The Dynamics Of Xylem Embolism And Non-structural Carbohydrates Pools For Woody Plants In Yuanjiang Hot-dry Savanna During A Prolonged Drought Event

Extreme drought events associated with climate change,have become more frequent in the past few decades,resulting in large-scale tree mortality and forest dieback globally."Hydraulic failure" and "carbon starvation" are two mechanisms often used to account for drought-induced plant death,while both of which are rarely tested-.Moreover,savannas have been less studied although they cover approximately 20%of the earth's land area,with regards to their response to extreme drought,compared to forests.Affected by El Nino,most regions in Southeast Asia countries experienced extreme drought events in 2019 and 2020.The drought event caused serious impacts on the agricultural and forestry in the region.Here,we quantified branch dieback and individual mortality,and the associated physiological responses of 30 common woody species in a Chinese savanna ecosystem to an incidence of extreme drought caused by the strong El Ni(?)o event in 2019 and 2020.These 30 species were classified into three groups based on leaf habit:evergreens,deciduous species and semi-deciduous species.The main findings are:(1)With repeated water potential measurements during the droughts,we found most species experienced severe water stress during the extreme drought events compared with dry season in normal years.Most species showed very negative stem water potentials(indicator of plant water status).Deciduous species generally maintained a more stable and less negative stem water potentials during the droughts compared to evergreen and semi-deciduous species.In contrast.evergreens exhibited the most negative stem water potentials.(2)Most species had high native embolisms in terminal branches during the drought.Species means of the percentage loss of hydraulic conductivity[PLC]ranged from 4.8%to 76.7%in the dry season,and the remaining stems maintained 3.7%to 78.9%at the end of the drought events.(3)The content of total nonstructural carbohydrates(NSCs)in terminal stems were not lower or even higher than those in the rainy season while different NSC components showed significant seasonality.Moreover,the NSC concentrations of most species remained high at the end of drought which suggest no "carbon starvation" occurred.(4)Four evergreen shrubs were used to investigate the associations between the hydraulic failure,NSCs pools and their drought response during the drought.The four species showed 80%to 100%branch dieback ratio,while individual mortality ratio was only found in Tarenna depauperata(4.5%)over a two-year period.These four species showed high resistance to stem embolism(P50 ranged from-5,62 MPa to-8.6 MPa),while the stem minimum water potentials reached-7.6 MPa to-10.0 MPa during the drought event.This had caused high native embolism(PLC,23%to 65%)in terminal branches during the drought and the remaining stems maintained 15%to 35%PLC at the end of the drought.Overall,the content of total NSC and its components in the stem were not lower or even higher than those in the rainy season in three of the four species(T.depauperate,Maytenus esquirolii,and Murraya exotica).Our results suggest high tolerance of savanna shrub species to extreme drought.This could be facilitated by high embolism resistance in stems,and branch shedding,to maintain individual water and carbon balance with vulnerable stems dying off during the drought.The findings of this study provide a mechanistic understanding of drought-driven mortality during the 2019-2020 El Ni(?)o drought event,and the potential response to drought events under future climate change scenarios for plants with contrasting leaf habits in dry-hot Yuanjiang savanna.