Autumn Phenological Response of European Beech to Summer Drought and Heat

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.