Pennsylvania forest. Photo by D. Sillman
(Click on the link to listen to an audio version of this blog …. Three Colorado trees
Here’s an update of a blog I wrote three years ago (Signs of Fall 1, September 9, 2018).
Our old home in Pennsylvania was a place of trees, the translation of the state’s name to “Penn’s Woods” tells you that. Flying over Pennsylvania or driving along its interstate highways or country roads emphasized the abundance and ubiquity of trees! Forests were everywhere! The original forests of the state represented the first in a long line of natural resources that the various colonial and then state governments of Pennsylvania gave away to fuel economic interests. Only 5% of the state’s original forests escaped repeated logging over the past three hundred years, and most of that 5% is in small, acre-or-less sized patches scattered across the commonwealth. The abundance of rainfall in Pennsylvania (average 41 inches/year) and the deep, fertile soils, and a healthy set of hearty, resilient tree species have helped the state’s forests repeatedly recover from the destruction of clearcutting and mismanagement.
Colorado, in contrast, does not seem to be a place conducive to trees. Colorado is arid (15 inches of rain a year on average) with very uneven rainfall. The entire eastern half of the state gets 10 to 12 inches of rain per year while places in the mountains may get 35 to 40 inches of precipitation per year. You find trees in the places where water is available: along rivers and lakes, in sheltered valleys, and up on mountain slopes. Colorado cities also have lots of trees but only because of direct human intervention (see Signs of Fall 3, October 10, 2020). The rest of the state is brush and rangeland (except where they are growing wheat, corn, hay, beans or sugar beets, and there irrigation is required to sustain the annual abundance of the biomass of the crop plants).
So the trees of Colorado are trees of the mountains and trees of riversides.
Blue spruce. Photo by USDA, Public Domain
The state tree of Colorado is the Colorado blue spruce (Picea pungens). We have seen some impressive blue spruces up in nearby Rocky Mountain National Park. Most of these trees were scattered out among some stately Ponderosa pines or bordered by dense stands of lodgepole pines or quaking aspens. The blue spruces thrive in the dry, short summers and cold, long winters of the Rockies. Colorado blue spruce has a very limited natural range, but, because of its ability to grow in a wide range of environmental conditions, it has been planted as an ornamental tree far outside of its natural distribution! You can find blue spruces in urban and suburban landscapes all across North America and Europe. A few summers back, for example, I was amazed to see Colorado blue spruces all over Prague, Czech Republic! The blue spruce is one of the most commonly planted landscape trees all around the world! Unfortunately, the long term health of many of these exotically located blue spruces is not very good. Often the long, hot, wet summers and short winters of these non-mountain climate zones have allowed all sorts of fungal diseases to take down many of the blue spruces (See Signs of Summer 12, August 3, 2017 for a discussion of some of the blue spruces around our old home in Pennsylvania).
Blue spruces are not a terribly important timber tree. Its wood is light and full of knots. It also does not generate a great abundance of food for either seed eaters or browsers. It does provide, however, good cover for birds and many small mammal species. This tree is dominantly valued for its appearance and regularly makes top five or top ten lists of Important Trees primarily because of its pleasing color and overall shape.
Quaking aspens in the Fall. Photo by Famartin, Wikimedia Commons
Quaking aspens (Populous tremuloides) are the definitive hardwood tree of the mountains of the Middle Rockies. Quaking aspens have the most extensive natural distribution of any tree in North America. Northern Pennsylvania is even included in the vast, continent-wide distribution of this species! It is a quick growing and relatively short-lived tree (100 to 150 years) that often becomes rapidly established after a fire. Quaking aspens reproduce by root sprouting and often form dense, clonal groves. They also produce air-transported, fluffy seeds that blanket the downwind areas from the mature aspen groves, although seed reproduction is not nearly as important for the species’ persistence and distribution as root sprouting. When you see a stand of aspens it is possible that you are really looking at a single organism! The trees are not only genetic clones but are also highly interconnected via their roots. In southern Utah there is a 100 acre grove of quaking aspens that are one highly interconnected entity. This multi-tree organism weighs 14 million pounds and is estimated to be 80,000 years old!
Aspen groves are usually generated by fires. The quick growing aspens send up root suckers after a fire and assume a pioneering role in the successional forest sequence. It is interesting that although aspen forests are somewhat resistant to fires (the trees’ moist, green leaves and thick twigs resist burning), if a fire does get established in an aspen grove the above ground trees are likely to be destroyed.
Under the aspens the slower growing pines and spruces are sheltered and protected. The deciduous nature of the aspens means that its forest floor receives more sunlight than a coniferous forest floor especially in the early spring. This allows a very vigorous growth of wildflowers and herbaceous plants in the aspen groves. Aspens forests are great places to look for wildflowers! Eventually, unless there is another fire (an increasingly likely event in our dry western ecosystems!) the conifers will grow up through the crowns of the aspens and shade them out
A syndrome that is affecting quaking aspens all across the country is called “sudden aspen decline” (SAD). There seems to be no specific pathogen associated with this die-off but rather it seems to be the consequence of drought and possibly climate change. Many of the larger quaking aspens in our neighborhood are showing signs of SAD. Several, I am sure, will have to be taken down in the next few years.
Cottonwood tree. Photo by A. Gordon, Public Domain
Along every river or creek and flanking every lake or reservoir are plains cottonwood trees (Populus deltoides). These cottonwoods are great, lumpy gray-barked trees of sometimes substantial girth that often rise up to 100 feet in height. Cottonwoods have thick, spreading branches that generate much needed shade and cover for a wide range of birds and mammals in the sun drenched plains of Colorado. In May through June the cottonwood trees release their eponymous “cotton” seeds generating snowdrifts of tumbling cotton fuzz for many miles around a stand of trees. The thick, deeply furrowed bark is resistant to fire and drought and the trees, which only live 80 to 100 years, can grow six feet in height a year in their early years of their lives. Cottonwoods often mark the edges of the scattered rivers and creeks that cross the dry high plains and were a welcome sight to early settlers announcing the presence of much needed water.
Some researchers feel that there may be more cottonwood trees now along Colorado’s rivers that there ever were. Suppressed wildfires and the absence of tree-destroying bison (the rough bark of the cottonwood must have been the just the thing for a good bison back-scratch) may be allowing longer survival intervals for the trees. Competition for the highly desirable, riparian woodland habitat, though, especially with a range of invasive species (see Signs of Fall 3, September 16, 2021) may reduce the overall potential abundance of the cottonwoods. Also, changes in the season flood patterns of the rivers because of dams and water impoundments may be having negative impacts on cottonwood seed germination. Great stands of cottonwoods may be reaching the end of their life expectancies with no young seedlings to take their place!
There are fifty native tree species in Colorado. My plan is to talk about as many of them as possible over the coming months and years!