Abstract
Contemporary climate change leads to earlier spring phenological events in Europe. In forests, in which overstory strongly regulates the microclimate beneath, it is not clear if further change equally shifts the timing of leaf unfolding for the over- and understory of main deciduous forest species, such as Fagus sylvatica L. (European beech). Furthermore, it is not known yet how this vertical phenological (mis)match—the phenological difference between overstory and understory—affects the remotely sensed satellite signal. To investigate this, we disentangled the start of season (SOS) of overstory F.sylvatica foliage from understory F. sylvatica foliage in forests, within nine quadrants of 5.8 × 5.8 km, stratified over a temperature gradient of 2.5 °C in Bavaria, southeast Germany, in the spring seasons of 2019 and 2020 using time lapse cameras and visual ground observations. We explained SOS dates and vertical phenological (mis)match by canopy temperature and compared these to Sentinel-2 derived SOS in response to canopy temperature. We found that overstory SOS advanced with higher mean April canopy temperature (visual ground observations: −2.86 days per °C; cameras: −2.57 days per °C). However, understory SOS was not significantly affected by canopy temperature. This led to an increase of vertical phenological mismatch with increased canopy temperature (visual ground observations: +3.90 days per °C; cameras: +2.52 days per °C). These results matched Sentinel-2-derived SOS responses, as pixels of higher canopy height advanced more by increased canopy temperature than pixels of lower canopy height. The results may indicate that, with further climate change, spring phenology of F. sylvatica overstory will advance more than F. sylvatica understory, leading to increased vertical phenological mismatch in temperate deciduous forests. This may have major ecological effects, but also methodological consequences for the field of remote sensing, as what the signal senses highly depends on the pixel mean canopy height and the vertical (mis)match.
Highlights
We aimed to investigate the following research questions: (1) how do the phenology of F. sylvatica overstory and understory, and the associated verticalmatch, respond to the main environmental variables associated with climate change, by using visual in situ observations from the ground and time lapse cameras, and (2) what does this mean for the sensed start of season (SOS) signal by Sentinel-2
Comparing the years using the Sentinel-2 observations, the pixels overlapping with polygons around overstory individuals reached SOS in 2020 on day of the year (DOY) 108.13, which was
Cameras and ground observations revealed that increased April canopy temperature, integrating air temperature, and site-specific radiation, advanced the overstory phenology, but not the understory phenology, leading to a decrease of the classical phenological escape of the understory relative to the overstory and, to an increase in vertical mismatch
Summary
In forests, early canopy closure in spring will provoke too low light levels beneath the overstory, creating more unsuitable growing conditions for understory vegetation [9,10]. To buffer these different types of mismatches, land use planning and forest management could play a key role, e.g., by promoting larger proportions of vegetation that is less sensitive to climate change, or by increasing spatial phenological variability [5,11]
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