Don’t judge a leaf by its color: Warming is not delaying end-of-season processes in some deciduous tree species

Background/Question/Methods

            Climatic warming's effect on leaf senescence in autumn is poorly understood.  Some trees are delaying their fall leaf coloration, but the response is inconsistent.  If delayed autumnal phenology extends the growing season, one would expect to see a delay in photosynthetic decline, or, alternatively, increased nitrogen resorption due to a prolonged period before leaf abscission. 

            During the 2011 autumn senescence period, photosynthetic rates, leaf nitrogen content, and leaf phenological status were collected weekly for Acer rubrum, Liquidambar styraciflua, and Liriodendron tulipifera at Duke Forest in Orange County, North Carolina.  Seedlings were grown in open-topped chambers established in 2009 at either ambient temperatures or warmed to 5 degrees Celsius above ambient.

Results/Conclusions

          We found a significant delay in autumnal phenology in Acer rubrum***, advanced autumnal phenology in Liriodendron tulipifera*, and no significant relationship between warming and autumnal phenology in Liquidambar styraciflua during the 2011 season.  Photosynthetic rates were slower in warmed chambers for Liquidambar styraciflua*** and Liriodendron tulipifera***, but faster in Acer rubrum***.  The timing of photosynthetic decline was not delayed by warming for any species.  Percent leaf nitrogen was lower in warmed chambers for all species (***).  The timing of nitrogen resorption was not delayed by warming, and final leaf nitrogen content was proportional to leaf nitrogen content during the start of the senescence period.

            Results suggest that while warming often impacts autumnal changes in leaf coloration, the timing of photosynthetic decline and nitrogen resorption appears to be unaffected.  From this study, the benefits of delaying visible senescence are unclear.  End of season processes other than late season photosynthesis and nitrogen resorption must be explored as potential causes of changes in the timing of senescence in lieu of an extended growing season or more complete nitrogen resorption.  In particular, the advanced autumnal phenology observed in the drought-deciduous Liriodendron tulipifera is an intriguing result that is worth exploring further.  The interactions of warming and drought are an area that future research will help disentangle.