Is cloud height formation increasing in tropical montane cloud forests?

Abstract:

Although tropical montane cloud forests (TMCF) only occupy a small fraction of all tropical forests, they are well known for their high degree of endemism of flora and fauna. TMCF also play an important role in regulating both local and regional hydrology; vegetation in TMCF can ‘strip’ moisture from clouds or fog and provide an important water source for downstream communities, especially during the dry season. However, as climate change increases terrestrial and atmospheric temperatures, studies have suggested an increase in the height of cloud formation, with the consequence of decreased cloud water input. While many of these modeling studies occur at large spatial scales, only a few empirical studies have found evidence of a decrease in cloud cover or frequency. In our study, we propose to use trees as our long-term data-loggers and will analyze tree cores for changes in the oxygen isotope as a means to detect changes in cloud water input. Because cloud water is often isotopically enriched compared to rain, the goal is to use these two distinct end members to partition water use of trees in TMCF over the last 100-150 years.

The use of stable oxygen isotopes in tropical trees has gained attention in recent years; however, the versatility of this approach across a long temporal scale has not been specifically addressed. In this paper, we will present the limitations of the isotope approach, as well as present ways to overcome or minimize these limitations. One major concern with using oxygen isotopes is that the oxygen signal of cellulose contains two signals: one from the source water and one from the atmospheric conditions (e.g. relative humidity). Thus, how do we use cellulose to track changes in source water if atmospheric conditions are changing as well? Another question is how do we couple carbon isotopes with oxygen isotope (i.e. dual isotope approach) to provide additional information about plant water stress as the source of water is changing? This presentation will focus on specifically addressing these issues and presenting predictions on how carbon and oxygen isotopes should change temporally if cloud height is increasing. Finally, an experiment in the Galapagos Islands focused on directly addressing cloud height will be presented as a case study.