Arrested succession in old fields within a temperate rain forest

Abstract

Vegetation of old fields in the temperate rain forest of the Queets River Valley, Olympic National Park, Washington, USA remains dominated by exotic herbs 60 years after abandonment. Exploratory vegetation analyses revealed that seven species dominated the old field vegetation: the persistent European pasture grasses Agrostis gigantea and Anthoxanthum odoratum, and five species that have invaded since abandonment: the exotic Cirsium arvense and Rubus laciniatus; and the native Carex obnupta, Pteridium aquilinum , and Picea sitchensis. The expanding patches of Cirsium, Carex, and Rubus appeared to be successional changes that will not lead to tree establishment. Tree colonization by Picea is more probable in fields that have older alluvial soils with higher organic matter, moisture and acidity; were burned prior to clearing; and were less disturbed by past agricultural use.Study of field edges revealed that invading Picea seedlings were virtually limited to older soils that had Anthoxanthum or Pteridium cover. The most important soil and cover factors to Picea invasion were Agrostis cover (negative correlation) and soil organic matter (positive correlation). Agrostis formed sod and increased ground cover, which may inhibit Picea seedling establishment.In experiments with sown seeds, seedlings of Picea and Tsuga heterophylla emerged as readily in Agrostis as in Pteridium or Anthoxanthum, but suffered higher mortality in Agrostis. Seedling establishment was very low in all cover types and almost nonexistent without experimental reduction of competing vegetation. Exclusion of rodents increased seedling emergence in some sites but not others. Cervid herbivory suppressed growth of Tsuga and Alnus rubra, both on transplanted seedlings outside of fenced exclosures and on natural seedlings in permanent plots. In contrast, Picea was not browsed and grew steadily in height with little mortality. Growth of Picea seedlings may be facilitated by Pteridium.Picea, Tsuga, and Alnus have small seeds and are poorly adapted to establishment in dense vegetation. Competition appears to be the prime inhibitor of tree colonization; it may be intensified by Agrostis, the most abundant cover type on the fields, and by the dry summers of the Pacific Northwest. If past and present dynamics continue, succession of most of the meadows to forest may be arrested indefinitely.