Effects of altered precipitation regimes on bryophyte carbon dynamics in a Peruvian tropical montane cloud forest

Abstract: Tropical montane cloud forests (TMCF) are unique ecosystems that are frequently surrounded by clouds which increase humidity and promote the growth of bryophytes. Several dimensions of humidity are important for the carbon dynamics of bryophytes. Climate change is therefore projected to have a large impact on them as well as on the TMCF itself. The present study aimed to investigate the effects that changing precipitation regimes will have on bryophyte carbon dynamics. Specifically changes in precipitation amounts, frequency and type i.e. if the precipitation falls as rain or is brought by cloud mist were studied through three experiments; (1) a blocked, full factorial experimental set-up tested for the effect of the three precipitation factors on bryophyte net photosynthesis and respiration, (2) the bryophyte photosynthetic performance was also measured on this set up and (3) on a second experimental set up the response of bryophyte carbon dynamics to re-wetting after increasing desiccation periods was measured. The results of the first experiment showed that precipitation amount had the clearest effect on bryophyte carbon dynamics where net ecosystem exchange (NEE) decreased most in high amount treatments. A slightly lower decrease in NEE was found with low amounts of mist compared to low amounts of rain, although the difference between the types was not significant. Furthermore, an effect from frequency was found on respiration where the effect differed most between amounts and types for the high frequency treatments while the difference was smaller for medium and low frequencies. However no interactions with frequency were found for NEE or gross primary production (GPP). In the second experiment, bryophytes generally reached a saturated photosynthesis at relatively low light levels (400 μmol photons m-2 s-1). At these light intensities or higher, the samples watered with low amounts of mist at high frequencies led to the highest maximum photosynthetic rate (Pmax) while the samples watered with high amounts of mist at low frequency gave the lowest Pmax. Thus, low amount, high frequency mist was generally most beneficial for carbon uptake. However at lower light intensities (200 μmol photons m-2 s-1) the carbon gain for the samples of low amount treatments was lower compared to samples of high amount treatments. In the third experiment, there was a clear decrease in GPP and NEE, i.e. carbon uptake, with increasing desiccation length. No trend for the respiratory response to increasing desiccation length was determined. In summary, bryophytes in TMCF seem to be well adapted to the environment they live in today, where desiccation periods are short and where low amounts of precipitation, probably in the form of mist are beneficial to bryophyte carbon uptake. This also suggests that projected changes in climate conditions in TMCF will influence bryophyte carbon dynamics negatively, leading to a decrease in carbon accumulation. Since bryophytes provide a number of important ecosystem services, a negative change in their carbon dynamics could result in biodiversity loss as well as changes in the hydrologic cycle and carbon dynamics of the TMCF. This in turn could have large scale effects on both downslope ecosystems and the people living there, as well as an impact on the world’s biodiversity.