Susceptibility of tropical conifers to changes in the hydrologic cycle

Abstract:

The distribution of tropical montane cloud forests (TMCF) is strongly linked to the occurrence of regular cycles of fog formation. Climate models suggest that global warming is causing an upward shift of the cloud basis for most of the world’s TMCFs, which will impose drier conditions for TMCF species in the near future. Changes in the hydrological cycle in these environments can lead to mortality and local extinction of iconic and vulnerable species that are already endangered, such as Araucaria angustifolia (Araucariaceae) and Podocarpus lambertii (Podocarpaceae). Our purpose was to study the susceptibility of these two species to drought and their recoverability through water supply in the soil and through artificial fog after a prolonged period of drought. We tested the hypothesis that fog is an important source of water for A. aungustifolia and P. lambertii, maintaining a positive water and carbon balance and prolonging the species survival during rainless periods. To test this hypothesis we conducted field and greenhouse experiments exposing plants to contrasting hydrological conditions in the soil and in the atmosphere. To test the recovery capacity of these species, we waited the species to reach their turgor loss point and subsequently rehydrated them via irrigation in the soil or fog nebulization. All the seedlings of P. lambertii died after 11 weeks of irrigation suppression while A. angustifolia seedlings were more resistant to drought and took 17 weeks before reaching full mortality. The fog treatment had contrasting effects on the two species – it allowed the survival of all P. lambertii seedlings, while A. angustifolia showed a mortality of 66.6% of the individuals when fog was the only water source. Considering the climate change scenario of lower frequency of fog events in TMCFs, we expect that P. lambertii populations would be more affected and show higher rates of drought-induced mortality. Because of their high abundance and important role in fog interception, the mortality of these key TMCF species could lead to severe changes in the ecohydrology of these forests.