Exotic Grass Invasion Increases Forage Productivity and Reduces Diversity of a High Altitude Mesic Grassland

Abstract

Mesic temperate rangeland, e.g. from the North American mixed grass prairie, Rocky Mountain grasslands and southern hemisphere continents is being invaded by the exotic rhizomatous grasses, Poa pratensis and Bromus inermis. To project the effects of their invasion on one grassland, we compared community properties in- and outside of clones invading a level environmentally homogeneous meadow representative of our high altitude fescue grasslands. Yields increased from native vegetation dominated by Festuca idahoenis (Feid, x= 96 gm/m2) through exotic vegetation dominated by Poa (x= 158 gm/m2) to Brome (x= 258 gm/m2) with little difference in forage quality (protein content). Some, e.g. a grazier, might therefore view the change as beneficial. Measurement of community composition in the three communities showed a considerable impoverishment of the native fescue (Feid) community by exotic invasion, an impoverishment understated by measured reduction in species richness [i.e. from Feid (16-12 species per 1.13m2) through Poa (11) to Brome (4 ); species evenness (Simpson) [i.e. from 88-84 to 81 to 42 ]; and life-form evenness (%forb) [i.e, Feid (42-34%) through Poa (23%) to Brome (5%). The impoverishment probably resulted first, from competitive exclusion of natives by lack of soil resources captured by exotics (whose entry surely demonstrated a superior capacity to acquire water and nutrients) and second, for natives associated with taller Brome, from a reduction of ground-level light, a deficiency confirmed by etiolation of the natives. Conservationists will surely decry the losses. Due to the inexorable rhizomatous spread of Poa and Brome and the resultant impossibility of controlling them, we deduce that managers should accept the foresee-able [vegetation] type-conversion and develop methods for managing/using the new vegetation- - a resigned reaction paralleling the response necessary for other anthropogenic factors: urbanization, N-supplementation, pollution and climate change. We expect parallel responses in other mesic grasslands. Sampling and statistics completed 2014- 20l5. 1st draft reviewed 2016.