South Fork, Part 2

There is a fragment of a real forest in the Oregon Cascades that I call South Fork. It is a thicket of grand firs with scattered older sugar and ponderosa pines, Douglas-firs, and incense cedars. To aid in understanding the dynamics, I present the following graph of age distribution (trees/acre by 25-year age class) compiled from increment cores of sampled trees at South Fork.

Note that the vertical axis is logarithmic. This brings out the detail in the age classes with smaller tree counts. To get this kind of detail with an arithmetic axis would require a graph about 50 page scrolls long. Note also the red lines indicating detected fire years.

Walking through South Fork it is obvious that there are two tree morphologies extant: stand-grown, skinny trees (mostly grand fir) and apparently open-grown, tall but broad-based trees from a variety of conifer species. It is tempting to divide the trees at South Fork into two age cohorts: trees less than 150-years-old and trees greater than 150-years-old. The 150 year break neatly divides the skinny trees from the fat ones. Fire scar evidence points to a fire at that time. Are we looking at a two-cohort forest here?

Before we answer that, let me clarify “fire scars”. Sometimes a forest fire can scorch trees but not the kill them. The scorched cambium may die on only one side of the tree, leaving a burn mark. Burned trees (that live through the fire) often grow new wood over the edges of the charcoaled areas. The growth rings in the over-growth reveal the year of the fire. Sometimes a fire scar may preserve records of multiple fires many years apart.

At South Fork most of the trees older than 150 years have fire scars; none of the younger trees have them. The last fire was 150 years ago, but six more fire years were detected in fire scars. The data suggest that there are at least 8 cohorts represented at South Fork, and probably more. Not every fire leaves a mark, and not every mark was cut into and analyzed. (Indeed, I hated to do more than bore a pencil-thin core or two from a few of the giant trees. Some of the fire scar measurements were taken from stumps in the adjacent clearcuts and the dates inferred to South Fork.)

Most of the trees at South Fork are in the youngest cohort. They became established following the last fire, which must have occurred around 1840. It can take a tree 5 to 50 years or more to grow from seedling to 4.5 feet, especially with shade-tolerant species like grand fir. Hence, the year of germination of each tree is unknown (except that it must predate the breast height age).

The giant trees at South Fork may include members of 7 or more cohorts. Counting all species, the older cohorts together make up 6 to 7 trees per acre. There are over 1,050 trees per acre in the youngest cohort. This forest is, metaphorically, a few punkins in a peanut patch, with the punkins being much older than the peanuts.

South Fork is a unique place in a unique forest, yet its multicohortedness is common in many real forests in the West. Understanding the dynamics at South Fork will aid in understanding other forests, as well. So then what, exactly, has been going on at South Fork for the last 400 years? We have a picture (the graph) but what does it tell us about this forest and its developmental history?

Stay Tuned for the Answers!