Understory patch dynamics and ungulate herbivory in old-growth forests of Olympic National Park, Washington

Abstract

The relationship between native ungulates (mainly Roosevelt elk, Cervuselaphus L.) and the occurrence of three patch types in an old-growth (220- to 260-year-old) Sitka spruce (Piceasitchensis (Bong.) Carrière)–western hemlock (Tsugaheterophylla (Raf.) Sarg.) temperate coniferous rain forest was investigated on the South Fork Hoh River in Olympic National Park. The distribution, frequency, and size of two understory patches (grass, moss) and patches where shrubs had escaped herbivory (refugia) were sampled along transects. Vegetation standing crop, percent cover, species richness, and equitability along transects were compared with conditions in two 8-year-old 0.5-ha ungulate exclosures. Ungulate herbivory profoundly affected the distribution and abundance of understory patch types. Grass-dominated patches disappeared following 8 years of protection from ungulate herbivory. Ungulates maintained a reduced standing crop, increased forb species richness, and determined the distribution, morphology, and reproductive performance of several shrub species. There is clearly a dynamic relationship between patch type, tree fall, and ungulate herbivory in these old-growth forests. Our results show that ungulate herbivory is a driving force shaping vegetation patterns in coastal coniferous forests.