Tree invasion of a montane meadow complex: temporal trends, spatial patterns, and biotic interactions



Questions: Do spatial and temporal patterns of encroachment of Pinus contorta and Abies grandis in a montane meadow suggest strong biotic controls on the invasion process?

Location: Forest–meadow mosaic, 1350 m a.s.l., Cascade Range, Oregon, US.

Methods: We combined spatial point pattern analysis, population age structures, and a time-series of stem maps to quantify spatial and temporal patterns of conifer invasion over a 200-yr period in three plots totaling 4 ha.

Results: Trees established during two broad, but distinct periods (late 1800s, then at much greater density in the mid-1900s). Recent invasion was not correlated with climatic variation. Abies grandis dominated both periods; P. contorta established at lower density, peaking before A. grandis. Spatially, older (≥90 yr) P. contorta were randomly distributed, but older A. grandis were strongly clumped (0.2-20 m). Younger (<90 yr) stems were positively associated at small distances (both within and between species), but were spatially displaced from older A. grandis, suggesting a temporal shift from facilitation to competition. Establishment during the 1800s resulted in widely scattered P. contorta and clumps of A. grandis that placed most areas of meadow close to seed sources permitting more rapid invasion during the mid-1900s. Rapid conversion to forest occurred via colonization of larger meadow openings – first by shade-intolerant P. contorta, then by shade-tolerant A. grandis– and by direct infilling of smaller openings by A. grandis.

Conclusions: In combination, spatial and temporal patterns of establishment suggest an invasion process shaped by biotic interactions, with facilitation promoting expansion of trees into meadows and competition influencing subsequent forest development. Once invasion is initiated, tree species with different life histories and functional traits can interact synergistically to promote rapid conversion of meadow to forest under a broad range of climatic conditions.