<p>Changes in the rhythm of nature are recognized as a useful proxy for detecting climate change and a very interesting source of data for scientists investigating its effects on the natural ecosystems. In this sense, phenology is the science that observes and studies the phases of the life cycling of living organisms and how the seasonal and interannual variations of climate affect them.</p><p>Traditionally, farmers or naturalists and scientists recorded phenological observations on paper for decades. Most of these observations correspond to practices today associated to <strong>Citizen Science.</strong> So far, in-situ observations were reduced to small traditional specimens closely located to the observer home, such as garden plants or fruit trees, butterflies, swallows or storks and, in general, the volunteers efforts were a bit biased towards accessible locations (close to the roads or urban areas). However, the strong variability of the vegetation phenology across biomes requires having more data to improve the knowledge about these changes. Despite its limitations, local, regional or national networks are dedicated to the collection of evidences on changes of vegetation phenology. At sub-national level <strong>in Catalonia</strong> (north-east of the Iberian Peninsula), the Catalan weather service deployed the FenoCat initiative and in the H2020 Groundtruth 2.0 project, <strong>RitmeNatura.cat</strong> (www.ritmenatura.cat) was co-designed as a phenological Citizen Observatory that has a community of phenology observers collecting either occasional or regular observations. It monitors 12 species and provides observers with species-phenophase guidance. Fortunately, scientists have found <strong>another ally</strong> to increase the collection of vegetation phenology data at global level: <strong>remote sensing</strong>.</p><p><strong>Remote Sensing</strong> (RS) provides several products with different spatial and spectral resolutions. MODIS with a daily revisit is ideal for detecting phenology in vegetation but in many areas of the world, a spatial resolution of 250 m (MODIS) is too coarse to account for small heterogeneous landscapes. In the other extreme high resolution imagery such as Landsat has a limited temporal resolution of only two revisiting periods per month being too low to generate a regular (and dense enough) time series once cloud cover is masked. Sentinel 2A and B with higher resolution, global coverage and 5 days temporal revisiting offer a good compromise. Still, <strong>what was obtainable from space differs methodologically from the in-situ observations and both are hardly comparable</strong>. The <strong>PhenoTandem Project</strong> (http://www.ritmenatura.cat/projects/phenotandem/index-eng.htm), part of the CSEOL initiative funded by ESA, provides an innovation consisting in co-designing a new protocol with citizen scientists that will make in-situ observations interoperate with remote sensing products by selecting the areas and habitats where traditional phenological in-situ observations done by volunteers can be also be observed in Sentinel 2 imagery</p><p>And so harmonizing citizens&#8217; science and remote sensing observations promoted through observatories ensures a <strong>promising partnership for phenology monitoring.</strong></p>
Synergistic Use of Citizen Science and Remote Sensing for Continental-Scale Measurements of Forest Tree Phenology