Genetic diversity reduces competition and increases tree growth on a Norway spruce (Picea abies [L.] Karst.) provenance mixing experiment

Abstract

Many recent studies address that diversification of tree species and stand structure can increase the level and stability of growth and other forest functions and services to mitigate natural and human disturbances. Most studies so far focussed on the diversification of tree species mixing and stand structure. The potential of intra-specific genetic diversification in terms of provenance mixture was hardly explored so far.Here we analyse how a mixing of species provenances of Norway spruce (Picea abies [L.] Karst.) affects the competition between neighboring trees and their growth. We based our study on the 40-years-old provenance-mixing experiment Vohenstrauß 622 in the Upper Palatinate Forest in South Germany. Here 21 different provenances from clonal propagation of Norway spruce across Germany were combined in individual tree mixture, replicated in different spacing, and thinning. Our main objective was to analyze how the individual tree competition, tree growth, and stand productivity were affected by neighboring trees of different provenances.First we show that at an tree age of 43 years the different provenances strongly vary between 10 and 27% in their crown diameter, cd, stem diameter, d, crown length, cl, and tree height, h, with a ranking cvcd > cvd > cvcl > cvh in the coefficient of variation, cv. We found significant differences of the stem and crown allometry between the provenances; e.g. the range of the allometric exponents of h∝dα was α = 0.12–0.58 and of cd∝dα α = 0.47–0.84.Second we found that different provenances in the neighborhood reduced the inter-individual competition and increased the stem diameter growth. Having 10 different provenances in the neighborhood increases the diameter growth of a tree on average by 28 % compared with having only 5 provenances in the neighborhood. Stem diameter followed a saturation curve with increasing number of provenances.Third we showed that an increasing number of provenances also increased the productivity at the stand level. The effects on stand growth are not only determined by the number of different provenances per area but also by their spatial mixing pattern.The inter-provenance complementarity of tree allometry is discussed as main cause of the revealed reduction of competition and increase of growth. We further discuss how spatially explicit individual tree models may incorporate tree structure as proxy of provenance characteristics. We conclude that genetic diversification in terms of provenance mixture can increase and stabilize tree and stand growth analogously to tree species mixing and may be further explored and considered as seminal silvicultural option.