Analyzing size-symmetric vs. size-asymmetric and intra- vs. inter-specific competition in beech (Fagus sylvatica L.) mixed stands

Abstract

In mixed stands, inter-specific competition can be lower than intra-specific competition when niche complementarity and/or facilitation between species prevail. These positive interactions can take place at belowground and/or aboveground levels. Belowground competition tends to be size symmetric while the aboveground competition is usually for light and almost always size-asymmetric. Interactions between forest tree species can be explored analyzing growth at tree level by comparing intra and inter-specific competition. At the same time, possible causes of niche complementarity can be inferred relating intra and inter-specific competition with the mode of competition, i.e. size-symmetric or sizeasymmetric. The aim of this paper is to further our understanding of the interactions between species and to detect possible causes of competition reduction in mixed stands of beech (Fagus sylvatica L.) with other species: pine-beech, oak-beech and fir-beech. To test whether species growth is better explained by size-symmetric and/or size-asymmetric competition, five different competition structures where included in basal area growth models fitted using data from the Spanish National Forest Inventory for the Pyrenees. These models considered either size-symmetry only (Reineke's stand density index, SDI), size-asymmetry only (SDI of large trees or SDI of small trees), or both combined. In order to assess the influence of the admixture, these indices were introduced in two different ways, one of which was to consider that trees of all species compete in a similar way, and the other was to split the stand density indices into intra-and inter-specific competition components. The results showed that in pine-beech mixtures, there is a slightly negative effect of beech on pine basal area growth while beech benefitted from the admixture of Scots pine; this positive effect being greater as the proportion of pine trees in larger size classes increases. In oak-beech mixtures, beech growth was also positively influenced by the presence of oaks that were larger than the beech trees. The growth of oak, however, decreased when the proportion of beech in SDI increased, although the presence of beech in larger size classes promoted oak growth. Finally, in fir-beech mixtures, neither fir nor beech basal area growth were influenced by the presence of the other species. The results indicate that size-asymmetric is stronger than size-symmetric competition in these mixtures, highlighting the importance of light in competition. Positive species interactions in size-asymmetric competition involved a reduction of asymmetry in tree size-growth relationships.

Influence of thinning intensity and canopy type on Scots pine stand and growth dynamics in a mixed managed forest

“…Beech presence apparently caused a reduction in the Tree volume annual increment, agreeing with previous results (Primicia et al, 2013;Río et al, 2014). In our study site, characterized by summer water deficit, the negative effect of beech on iv may have been related to the beech root system, since it is highly competitive for water and nutrient resources (Curt & Prevosto, 2003).…”

“…In our study site, characterized by summer water deficit, the negative effect of beech on iv may have been related to the beech root system, since it is highly competitive for water and nutrient resources (Curt & Prevosto, 2003). The importance of light in competition in mixed Scots pine-beech forests in the same region has been also highlighted (Río et al, 2014), where size-asymmetric competition (resource uptake depends on the relative sizes of the competitors) is apparently stronger than sizesymmetric competition (resource uptake is independent of the relative sizes of the competitors, Schwinning & Weiner, 1998). The reduction of the Tree volume annual increment due to beech presence gradually increased during the study period, related to beech crown development.…”

“…Several studies on the effects of mixture on the forest yield have been carried out (e.g. Kelty, 1992;Pretzsch, 2005;Pretzsch & Schütze, 2009;Pretzsch et al, 2010), but little is known about inter-specific processes in mixed Scots pine (Pinus sylvestris L.)-beech (Fagus sylvatica L.) forests, and particularly in Mediterranean areas (but see Primicia et al, 2013;Río et al, 2014). Spiecker (2003) suggested that the structure of mixed stands may be maintained and improved by management practices like thinning, which has been also recommended as a necessary first step in the conversion process from pure Scots pine plantation to mixed broadleaved forests (Crecente-Campo et al, 2009).…”

Allometric modelling of crown width for white spruce by fixed- and mixed-effects models

“…Among all distance-independent CIs examined, Reineke's stand density index calculated from coniferous trees (SDI_CON) was the best one. del Río et al (2014) also found that CIs based on the stand density index concept were better than those based on basal area because the latter is more influenced by stand development and environmental conditions. Among all distance-dependent CIs examined, Hegyi's CI calculated from coniferous competitors (CI1_CON) was the best one.…”

Response of Boreal Jack Pine (Pinus banksiana Lamb.) Stands to a Gradient of Commercial Thinning Intensities, with and without N Fertilization

“…However, the ability to respond to increased light availability also depends on the shade tolerance of species [38]. There is evidence that, in forest stands, competition for light is size asymmetric while competition for soil resources is size symmetric; that is, larger individuals capture a disproportionate share of light resources considering their absolute size while uptake of soil resources is proportional to individual's size [39][40][41]. Thinning from below should therefore have little effect on growth of residual dominant jack pine stems unless soil resources are increased by thinning.…”