The effect of warming, drought, and elevated carbon dioxide on the germination, growth, and survival of Quercus macrocarpa acorns from across a North American latitudinal gradient

Abstract

Forests are being increasingly impacted by climate change, as warmer and drier conditions accelerate both tree mortality and subsequent shifts in species composition. Seedlings play an outsize role in this process, and so predicting the effect of changing climate conditions on seedling performance is essential to understanding the future of forests. Bur oak (Quercus macrocarpa) is especially important for this process at the boreal-temperate ecotone in northern Minnesota, and given their wide latitudinal distribution, bur oaks may have developed local adaptation in response to climate. We measured the germination, growth, and survival of Q. macrocarpa acorns from Minnesota (MN), Illinois (IL), and Oklahoma (OK), in two open-air global change experiments in Minnesota, USA, that manipulate above and below ground temperature, rainfall, and atmospheric CO2. The TeRaCON experiment in central Minnesota includes a fully factorial design with two levels of warming (ambient, +2.5°C), rainfall reduction (ambient, –30% by volume), and elevated atmospheric CO2 (ambient at 414ppm, elevated at 511ppm). The B4WarmED experiment also has a fully factorial design with three levels of warming (ambient, +1.7°C, +3.3°C) and two levels of rainfall (ambient, –30% by volume). We hypothesized that: (1) Greater warming and drought will be associated with reduced germination success, survival and growth, but that (2) elevated CO2 will be associated with increased performance. (3) Larger acorns will grow into taller seedlings, even if growth is limited under warmer and drier conditions. (4) Acorns from more arid climates (IL, OK) will perform better than those from MN when under warmer and drier conditions. (5) Acorns from more arid climates will germinate later in the season than those from farther north. In central MN, warming and drought reduced overall first season survival as expected (by –13% and –14% respectively), while eCO2 increased it (+14%). However, farther north at B4WarmED, moderate warming had a marginally significant positive effect on survival (+17%). OK acorns germinated three weeks later at both sites, and had 30% lower survival at B4WarmED, as compared to acorns from MN and IL. In summary, we find that while climate change will likely impair Q. macrocarpa establishment in central Minnesota, it may potentially benefit establishment in northern Minnesota. There was not clear evidence of a locally adapted germination or survival advantage for acorns from IL and OK when planted in Minnesota, even under the warmer and drier conditions anticipated for the near future. Indeed, while Q. macrocarpa acorns from Oklahoma can germinate and survive equally well as native populations in central Minnesota, even under future warming northern Minnesota will likely remain too cool for acorns from Oklahoma to establish themselves as successfully as their local counterparts.