Introduction

Tree encroachment into systems long dominated by herbaceous plants implies a shift in the factors that once limited tree establishment. These factors are typically assumed to be extrinsic, or external, such as changes in climate or disturbance regime (fire or grazing). However, limited attention has been paid to intrinsic factors—including biotic interactions or positive feedbacks—that can maintain or accelerate tree invasions once they have begun.

Objectives. We focused on recent invasions (mid- to late-1900s) at Bunchgrass Ridge—a period for which trees can be precisely aged, losses to mortality are small, climate records are complete, and cone-production data are available. Our objectives were three-fold:

1. to assess the contributions of climate variation, seed availability, and biotic interactions (inferred from spatial associations) to establishment of lodgepole pine (Pinus contorta) and grand fir (Abies grandis)

2. to quantify the nature and strength of interactions among trees and how these change during the invasion process

3. to illustrate how multiple sources of evidence—including population age structures, climate and cone production data, species’ life histories, and spatial associations—can provide insight into the dynamics of tree invasions

In a 0.21 ha (30 x 70 m) plot supporting recent establishment, we mapped and aged 929 trees >0.3 m tall.

For the primary species, grand fir and lodgepole pine, we correlated age structures with climate records (precipitation, temperature, and snowpack) and regional data on cone production (available for grand fir only).

We inferred the importance of biotic interactions from spatial associations among trees and how these changed over time. We used a combination of methods to explore spatial patterns at increasingly finer temporal resolution: inhomogeneous variants of the Ripley's K and pair-correlation functions (for both uni- and bivariate tests), the J-function, and a novel evolving nearest-neighbor metric.