Contrasting strategies of xylem formation between black spruce and balsam fir in Quebec, Canada.

Present-day global warming is occurring faster at higher elevations. Although there is much information regarding the divergent responses of tree growth to climate change, the altitudinal patterns of species-specific xylogenesis remains poorly understood. We investigated the xylogenesis of balsam fir (Abies balsamea Mill.) and black spruce (Picea mariana Mill. B.S.P.) at two elevations in Quebec (Canada). The number of enlarging and mature cells of the developing tree ring were counted on microcores collected weekly between 2011 and 2014. At the lower site, the growth pattern and duration of xylogenesis were similar between species. No difference in responses to temperature and solar radiation between species was observed. At the higher site, however, cell production was higher and lasted longer in balsam fir than black spruce. Furthermore, the xylem growth of balsam fir had a stronger response to temperature and solar radiation than black spruce. These findings demonstrate the contrasting strategies of wood formation of the two species, with black spruce being more conservative than balsam fir. Our study provides evidence that sympatric species can have species-specific growth dynamics and site-specific responses to the local environment. Predictions of tree growth under a changing environment require the incorporation of species-specific growth strategies.