Diversity and tree neighborhood effects on the growth dynamics of European beech and the stand seed bank in temperate broad-leaved forests of variable tree diversity

Abstract

Among the recent studies investigating species diversity's effect on ecosystem functioning, only few examine long-term changes. My study addresses this question based on seed bank analysis and dendrochronology in temperate deciduous forest stands in Hainich National Park (Thuringia, Germany). I focused on three main hypotheses i) species identity of the neighbor trees and their functional traits can significantly influence growth dynamics of Fagus sylvatica L. and the soil seed bank, while species diversity of the neighborhood is of no significance, ii) neighborhood diversity alters trees' capacity of reaction to disturbances, and iii) there are processes altering tree species diversity, growth patterns of F. sylvatica and the soil seed bank at the same time.The selected 12 forest stands, situated on similar bedrock, represent a gradient in tree species diversity. Samples were taken from monospecific beech stands (DL1), medium-diverse forests with beech, ash, and lime (DL2), and highly diverse stands dominated by beech, ash, lime, maple, and hornbeam (DL3). I investigated germinating seeds in three horizons (0-5cm, 5-10cm, 10-20cm) of the soil seed bank and determined seed density, composition and functional types of species.Furthermore, structural parameters of target Fagus trees and their neighbors were analyzed.Neighborhoods of target trees were grouped according to the relative importance of intraspecific competition by Fagus: from purely conspecific neighborhoods (Fagus100) to neighborhoods with mainly conspecific neighbors and few allospecific competitors (Fagus70-99) to neighborhoods with more than 30% of the canopy's influence sphere occupied by allospecific trees (Fagus<70). I examined tree-ring width and growth response to, as well as recovery from, selected stress events (negative pointer years). In addition, I studied δ 13In the period from 1926 to 1975, beech predominantly surrounded by conspecifics (Fagus100, Fagus70-99) reached higher values of δ 13 C than beech largely affected by allospecific competitors (Fagus<70). On the contrary, values of the more actual period 1976 to 2005 did not vary between the neighborhood classes. This leads to the conclusion that target trees, today surrounded by allospecific neighbors, probably benefited from thinning in order to promote growth of other valuable broad-leaved tree species. Thinning resulted in an increase in available soil water and growth. This increase in soil water is manifested in relatively smaller δ 13 C values in the largely allospecific Fagus<70 neighborhoods. The effect vanished when the canopy closed again. Hence, the observed δ 13 C pattern is probably rather a secondary effect of forest management than a direct effect of neighborhood diversity on water supply of Fagus.Disturbance regime, forest management and stand age are important for ecosystem processes in forests. They leave detectable traces in the soil seed bank, tree-ring width and δ 13 C series.Generally, I assume that the neighborhood represents an environmental factor of lower imp...