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Winter 2013

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

March 12, 2013

The internal greenhouse effect of a daffodil

Early pollinators struggle with cold weather, so it's no wonder that late-winter flowers such as Narcissus (daffodil) tend to have extended blooming times and long-lasting flowers, some of which remain open for more than two weeks. When the sun is out, they can offer a warm microenvironment for bees.

Although they usually start showing off in the Washington area by late February, blooming times for Narcissus are highly variable, depending on variety, weather and planting location. Moisture, tree cover and mineral availability all affect flowering time, but soil temperature tends to have the greatest influence on early-blooming flowers. The microclimate of a sun-drenched, south-facing slope will sport blooms well before flowers appear in the shadow of a building that gets only a half-day of sun.

Once open, the flower can create a nanoclimate within itself. When sunlight hits a Narcissus longispathus flower, its tube-shaped corolla acts as a tiny greenhouse, raising temperatures around the pollen-producing anthers as much as 15 degrees higher than the surrounding air.

That's good news for small bees, which in late winter have trouble getting warm enough to fly.

Bumblebees and honeybees prepare to fly on cold days by exercising their flight muscles to generate heat. But mining bees, which nest in the ground, are less able to do that and are more dependent on ambient temperatures to warm them up for takeoff.

Spanish ecologist Carlos M. Herrera studied the relationship between mining bees and native N. longispathus flowers. The bees he studied could fly only if their internal body temperatures approached 72 degrees, yet they were able to forage in 54-degree weather — but only if the sun was out. Basking in sunlight on the outside of a warm Narcissus bloom was enough to get a bee airborne. Heat from flight muscles could then help keep bees aloft, but they still relied on blossom-basking to supplement their heat needs.

On those cooler days, bees spent most of their foraging time basking, but "flights between flowers were sometimes interrupted by sudden falls to the ground," Herrera wrote in his 1995 paper. "Fallen bees crawled into some nearby flower and basked before resuming normal foraging."

Sources: Ecology; Insect Physiological Ecology: Mechanisms and Patterns

Narcissus with Adena; daffodil with mining bee

Although a bee inside a flower has access to warmth and food, a bee basking atop a flower can more effectively raise its temperature by absorbing heat both from direct sunlight and from the flower below.