(ORDO NEWS) — Coastal redwoods (sequoias) are amazing trees that scientists have studied for generations. We know that they are the tallest living trees and have survived for thousands of years, withstanding fires and pests.
Because redwoods are long-lived, large, and resistant to rot, the forests they dominate store more aboveground mass, and therefore more carbon, than any other ecosystem on Earth.
However, while working on a recently published study, colleagues at UC Davis and UC Humboldt and I discovered a secret that was lurking right under our noses.
It turns out that redwoods have two types of leaves that look different and perform completely different tasks. This previously unknown feature helps trees adapt to both wet and dry conditions, an ability that could be key to their survival in a changing climate.
Wherever trees grow, sooner or later their leaves get wet. For trees growing in humid environments, this can be a problem if films of water cover their stomata. These tiny pores allow carbon dioxide to enter the leaves so that the tree can combine it with water to produce plant tissue through photosynthesis.
Many trees found in moist forests have adaptations in their leaves that prevent these water films from forming.
In contrast, trees growing in a dry environment take advantage of short bursts of leaf wetness to absorb valuable water directly through the surface of the leaves, through specialized leaf structures, and even through stomata.
But some trees, including coastal redwoods, live in both wet and dry environments with intense seasonal variations.
For broad-leaved trees such as Norway oak, which grows in Mediterranean climates with dry summers and rainy winters, this seasonal moisture problem is relatively easy to overcome. Their stomata are on the protected underside of the leaves, which allows them to keep water out, while the upper surface of the leaves absorbs water.
But redwoods are conifers, or cone-bearing trees, with thin, flat, needle-like leaves, and need a different way to balance the competing goals of water repulsion and absorption.
We knew we wanted to study how redwoods handle the paradoxical task of wetting their leaves, how much water redwoods can absorb, and what features of the leaves cause differences in water uptake.
What we learned came as a complete surprise to us.
Big trees with big secrets
Scientists have long known about the ability of red trees to absorb water through their leaves. But figuring out how much water a redwood can absorb in this way, and how that ability can vary with climate type, is a real challenge for this species.
First, the great mahogany has over 100 million leaves with a huge surface area to absorb water. And these leaves dramatically change their structure with height, moving from long and flat to short and subulate. Therefore, we will not be able to achieve the desired result by simply collecting leaves at ground level.
To complicate matters further, gravity is constantly pushing against the giant column of water rising up the trunk of the redwood tree. As a result, the leaves at the top of the tree always have less water available than those below.
The inherent dryness at the top of the tree should draw water into the leaf faster than into the water-rich leaves below, just as a dry sponge absorbs water faster than a damp one.
To get an accurate idea of how redwoods absorb water, we needed tree leaves from wet and dry environments, and from different tree heights.
To bring them up to the natural gravitational water level for analysis, we placed leaf samples in a mist chamber in this case, an ice bin connected to a room humidifier and measured weight gain over time to see how much water they could absorb.
Trail of clues
As we dismantled clusters of redwood sprouts to submerge them in the mist, we divided each cluster into pieces.
Groupings of redwood shoots radiate from the woody core and split into separate shoots of different ages, each with its own set of leaves. We separated shoots along the woody central axis from the much more common pliable shoots at the outer edges of each cluster.
It quickly became apparent that the shoots from the central axis had leaves capable of absorbing water three times faster than the peripheral leaves. When we looked inside the leaves with a microscope, we realized that they were two completely different types.
Outwardly, they also look different, but it was so unexpected that we had to see their internal structure to be convinced of this.
Axial leaves were filled with cells to store water, but their phloem tubes in leaves that export photosynthetic sugars to the tree turned out to be blocked and useless. It is generally accepted that if a tree has leaves, then they serve for photosynthesis, but we wondered if the axial leaves have another purpose.
After taking additional measurements, we found that redwood axial leaves are specialized in absorbing water. Differences between the surfaces of axial and peripheral leaves, especially their waxy coating, cause differences in the rate of water uptake.
Unlike the axial leaves, the peripheral leaves of mahogany have a waxy surface with more stomatomas. This helped explain how they photosynthesize year-round despite the long wet season in most of their current habitats.
Further analysis showed that the axial leaves of Red Book trees account for only about 5 percent of the total leaf area and barely produce enough energy through photosynthesis to sustain life. However, they provide up to 30 percent of the total capacity of trees to absorb water.
Together, these two types of leaves balance between photosynthesis and water uptake, allowing redwood to thrive in both wet and dry habitats.
Using tree scale measurements and redwood leaf area equations, we calculated that these thirsty giants can absorb up to 105 pounds (48 kilograms) of water in the first hour after rain wets their leaves. This is equivalent to 101 pints of beer.
Understanding the reasons for differences in the ability of redwood leaves to absorb water can help us appreciate differences in the ability of trees and the environment to absorb water now and in the future. In my opinion, this is the most potentially useful part of our study.
Redwoods are distinguished by two types of leaves depending on local climatic conditions. In tropical rainforests in the northern part of their range, above Mendocino County, trees use fewer axial leaves, which are specialized for absorbing water.
These leaves are concentrated at the bottom of the canopy, leaving the tops of high photosynthetic trees free to maximize sugar production in full sun.
In dry forests at the southern fringes of the redwood range, trees have more axial leaves in their tops, which are water-deficient.
This allows them to better take advantage of short leaf wet periods, but it means they get less photosynthesis per unit leaf area than redwoods in wetter areas.
Redwoods’ ability to change leaf type to match regional climate differences could help them adapt to climate change in California’s perpetually dry climate.
This would be good news for the conservation of these epic trees, and could be a promising feature to study when scientists attempt to link drought tolerance traits to regional differences between redwood populations. Conversation
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