Objective Litter layer is an important component of forest ecosystem, and it is of unique and key significance to study the spatial distribution characteristics of nutrients for maintaining forest ecological environment and sustainable management.
Method To investigate the spatial heterogeneity of litter nutrients at different decomposition stages, we established four 1 ha permanent sample plots in temperate spruce-fir mixed forests. Litter collection and stand survey were conducted using an equidistant grid point sampling method. Litter samples in semi-decomposed (F) horizon and completely decomposed (H) horizon were determined to analyze the spatial variations of litter nutrient concentrations using statistical analysis and kriging interpolation.
Result (1) In the F horizon, concentrations of litter organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) were 421.68, 18.86 and 1.26 g/kg, respectively, and they were 350.78, 17.80 and 2.68 g/kg in the H horizon. Litter OC and TN decreased but TP increased with the increase of decomposition degree. The coefficients of variation of litter OC, TN and TP in the F and H horizons varied in the four sample plots with a range of 10.94%−65.63%. The spatial variation degrees of litter OC and TP in the F horizon were smaller than those in the H horizon. (2) Litter OC, TN and TP concentrations showed higher spatial autocorrelation, primarily resulting from the structure factors at the forest community scale. Litter nutrient concentrations in the four sample plots showed a strip and patch gradient distribution. Litter OC in the F horizon had similar spatial distribution pattern to the H horizon. However, the high-value centres of TN and TP in the F horizon were generally lower in the H horizon. The fractal dimension of litter OC and TP concentrations in the F horizon was higher than that in the H horizon. This indicated that the F horizon exhibited a more complex spatial pattern, whereas litter TN had a better structure and stronger spatial dependence in the F horizon than that in the H horizon. (3) Litter OC and TN in the H horizon had a significant negative correlation (P < 0.05) with the standing crop of litter but the TP concentration with an opposite relationship. Litter OC and TN in the H horizon were significantly affected by water holding capacity. In addition, litter OC, TN, TP and their stoichiometric ratios were significantly correlated. Litter TN and TP in the F horizon were significantly affected by biodiversity indices and species number, while those in the H horizon were mainly affected by the proportion of conifer species and stems. However, litter TN and TP were more strongly related with stand characteristics in comparison with OC.
Conclusion The spatial distribution characteristics of nutrients in litter layer over temperate natural spruce-fir mixed forest are affected by litter properties (e.g. the degree of decomposition and water holding capacity.) and stand structure (e.g. tree species and stand density.).
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