Climatic and non-climatic controls shaping early postglacial conifer history in the northern Greater Yellowstone Ecosystem, USA

Mountain ecosystems are characterized by their complex vegetation responses to past climate variability because of the interplay between large-scale climate changes and local-scale biotic and abiotic conditions. This study reconstructs the early postglacial expansion of conifer populations in the northern Greater Yellowstone Ecosystem (GYE). The objective is to examine how climate change and non-climatic factors, including species characteristics, edaphic conditions and disturbance, governed postglacial vegetation changes. Spruce populations expanded first at 13 300 cal a BP, concurrent with soil development and warming summers. Subalpine fir populations expanded after 12 300 cal a BP and probably lagged spruce expansion due to differences in climatic tolerances and/or its poorer seed dispersing capacity. Pine species expanded nearly synchronously after 11 300 cal a BP in response to elevated summer temperatures and increased fire activity. Douglas-fir populations expanded last after 10 200 cal a BP during the early Holocene summer insolation maximum and cooler winters. The sequence and timing of conifer expansions in the northern GYE are consistent with the regional conifer history, indicating strong vegetation responses to millennial-scale climate change associated with the seasonal cycle of insolation across spatial scales. Nonetheless, non-climatic factors, including landscape stabilization and subsequent soil development, seed dispersing characteristics and fire, still shaped local-scale patterns of conifer expansion.