College of water sciences, Beijing Normal University, Beijing, China.
Department of biology, University of Antwerp, Antwerp, Belgium.
Glob Chang Biol. 2019 May;25(5):1696-1703. doi: 10.1111/gcb.14599. Epub 2019 Mar 19.
Leaf phenology is one of the most reliable bioindicators of ongoing global warming in temperate and boreal zones because it is highly sensitive to temperature variation. A large number of studies have reported advanced spring leaf-out due to global warming, yet the temperature sensitivity of leaf-out has significantly decreased in temperate deciduous tree species over the past three decades. One of the possible mechanisms is that photoperiod is limiting further advance to protect the leaves against potential damaging frosts. However, the "photoperiod limitation" hypothesis remains poorly investigated and experimentally tested. Here, we conducted a photoperiod- and temperature-manipulation experiment in climate chambers on two common deciduous species in Europe: Fagus sylvatica (European beech, a typically late flushing species) and Aesculus hippocastanum (horse chestnut, a typically early flushing species). In agreement with previous studies, we found that the warming significantly advanced the leaf-out dates by 4.3 and 3.7 days/°C for beech and horse chestnut saplings, respectively. However, shorter photoperiod significantly reduced the temperature sensitivity of beech only (3.0 days/°C) by substantially increasing the heat requirement to avoid leafing-out too early. Interestingly, the photoperiod limitation only occurs below a certain daylength (photoperiod threshold) when the warming increased above 4°C for beech trees. In contrast, for chestnut, no photoperiod threshold was found even when the ambient air temperature was warmed by 5°C. Given the species-specific photoperiod effect on leaf phenology, the sequence of the leaf-out timing among forest tree species may change under future climate warming conditions. Nonphotoperiodic species may benefit from warmer springs by starting the growing season earlier than photoperiodic sensitive species, modifying forest ecosystem structure and functions, but this photoperiod limitation needs to be further investigated experimentally in numerous species.
叶片物候是温带和寒带地区全球变暖的最可靠生物指标之一，因为它对温度变化非常敏感。大量研究报告称，由于全球变暖，春季叶片提前展开，但在过去三十年中，温带落叶树种的叶片展开的温度敏感性显著降低。其中一种可能的机制是，光周期限制了进一步的提前，以保护叶片免受潜在的破坏性霜冻。然而，“光周期限制”假说仍然缺乏深入研究和实验验证。在这里，我们在气候室内对欧洲两种常见的落叶树种进行了光周期和温度处理实验：欧洲山毛榉（欧洲山毛榉，一种典型的晚展叶物种）和七叶树（七叶树，一种典型的早展叶物种）。与之前的研究一致，我们发现，变暖使山毛榉和七叶树幼苗的展叶日期分别显著提前了 4.3 和 3.7 天/℃。然而，较短的光周期显著降低了山毛榉的温度敏感性（仅 3.0 天/℃），因为它大大增加了避免过早展叶的热量需求。有趣的是，只有当山毛榉的变暖超过 4°C 时，光周期限制才会出现在一定的日照长度（光周期阈值）以下。相比之下，对于七叶树，即使环境空气温度升高 5°C，也没有发现光周期阈值。鉴于叶片物候对光周期的特定影响，在未来气候变暖条件下，森林树种的展叶时间顺序可能会发生变化。非光周期物种可能会受益于温暖的春天，因为它们比光周期敏感的物种更早开始生长季节，从而改变森林生态系统的结构和功能，但这种光周期限制需要在众多物种中进一步进行实验研究。
