Many regions of the world are undergoing climate change-effects on forests. However, the climate adjacent to Lake Superior is changing faster than at most other locations due to ice-albedo effects, which have cause the water surface temperatures to rise faster than regional air temperatures. Rising water temperatures have caused growing seasons to become longer and summer cloud cover to have been substantially reduced within about 50 km of the lake. Here we investigated what the effects of these changes might have been on tree physiology using patterns in tree-ring growth and carbon isotope discrimination (Δ13C) of old (age > 150 years) and co-occurring sugar maple (Acer saccharum) and white spruce (Picea glauca) at Turkey Lakes Watershed (TLW), a long-term ecological research site in southern Ontario that is 32 km East from Lake Superior. We also used a Li-Cor 6400 to determine response curves of photosynthetic assimilation to leaf internal CO2 concentrations (i.e., A-ci curves). A-ci curves were combined with Δ13C data to reconstruct rates of A and stomatal conductance (gs) in response to changes in climate at TLW.
Results/Conclusions
Despite sugar maple being near its northern range limit and white spruce being near its southern range limit, both species showed significant (P < 0.05) declines in basal area increment growth (BAI) over the past 30 years. Sugar maples in particular had a strong increase in “missing rings” over the most recent decade, during which time summertime cloud cover has dropped to its lowest level on record. Both species also showed significant (P < 0.05) declines in Δ13C, indicative of greater drought stress. The greatest declines in both BAI and Δ13C occurred in sugar maples samples on concave and relative wet landscape positions. Sugar maples from these sites have exhibited growth and leaf gas-exchange that is comparable to trees located on the driest and most convex ridgeline sites. In both species, increasing atmospheric CO2 has driven up rates of reconstructed A over time, whereas gs has stayed constant or declined. Overall these trees represent an example of what effects climate change may have on most temperate to boreal ecotone forests decades into the future. Further studies on overstory and seedling demographics should elucidate whether increased mortality is already taking place in these forests.