Understory plant community responses following herbicide application in loblolly pine (Pinus taeda L.) stands

Abstract

Working pine (Pinus spp.) forests rely on herbicides to reduce competing vegetation and promote stand productivity, but the effects of chemical application practices on herbaceous biodiversity are not fully understood. Our objective was to assess the direct short-term (< 3 years post treatment) effects of different herbicide application methods on understory plant communities in early post-establishment and midrotation loblolly pine (P. taeda L.) forests of the Georgia Piedmont region. 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. In 2022–2023, we sampled understory vegetation in each stand using twelve 1 m² quadrats (480 quadrats total). Within each quadrat, we documented total percent understory plant cover and percent dominance of all non-graminoid understory plant species. We identified 112 native and 18 non-native understory plant species during the study. Early post-establishment plant communities showed significant increases in richness from the first to second sampling year. Plant species richness increased by 36 % in the broadcast chemical site preparation and 65 % in the broadcast chemical site preparation + banded herbaceous weed control treatments. Herbaceous weed control applications favored herbicide-tolerant, ruderal plant species adapted to disturbed areas. Midrotation herbicide treated stands were dominated by early successional plant species and had a 59 % increase in species richness compared to control stands over two sampling years, but total plant cover remained stable. Our results indicate that species richness increases rapidly (< 3 years) in early post-establishment plant communities following targeted herbicide applications, and that there was a shift in understory plant communities to favor ruderal, herbicide-tolerant species following midrotation woody release treatments.