Plants

The tissues of living trees may hold the secrets of why some can recover after drought and others die. But those tissues are challenging to assess in mature forests. After all, 90-year-old trees can’t travel to the lab to get an imaging scan. So most studies of the impacts of drought on plants are done in the lab and on younger trees — or by gouging cores out of mature trees.

Barbara Beikircher, an ecophysiologist at the University of Innsbruck in Austria, and colleagues came up with a different approach: They brought the lab to the trees.

In the Kranzberg Forest outside Munich, the team outfitted stands of mature spruce and beech trees with rugged, waterproof ultrasound sensors. Some of the stands had been covered by roofs to block the summer rain, creating artificial drought conditions.

Researchers outfitted stands of mature spruce and beech trees with ultrasound sensors and electrical probes to figure how the species cope with long dry spells.University of Innsbruck

Five years of monitoring revealed that beeches (Fagus sylvatica) are more drought-resilient than spruces (Picea abies), the team reported in the December Plant Biology. Delving into the underlying mechanisms explained this difference.

Drought-stressed trees produced more ultrasound signals than trees exposed to summer rains. Those faint acoustic waves were bouncing off air bubbles called embolisms deep within the trees’ vasculature. Surface tension keeps water moving through a tree’s thousands of tiny vessels — evaporation from pores in leaves drives water up the trunk (SN: 9/6/22). But if there’s insufficient water in the soil, this upward pull can generate embolisms that clog vessels. In the experiments, spruces pinged much more than beeches, suggesting they had far more embolisms.

That’s despite the fact that beeches appear to be less conservative with their water management, at least aboveground. Trees can prevent embolisms by closing the pores on their leaves, but there’s a trade-off. Doing so cuts off the supply of the carbon dioxide that drives photosynthesis, which makes the carbohydrates and sugars that trees need to live and grow. In dry conditions, trees face an impossible choice “between starving and dying of thirst,” Beikircher says.

Beeches suffered fewer embolisms than the spruce, even though they kept their pores open longer than the conifers did. Perhaps that’s because beeches have roots that extend into deeper, wetter soil as well as more robust water reserves, Beikircher says. Another set of experiments after the researchers relieved the drought suggests that’s the case.

At the end of the experiment, the team drenched the soil. All the trees recovered well by most measures: Rates of photosynthesis in the previously parched trees caught up to the rates of trees in the control groups and embolisms filled with water.

But when Beikircher measured the trees’ resistance to an electrical current, an indication of moisture levels deep within trunks, the spruces’ water reserves were still depleted. One season of rain was not enough to help these trees fully recover. It’s unclear whether spruces can replenish their reserves after prolonged drought or how long that might take.

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Species that can withstand drought conditions and recover more quickly may become more populous in future forests as climate change causes droughts to become more frequent and intense (SN: 3/10/22). That means the compositions of the trees that make up the world’s temperate forests could change as the climate warms, with uncertain consequences for the other plants and animals in these ecosystems.

Beikircher plans to test whether a more diverse forest could help drought-sensitive species like the spruce survive. Deep-rooted beeches interspersed with spruces might help increase moisture in the soil’s upper levels by wicking water up to where spruce roots are, she says. More

No one likes a cheater, especially one that prospers as easily as the grass Bromus tectorum does in the American West. This invasive species is called cheatgrass because it dries out earlier than native plants, shortchanging wildlife and livestock in search of nutritious food.

Unfortunately for those animals and the crowded-out native plants, cheatgrass and several other invasive annual grasses now dominate one-fifth of the Great Basin, a wide swath of land that includes portions of Oregon, Nevada, Idaho, Utah and California. In 2020, these invasive grasses covered more than 77,000 square kilometers of Great Basin ecosystems, including higher elevation habitats that are now accessible to nonnative plants due to climate change, researchers report November 17 in Diversity and Distributions.

This invasion of exotic annual grasses is degrading one of North America’s most imperiled biomes: a vast sea of sagebrush shrubs, wildflowers and bunchgrasses where pronghorn and mule deer roam and where ranchers rely on healthy rangelands to raise cattle.

What’s more, these invasive grasses, which are highly flammable when dry, are also linked to more frequent and larger wildfires. In parts of Idaho’s Snake River Plain that are dominated by cheatgrass, for example, fires now occur every three to five years as opposed to the historical average of 60 to 110 years. From 2000 to 2009, 39 out of 50 of the largest fires in the Great Basin were associated with cheatgrass.

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To add insult to injury, cheatgrass is more efficient at recolonizing burned areas after a fire than native plants, creating a vicious loop: More cheatgrass causes more fires, and more fires foster more of the weeds. This means that land managers are often behind the curve, trying to keep cheatgrass from spreading to prevent wildfires, while also attempting to restore native plant communities after fires so that the sagebrush ecosystems don’t transition into a monoculture of invasive grasses.

“We need to get strategic spatially to pinpoint where to protect intact native plant communities rather than constantly chasing the problem,” says Joseph Smith, a rangeland ecology researcher at the University of Montana in Missoula.

To do that, Smith and his colleagues quantified how much of the Great Basin has transitioned to invasive annual grasses over the last three decades. The researchers used the Rangeland Analysis Platform, or RAP, a remote sensing product powered by Google Earth Engine that estimates the type and percentage of vegetation at a baseball diamond–sized scale.

While the satellite imagery that RAP relies on can show where annual grasses turn brown in late spring in the West or where perennial plants stay green longer into the summer, the technology can’t delineate between native and nonnative plants. So researchers cross-checked RAP data with on-the-ground vegetation surveys collected through the U.S. Bureau of Land Management’s assessment, inventory and monitoring strategy.

Invasive annual grasses have increased eightfold in area in the Great Basin region since 1990, the team found. Smith and colleagues estimate that areas dominated by the grasses have grown more than 2,300 square kilometers annually, a rate of take-over proportionally greater than recent deforestation of the Amazon rainforest.

Perhaps most alarmingly, the team found that annual grasses, most of which are invasive, are steadily moving into higher elevations previously thought to be at minimal risk of invasion (SN: 10/3/14). Invasive annual grasses don’t tolerate cold, snowy winters as well as native perennial plants. But as a result of climate change, winters are trending more mild in the Great Basin and summers more arid. While perennial plants are struggling to survive the hot, dry months, invasive grass seeds simply lie dormant and wait for fall rains.

“In a lot of ways, invasive grasses just do an end run around perennials. They don’t have to deal with the harshest effects of climate change because of their different life cycle,” Smith explains.

Though the scale of the problem can seem overwhelming, free remote sensing technology like RAP may help land managers better target efforts to slow the spread of these invasive grasses and explore their connection to wildfires. Smith, for instance, is now researching how mapping annual grasses in the spring might help forecast summer wildfires.

“If we don’t know where the problem is, then we don’t know where to focus solutions,” says Bethany Bradley, an invasion ecologist and biogeographer at the University of Massachusetts Amherst who wasn’t involved in the research. “Mapping invasive grasses can certainly help people stop the grass-fire cycle by knowing where to treat them with herbicides.” More