Aims Conversion of secondary forests to pure larch plantations is a common management practice driven by the increasing demand for timber production in Northeast China, resulting in a reduction in soil nutrient availability after a certain number of years following conversion. Nutrient resorption prior to leaf senescence was related to soil fertility, an important nutrient conservation strategy for plants, being especially significant in nutrient-poor habitats. However, the seasonal dynamics of leaf nutrients and nutrient resorption in response to secondary forest conversion to larch plantations is not well understood.
Methods A comparative experiment between larch plantations (Larix spp.) and adjacent secondary forests (dominant tree species including Quercus mongolica, Acer mono, Juglans mandshurica and Fraxinus rhynchophylla) was conducted. We examined the variations in leaf nutrient (macronutrients: N, P, K, Ca and Mg; micronutrients: Cu and Zn) concentrations of these tree species during the growing season from May to October in 2013. Nutrient resorption efficiency and proficiency were compared between Larix spp. and the broadleaved species in the secondary forests.
Important findings Results show that the seasonal variation of nutrient concentrations in leaves generally exhibited two trends, one was a downward trend for N, P, K, Cu and Zn, and another was an upward trend for Ca and Mg. The variations in foliar nutrient concentrations were mainly controlled by the developmental stage of leaves rather than by tree species. Resorption of the observed seven elements varied among the five tree species during leaf senescence. Nutrient resorption efficiency varied 6–75% of N, P, K, Mg, Cu and Zn, while Ca was not retranslocated in the senescing leaves of all species, and Mg was not retranslocated in Larix spp. Generally, Larix spp. tended to be more efficient and proficient (higher than 6–30% and 2–271% of nutrient resorption efficiency and resorption proficiency, respectively) in resorbing nutrients than the broadleaved species in the secondary forests, indicating that larch plantations had higher leaf nutrient resorption and thus nutrient use efficiency. Compared with Larix spp., more nutrients would remain in the leaf litter of the secondary forests, indicating an advantage of secondary forests in sustaining soil fertility. In contrast, the larch plantation would reuse internal nutrients rather than lose nutrients with litter fall and thus produce a positive feedback to soil nutrient availability. In summary, our results suggest that conversion from secondary forests to pure larch plantations would alter nutrient cycling through a plant-mediated pathway.