Planting Douglas-fir? Four things to consider before you start

Research from SLU has shown that naturally regenerated birch can help protect Douglas-fir seedlings during their first years. In a five-year field experiment, both moderate and dense birch shelter reduced frost damage, while survival remained high and no significant reduction in early height growth was observed.

We asked SLU PhD student Michal Kibitlewski what to consider before using the method.

1. Choose the right provenance

Not all Douglas-fir planting material responds in the same way to Swedish conditions.

The experiment used the relatively frost-hardy Kouvola provenance. According to Kibitlewski, matching provenance to local conditions is one of the most important decisions before planting because provenances differ in both frost tolerance and growth.

A birch shelter can reduce frost exposure, but it cannot compensate for poorly adapted planting material.

2. You don't necessarily need an extremely dense shelter

The experiment compared birch shelters of approximately 1,500 and 3,000 stems per hectare.

Both reduced frost damage compared with an open clear-cut, and no significant differences in early growth or survival were found between the treatments during the study period. The densest birch shelter provided the strongest frost protection.

But that does not mean that more birch is always better.

As the Douglas-fir grows, the shelter will increasingly compete for light, water and nutrients.

3. Plan to adjust the shelter over time

The experiment followed the seedlings during their first five years, so it primarily provides information about the establishment phase.

Kibitlewski suggests gradually thinning the birch as competition begins to limit Douglas-fir growth.

Existing guidance such as the Kronoberg method can provide a useful reference for managing birch shelter over Norway spruce, but Douglas-fir has higher light requirements. The timing and intensity of thinning may therefore need to be different.

This is also one of the questions where more practical experience and research are still needed.

4. Don't forget wildlife

Frost is not the only establishment risk.

Douglas-fir can also be vulnerable to browsing and to fraying damage when deer rub their antlers against the stem. On sites with high ungulate pressure, fencing may therefore be necessary.

With only a relatively small area of Douglas-fir currently planted in Sweden, practical experience under Swedish conditions remains limited.

Contact Michal Kibitlewksi for more information.