Restoring the Hawaiian forest: plant it and the water will come.

It's been more than a century since Hawai'i sugar planters recognized that the dramatic deforestation of Hawai'i in the 1800s, much of it for firewood, had played havoc with water resources.

They began a reforestation effort whose impacts can still be seen today. Much of the reforestation was with hardy and often fast-growing imported species, like eucaluypts, pines, silver oak and others.

(Image: Degraded pasturelands in 1940 on the Edwards Plateau in Texas, where spring flows revived after woody plants were allowed to return. Credit: Charles Taylor and American Geophysical Union.)

The reforestation theory, in part, is that the trees shade the ground, reducing evaporation, and reduce ground-level wind, further reducing evaporation. That gives rainfall an opportunity to soak into the ground and recharge groundwater supplies.

But there have been naysayers who suggest that trees actually suck water out of the ground more than they preserve.

The correct answer, as often is the case, may be that it depends. Not all trees are created equal as protectors of the watershed. And some dryland species like kiawe and tamarisk have been accused of sending down deep roots that suck up groundwater. A welldigger once told me that you could actually see the impact of kiawe on water supplies, causing groundwater levels to drop as soon as the sun rose and the trees started the process of evapotranspiration.

But a complex forest with trees and shrubs does appear to significantly improve an area's water retention.

And not only in Hawai'i.

A new paper to be published by the American Geophysical Union's Geophysical Research Letters says that groundwater levels improved dramatically in Texas after severely overgrazed lands were better managed. http://www.agu.org/news/press/pr_archives/2010/2010-06.shtml

The study says that an “analysis of many decades of historical records for four central Texas river basins challenges widespread perceptions that woody plants have the opposite effects on streams and aquifers.”

Bradford Wilcox of Texas A&M University and Texas AgriLife Research, and former Texas A&M graduate student Yun Huang studied the Edwards Plateau, whose Edwards aquifer provides much of the water for San Antonio. The semi-arid plateau had been overgrazed for a century starting about 1880.

But when stocking rates were reduced, and trees and shrubs came back, they saw a change.

“These landscapes are recovering, but they’ve also converted to woody plants. We’re also seeing large-scale increases in the amount of spring flows. This is opposite of what everybody is presuming —[which is that] the trees are there sucking up all of this water. The trees are actually allowing the water to infiltrate,” Wilcox said.

The flow of water from springs has doubled since before 1950, and the flows in three of the region's four rivers has increased in the same period, they said.

These results will not be a shock to anyone who has walked under the cool shade of the native forest in The Nature Conservancy's Kanepu'u reserve on Lana'i, and then compared the environment to the sun-baked, eroded environment outside the fenced reserve.

© Jan TenBruggencate 2010