Tim Fitzharris / Quaking Aspen (Populus tremuloides) trees and East Beckwith Mountain, West Elk Wilderness, Colorado

Wild Wonders of Europe / Smit / Old growth forest of Black poplars (Populus nigra) at sunrise, Gornje Podunavlje Special Nature Reserve, Serbia, June 2009

Wild Wonders of Europe / Smit / Old growth forest of Black poplar trees (Populus nigra) Gornje Podunavlje Special Nature Reserve, Serbia, June 2009

Wild Wonders of Europe / Möllers / Truck carrying Poplar logs, Mohacs, Béda-Karapancsa, Duna Drava NP, Hungary, September 2008

Scott Leslie / Quaking Aspen (Populus tremuloides) trees along water in autumn, Nova Scotia, Canada

Juan Carlos Munoz / Looking up through the branches of White poplar tree (Populus alba) with seeds covered in thick white fluff to aid wind dispersal, Andalucia, Spain, March

Juan Carlos Munoz / White poplar tree (Populus alba) with seeds covered in thick white fluff to aid wind dispersal, Andalucia, Spain, March

Juan Carlos Munoz / White poplar tree (Populus alba) with seeds covered in thick white fluff to aid wind dispersal, Andalucia, Spain, March

Juan Carlos Munoz / Branches of White poplar tree (Populus alba) with seeds covered in thick white fluff to aid wind dispersal, Andalucia, Spain, March

Alan Williams / Male Blackcap (Sylvia atricapilla) in Willow tree, Essex, UK, April

Shattil & Rozinski / Quaking aspen (Populus trembloides) leaves in stream, San Luis Valley, Colorado, USA

Shattil & Rozinski / Quaking aspen (Populus tremuloides) leaves above a stream, San Luis Valley, Colorado, USA

Shattil & Rozinski / Quaking aspen trees (Populus tremuloides) in autumn, San Luis Valley, Colorado, USA

Donald M. Jones / Alaska Moose (Alces alces gigas) bull in willows, central Alaska

Donald M. Jones / Eastern Fox Squirrel (Sciurus niger) on Cottonwood (Populus sp) tree, central Montana

Donald M. Jones / Eastern Fox Squirrel (Sciurus niger) on Cottonwood (Populus sp) tree, central Montana

Donald M. Jones / American Elk (Cervus elaphus nelsoni) bull eating Quaking Aspen (Populus tremuloides) bark in winter, western Montana

Donald M. Jones / American Elk (Cervus elaphus nelsoni) chew marks on bark of Quaking Aspen (Populus tremuloides), western Montana

Donald M. Jones / American Elk (Cervus elaphus nelsoni) bull bedded in background and elk chews on Quaking Aspen (Populus tremuloides) bark in foreground, western Montana

Donald M. Jones / American Elk (Cervus elaphus nelsoni) chew marks on bark of Quaking Aspen (Populus tremuloides), western Montana

Donald M. Jones / American Beaver (Castor canadensis) swimming with willow branch, western Montana

MYN / Niall Benvie / Aspen tree (Populus tremula) twig with new leaves, Alam-Pedja nature reserve, Tartumaa, Estonia, May meetyourneighbours.net project

Andy Sands / Willow warbler (Phylloscopus trochilus) singing from Willow tree (Salix sp.), Upper Teesdale, Co Durham, England, UK.

Martin Gabriel / Black poplar tree (Populus nigra), backlit at sunset, Bavaria, Germany, May 2008

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) trees with frost-covered branches, Inyo National Forest, California

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) in fall colors in an alpine meadow, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) tree trunk with elk antler rubs, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) and cumulus clouds above Snake River, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) in brilliant fall colors, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) with cumulus clouds, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) and cumulus clouds above Snake River, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) and cumulus clouds above Snake River, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) and alpine meadow in autumn, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) in brilliant fall colors, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) and cumulus clouds in autumn, Grand Teton National Park, Wyoming

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) trees in fall near the Alaska Highway, British Columbia, Canada

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) trees in fall near the Alaska Highway, British Columbia, Canada

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) trees in fall near the Alaska Highway, British Columbia, Canada

Nick Upton / White willow (Salix alba) trees and Common dogwood (Cornus sanguinea) bushes in winter, Chew Valley Lake, Somerset, UK. 2009

Sebastian Kennerknecht / Quaking Aspen (Populus tremuloides) forest in autumn, Lundy Canyon, eastern Sierra Nevada, California

Niall Benvie / Aspen leaves {Populus tremula} in autumn, Scotland, UK, October 2007

Tim Fitzharris / Quaking Aspen (Populus tremuloides) and Spruce (Picea sp) dusted with fresh snow, Rocky Mountain National Park, Colorado

Shattil & Rozinski / Fallen Aspen leaves {Populus tremula} on rocks in river in autumn, USA

Shattil & Rozinski / Aspen tree {Populus tremula} fallen autumn leaves, USA

Shattil & Rozinski / Aspen trees {Populus tremula} and evergreens on hillside, Colorado, USA

Shattil & Rozinski / Aspen leaf {Populus tremula} USA

Shattil & Rozinski / Aspen tree {Populus tremula} fallen autumn leaves, USA

Shattil & Rozinski / Aspen tree {Populus tremula} after the first snow of winter, Colorado, USA

Shattil & Rozinski / Close up of vein in leaf of Aspen tree {Populus tremula} as it changes colour in autumn, USA

Shattil & Rozinski / Fallen Aspen leaves {Populus tremula} on rock in river, San Luis Valley, Colorada, USA

