The influence of forest management on the abundance and diversity of hoverflies in commercial plantations of Sitka spruce: the importance of sampling in the canopy

• Hoverflies sampled in 5 shelterwood, 6 group selection and 13 even-aged spruce stands.
• 7323 hoverflies of 74 species captured, the majority in the canopy.
• Highest numbers of hoverflies caught in group selection stands.
• Trends in abundance at ground level differed from trends observed in the canopy.
• Species richness determined at a landscape scale, not at the plot scale.

The majority of Sitka spruce plantations in the UK are managed by clear-cutting and replanting, which leads to the formation of relatively large blocks (5–30 ha) of even-aged, closely spaced trees. These stands are considered to be poor in terms of general biodiversity and vulnerable to outbreaks of insect pests and diseases. In contrast, stands managed using alternative silvicultural systems, such as shelterwood or group selection systems, contain a wider range of tree ages and sizes and have the potential to support greater numbers of other species. If this includes the natural enemies (predators, parasitoids) of pest species, then adopting alternative silvicultural systems might help to reduce current and future pest impacts. To determine whether the greater structural complexity of stands managed using alternative silvicultural systems is associated with an increase in insect diversity and an increase in the abundance of predatory species, we sampled hoverflies (Diptera: Syrphidae) in 5 shelterwood stands, 6 group selection stands and 13 even-aged stands of Sitka spruce at 4 forest sites in mid and north Wales. The larvae of aphidophagous hoverflies are important invertebrate predators in spruce plantations, and hoverfly species richness is often used as an indicator of general invertebrate diversity. Hoverflies were sampled at ground level and in the canopy using yellow pan traps and flight interception traps. 7323 adult hoverflies of 74 species were captured, the majority of these in traps in the canopy. Total hoverfly abundance and the abundance of aphidophagous species were significantly higher (P < 0.001) in group selection stands. The lowest abundance was recorded in even-aged stands, but patterns in abundance amongst the even-aged stands depended on whether sampling was carried out near the ground or in the canopy. Traps at ground level caught significantly fewer hoverflies in mid-rotation even-aged stands, whereas traps in the canopy caught similar, if not more, hoverflies in mid-rotation stands compared with young and mature even-aged stands. Stand species richness was closely related to the total numbers of individuals captured (abundance) and as a result was also highest in group selection stands, but there was no fundamental difference in hoverfly diversity (H’) between stand types or silvicultural systems. The data indicate that unlike abundance, overall hoverfly diversity was determined at a wider landscape scale and, at the plot scale, there was no evidence that structurally complex spruce stands supported more species because they provided a wider range of microhabitats and ecological niches.