Nutrient response efficiency, tree-microbe competition for nutrients and tree neighborhood dynamics in a mixed-species temperate deciduous forest in central Germany

Abstract

In most Central European countries, less than one percent of the remaining broadleaved forest is undisturbed and temperate forests face challenges such as species invasion, climate change and increased nitrogen deposition. High N deposition has been shown to alleviate N limitation, inhibit P uptake and cause deficiencies in leaf P status of beech. Tree species diversity may increase stand productivity in temperate forests through complementary resource use and/or facilitation of a limiting nutrient. The passage of nutrients through microbial biomass during decomposition is a key process in nutrient cycling and competition between trees and the microbial biomass has been demonstrated for N e.g., in beech trees and for P in tropical peatlands. Beech is a very competitive tree species within temperate forest ecosystems, but may in dry areas be outcompeted by oak, while hornbeam and lime play a smaller role. However, oak experiences a recent decline in European forests, possibly also related to high N deposition. In this work, we explored nutritional, competitive and structural dynamics in an unmanaged, very close-to natural mixed species deciduous forest in Central Germany, comprised of beech (Fagus sylvatica), oak (Quercus petraea and Quercus robur), hornbeam (Carpinus betulus) and lime (Tilia cordata and Tilia platyphyllus). Our aims were to: (1) investigate whether complementarity and/or facilitation increase productivity in this forest ecosystem, (2) determine whether there is competition for nutrients (N, P and K) between trees and the microbial biomass and, (3) investigate tree neighborhood dynamics and whether oak declined in this forest due to high N deposition. In stands of single species (mono-species stands) and in stands with different combinations of three of the tree species (mix-species stands), we measured biomass production and availability of nutrients. Nutrient response efficiency curves (nutrient response efficiency = biomass production per nutrient availability) were used to evaluate whether a specific nutrient limited tree growth. Annual net nutrient change in a litterbag study was calculated as the difference between the initial and remaining nutrient contents in the decomposing leaf litter after one year. For the calculation of nutrient resorption efficiency, we measured N, P and K concentrations in sun-exposed leaves and in leaf litterfall. Tree neighborhood dynamics were assessed by diameter distribution for each species, above-ground woody biomass for each species composition as well as polygon-estimation of growing spaces and diameter based nearest-neighbor analysis for tree pairs. A geographic information system (GIS) was used to create such polygons and detect nearest neighbors. At a tree level, using a neighborhood approach, relative growth rates of beech trees in mono-species stands were smaller than when they were in mix with lime and hornbeam whereas growth of lime trees in mono-species stands was larger than in mix with beech and oak. The nutrient response efficiency curve for beech showed that beech trees in mix-species stands had optimal P and K response efficiencies whereas beech trees in mono-species stands showed P and K limitations. While net nutrient change in decomposing leaf litter influenced the availability of P and K in the soil, this was not the case for soil N availability. Resorption efficiencies for N, P and K were negatively related to net nutrient change in decomposing leaf litter. In our study on tree neighborhood dynamics we found that in intraspecific nearest-neighborhoods, neighbors had the same dbh and increased their dbh with that of their neighbor. In contrast, in interspecific nearest neighborhoods, dbh between neighbors generally differed and neighbor dbh decreased with increasing dbh of a target tree. Oak trees were not able to increase growing space with dbh but dominated in size over their nearest neighbors. Our results showed that in the studied forest ecosystem, nutrient limitation was species-dependent and that using nutrient response efficiency and a neighborhood approach are useful tools in quantifying the effects of individual tree species on a species’ productivity between mono- and mix-species stands, such as an observed facilitation of beech in mixed-species stands. These tools provide important basis for improving management of typical mix-species, temperate forests. We also concluded that competition between microbial biomass and trees was strong for P and K and minor for N, likely due to high atmospheric N deposition in this forest ecosystem, resulting in decoupling of the internal N cycle. High N deposition likely also contributed to little recent rejuvenation of oak trees, while older oak trees were competitive for light in our research area. Stand structure was characterized by greater inter- compared to intraspecific competition. As a result, mono-species stands of beech, oak and lime formed high-biomass climax stands in a shifting small-scale mosaic of compositions. In meeting the new management demands of global change, further investigations of resource response efficiency for tree species in different species compositions are recommended.