# Monitoring Forest Phenology in a Changing World
## 2 Monitoring Phenology
### Trade-Offs among Phenological Monitoring Techniques
#### 2.3.3 Cost Implications of Competing Methods  
In many phenological studies, cost represents a major and critical factor in the choice of method. Costs range over several orders of magnitude depending on the price per unit of monitoring equipment and the price of acquiring data from those units (Table 1). Single time point panchromatic high resolution satellite imagery (50 cm) can cost around US$24 per km\({}^{2}\) with a minimum order of 100 km\({}^{2}\) (=US$2400) and medium resolution (150 cm) around US$5 per km\({}^{2}\) with a minimum order of 500 km\({}^{2}\) (=US$2500) [147]. However, free satellite data is also available at high spatial and temporal resolutions, especially if the study area is located in North America or Europe [148], with the National Aeronautics and Space Administration (NASA: ~30 m with ~16 day revisit frequency) and the European Space Agency (ESA: ~10 m with ~5 day revisit frequency) hosting freely available datasets [149; 150]. Generally, as the monitoring device decreases in size, so does the cost and the area covered. Placing sensors on-board manned aircraft can also be excessively expensive, ranging from several thousand to hundreds of thousands of dollars [25; 125]. Per device, fixed digital phenological cameras provide a near-ground (tower) or ground-based remote sensing alternative to satellites or manned-aircraft for around US$1000 [151] or less, depending on the quality of the digital camera being used [152], but will sometimes incur the additional cost of building a tower to attach the camera to (Table 1). This can be avoided by attaching cameras to trees [153], inexpensive poles or pre-existing towers such as those used for eddy covariance flux [135], but this is not always a guaranteed option when establishing new monitoring locations. Similarly, drones are relatively cheap in comparison to the cost of acquiring satellite imagery or whole networks of digital cameras required to survey a suitable area (<US$2000 per aircraft [154]). Yet, UAVs are still more expensive per unit than collection traps, accelerometers, dendometers, and visually observing plants from the ground [25; 125]. Moreover, losses from crashes are not uncommon. In terms of equipment, the cheapest solutions for monitoring are visual assessment using binoculars, dendrometers, or collection traps. Binoculars can cost as little as US$20 for a suitable focal range to monitor canopy phenology, dendrometers can cost US$2 per unit, and collection traps can cost <US$10 per unit. Despite these methods being cheap per unit, it should be noted they can rapidly become costly when including the labour costs of carrying out monitoring surveys, placing traps in the field and collecting material from those traps (Table 1). This is especially true if you compared the cost per tree surveyed rather than the cost per unit to survey. To monitor a whole landscape, labour costs could end up making visual assessment and traps more expensive than alternative options, such as using satellite data of the region if available at a low price point, or to purchase a single UAV which could survey the same area in a much shorter time.