Abstract
The frequency of sudden, strong warming events is projected to increase in the future. The effects of such events on spring phenology of trees might depend on their timing because spring warming has generally been shown to advance spring budburst while fall and winter warming have been shown to delay spring phenology. To understand the mechanism behind timing-specific warming effects on spring phenology, I simulated warming events during fall, mid-winter and at the end of winter and quantified their effects on bud dormancy depth and subsequently on spring leaf out. The warming events were carried out in climate chambers on tree seedlings of Betula pendula and Fagus sylvatica in October, January, and February. Control seedlings were kept at photoperiod and temperature matching the daily fluctuating field conditions. Warmed seedlings were kept 10°C warmer than the control seedlings for 10 days during the respective warming periods. Warming in October increased bud dormancy depth and decreased spring leaf-out rate only for F. sylvatica, whereas warming in February reduced bud dormancy depth and advanced spring leaf-out rate only for B. pendula. Neither bud dormancy depth nor spring leaf out rate were affected by January warming. The results indicate that warming-induced changes in bud dormancy depth may explain species- and timing-specific warming effects on spring phenology. The extent to which the timing of bud dormancy phases is species-specific will influence among-species variation in future spring leaf out times.
Highlights
Climate warming has often been associated with advanced spring phenology, via observations (Wesołowski and Rowinski, 2006; Wood et al, 2006; Richardson et al, 2010; Zohner and Renner, 2014), and through experiments (Hole, 2014) and modeling studies (Luedeling et al, 2013; Lange et al, 2016)
No effect of warming was detected in January for either species and in February a greater decrease in dormancy depth occurred in warmed seedlings than in control seedlings only for B. pendula (p < 0.001)
In F. sylvatica dormancy depth was increased by approximately 3 times after fall warming compared to control, whereas the dormancy depth of B. pendula was decreased by approximately 2.5 times after February warming compared to control
Summary
Climate warming has often been associated with advanced spring phenology, via observations (Wesołowski and Rowinski, 2006; Wood et al, 2006; Richardson et al, 2010; Zohner and Renner, 2014), and through experiments (Hole, 2014) and modeling studies (Luedeling et al, 2013; Lange et al, 2016). Temperature increases have been shown to both advance and delay budburst dates, depending on their timing (Heide, 2003; Fu et al, 2012; Luedeling et al, 2013). Experiments have shown that the forcing requirement increases exponentially with decreasing time spent at cool temperatures, generally accepted to be below 10◦C, while being species-specific (Battey, 2000; Cesaraccio et al, 2004; Harrington et al, 2010). Delayed and/or deeper bud dormancy could require a longer cold period to bring the forcing requirements to the same level as in years with cooler fall temperatures. Experiments showing how bud dormancy depth is affected by warming in different tree species and how such dormancy changes affect spring phenology are lacking
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