Hydrologic position mediates sensitivity of tree growth to climate: Groundwater subsidies provide a thermal buffer effect in wetlands

- Hydrologic setting strongly mediated climate-growth responses.
- Fen soils were cooler than surrounding uplands.
- Growth relationships were consistent with thermal buffering from groundwater.
- Abies balsamea on drier soils in the northeast may be vulnerable to climate change.
- Precipitation changes rather than high temperatures may most strongly shape growth.

We evaluated the effects of hydrologic setting on the growth sensitivity of two conifer species to recent atmospheric climatic variability and change in a region experiencing a warming (annual Tmin: 0.07 °C/decade) and wetting trend (total annual PPT 9.8 mm/decade; 1911-2012). Tree-ring chronologies were constructed for the boreal disjunct balsam fir (Abies balsamea; n = 72; 1916-2012) and range-centered eastern white pine (Pinus strobus; n = 84; 1707-2012) at three forested, groundwater-fed wetlands (fens) and their neighboring uplands in New York State, USA. Soil temperature monitoring in 2010 confirmed that upland soils were significantly warmer than fen soils during the spring and summer months (11.5-13.9 °C; p < 0.05). Climate-growth relationships for Abies balsamea varied substantially based on hydrologic setting and season. Compared with a remnant upland population that showed an increasingly negative sensitivity to warm summer temperatures over time A. balsamea positioned in fens were less sensitive to recent warming. While atmospheric climate-growth relationships were more consistent across hydrologic settings for Pinus strobus, we still observed qualitatively different responses to atmospheric climate variables between fen and upland populations. Overall, the climate-growth relationships identified in this study suggest that relative to trees growing in mesic upland soils, growth sensitivity to warm ambient climate in summer is ameliorated by groundwater inputs. The climate modulating effects of groundwater on tree-growth observed in our study suggests that for temperate and boreal regions where fens are abundant further consideration of groundwater influences on climate-growth relationships is warranted.