Differentiation in seed mass and seedling biomass allocation in Prosopis laevigata throughout its distribution range in Mexico is associated to water availability

Abstract

Background: Seedling establishment depends on the quality of the seeds and environmental conditions. Differential biomass allocation in emergent seedlings probably constitutes a relevant adaptive response of populations along environmental gradients. Questions: Are there differences in seed mass and biomass allocation in seedlings among Prosopis laevigata populations? Is this variation correlated with environmental variables? Studied species: Prosopis laevigata (Humb. & Bonpl. ex Willd.) M.C.Johnst (Fabaceae). Study site and dates: Thirteen localities along the distribution of P. laevigata in México. From 2016 to 2020. Methods: Seeds were collected from four or five mother trees per locality. Seed mass (SM) was obtained in ten seeds per mother and six functional traits indicative of biomass allocation were measured in the seedlings after 10 days of germination. Population mean values were obtained for the six traits plus SM and subjected to a principal component analysis (PCA). Population scores on the first two axis of the PCA were regressed against environmental variables from the collection localities using a stepwise regression model. Results: Populations displayed functional variation congruent with alternative biomass allocation strategies. The conservative strategy was characterized by larger seeds and seedlings with denser tissues and a higher investment in root biomass, while the opposite characterized the acquisitive strategy. Actual evapotranspiration in May, isothermality and soil water content in February were environmental variables that significantly predicted population scores on the first two axes of the PCA. Conclusion: Water availability gradients influence seed mass and seedling biomass allocation variation among P. laevigata populations.