A causal trait model for explaining foliar water uptake capacity

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Prepared by Ilaine S. Matos, Sami Rifai, Walquíria F. Gouveia, Imma Oliveras, Dulce Mantuano and Bruno Rosado

Photo of our study area at the Campos de Altitude of the Itatiaia National Park (Rio de Janeiro, Brazil). The image shows the vegetation composed of an herbaceous matrix (grasses and forbs) with some sparse shrubs. Photo credit: Ilaine Matos

Plants growing in the Brazilian mountaintops (i.e. Campos de Altitude) are frequently immersed in clouds. Those clouds forming near the ground are called fog and can constitute an important alternative source of water for some montane plants. Usually, plants absorb water from the soil through their roots. But, if the soils are dry and the atmosphere is very humid, some plants can absorb atmospheric water through their leaves, a phenomenon called foliar water uptake (FWU). Plants capable of FWU could have an advantage during soil water shortages, which are expected to become more and more frequent due to climate change. However, climate change may also cause cloud-lifting, i.e., the increase in the height of cloud formation, thus reducing fog frequency and the opportunities for FWU in mountainous areas. Understanding what makes a plant more or less capable of performing FWU is essential to predicting how changes in both soil and atmospheric water availabilities will affect plant communities.

In this study, we selected 55 plant species from the Campos de Altitude and measured their capacity for FWU. We also measured several other traits that could influence a leaf’s ability to absorb water, which were classified into four categories:

(1) wettability: describing how the water droplets interact with the leaf surface;

(2) surface conductance: describing how much water can penetrate the leaf;

(3) water potential: describing how dry the leaf is compared to the surrounding atmosphere;

(4) water storage: describing how much water can be stored inside the leaf.

Experiment to measure plant species capacity for foliar water uptake (FWU). Leaves of 55 grassland species were immersed in water for 3 h and then we used the mass-difference before and after the submersion to calculate the amount of water absorbed via FWU. (a) Agarista hispidula (Ericaceae), species with the lowest FWU capacity. (b) Hypochaeris lutea (Asteraceae), species with the highest FWU capacity. Photo credit: Ilaine Matos

We then applied up-to-date statistical methods to investigate how those leaf traits explain differences in plant species capacity for FWU.

We discover that although all species growing in the Campos de Altitude are exposed to fog, they largely vary in their capacity for FWU. After one hour of immersion in water, some leaves absorbed only 4 g of water per m2 of leaf, while others absorbed over 70 g of water per m2. Higher capacity for FWU was found in species with:

(1) high wettability: i.e. in leaves where water droplets tend to spread more across the leaf surface;

(2) high surface conductance: i.e. in leaves where more water can penetrate the leaf surface;

(3) low water potential: i.e. in leaves that are dryer (i.e. have more negative water potentials);

(4) high water storage: i.e. in leaves able to store larger amounts of water.

Diagram of a leaf cross-section for a hypothetical plant species with: (a) High foliar water uptake: (1) the leaf surface is hydrophilic (i.e. low contact angle between a water droplet and the leaf surface) and the water can enter the leaf (blue arrows) through the open stomata (2), cuticle (3), and trichomes (4). A more negative leaf water potential (drier leaf) drives the water flow (5), which can be sustained for longer due to the presence of elastic cells with high capacitance in the mesophyll (6). (b) Low foliar water uptake: (1) the leaf surface is hydrophobic (i.e. high contact angle) and the water cannot enter the leaf, neither through the stomata (2), nor through the cuticle (3) or trichomes (4). A less negative leaf water potential (wetter leaf) reduces the driving gradient for water flow (5), and the presence of rigid cells with low capacitance in the mesophyll (6) hinders continued water absorption. Image credit: Ilaine Matos, from Matos et al., 2024 Journal of Vegetation Science

Our results suggest that a single species will rarely exhibit the optimal combination of leaf traits to favor FWU listed above. That is, some species will have high surface conductance but low water storage, or high water potential but low wettability. This means that, at least in the Campos de Altitude, co-occurring species may engage in FWU through alternative combinations of leaf traits. Therefore, they may exhibit different responses and vulnerabilities to climate change.

This is a plain language summary for the paper of Matos et al. published in the Journal of Vegetation Science (http://dx.doi.org/10.1111/jvs.13258)