The interspecific differences in growth and water use efficiency between trembling aspen (Populus tremuloides) and red maple (Acer rubrum) as a result of interannual climate variation in Northern Minnesota

Background/Question/Methods

Dieback of both aspen and maple are increasing across the American Upper Midwest. Red maple (Acer rubrum), however, is predicted to respond relatively more favorably to future climate change than the commercially more important trembling aspen (Populus tremuloides). Our primary objective was to investigate the differences in growth and water use efficiency (WUE) between trembling aspen and red maple over the past decade in order to better understand potential responses to climate change and mechanisms of dieback.  Recent research indicates that aspen trees maintain growth rates but decrease WUE during drought, while red maple has been shown to decrease growth and increased WUE with drought. Therefore, we predicted that aspen would have higher rates of annual growth, but would be less able to regulate water uptake and thus have lower annual WUE than maple.  Three cores each were extracted from 6 dominant aspen and 6 dominant maple trees at 1.5 m height.  Two of the 3 cores per tree were measured for annual ring width and the third core was analyzed for δ¹³C to estimate annual integrated WUE in rings formed between 2001 and 2010.  Annual long-term weather data was collected to assess drought conditions from past growing seasons measured.

Results/Conclusions

Annual radial growth was greater in the trembling aspen trees than in the red maple trees across all years measured, except during periods of high temperature and low precipitation, as measured through Palmer Drought Severity Index (PDSI).  In addition, WUE as estimated by δ¹³C of annual tree rings was higher in maple compared to aspen, especially during periods of high PDSI. The ability to use water more efficiently during periods of increased temperature and decreased soil available water, and the intolerance of aspen to decreased soil available water was likely why maple had higher water use efficiency when compared to aspen.  A decrease in growth rates of trembling aspen during periods of drought could indicate a reduction in this commercially important hardwood species with climate change, and also suggests a possible mechanism for aspen dieback in northern Minnesota forests.  Overall, this study supports the notion that red maple may outcompete aspen with more frequent extreme weather events such as drought, and that red maple should perhaps be considered for future commercial viability.