Tiny insects cause big changes

The tiniest of creatures have a big voice in the debate about global warming.

Researchers at Northern Arizona University’s School of Forestry and Department of Biological Sciences are discovering that drought-influenced insects speed up the effects of climate change on soil.

The research reveals that as temperatures rise in response to climate change, insect herbivores, such as moths and minuscule scale insects, are expected to expand their range into new areas, which could ultimately alter the look and health of local forests.

“Understanding how herbivores indirectly alter the soil microclimate will enable scientists to make better predictions about the overall impacts of global climate change on forest ecosystems,” said Stephen Hart, a forestry professor and researcher.

Hart said NAU’s research is unique because it focuses on the cascading consequences caused by the herbivores rather than how they just affect the trees.

Hart, along with NAU researchers Tom WhithamNeil CobbGeorge Koch and Aimée Classen, an NAU alumnus now at Oak Ridge National Laboratory, published their findings in the December 2005 issue of Soil Science Society of America Journal.

“When herbivores alter plant architecture it affects the soil microclimate,” Hart said. “These changes in the microclimate below the affected trees are similar in magnitude to the predicted changes in regional climate over the next century.”

The researchers took advantage of a 20-year ongoing study of herbivores at Sunset Crater National Monument 35 miles east of Flagstaff to test their hypothesis that insect herbivory could indirectly alter rates of soil processes such as plant litter decomposition and nutrient cycling by altering the soil environment.

“Sunset Crater is a neat lab because the trees are already stressed from living in the coarse cinders,” Hart said. “Sunset Crater acts like the canary in the coal mine, providing us with an early warning signal about what might happen to these ecosystems on a more region-wide basis with climate change.”

Two insect herbivores chronically attack piñons at this site. One is the stem-boring moth that attacks adult piñon trying to grow new shoots, and the other is the piñon needle scale, a barely visible bug that gobbles up piñon leaves.

The piñon needle scale insects attack young trees causing them to grow into an open crown allowing more sunlight to hit the soil and resulting in about a 30 percent increase in soil temperature under the tree than would be found in the absence of scale herbivory.

The open canopy also results in a greater amount of precipitation reaching the soil surface, increasing soil moisture by about 35 percent. Both of these changes likely result in increased nutrient availability to the tree.

The stem-boring moth, on the other hand, kills the part of the tree that grows new shoots, causing them to form a dense crown on top. Despite this marked change in tree form, the researchers found that moth herbivory did not alter the soil microclimate.

Hart said the group also discovered some trees that are resistant to both herbivores. They are also trying to determine what in their genetics makes this possible.

“Climate change assessments tend to focus on how regional changes in climate may alter vegetation. However, one of the challenges in predicting the effects of climate change on forests and other ecosystems is understanding how large-scale climatic effects interact with local ecosystem processes, like plant herbivory,” Hart said.

He added that if climate change increases insect outbreaks in southwestern forests in the future, then there may be a shift in the genetic population of trees within piñon woodlands, with associated changes in rates of ecosystem processes like decomposition and nutrient cycling.

“Such alterations in the functioning of these ecosystems may, in turn, feedback to the trees and impact the sustainability of these woodlands over the long-term,” he said.