Validation and Application of European Beech Phenological Metrics Derived from MODIS Data along an Altitudinal Gradient

Abstract

Monitoring plant phenology is one of the means of detecting the response of vegetation to changing environmental conditions. One approach for the study of vegetation phenology from local to global scales is to apply satellite-based indices. We investigated the potential of phenological metrics from moderate resolution remotely sensed data to monitor the altitudinal variations in phenological phases of European beech (Fagus sylvatica L.). Phenological metrics were derived from the NDVI annual trajectories fitted with double sigmoid logistic function. Validation of the satellite-derived phenological metrics was necessary, thus the multiple-year ground observations of phenological phases from twelve beech stands along the altitudinal gradient were employed. In five stands, the validation process was supported with annual (in 2011) phenological observations of the undergrowth and understory vegetation, measurements of the leaf area index (LAI), and with laboratory spectral analyses of forest components reflecting the red and near-infrared radiation. Non-significant differences between the satellite-derived phenological metrics and the in situ observed phenological phases of the beginning of leaf onset (LO_10); end of leaf onset (LO_100); and 80% leaf coloring (LC_80) were detected. Next, the altitude dependent variations of the phenological metrics were investigated in all beech-dominated pixels over the area between latitudes 47°44′ N and 49°37′ N, and longitudes 16°50′ E and 22°34′ E (Slovakia, Central Europe). In all cases, this large-scale regression revealed non-linear relationships. Since spring phenological metrics showed strong dependence on altitude, only a weak relationship was detected between autumn phenological metric and altitude. The effect of altitude was evaluated through differences in local climatic conditions, especially temperature and precipitation. We used normal values from the last 30 years to evaluate the altitude-conditioned differences in the growing season length in 12 study stands. The approach presented in this paper contributes to a more explicit understanding of satellite data-based beech phenology along the altitudinal gradient, and will be useful for determining the optimal distribution range of European beech under changing climate conditions.

Evaluating the effect of stand properties and site conditions on the forest reflectance from Sentinel-2 time series

“…A similar trend was reported by Hallik et al [14] in hemiboreal forests, where a negative relationship between LAI and reflectance was reported in visible bands, particularly in blue and red, and by Eklundh et al [66] in boreal coniferous forests, where the red band has been found as the most important band for LAI estimation. Also, at a leaf scale, full leaf onset (young leaves) is characterized by a higher reflectance than final leaf onset (mature leaves) in the visible green and red wave regions [67], which can also be observed in our canopy scale analysis. As chlorophyll strongly absorbs energy in the red region [2], more of the red light is absorbed with increasing chlorophyll content at lower elevations.…”

“…The autumn beech phenophases exhibited an opposite direction, and the differences across the elevation gradients occurred mainly in the middle of October ( Fig 3 ). In previous studies, the relationships between elevation and autumn phenophases have been reported to be weaker than in the spring [ 65 , 67 ], or even negligible [ 72 ]. The two main phenological phases of broadleaved trees during autumn are leaf coloring and leaf fall.…”

Autumn Phenological Response of European Beech to Summer Drought and Heat

“…Since trees as long-lived plants adapt to the climatic seasonality through phenology, it is considered a good indicator of changing climate conditions [20][21][22][23][24][25]. Under climate actions, plants could shift the onset of phenological phases and thus extend or shorten their growing seasons as the photosynthetically active period modulates ecosystem carbon balance [26,27].…”

How did the characteristics of the growing season change during the past 100 years at a steep river basin in Japan?

“…Previous studies examined the spatiotemporal distribution of the timing of leaf flushing, leaf colouring, and leaf fall along an elevational gradient using data from in situ observations [14][15][16][17], phenological images [7,18,19], herbarium records [10], phenological information published on the Internet [9], data from satellite observations [6,12,[20][21][22][23], and modelling [24,25]. The scales of the targeted regions have been national (Germany [15], Slovakia [17], and China [12,22,24]), continental (Europe [14] and North America [7,10]), and global [6].…”