Fall is in the air. Here in North Carolina that means drastic temperature swings that cause me to dress incorrectly on any given day. It also means the arrival of fall colors. Indeed, fall colors are incredibly beautiful, but biologically speaking, you are watching death happen. This autumn splendor got me to thinking about these colors a little closer, specifically the phenology of trees.
Phenology is the study of the annual timing of recurring life cycle events. The timing of these events is typically influenced by seasonal environmental changes. In the case of trees, specifically hardwood forests, this is the leafing-out (flushing) and dropping-off (senescence) of leaves. But what actually triggers a plant to leaf-out? This can vary a bit by species or even individual, but there are a couple of general categories you can look to. The first is changes in air temperature, the chilling in the winter and warming in the spring. The other is photoperiod, or the day length, which often interacts with temperature, allowing plants to quickly respond to changing conditions.
Considering that these events are triggered by environmental changes, it is logical to assume that global climate change can force changes in the phenology of many species and communities. This is another think-about-the-plants moment. How plants respond to climate change has huge consequences for world ecosystems – growing seasons, species ranges, carbon and water cycling, interactions with animals, etc.
A paper published earlier this year in PNAS took a look at variations in leaf flushing and senescence dates in relation to warming. Many phonological studies focus on specific phenophases (like leaf-out in the spring), but this study is unique in that it looks at subsequent phenological events. The authors aimed to see if effects of warming lasted longer than the current growing season. To do this, in December 2009 they took seventy 3-4 year old cloned oak and beech trees and put them in growth chambers where they could very carefully control the winter environmental conditions. They manipulated the temperatures of the growth chambers to create treatment groups of winter-spring warming, winter-only warming, and spring-only warming. Then, in spring of 2010 when the flushing was complete, they moved the trees out of the chambers and into a field. The trees stayed outside and were measured until the following spring of 2011. Leaf-out rates were determined using a scale that went from undeveloped bud to unfolded leaf, and leaf senescence was recorded as the date at which half of the leaves were colored or dropped. These measurements allowed for a quantification of growing season length. Additional measurements of numbers of leaf per tree, specific leaf area, total leaf area per tree, number of buds, dry weights of various parts of the trees, carbohydrate content, and carbon and nitrogen content were taken. They also combined their data with that of the European phenology network to get both a larger sample size and a wider geographic area.
The researchers found both leaf flushing and senescence in both species to be advanced 15-18 days by winter-spring warming. In the long-term, the timing of autumn leaf senescence was found to be positively correlated with spring leaf flushing dates, and advanced leaf flushing lead to earlier leaf flushing the following year. This suggests that the physiological impacts of a warmer winter last longer than just one growing season. Advanced leaf flushing in this winter-spring-warming treatment was also associated with some physiological and morphological changes, particularly in the oaks. These included higher leaf number, higher leaf area per tree, and higher starch accumulation.
The trends of the experiment were also observed in the mature trees in the long-term field-based phenology observations of the European phenology network. The underlying cause in both cases is likely that the plants never really fulfill the winter chilling requirements necessary for them to enter dormancy. Currently, the most widely accepted mechanism for leaf senescence is the environmental control hypothesis, which proposes that leaf senescence is triggered with the unfavorable autumn season comes (changes in photoperiod, temperature, or both). This study shows that perhaps that isn’t all that’s going on.
*sigh* nothing is ever simple is it?
Fu, Y., Campioli, M., Vitasse, Y., De Boeck, H., Van den Berge, J., AbdElgawad, H., Asard, H., Piao, S., Deckmyn, G., & Janssens, I. (2014). Variation in leaf flushing date influences autumnal senescence and next year's flushing date in two temperate tree species Proceedings of the National Academy of Sciences, 111 (20), 7355-7360 DOI: 10.1073/pnas.1321727111For lots of really great info on the science of leaf-out, I recommend this review article:
Polgar, C., & Primack, R. (2011). Leaf-out phenology of temperate woody plants: from trees to ecosystems New Phytologist, 191 (4), 926-941 DOI: 10.1111/j.1469-8137.2011.03803.xAnd you can contribute to leaf phenology research through Project Budburst!
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