# Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model
While these logistic models are usually parameterized with ground-observed ecosystem productivity, camera-based indices, and RS-predicted vegetation index at different ecological scales [8,31], the growing availability of eddy covariance (EC) data, which captures direct measurements of CO\({}_{2}\) fluxes from whole ecosystems, has led to their widespread use in parameterizing phenological models [20]. Estimates of gross primary production (GPP) are then fit to these logistic functions to obtain dynamic representations of important phenology nodes, such as the start, end, and overall length of the growing season [18,26]. This improved understanding of the growing season of forests using GPP can aid the scientific community's understanding of the dynamic characteristics of LSP at different scales, which may improve our estimates of regional carbon budgets with climate change [20]. However, past research linking phenological processes in forests using EC technology has been mainly concentrated in temperate regions, with a focus on the effects of geographic and environmental controls, such as latitude/longitude, air/soil temperature, precipitation, and radiation [32]. While subtropical regions also have extensive forested areas, EC-based phenological studies in these regions have received less attention [28,33]. In addition, relatively few studies have examined the complex interactions between weather and prescribed fire, a common forest management activity in many regions, both of which may control forest phenology and ultimately their system's carbon dynamics [34]. Prescribed fire is used extensively in the southeastern US to maintain native biodiversity and reduce the risk of wildfire; however, prescribed fire frequency and intensity may change with global climate change due to the forecasted increased drought conditions and lengthened growing season [35]. In light of the potential for phenology to be impacted by climate change-fire interactions, there is a need for a better understanding of the mechanisms driving changes in phenological processes in subtropical forests. We used long-term, EC-measured CO\({}_{2}\) flux and micro-meteorological data in three subtropical coniferous forests [36], to study phenological characteristics and responses to prescribed fire, weather events, and variation in air temperature using a photosynthesis-based phenology model. This study addressed the following hypotheses: (1) Soil water availability will significantly affect ecosystem-scale phenological processes, i.e., higher soil water availability will be associated with a longer growing season. (2) Phenological nodes describing the growing season length (SOS, EOS, and LOS, etc.) for the sites will be affected by climate (e.g., air temperature, precipitation, and radiation) and prescribed fire, i.e., increased air temperature will prolong LOS (early SOS and late EOS), while drought will lead to a shortening of the growing season. (3) Prescribed fire during the dormant season will delay the SOS, and this will be compounded if water availability is limited during the recovery period. By testing these hypotheses, we will increase our understanding of subtropical forest phenological processes and their response to climate change and weather anomalies, as well as provide a basis for carbon budget planning and mitigation of global change.