Spatio-temporal patterns of tree establishment are indicative of biotic interactions during early invasion of a montane meadow

Tree invasions of grasslands are occurring globally, with profound consequences for ecosystem structure and function. We explore the spatio-temporal dynamics of tree invasion of a montane meadow in the Cascade Mountains of Oregon, where meadow loss is a conservation concern. We examine the early stages of invasion, where extrinsic and intrinsic processes can be clearly delineated. In a 0.21-ha plot, we mapped and aged 929 trees a parts per thousand yen0.3-m tall, yielding a detailed record of the spatio-temporal dynamics of invasion. For the primary species, Abies grandis and Pinus contorta, we correlated age structures (unimodal in both species) with climate (precipitation, temperature, and snowpack) and cone production, but found weak or non-significant relationships. Evidence of biotic interactions within and between species was obtained by examining the spatial associations of trees to a distance of 5 m and how these changed over time. We used multiple methods including uni- and bivariate forms of the Ripley's K and pair-correlation function (pcf) (corrected for inhomogeneity), the J-function, an evolving nearest-neighbor metric, and a test for directional bias in establishment. Pinus and Abies contributed in contrasting ways to the pace and spatial structure of invasion. Shade-intolerant Pinus tended to establish in the open, initiating clusters. In contrast, shade-tolerant Abies established in association with Pinus or in conspecific clusters. Preferential establishment of Abies to the north of older Pinus suggests that facilitation occurs by shading. The factors responsible for initial establishment remain unresolved, but positive interactions are pivotal in accelerating invasions, once initiated. Similar processes are likely to occur in other grasslands undergoing rapid conversion to woodland or forest. In combination, analyses of spatial and temporal patterns of establishment provide insight into the processes that structure invasions.