Distinct seasonality of nutrients in twigs and leaves of temperate trees

Seasonal variation of nutrient concentrations in different organs is an essential strategy for temperate trees to maintain growth and function. The seasonal variations and variability (i.e., seasonality) of leaf nutrient concentrations have been well documented, while the trends and magnitudes of such seasonal variations in other tree organs (e.g., twigs) and their associations with leaf nutrients remain poorly understood. We measured the concentrations of 10 nutrients (nitrogen, N; phosphorus, P; potassium, K; calcium, Ca; magnesium, Mg; iron, Fe; manganese, Mn; copper, Cu; zinc, Zn; boron, B) in twigs and leaves of four temperate tree species (i.e., Pinus tabuliformis, Ginkgo biloba, Cotinus coggygria, and Sophora japonica) to explore their seasonal variations and seasonality. Our results showed that macronutrient concentrations (N, P, K, Ca, and Mg) were significantly higher in leaves and micronutrient concentrations (Fe, Mn, Cu, and Zn) were significantly higher in twigs. Concentrations of P and K both showed a negative seasonal covariation between twigs and leaves, while Ca, Fe, Mn, Cu, Zn, and B showed an opposite relationship. Compared with mobile nutrients, nonmobile nutrients exhibited significantly greater seasonality in the leaves but there were no such differences in twigs. The seasonality of nutrient concentrations in twigs was significantly stronger than in leaves and they were positively correlated. Additionally, nutrients with higher physiological requirements in leaves showed weaker seasonality, confirming the hypothesis of seasonal stability of high-demand nutrients, while such relationships were not statistically significant for twigs. This study demonstrates distinct seasonality of nutrients in twigs and leaves of temperate woody plants. These findings highlight that high-demand nutrients show stronger seasonal stability in leaves but not in twigs and uncover the seasonal coordination between twigs and leaves as a nutrient conservation strategy.