Growth efficiency of Norway spruce and European larch in pure und mixed stands

Growth and yield of tree species as compared between mono-specific and in mixed species stands is an old, yet still unsolved problem. While there are quite a lot of recent studies for the mixtures of Norway spruce and common beech, there is a lack of studies of larch- and spruce mixtures, which frequently grow in the mountainous areas of the Eastern Alps. Thus, the growth efficiency of these two species, growing in mono-specific and in mixed stands will be studied. There are two reasons why a species` growth may be different in a mixed and in a monospecific stand. First, a tree with a given leaf area may receive different amounts of light if the neighbouring trees are of the same species or from another species. In this case the light use efficiency (growth per absorbed light) could be the same even if the leaf area efficiency (growth per m leaf area) were different. Second, depending on the mixture the tree species may change its rooting behaviour or its shade tolerance and thus exhibit different light use efficiency. Both these effects depend additionally on the trees` dominance and on the crown disengagement, thus on the proportion of the stand area which is available to the tree. A tree, even with the highest light use efficiency, may be very inefficient on the level of the stand area if it has too much space available. Thus, at first we will investigate in 12 stands how the light use efficiency and the leaf area efficiency depend on the species, the mixture, the stand age and the stand density on the one hand; and on the dominance, on the other hand. Then we will find a definition of the stand area available for a species by comparing an individual tree approach with the approach via potential stand density. Once, this definition found, we will investigate how the area efficiency (growth per unit of stand area) depends on the stand age, the stand density and the species mixture.

Since several decades the scientific community focusses its research on mixed forests because they are supposed to be more resilient to disturbances and extreme climatic events, and to provide higher levels of ecosystem services. An important question in this context is the growth of mixed forests in comparison to monospecific stands. While there are a lot of respective results already available for a number of mixtures, mixed forests of European larch and Norway spruce have only been scarcely investigated. In Austria there are about 240,000 ha of this mixture type, mainly abundant in elevations between 300 and 2000m in the Alps. We investigated 4 triplets each one consisting of a mixed larch-spruce stand, a pure spruce and a pure larch stand. The results from 12 such forest stands in elevations between 900 and 1300 m were: From data of the Austrian National Forest Inventory we know that the potential density of spruce stands is higher than in larch stands, with the difference increasing with age. Comparing growth (per hectare) of these two species between mixed and pure stands, the growth has to be related to the available fraction of the stands area of the trees, especially in the mixed stands. Similar to other tree species mixtures, we could confirm that leaf area is a better proxy for the available ground area of spruce and larch, than canopy cover, which unfortunately often is used in the description of forest stands. Growth of spruce was poorer in mixed stands, if the stands were young. If, however, the stands were older than 100 years, spruce grew clearly better in mixed stands. Thus, in older stands the total growth of mixed stands is higher than in pure larch or spruce stands, while it is lower in younger stands. These mixing effects were only caused by a different behavior of spruce, because we could not find mixing effects on growth of larch. For forest management this means that mixing of spruce and larch can only be recommended if long rotations are intended. This results from the behavior of individual trees in the stands: Larches in the mixed stands clearly have larger crowns than in pure stands. At a given crown size however, the crown has less leaf area in the mixed stand. Norway spruce, generally had more leaf area than larch, its crown size in mixed stands was smaller than in pure stands, and for a given crown size, the leaf area of spruce did not differ between pure and mixed stands. The most efficient larches are those, which occupy only small space, and bear much leaf area, while the efficiency of spruce trees increases nearly linearly with their leaf area, independent of their space requirements.