The conversion of large swaths of land to uniform tree plantations and single-crop species may lead to unintended consequences for the water cycle. This will put ecosystems at greater risk for fires, floods, droughts and even hurricanes, warns a think-tank group of almost 30 water scientists from 11 countries.

Worldwide, policies are increasingly aimed at planting more trees and crops both to combat climate change and increase food and fuel production. Already about 40 per cent of the world’s ice-free land surface has been converted to forestry and agriculture—often with only a few choice tree species and crops where biodiversity once thrived. This trend is poised to continue or even accelerate.

But in an article published in Nature GeoScience, the scientists argue that mixed-species diversity is crucial to the water cycle pathways that enable soil-plant-water systems to recover quickly from environmental stresses.

Forestry and agricultural monocultures (growing a single species repeatedly on the same land) can constrain these pathways. This adversely affects conditions such as soil moisture and erosion, streamflow, evaporation, and groundwater quality—and ultimately reducing ecological resilience.

The authors urge policy makers and land managers to take into account critical water-vegetation interactions to guide decisions about what to plant and where.

“When we modify landscapes to help combat climate change or meet human demands for food and energy, we need to be smart about it,” said Irena Creed, a University of Saskatchewan hydrologist who co-led the think tank paper with University of Delaware researcher Delphis Levia.

“We need to emulate what was natural by not relying on just a few choice crops or trees but instead embracing biodiversity,” added Creed. “When you narrow biodiversity to a few select crops, it makes the whole ecosystem vulnerable.”

Creed explains that the rate, timing, and magnitude of water released to the atmosphere varies with each plant species.

“By having a diverse range in the rate of water movement, you are building a more diverse water system that can withstand water stresses such as droughts and fires,” said Creed.

For example, in a forest with a variety of tree species, some species send roots down shallow, some at an intermediate level, and some deep.

“That means there’s a lot more soil moisture available to some tree species than others,” said Levia. “But if you’re in a monoculture situation, as with many staple crops, the rooting depths are more uniform. They don’t penetrate the soil to varying degrees like natural vegetation in forests. And so, they can be more susceptible to drought.”

The paper notes that increased production of tree plantations to meet demand for wood can reduce, or even eliminate streamflow. It can also sometimes lead to the salinization and acidification of soils, as well as to increased susceptibility to fire.

In highly managed landscapes that have replaced wetlands, plant uniformity has been linked to increases in the frequency and severity of both floods and droughts, as well as water quality deterioration.

More research is needed to pinpoint the water movement pathways that are most susceptible to being constrained in the conversion of natural vegetation to planted monocultures, the authors state.

“We need governments to prioritize research into how much diversity is enough to ensure that a given type of landscape can be resilient and withstand environmental stresses,” said Creed.

Having data on precisely how the change in the water cycle is occurring would enable proper management practices to be put into place, said Oregon State University professor John Selker, a co-author on the paper. Such evidence gathering will be possible using new sensor technologies that are becoming available.

“By recognizing, preserving, or enhancing the diverse array of hydrological responses among plant species, we can provide better stewardship of the Earth’s finite water resources,” concluded the authors.


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