Conservation value of low-productive forests measured as the amount and diversity of dead wood and saproxylic beetles

Abstract

In many managed landscapes a major part of all remaining unmanaged land is low-productive. Low-productive land is also often over-represented within protected areas, as it is less expensive to set aside. Despite this the relationship between productivity and conservational value of a site is not well known, although it has been hypothesized that biodiversity generally increases with productivity due to higher resource abundance and heterogeneity. If biodiversity is indeed higher in more productive land, there is a risk that an important part of diversity will remain unprotected when mainly low-productive land is set aside.

We examined the conservational value of low-productive forest land by comparing the species richness and composition of dead wood-dependent beetles, as well as the volume and diversity of dead wood, between low-productive (potential forest growth < 1 m3/ha/year) and productive Scots pine-dominated forests in Sweden. We surveyed 192 forest stands, including two types of low-productive forests (stands on thin, rocky soils and on mires; the main categories of low-productive forest in Fennoscandia), and two types of productive forests (older managed stands and unmanaged stands set aside for conservation purposes).

We found a total of 90 beetle species, 13 of which were red-listed. Species richness was generally higher in the productive forest types: both the total species number and number of red-listed species were highest in the productive set-asides and lowest in mires. Species richness was positively correlated with both volume and diversity of dead wood, but volume appeared to be a better predictor for the higher richness in productive forests. The species composition was generally similar among stand types, even though certain species were only found in the set-asides. None of the species were unique to low-productive forests, and the species assemblages in low-productive stands were thus subsets of those in productive set-asides.

We conclude that low-productive forests are less valuable for conservation than productive forest land, since they contained less dead wood and thus hosted lower beetle species richness. However, given the generally similar species composition among stand types, a comparable conservational effect could be obtained by setting aside a larger area of low-productive forest in comparison to the productive. For example, in terms of dead wood volumes, 1.8–3.6 ha of low-productive forest would have the same value as 1 ha of unmanaged productive forest. However, as productive forests harbored some unique species, they are not completely exchangeable.