Top‐down and bottom‐up controls limit woody encroachment into persistent temperate rainforest meadows

Abstract

AbstractQuestionsMeadows in montane forests have emerged as a global study system for the accelerating phenomenon of woody encroachment, particularly in the highly productive temperate rainforests of the Pacific Northwest. Study of woody encroachment into meadows in western Oregon, however, has been conducted almost entirely in the western and High Cascades, with only two extant observational studies of grassy balds in the Coast Range. This study evaluates evidence for factors limiting woody encroachment into meadows in the Oregon Coast Range, including bottom‐up control by soil properties, plant–plant interactions and top‐down control by large herbivores.LocationCoast Range, western Oregon, United States of America.MethodsThe chemical and physical properties of soils to a depth of 50 cm in meadow and forest measured, and the community, density and browse pressure for shrubs, conifers, and deciduous trees ≤2 m tall from meadow into forest were recorded. A total of 20Pseudotsuga menziesii(Douglas‐fir) seedlings were experimentally planted in each of five meadows and neighbouring plant presence and herbivore access manipulated.ResultsMeadow soils were lower in C and C:N, slightly lower in N and similar in plant‐available water and pH relative to forest soils. Shrubs were most dense but experienced the lowest browse pressure near the meadow edge, while trees were sparse and varied by site (although at one site, browse pressure was heavier in meadow than forest). Seedling survival and growth varied by site and herbivory‐reduced growth; total soil N best explained residual variation in seedling growth among sites.ConclusionsThese findings indicate that ungulate herbivores exert top‐down control on woody encroachment into temperate montane meadows, perhaps in concert with local N‐limitation.