Plant leaves are photosynthetic organs. Their main job is to harvest energy from sunlight, and use that energy to convert carbon dioxide and water into carbohydrates. In addition to capturing sunlight, leaves need to be good at doing two other things - taking up carbon dioxide and conserving water. These requirements conflict - anything that lets carbon dioxide in also lets water out. To deal with these conflicting requirements, plants produce a waterproof cuticle and regulate carbon dioxide uptake by opening and closing their stomata.

Photosynthetic rates tend to be limited by whatever is in shortest supply - water, light, carbon dioxide, nitrogen to make enzymes... If you never water your plants, they will die regardless of how much carbon dioxide and fertiliser you give them. But no matter how much water, light and carbon dioxide they have, if nitrogen is in short supply they can't make enzymes to run photosynthesis.

For plants in wet environments, access to water isn’t their biggest problem. When plants experience precipitation every day, access to carbon dioxide is likely to be more important than access to water. Since carbon dioxide is less soluble in water than in air, the presence of water on the leaf surface may hinder the uptake of water by the plant. It has been suggested that in environments like these, selection should favour the ability to rapidly shed water from leaves.

A study published in the November issue of the journal Biotropica1 looked at the ability of trees in tropical montane forests to repel water from their leaves. The author hypothesised that trees in montane forests should be better at repelling water than trees from drier environments - dry forests in Guatemala and a semiarid grassland-foothills ecotone in Colorado. However, he found montane forest trees were less able to repel water than dry forest or temperate grassland-foothill species. The author suggested that the increased wettability of the montane forest species may be driven by their coating of epiphylls (plant and other organisms growing on the leaf surface), while increased ability to shed water may be selected for in dry environments because it may help channel water to the roots of the plants in arid environments.

I was surprised by the underlying hypothesis - that trees in wet environments should be better at repelling water than those in dry environments. It’s certainly reasonable once you hear it - if carbon dioxide is limiting, then anything that improves carbon dioxide supply should be favoured, both at the species level and the ecosystem level. Unfortunately, this hypothesis seems to skip over the question of whether carbon dioxide is limiting for plant growth in this environment. Light has shown to be limiting for tropical rainforest species during the wet season. Montane forests are often called cloud forests - if light is limiting in lowland rain forests, it seems likely that this limitation will be even more pronounced in cloud forests. In addition, casual familiarity with montane forests does not support a hypothesis of water repulsion. Montane forest leaves are usually overgrown with epiphylls. The moisture allows mosses (and other groups) to coat the leaves of most trees. This is almost certain to make these leaves more wettable.

In addition, the author’s explanation for why species in drier areas have less wettable leaves is a little too adaptionist. There’s a far simpler explanation why leaves of these species repel water - thick, waxy cuticles. Again, casual familiarity with the system should have pointed away from the hypothesis Holder sought to test.

That said, it’s nice to see someone publishing results which don’t support their working hypothesis.

  1. Holder, Curtis D. 2007. Leaf Water Repellency as an Adaptation to Tropical Montane Cloud Forest Environments. Biotropica 39 (6), 767-770. doi:10.1111/j.1744-7429.2007.00303.x