Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model

Abstract

Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenological processes. Dormant season fire had no significant effect on phenological processes by site, but we observed differences in the start of the growing season (SOS) between fire and non-fire years. Fire delayed SOS by 10 d ± 5 d (SE), and this effect was greater with higher soil water availability, extending SOS by 18 d on average. Fire was also associated with increased sensitivity of spring phenology to radiation and air temperature. We found that interannual climate change and periodic weather anomalies (flood, short-term drought, and long-term drought), controlled annual ecosystem phenological processes more than prescribed fire. When water availability increased following short-term summer drought, the growing season was extended. With future climate change, subtropical areas of the Southeastern US are expected to experience more frequent short-term droughts, which could shorten the region’s growing season and lead to a reduction in the longleaf pine ecosystem’s carbon sequestration capacity.

Uncertainty in parameterizing a flux‐based model of vegetation carbon phenology using ecosystem respiration

“…The long‐term average annual air temperature is 19.1°C, with the lowest monthly average air temperature in January (10.7°C) and the highest monthly average temperature in June (27.4°C, Starr et al, 2015). While long‐term winter (November to December) air temperature over the past 60 years has been estimated to be 13°C (Gong et al, 2021), an abnormal winter air temperature increase occurred during 2015 and 2016, with winter temperatures, which were ~3.8 and ~2.2°C higher than the long‐term average at both sites. The two sites are situated within 5 km of each other and differ in soil water holding capacity and forest structure (Table 1; Wiesner et al, 2020, 2021).…”

“…This may be because the productivity of evergreen species in these study sites does not depend as strongly on leaf production compared to deciduous forests (Wu et al, 2014, 2017); coupled with the two‐year needle replacement cycle, the greenness of needles cannot fully predict the seasonal pattern of VCP (Wu et al, 2014, 2017). Instead, medium‐ and/or long‐term weather changes induced by climate change, such as photoperiod in spring and air temperature in summer, may contribute more to VCP dynamics of these forest (Gong et al, 2021; Kong et al, 2020; Wu et al, 2014). In addition, due to the 8‐day remote sensing data cycle and limited spatial resolution, some LSP signals may be missed or delayed by the satellite sensors.…”

“…In the three cases, we reported such abnormal behavior, the default LOP prediction from the GR method overestimated Re, while the default LOP prediction from the TD method underestimated Re (Appendix S1: Figure S6). However, water did not affect DOP (Gong et al, 2021; Gu et al, 2009).…”

“…Re can also be used to parameterize plant community models to better quantify biological and nonbiological disturbances in these systems (Yang & Noormets, 2020). While simple functions may not be able to describe short‐term variation in ecosystem VCP (J. Wang et al, 2018; Yang & Noormets, 2020), the nine‐parameter function developed by Gu et al (2009) has been shown to have flexibility in describing vegetation phenology responses (Gong et al, 2019, 2021). The model also has better performance when describing ecosystem VCP under abnormal environmental conditions (Yang & Noormets, 2020).…”

Tree-Ring Width Data of Tsuga longibracteata Reveal Growing Season Temperature Signals in the North-Central Pearl River Basin since 1824 AD

“…For example, increasing drought stress could have major negative impacts on the water-limited primary forests of the Croatian Dinaric mountains [4]. For subtropical coniferous forests in the south-eastern United States, increasing water availability after short-term summer drought could significantly affect ecosystem phenological processes by extending the growing season [5]. Under a tropical monsoon climate, high temperatures in the dry season may result in high mortality for mangrove forests in eastern Thailand, leading to long-term reduction of forest biomass [6].…”