Lasting Drought Effect In The Rainy Eastern US

April Flowers for – Your Universe Online

So far, the spring of 2014 has seen more than 40 percent of the western US in a drought that the USDA has deemed “severe” or “exceptional.” In 2013, the drought was just as severe, and in 2012 it spread to the humid eastern states.

Looking at the effects of a drought in geological terms, it would be easy to assume that a three-year drought is inconsequential in the long term, despite the devastating effects it might have on farmers and crops in the short term.

A new study published in a recent issue of Ecological Monographs and led by researchers at the Harvard Forest and Columbia’s Lamont-Doherty Earth Observatory, reveals that short-lived but severe climatic events can trigger cascades of ecosystem changes that can last for centuries.

Scientists have found some of the most compelling evidence of ecosystem response to drought and other challenges in the trunks of the oldest trees. The team analyzed tree rings from a study area of over 300,000 square miles in the eastern US and up to 400 years old. Their results point to ways in which seemingly stable forests could abruptly change over the next century.

“Trees are great recorders of information,” Dave Orwig, an ecologist at the Harvard Forest, said in a recent statement. “They can give us a glimpse back in time.”

Across the broadleaf forests of the eastern US — Kentucky, Tennessee, North Carolina and Arkansas — the tree records revealed that the simultaneous death of many trees opened huge gaps in the forest. These gaps allowed for the growth of a new generation of saplings.

The team searched for historical records that would show that the dead trees succumbed to logging, ice storms or hurricanes. No such records were found, however. Instead, the researchers believe that the trees were weakened by repeated drought leading up to the 1770s. An intense drought from 1772 to 1775 further weakened the forest. An unseasonable and devastating frost in 1774 was the final coffin nail for these forests. Until this study, that frost was only known to historical diaries, such as Thomas Jefferson’s Garden Book, where he recounts “a frost which destroyed almost every thing” at Monticello that was “equally destructive thro the whole country and the neighboring colonies.”

The large gaps in the forest created by these severe events allowed for an oversized generation of new trees — a baby boom type satiation — that reshaped the old-growth forests that still stand in the Southeast today.

“Many of us think these grand old trees in our old-growth forests have always been there and stood the test of time,” said Neil Pederson of the Lamont-Doherty Earth Observatory. “What we now see is that big events, including climatic extremes, created large portions of these forests in short order through the weakening and killing of existing trees.”

Pederson will join Harvard Forest as a senior ecologist in the fall of 2014. He notes that as climate warms, extreme events like increasing drought conditions and earlier springs like the one in 1774 could easily recreate the conditions that changed the eastern forests so abruptly in the 17th and 18th centuries.

“We are seeing more and more evidence of climate events weakening trees, making them more likely to succumb to insects, pathogens, or the next severe drought,” said Orwig.

Pederson added, “With this perspective, the changes predicted by models under future climate change seem more real.”

Image 2 (below): The rings of old-growth trees – one ring for every year the tree has been alive – can reveal centuries of forest change. Drought often results in very narrow spaces between rings. Photo by Neil Pederson