Redwood Reader

4. Commentary
REDWOODS: How tall can they grow?
Keay Davidson
Monday, April 26, 2004 ©2004 San Francisco Chronicle
URL: sfgate.com/cgi-bin/article.cgi?file=/c/a/2004/04/26/MNGDV6ARD71.DTL

Today I would like to demonstrate how understanding water carrying capacity of the soil illuminates our thinking on new science and old issues. An article in the SF CHronicle about a paper in Nature, written by scientists from HSU, Pepperdine and Northern Arizona University for Save the Redwoods and Global Forests described measurements about maximum height in redwoods. They find water pressure is the determining factor in tree growth. They say that there are legends of taller trees and that they guesstimated maximum growth under "current environmental conditions" and came out slightly ahead of the old number, which didn't quite fit the evidence. They report decreasing stoma and leaf sizes, and the huge volume of water used by the trees, and air embolisms that shut growth down. While we salute the new study, there are some missing parameters here. What is the water capacity of the watershed the trees are growing in? Are they near any roads or other development? How deep is the soil saturable? A glomalin study correlating ground storage and height would seem a logical next step. This is dry/wet season country and the soil must retain enough water to supply all these trees at "Even a comparatively short 45-meter-high (148 feet) redwood uses 600 kilograms (1,323 pounds, or about two-thirds of a ton) of water daily.", or about 150 gallons a day, 22500 gallons per tree for a five month dry season, 54750 gallons per year. SIx large trees per acre equals 135000 gallons, 328500 gallons per year. One acre inch of rain is about 6600 gallons, times average annual rainfall in HRSP, about 65 inches per year, we arrive at 429000 gallons a little over five acre feet. All rough numbers, of course. Each acre of ground must be able to store a minimum of twenty inches of rain to get through the dry season. A good question is whether soil moisture depletion late in the season might affect growth.
Ground storage we now know is a function of the trees as well. The trees and helper fungi condition the soil to accept and retain water through glomalin production, and pipe it to the trees via the hyphae. We also know rising CO2 levels accelerate carbohydrate production in the leaves, fungi growth, glomalin production, and also decreases stomata size as concentrations rise. Therefore we expect current height records would be vulnerable to accelerated growth rates, in an undisturbed area.
It has been a subject of discussion for some time about the crown death in redwood seen along the 101 corridor by the north end of HRSP by Pepperwood. Scald, wind, bugs, pollution have all been blamed. It occurs to me that the one two punch of removing some of the ground storage capacity together with rearranged drainages causing increased runoff rates are probably reducing soil water storage and associated water pressure, limiting growth and causing dieback. This is a weakened condition for the trees and may make them less resistant to endemic enemies and is ironocally close to the demonstration forest as well.
For foresters and restorationists this is giving us a pretty good handle on why previous Forest Practice Rules were inadequate to protect watersheds, what we need to shoot for, and hope in that rapidly rising global CO2 levels are actully the healing power of nature and can be used by us to great advantage in the near future, indeed now.