Climate and competition affect growth and survival of transplanted sugar maple seedlings along a 1700-km gradient

Plant species distributions, broadly shaped by climate, may also be constrained by other species. The degree to which biotic factors affect range limits is unclear, however, and few experimental studies have investigated both biotic and abiotic factors across and beyond a species' range. We examined seedling survival and net growth for three years in contrasting canopy type (closed canopy vs. gap) and neighbor density (clipped vs. unclipped) environments for northern, central, and southern populations of sugar maple (Acer saccharum) representing a climate-of-origin gradient, experimentally planted from Arkansas, USA to Ontario, Canada at ten forested sites along a 1700-km transect spanning beyond the species' range. We hypothesized that each population's highest survival and growth would occur in its region of origin, with poorer performance in cooler or warmer areas. Refuting this hypothesis, seedlings of all three populations had greater growth and survival in sites increasingly warmer than their point of origin, although they did show poorer growth and survival at increasingly colder sites. We also hypothesized that maple survival and net growth near and beyond range margins are constrained primarily by cold temperature limitation in the north, where we expected neighbors to facilitate survival, and by competition in the south, where we expected to enhance survival and growth by reducing neighbor density. Results partially supported the hypothesis concerning biotic interactions: in canopy gaps, understory neighbors enhanced maple growth at the coolest sites but did not suppress growth as expected at the warmest sites. As the northern population grew and survived reasonably well beyond the northern range limit, and as all populations performed best at warmer sites, including beyond the southern range limit, there was tepid, if any, support for the hypothesis that climate regulated the northern limit and absolutely no support for the hypothesis that competition regulated the southern limit. Together, these three-year findings with juvenile trees suggest that sugar maple range limits may instead be constrained by factors besides climate and competition, by those factors at another life stage, and/ or by climate events such as heat waves, droughts, and cold snaps that occur at longer return intervals.