The impact of canopy composition on the nutritional statusof an admixed spruce and beech forest at Solling,central Germany

Abstract

It was hypothesised that in mixed spruce-beech forest ecosystems the heterogeneity in canopy composition may create different micro-sites (units) in the forest floor and the mineral soil with different ecological characteristics. Therefore, different types of canopy compositions (canopy classes) were used to identify the variability of water and element fluxes (via throughfall and litterfall), soil and soil solution chemistry, litter decomposition and soil respiration. The investigation was carried out in a mixed sprucebeech stand in Solling, central Germany. Two different plots were selected for this study representing the most contrasting cases of mixed forests types, which were i) a site greatly dominated by spruce trees (the spruce dominated plot, SDP) with two beech trees in-between and ii) a directly neighbouring site which was dominated by beech trees and having a single spruce tree in-between (the beech dominated plot, BDP). The canopies of the two plots were classified in four categories: pure beech, pure spruce, mixed canopy and gap. Throughfall water was significantly lower and major element fluxes were higher under spruce than under beech in both plots. This indicated that the nutrient inputs under the canopies of individual trees were driven by species-specific properties of the canopies and were quite independent of the degree of admixture. With the exception of K+, mixed canopies showed intermediate element inputs via throughfall, compared with pure canopy classes. The K+ input, however, was significantly greater under mixed canopies due to interactions of the canopies, leading to higher leaching rates for K+. Throughfall was the main source of heterogeneity in nutrient inputs, while foliar litterfall input was almost equal between sub-plots and thus had a homogenising effect on annual nutrient fluxes in the beech-spruce mixed stands. Differences in soil chemistry under different canopy classes were mainly observed in the forest floor and top mineral soil layers. Significant effects of the canopy composition on pH (CaCl2) values of the forest floor and mineral soil were detected between the gap (significantly higher) and spruce (significantly lower) sub-plots in the spruce dominated plot (SDP). The water fluxes (lower under spruce) and chemistry (higher concentration of elements under spruce) of throughfall could explain theses differences. In spite of almost equal litterfall inputs, different masses of organic matter (humuslayer) were observed in the forest floor of different sub-plots for both plots, SDP and BDP. Differences were most pronounced between the spruce, beech and gap sub-plots (spruce ≥ beech ≥ gap). The soil solution at 10 cm soil depth showed significantly higher pH values in the beech sub-plots, compared with the spruce sub-plots. This finding may be linked to different water and element fluxes via throughfall between sub-plots. A significant effect of the canopy composition on the rate of litter decay and the soil CO2 efflux was observed in the beech dominated plot (BDP). Here, the beech and gap sub-plots showed significantly lower remaining masses at the end of the incubation period (about one year after incubation) compared with the spruce sub-plot. This may indicate that the early stage of the decomposition process was not governed by the given canopy composition. The beech sub-plot showed significantly higher soil respiration, compared with the gap sub-plot. An estimation of the root-associated CO2 production revealed considerably lower root respiration in the gap sub-plot compared with the other sub-plots in the BDP. In total, it was shown that the selected canopy classes were able to create specific biogeochemical patterns in the investigated mixed beech-spruce forest. However, the impact of an individual spruce tree in a beech-dominated site induced obviously higher degrees of spatial heterogeneity with respect to nutrient inputs via throughfall, litter decomposition and soil respiration compared to individual beech trees in a spruce dominated site.