Print PageThe timing of phenological events is changing with warming temperatures. It is well established that recent advances in spring phenology are correlated with warming (Kramer 1995, Menzel et al. 2006A, Vitasse et al. 2009A), but the changes in leaf senescence patterns are not well understood. Some studies suggest that the delays in senescence that have occurred over the last century are attributable to warming (Olsen 2010, Khanduri et al. 2008). Others highlight the inconsistent response of leaf senescence to temperature and suggest instead that leaf senescence is primarily driven by photoperiod (Delpierre et al. 2009).
This study assesses the importance of temperature to fall phenology in Acer rubrum and Quercus alba seedlings grown in the in the North Carolina piedmont and Massachusetts mixed forest. Seeds were planted in open-top chambers warmed above ambient temperatures by 0, 3, and 5°C. Leaf senescence data were collected weekly starting in mid-September 2009. Leaf condition classes were "mature", with less than 33% leaf senescence, "initial senescence", with a minimum of 33% of the leaf area senesced, and "leaf drop", with at least 67% of the leaf area brown or abscised. These classes were given the numerical rankings of 2, 1, and 0, respectively.
Results/Conclusions
In Duke Forest, both species had significant responses to warming in shade chambers, and significant responses in gap chambers after November 7, 2010. On October 14, 2010 the shaded Q. alba seedlings averaged senescence classes of 1.37 and 1.2 in ambient and 5°C warmed chambers, respectively. The shaded A. rubrum seedlings averaged senescence classes of 0.7 and 0.3 in ambient and 5°C warmed chambers. The gap response was too variable to be considered significant. In contrast, on December 12 the shaded ambient Q. alba seedlings averaged 0, while the 5°C warmed chambers averaged 0.65. A. rubrum ambient shaded seedlings averaged 0, and warmed chambers averaged 0.06. Gap seedlings followed the same pattern.
Warming's influence switched from an initial higher proportion of seedlings experiencing leaf drop (in shaded chambers) to a smaller proportion of seedlings undergoing leaf drop in both shaded and gap chambers in the latter part of the season. This interaction between light habitat and temperature may be explained by the hotter and drier conditions found in gaps. Drought stress early in the fall season could also explain the dual patterns of leaf senescence. Taken together, these results suggest that temperature alone does not determine senescence date.
