Bees and Trees: What's All the Buzz?

By Laura Jackson, PA Forest Steward, Class of ‘00 (First published in Pennsylvania Forest Stewards News, Sep-Oct 2024)

I just love to watch busy bees as they feast on flowers—there is something soothing about getting my nose close to a tiny creature that is oblivious to my larger self as they so intently focus on collecting pollen and nectar. I see thousands of bees on native wildflowers from spring to late fall since my husband, Mike, and I transformed about three acres of fallow farm fields into native wildflower meadows because we want to support pollinators: butterflies, wasps, flies, beetles, and especially bees.

Bumble bees, like this one pollinating American wild plum, can rapidly increase their body temperature, allowing them to fly even on cool spring days. Their ability to thermoregulate is truly remarkable. Credit: Mike and Laura Jackson

Most of our 120-acre property is wooded, so it’s fascinating to learn that the interactions between flowers and pollinators just inches from my camera are insignificant compared to what is going on high above me in the forest canopy. Research shows that bees prefer foraging from tree species, even in areas dominated by open land.¹

I invited two beekeepers to educate us about honey bees at a recent meeting sponsored by the Woodland Owners of the Southern Alleghenies. We learned from Nate Caudill, a beekeeper who has over 1,000 honey bee hives, that trees are the most important food source for honey bees. I asked him to elaborate because I thought he’d say apple, cherry, peach—all the familiar fruit trees. But he surprised me by naming native trees that grow and bloom in our woods: black locust, tulip poplar, and basswood, to name a few.

Some of our earliest pollinators are native ground-nesting bees like this solitary bee on a pussy willow flower. Credit: Mike and Laura Jackson

I’m more interested in native bees than the non-native honey bee (although I love honey), so it made me wonder if our native bees also depend mostly on our native flowering trees. Native bees are in trouble due to pesticides, habitat loss, and climate change, so what trees do our native bees need and how can we manage our woods to help bees?

I think we all know that bees are valuable pollinators of many food crops, but it isn’t quite as clear that bees are important to forests, so hopefully the list of trees that feed native bees (at the end of this article) will help you make important connections between bees and trees. Scientists have a lot of unanswered questions about the actual interaction between bees and trees, especially high in the canopy. Since bees aren’t the only pollinators, do other types of insects, like beetles and flies, play more important roles in tree pollination? It’s difficult to collect insect samples high in the canopy, but more researchers are using blue vane traps that hang from branches high in the treetops. I precariously balanced on a tall step ladder so I could sample bees attracted to a blooming Hercules club.

Hercules’ club blooms in mid-summer in Bedford County and attracts a wide variety of bees, which I collected and sent to the López-Uribe Lab at Penn State for identification. I’m a trained volunteer in a statewide study to collect bees in order to understand their diversity, distribution, and abundance in Pennsylvania. Credit: Mike and Laura Jackson

Hercules’ club, also called devil’s walking stick (because of the stout, sharp spines on its stems, branches, and leaf stalks), is a good example of a forest species that has very high ecological value but low economic value. When in bloom, the air around it comes alive with the sound of bees and wasps, while butterflies flutter all around. In late summer, the dark purple fruit is in striking clusters that attract songbirds. Bears tend to bend or break the trees to access the fruit, but that makes it accessible to small mammals as well.

At the opposite end of the spectrum is one of Pennsylvania’s most important economic trees: black cherry. Black cherry trees flower from late May to June, and I’ve staked out low blooming branches, but have never been able to capture a bee. Maybe next year I’ll be lucky! As many of you know, most of the really valuable black cherry grows in the northern tier of Pennsylvania, but since 2000, it has produced poor seed crops which threaten black cherry regeneration. Since black cherry is insect pollinated, researchers are collecting and identifying the pollinators to determine if the poor seed production is linked to pollinator decline. In 2022, a research team from Penn State published their research findings: native ground-nesting andrenid bees are “likely the most important pollinators.” They found that the number of viable seeds increased as the number of andrenids increased, since they carried 347 times the amount of black cherry pollen than flies or other bees.³

Of course, other factors need to be analyzed, but this is an interesting example of current bee research as it relates to the health of forest trees.

