Sven Zacek / Ural owl (Strix uralensis) perching on an Apsen tree during the blue hour of dawn, Tartumaa, Estonia, April.

Jack Dykinga / Bark patterns of Aspen tree (Populus tremula) in front of a Ponderosa pine (Pinus ponderosa) trunk, Kaibab National Forest, Arizona, USA

Jack Dykinga / Charred remains of Aspen-fir forest near Hannagan's Meadow, at sunset with fireweeds growing in profusion amidst felled trees, Apache-Sitgreaves National Forest, Arizona, USA September 2011

Daniel Heuclin / Rio Grande Cottonwood tree (Populus fremonti wislizenii) on a sand "pedestal" White Sands National Park. New-Mexico, USA, August 2009

Floris van Breugel / Autumn colours of Aspen tree (Populus tremula) Sierra Nevada, California, USA. September 2009

David Noton / Autumn colours of the Aspen trees (Populus tremula) in the snow, near Muleshoe, Bow Valley Parkway, Banff National Park, Alberta, Canada. October 2009

David Noton / Autumn colours of the Aspen trees (Populus tremula) in the snow, near Muleshoe, Bow Valley Parkway, Banff National Park, Alberta, Canada. October 2009

Laurie Campbell / Looking up the trunk of an Aspen tree (Populus tremula) in autumn, Speyside, Scotland, UK, October

Jack Dykinga / Havasu Falls with Fremont cottonwood trees (Populus fremontii) Havasupai Reservation, Arizona, USA

Jack Dykinga / Havasu Falls cascade behind Fremont cottonwood trees (Populus fremontii) glowing in reflected canyon light, Havasupai Reservation, Arizona, USA

Jack Dykinga / Willow, Cottonwood and Aspen trees on the banks of the Oxbow Bend, Snake River, at dawn, Teton mountain range in the background, Grand Teton National Park, Wyoming, USA

Jack Dykinga / Havasu Falls with travertine formations in the river creating spillways, and Fremont cottonwood trees (Populus fremontii), dawn, Havasupai Reservation, Arizona, USA