Scientists have studied coast redwoods for many years. They are the tallest trees and have survived for thousands of years.
redwoods are long-lived, large, and decay-resistant, the forests they dominate store more above-ground mass, and thus more carbon, than any other ecosystems on Earth.
While working on a recently published study, colleagues at the University of California, Davis, and Cal Poly Humboldt and I learned a secret that had been sitting under our noses.
There are two different types of leaves that the renowns have. The feature helps the trees adapt to both wet and dry conditions, which could be key to their survival in a changing climate.
Wherever trees grow, their leaves get wet. If films of water cover the stomata, it can be a problem for trees in wet environments. These tiny pores allow carbon dioxide to enter leaves so the tree can combine it with water to make plant tissue.
Many trees that are common to wet forests have leaves that prevent water films from forming.
Trees growing in dry environments take advantage of brief bouts of leaf wetness to take up valuable water directly across the surfaces of their leaves, through special leaf structures and even through their stomata.
Some trees, including coast redwoods, live in both wet and dry environments.
It is relatively easy to overcome the seasonal wetness challenge for broad-leaved trees like the holm oak, which grow in Mediterranean climates with dry summers and rainy winters. Their leaves have stomata on the undersides that keep them clear of water.
The competing goals of repelling and absorbing water need a different way to be balanced, and that's why redwoods are conifers.
We wanted to find out how redwoods met the challenge of leaf wetness, how much water they could absorb, and which leaf features caused differences in water absorption capacity.
We were surprised by what we learned.
Scientists have known for a long time that redwoods can absorb water through their leaves. It is a challenge to figure out how much water redwoods can absorb in one climate and how much in another.
A big redwood has over 100 million leaves and a lot of surface area for water absorption. The leaves change structure with height, going from long and flat to short and awllike. Picking leaves at ground level was not enough to get this right.
To complicate matters further, gravity is always pushing down on the giant column of water rising upward through the trunk. The leaves at the top of the tree have less water than the leaves at the bottom of the tree.
The treetop has a tendency to pull water into the leaf more quickly than into water-rich leaves at the bottom, just as a dry sponge picks up water faster than a damp one.
We needed leaves from trees in wet and dry environments and from multiple heights to get an accurate picture of how redwoods absorb water.
To get them to their natural gravity-based water levels for analysis, we put our leaf samples in a fog chamber, hooked up to a room humidifier, and measured weight gain over time to see how much water they could get.
We divided the clusters into pieces as we took apart the shoots. The shoots of a renown shoot cluster are divided into individual shoots of multiple ages, each with its own set of leaves. The shoots on the central axis were separated from the shoots on the outer edges.
It was obvious that shoots from the center axis had leaves that could absorb water three times faster than peripheral leaves. When we looked at the leaves with a microscope, we realized they were different types.
We needed to see their internal structure to convince ourselves that they look the same on the outside.
Alana Chin is from UC Davis.
The left and right leaves of a redwood have different purposes.
The leaves were packed with water storage cells, but the leaves' phloem appeared to be blocked and useless. If a tree has leaves, the conventional wisdom is that they are there for photosynthesis, but we wondered if the leaves had a different purpose.
We found that the leaves of redwoods are specialized for absorbing water. Water absorption rates are affected by differences between the surfaces of leaves.
The peripheral leaves of the redwoods have a lot of stomata. This helped to show how large they are year-round regardless of the long wet season.
The analysis shows that the leaves of the redwoods are only 5 percent of the total leaf area. Up to 30 percent of the total water absorption capacity can be contributed by them.
redwoods thrive in both wet and dry habitats because of the balance of dueling requirements of the two types of leaves.
We estimated that the thirsty giants can absorb as much as 105 pounds of water in the first hour of a rain, using large-scale tree measurements and equations. That is equivalent to 101 gallons of beer.
Understanding what causes the variation in redwood leaves can help us gauge differences in water absorption capabilities among trees and environments. This is the most useful part of the study.
The two leaf types of the renowns are different. In wet rainforests in the northern part of their range, the trees invest in less of the leaves that are specialized for absorbing water.
The leaves are concentrated in the lower crowns of the trees, making them free to maximize sugar production in the bright sun.
There are more leaves in the water-stressed tops of trees in the south.
It allows them to take advantage of briefer leaf-wetting events, but it also means they are smaller per leaf area.
In an ever-drier California, the ability to shift leaf types to match regional climatic differences may help them adjust to climate change.
It may be a promising feature to investigate as scientists try to link the differences between the populations of the epic trees.
The Swiss Federal Institute of Technology is home to the Postdoctoral Fellow in Plant Ecology.
This article is free to use under a Creative Commons license. The original article is worth a read.
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