Conservation-oriented forestry and early successional saproxylic beetles: responses of functional groups to manipulated dead wood substrates

Summary

Conservation-oriented forestry methods aim to maintain populations of forest organisms in managed forests. The value of provision of dead wood for early successional saproxylic beetles was tested in forest in northern Sweden (between latitudes 63.620°N-64.285°N and longitudes 16.889°E-20.132°E).

Study area: . Nine unmanaged Norway spruce Picea abies dominated forest stands in nature reserves were selected in southern Västerbotten and northern Västernorrland, and paired with nearby (0.5-6 km) mature spruce-dominated managed forest and clear-cut stands (cut 1999-early 2001).

Dead wood provision: Within an area of about 1 ha in each stand type (unmanaged, managed and clear-cut) at each of the nine sites, three 'blocks' of spruce (three logs and one snag each) were laid out between September 2001 and March 2002. Logs were 4 m long x c.20 cm diameter (recently cut). Snags were cut at 4 m height and were c.20 cm diameter at breast height. Logs in each block were randomly assigned a ‘burned’or ‘control’ treatment. A LPG-burner was used to simulate the effect of a wild fire; the log surface was burned (for 15-30 min) on all sides until blackened. ‘Shaded logs’ were placed in naturally shaded areas.

Beetle sampling: A combination of emergence (‘eclector’) traps (to collect beetles emerging from the wood) and flight-intercept (‘window’) traps (attached to the logs) were used between July and August 2002.

Functional groups: Species were classified as 'obligate saproxylic’ (dependent on dead wood during at least part of their lifecycle) or ‘facultative saproxylic’ (associated with but not dependent on dead wood). Saproxylic beetles were also allocated to groups i.e. ‘fire-favoured’ or ‘red listed’ and on the basis of their diet i.e. cambium consumers, detritivores; fungivores, predators and sapwood feeders.

Species richness and functional group abundance: A total of 126,092 obligate saproxylic beetles (76 species) were collected in eclector traps. Trypodendron lineatum made up 86% of individuals, followed by Hylurgops palliatus (12%), H. glabratus (0.5%), Phloeonomus sjoebergi (0.3%) and Epuraea pygmaea (0.2%). More were collected in unmanaged and managed forests, than in clear-cuts, and fewer from snags than control logs.

Six fire-favoured species (72 individuals) were collected in window traps: Orthotomicus suturalis (72% of the catch), Placusa atrata (11%) Hylobius abietis (10%), Hylobius piceus (4%), Acmaeops septentrionis (1%) and Pachyta lamed (1%). Eclector traps collected 46 individuals of three fire-favoured species. The abundance of fire-favoured species in window traps differed among stand types (but not log treatments); more individuals were collected from clear-cuts than managed forests and reserves (where numbers collected were similar).

Fifty-six individuals of 16 red-listed species were collected in window traps, and six individuals of five species in eclector traps. Window traps were dominated by E.deubeli (19) and E.longipennis (11). Eleven red-listed species were found in window traps on snags, compared with four on each of the control, burned and shaded log treatments.

Obligate cambium consumers (18 species, dominated by Hylurgops palliatus, H. glabratus, Crypturgus subcribrosus, Pityogenes chalcographus and Hylastes cunicularius) were less abundant on burned logs than on other treatments.

Total detritivores (5 species, dominated by E.pygmaea and E.marseuli), obligate fungivores (11 species, dominated by T.lineatum, Atomaria bella, Corticaria lateritia and E. biguttata), total fungivores (T. lineatum excluded) (23 species, dominated by E. pygmaea and E. angustula), and obligate predators (16 species, dominated by E. laeviuscula, Quedius plagiatus and Nudobius lentus) were more abundant in stands than in clear-cuts. Obligate fungivores (with T.lineatum excluded) were less abundant on burned logs than snags, but only on clear-cuts. When T. lineatum was included, obligate fungivores were less abundant on snags than on control logs. Total fungivores (T.lineatum excluded) were less common on shaded logs than on snags. The abundance of total predators (36 species) showed no response to either stand type or log treatment.

Beetle assemblages: Clear-cuts supported a different assemblage of saproxylic beetles to older stands in all log treatments. On clear-cuts, the composition of beetles collected from shaded logs also differed from that collected from control logs. The burned logs supported a fewer beetles (T.lineatum and several cambium consumers present in low numbers) rather than a different beetle assemblage. Burning reduces the nutritional quality of the cambium, making it unsuitable for many early colonizing cambium consumers. The six supposed fire-favoured species exhibited no preference for burned logs, preferring the open clear-cuts.

Snags supported a different assemblage of saproxylic beetles from the logs. Snags were unattractive to the most common bark beetles: H. palliatus was rare and H. glabratus was absent. Fungivores showed contrasting responses. The most abundant species, the ambrosia beetle T.lineatum was less common on snags, while total fungivores (particularly E. pygmaea and E. angustula) were more common on snags than logs. Logs have greater contact with soil moisture and support more species of fungi than snags. It is thus surprising that fungivores, other than T.lineatum, were more abundant on snags. However, the snags were created in situ, while the logs were stacked for several months before being distributed and may have been dry relative to natural logs. This may have resulted in a depauperate fungal and fungivore biota in this first year. Monitoring is ongoing.

Conclusions: The spruce log piles and snags were of conservation value for many early successional saproxylic beetle species. Snags supported a different assemblages of cambium consumers and fungivores and supported more red-listed individuals, than the logs. Benefits of snag and logs may be linked to their fungal communities and may thus not become apparent until several years after placement.


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