## 5 Discussion  
The results obtained in this pilot study are encouraging. It is suggested that the methods for using the images captured by these traffic cameras are refined and in doing so the ecological value of these data will be fully realised. The data could benefit a multi-scale monitoring programme of phenology so critical for an improved understanding of environmental change, in particular that associated with climate variability [13]. The continuous measurement of phenology at high temporal and spatial resolution affords an understanding of trends in vegetation productivity and this could be achieved via aggregation across multiple species or for specific species in selected locations [47]. The ability to focus on species is possible via the manual extraction of areas of interest and would allow a better understanding of the seasonal-gap that exists between species [48], as well as the role of site-specific properties and geographical location [49]. The images captured by these traffic cameras offer extended analysis possibilities, particularly when coupled with similar resolution observations of precipitation and temperature, carbon flux measurements and soil properties [47]. This will benefit the understanding of budgets of carbon and water exchange (e.g., gross primary productivity (GPP) [50, 51]) and moreover shed light on any interannual variability in the growing season caused by weather conditions (e.g., windstorms [52]) and other occasional but often severe biotic stresses that influence the physiological status of trees [45, 53]. Beyond this there is scope for the data to be used in the understanding of ecosystem interactions, food chains, pollination, breeding and migration [54, 55]. Using a network such as that offered here meets the requirements for a spatially dense set of observations that are geographically widespread but comply with standardised rules [36]. Cameras fill in the large scale-gap that exists between observers who tend to focus on one tree and the satellite-derived data which are useful at the ecosystem scale [50]. From a UK perspective, this network could be well placed to understand the impact of current observed trends in the climate on its  
Figure 6: Ground truth dates _versus_ dates automatically calculated from cameras; (**a**) end of senescence 2011; (**b**) start of green up 2012. Note: missing points are due to lack of camera data for visual inspection. vegetation: the Central England Temperature has risen by about 1 \({}^{\circ}\)C since the 1970s, with 2006 being the warmest in the 348-year long record. Seasonal rainfall is highly variable, but there has been a slight trend over the last 250 years for decreased rainfall in summer and increased in winter [56]. Any environmental impacts already evident are likely to be exacerbated in the near future given climate change projections [57, 58] which indicate that the UK climate will continue to warm substantially and that increases in the number of days with high temperatures will be found everywhere, particularly in south east England. Further, reductions in the number of frost days are predicted, as well as increases in the number of 10-day dry spells across the UK, this being more pronounced in southern England and in Wales. There will also be further changes in rainfall patterns and its seasonal distribution; and considerable regional variations can be expected. A detailed study into the observed impacts of climate change on UK vegetation to date found that there is clear evidence that climate change is having an impact on some aspects of the composition and function of woodland [59]. Leafing has advanced by 2-3 weeks since the 1950s. However, evidence for increases in tree growth rates and forest productivity resulting from lengthening growing seasons is limited for the UK, yet the link between climate change, primary productivity and growing season has been evident elsewhere [60].