Root biomass fraction of forest-planted sugar maple (Acer saccharum Marsh.) seedlings varies with climate of population origin across a latitude gradient

Background/Question/Methods

Planting tree seedlings beyond a species’ current distribution is among the potential approaches being considered to address anticipated climate-change-driven shifts in forest composition. The extent to which climate of origin affects seedling biomass allocation and plasticity in response to a warmer climate, however, are poorly understood. We examined shifts in root biomass allocation of sugar maple seedlings from contrasting climates planted across a large-scale latitude and temperature gradient that spanned the range of sugar maple and extended beyond range margins to the north and south. Seedlings originating from two populations in northern (Minnesota, USA) and southern (Missouri, USA) portions of the range were planted at forested sites from Ontario to Arkansas and harvested after three growing seasons. We expected seedlings to exhibit allocation patterns consistent with local adaption to resource availability and growing conditions in their region of origin, hypothesizing seedlings sourced from a northern population would have a greater proportion of biomass distributed to roots than those from a southern population, while seedlings sourced from a southern population would exhibit greater plasticity in biomass distribution. Global patterns in root biomass fraction also led us to expect root biomass fraction would increase clinally from southern to northern sites.

Results/Conclusions

The two populations differed significantly in root mass, even after accounting for seedling size: for any given shoot mass, seedlings from the southern population consistently had higher root mass than similarly-sized northern population seedlings. This was opposite our hypothesis, but suggests temperature may be indirectly influencing genetic variation through effects on soil resources. Southern-origin seedlings exhibited plasticity in biomass allocation as hypothesized, with greater proportional root biomass at northern than at southern sites. Root biomass of southern-origin seedlings responded in a manner consistent with hypotheses, but at the species level we found no evidence that temperature across a latitude gradient consistently affects root biomass fraction of sugar maple seedlings. Global climate change is likely to affect roots in a multitude of ways: predicted shifts in patterns of precipitation and an increasing incidence of drought may affect root biomass allocation, as may changes in soil nutrient cycling and availability associated with climate change. Determining the plasticity of belowground allocation across naturally existing resource gradients offers a window into how species may respond to ongoing climate change. Our work points to the need for additional research on allocation to roots within species and along resource gradients that co-vary with latitude.