How You Can Support Native Bee Populations in Your Woods

What can we do, as private forest landowners, to help native bees (many of which are ground-nesters) so their populations don’t decline in our woodlots? Here are a few suggestions:

Native tree biodiversity supports a diverse number of bee species. As more diseases and foreign insect pests invade our forests, we can’t rely on monocultures of just economically important trees to maintain forest health. A diversity of native trees also helps to provide resilience to climate change stressors like heat, drought, and excessive rainfall.

Tulip poplar has beautiful flowers that attract both native and honey bees, plus it’s commercially valuable. It’s also a tree that has an important role in climate change impacts, since it’s more adapted to warmer temperatures than are trees like sugar maples. Credit: Mike and Laura Jackson

Build habitat piles and keep snags that provide coarse woody debris for nesting bees, as well as many other pollinators. There are over 400 species of native bees in Pennsylvania, and many of them nest in decaying wood.

Brian Jones (formerly of Western PA Conservancy) stands in front of a habitat pile on our property that will slowly decompose, providing shelter and nesting habitat for a number of species, including native bees. Credit: Mike and Laura Jackson

Structural diversity in a forest is also important. We had a shelterwood cut of 30 acres in 2014 that created a young forest, hoping to attract Golden-winged Warblers. Although these threatened birds never found us, the regeneration did attract many other birds and created a multi-aged forest. More light reached the forest floor, resulting in more flowering plants like blackberry, which supported more bees and other pollinators.

We seeded the log roads and log landing after the tree harvest with a native pollinator mix from Ernst Conservation Seeds that included native warm-season bunchgrasses like Indian grass, little bluestem, and big bluestem. About 70% of native bees create burrows in the ground, and bunch grasses provide space around the base of the plant so bees can access bare soil. The other 30% of native bees are cavity nesters in dead wood, hollow stems of perennials, or in habitat piles.

Invasive plant species management is our biggest challenge since so many invasive plants respond quickly to gaps in the canopy where more sunlight reaches the forest floor. Japanese vine honeysuckle, Asiatic bittersweet, mile-a-minute, Japanese barberry, and multiflora rose are the most common invaders, taking over native plant habitat that prevents native tree regeneration, thus decreasing native biodiversity.

We pull and bag mile-a-minute that we find in canopy gaps throughout our forest. It’s tedious, but such a joy to outsmart these bio-bullies! Credit: Mike and Laura Jackson

I hope this has opened your eyes to the importance of trees and forests for bees. As private forest landowners, it’s important for our forest’s health to consider native bee diversity and abundance as we write and implement our forest management plans.

List of Native Trees that Feed Native Bees

(in approximate order of bloom time from spring through summer)

• Maple species (Acer spp.)
• Serviceberry species (Amelanchier spp.)
• Native willow species (Salix spp.)
• Sassafras (Sassafras albidum)
• Eastern redbud (Cercis canadensis)
• American sweet crabapple (Malus coronaria)
• Hawthorn species (Crataegus spp.)
• Black walnut (Juglans nigra)*
• Butternut (Juglans cinerea)*
• Hackberry (Celtis occidentalis)
• American wild plum (Prunus americana)
• Oak species (Quercus spp.)*
• Black gum (Nyssa sylvatica)
• Black locust (Robinia pseudoacacia)
• Honey locust (Gleditsia triacanthos)
• Sumac species (Rhus spp.)
• Cherries (Prunus spp.)
• American persimmon (Diospyros virginiana)
• American chestnut (Castanea dentata)*
• Tulip poplar (Liriodendron tulipifera)
• Devil’s walking stick (Aralia spinosa)
• American basswood or linden (Tilia americana)

*Oaks, walnuts, butternuts, and chestnuts are wind-pollinated, but their pollen provides an important food source for native bees.