The symmetry of competition: a battle crown-to-crown or roots-to-roots?

The post provided by Marco Mina

This post refers to the article The symmetry of competitive interactions in mixed Norway spruce, silver fir and European beech forests published in the Journal of Vegetation Science: https://doi.org/10.1111/jvs.12664

Forests rich in tree species are not only known for providing higher levels of ecosystem services but also to be prompter to cope with unexpected disturbances and climatic changes. However, the mechanisms of competitions in multi-species forests are all but clear. Scientists are still puzzled about which combinations of tree species grow better in a particular environment or what factors promote or reduce a positive growth complementarity in secondary forests and/or plantations. A recently published JVS paper (Mina et al. 2018, Journal of Vegetation Science 29: 775-787, https://doi.org/10.1111/jvs.12664) tackled this question for the three economically most important species growing in Central Europe: Norway spruce (Picea abies), silver fir (Abies alba) and European beech (Fagus sylvatica).

The study was carried out in the framework of the recently completed project Integrating Species Mixtures in Tree Growth Functions for Forest Development Models in Switzerland – Swiss-SpeMixMod, by researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. Overall, the project focused of evaluating patterns of tree growth complementarity for the major central European tree species growing in a total of 19 mixture types. In a a follow-up, the researchers focused on mixed forests composed of Norway spruce (Picea abies), silver fir (Abies alba) and European beech (Fagus sylvatica). The aim of the study was to better understand the different modes of intra‐ and inter‐specific competition in these two‐ and three‐species mixed forests. The main goal was to disentangle if species interactions in spruce‐fir‐beech forests are more associated with size‐symmetric (i.e., competition for belowground) or size‐asymmetric competition (i.e., for aboveground resources). In other words:

Are tree interactions a battle crown-to-crown or roots-to-roots?

The researchers took advantage of the extensive database available in the context of the Swiss National Forest Inventory (NFI). The Swiss NFI is based on terrestrial sampling on a 1.4×1.4 km grid of permanent plots covering the entire country of Switzerland with measurements taken since the early 1980s until now. Large-scale forest inventories are a great source of data for scientists with the strength of being representative for a very broad range of environmental conditions, stand development, stand densities, forest types, silvicultural regimes and species compositions. As shown in the figure below, spruce, fir and beech grow in many locations across the country, and they grow whether in monospecific stands or in two- and three-species mixture stands.

Marco Mina, the lead author of the JVS article, shared his summary of the study: “We applied a similar approach as in one of our previous study and examined the individual-tree growth of Norway spruce, silver fir and European beech using non-linear mixed effect models to assess tree growth. This time, however, we implemented distance-independent competition indices in the models. These indices were used as a proxy for size-symmetric (competition for belowground resources) and size-asymmetric (competition for light) competition. To explore the influence of species mixture on tree growth, and thus to analyse the size-symmetric and size-asymmetric mixing effects, we split these two indices into species-specific components. After a process of model selection, we obtained three final models that were used to evaluate the symmetry of competition”.

Do Norway spruce, silver fir and European beech compete more for aboveground or belowground resources? And what is the most competitive species?

The study demonstrated that species-specific competition indices could be integrated into individual tree growth models to express the different modes of competition among species in mixed forests. Clear differences between intra– and inter-specific competition among these three important species were found. This highlight the presence of mixing effects in two- and three-species mixtures of spruce, fir and beech. These effects, however, seems to differ whether competition for aboveground or belowground resources is considered.

In the case of mixtures of Norway spruce and silver fir, results showed that tree growth of both species is larger in spruce-fir mixtures than in the respective monocultures.

The positive competitive interactions found between these two species might be due to more efficient use of belowground rather than aboveground resources. A possible explanation could be the complementary root systems of the species, which is shallow for spruce and deep rooting for fir.

However, when beech grows together spruce and fir, negative effects of increasing proportions of beech on individual tree growth of both conifers were detected. “It is not a surprise that beech is a highly competitive species”, continues Marco Mina. “Our results indicated that the negative effects of beech on the growth of the two conifers could be due to the competition of rooting systems and belowground use rather than for aboveground resources.” It is important to remark that, even if the growth of spruce and fir is reduced by the presence of beech, beech’s growth is highly improved by the presence of the two conifers rather than competing against an individual of the same species. In summary: beech is highly self-competitive species and it grows better when mixed with spruce and fir rather than in monospecific stands.

The figure below shows how different the growth of a single beech tree is depending on the increasing number of competitors of different species (x-axis: basal area of trees larger than the target one, BAL). The indices BAL indicates competition for light, thus a battle crown-to-crown. It is noticeable from the lower line (“All beech”) that the higher reduction of growth occurs when a beech tree is surrounded by bigger trees of the same species. Instead, the lowest reduction in growth occurs when the beech is surrounded by spruce and fir trees (lines “All spruce”, “Spruce and fir”).

To sum up, this study demonstrates that it is possible to further understand the symmetry of competition (i.e., how trees compete against each other) alongside with species competitive interactions. This analysis also highlights that forest inventories are a great source of data for performing such analysis.

Improved modelling of competitive interactions can help to better evaluate adaptation measures for mixed forests under global change stressors. Although it has been widely demonstrated that planting and restoring forests using multiple tree species brings only advantages, in many parts of the world monospecific plantations are still the rule. In the face of uncertainty such as climate change and unexpected disturbances, mixed species forests are believed to be a suitable option to build resilient forests and mitigate climate change by stocking more carbon.

This post is based on a contribution that appeared first in the blog Forest Monitor on July 16th, 2018.

Marco Mina is a forest ecologist. He is currently a postdoc at the Centre for Forest Research at the Université du Québec à Montréal (Canada) and a research associated at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL (Switzerland).