Forest response to cumulative disturbance and stress: Two decades of change in whitebark pine ecosystems of west-central British Columbia

Forests dominated by the endangered tree species whitebark pine (Pinus albicaulis) are threatened by multiple stresses (fire suppression, climate change) and disturbances (white pine blister rust [Cronartium ribicola], mountain pine beetle [Dendroctonus ponderosae]). To gain insight into how these ecosystems respond, we quantified vegetation change over 2 decades (21-29 y) in xeric and submesic P. albicaulis ecosystems near the northern edge of the species' range on the leeward side of the Coast Mountains of British Columbia, Canada. We compared changes in overstory and understory vegetation composition of these stands to changes in mesic, non-whitebark pine ecosystems in the same region. Multi-response permutation procedure (MRPP) analysis showed that the overstory of xeric whitebark pine ecosystems became compositionally similar to mesic ecosystems, i.e., there was increased dominance by Abies lasiocarpa or Tsuga mertensiana. Yet understory composition in xeric whitebark pine stands changed little and there was continued regeneration of P. albicaulis. Submesic whitebark pine stands developed a dense canopy dominated by T. mertensiana, and although their understories did not become compositionally similar to mesic ecosystems, there was minimal P. albicaulis regeneration. Understory stability in xeric and submesic whitebark pine ecosystems over 21-29 y suggests compositional resilience in these ecosystems to multiple stresses and disturbances. However, ongoing disturbance affecting both overstory and understory P. albicaulis might still result in the loss of this keystone species.