Forage Quality and Quantity in Migratory and Resident Mule Deer Summer Ranges

The quality and quantity of forage available to reproductive female ungulates during the summer can influence body condition and lactation, ultimately influencing herd production. The energetic costs of migration can influence individual fitness, thus understanding the mechanisms that drive some individuals in a partially migratory population to migrate, while others remain on winter range year-round, has important conservation and ecological implications. We examined differences in forage quality and quantity between migratory (n = 19) and resident (n = 17) summer home ranges for a population of mule deer (Odocoileus hemionus) in the John Day Basin of northeast Oregon. During 2018, we collected vegetation measurements three times throughout the summer (mid-May, mid-June, mid-July) to determine changes in dry matter digestibility (DMD; %) at three different phenological stages (emergent, mature, and cured) for three forage classes (grass, shrub, forbs). On the second visit, at the peak of the growing season, we collected biomass and cover data to determine differences in forage quantity for each forage class between the migratory and resident summer ranges. In mid-June 2019, we revisited a subsample of sites (migratory [n = 5]; resident [n = 5]) to estimate interannual variation. Migratory deer had access to overall higher quality forage at higher-elevation summer ranges. Forbs had the highest forage quality on both summer ranges. Forage quality declined across the entire study area as moisture declined and vegetation senesced. Shrub forage quality was higher (DMD = 54.1%) on resident ranges than migratory ranges (49.7%; P = 0.001). Grasses had higher biomass (26.1 Mg/ha) and cover (50.1 %) than forbs (19.2 Mg/ha; 34.7% cover) or shrubs (20.6 Mg/ha; 6.7% cover) across the entire summer range. Overall, migratory deer had access to higher-quality forage throughout the summer, yet resident deer had access to higher-quality shrub browse. This trade-off in forage availability may partially explain the persistence of a partially migratory population as a bet-hedging strategy on a landscape where low moisture availability and episodic drought and disturbance create variable foraging conditions.