Finnish Forest Research Institute  Metla

Press release 9.2.2005

Dissertation: Herbivory among competing seedlings: effects on silver birch populations

In her doctoral thesis in the field of biology Kaarina Prittinen, M.Sc., studied variation within silver birch seedling stands regarding growth and resistance to herbivorous insects and voles, as well as changes caused by insects and voles in the genetic structure of experimental populations. During the study, significant variation was observed among seedling families in growth and resistance to herbivores. Additionally, the study showed that herbivores can change the population structure of silver birch during thinning of dense stands.

According to the study both insects and voles prefer fast-growing seedling families. The effect of insects on the stands was considerable: they almost doubled the mortality rate. Mortality was most pronounced in short, shaded seedlings, that were relatively well spared from feeding damage, which means that the damage caused by insects increased the effects of competition in the stands. Voles, on the other hand, had no immediate effect on seedling mortality; birch stands were regenerated from cut seedlings in the following spring.

As a result of natural thinning the genetic diversity of the test populations was reduced. However, voles increased diversity in the canopy layer by cutting longer seedlings. This observation is significant, because the canopy layer consists of the seedlings that have the best chances to survive in the competition for light. Insects, on the other hand, changed the genetic composition of the populations, i.e. they changed the numerical ratios between different seedling families. However, the effects of herbivores on the population structure cannot be predicted merely on the basis of the amount of feeding damage. Instead, the response of each birch family depends on its resistance and characteristics relating to competition, as well as on the mode and timing of feeding.

Since the amount of competition and the number of herbivores vary in natural populations both with time and location, birches are faced with varying selection pressures. Thus, the variation of characteristics regarding both competition and feeding can be maintained by such selection pressures.

The experimental populations studied in the doctoral thesis “Herbivory among competing seedlings: effects on silver birch populations” consisted of 20 silver birch families. The seedling stands were grown from seeds that had been picked from randomly selected mother trees in a naturally regenerated forest. In the field experiment seedlings grew as dense stands subject to competition for light, which is typical in natural regeneration of birch. A part of the experimental populations were subjected to herbivorous insects and voles. The field experiment was carried out at the Punkaharju Research Station of the Finnish Forest Research Institute. Prittinen defended her dissertation at the University of Joensuu .

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