Temperate tree expansion into adjacent boreal forest patches facilitated by warmer temperatures

Abstract

Temperate and boreal forests are forecast to change in composition and shift spatially in response to climate change. Local-scale expansions and contractions are most likely observable near species range limits, and as trees are long-lived, initial shifts are likely to be detected in the understory regeneration layers. We examined understory relative abundance patterns of naturally regenerated temperate and boreal tree species in two size classes, seedlings and saplings, and across two spatial scales, local stand-scale ecotones (tens of meters) and the regional temperate -boreal transition zone ( ~ 250 km) in central North America, to explore indications of climate-mediated shifts in regeneration performance. We also tested for the presence of strong environmental gradients across local ecotones that might inhibit species expansion. Results showed that tree regeneration patterns across ecotones varied by species and size class, and varied across the regional summer temperature gradient. Temperate tree species regeneration has established across local ecotones into boreal forest patches and this process was facilitated by warmer temperatures. Conversely, boreal conifer regeneration exhibited negative responses to the regional temperature gradient and only displayed high abundance at the boreal end of local ecotones at cool northern sites. Th e fi ltering eff ects of temperature also increased with individual size for both boreal and temperate understory stems. Observed regeneration patterns and the minor environmental gradients measured across local ecotones failed to support the idea that there were strong barriers to potential temperate tree expansion into boreal forest patches. Detectable responses, consistently in the directions predicted for both temperate and boreal species, indicate that summer temperature is likely an important driver of natural tree regeneration in forests across the temperate -boreal transition zone. Regeneration patterns point toward temperate expansion and reduced but continued boreal presence in the near-future, resulting in local and regional expansions of mixed temperate-boreal forests.

Regeneration dynamics of Euptelea pleiospermum along latitudinal and altitudinal gradients: Trade-offs between seedling and sprout

“…This indicates that northward migration is occurring in this Tertiary-relict rare tree species across its geographic range. This is consistent with earlier findings reported for temperate trees at the temperate-boreal transition zone in North America (Fisichelli et al, 2014) and for about one fifth of the tree species across the eastern United States based on USDA Forest Service's Forest Inventory and Analysis data (Zhu et al, 2012).…”

“…latitude and altitude) are early indicators of range shift. There is a growing body of evidence for upward shifts in tree species distributions along altitudinal gradients (Peñuelas et al, 2007;Lenoir et al, 2009;Dang et al, 2010;Lv and Zhang, 2012;Vitasse et al, 2012;Rabasa et al, 2013), whereas only few empirical studies examined latitudinal shifts of tree species and these studies offer conflicting results (Jump et al, 2009;Zhu et al, 2012;Fisichelli et al, 2014). Empirical studies of both latitudinal and altitudinal shifts are scarce (Matías and Jump, 2015).…”

“…As temperate elements of subtropical montane evergreen and deciduous broad-leaved forest, they are likely to migrate to higher latitudes in the face of ongoing global warming. In North America, temperate trees have expanded into adjacent boreal forests (Fisichelli et al, 2014). Although the relict trees mentioned above are not timberline species, climate warming can potentially threaten all montane species with restricted distribution ranges (Parmesan, 2006).…”

“…Initial range shifts of long-lived trees are most likely detected in the regeneration layer, because seedling establishment at the leading edges occurs relatively rapidly (Fisichelli et al, 2014). Therefore, one popular way to detect poleward and upward shifts of tree species is comparing regeneration dynamics along latitudinal and altitudinal gradients, respectively (Peñuelas et al, 2007;Dang et al, 2010;Lv and Zhang, 2012;Fisichelli et al, 2014).…”

Non-climatic constraints on upper elevational plant range expansion under climate change

“…This suggests that seedling herbivory or other factors potentially limiting seedlings do not play major roles in controlling maple establishment beyond its current distribution, unlike herbivory-controlled deciduous tree species' limits common to the southern Scandes of Europe [45]. While we did not address the factors that determine growth and fitness of saplings or adult trees, at which stage climate may well play a role [46,47], the seed and seedling stages in trees are of profound demographic importance given very high average mortality [48]. In combination, the results from our two field experiments point to seed predation as a critical bottleneck constraining establishment beyond sugar maple's current range limit, and more generally to the importance of the seed-to-seedling transition in limiting potential range expansion [10,49].…”