Mortality, and Spatial Patterns in an Old-Growth Douglas-Fir Forest

Introduction

There are few trees in the world like those in coastal British Columbia that are able to achieve so ultimate an expression of life in size, stature and life span. They leave nature lovers wondering how the trees and the forests in this region develop into such prominent features.
 

In ecology, the development and dynamics of a forest is called succession, a replacement process of trees and species driven by competition and environmental conditions. It is somewhat like a death-birth process: small/thin trees of shade-intolerant species are constantly phased out, leaving space for big/tall trees and shade-tolerant species to colonize and establish. Accompanying the succession are changes in various aspects, including structure, function and productivity, of ecosystems. If time is permitted and succession keeps going on in the absence of major disturbances (e.g., fire or harvesting), a forest ecosystem is expected to come to a climax or old-growth stage at which the system reaches a relatively steady state and achieves its ultimate function.
 

To understand the successional changes of the forests in coastal BC is the major goal of a research project, the Chronosequences Study, led by the Canadian Forest Service, from which the following data are derived.

Data

The study site is in an old-growth forest (~ 300 years old) located in the Greater Victoria Watershed District on southern Vancouver Island. The forest is dominated by pioneer Douglas fir with significant components of the late-successional species western hemlock and western red cedar. In the summer of 1997, a 102 by 87 m (0.8874 ha) plot was established in the forest. The plot was located at average elevation of 382 m above sea level on a south facing hillside with a slope of about 20º and an elevational range of 33 m from the lowest to the highest point. Three tree species, Douglas fir (DF), western hemlock (HL), western red cedar (CD) occurred in the plot. Trees were classified into three categories:

1. Live trees: Trees with height > 1.3 m tall.
2. Saplings: Trees with height <= 1.3 m tall.
3. Dead trees: Including standing dead, fallen logs and stumps.