by Steve Lovett
May 31, 2002

Beyond the implications of politically charged buzzwords like “old growth”, there is a science which has been emerging over the last 20 years that focuses on the inner workings of old growth forests. Old growth forests, also called late successional forests, are characterized by large, uninterrupted stands of trees with various ages and sizes creating a multi-layered canopy, interlaced with downed trees and standing dead trees. Most agree that forests begin to develop these characteristics anywhere from 100 to 350 years of age. These stands maintain a steady state of reproduction through the death and rebirth of individual or a small collection of trees. Late successional forests provide unique structure and habitat features unrivaled by the closed canopy of second growth and the exposure of open lands. They create their own climate conditions such as moderate temperatures, higher humidity, lower wind speed, and slower temperature change. The massive old trees also receive and provide moisture through capturing summer fog. While academics and foresters debate the definition and amount of late successional forests left in the U.S., much can be agreed upon. Research has revealed that the canopy, snags and downed trees, soil and wildlife in old growth forests are biologically unique.

The multi-layered canopy of the old growth forest creates a distinct vertical structure. Frequently, as trees age they develop dead crowns, which may serve as nesting platforms for soaring raptors such as eagles. Below the crown, the massive branches provide ideal nesting structures for an ever-increasing list of threatened old growth associated birds. After decades of the canopy dropping its needles on the branches below, a soil mat of up to three feet thick can form on ancient trees where enormous limbs meet the trunk. These soil mats create a habitat in the canopy for animals such as salamanders and red tree voles threatened by predation of other terrestrial animals. Changes in the canopy are essential to maintain the integrity of late successional forests. Trees eventually succumb to lightening, fire, insects, disease, wind or age; this creates a shifting mosaic of openings and closings in the canopy of old forests. Openings suddenly increase solar radiation and temperature, which promotes vigorous growth for sun dependent tree, shrub and herbaceous plant species. In fact, some botanists believe certain rare plant species may have evolved in the gaps of old growth forests.

The dead tree that once covered the gap in the canopy continues to provide life to the forest. Large downed trees continue to serve the forest through stabilizing steep slopes, slowing swift moving streams, creating a nurse log for other trees and plants to grow and absorbing vast amounts of water which can be retained during the dry months. In addition, rotting logs contain a higher sheer mass of living tissue than a living tree. This is due to the high concentration of insects and fungi feeding and growing inside the log. The rough bark on mature Douglas fir creates an ideal growing medium for mosses, lichen and fungi, which in turn will provide essential nitrogen to the tree after falling to the forest floor. The wide variety of species of fungi found in the soil assist in both the exchange of nutrients and water to trees. There are more fungal associations in late successional forests than in other stages of forest development. These associations are referred to as mycorrhizal, which literally means “fungus root”. Mycorrhizae create intricate threads beneath the forest soil connecting organic material from tree, shrub, and herb roots to decaying logs, forest litter or animals. Individual trees may depend on many different species of fungi to assist in their growth. Plants receive phosphorus and nitrogen from the fungi and give sugars and amino acids in return. In addition, mycorrhizae inoculate trees against disease.

Certain animal species have specialized to the interior old growth forest conditions. Many old growth forest species are elusive due to the remoteness of most functional late successional forest and the low population numbers from habitat loss. While some interior forest species may occasionally be found in second growth, their ideal habitat is vast old growth forests. Interior forest species include not only the spotted owl, but a myriad of endangered and threatened species including the marbled murrelet, vaux’s swift, northern goshawk, cooper’s hawk red tree vole, humboldt martin, pacific fisher, tailed tree frog, southern torrent salamander, red-legged frog and wild salmon.

The large remnant fire-scarred trees tell us our landscape has been shaped by fire. Old growth forests have the ability to slow wildfires. This function is created by several factors, the thick bark of the Douglas-fir, less of a fuel ladder, higher humidity and moisture laden logs. The closed canopy and brushy second growth that surrounds us only increases the probability of a large wildfire.

The more we study late successional forests the more we realize the complex interactions and symbiotic relationships. Scientists believe that the amount of remaining late successional forests in the United States is between 8-15% of what originally existed, most of this is in the Pacific Northwest. The preservation of large blocks of contiguous late successional habitat is essential for the recovery of endangered and threatened species. Over 90 percent of the late successional forest found in the Mattole watershed has been removed, however the largest coastal old growth Douglas fir forest in California is located in the Upper North Fork of the Mattole River. Over the next 10 years this natural history legacy is threatened by the timber operations of Pacific Lumber Company. As scientists and naturalists continue to unravel the secret of the old forests, the public is becoming more aware of this endangered habitat. Hopefully we can save the remaining old growth forests before fragmentation and attrition degrade what is left.



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Table of Contents for Mattole Restoration Newsletter, Issue 18 - Summer/Fall 2002