# Monitoring Forest Phenology in a Changing World
## 2 Monitoring Phenology  
### Synchrony, Asynchrony and the Challenge for Phenological Monitoring  
Habitat and climate change are evidently impacting phenology in ways that generate cascading ecosystem impacts, so there is self-evidently a need to monitor plant phenology effectively if we are to understand and mitigate the ecosystem-level effects of these changes [34; 86]. This point is not new: [PERSON] et al. [87] included remotely sensed land-surface plant phenology as one of the six Essential Biodiversity Variables to monitor global change, and the Intergovernmental Panel on Climate Change (IPCC) defined phenology as \"perhaps the simplest process to track changes in the ecology of species in response to climate change\" [23]. In light of such high profile calls for improved global monitoring of phenology, there is a timely need for a consensus on the monitoring techniques used to quantify phenological patterns and changes.  
In most temperate and boreal biomes, plant phenology is seasonal, with individual plant phenologies often synchronising twice annually (spring and autumn) to create community and landscape level changes [40]. In many tropical regions however, phenological events can vary from complete intraspecific or interspecific synchrony (masting) to extreme asynchrony, and from constant activity to recurrent short pulses [31; 88]. Moreover, synchronies in phenological events can be intra-annual or supra-annual such as that of the multi-year cycles induced by El Nino Southern Oscillation events [89]. Together, this creates a mosaic of individual and community level phenological events that vary across space and through time. If we then add changes in spatial scales and other dependencies such as changing environmental conditions, coexistence dynamics, individual tree characteristics, and location that all influence phenology [90; 91; 92], it rapidly becomes apparent that the simple-sounding task of monitoring phenology masks a challenging problem.  
Multiple approaches have emerged over the years to monitor phenology including the use of satellite-based sensors, airborne (plane) mounted sensors, UAV (drone) mounted sensors, fixed digital phenological cameras, collection traps, accelerometers, eddy covariance flux towers, micro-coring and dendrometers, visual assessment, citizen science networks, and genetics (summarised in Table 1; historical introduction of each method outlined in Figure 1). Each method has its own advantages and limitations, often making them more applicable in certain environments and contexts. Here, we examine how each method compares when considering key factors that influence the effectiveness of monitoring phenology: spatial resolution, temporal resolution, cost in terms of equipment, acquisition or labour, and data processing and interpretation requirements. Furthermore, we consider each factor in the context of monitoring phenology in different environments or for different purposes.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & & & & \\ \hline \multirow{3}{*}{**Monitoring Method**} & \multirow{3}{*}{**Measurement**} & **Spatial Resolution** & **Temporal Resolution** & **Deployment Cost (s)** & **Acquisition Cost per** & **Acquisition Cost per** & **Options** & **Acaptability Durability** & **Aequiting Man- Power** & **Data** & **Key Summary** & **Image-offs (ProC/on)** \\  & & **Indirect- Image** & 0.31–1000 & 24–384 & 885,000,000 & 0-2500 & Low & High & Low & High & NA & High & Spectral & \\  & & **Frequency** & & & & & & & & & & & & & & & \\ \hline \multirow{3}{*}{**Monitoring Method**} & \multirow{3}{*}{**Measurement**} & \multirow{3}{*}{0.5–500} & \multirow{3}{*}{24} & 10,000-30,000 & \multirow{3}{*}{7000} & \multirow{3}{*}{Medium} & \multirow{3}{*}{High} & \multirow{3}{*}{Low} & \multirow{3}{*}{High} & \multirow{3}{*}{Low} & \multirow{3}{*}{High} & \multirow{3}{*}{High} & \multirow{3}{*}{High} & \multirow{3}{*}{High} & \multirow{3}{*}{Low} \\  & & **Resolution** & & & & & & & & &