Shattil & Rozinski / Aspen trees {Populus tremula} in autumn, Colorado, USA

Shattil & Rozinski / Close up of veins in leaf of Aspen tree {Populus tremula} in autumn, USA

Shattil & Rozinski / Fallen Aspen leaves {Populus tremula} on rocks in river, Colorada, USA

Shattil & Rozinski / Leaves of Aspen tree {Populus tremula} in autumn, USA

Shattil & Rozinski / Rocky Mountain NP with Aspen trees {Populus tremula} in autumn, Longs Peak in the background, Colorado, USA

Shattil & Rozinski / Aspen tree {Populus tremula} with the first snow of winter, USA

Shattil & Rozinski / Leaves of Aspen tree {Populus tremula} in autumn, USA

Roy Mangersnes / The colourful male flower of the willow {Salix myrsinites}, Norway. May

Tim Fitzharris / Quaking Aspen (Populus tremuloides) trees and Fireweed (Chamerion angustifolium), Collegiate Peaks Wilderness Area, Colorado

Tim Fitzharris / Quaking Aspen (Populus tremuloides) in fall colors and Maroon Bells, Elk Mountains, Snowmass Wilderness, Colorado

Theo Allofs / Quaking Aspen (Populus tremuloides) trees in autumn, Canada

Michel Poinsignon / Aspen trees (Populus tremula) at sunset with reflections in water, Haff Reimech Nature Reserve, Remerschen, Luxembourg

Michel Poinsignon / Aspen (Populus tremula) and birch (Betula) trees reflected in water, Autumn, Haff Reimech Nature Reserve, Remerschen, Luxembourg

Rob Tilley / Hoarfrost on Cottonwood Trees (Populus deltoides), Lamar Valley, Yellowstone NP, Wyoming, USA

Nick Turner / Willow (Salix sp) trees along the River Evenlode, Bledington, Oxfordshire, UK

Nick Turner / Willow (Salix sp) trees along the River Evenlode, Bledington, Oxfordshire, UK

Jason Smalley / Female catkins of Crack willow {Salix fragilis} UK

Elio Della Ferrera / Dwarf willow plants {Salix herbacea} leaves and flowers, Tessa Regional Park, Alps, Italy

Elio Della Ferrera / Thyme leaf willow {Salix serpyllifolia} Puez-odle Regional Park, Alps, Italy

Niall Benvie / Catkin of Willow {Salix sp} with flowers and buds in stages of development, Scotland, UK

Albert Lleal / Willow (Salix sp) SEM close-up view of anther with pollen at 112x magnification

Jose B. Ruiz / White poplar trees {Populus alba} growing beside a river, winter, Ruidera NP, Spain

Jose B. Ruiz / White poplar trees {Populus alba} growing beside a river, winter, Spain

Bernard Castelein / Beartooth highway, Montana, USA in autumn with Aspen trees {Populus tremula} in the background and rock with lichen in foreground.

Bernard Castelein / Aspen trees {Populus tremula} in autumn, Montana, USA

Jose B. Ruiz / White poplar tree (Populus alba) in town, Ibi, Alicante, Spain

Jose B. Ruiz / Close up of bark of White poplar tree (Populus alba) Ibi, Alicante, Spain

Michael Durham / Cottonwood (Populus sp) hybrid tree farm grown for pulp and fiber, primarily for paper production, Washington

Michael Durham / Cottonwood (Populus sp) hybrid tree farm grown for pulp and fiber, primarily for paper production, Washington

Dan Thory / Male catkins of Pussy willow {Salix caprea} England UK

Yva Momatiuk & John Eastcott / Black Spruce (Picea mariana) at their northern most range in tundra in fall colors, mountain stream rushing down among boulders, and high rocky peaks in first snow, Brooks Range, arctic Alaska

Yva Momatiuk & John Eastcott / Quaking Aspen (Populus tremuloides) with golden leaves and highway, and cumulus clouds in autumn evening, Alaska Highway, Yukon Territory, Canada

Yva Momatiuk & John Eastcott / Dwarf Birch (Betula nana) and isolated Black Spruce (Picea mariana) trees with lichens and berry bushes in colorful tundra in autumn, Denali National Park, Alaska

Tim Fitzharris / Haystack Mountain with aspen forest, Maroon Bells-Snowmass Wilderness, Colorado

Tim Fitzharris / Quaking Aspen (Populus tremuloides) trees covered with snow near Kebbler Pass, Gunnison National Forest, Colorado

Tim Fitzharris / Aspen forest overlooking Fremont Lake, Bridger-Teton National Forest, Wyoming

Tim Fitzharris / Aspen forest overlooking Fremont Lake, Bridger-Teton National Forest, Wyoming

Tim Fitzharris / Cottonwood (Populus sp) frozen leaves, North America

Tim Fitzharris / Cottonwood (Populus sp) frozen leaves, North America

Tim Fitzharris / Quaking Aspen (Populus tremuloides) forest, Maroon Bells, Snowmass Wilderness, Colorado

Tim Fitzharris / Quaking Aspen (Populus tremuloides) grove in autumn, Colorado

Tim Fitzharris / Quaking Aspen (Populus tremuloides) forest, Colorado

Tim Fitzharris / Marcellina Mountain and aspen forest in Raggeds Wilderness, Colorado

Thomas Marent / Goat Willow (Salix caprea) male flowers, Switzerland

Pete Cairns / Willow catkins {salix genus} after rainfall, Montreathmont, Angus, Scotland

Rob Tilley / Quaking Aspen Grove {Populus tremuloides} in autumn, Colorado, USA