Abstract:

Aims Investigating the needle phenotypic variation and its geographical distribution pattern of related species complexes will aid in understanding conifer geographic variation, population dynamics, and physiological and ecological responses to geographical climates. Here we focus on the Pinus kesiya, P. yunnanensis, and the hybrid species P. kesiyavar. langbianensis, which are primarily distributed across tropical and subtropical Southeast Asia, exhibiting abundant climate and genetic variation.

Methods We selected thirty-one representative populations covering the distribution areas of three pine species, and sampled ten individuals from each population. Eight traits were measured, including needle length, stomatal density, stomatal guard cell length, stomatal guard cell width, woody thickening layer length, woody thickening layer width, stomatal cavity length, and stomatal cavity width. Variation between species and populations was assessed using a nested variance analysis. Principal components analysis was applied to evaluate the underlying dimensionality of the needle variation. Clustering analysis among populations was performed using the Ward method to infer the population structure. Pearson’s correlation coefficient between latitude and needle characters was assessed. Finally, a multiple linear regression model was used to identify the main environmental factors influencing needle trait variation.

Important findings The variation coefficients of the eight needle traits ranged from 12.01% to 34.08% across populations. The phenotypic differentiation coefficient was higher for stomata guard cell length, woody thickening layer length and width, needle length, and stomatal density. Most morphological needle characteristics were significantly different between P. yunnanensis and P. kesiya,while P. kesiyavar. langbianensis showed intermediate values between the two parental pines. Hierarchical clustering analysis showed that the variation pattern of needle traits was related to geographical region. Needle length and stomatal density were positively correlated with latitude, while the stomatal cavity length and width were negatively correlated with latitude. The key environmental factors affecting the needle traits of the three pine species were mean temperature during the driest quarter, precipitation seasonality, mean diurnal range, precipitation during wettest month, and precipitation during the driest month. The needle traits of the three pine species displayed high interspecific and interpopulation variation. The significant latitudinal gradient trend in needle trait variation indicates long-term evolutionary adaptation to the environment. The response of needle character variation to environmental factors can provide essential insights for geographical provenance selection in afforestation breeding

Key words: Pinus yunnanensis, Pinus kesiya var. langbianensis, Pinus kesiya, geographical variation, needle, anatomical characters, temperature, precipitation