Conservation value of low‐productivity forests measured as the amount and diversity of dead wood and saproxylic beetles

Abstract

In many managed landscapes, low-productivity land comprises most of the remaining relatively untouched areas, and is often over-represented within protected areas. The relationship between the productivity and conservational value of a site is poorly known; however, it has been hypothesized that biodiversity increases with productivity due to higher resource abundance or heterogeneity, and that the species communities of low-productivity land are a nested subset of communities from more productive land. We tested these hypotheses for dead-wood-dependent beetles by comparing their species richness and composition, as well as the amount and diversity of dead wood, between low-productivity (potential forest growth <1 m ·ha ·yr ) and high-productivity Scots pine-dominated stands in Sweden. We included four stand types: stands situated on (1) thin soils and (2) mires (both low-productivity), (3) managed stands, and (4) unmanaged stands set aside for conservation purposes (both high-productivity). Beetle species richness and number of red-listed species were highest in the high-productivity set-asides. Species richness was positively correlated with the volume and diversity of dead wood, but volume appeared to be a better predictor than diversity for the higher species richness in set-asides. Beetle species composition was similar among stand types, and the assemblages in low-productivity stands were largely subsets of those in high-productivity set-asides. However, 11% of all species and 40% of red-listed species only occurred in high-productivity stands, while no species were unique to low-productivity stands. We conclude that low-productivity forests are less valuable for conservation than high-productivity forest land. Given the generally similar species composition among stand types, a comparable conservational effect could be obtained by setting aside a larger area of low-productivity forest in comparison to the high-productivity. In terms of dead wood volumes, 1.8-3.6 ha of low-productivity forest has the same value as 1 ha of unmanaged high-productivity forest. This figure can be used to estimate the conservation value of low-productivity forests; however, as high-productivity forests harbored some unique species, they are not completely exchangeable.

Ecosystem service multifunctionality of low‐productivity forests and implications for conservation and management

“…While this is the first assessment of ES‐multifunctionality in forested mires, our results of lower values for certain individual ES are mirrored in other studies. In agreement with our findings, forested mires have been shown to have lower volume and diversity of deadwood and species richness of deadwood‐dependent beetles, especially in northern Sweden, compared to rocky outcrops and productive managed forests (Hämäläinen et al, ). We showed that the levels of many individual ES, such as forest berries, deciduous trees, bryophytes and fungi were also lower in forested mires compared to productive forests.…”

“…Some evidence from Northern Europe suggests that logging effects have indeed reduced the availability of biodiversity elements such as old trees and deadwood in forested rocky outcrops, although a few exceptional areas contain substantial amounts of deadwood and old‐growth indicator species (Sverdrup‐Thygeson & Ims, ). In agreement with our findings, deadwood volumes and beetle species richness of forested rocky outcrops have been shown to be similar to productive managed stands of similar age but also to host lower beetle richness and fewer red‐listed species compared to older productive stands of high conservation value (Hämäläinen et al, ). Common for the studied forested rocky outcrops with sparse tree cover and slow tree growth is that any removal of living and dead trees will likely have a disproportionately large negative influence of certain biodiversity and ES values.…”

“…For example, it has been shown that low‐productivity forests may harbour lower biodiversity values than more productive forests due to lower resource availability and heterogeneity limiting population sizes (e.g. Chase, ; Hämäläinen, Strengbom, & Ranius, ; Liang et al, ). Comparative insights of the drivers of ES‐multifunctionality of different types of low‐productivity forests (i.e.…”

“…In particular, higher levels of ES are expected in productive forests because the provision of tree biomass, forest berries, as well as forest structures important for biodiversity (e.g. deciduous trees and deadwood) are generally positively related to site productivity (Chase, ; Hämäläinen et al, ). However, low‐productivity forests have been less affected by forestry and may therefore shelter higher biodiversity due to higher quantities of key forest structures such as deadwood and old trees (Fitzsimons & Michael, ; Hämäläinen et al, ).…”

“…deciduous trees and deadwood) are generally positively related to site productivity (Chase, ; Hämäläinen et al, ). However, low‐productivity forests have been less affected by forestry and may therefore shelter higher biodiversity due to higher quantities of key forest structures such as deadwood and old trees (Fitzsimons & Michael, ; Hämäläinen et al, ). We also expected higher supplies of soil carbon storage in forested mires on more peaty soils (e.g.…”