Age Distributions of Spruce and Fir in Beetle-killed Forests on the White River Plateau, Colorado

Abstract

In the 1940's an epidemic of the Engelmann spruce beetle (Dendroctonus engelmannii Hopk.) killed almost all the spruce (Picea engelmannii (Parry) Engelm.) and most of the fir (Abies lasio- carpa (Hook.) Nutt.) larger than 10 cm in diameter breast height, on the White River Plateau, in northwest Colorado. Reconstruction of the age distributions of spruce and fir at the time of the beetle attack were made by counting all the dead trees, stumps and live trees in two stands and estimating the ages of these trees at the time of the beetle attack. The resulting age distributions indicate a reduction in spruce reproduction in the late 1700's, while fir reproduction was relatively unchanged. An epidemic of Engelmann spruce beetles in the early 1700's is suggested to explain these age distributions. Fire, climatic change, or grazing and trampling by cattle are less likely hypotheses. INTRODUCTION AND BACKGROUND The White River Plateau is a subalpine plateau located in north- western Colorado, north of Glenwood Springs, in Garfield and Rio Blanco counties. The plateau is bordered on the south and southeast by the Colorado River, on the northeast by the Yampa River, and on the north by the north fork of the White River. The central part of the Plateau is drained by the south fork of the White River. Most of the present surface of the Plateau lies between 3050 and 3350 m elevation and may be described as rolling, broken here and there by rocky out- crops and deep canyons cut by the major streams draining the area. The vegetation is composed of about 50% forests of Engelmann spruce (Picea engelmannii (Parry) Engelm.) and subalpine fir (Abies lasio- carpa (Hook.) Nutt.) and about 50% meadow. Forests of spruce and fir with patches of meadow predominate on north and northeast ex- posures while meadows containing isolated spruce trees and patches of several spruce and fir predominate on south and southwest exposures. In the 1940's, following a severe wind and blowdown, an epidemic of Engelmann spruce beetles (Dendroctonus engelmanni Hopk.) killed almost all spruce larger than approximately 10 cm diameter breast height. The beetles killed the larger trees first and then pro- gressed through smaller trees. Between 1940 and 1944 the average diameter of trees killed was 53 cm; in 1945, the average diameter of trees killed was 43 cm, and in 1946, 38 cm. The average diameter of live spruce at the end of 1946 was 34.3 cm. By 1948 most of the spruce larger than 10 cm diameter breast height - were dead (Massey and Wygant, 1954). Some of this beetle-killed spruce was lumbered in the early 1950's. Preliminary reconnaissance in 1962 and 1963 indicated that at the time of the 1940 spruce beetle epidemic the forests were not composed of an uneven-aged stand of trees, as one might expect if the forest had