Encroachment: Age Structure Analysis on a Forest-Grassland Ecotone

Abstract

Encroachment is a process where woody plants invade grassland ecosystems, transforming them to forest ecosystems. The exact driver is uncertain, but some studies indicate that a lack of disturbance may accelerate the process. Research has predominantly focused on negative and positive impacts of encroachment on the adjacent ecosystems, yet there remains a gap in our understanding of how forests encroach and at what rate. Aiming to fill this gap, I conducted an age structure analysis, which categorizes trees by age and development stage, of an encroaching forest on the Dagnall Ranch located about 20 miles north of Helena. On a gentle northwestern facing slope, I established a 200m transect across the transition zone, or ecotone, between the grassland ecosystem and the forest. I measured height and basal diameter of all trees and saplings within 2 m of the transect, and collected cores from 16 trees greater than 5cm in diameter. I counted annual rings in all of the cores, performed statistical analyses to cross-date the rings and determine germination date of trees, and ultimately established an age distribution of trees along the transition from forest to field. Initial results indicate the youngest trees were at the front lines of the advancing forest and suggest that the closed forest edge is advancing significantly over a span of 30-40 years. This information provides a basic timeline of encroachment, and may be useful to land managers considering future grassland or forest management approaches.