California redwoods are famous worldwide for their impressive size and age. The species is the sole living representative of the genus Sequoia, and includes the tallest and oldest living things on Earth. Because of these attributes, redwoods have been thoroughly studied and are considered one of the best understood organisms on the planet.
However, scientists from UC Davis have recently discovered something brand new about these trees – they have two different types of leaves on their branches, and those leaves have completely different functions.
Redwoods occur naturally in coastal areas that can experience both wet conditions and droughts. The rainfall can be high and coastal fog also coats the trees with moisture much of the time. But dry conditions can prevail in certain seasons, or certain areas of the species’ range, and the plants are able to cope with both types of environmental conditions.
They do this partly through the specializations of their leaves, according to the study by the UC scientists. The peripheral leaves have longer leaflets that are covered with a waxy, water-repellent cuticle. These leaves are on the outer edges of the branches and receive most sunlight. They function as organs of photosynthesis, manufacturing sugars for the tree from carbon dioxide, water and sunlight energy.
In contrast, the axial leaves have very short leaflets that lie flush with the central stem and their function is to absorb water through the leaf blades. Plants usually absorb their water through their roots, so the fact that redwood axial leaves have this ability is already remarkable. In fact, the study found that a large redwood can absorb up to 14 gallons of water in just the first hour after its leaves become wet.
The problem is that leaves need to absorb carbon dioxide gas from the atmosphere in order to photosynthesize. They do this through tiny pores, called stomata, which are found on the leaf surface. In wet conditions, the pores clog up with water and photosynthesis becomes inhibited. In contrast, a leaf with a waxy coating may be able to photosynthesize in wet conditions better but will be unable to absorb water in significant quantities.
For redwoods, the different leaf types allow the trees to get wet and still be able to photosynthesize. The peripheral leaves forgo the ability to absorb water but are better adapted for photosynthesis in moist conditions, while the axial leaves don’t contribute to photosynthesis but are able to supply the tree with essential water.
“I’d be surprised if there weren’t a lot of conifers doing this,” said study lead author Alana Chin. “Having leaves that aren’t for photosynthesis is in itself surprising. If you’re a tree, you don’t want to have a leaf that’s not photosynthesizing unless there’s a very good reason for it.”
In order to work out how water uptake by redwood leaves is balanced with the tree’s need to photosynthesize, the researchers collected shoot clusters from six redwood trees at five forest locations between wet Del Norte County and the dry Santa Cruz Mountains. They exposed these shoots to experimental fog conditions and measured how much water they were able to absorb.
The experts also estimated the water absorption potential for seven additional whole trees – including the tallest living tree – by taking samples of their leaf shoots at varying heights and scaling up the numbers of leaves, branch by branch, until they could represent the entire tree canopy. This enabled the team to make the first whole-crown estimates of the capacity of redwood leaves to take up water, using some of the largest trees known. Using this method, they could quantify the contribution of leaf uptake to the water balance of the tree as a whole.
The researchers then compared the anatomy and measured the rates of photosynthesis in the peripheral and axial leaves to understand their functions more completely.
Apart from finding that the two different leaf types have different, complementary functions, the study also found that leaves can be located in different regions of the tree, depending on whether the environment is wet or dry. In the wet, rainy north coast, the water-absorbing leaf type is found on the tree’s lower branches, leaving the upper, sunnier levels to the photosynthesizing leaf type. However, redwoods in the southern, drier regions have their water-collecting axial leaves mainly on the higher branches, to take advantage of the less frequent fog and rain.
“The cool thing here is their ability to thrive under all these circumstances and adjust themselves to these different environments,” said Chin, who grew up near the redwoods in Mendocino County. “That things like this can be happening right under our nose in one of the best-studied species out there – none of us assumed this would be the story.”
Of all the study’s remarkable and novel findings, Chin is most excited to have found an easy and effective way to indicate redwood trees’ ability to access fog. Researchers can monitor how and if redwoods are adapting to changing climate conditions and a future, drier world by simply looking at the visible waxes covering the two types of leaves – something that could be captured on a cell phone camera and shared by other scientists or even members of the public.
The study was published recently in the American Journal of Botany.