Abstract
Budburst phenology is a key driver of ecosystem structure and functioning, and it is sensitive to global change. Both cold winter temperatures (chilling) and spring warming (forcing) are important for budburst. Future climate warming is expected to have a contrasting effect on chilling and forcing, and subsequently to have a non-linear effect on budburst timing. To clarify the different effects of warming during chilling and forcing phases of budburst phenology in deciduous trees, (i) we conducted a temperature manipulation experiment, with separate winter and spring warming treatments on well irrigated and fertilized saplings of beech, birch and oak, and (ii) we analyzed the observations with five temperature-based budburst models (Thermal Time model, Parallel model, Sequential model, Alternating model, and Unified model). The results show that both winter warming and spring warming significantly advanced budburst date, with the combination of winter plus spring warming accelerating budburst most. As expected, all three species were more sensitive to spring warming than to winter warming. Although the different chilling requirement, the warming sensitivity was not significantly different among the studied species. Model evaluation showed that both one- and two- phase models (without and with chilling, respectively) are able to accurately predict budburst. For beech, the Sequential model reproduced budburst dates best. For oak and birch, both Sequential model and the Thermal Time model yielded good fit with the data but the latter was slightly better in case of high parameter uncertainty. However, for late-flushing species, the Sequential model is likely be the most appropriate to predict budburst data in a future warmer climate.
Highlights
Leaf phenology is a key driver of canopy development, tree growth, ecosystem carbon and water balance, and species distribution [1,2,3,4,5]
Winter temperatures that determine the release from dormancy in winter [16] and spring warming temperatures that accelerate bud development following the release from dormancy are both acknowledged to influence spring phenology [16,17,18,19]
The continuously warmed W6S6 treatment resulted in the earliest budburst dates, while in the control chambers (W0S0) the latest budburst dates were observed (Figure 2)
Summary
Leaf phenology is a key driver of canopy development, tree growth, ecosystem carbon and water balance, and species distribution [1,2,3,4,5]. In the Northern hemisphere, a clear advancement of spring tree phenology, paralleling the recent increase in surface temperature, has been well documented [6,7,8,9,10,11,12,13]. This observed correlation between temperature and leaf flushing date cannot be extrapolated to simulate the future phenology changes in a warmer world, because the mechanisms underlying the budburst process are far from fully understood [14,15]. To accurately simulate the budburst process in a changing climate, more information on the impact of warming on chilling and forcing phases, separately, as well as their combined effect on tree phenology, is needed
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