Wood formation mechanisms of Cedrela fissilis Vell. in extreme years: Climatic, anatomical and chemical predictors

Extreme climatic events in the Amazon region open questions about how the interactions between exogenous and endogenous variables occur in the wood formation of widely distributed tree species such as Cedrela fissilis Vell. In this study, eleven mature-phase trees from a C. fissilis population growing on a non-flooded forest in the southern Amazon basin were selected to determine the interaction between climatic (precipitation, temperature, and moisture indicators: precipitation minus potential evapotranspiration - P-PET and standardized precipitation evapotranspiration index - SPEI), chemical (xylem concentrations of Al, P, S, Ca, K, Mn, Fe, Sr and their meaningful molar ratios) and anatomical (proportion of vessels – PV, fibres – PF, and parenchyma – PP) predictors to explain the variability of ring width (RW) and wood densities (average – RD, minimum – MND, and maximum – MXD). Decision tree regression was applied for understand this interaction in three datasets, values of narrow or less dense, complacent, and wide or dense rings. Narrow or wider rings vary manly associated to hydraulic tissues (PV) and SPEI. Less dense and complacent (in density values) rings vary manly associated to structural tissues (PF) and elements involved in soil-root interactions (Al) and resistance to water stress (Ca/Mn). Less dense MND vary associated to storage and transportation tissues (PP) and elements involved in soil-root interactions (Al and Mn), whereas dense MXD vary associated to structural (PF) and hydraulic (PV) tissues, precipitation and elements related to cambial activity (Ca and K/Ca). In the context of an increase in the frequency and intensity of droughts in the region, the formation of narrower and less dense rings in C. fissilis is expected. In this sense, our results suggest that higher values, which ensure the functionality of these variables under these extreme conditions, are associated with a higher proportion of structural tissues to the detriment of hydraulic ones, in addition to higher K, Mn, and Al storage in the xylem, which mediate cambial activity based on cell expansion, reduced vulnerability to water stress, and healthier root system conditions, respectively. Altogether, this study offers clues to understand variables combination associated with wood formation in the face of extreme water-availability stress that can lead hydraulic failure, carbon starvation, increasing vulnerability, and even specie´s dieback.