Water and light drive photosynthetic activity of Amazonian mosses

Mosses and other epiphytic organisms like lichens and algae cover large parts of trees and shrubs in the Amazon rainforest. At ATTO, they are practically everywhere we look. We know from studies in other regions of the world that they are important for carbon uptake through photosynthesis, climate processes, and nutrient cycling. But very little research on this has been done on this in tropical rainforest settings. Now Nina Löbs for MPI-C and her colleagues are working to change that.

Growing on a tree trunk at the forest floor is Leucobryum martianum, one the tropical mosses studied for their microclimatic requirements. © Michael Welling / MPI-C
Growing on a tree trunk at the forest floor is Leucobryum martianum, one the tropical mosses studied for their microclimatic requirements. © Michael Welling / MPI-C

Water-loving mosses

Two important characteristics of mosses are that

  • they are only active when it’s wet, which is why they are so abundant in the rainforest
  • they cannot regulate the water content by themselves. Instead, they depend on water from rain or dew.

Mosses are able to survive long dry periods in a dried out inactive state, like bears in hibernation. But as soon as it rains they are activated again and perform respiration or photosynthesis, for example.

To understand the impact of tropical mosses on local, regional, and even global biogeochemical processes we need to learn more about this dependency on water. Therefore, Nina and her team analyzed the water content, temperature and light conditions of mosses at the forest floor and at different heights in the canopy in the Amazon rainforest.

The habitat matters

They found that mosses close to the forest floor react very reliably to rain events and increase their own water content. But for mosses within the canopy, it’s not as simple as that. It appears that the dense foliage might offer a lot of shading from the rain. Instead, the data suggest that air humidity and dew are the more important water source for them. When they have enough water to wake up from their hibernation, light availability becomes most important to determine how productive they are in their photosynthetic activity. This determines if they take up more carbon from the atmosphere than they release due to respiration.

Surprisingly enough, the mosses close to the forest floor still survive under extremely low light intensities.

This study is a first step to investigate the potential role of tropical mosses in carbon cycling and other biogeochemical cycles. The next step will now be to measure CO2 and other trace gas exchange rates of the mosses with the air around them. This is especially crucial in the face of climate change and deforestation, which leads to more severe and longer droughts. This might limit the productivity of mosses in the future.

Löbs et al. published the study “Microclimatic conditions and water content fluctuations experienced by epiphytic bryophytes in an Amazonian rain forest” Open Access in the journal Biogeosciences.

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