Wild bee community responses to forest herbicide treatments in planted loblolly pine (Pinus taeda L.) stands

Abstract

Production loblolly pine (Pinus taeda L.) forests rely on herbicides to remove understory plants and promote forest productivity, yet there has been limited research on the indirect effects of herbicides on wild bees. The objectives of our study were to: 1) evaluate the indirect effects of herbicide treatments on wild bee populations and communities over a two-year period; and 2) determine linkages between stand attributes, including changes in understory plant communities, and wild bee responses. In 2022–2023, we sampled early post-establishment and midrotation loblolly pine stands in the Piedmont region of Georgia. Early post-establishment stands underwent broad-spectrum herbicide application through either: 1) broadcast chemical site preparation; 2) broadcast chemical site preparation + banded herbaceous weed control; or 3) control with no herbicide. Midrotation stands received either: 1) broadcast woody release treatment; or 2) control with no herbicide. We sampled bees with pan and blue vane traps and recorded stand attributes such as canopy openness, herbaceous plant species richness, and cover of bare ground, litter, and downed woody debris. We captured 7969 bees from 111 species, which constitutes ∼20 % of all bee species found in Georgia. Wild bee communities in herbicide treated stands had similar or greater total captures, species richness, and functional diversity as those in untreated controls. Bare ground cover was a significant driver of bee species composition in early post-establishment stands and indicator species analyses identified six soil-nesting species associated with early post-establishment treatments. Midrotation control stands were dominated by understory vegetation, while broadcast woody release treatments were associated with greater snags, coarse woody debris, and bare ground cover. Bee captures exhibited a slight negative response to increased tree density in midrotation stands. Our overall results indicated minimal negative impacts of broad-spectrum forest herbicide applications on wild bees in planted loblolly pine stands. Further, herbicide treatments may benefit some ground-nesting bee species by removing understory plant cover and improving nesting habitat conditions through increases in bare soil exposure.