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Introduction: Tropical montane cloud forests (TMCFs) may face increased frequency and magnitude of water stress due to the expected reduction in precipitation and increase in mean cloud altitudes over the coming century. The progressive water stress can negatively feedback with plant-available water inducing significant changes in the TMFCs ecosystem. However, it is still unclear how TCMFs might react to climate change. We may expect that TMCFs will respond most negatively to these changes because the TMCFs trees have not evolved to survive in a more arid regime.

Objective: Our study aimed to answer two questions: i) How does tree water use, and transpiration responses to vapor pressure deficit (VPD) in cloud forest species respond to decreased water inputs from rainfall and cloud deposition? ii) How is the difference in the magnitude and/or type of tree’ water-use responses to reductions in water inputs from rainfall vs. clouds?

Methods: We performed our study at the Wayqecha Cloud Forest Biological Station in Peru (3000 m elevation), where we reduced fog inputs to one plot using a 30-meter high curtain intercepting laterally moving low clouds, while in another plot we excluded >90% of throughfall using a roof water exclusion in a 900 m2 area. Both treatment plots were paired with nearby, unmodified control plots. We continuously monitored tree water use in trees on all treatment and control plots from 2018 to 2020 using sap flow sensors.

Results: The sap flow was reduced by 84% during both the rainy and the dry seasons in the throughfall exclusion plot after more than three years of these treatments. The effects of reducing fog inputs alone were more subtle, with no significant reduction in sap flow. Overall, tree water use starts to decline above 1 kPa VPD independently of the treatment, but the control plot presented higher transpiration for a higher maximum daily VPD than the throughfall exclusion plot. This was a result of strong control of sap flow rates in both morning and afternoon for a similar level of VPD on an hourly basis.

Implications: Our results highlight the conservative behavior of water use of TMCF trees under water stress. With the expected declines in water inputs associated with climate change, we would expect a strong decline in forest productivity given the relationship between tree water use and tree growth. These results indicate that climate change could drive a strong change in TMCF function.

Keywords:

climate change, sap flow, drought, cloud forest, tree water use

Heidi Asbjornsen, Matthew Vadeboncoeur, Mauro Brum, Beisit Vilca, Darcy Galiano, Aline Horwath, Daniel Metcalfe

Presentation within symposium:

S-9 Plant ecophysiology in a changing world: from theory to application

Ecophysiological controls on water use dynamics in response to reducing throughfall and fog inputs in a tropical cloud forest

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