Space sequestration below ground in old-growth spruce-beech forests—signs for facilitation?

Abstract

Scientists are currently debating the effects of mixing tree species for the complementary resource acquisition in forest ecosystems. In four unmanaged old-growth spruce-beech forests in strict nature reserves in southern Sweden and northern Germany we assessed forest structure and fine rooting profiles and traits (≤2 mm) by fine root sampling and the analysis of fine root morphology and biomass. These studies were conducted in selected tree groups with four different interspecific competition perspectives: (1) spruce as a central tree, (2) spruce as competitor, (3) beech as a central tree, and (4) beech as competitor. Mean values of life fine root attributes like biomass (FRB), length (FRL), and root area index (RAI) were significantly lower for spruce than for beech in mixed stands. Vertical profiles of fine root attributes adjusted to one unit of basal area (BA) exhibited partial root system stratification when central beech is growing with spruce competitors. In this constellation, beech was able to raise its specific root length (SRL) and therefore soil exploration efficiency in the subsoil, while increasing root biomass partitioning into deeper soil layers. According to relative values of fine root attributes (rFRA), asymmetric below-ground competition was observed favoring beech over spruce, in particular when central beech trees are admixed with spruce competitors. We conclude that beech fine rooting is facilitated in the presence of spruce by lowering competitive pressure compared to intraspecific competition whereas the competitive pressure for spruce is increased by beech admixture. Our findings underline the need of spatially differentiated approaches to assess interspecific competition below ground. Single-tree approaches and simulations of below-ground competition are required to focus rather on microsites populated by tree specimens as the basic spatial study area.

Mixing Effects in Norway Spruce—European Beech Stands Are Modulated by Site Quality, Stand Age and Moisture Availability

“…However, this negative mixing effect diminished with increasing moisture index, i.e., with increasing water availability. This pattern may result because European beech tends to have a high above-and below-ground competitiveness [13,57,58]. In this mixture type, Norway spruce was shown to exhibit a shallower root system [59] whereas European beech appeared to develop longer and more fine roots and to extend the space exploited to soil layers less occupied by competitors [60].…”

Site‐adapted admixed tree species reduce drought susceptibility of mature European beech

“…In mixed stands with Norway spruce, Fagus sylvatica should be able to benefit from its ability to efficiently explore more drought resistant subsoil layers leading to a partial stratification of fine-root systems (Schmid, 2002;Bolte & Villanueva, 2006;Bolte et al, 2013). However, the positive effect of Norway spruce neighborhoods on beech growth was quite weak.…”

Drought Stress Reaction of Growth and Δ13C in Tree Rings of European Beech and Norway Spruce in Monospecific Versus Mixed Stands Along a Precipitation Gradient

“…Additionally, Metz et al [23], Bolte et al [65], and Mölder et al [5] reveal that growing in a neighborhood with other species has a positive effect on beech. This positive effect is detected in this study which shows that beech grown in a neighborhood with spruce has significantly higher resistance and resilience (∆ 13 C) on moist sites.…”