Canopy gaps facilitate upslope shifts in montane conifers but not in temperate deciduous trees in the Northeastern United States

Many montane tree species are expected to migrate upslope as climate
warms, but it is not clear if forest canopy gaps, which can facilitate
tree seedling recruitment, serve as an important mechanism driving tree
species range shifts. Patterns of tree seedling establishment can inform
us about early stages of tree species migrations and are critical to
examine in the context of global climate change. We contrasted elevational
distributions of tree seedlings both within and outside of forest canopy
gaps with the distributions of conspecific adults and saplings across the
deciduous-coniferous ecotone on ten mountains in four states of the
northeastern United States. We tested if seedling distributions of four
dominant tree species (Abies balsamea, Picea rubens, Acer saccharum, and
Fagus grandifolia) were shifted upslope of conspecific adult and sapling
distributions. We also examined if this shift was facilitated by canopy
gaps and what environmental drivers affected species distributions. There
was limited seedling recruitment of dominant tree species at the
temperate-coniferous ecotone, which we attributed to (i) an observed
downslope shift of seedling distributions of the low-elevation deciduous
species (Acer saccharum, Fagus grandifolia) and (ii) an upslope shift in
seedling distributions of the high-elevation conifers (Abies balsamea,
Picea rubens). The upslope shift of conifer seedlings contrasts with our
previous research at these sites which observed downslope shifts of
sapling distributions in Picea rubens, suggesting that seedlings may be
responding to more recent climate warming. Canopy gaps in high-elevation
conifer forests facilitated these upslope shifts by promoting conifer
seedling recruitment. However, gaps at lower elevations did not play a
significant role in seedling recruitment or the observed downslope shifts
of the dominant deciduous species. Climate was the dominant predictor of
adult tree distributions whereas both climate and soil were important
predictors of seedling distributions. Synthesis. Our study illustrates
that tree seedlings have the potential for monitoring the early stages of
tree species migrations, and particularly so in canopy gaps in
high-elevation conifer forests. Further, we stress that species range
shifts are sensitive to local scale heterogeneity in light availability
(i.e., canopy gaps) and other non-climatic factors.