Spatio-Temporal Patterns of Tree Establishment in the M1 Meadow of the HJ Andrews Experimental Forest

Montane meadows in the Cascade Range of Oregon have been declining due to tree establishment since records began. Montane meadow complexes in the H.J. Andrews Experimental Forest shrank by 60 to 75% from 1949 to 2005, but fine scale temporal and spatial processes of tree establishment in these meadows are unknown. In the 4.8-ha M1 meadow in the Andrews Forest, the species and diameter of all trees of any size were measured in seven 20x20 m plots and one 40x55 m plot in August 2015. A total of 1385 trees were sampled, including 1008 in the 40x55 m plot. In the 40x55m plot, all trees were mapped to the nearest 0.1 m and 252 were aged to the nearest +/- 2 years. Spatial patterns of tree locations were assessed using second order analysis (Ripley's K, pair correlation function [PCF], J-function). In the 40x55m plot, the oldest tree established in the 1870s, and tree establishment has accelerated since 1950, as invasion has progressed from the edges toward the center of the meadow. Nine conifer species occurred in the plots, but almost 84% of individuals were Abies amabilis (Pacific silver fir, ABAM) or Abies grandis (grand fir, ABGR), and 8% were Pseudotsuga menziesii (Douglas-fir, PSME). PSME and ABAM dominated the basal area, and >90% of ABAM and ABGR and 50% of PSME individuals were
6 m, and trees have become increasingly clustered over time (J-function). However, trees aged 30 yrs, and evidence of facilitation was weak. Thus, all conifer species appear capable of establishing in the montane meadow, but in many cases trees established several decades before they began to grow rapidly. Based on current rates of tree establishment, the meadow will continue to shrink in the future. Because many trees are small, complete tree removal is an initial step toward meadow restoration in this and comparable sites. However, additional approaches will be necessary to counteract legacy effects of trees on soils. Future work should combine fine-scale analysis of tree spatial patterns with environmental (light, moisture and soil) data in order to reveal the local environmental drivers of tree establishment patterns in montane meadows.