ABSTRACT
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.
Keywords: cumulative disturbance, forest change, Pinus albicaulis, resilience.
