# Monitoring Forest Phenology in a Changing World
## 2 Monitoring Phenology
### Trade-Offs among Phenological Monitoring Techniques
#### 2.3.2 Temporal Resolution and Revisit Frequency  
Temporal resolution describes the smallest amount of time between site 'visits'. As with spatial resolution, in regions where phenology happens at low temporal resolutions, or in synchrony, such as temperate and seasonal biomes, monitoring methods like sensors onboard satellites, planes and helicopters, along with collection traps, can be used effectively to monitor plant phenology, with revisit frequencies as low as a few days [125] (Table 1). For example, [PERSON] et al., [100] used multispectral cameras on-board a helicopter to track changes in the NDVI values of individual trees and grasses in sub-tropical savannah on a greater than weekly basis, from which they inferred leaf flush timing. Collection traps record phenology at a similar temporal resolution, as can micro-cores. The temporal resolution of both techniques is dependent on the frequency at which samples are collected, which may be daily but is typically longer [26; 107].  
For regions where asynchronous phenology occurs at high frequency (<1 week), methods that have higher revisit frequencies (daily) may be more beneficial (Table 1). In these regions, continuous high temporal resolution monitoring can feasibly be achieved through the use of devices such as accelerometers, dendrometers, fixed digital cameras or eddy covariance flux towers. Fixed digital cameras can take repeat images at temporal resolutions of less than one day, but are limited by viewing angle (often close to horizontal),meaning they can be dominated by plants closest to the camera [125; 129; 144]. Eddy covariance flux towers, often used in combination with digital cameras [135], can track the changes in gross primary productivity at very high temporal frequency, commonly 30 min intervals [113]. These three methods--accelerometers, fixed digital cameras and eddy covariance flux towers--have the potential to provide phenological data at very fine temporal resolution, but the first two of these methods often monitor just one or a limited number of plant individuals (Table 1). To get an idea of landscape or community level phenology, multiple devices need to be operated in a network, which can be infeasible in environments with challenging topography. Steep topography and its associated turbulence also presents a challenge for the use of eddy-covariance flux towers [145].  
UAV mounted cameras offer a good solution to overcome this limitation as they can be flown at low altitude, close to the canopy, where they can resolve individual-level plant phenology for many, potentially thousands, of individuals [146]. Furthermore, UAV revisit times can be optimised to match the temporal resolution of target species [25] (Table 1). Most consumer level UAVs are lightweight (<2 kg [138]), allowing relatively easy transport to remote locations where they can monitor phenology by flying from accessible areas, provided there are accessible areas within the UAV flight range that can be used as a launching site. UAVs are, however, still limited by maximum survey area (<100 ha for quadcopter or <1000 ha for fixed wing) and by battery life (~20 min per battery) which can impact revisit frequency--realistically no more than daily.