June 16-22 is Pollinator Week

When most people picture pollinators, the image that comes to mind is often a field of wildflowers buzzing with bees. But researchers say the forest, often overlooked in pollinator conversations, plays a vital and under-appreciated role in the lives of native bees.

In a presentation hosted by the Xerces Society, entomologist Dr. Kass Urban-Mead shared findings showing that up to one-third of native bee species rely on forested habitats for part or all of their life cycle. While open meadows remain important, she said, healthy woodlands provide critical nesting, foraging, and overwintering grounds for a wide range of native pollinators.

“Insects are inseparable from our survival,” Urban-Mead said. “They keep our ecosystems functioning — breaking down waste, cycling nutrients, managing pests, and, of course, pollinating plants.”

Many of those plants include the crops humans eat. Some studies estimate that one in every three bites of food is made possible by a pollinator. That makes pollinators a big deal, not just for the ecosystems where they are found, but also for humans.

The issue is that pollinator populations are in decline. According to Urban-Mead, 28 percent of bumble bees are considered threatened, while 17 percent of North American butterfly species are at risk. Monarch butterflies, among the most recognizable insects in the country, have declined by roughly 90 percent since the 1990s.

Habitat loss and degradation — especially the loss of forested areas — are among the leading threats to these species.

Urban-Mead’s research focused on how bees used forests throughout their life stages. In contrast to the more commonly known bumble bee, which forms social colonies and accounts for just 1 percent of bee species in North America, the majority of native bees are solitary. Around 30 percent are stem- or wood-nesting bees, while the remainder nest in the ground. These solitary bees are often some of the first to emerge in the spring. Many of these bees spend most of their lives as juveniles inside soil or wood nests. Parts of the forest are often places where these bees call home. But, obviously, they cannot, and do not, live everywhere in the forest.

Forest structure and composition provide vital early-season resources. Spring ephemerals are those short-lived wildflowers that bloom on the forest floor before trees leaf out. They are essential to specialist bees that emerge early in the season and may forage for only a few weeks.

Trees and shrubs also carry surprising importance. Species such as willow, serviceberry, red maple, cherry and redbud offer enormous quantities of pollen and nectar at a time when few other plants are blooming. Urban-Mead’s research found that a single red maple could contain more than 600 nectar-producing flowers per cubic meter. That makes them a great food source for these early emergers. 

While many trees are wind-pollinated, she said, bees still play a role. A 1945 British study cited in her talk found solitary bees in the genus Andrena collecting significant amounts of pollen from wind-pollinated trees such as oak, maple, beech and chestnut. That study suggested bees may be quietly contributing to the reproductive success of trees once thought to rely entirely on wind. It was a bit eye-opening in that sense.

“Sampling bee diets only in the understory, but not in the canopy, results in an underestimate of the importance of pollen in the treetops,” Urban-Mead said. It makes sense, though, that this had been the focus, when it has historically been thought that most trees were wind-pollinated.

The importance of forests does not end with food. Nesting and overwintering opportunities are also abundant in wooded areas. Deadwood, stumps, brush piles and downed logs offer shelter for stem- and wood-nesting bees. Forest “tip-ups,” where the root system of a fallen tree is exposed, create ideal nesting habitat for ground-nesting bees. Leaf litter is important, too. Bumble bees, which are often active longer than other species, often overwinter in leaf litter and pine duff on the forest floor. These are the same bees that make use of leaf litter in the yards of those who do not rake leaves up, or who “leave the leaves” somewhere near the yard’s edge.

Urban-Mead said she also found that overwintering success rates for bumble bees was highest in forests. Not only was overwintering success higher, but nests located in forested areas also were more likely to produce a queen. More queens means more reproduction, which means higher bee populations.

Diversity is key, even in forest ecosystems. Studies show a strong correlation between the diversity of trees, shrubs, and understory plants and the abundance and variety of native bee species. Forest management strategies that promote plant diversity, retain downed wood, and maintain edge habitat can go a long way in supporting native pollinators.

Landowners can take steps to enhance pollinator habitat, even on a small scale. Urban-Mead recommended maintaining brush piles, preserving native shrubs and flowering trees, and increasing the diversity of early-blooming species. Forest edges, in particular, offer transition zones rich in food and shelter. 

Native bees associated with forests include mining bees, cuckoo bees, cellophane bees, mason bees, bumble bees and sweat bees — all of which depend on close proximity to food and nesting resources.

Even small differences in availability of habitat can make a difference, she said. That makes forest management an important part of providing for the next generation of pollinators such as bees.

Beckie Gaskill may be reached via email at [email protected].