The changes in precipitation and temperature regimes brought on by the current climate change have influenced ecosystems globally. The consequences of climate change on plant phenology have been widely investigated during the last few years. However, the underlying causes of the timing of autumn phenology have not been fully clarified yet. Here, we focused on the onset (10%) of leaf colouring—LCO—(Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie (BBCH) 92) of European beech (Fagus sylvatica, L.) as an important native tree species growing throughout Europe. Studied beech stands are located along the natural distribution range of the European beech in Western Carpathians (Slovakia) at different altitudes from lowlands (300 m a.s.l.) to uplands (1050 m a.s.l.) and climatic regions from warm to cold. To define limiting climate conditions for LCO, we established several bioclimatic indices as indicators of meteorological drought: climatic water balance (CWB), standardized precipitation index (SPI), standardized precipitation-evapotranspiration index (SPEI), dry period index (DPI), and heat waves (HW). In addition, meteorological variables such as monthly mean temperatures and precipitation totals were taken into account. Throughout the 23-year period (1996–2018) of ground-based phenological observations of temperate beech forests, the timing of LCO was significantly delayed (p ≤ 0.05) in the middle to high altitudes, while in the lowest altitude, it remained unchanged. Over the last decade, 2009–2018, LCO in middle altitudes started at comparable to low altitudes and, at several years, even later. This resulted mainly from the significant negative effect of drought prior to this phenological phase (p ≤ 0.01) expressed through a 1-month SPI in September (SPIIX) at the stand at the low-altitude and warm-climatic region. Our results indicate that the meteorological drought conditioned by lower total precipitation and higher evapotranspirative demands in the warmer climate advance leaf senescence. However, at present time, growth in rising temperature and precipitation is acceptable for most beech stands at middle to high altitudes. Beech utilizes these conditions and postpones the LCO by 0.3–0.5 and 0.6–1.2 day per year at high and middle altitudes, respectively. Although we show the commencing negative effect of drought at mid-altitudes with lower (below 700 mm) total annual precipitation, the trend of LCO in favourable warm climates is still significantly delayed. The ongoing warming trend of summer months suggests further intensification of drought as has started to occur in middle altitudes, spreading from the continual increase of evapotranspiration over the next decades.

中文翻译：

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欧洲山毛榉对夏季干旱和炎热的秋季物候响应

当前气候变化带来的降水和温度状况的变化已经影响了全球的生态系统。气候变化对植物物候的影响在过去几年中得到了广泛的研究。然而，秋季物候发生时间的根本原因尚未完全阐明。在这里，我们专注于欧洲山毛榉 (Fagus sylvatica, L.) 作为生长在整个欧洲的重要本地树种的叶子着色 - LCO -（Biologische Bundesanstalt，Bundessortenamt und Chemische Industrie (BBCH) 92）的开始（10%）。研究的山毛榉林分位于西喀尔巴阡山脉（斯洛伐克）欧洲山毛榉的自然分布范围内，海拔从低地（300 m asl）到高地（1050 m asl）以及从温暖到寒冷的气候区域。为了定义 LCO 的极限气候条件，我们建立了几个生物气候指数作为气象干旱的指标：气候水平衡 (CWB)、标准化降水指数 (SPI)、标准化降水蒸散指数 (SPEI)、干旱期指数 (DPI) 和热浪 (HW)。此外，还考虑了气象变量，例如月平均温度和降水总量。在温带山毛榉林地基物候观测的23年期间（1996-2018）中，中高海拔LCO时间显着延迟（p≤0.05），而最低海拔则保持不变. 在过去十年，即 2009 年至 2018 年，中海拔地区的 LCO 开始时与低海拔地区相当，几年甚至更晚。这主要是由于该物候阶段之前干旱的显着负面影响 (p ≤ 0.01) 通过 9 月 1 个月的 SPI (SPIIX) 在低海拔和温暖气候区的林分表现出来。我们的研究结果表明，在温暖气候下，以较低的总降水量和较高的蒸散需求为条件的气象干旱会促进叶片衰老。然而，目前，对于中高海拔地区的大多数山毛榉林来说，气温上升和降水量的增长是可以接受的。Beech 利用这些条件，在高海拔和中海拔分别将 LCO 每年推迟 0.3-0.5 天和 0.6-1.2 天。虽然我们展示了干旱在年降水量较低（低于 700 毫米）的中海拔地区开始产生的负面影响，有利温暖气候下 LCO 的趋势仍显着延迟。夏季持续变暖的趋势表明干旱进一步加剧，正如在中海拔地区开始发生的那样，随着未来几十年蒸散量的持续增加而蔓延。