Abstract:

【Objective】 The seasonal variations in ionic fluxes in throughfall and stemflow resulting from washoff and leaching by forest canopy indicate significant processes affecting the biogeochemical cycling of forested ecosystems, particularly in temperate deciduous forests with distinct phenological seasons. Previous studies on the water chemistry of forest ecosystems have been focused on the growing season as a time scale to discuss the changes in element fluxes, but ignored the influence of leaf phenology changes on water chemistry during the growing season. Therefore, our objective is to determine the impacts of phenological changes on the water chemistry of forest ecosystems and to better understand the nutrient element cycle process of temperate deciduous forest ecosystems.【Method】 In this study, we selected a Larix gmelinii plantation in the Northeast Forestry University Urban Forestry Demonstration Base as the research subject. Thirteen self-made rain gauges with a diameter of 20 cm were laid across the center of the observation plot, and five L. gmelinii trees were selected to install the stemflow collectors. Simultaneously, a tipping bucket rain gauge and three self-made rain gauges were arranged outside the forest. We measured and sampled in situ the solution of bulk precipitation, throughfall, and stemflow after each rainfall event from May 1 to October 30, 2015. After filtering and acidification of the water sample, the concentrations of Na ⁺ and K⁺ were measured with a flame atomic absorption spectrophotometer to explore the role of the canopy at different phenological stages (leaf expanding stage, full leaf stage, and senesced leaf stage) on the concentrations and net inputs of Na⁺ and K⁺ in the process of rainfall distribution.【Result】 Throughout the observation period, the concentrations of Na⁺ and K⁺ in rainfall were 0.45 and 1.89 mg/L, in throughfall were 0.44 and 2.48 mg/L, and in stemflow were 1.98 and 18.63 mg/L, respectively. The highest and lowest Na⁺ concentrations in the rainwater occurred at the senesced and full leaf stages, respectively. Meanwhile, higher K⁺ concentration in rainwater was also found at the senesced leaf stage, and the lowest concentration at the leaf expanding stage. In the throughfall, both Na⁺ and K⁺ concentrations were in the order of senesced leaf stage > full leaf stage > leaf expanding stage, and in the stemflow, the order was leaf expanding stage > full leaf stage > senesced leaf stage. During the whole growing season, the canopy intercepted Na ⁺ with the values of 0.252 kg/hm², and it intercepted both in the leaf expanding stage and senesced leaf stage, but showed leaching during the full leaf stage. The interceptions of Na⁺ were 0.143 and 0.193 kg/hm², with interception rates of 30.63% and 48.22% at the leaf expanding stage and senesced leaf stage, respectively, and the leaching flux of Na⁺ was 0.083 kg/hm². In contrast, the leaching flux of K⁺ was 0.903 kg/hm² from the canopy due to rainfall during the growing season, with fluxes of 0.999 and 0.157 kg/hm² at the leaf expanding stage and full leaf stage, respectively, and the interception was 0.254 kg/hm² at the senesced leaf stage, with an interception rate of 20.25%. In conclusion, the redistribution of Na⁺ and K⁺ in the rainfall affected by the larch plantation canopy varied considerably with phenological stages. 【Conclusion】 The ion concentration in bulk precipitation changed significantly after passing through the forest canopy, and the change intensity differed according to the phenological stages and the ions. Throughout the observation period, the L. gmelini forest exhibited an interception effect on Na ⁺, but showed a leaching effect on K⁺. In other words, the leaf phenology of L. gmelini affects the transfer of Na ⁺ and K⁺ in rainfall water. The results of this study can provide a reference for further understanding the nutrient cycling process associated with hydrological processes and its sustainable management in temperate forest ecosystems in China.

Key words: urban forest, phenological stage, throughfall, stemflow, water chemistry, metal element, nutrient input