Effect of light on the browsing sensitivity of fir (Abies alba) seedling and saplings

Introduction:

Abies alba is an important tree species in mountain forests that often provides protection against natural hazards (such as snow avalanches, rockfall and erosion). However, Abies alba is one of the most browsed tree species by ungulates in Switzerland. The lack of fir trees in the regeneration layer is therefore often supposed to be due to shoot browsing by red deer, roe deer and chamois.

Nevertheless, little is known about the reaction of fir seedlings and saplings to shoot browsing under different ecological conditions. In particular it is unclear whether browsed fir saplings are always smaller and grow more slowly than unbrowsed trees regardless of site conditions (as postulated by Prof. Eiberle and co-workers), or if they can compensate for browsing damage under good site conditions (as simulated browsing experiments have shown, cf. Simulated browsing experiment with Abies alba).

It has been hypothesized that the reaction of trees to browsing depends on tree vitality. If this is true, the ability of fir saplings to compensate for lost tree height would be different in sites with high light availability (e.g., windthrow areas, regeneration sites) compared to sites with low light (e.g., closed forest stands). By gaining a better understanding about how browsing pressure and environmental conditions interact, we expect to contribute to the development of forest management approaches that are designed to minimize the negative impact of browsing on fir saplings.

In this project we analyze how fir seedlings and saplings react to browsing, and how frequent the various reaction types are under different environmental conditions. In particular, we quantify the extent to which light availability influences the reaction under otherwise similar site conditions. In addition, we test whether there is a threshold light value that determines how browsed trees respond, or if the reaction types change continuously along a light gradient. Specifically, we seek answers to the following questions:

• Under which light conditions do Abies alba grow best?
• When do Abies alba trees react best after browsing?
• Where are Abies alba trees eaten less often?

Method:

Along natural light gradients (from closed forests to gaps), we investigate which Abies alba trees are browsed most intensively and most frequently; how Abies alba trees react to browsing (e.g., by flagging of side shoots, new epicormic shoots, etc.); and how well they are growing. We are doing this at 4 different sites in the Northern Swiss Prealps and on the Swiss Plateau.

In addition, in 2001 four 5 m x 5m fenced exclusion plots were constructed and planted with ca. 30 Abies alba seedlings each. These fences were set up so that some of the planted trees are located under the crown of a old tree (Picea abies), some are in the dripping zone, and some experience open conditions. These fir saplings are now on average 50 cm (17 cm to 1 m) tall. In addition, in spring 2008 ca. 700 fir seedlings were planted in two ca. 5 x 25 m fenced exclusion plots. These saplings are now 10 cm tall on average, and are situated along natural light gradients from under trees to open sites.

For half of the seedlings and saplings, the fences have been removed temporarily so that deer and chamois can feed on them. Following these browsing periods we quantify the loss due to browsing and follow the growth reaction of every single tree.

Questions and hypotheses regarding the expected results

• Under which light conditions does Abies alba grow best?
  • Under open site conditions, unless the herb and shrub layer “suffocates” the small trees. However, the difference will be rather small in sites that have optimal site conditions for Abies alba.
• When do Abies alba trees react best after browsing?
  • When they grow in optimal site conditions (regardless of light conditions) and experience only browsing on the uppermost part of the annual leader shoot, so that they continue to grow out of regularly formed lateral buds. Under such conditions, a fraction of the Abies alba trees is likely to grow out of the size classes that are vulnerable to ungulates even though they are browsed every year (i.e., browsing intensity larger than 50%).
  • When they grow in gaps and experience intensive browsing of the leader shoot, as under good light conditions flagging (bending up a side shoot) is more frequent, and epicormic shoots are formed within a very short time.
• Where are Abies alba trees eaten less often?
  • Probably in gaps (e.g., windthrow areas), but not along their edges.

The findings of this project will contribute to quantifying the influence of ungulates on fir regeneration under varying site conditions. Based on this study we will develop recommendations for forest management regarding which light conditions would be optimal to have the least damage and where fir grow best; this information will be useful because the light regime can be manipulated easily by forest management.

Already today, we recommend that the degree to which the annual leader shoot has been browsed should be included in the standard surveys of browsing intensity. The threshold value of 9% annually browsed leader shoots (so-called “Eiberle threshold”) can be exceeded if only the uppermost part (i.e. the leader buds) are browsed in optimal sites. However, if almost the entire leader shoot is browsed (at least such that no regularly formed side buds remain), it is probably appropriate to not permanently exceed this threshold value.

Contact person: Andrea Doris Kupferschmid Albisetti