Effects of Seasonality, Forest Structure, and Understory Plant Richness on Bee Community Assemblage in a Southern Rocky Mountain Mixed Conifer Forest

Abstract

Wild bee community assemblages were surveyed in a high-elevation mixed conifer forest in central Colorado at multiple points during the growing season (April–August) and across a range of forest stand densities using blue vane traps. Understory forb communities were also characterized and related to bee species abundance and diversity. Overall γ-diversity was characterized by 19 genera of bees captured representing five families (Andrenidae, Apidae, Colletidae, Halictidae, and Megachilidae) and 39 unique species. Seasonal variation in abundance and community composition was considerable with early-season (April) communities dominated by Osmia spp. and midsummer (June–July) communities dominated by Bombus spp. Bee abundance and α-diversity were on average 88 and 74% higher in midsummer than in early- and late season. Forest basal area was negatively correlated with bee α-diversity and abundance, as well as understory plant species richness. Trapping locations with a stand basal area of ≤7.5 m2/ha exhibited approximately 54% higher bee abundances and 44% higher α-diversity than locations with ≥20.0 m2/ha of basal area. Bee α-diversity and abundance positively correlated with understory species richness suggesting that forested sites with low basal area, likely corresponding to overstory canopy gaps and increased site occupancy by forbaceous species, may drive local abundance and diversity of wild bee assemblages. This study provides basal area threshold values in a mixed conifer forest type that may be useful for resource management practitioners concerned with creating or conserving pollinator habitats.