The role of understory phenology and productivity in the carbon dynamics of longleaf pine savannas

Abstract

Savanna ecosystems contribute ~30% of global net primary production (NPP), but they vary substantially in composition and function, specifically in the understory, which can result in complex responses to environmental fluctuations. We tested how understory phenology and its contribution to ecosystem productivity within a longleaf pine ecosystem varied at two ends of a soil moisture gradient (mesic and xeric). We used the Normalized Difference Vegetation Index (NDVI) of the understory and ecosystem productivity estimates from eddy covariance systems to understand how variation in the understory affected overall ecosystem recovery from disturbances (drought and fire). We found that the mesic site recovered more rapidly from the disturbance of fire, compared to the xeric site, indicated by a faster increase in NDVI. During drought, understory NDVI at the xeric site decreased less compared to the mesic site, suggesting adaptation to lower soil moisture conditions. Our results also show large variation within savanna ecosystems in the contribution of the understory to ecosystem productivity and recovery, highlighting the critical need to further subcategorize global savanna ecosystems by their structural features, to accurately predict their contribution to global estimates of NPP.

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

“…Previous studies have reported the long-term precipitation dynamics of these sites during the study period (2009-2017) [40,42], and showed short-term seasonal annual drought and water stress from June to August [38]. This caused significant day-to-day fluctuations in summer ecosystem productivity (GPP decline and recovery) and changes in summer phenological processes across sites.…”

“…Prescribed fire and weather events (meteorological drought and flood) affected ecosystem-scale phenological processes and productivity [40,49], especially summer phenology (Table 2; Appendix B, Figure A2) because of the variability of water availability. To develop a more thorough understanding of the phenological patterns of the study sites, we investigated SOS, EOS and LOS patterns and characteristics at an inter-annual scale ( Figure 6; Appendix B, Table A1 and Figure A2).…”

“…Increased post-fire soil hydrophobicity has been shown to lead to short-term water competition, which limits the development of understory leaves, thus reducing GPP which drives our phenological model [59]. Greater soil water holding capacity is related to greater understory plant diversity and biomass [40], which resulted in a relatively longer recovery time for productivity following fire. To fully understand the meteorological and biological mechanisms of how prescribed fire influences spring phenology, it may be necessary to study fire intensity by site, as greater fire intensity may be associated with more vegetation loss on ignition, leading to a longer period of recovery [59].…”

“…Since the three study sites are in a warm subtropical region, rarely are there light or temperature limitations which constrain GPP. Therefore, water availability is an important factor which supports and affects GPP (understory and canopy; Appendix F, Figure A5) and thus day-to-day phenological processes at the ecosystem scale [40] (Appendix B, Figure A2).…”

Prescribed fire science: the case for a refined research agenda

“…Prescribed fire objectives necessitate a detailed and nuanced understanding of the fine-scale micrometeorological ) and phenological conditions (Wiesner et al 2019) that influence fuel moisture dynamics of both live and dead fuels (Jolly et al 2014;Kreye et al 2018). Changes over time represent a "fourth dimension" of fuels characterization.…”