The lasagne forests of Hawai'i

The key value to forests is their ability, according to a new federal report, is to manage water, and the native lasagne forests of Hawai'i are a key example.

That's lasagne in the sense of layered, not food, since much of the Hawaiian forest is not particularly edible.

(Image: The native Kaua'i forest shrub, mokihana, whose berries are collected and woven into lei that have a distinct anise scent. But you wouldn't want to eat them.)

A new federal study is saying that water is perhaps the most product of a forest. The online journal Science Daily today (July 21) issued a report under the title “Greatest Value Of Forests Is Sustainable Water Supply.”

"Historically, forest managers have not focused much of their attention on water, and water managers have not focused on forests. But today's water problems demand that these groups work together closely," said Oregon State geosciencies professor Julia Jones, vice chair of a committee of the National Research Council, which released the report. She was quoted in Science Daily.

Hawai'i resarchers have figured this out, and water departments work alongside wildlife managers and conservation groups on watershed management teams across the state, to protect native forests.

What's special about Hawaiian native forests as opposed to, for instance, woodlands of non-native species?

One way to determine this is to walk through a woodland in Hawai'i. The planted loblolly pine forests of Kōke'e on Kaua'i have very little other growth under them. Eucaluptus stands in Maui's Upcountry area prevent other species from coming up in their shade. Miconia forests on the Big Island are often nearly entirely miconia, with very little other vegetation able to survive.

When a heavy rain pounds these woodlands, muddy water can flow from them, as the rain erodes the unprotected soil below.

By contrast, a healthy native Hawaiian forest can be layered like a dish of lasagne.

As a raindrop is driven by gravity toward the ground, it first encounters an upper layer of canopy trees, like koa and 'ōhi'a. And then it encounters the shorter trees growing below, the mehame and 'āla'a. And then the ferns like hāpu'u and shrubs like 'a'ali'i. And then the ground ferns, mosses and the dense layers of roots, leaves, rotting branches and the rest.

The upshot, according to botanists, is that all the gravity-fed power of that raindrop to slam into the ground and break up soil particles is gone. Instead of muddy water seeping into streams, the water drips clear from springs and saturated mosses. Those dense forests also inhibit the ground-level winds that suck moisture out of the landscape, and block the evaporative powers of the sunshine.

Here is a list of findings from the report cited in Science Daily:

• Forests provide natural filtration and storage systems that process nearly two-thirds of the water supply in the U.S.

• Demand for water continues to rise due to population growth, while forest acreage is declining and remaining forest lands are threatened by climate change, disease epidemics, fire and global climate change.

• Forest vegetation and soils, if healthy and intact, can benefit human water supplies by controlling water yield, peak flows, low flows, sediment levels, water chemistry and quality.

• Increases in water yield after forest harvesting are transitory; they decrease over time as forests re-grow, and in the meantime water quality may be reduced.

• Impervious surfaces such as roads and road drainage systems increase overland flow, deliver water directly to stream channels, and can increase surface erosion.

• Forest chemicals, including those used to fight fire, can adversely affect aquatic ecosystems, especially if they are applied directly to water bodies or wet soil.

• One of the biggest threats to forests, and the water that derives from them, is the permanent conversion of forested land to residential, industrial and commercial uses.

The Science Daily report is found at http://www.sciencedaily.com/releases/2008/07/080714162600.htm.

© 2008 Jan W. TenBruggencate