When will the chestnuts bloom in the Summer Palace in Beijing?
Our changing climate is changing trees’ flowering times and the length of their growing season. But the results are not as easy to predict as you might expect, say Liang Guo, Junhu Dai, Jianchu Xu and Eike Luedeling Changes to the planet’s climate have affected when plants flower, which in turn affects their fruiting and growth. This phenomenon, known as phenology, is being studied throughout the world but few studies have evaluated the responses of fruit trees in East Asia to the changing climate.
We wanted to learn more about this so we examined the long-term records (1963–2008) of chestnut (Castanea mollissima Blume) trees at the Summer Palace in Beijing, looking at the times of their first flowering and when their leaves began to change colour in the autumn and the length of their growing season. Our analysis found that the chestnut trees bloom later at the Summer Palace, by 1.6 days per decade since 1963. In total, the bloom date has advanced by about one week (7.5 days) over the last 50 years, similar to changes in Wisconsin, USA, between 1936 and 1988 and in the Mediterranean region between 1952 and 2000. While the bloom date in spring has advanced, we also found that there was a delay in the ‘chill accumulation’ phase. Like all other deciduous trees from temperate or cold climates, chestnuts lie dormant in winter and thereby avoid frost damage to sensitive growing tissue. They need a certain amount of cold temperatures followed by specific warmer temperatures before they will wake up from their dormancy and resume growth in spring. However, warmer autumns make trees more slowly accumulate the coldness they need. We found that the chestnut trees’ growing season in Beijing expanded by 4.3 days per decade, caused by warmer winters and springs. The longer season was due almost entirely to earlier springs rather than later autumns. In the future, we cannot be sure if temperatures will change more or when they will change. Autumn and winter temperatures in Beijing are currently low enough that the trees’ are cold enough every year. Given that temperature increases during certain parts of winter are likely to even increase the rate of chill accumulation—due to less frost: because the trees don’t store coldness once the temperature drops below freezing—then warmer springs will probably continue for some time into the future, even as the impact of reduced chilling becomes gradually more important. Regarding changes to the trees’ patterns in autumn, past as well as future trends are quite unclear. Our analysis showed evidence that leaf colouring is a response to conditions that occur during most of the growing season, as well as to short-term temperature triggers shortly before visible leaf colouring in autumn. Future warming should cause leaves to change colour earlier. While warmer springs and the extension of the growing season have clearly been the major changes over the last few decades, it is unclear whether or not these trends will continue into the future. We did notice that warmer conditions seemed to cause some other effects that went against the expected: a reduction in winter chill, which might delay rather than advance spring; as well as a faster accumulation of heat during summer, which might shorten the life of chestnut leaves, leading to contraction rather than extension of the growing season. At present, these secondary warming effects are swamped by the dominant impacts of faster heat accumulation in spring and longer, warmer autumns. Further warming might shift this balance, however, and eventually lead to substantial changes in trees growing, flowering and fruiting behaviour. Given how critical trees are to food supply and the provision of other environmental services, it would be a good idea to increase scientific attention on such secondary climate responses, especially for species grown outside their native ranges (for example, fruit trees or ornamentals) and plants in environments exposed to particularly pronounced temperature increases. Edited by Robert Finlayson READ THE ARTICLE GuoL, DaiJH, XuJC, LuedelingE. 2013. Response of chestnut phenology in China to climate variation and change. Agricultural and Forest Meteorology 180:164–172 This work is linked to the CGIAR Research Program on Forests, Trees and Agroforestry