Influences of Branch Death on Leaf Nutrient Status and Stoichiometry of Wild Apple Trees (Malus sieversii) During Annual Growth Period in the Western Tianshan Mountains, China

Background: Malus sieversii (wild apple) is a tertiary relict tree species and a key ancestor of the common cultivated apples today. In recent years, a serious ecological degradation has occurred in the wild apple forests, and many individuals of wild apples have died or have severe dead branches. Whether the death of branches would lead to the change in leaf nutrient stoichiometry of M. sieversii remains unclear. In this study, the N, P, and K contents and the N:P, N:K, and P:K ratios of wild apple trees with three dead branch rates (DBRs; low [LD, 80%]) during annual growth period, elemental scaling relations, and the possible influencing factors were systematically compared and discussed. Results: : Leaf N, P, and K decreased during growth, and N and P did not show significant differences among the different DBRs; however, the HD wild apple trees had the lowest K contents in the photosynthetic and the reproductive organs. Flowers had higher P and K contents than leaves, whereas fruits had low N content. The growth of M. sieversii was always limited by N due to low N:P ratio and N resorption efficiency. The scaling exponents of leaf N–P, N–K, or P–K among the three DBRs did not show any significant difference, and the relationships between stoichiometric ratios and three elements were consistent among three DBRs, revealing an inherent property of M. sieversii . Leaf K content was strongly and negatively influenced by most soil variables. Leaf nutrients were significantly and positively correlated with precipitation and relative humidity, whereas temperature showed the opposite trend. Conclusions: : These findings suggest that increasing water input, and improving plant K content and utilization efficiency may be conducive to enhance the resistance and recovery ability of diseased wild apple trees.