Forest ecosystems play an important role in the global carbon cycle. Alterations in climate and management practice are expected to cause shifts in ecosystem phenological responses that will affect forest carbon dynamics. Our project used long-term CO2 flux data (27 site-years, 2009-2017) to assess phenological variation at three longleaf pine sites managed with prescribed fire at the Jones Center Ichauway (JCI), These sites are distributed across an edaphic gradient, comprising a mesic, xeric, and intermediate site. The period of study encompassed three years of drought conditions, preceded and followed by two years of normal precipitation. We used the GPP-based phenology model to test the following hypotheses: (1) Whether naturally occurring soil water availability gradients can cause differences in ecosystem-scale phenological processes. (2) Whether forest management in early spring will delay the start of the ecosystem-scale growing season.
Results/Conclusions
Due to the gradient of soil water availability, we found differences in the start of growing season (SOS), end of growing season (EOS) and length of growing season (LOS) by site. The SOS ranged from 1-98 d, with the mesic site having lower SOS than the xeric site. The LOS ranged from 262-364 d, with the xeric and intermediate averages lower than that of the mesic. The EOS ranged from 335-365 d. The longest LOS and earliest EOS for all three EC sites were all in 2017. Soil water availability is a potentially significant environmental control factor affecting phenological processes at the ecosystem scale for these three subtropical longleaf pine ecosystems. SOS in years where prescribed fire was applied was delayed compared to the non-fire years. The dynamic changes of EOS and LOS seem to have weaker relationships with fire. Fire control delayed SOS by 10 d on average, and along the gradient of soil water availability at the three EC sites, the characteristics of SOS delay became more apparent. Further investigation of human activities and the synergistic effects of the climate system on forests in the future can provide a reference for global climate change and forest management.