Encroachment: Age Structure Analysis on a Forest-Steppe Ecotone

Abstract

Encroachment is a process where woody plants invade grassland, shrubland, or savanna ecosystems, transforming them into forest ecosystems. Several drivers of forest encroachment have been identified, including fire suppression, grazing, and climate factors. Depending on species present, land-use history, fire regime, and climatic differences, the rate of encroachment varies across the world and Montana specifically. The aim of this study is to establish rates of encroachment for a forest-sagebrush steppe ecotone in central-western Montana. An age structure analysis, categorizing trees by age and development stage, was conducted on a 200m transect established on the transition of an encroaching Rocky Mountain Douglas-fir Forest and a Sagebrush steppe on the Dagnall Ranch located 20 miles north of Helena. For all 142 individuals height was measured, and for all 16 mature (greater than 5cm in diameter) trees, the diameter and a core were collected. Annual rings were counted for the cores which determined the germination date of trees and established an age distribution of trees along the transition from forest to field. Aerial images ranging from 1956 to 2025 were trimmed to a 200x50m quadrat surrounding the transect and analyzed to quantify increases in tree canopy area over time. Results indicate the saplings are at the front lines of the advancing forest and suggest that the closed forest edge has advanced 70-80m over a span of 30-40 years, with two bursts of establishment, or germination events, occurring between 1956-67 and 1985-95. This information provides a baseline for future research on a Douglas-fir-sagebrush steppe ecotone in central-western Montana, and a basic timeline of encroachment which may be useful to land managers considering future grassland or forest management plans.