LAI and Nutrient Levels Affect Douglas Fir Regeneration

• If people were assured it is not invasive, introduced Douglas fir, which produces quality wood and can survive summer drought, could become a sustainable species for future forestry 
• A study compared the regeneration of Douglas fir to other tree species in different native forests and under varying environmental conditions.
• Douglas fir cannot compete with productive native broadleaved forests, which show higher LAI or closed canopies and rich soils.
• The study found that Douglas fir spreads rapidly on poor soils without native vegetation/low LAI.  

Why is Douglas Fir Important?

Douglas fir (Pseudotsuga menziesii var. menziesii) was introduced to Europe at the end of the nineteenth century because it is highly productive, and the quality of its wood is good. It is usually grown in mixed-species plantations. Germany, France, and Switzerland are the top growers of this species. 

Norway spruce (Picea abies) is declining in central Europe because it is drought-sensitive and cannot cope with additional stress from bark beetle attacks.

The current interest in the introduced Douglas fir is due to its tolerance to summer drought. In addition, foresters see it as a species with the potential to replace the Norway spruce. 

There is also a fear that because the Douglas fir is an exotic species, it can spread uncontrolled at the expense of native species and become invasive. Reports so far show that it supports equally diverse herbaceous undercover and has no adverse effects on the soils; however, it does not support the same diversity of fungi and animals as native species.

Since Douglas fir outcompeted native species in marginal soils, it has been declared invasive in Germany but not Switzerland.

The problem is the need for more information on its natural regeneration abilities and extent of regeneration, even though the growth and productive parameters of the tree are known. Therefore, judging if it can be or is already invasive is difficult.

Factors Affecting Douglas Fir’s Regeneration

Frei, Moser, and Wohlgemuth, forest and landscape researchers, conducted the first investigation of the extent of natural Douglas fir regeneration in several forest communities in Central Europe to understand the species regeneration patterns. 

Many factors affect regeneration. Light availability and competition from native or neighboring species are crucial for tree establishment. The canopy of neighbors can also determine the type of microclimate in the understorey. Once the seeds have germinated and have established themselves, they face biotic stress like pests and grazing. 

The scientists studied the Douglas fir, in comparison to silver fir, Norway spruce, beech, and other tree species, to answer the following questions:

● How did Douglas fir regeneration compare with that of other species?

● How do climate, light, soil, seed source, litter, and competition from other tree species affect Douglas fir regeneration compared to the other species?

● Does the tree community predict Douglas fir regeneration?

● Did browsing affect Douglas fir regeneration in comparison to other trees?

Data Collection

The scientists studied the number of Douglas fir seedlings and saplings compared to the main competitors in sites where douglas fir was planted along with other species in Switzerland. As a result, they selected 39 sites with naturally regenerated communities and at least ten Douglas fir trees over 55 years. The tree age ensures ample time for repeated seed production and natural regeneration in the stands. 

The communities included

• 23 submontane beech forests (Galio- and Luzulo-Fagenion),
• nine montane beech forests (Lonicero-Fagenion),
• five upper montane silver fir-beech forests (Abieti-Fagenion), and
• two mountain conifer forests (one Ononido-Pinion and one Vaccinio-Piceion).

Six to eight circular plots were randomly selected for sampling in each stand. So total, there were 238 sampling plots.

The researchers collected data on the number of seedlings (<130cm in height) and saplings (≥130 cm in height and <12 cm in diameter), and adults of all species. The number of browsed individuals in each species and plot was estimated to find the average browsing pressure for a species.

Other parameters data collected were the understorey vegetation, nutrient indicator value, litter thickness, and leaf area index (LAI), the one-sided leaf area per ground surface. The scientists used hemispherical images of the canopy taken by CID Bio-Sciences CI-110 Plant Canopy Imager The device also analyzed the images using the Gap Fraction Method for non-destructive estimation of LAI. In addition, soil samples were taken to analyze the pH. 

Young Douglas Fir Numbers

Figure 1: “Number of seedlings (a) and saplings (b) of Douglas fir, Norway spruce, silver fir, beech, ‘other broadleaved species’ and ‘other conifers’ in the 39 stands. The stands are sorted by number of Douglas fir stems. The symbol ‘°’ indicates stands without Douglas fir regeneration,” Frei et al., 2022. (Image credits: https://doi.org/10.1016/j.foreco.2021.119767) 

The scientists found that Douglas fir regenerates in most forest communities they studied. Though Douglas fir seedlings were present in 90 percent of the 39 stands and 45 percent of the 238 plots, their abundance was low, see Figure 1. The seedlings’ abundance accounted for less than five percent of the seedlings in all but four stands, where they were dominant. Norway spruce and beech were dominant in ten stands, and silver fir in two stands. The three species were more abundant than Douglas fir in 35 stands.

