Modeling Foliar Water Uptake in Fagus grandifolia

Foliar water uptake (FWU) is a process many trees and plants use to uptake water vapor into their leaves. They do so through several pathways: stomata, the leaf cuticle, trichomes, and hydathodes. There is relatively little known about these pathways and processes. Most of the available research has treated FWU as a reversal of stomatal transpiration. However, there are significant differences in predictions of FWU based on assumptions surrounding the details of water transport through and within the leaf. By creating a comprehensive model, it will be possible to better predict the impact of drought and other climate change on ecosystems, water availability, and the water and carbon budgets for different regions. This study aims to model foliar water uptake of water vapor through the leaf stomata and cuticle assuming both saturated and unsaturated leaf internal air space to aid in the understanding of the meteorological and environmental conditions that impact foliar water uptake.

Rates of foliar water uptake are assessed using a model based on a soil-plant-atmosphere continuum (SPAC) model, with an added assumption that water can be taken up through the leaves by entering the substomatal cavity, where it condenses and is then absorbed. Cuticular conductance is another route for foliar water uptake to occur. The leaf cuticle is a waxy hydrophobic layer that prevents excessive water loss. During meteorological conditions such as dew formation and fog, the leaf may then absorb water through the cuticle. In such a model, the water potential plays a significant role by driving water movement between the atmosphere and tree in relation to concentration gradients. If the leaf has a high water potential and the atmosphere has a lower vapor pressure, then transpiration is expected to occur as leaves release water vapor into the air. If the atmosphere has a higher vapor pressure than the leaf, that is when FWU is most likely to occur. The values for these calculations and subsequent research are from Fagus grandifolia (American beech) in the Robinson Forest, Kentucky. This model has offered fresh insight into these pathways, the differences between each one, and the effect they have on boosting leaf water potential and photosynthesis during drought periods. Based on these results, the cuticle’s role in FWU appears to be passive and does not account for the majority of water uptake in the leaf. Still, it is necessary to understand cuticular conductance, especially in unsaturated scenarios, to develop a nuanced model and understanding of FWU across different climates and atmospheric conditions. FWU must be modeled accurately to predict plant survivability in water-stressed environments and to create a more accurate prediction of water and carbon budgets during climate change.

Results show that humidity and temperature can change the rates of FWU. Higher humidity levels lead to more periods of fog and heavy dew, encouraging FWU by creating a higher water potential gradient that favors the uptake of water as opposed to transpiration. Conversely, high temperatures with little cloud cover generally tend to increase the transpiration rates because there is less water available in the surrounding air. This illustrates the importance of the microclimate surrounding the leaf regarding foliar water uptake. These meteorological and atmospheric conditions have a crucial impact on water-stressed montane cloud forests.

The model showed that cuticular conductance had a marginal impact on the transpiration and carbon assimilation rates in Fagus grandifolia. With a higher cuticular conductance there was a slight increase in nocturnal transpiration while the stomata were closed. There was a negligible difference in carbon assimilation rates with a higher cuticular conductance having some variation at the peak events of carbon assimilation, but not enough to be significant. Including cuticular conductance in the model will allow for estimations of night-time water loss in transpiration models.

Modeling FWU has many implications for drought and climate change. Due to climate change, droughts are becoming more frequent and severe. Plants will need to begin relying on functions such as FWU to survive in these water-limited environments where soil water uptake will not be sufficient to sustain plant survivability or continued growth. In regions where FWU accounts for a significant portion of a plant’s water uptake, any disruptions or changes to this process due to shifts in humidity, temperature, or other climate conditions could impact plant health and ecosystem stability. As the climate changes, water availability will affect plants and entire ecosystems, resulting in impacts on the carbon cycle as well. The study of FWU is critical for understanding and modeling plant survivability in a changing climate. By modeling foliar water uptake, it is also possible to create more accurate predictions of carbon and water budgets in cloud affected forests. By creating a precise model using both stomatal and cuticular conductance, scientists will be able to accurately predict how plants will respond to the changing climate and specific atmospheric conditions. Accurate predictions will then lead to the development of strategies on how to effectively mitigate the impacts of climate change. A better understanding of FWU will provide invaluable insights into the future of water and carbon budgets in a quickly changing world.