Effects of growth decline on twig functional traits of wild apple trees in two long-term monitoring plots in Yili Valley: Implication for their conservation

Abstract

Malus sieversii (wild apple trees) is a tertiary relict tree species and a key ancestor of the cultivated apple trees today. In recent years, serious growth decline and individual death have occurred for M. sieversii . Whether growth decline would lead to the change in twig (composed of leaves and one-year stem) functional traits and their associations of wild apple trees remains unclear. Two long-term monitoring plots (1 hm 2 ) for severely declined and relatively healthy wild apple trees, respectively, were set up in Yili Valley, China. Based on two-year investigations, the differences in tree functional traits and interrelations between the two plots and the influencing factors were analyzed. The crown projective cover, dead branch percentage, and fruit yield were markedly different between two sites and two years. Leaf nitrogen and phosphorous concentrations of declined trees were generally less than healthy trees, although the soil of the former was more fertile. The nitrogen was more susceptible to the stressing, especially for declined trees. Concentrations of stem nitrogen, phosphorous, and potassium and leaf potassium were relatively stable in both plots. The plant trait network analysis indicated that leaf nitrogen (per unit area) that promoted plant growth was the hub trait (meaning that it is correlated with most traits) for healthy trees, while leaf potassium (per unit mass) that related to resistance enhancement became the hub trait for declined trees. Meanwhile, the covariant characteristics of twig traits between the two plots differed obviously, and the soil-trait and growth-trait correlations became more negative for declined trees than healthy trees, indicating that intra-trait and environment-trait associations had changed greatly when trees declined. The ordination analysis revealed that total and available soil nitrogen, phosphorous, and total potassium, pH and plant growth parameters significantly influenced twig traits. Our results demonstrated that twig functional traits and their interrelations of wild apple trees can indicate tree decline trend. We strongly recommend that managers should take measures to prevent the decline of healthy wild apple trees as soon as possible to realize the sustainable existence of this precious germplasm resource. The results provide a theoretical support for scientific management of wild apple populations in the Tianshan Mountains in Central Asia.