# Monitoring Forest Phenology in a Changing World
## 1 Introduction
### The Ubiquitous Importance of Plant Phenology  
Most biological organisms follow certain life cycle events, but plant phenology is arguably the most studied. Plant phenology includes processes such as leaf emergence, flowering, fruiting, seeding and leaf senescence, and these processes link strongly to ecosystem function and services (Figure 2; [6; 31]). Therefore, change to phenological events may have implications for biogeochemical cycles, including that of carbon [32], and for the population dynamics of species connected at different trophic levels or in competitive and mutualistic interactions [33; 34]. Plant phenology can further force temporal shifts in animal phenology [22; 35], with the potential for phenological mismatching if the species involved do not respond similarly to environmental changes [35; 36]. Leaf emergence (flush) phenology (the presence of new leaves on a tree canopy) is particularly important, as it directly controls processes such as primary productivity, carbon sequestration, nutrient cycling, water storage, and competition and coexistence dynamics (Figure 2; [37; 38; 39; 40; 41; 42]). For example, when canopy trees flush their leaves it alters the water and light environments for coexisting organisms by intercepting more of the incoming solar radiation and reducing throughfall from precipitation, increasing soil water uptake and consequently reducing soil water and soil water evaporation rates (Figure 2; [38; 43]). Indirectly, leaf flush can alter the presence of herbivous insects and consequently shift the presence of insectivorous birds and mammals (Figure 2). For instance, phenological change of oil-seed crops has resulted in changes in the abundance and type of herbivous insects present in agricultural landscapes, in turn causing diet shifts in insectivorous birds [44]. There is also a feedback loop to this process: some plant species aim to reduce herbivory by altering their phenological activity to flush before insects emerge, or by synchronising their leaf flushing with other plant species to reduce herbivory pressure [31]. Tropical plants that flush their leaves later in the season can suffer significantly higher damage by insects compared to those that flush early or in synchrony during the peak flushing phase [45]. This is not always the case in temperate regions however, where variation in leaf flush timing involves ecological trade-offs. For example, some Oak species (_Quercus_ spp.) in Eastern Europe flush early in the season to avoid summer d  
Figure 1: Timeline history of monitoring phenology. References for the specific time periods are: pre-1700 [4], 1700s [8], 1853 [9], 1900s [10; 11], 1920s [12], 1950s [13], 1960s [14; 15], 1965-83 [16; 17; 18; 19], 1900s [20], 2003 [21], 2000s [3; 22], 2007 [23; 24], 2012 [2; 25; 26; 27], 2016 [28; 29], and 2020 [30]. suffer from later spring frost and insect herbivory, whereas other Oak species flush up to six weeks later in the season, avoiding insect herbivory but suffering from summer droughts [46; 47]. Flowering and fruiting phenologies are often overlooked, possibly because of the more obvious and critical role played by leaf emergence cycles in many ecosystem processes, however, they also remain key drivers of ecosystem processes [3]. Flowering phenology, for example, can directly impact the diversity of both pollinators and plants (Figure 2). Most of the world's plants rely on animal pollination for successful reproduction, especially in the tropics where the proportion of animal-pollinated species has been estimated at 94% [51]. Flowering phenology has been demonstrated to positively structure mutualistic plant-flower visitor networks [57], as well as regulate their modularity--a measure of decoupling in densely connected networks [58]. In addition, fruiting and seeding can force ecosystem processes such as fruit hording and seed dispersal, which in turn affects the diversity and distribution of taxa at higher trophic levels (Figure 2).