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Long-term droughts are throttling growth in Hawaiian forests, finds airborne laser-based study

The scarlet honeycreeper. Credit: Cari Lynn Squibb for USGS.

  • Long-term declines in rainfall on the Big Island of Hawaii have added up over time to make forests shorter and less green.
  • Data from satellites and airplane surveys showed that forest canopy greenness decreased twice as much in areas where annual rainfall had steadily declined since 1920.
  • Long-term drying trends in other parts of the world may have far-reaching impacts on forests.

A plant without water will wither and, eventually, die. This is easy to observe in the browned leaves of a neglected houseplant, but what happens when an entire forest gets less and less rain over the course of nearly 100 years?

To address this question, researchers used satellites and a high-tech airplane to spy on the native forests of Hawaii’s Big Island. The effects of drought do not end when rains return, the study showed. Rather, these impacts add up over time to make forests shorter, less dense, and less green.

Unprecedented droughts and large-scale decreases in average rainfall are affecting many locations around the world, climate data has shown. Scientists have long wanted to know how these changes will affect tropical forests. But such projects require high-quality weather data that spans decades, and it is difficult to make precise observations for vast swaths of forest.

The Carnegie Airborne Observatory used a laser tool called LiDAR to detect the three-dimensional structure of the forests and terrain below the canopy. Photo courtesy of Gregory Asner/Carnegie Institution for Science

Ecologists Jomar Barbosa and Gregory Asner from the Carnegie Institution for Science at Stanford University met these challenges by using recent advances in remote-sensing technology. They also used a unique, long-term weather dataset on Hawaii’s Big Island, where rainfall has declined on one side of the island for the last century—affecting the native forests that grow on the volcanic slopes.

For their study, the researchers used data from the Carnegie Airborne Observatory (CAO)—an airplane designed by Asner to study ecosystems at this scale. The plane carries nearly a metric ton of computing and remote-sensing technology.

“Work on the ground is very hard to translate into the big picture,” said Asner in an interview. “The airborne observatory was conceived to try to draw out the major findings derived from all these different landscapes.”

Changes in forest photosynthesis on Hawaii’s Big Island from 2002 to 2016. Red: decrease in photosynthesis. Green: increase in photosynthesis. White: other land-cover classes such as bare soil, invasive species, or human settlements. Graphic from Barbosa and Asner, Environmental Research Letters (2017).

The study also used data from NASA’s Terra satellite, which tracked the greenness of forest canopies on the Big Island between 2002 and 2016. Greenness reflects how much photosynthesis is taking place. Less photosynthesis reflects a decline in plant growth—which means the forest can’t support as much life.

The CAO flew over two patches of the Hawaiian forest in 2007 and 2016. One patch was on the island’s wetter side, and the second was on the island’s drier side. A laser instrument fired 400,000 pulses per second to measure the canopy’s height in detail. Taller trees typically contain more biomass, pointing to more sustained growth over time.

Waipio Valley on the Big Island of Hawaii. Photo by Eric Tessmer/Creative Commons

To connect the changes seen in canopy height and greenness to rainfall, the team used data developed by researchers at the University of Hawaii to trace the island’s rainfall by year and by location from 1920 to 2012. This allowed Barbosa and Asner to pinpoint the driest spots on the island at various time scales.

They found that forests which had seen less and less rain since 1920 were shorter and two times less green than they would have expected from the effects of short-term droughts alone. Browned leaves and dying vegetation are common during extended droughts. But these results show for the first time that when rainfall declines over much longer time scales, the damages to forests pile up.

“They were able to distinguish the loss of greenness from short-term versus long-term drying trends,” said Christian Giardina, a Hawaii-based ecologist with the U.S. Forest Service who was not involved in the study. “This kind of analysis highlights the importance of long-term climate data and paves the way for future studies that seek to integrate short-term and long-term datasets where they are available,” Giardina told Mongabay.

Applying this work to managing important forests may be easier said than done, Asner said. “It’s time for land managers and policymakers to consider alleviating some of the other stresses on forests, like fire or invasive species,” he said. “Then you’re giving the forest a better chance of surviving this long-term decrease in precipitation. When I say this in meetings people know it’s the answer, but it’s hard to get it done.”

The I’iwi or scarlet honeycreeper, a native Hawaiian forest bird. Photo by Bettina Arrigoni/Creative Commons

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Header image: Pair of scarlet honeycreepers. Credit by Cari Lynn Squibb for USGS.
 

Alex Fox is a graduate student in the Science Communication Program at the University of California, Santa Cruz. Other Mongabay stories by UCSC students can be found here.