USNPS photo

USNPS photo

Edward Frank - June 1, 2005
Last week I posted a question concerning how to tell if an aspen grove was
clonal or grown from seed. Lee pointed out that clonal aspens will change
color all at the same time.

I have been reading on the internet from a number of sites. One key to
determining if a grove is a clone grove is that the trees are all the same
sex. Aspen is dioecious, so individual trees are either male or female.
Trees flower in March and April, before the leaves appear, with both the
male and female trees producing catkins. Male catkins produce yellow
pollen. Female catkins do not preoduce pollen. There also are
morphological differences in the catkins. See illustration from
http://www.devonian.ualberta.ca/pwatch/poplar.htm#TREES

Here are excerpts from some of the internet sites:

http://www.treesforlife.org.uk/tfl.aspen.html
European Aspen

Aspen is dioecious, so individual trees are either male or female (in
contrast to most trees, such as Scots pine, for example, where male and
female flowers occur on the same tree). Trees flower in March and April,
before the leaves appear, with both the male and female trees producing
catkins. Pollinated female catkins ripen in early summer and release tiny
seeds (each weighing about one ten-thousandth of a gram!) which are tufted
with hairs.

Aspen has an extensive root system, and ramets have been recorded growing
up to 40 metres from a parent tree. Because of their access to nutrients
through the parent tree's root system, aspen ramets can grow very quickly -
up to a metre per year for the first few years. As the ramets grow, they
remain joined through their roots, and all the interconnected trees are
called a clone. They are all the same individual organism and are therefore
all single-sexed, either male or female. Each clone exhibits synchronous
behaviour, with, for example, all the component trees coming into leaf at
the same time in the spring. A clone can also sometimes be identified by
the specific colour its leaves change to in the autumn.

http://reference.allrefer.com/wildlife-plants-animals/plants/tree/poptre/
botanical-ecological-characteristics.html

Quaking Aspen

Quaking aspen forms clones connected by a common parent root system. It
is typically dieocious, with a given clone being either male or female.
Some clones produce both stamens and pistils, however. Quaking
aspen stands may consist of a single clone or aggregates of clones. 
Clones can be distinguished by differences in phenology, leaf
size and shape, branching habit, bark character, and by electrophoresis.

http://www.devonian.ualberta.ca/pwatch/poplar.htm#TREES
How to recognize male and female trees before flowering:

Here's a great suggestion from the McKinstrys in Oyen, Alberta:

• in early spring bring a small branch from your poplar tree (label twigs
  with tape if from different trees) into the house and set in a jar of water
  soon the grey tufts (catkins) get longer, turning pinkish
• if the branch is from a male tree the catkins then turn pale yellow as they
  shed a yellow powder (pollen) at the slightest touch. Place a dark paper or
  cloth under jar.
• female catkins do not have pollen
• outdoors, the wind quickly blows pollen away, so try this to be sure of
  your tree
• if the catkins on your outdoor tree eventually turn green, the tree is
  female, and the search for male trees continues!
  

allrefer.com reference - Quaking Aspen
http://reference.allrefer.com/wildlife-plants-animals/plants/tree/poptre/
botanical-ecological-characteristics.html 

Aspen clone takes size prize http://www.taiga.net/yourYukon/col373.html 

Quaking Aspen - Bryce Canyon National Park - http://www.nps.gov/brca/quaking_aspen.html 

Botanical Record Breakers - http://waynesword.palomar.edu/ww0601.htm 

Devonian Botanic Garden - Aspen Poplar - http://www.devonian.ualberta.ca/pwatch/poplar.htm