Douglas saplings were less frequent and found only in one-third of the stands and 8 percent of the plots. Their abundance was between 10-23 percent in five stands and less than 10 percent in eight stands. Silver fir and beech occurred in almost all stands, and Norway spruce was found in 50 percent of the stands. Beech saplings were the most abundant in 11 stands. Silver fir and Norway spruce saplings were dominant in three and six stands, respectively. Thus, Douglas fir did not dominate the regeneration cohort in any stand. 

Influence of Forest Type

Around 90 percent of the stands had natural beech or silver fir populations. Douglas fir cannot compete against the proliferating native beech. In the montane and submontane beech forests, and fir-beech forests, Douglas fir could only regenerate when helped by management practices, like tending that reduced competition and improved light availability. 

In weakly acidic soils, where beech and other deciduous trees do well, tending doesn’t even help establish Douglas fir. 

In three sites that were dry or nutrient-poor, Douglas fir seedling numbers were high. One was an alpine stand, and the other two were the mountain conifers, where the Douglas fir was the most abundant species, accounting for 68 percent of the seedlings. These sites have a semi-open canopy cover and little herbaceous vegetation. 

Controlling Douglas Fir

Early harvest of trees potentially near seed-bearing, and removal of seedlings and saplings, are some means to control Douglas fir regeneration. In some experimental sites, the low number of Douglas fir could be due to such silvicultural interventions.

Douglas fir will produce seeds between the age of 15 to 40 years, so it is relatively easy to control.

Figure 2. “Number of seedlings (a) and saplings (b) of Douglas fir, Norway spruce, silver fir, beech, ‘other broadleaved species,’ and ‘other conifers’ in the five forest communities (TypoCH classification, Delarze et al., 2015): Mountain conifer forests: Vaccinio-Piceion (N = 1), Ononido-Pinion (N = 1),” Frei et al., 2022. (Image credits: https://doi.org/10.1016/j.foreco.2021.119767)

Ecological factors 

The presence of competing understorey plants limited all four species’ seedlings. Douglas and silver fir seedling numbers are higher in places with fewer herbs. The herbs can spread fast when there is an opening in productive soils, affecting seed germination and seedling establishment due to competition and encouraging plant predators’ presence. After the initial establishment, herbs do not influence Douglas fir numbers. 

Shade or LAI and the distance to the nearest adult were also crucial for Douglas fir seedlings to occur. Douglas fir needs more light or open canopies than beech during the early stages. Beech and other broad-leaved species are shade tolerant and established under closed canopies and gaps.

Thick litter was also a problem for Douglas fir and Norway spruce, as it hindered germination by acting as a mechanical barrier.

Because of limited dispersal, Douglas fir seedlings are found close to seed trees. The scientists conclude from other studies that the seed weight allows wind to transport them only for short distances up to 100 meters from the seed tree. 

Young Douglas firs have the advantage of not being browsed as much as most broad-leaved species, except beech, which suffers only minor browsing damage. Among the needle trees, Douglas fir is damaged more than Norway spruce but less than silver fir. However, the differences in browsing damage did not affect seedling and sapling numbers in any species studied.

Site pH, air temperature, and rainfall did not affect Douglas fir seedling and sapling numbers. In contrast, beech saplings were more abundant in soils with higher pH and temperature, and Norway spruce was more in sites with less rainfall.

Disturbed sites with more light and no litter due to wind throw or silvicultural practices provide ideal conditions for Douglas fir. The ability to withstand drought also helps Douglas seedlings here.

Management Of Douglas Fir

Though Douglas fir has been regenerating for many decades in several forest types, it has not become invasive in any closed tree communities. Douglas fir cannot establish in productive beech forests without being tended.  

Douglas fir thrives naturally in open and marginal sites where no beech or broad-leaved species will grow. But, unfortunately, the tree can and has become invasive in these conditions. So, the scientists recommend carefully monitoring them at these sites.  

Read the original peer-reviewed study for more details:

Frei, E. R., Moser, B., & Wohlgemuth, T. (2022). Competitive ability of natural Douglas fir regeneration in Central European close-to-nature forests. Forest Ecology and Management, 503, 119767. https://doi.org/10.1016/j.foreco.2021.119767