On the temporal mismatch between in-situ and satellite-derived spring phenology of European beech forests

ABSTRACT

Forest phenology plays a key role in the global terrestrial ecosystem influencing a range of ecosystem processes such as the annual carbon uptake period, and many food webs and changes in their timing and progression. The timing of the start of the phenology season has been successfully determined at a range of scales, from the individual tree by in situ observations to landscape and continental scales by using remotely sensed vegetation indices (VIs). The spatial resolution of satellites is much coarser than traditional methods, creating a gap between space-borne and actual field observations, which brings limitations to phenological research at the ecosystem level. Several unconsidered methodological and observational-related limitations may lead to misinterpretation of the timing of the satellite-derived signals. The aim of this study is therefore to clarify the meaning of a set of spring phenology metrics derived from Moderate Resolution Imaging Spectroradiometer (MODIS) Enhanced Vegetation Index (EVI) time series in beech forests distributed across Europe with respect to PEP725 in situ observations, from 2003 to 2020. To this aim, we (i) tested the differences between remotely sensed and in situ start-of-season (SOS) metrics and (ii) quantified the influence of latitude, elevation, temperature, and precipitation on such differences. Results demonstrated that there is a clear temporal gradient among the different SOS metrics, all of them occurring prior to the in situ observations. Furthermore, latitude and temperatures proved to be the main factors guiding the differences between remotely sensed and in situ SOS metrics. Evidence from this study may help in recognizing the actual meaning of what we see by means of remotely sensed phenology metrics. In this perspective, field observations are crucial in understanding phenology events and provide a reference base. Satellite data, on the other hand, complement field observations by filling in gaps in spatial and temporal coverage, thus enhancing the overall understanding.

KEYWORDS:

• Beech forest
• bioclimatic variables
• MODIS EVI
• PEP725
• phenology
• remote sensing

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data derived from public domain resources: the elevation was extracted from the ASTER Global Digital Elevation Model (GDEM) V2 product freely available at https://asterweb.jpl.nasa.gov/gdem.asp; the phenological time-series of ground observations was extracted from the Pan-European Phenology (PEP725; www.pep725.eu); the EVI MODIS time series are available at https://modis.gsfc.nasa.gov/data/dataprod/mod13.php.

Additional information

Funding

The work was carried out within the Agritech National Research Center and received funding from the European Union Next-GenerationEU (National Recovery and Resilience Plan (NRRP) – MISSION 4 COMPONENT 2, INVESTMENT 1.4 – D.D. 1032 17/06/2022, CN00000022). This manuscript reflects only the author’s views and opinions, neither the European Union nor the European Commission can be considered responsible for them.