How does forest composition affect pollination in nearby apple orchards?

Talk like an ecologist

Coniferous forest – forest dominated by cone-bearing, evergreen trees such as pine, spruce and fir

Deciduous forest – forest dominated by broad-leaved trees such as oak, beech and maple that shed their leaves annually

Insect collection vacuum – a device used to suck up insects for further observation

Yield – the amount of a crop that is produced in an orchard or other crop field

Picture yourself sat in the dappled shade of an orchard. A gentle breeze wafts the sweet scent of apple blossom through the air and gently rustles the leaves above you. All around, bees buzz from tree to tree, filling the warm afternoon with a soft, soothing hum. Believe it or not, this idyllic scene is the site of Joanna Greenwood’s field research.

Joanna, a PhD student at Rutgers University, is studying the relationship between forests, pollinators and apple orchards in the northeastern US. “Work by other researchers has shown that apple orchards that are closer to forest or surrounded by a larger amount of forest are visited by a more abundant and diverse community of pollinators,” she says. “Apple flowers can’t produce fruit unless they are visited by pollinators, which transfer pollen between flowers, so the higher abundance and diversity of pollinators near orchards results in more apples.”

But apple growers don’t only care about how many apples they produce; they also care about the quality of those apples – for example, how big and round they are. “Apples with more seeds tend to be larger and rounder, and each seed requires a grain of pollen to fertilise it,” says Joanna. “That means that more pollinators depositing more pollen can improve the quality of the fruit.”

Busy bees

Many kinds of insects visit apple flowers in the northeastern US, but bees are the most important apple pollinators because they deposit the most pollen on flowers. “Many apple growers rent managed honey bees to help with pollination,” says Joanna. “However, honey bees are expensive, and some wild bees, such as mining bees, are more effective at pollinating apple flowers.” If their orchards are surrounded by good habitat for wild bees, some apple growers decide not to rent honeybees and rely entirely on wild bees instead. For these growers, understanding the relationship between their orchards, the surrounding habitat and their wild pollinators is crucial.

A bee’s favourite forest

While several studies have found that the amount of forest surrounding an orchard impacts the number of pollinators it receives, Joanna wanted to find out whether the type of forest also has an impact. “The northeastern US has both coniferous and deciduous forests,” she says. “We know that bees prefer deciduous forests because they have more flowers, which bees need for food.” So, Joanna suspected that high surrounding forest cover wouldn’t benefit apple pollination if it wasn’t the type of forest that bees prefer.

“I designed a study to answer two questions,” says Joanna. “Firstly, do apple orchards surrounded by different types of forest (deciduous vs. coniferous) differ in the abundance and diversity of wild bees visiting apple flowers? And secondly, if that is the case, does this translate to differences in apple yield and quality?” To answer these questions, Joanna found six orchards surrounded by coniferous forest and six orchards surrounded by deciduous forest and collected data on the pollinators and apple yield at those orchards.

The big bee count

“To understand how many pollinator visits apple flowers were receiving at each orchard, we conducted visual visitation observations,” says Joanna. “This involved counting how many and what kind of insects visited a clump of flowers for a set period of time, then calculating how many visits each flower received per minute, on average.” However, it is difficult to identify most species of bee without looking at them under a microscope, so Joanna and her field technician, Kirah, also caught samples with an insect collection vacuum and took them back to the lab to determine which species were present in the orchards.