Monday Bites: Coral Bleaching, Drought News, and Doomed Dinosaurs

Corals Losing Practice Run

Two weeks ago, we reported on the dramatic coral bleaching event affecting the Great Barrier Reef. Researchers have now found that many of these reefs are usually able to withstand the warmer waters that cause bleaching because they get practice. Tracy Ainsworth from James Cook University explains that bleaching is like a marathon for corals: “When corals are exposed to a pre-stress period in the weeks before bleaching, as temperatures start to climb, this acts like a practice run and prepares the coral. Corals that are exposed to this pattern are then less stressed and more tolerant when bleaching does occur.”

Early evidence suggests that the current bleaching event in the Great Barrier Reef has followed this warm-up pattern. But future events could be without this practice run, the scientists warn in Science. “When corals lose the practice run, there is no break, or relaxing for the corals as summer stress develops,” explains Scott Heron, from Coral Reef Watch at NOAA. “In future summers, bleaching events will occur more often and, without the practice run, become even more severe—with a greater risk for coral mortality and a fast decline in coral cover across reefs.”

More Drought News

Last week the Public Policy Institute of California (PPIC) released a drought response report card, assessing the way the governments, farmers and others reacted to our severe drought. The good news? City governments, agencies and residents did a good job sharing our limited water resources, receiving an A-. The bad news? The state’s response to the environmental drought crisis. This effort received a D because “California’s rivers, wetlands and forested lands — and the biodiversity they support — suffered major harm. Vast numbers of trees in our poorly managed forests are dead or dying, increasing the risk of catastrophic wildfire,” according to the PPIC’s commentary in the San Francisco Chronicle.

Speaking of forests and drought, a study published today in the Proceedings of the National Academy of Sciences identifies tree traits that may contribute to drought vulnerability. With twelve million trees lost this past year in California, these data could be helpful to forestry managers in future droughts. William Anderegg of the University of Utah and his colleagues reviewed previous tree mortality studies, noting the mortality rates for each species, then compared those to ten tree physiological traits. The traits included wood density, rooting depth and basic leaf characteristics, such as whether the species was an evergreen or a deciduous tree. Other traits concerned the hydraulics of how water moves through trees.

Not surprisingly perhaps, the greatest cause of mortality during a drought is hydraulics. Tension builds in the tree’s “pipes” as they work harder and harder to move water, like a person trying to suck a very thick smoothie through a small straw. At a certain point, the tension on the pipes throughout becomes so great that bubbles of air enter the pipes and block the flow of water. It's called an embolism. “It's a little bit akin to a tree heart attack,” Anderegg says. “You can actually hear this on a hot summer day if you stick a microphone up to a tree—you can hear little pings and pops as these pipes get filled with air.”

Dinosaur Decline Before Extinction Event

How were dinosaurs faring before the Chicxulub impact 66 million years ago? Not so great, according to a new study in the Proceedings of the National Academy of Sciences. Scientists created a model to look at extinction and speciation rates of our prehistoric friends and found that extinction rates surpassed speciation rates beginning around 24 million years before the impact. In fact, when the three major dinosaur groups were separately considered, species were going extinct faster than new species were evolving as early as 48-53 million years before impact. The team is unsure of the reasons for the decline of speciation during that period, but the breakup of supercontinents, sustained volcanism, and ecological factors may have influenced the downturn. In a paper several years ago, the Academy’s Peter Roopnarine determined that ecosystems were more vulnerable before the asteroid hit. Perhaps it was a devastating combination of many of these factors for dinosaurs, leaving them unable to recover from the mass extinction event.

Image: Peter Mumby