A tree-community-level analysis of successional status and gap-phase and postfire regeneration of range-margin Thuja plicata (western redcedar)

Range-margin populations may have different life history characteristics than other populations, and the montane forests of Glacier National Park represent the easternmost limit of several western conifer species including Thuja plicata Donn ex D. Don (western redcedar) and Tsuga heterophylla (Raf.) Sarg. (western hemlock). Understanding regeneration strategies of range-margin populations is important for predicting species distribution shifts. We identify successional status, seedling substrate preferences, and the degree to which different species establish within canopy gaps and the forest matrix. Thuja plicata and Tsuga heterophylla were each found to comprise at least 35% of all tree size classes, with six other species each contributing less than 6%. Similarly, other species each comprised less than 5% of canopy gap and understory seedlings. Thuja plicata is typically thought of as a late-successional species but it, along with Tsuga heterophylla, dominated stands during all stages of successional development. Similarly, they dominated both closed forest and gap establishment sites on all rooting substrates but preferred wood for establishment. Even with the likelihood of increased fire frequency, these range-margin populations likely have the capacity to maintain their current distributions. As disturbance regimes are modified across landscapes, it may be possible to observe the potential to adapt to local conditions in other range-margin populations.