California is getting hot under the collar. Average temperatures in the Golden State have been on the rise for 90 years. Nighttime heat waves have become more prevalent and more severe since 1990. These trends are likely to intensify, say scientists. Up to nine record-breaking heat waves are expected to hit the western United States every decade from 2050 to 2100.
Humans and other animals can walk, slither, or fly their way to more temperate sites. Plants, however, are literally rooted in place. With no easy means of escape, plants can only endure the frequent, blistering heat spells of the future.
UC Berkeley biologist Michal Shuldman is studying how native plants cope with current heat waves. A doctoral candidate in Professor Todd Dawson's plant physiology laboratory, Shuldman says the timing of heat waves will be crucial in California's Mediterranean climate.
"If you have a heat wave in the spring, when soil water availability is high, how is that different from a heat wave in the fall, when there's not much water available in the soil, but the plants may have acclimated to heat over the summer?" Shuldman asks.
Current climate models predict that the heat wave season in California will lengthen by 5 to 13 weeks by 2100. However, these projections don't indicate whether the mercury will spike earlier or later in the year.
Shuldman is examining both scenarios. At the NRS's Quail Ridge Reserve in Napa County, she is observing native plants before, during, and after blistering spring and summer heat spells. Like other weather phenomena, both the timing and the severity of heat waves are notoriously difficult to predict. For this reason, Shuldman is one of few scientists to go into the field to study their effects. What she is discovering should help other scientists to model how native plants might weather tomorrows soaring mercury.
Sweating for Science
In spring and fall of 2010, Shuldman scrutinized weather forecasts for early inklings of hot spells. At the first word of a coming scorcher, she sprang into action. She loaded coolers with Gatorade and ice, packed up a portable shade structure, and grabbed her floppy hat. She also headed for the lab to cajole volunteers into eschewing campus pools and air conditioning in favor of the nostril-searing heat of chaparral.
In the field, Shuldman gives her attention to toyon. In fall and winter, this woody shrub produces bounteous clusters of crimson berries. During the rest of the year, elongated, toothy leaves with rounded tips are its most salient feature.
At Quail Ridge, Shuldman and her teams took the plant equivalent of vital signs. "If you went to the doctor, they might use a stethoscope, peer into your ears, and take your temperature and blood pressure to assess your overall health. We bring out cartloads of equipment and devices to monitor how the plants are performing," she says. She and her team go, home with thousands of data points.
Having to repeat tests all day long with so much gear explains why Shuldman requires help. "You can't run five different pieces of equipment in the field by yourself — you need a team," Shuldman says.
Her team has proven its mettle under the merciless sun. Temperatures at Shuldman's site at Quail Ridge Reserve climbed to 105 degrees Fahrenheit during October 2010. Shuldman and her helpers soak their hats and handkerchief turbans before venturing outside, and they measure their samples in the shade of the tent whenever possible.
On hot days, a plant's prime directive is staying hydrated. Plants maintain a continuous water column that stretches from their roots, through their stems, and out to the atmosphere via their leaves. Shuldman envisions this water column as a string. "If the plant is losing a lot of water to the air, the string is getting pulled tight. If it gets too tight, the string will snap. If that happens, the plant can no longer move water," she says.
To gauge the tension of a plant's water column, scientists use a contraption that looks like something that might have been invented by Rube Goldberg. The procedure that accompanies the device requires plucking a leaf from a plant and placing it in a sealed chamber. The leaf is positioned so that only its stalk protrudes into the open air. The chamber is then pressurized with gas from a cylinder. The pressure required to push water from the stalk is the plant's water potential.
Shuldman camps at her plots the night before and rises before the sun to take her first water-potential measurements. Plants lose almost no water at night, because they aren't photosynthesizing. As a result, their tissues come back into equilibrium with the water in the environment. She compares the predawn numbers against those taken during the day to determine how much soil water is available to plants.
"Spring heat waves could be really bad for seedlings that are germinating. An increase in two degrees to a tiny sprout next to the soil is very different than to a ten-foot-tall shrub. That could affect recruitment" of new seedlings, Shuldman says.
Adult plants might fare better. Shuldman has measured how open the pores, or stomata, on each leaf are. Well-hydrated plants hold their stomata open longer during heat waves. This allows evaporating water to cool the plant in a manner akin to sweating. "Adult plants could utilize that water to cool their leaves and ameliorate some of the heat stress when water is plentiful in spring. But that might use up more of the set amount of water available for the year," she says. In fall, however, the plants did not keep their stomata open longer and, if anything, were likely to close them.
Turned Off in Autumn
A contented plant — whose requirements for water, nutrients, and light are being met — is one that is photosynthesizing. The amount of light energy a plant reflects is a measure of how active it is. Plants not receiving enough water to photosynthesize will bounce unused light back into the atmosphere. This phenomenon, which helps plants avoid sunburn, is called fluorescence. (Plants also dissipate some unused light energy as heat.)
"Typically, toyon, like most plants, will be most active in the morning, shut down to take a siesta midday, and some will turn back on in the late afternoon," Shuldman says. This activity pattern is common to plants in water-limited ecosystems, such as those in many parts of California. By fall, however, that schedule drops by the wayside. "Even before the heat wave happened, I could tell some of my plants were stressed. They were shut down, as if they were saying, 'I give up. I'm not doing anything anymore.' "
By autumn, plants have acclimated to increasing temperatures over the summer. For example, they may be producing special proteins to stabilize their cells in high heat. Yet these adaptations may be canceled out by drought conditions, when moisture becomes too scarce to squander on cooling.
For toyon, fall heat could be particularly harsh. Autumn is its fruiting season, when toyon earns the monikers Christmas berry and California holly. If water is too scarce in fall for plants to support reproduction, the result is sterility. Drought-stricken plants either abort their fruit or put out poor-quality ones. About 90 percent of the seeds Shuldman collected during relatively dry 2009-10 failed to sprout, versus the 85 percent from wetter 2008-09 that did sprout.
Over the long term, Shuldman's research should help shepherd sedentary native plants into an uncertain future. Understanding how such plants as toyon cope with extreme temperatures will help refine models of the types of conditions other native species can tolerate. The most extreme climate scenarios, after all, are the ones that plants must aim to survive, just as big 100-year floods, not average inundations, are the ones that humans fear. With more detailed information on how plant ranges will shift, Shuldman says, "We can inform the choices that we make. We can buy land, manage the land that we have, and save corridors from development that might be useful for animals and plants to move to. It would help us manage things better if we knew what populations were more likely to be at risk." — KMW