Nature’s Notebook

Connecting People with Nature to Benefit Our Changing Planet

You are here


PopClock logoProject Background

PopClock is a partnership between scientists and volunteers to study climate change impacts on poplar trees. With a grant from the National Science Foundation, a research team from the University of Maryland Center for Environmental Science (UMCES) Appalachian Laboratory is examining how forest trees are responding to rapidly changing climatic conditions.

As part of this effort, UMCES scientists are working with U.S. and Canadian volunteers  to collect ground-based observations of spring leaf emergence and fall color change of two poplar species—balsam poplar (Populus balsamifera) and quaking aspen (Populus tremuloides). These observations will be used to create maps of “green-up” and “green-down,” which will be combined with genetic information to identify areas where trees are most and least adapted to climate change.

Join us! 

Populus tremuloidesWe are seeking observers in the U.S. and Canada to document changes in the growth of two important forest species—balsam poplar (Populus balsamifera) and quaking aspen (Populus tremuloides). You can join us by finding local stands of these species and recording when new leaves emerge in the spring and when leaves change color in the fall. We hope you continue to follow your stands for two or three years; this will ensure we have all the necessary data to make our predictions about how northern forest trees will respond to climate change.

How to participate…

1. Select your plants. Identify one or more individual balsam poplar (Populus balsamifera) and quaking aspen (Populus tremuloides) stands or trees to track. Information for correctly identifying poplar species is provide at the end of this page. Do poplars grow where you live? See the range map for balsam poplar and quaking aspen

Poplars tend to grow in clonal colonies, or stands (or patches) of individual trees sharing the same roots.  Please make your observations at the scale of the entire stand (or patch), rather than an individual stem. When you register your plant in Nature's Notebook, check the box next to "Patch?" to indicate that you are reporting on the stand rather than an individual tree.

2. Join Nature's Notebook. If you haven't already, create a Nature's Notebook account. See our specifics of observing if you need more details on getting started.

3. Sign up to receive the PopClock campaign messages (in the right sidebar of this page - you may need to scroll back up to see it). You will receive messages approximately every 4-6 weeks during the growing season, providing early results, encouragement, observation tips, interesting links, and campaign-specific opportunities. Don't miss out!

4. Observe your plant(s). For this project, we are especially interested in observations of two phenophases:

Phenophase Definition


One or more live, unfolded leaves are visible on the plant. A leaf is considered "unfolded" once its entire length has emerged from the breaking bud so that the leaf stalk (petiole) or leaf base is visible at its point of attachment to the stem. Do not include fully dried or dead leaves.

Colored leaves

One or more leaves (including any that have recently fallen from the plant) have turned to their late-season colors. Do not include fully dried or dead leaves that remain on the plant.

However, we welcome you to collect observations on other phenophases as well!

We encourage you to observe your plant(s) 2-4 times a week, especially in the spring and fall, when things are changing rapidly. However, we welcome any observations you can contribute.

 5. Report your observations. As you collect data during the season, log in to your Nature's Notebook account and enter the observation data you recorded. You can also use our smartphone apps to submit your observations!

Consider sharing phenophase photographs. We are seeking high-quality photographs of these poplar species and their various phenophases. If you are interested in sharing your digital images, please email them to Please specify your preferred photo credit and let us know if there are any copyrights or licenses attached to your photo. 

Distinguishing balsam poplar and aspen from other similar species

How to distinguish balsam poplar and aspen:

balsam poplar bark

The bark of aspen and balsam poplar can look similar, but balsam poplar’s bark tends to be more tan in color and the texture is rougher (see photo of balsam poplar on right). 

aspen canopy (left) and balsam poplar canopy (right)

As you can see in the photo on the left, the canopy of these two species also differs. Aspen (left side of the photo) displays a more finely branching canopy while balsam poplar (right side of the photo) displays robust, straight twigs. 

The leaves of quaking aspen (photo on the left below) are quite distinct from the leaves of balsam poplar (photo on the right below) -  aspen leaves are more round and heart-shaped compared to balsam poplar’s leaves that are rounded at the base and tapered towards the end (ovate).

aspen leaves                      balsam poplar leaves

Some other trees that may look similar to balsam poplar and aspen:

Bigtooth aspen (Populus grandidentata). Bigtooth leaf margins have highly distinct coarse teeth, whereas in quaking aspen the leaf margins are finely toothed to nearly smooth. Bigtooth aspen primarily overlaps with quaking aspen in the northeast and Great Lakes regions of the U.S., and southeastern Canada.  

Black cottonwood (Populus trichocarpa). Closely related to P. balsamifera, black cottonwood has 3-valved fruit capsule covered in short hairs (balsam poplar has a 2-valved capsule with no hairs). Black cottonwood is distributed in the Pacific Northwest region of the lower 48 states, Canada, and Alaska, and may form hybrids with balsam poplar in British Columbia and western Alberta.

Eastern cottonwood (Populus deltoides). Eastern cottonwood has a more heart-shaped leaf with highly distinct teeth along the margins of the leaf compared to balsam poplar. Overlaps with balsam poplar’s range primarily in the Mid-West and Mid-Atlantic regions of the U.S.

Narrowleaf cottonwood (Populus angustifolia). Similar in habitat and appearance to balsam poplar, but with much narrower leaves (<1 in. wide versus 1.5-3 in. wide in balsam poplar). In narrowleaf cottonwood, leaf stalks (petioles) are generally less than one-third the length of the leaf, while the stalks are generally greater than one-third the length in balsam poplar. Overlaps with balsam poplar’s range in Colorado and Wyoming.