed in the understory of a 1-ha forest treatment plot, while a 1-ha control plot received normal rainfall. After 3.2 years, the treatment resulted in a 38% increase in mortality rates across all stems >2 cm dbh. Mortality rates increased 4.5-fold among large trees (>30 cm dbh) and twofold among medium trees (10-30 cm dbh) in response to the treatment, whereas the smallest stems were less responsive. Recruitment rates did not compensate for the elevated mortality of larger-diameter stems in the treatment plot. Overall, lianas proved more susceptible to drought-induced mortality than trees or palms, and potential overstory tree species were more vulnerable than midcanopy and understory species. Large stems contributed to 90% of the pretreatment live aboveground biomass in both plots. Large-tree mortality resulting from the treatment generated 3.4 times more dead biomass than the control plot. The dramatic mortality response suggests significant, adverse impacts on the global carbon cycle if climatic changes follow current trends.