U. ARIZONA (US) — Extreme droughts in the American Southwest are causing a water-thirsty atmosphere to pull moisture away from plants, creating increasingly stressful growing conditions, a study shows.
Researchers used a growing season index computed from weather data to examine limits to plant growth during times of drought and found conditions were especially severe in low to middle elevations according to a study to be published in the Journal of Geophysical Reseach Biogeosciences.
“We know the climate in the Southwest is getting warmer, but we wanted to investigate how the higher temperatures might interact with the highly variable precipitation typical of the region,” says lead author Jeremy Weiss, a senior research specialist in the department of geosciences at the University of Arizona.
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“The approach we took allows us to model and map potential plant responses to droughts under past, present and future conditions across the whole region,” says Julio Betancourt, a senior scientist with the US Geological Survey who co-authored the study along with Jonathan Overpeck, co-director of the University of Arizona Institute of the Environment. “Our study helps pinpoint how vegetation might respond to future droughts, assuming milder winters and hotter summers, across the complex and mountainous terrain of the Southwest,” Betancourt says.
For the study, researchers used a growing season index that considers day length, cold temperature limits, and a key metric called vapor pressure deficit to map and compare potential plant responses to major regional droughts during 1953-56 and 2000-03.
Vapor pressure deficit, defined as the difference between how much moisture the air can hold when it is saturated and the amount of moisture actually present in the air, is a key source of plant stress. A warmer atmosphere can hold more water vapor, and during droughts it acts like a sponge sucking up any available moisture from the ground surface, including from plants.
Both droughts—with the more recent one occurring in warmer times—led to widespread tree die-offs, and comparisons between them can help sort out how both warming and drying affected the degree of mortality in different areas.
Multiyear droughts with precipitation well below the long-term average are normal for the Southwest. He said the 1950s drought mainly affected the US-Mexico borderlands and southern High Plains and happened before warming in the region started. The 2000s drought centered on the Four Corners area and occurred after regional warming began around 1980.
The actual causes of physiological plant stress and tree death during droughts are being investigated by various research teams using models and field and greenhouse experiments. One possibility is prolonged embolism, or the catastrophic disruption of the water column in wood vessels as trees struggle to pump moisture from the soil in the heat of summer. The other is carbon starvation as leaves shut their openings, called stomates, to conserve leaf water, slowing the uptake of carbon dioxide needed for photosynthesis. Stomatal closure is triggered by deficits in the ambient vapor pressure, which controls the rate of evaporation for water and is very much influenced by temperature.
“When the air is hotter and drier, it becomes more difficult for plants to conserve water while taking up carbon dioxide,” Weiss explains. “As plants become starved of carbon, it also weakens their defenses and renders them more susceptible to insect pests.”
To make matters worse, Weiss says, the size of the “atmospheric sponge” grows faster during increasingly hotter summers like those over the last 30 years, absorbing even more moisture from soil and vegetation.
“When warmer temperatures combine with drought, relatively stressful growing conditions for a plant become even more stressful,” Weiss says. “You could say drought makes that atmospheric sponge thirstier, and as the drought progresses, there is increasingly less moisture that can be evaporated from soil and vegetation to fill—and cool—the dry air.”
“In a sense, it’s a vicious circle. Warmer temperatures during droughts lead to even drier and hotter conditions.”
The researchers mapped relatively extreme values of vapor deficit pressure for areas of tree die-offs during the most recent drought determined from annual aerial surveys conducted by the US Forest Service.
“Our study suggests that as regional warming continues, drought-related plant stress associated with higher vapor pressure deficits will intensify and spread from late spring through summer to earlier and later parts of the growing season, as well to higher elevations,” the authors write. This could lead to even more severe and widespread plant stress.
The results are in line with other trends of warming-related impacts in the Southwest over the past 30 years, including earlier leafout and flowering, more extensive insect and disease outbreaks, and an increase in large wildfires.
“We’re seeing climatic growing conditions already at an extreme level with just the relatively little warming we have seen in the region so far,” Weiss says. “Our concern is that vegetation will experience even more extreme growing conditions as anticipated further warming exacerbates the impacts of future droughts.
“We also know that part of the regional warming is linked to human-caused climate change. Seeing vapor-pressure deficits at such extreme levels points to the conclusion that the warmer temperatures linked to human-caused climate change are playing a role in drying out the region.”
Betancourt says: “We have few ways of knowing how this is going to affect plants across an entire landscape, except by modeling it. There is not much we can do to avert drought-related tree mortality, whether it is due to climate variability or climate change.”
Instead, land managers should focus on how to manage the regrowth of vegetation in the aftermath of increased large-scale ecological disturbances, including wildfires and drought-related tree die-offs.
“Models like the one we developed can provide us with a road map of areas sensitive to future disturbances,” Betancourt says. “The next step will be to start planning, determine the scale of intervention and figure out what can be done to direct or engineer the outcomes of vegetation change in a warmer world.”
Source: University of Arizona
