Nutritional Dynamics of Plant Growth Forms in a Forest-Grassland Mosaic

Abstract

The objectives of this study were to determine the influence of plant community type, forest stand age, and season on the forage quality of plant growth forms in a forest-grassland mosaic. We determined the forage quality of plant growth forms (graminoids, forbs, and shrubs) at 64 sites in the Blue Mountains of Oregon, over four dates between May and September over two years. Samples were analyzed for crude protein (CP), acid detergent fiber (ADF), and neutral detergent fiber (NDF). Sites were classified according to climax vegetation and grouped as either forested (ponderosa pine, Pinus ponderosa; Douglas-fir, Pseudotsuga menziesii; grand fir, Abies grandis) or non-forested (meadow or grassland). Forested sites were also categorized as “young” or “old” based on mean diameter at breast height of less than or greater than 37.5 cm, respectively. Graminoid and forb CP in early May were 3.5 and 2.5 points greater in the grand fir community type than the other forested types (P ≤ 0.01) and were 2.7 and 4.9 points greater in the meadow type than the grassland type (P ≤ 0.04). In mid-September, graminoid CP was similar between all community types (P ≥ 0.78). In early May, forb ADF and NDF were similar between all community types (P ≥ 0.18). In mid-September, forb ADF was 10.7 points lower in the ponderosa pine and Douglas-fir types than in all other community types (P ≤ 0.02). Forb ADF and NDF were greater in young stands than in old stands (P < 0.01). Furthermore, forb ADF was 11.4 points greater in the young grand fir community type than all other forested types (P < 0.01). Our study characterizes forage quality across the growing season in diverse vegetation types and, in turn, can be applied to wildlife and livestock management to predict and manage animal landscape distribution.