Land surface phenology across major natural vegetation land cover types and biogeographical regions in the Iberian Peninsula

Land surface phenology (LSP), the study of seasonal dynamics on the vegetated land surface from satellite data, has contributed to improve the understanding of the phenological dynamics of vegetation in boreal and temperate ecosystems located in the high and mid-latitude regions of the Northern Hemisphere. Iberian Peninsula is one of the most biologically rich regions of the European continent, where endemic plant species are very common. Hence, the ecosystems of the Iberian Peninsula provide a wide variety of ecosystem services, which may be altered by changes in climatic conditions. LSP has not been studied in detail in the Iberian ecosystems. Despite their ecological importance and vulnerability to climate change, there is a lack of an exhaustive and long-term characterisation of the phenological patterns of the main natural vegetation land cover types of the whole Iberian Peninsula. Therefore, the goal of this study was to monitor the LSP dynamics across major natural vegetation land cover types in the Alpine, Atlantic, and Mediterranean biogeographical regions of the Iberian Peninsula. Two-band Enhanced Vegetation Index (EVI2) time-series were generated from the MOD09Q1 surface reflectance product from January 2001 to December 2021. This MODIS product is based on 8-day image composites at a spatial resolution of 250 meters. The methodology to estimate LSP from raw EVI2 time-series followed two steps, including a) data denoising and b) the extraction of spring and autumn phenometrics. A double logistic function was used to smooth the EVI2 time-series from TIMESAT software. Start of the growing season (SOS) (i.e., proxy of the spring phenology) and the end of the growing season (EOS) (i.e., proxy of the autumn phenology) were extracted using a 10% threshold-based technique.

Results of this research showed spring and autumn phenometrics had a similar spatial pattern across major natural vegetation land cover types of the three biogeographic regions of the Iberian Peninsula. Generally, SOS dates were in March-April, while EOS dates were in November-December. Despite this, the spatial patterns of LSP were very different in Mediterranean shrublands, savannahs and grasslands. SOS and EOS dates showed the highest variability in these natural vegetation land cover types. The growing season generally started between October and February (SOS) and ended between June and November (EOS). The spatial variability of spring and autumn phenometrics in these three natural vegetation cover types may be related to several factors: i) the mixture of the phenological cycles of different functional vegetation types (e.g., herbaceous, shrub or tree vegetation); ii) the mixture of phenological trajectories of multiple plant species; iii) the influence of non-vegetation cover, such as bare soil, affecting spectral information. These LSP patterns of natural vegetation cover types in the Iberian Peninsula could contribute to improve the understanding of the response of Iberian ecosystems to climate change.