Autumn leaf phenology: discrepancies between <i>in situ</i> observations and satellite data at urban and rural sites
Creators: name: Donnelly, Alison
name: Liu, Lingling
name: Zhang, Xiaoyang
name: Wingler, Astrid
Publisher: name: Taylor & Francis
Year: 2018
Subjects: subject: Cell Biology
subject: Neuroscience
subject: Evolutionary Biology
subject: FOS: Biological sciences
subject: Environmental Sciences not elsewhere classified
subject: Chemical Sciences not elsewhere classified
subject: Ecology
subject: Developmental Biology
subject: Plant Biology

description: Autumn phenophases, such as leaf colouration (LC) and leaf fall (LF), have received considerably less attention than their spring counterparts (budburst and leaf unfolding) but are equally important determinants of the duration of the growing season and thus have a controlling inﬂuence on the carbon-uptake period. Here, we examined THE trends (1968–2016) in <i>in situ</i> observations of the timing of LC and LF from a suite of deciduous trees at three rural sites and one urban site in Ireland. Satellite-derived autumn phenological metrics including mid-senescence (MS) and end of senescence (ES) based on two-band enhanced vegetation index (EVI2) from Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS) from 1982 to 2016 at a spatial resolution of 5km<sup>2</sup> were also examined. The aim of this study was to assess the effectiveness of satellite remote sensing in capturing autumn phenology as determined by <i>in situ</i> observations . Analysis of <i>in situ</i> data (1968–2016) revealed the urban site to be significantly different from the rural sites as LC and LF occurred later in the season and the duration of the autumn season (LF-LC) became shorter over time. These trends may be partly driven by the presence of artificial light in the city. On average (1982–2016), there was a 6-day delay in the timing of MS compared to LC and a much larger difference (21 days) between ES and LF. This resulted in a 31-day autumn duration as defined by satellite data compared to 16 days from <i>in situ</i> observations. Furthermore, there was little overlap in timing between LC and MS, and LF and ES at the rural sites only. Discrepancies between <i>in situ</i> and satellite data may be attributed to the satellite data integrating a much broader vegetation signal across a heterogeneous landscape than <i>in situ</i> observations of individual trees. Therefore, at present, satellite-derived autumn phenology may be more successful in capturing <i>in situ</i> observations across large homogeneous landscapes of similar vegetation types (e.g. forested areas) than in heterogeneous landscapes (e.g. small mixed farms, urban areas, etc.) as is the case in Ireland where the <i>in situ</i> observations of trees may not be reflective of the overall vegetation. Matching the scale of satellite data with <i>in situ</i> observations remains a challenging task but may, at least in part, be overcome by increasing the extent of observations to include a wider range of species and in future as satellite data become available at higher spatial and temporal resolutions.