Flowering plants critical to planet’s water cycles, Stanford research shows

A Stanford research study, along with an independent study out of the University of Chicago both show that leaves - particularly leaves of flowering plants - play critical roles in the environment's water cycles.

Perhaps you dreaded raking them every autumn and hauling them out to the curb for pickup.  Maybe you remember placing a piece of paper on top of them, rubbing a crayon over them, and creating an imprint.  Maybe you still step on the brittle ones on the sidewalk just to hear the crunch beneath your feet.

The point is, we’ve all interacted with leaves.  From two very different angles, two current research studies from the University of Chicago and Stanford University show that leaves not only play a critical role in our environment, but that they also are affected by their environment in every interaction.

Looking back over time

Over the past 100 million years, flowering plants, such as grasses, oak trees and sunflowers, have come to dominate our landscape.  But what if that had never happened? In a recent study, Dr. C. Kevin Boyce of the University of Chicago measured the great impact of flowering plants on the environment. This influence has to do with their leaves and the global water cycle, explained Boyce at a Stanford University paleobiology seminar in April.

A satellite image of the Amazon Rainforest. The little white dots are the clouds formed from water exiting the leaves of the trees. (Photo: NASA Earth Observatory)

Through their leaves, all plants give off water to the atmosphere during photosynthesis in a process called transpiration.  Water in the atmosphere translates to clouds, humidity, and rainfall – which in turn can feed further growth.  Plants play a critical role in cycling water from the soil into the atmosphere and back down to land.

Boyce examined leaf fossils of flowering and non-flowering plants and discovered that the veins that run through leaves are four times denser in flowering than in non-flowering plants.  What this means is that flowering plants give off a lot more water to the atmosphere than do their non-flowering relatives – contributing more to rainfall.

In a global climate model, Boyce and colleagues replaced all flowering plants with non-flowering plants.  They found that without flowering plants, today’s rainforests would be 80 percent smaller.  The Amazon basin would also become much hotter and drier, with profound impacts on the animals and plants that live there.  Boyce’s study answers the important question: “If you remove the interchange and the cycling in [flowering plant] leaves, what does that mean for the distributions of environments on earth?” said Stanford University ecologist Dr. Elizabeth Hadly, who was not involved with the study.

Relative to other plants, Boyce said, “[Flowering plants] haven’t been around that long, but they have had a huge impact.”  Boyce’s study highlights the important and central role of leaves in recycling water back into the Earth’s atmosphere.

A closer look at leaves

“Although plants have manipulated their environments over large timescales, over short time scales, they are very susceptible to changes in their environments,” said Graham Dow, a grad student working in a Stanford biology lab.

This magnified image of a leaf shows the microscopic holes called stomata, which control a leaf's water loss. (Photo: Emmanuel Boutet)

Hadly, a Stanford biology professor, describes the work as a “modern take on something [Boyce] is looking at over millions of years.”  The lab studies stomata, or microscopic holes in leaves through which carbon dioxide (CO2) enters and water exits.  Stomata, since they control the loss of water through the leaf, are the driving force behind transpiration, Dow said.

Plants can respond to changes in the environment with their stomata.  For example, if CO2 levels increase in the air, plants do not need as many stomata to take in the CO2 that they need for photosynthesis – and so usually decrease the density and numbers of their stomata.  But at the molecular level, how a plant actually induces this response is a black box.

To better understand stomata development, Dow said he plans to raise different strains of plants in high and low CO2 concentrations.  Some plants respond strongly to changes in CO2, while others do not.  Using differences in plant responses to CO2 levels, Dow will uncover which genetic factors might play a role in how stomata develop, and thereby better understand the molecular basis for plants’ responses to environmental change through this mechanism.

Importantly, this work can help to better understand how fewer leaf stomata could then feed back into changes in the environment – namely, in reducing the amount of water cycled by plants back into the atmosphere.

Completing the feedback

The effect of the environment on stomata brings us back to Boyce’s work about the effect of leaves on the environment.  The “realization that plants are both affecting and being affected is an important one,” Dow said, highlighting the crucial feedback between water, leaves, and the atmosphere.

“Environmental perturbations matter more now than in Earth’s past,” Boyce said, citing the major changes expected with climate change.  Together, both pieces of research emphasize the importance of studying climate change not only at the environmental level, as in Boyce’s study, but also at the level of the leaf, as in Dow’s work.  Understanding these complex feedbacks at both the large and small scales will help us to better understand and predict how both the environment broadly, and individual living things, will react in a future that might look very different than today.

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