Restoring forest cover is a promising option for mitigating climate change. So, as foresters we should be celebrating and rolling up our sleeves to plant trees, right? But where are the tree-planting opportunities in cold, semiarid climates? Much of the Colorado Front Range population area lies at 5,000 to 6,500 feet elevation within the Colorado Piedmont physiographic region. The climate is cold and semiarid. Mean annual precipitation is only 14.14 inches at Denver and reaches 18 inches in the foothills west of Denver. Evapotranspiration demand is remarkably high from May through September.

Potential natural vegetation ranges from shortgrass prairie to foothills shrubland and montane grassland with scattered ponderosa pines. These are difficult conditions for growing trees. Existing forested areas are found only in the floodplains of rivers and large creeks and within the riparian zones of perennial streams. These climatic and vegetation conditions are also found on the western slope of the Rockies in Colorado and in the Central Rockies in southcentral Wyoming and Montana.

Urban Afforestation

Urban and suburban opportunities to mitigate climate change by increasing carbon sequestration through large-scale tree planting include urban and suburban open spaces, parks, golf courses, boulevards, freeway medians, biodiversity and/or migration corridors, floodplains, and riparian areas. More tree shade for urban heat islands is much needed.

Afforestation of open spaces, parks, golf courses, boulevards, and freeway medians will require cluster planting in addition to individual tree planting. We must convince Parks and Recreation Departments and city foresters to change their culture and learn to plant silviculturally designed clusters of trees with high species diversity. No more monocultures! Cities and counties should require developers to plant urban and suburban forest patches, not just individual streets, or sidewalk trees.

Land management practices that maintain or restore forest cover, create forests where they did not previously exist (afforestation), or that avoid disruptions of forested and wetland ecosystems with deep organic soils (containing substantial carbon stores), are likely to have the best results for mitigating climate change.

Floodplain Forests

Floodplains of major rivers and their tributary creeks along the Colorado Front Range contained gallery forests over a quarter mile in width before European settlement. The predominant ecological system is the Western Great Plains Floodplain, characterized by cottonwood gallery forests and dense shrublands lining active river channels. These forests and shrublands once served as important biological migration corridors, especially at riverine confluence sites.

The biological diversity of intact floodplain forests is amazing. Dominant trees include plains or lanceleaf cottonwood, peachleaf willow, green ash, and box elder, with a truly diverse understory of saplings, shrubs, forbs, and grasses. Fauna includes large and small mammals, birds, reptiles, amphibians, insects, and macroinvertebrates. Floodplain gallery forests are often home to bald eagles and ospreys that depend on large cottonwood trees for nesting sites.

Most of the Colorado Front Range river systems have been dammed, their channels straightened and confined, and their peak flows reduced due to flood control reservoirs. This has affected cottonwood regeneration on the floodplains, resulting in aging and deteriorated floodplain cottonwood forests. Mitigation will require reforestation of the floodplain forests. A prime example along the Colorado Front Range is the South Platte River, which has lost most of its floodplain forest cover.

Riparian Woodlands

The extent of riparian woodlands in creeks and perennial gulches is dependent on elevation, valley gradient, and floodplain width. The ecological system of interest along the Colorado Front Range is the Rocky Mountain Lower Montane-Foothills Riparian Woodland and Shrubland, found at 5,000 to 8,000-feet elevation. Dominant trees of the riparian woodlands include narrowleaf cottonwood, plains cottonwood, peachleaf willow, box elder, green ash, balsam poplar, Rocky Mountain maple, thinleaf alder, and river birch. The larger riparian woodlands may include an understory of chokecherry, river hawthorn, red osier dogwood, and various willow species.

The biodiversity of riparian fauna is less than that of riverine floodplains but greater than the adjacent uplands. Improving riparian woodlands through selective tree planting will simultaneously enhance the ecological diversity of those areas.

Biodiversity and Migration Corridors

Reforestation of floodplain forests and riparian woodlands will enhance landscape connectivity. The prime biodiversity corridors along the Colorado Front Range include floodplains connected to tributary riparian areas. In many cases, these floodplain-to-floodplain and floodplain-to-riparian areas were disrupted by commercial or housing developments. Re-establishing those biodiversity corridors requires

reforestation but also the removal of land ownership impediments. If floodplains or riparian areas are under private ownership, then they may be converted to public ownership through voluntary buyouts, land exchanges, or conservation easements.

Floodplains and confluence areas were often used as sand and/or gravel extraction sites and will require major reclamation efforts to re-build soils prior to reforestation. Prime examples of such confluence sites include the Bear Creek confluence with the South Platte River and the Clear Creek confluence with the South Platte. Reforestation of the Bear Creek floodplain between the South Platte River and the foothills can provide critical habitat connectivity.

Co-Benefits of Floodplain and Riparian Area Tree Planting

Increased carbon storage by trees and shrubs and soil carbon sequestration are the primary goals, but other ecological benefits accrue from reforestation. Reforestation and afforestation both have well-demonstrated co-benefits. The co-benefits of floodplain and riparian area tree planting include 1) increased hydrologic stability of stream channels and floodplains; 2) reduced stormwater flows thereby decreased streambank cutting and sediment production; 3) increased groundwater recharge; 4) water quality improvements; 5) recruitment of large deadwood and particulate organic matter beneficial for aquatic organisms; 6) enhanced soil fertility; and 7) increased habitat for birds, fauna, amphibians, macroinvertebrates, and even fish in the larger streams and rivers.

Integration with Community Natural Area Programs

Many Colorado Front Range counties and municipalities have open space or natural area programs. Reforestation of floodplains and riparian areas would be entirely consistent with open space and natural area programs.

Integration with Community Stormwater Management Programs

Forested floodplain and riparian areas help trap and slow stormwater drainage. In return, captured and filtered stormwater could be used to provide water to tree plantings on floodplains and in riparian areas, provided that state water laws allow such uses. 

New Silvicultural Principles and Practices

Floodplain forests and riparian woodlands are essentially unmanaged today. Foresters must develop restorative and regenerative silvicultural principles and practices for floodplain forests and riparian woodlands.

In the case of afforestation, cluster planting of trees and shrubs (e.g., Miyawaki and similar methods) should be considered a promising silvicultural method for such semiarid climates.

Native tree and shrub species should be used whenever possible and if available. Adapted cultivars, adapted to warm and dry climates, should be used if native species are not available. In the case of drastically disturbed floodplain and riparian sites, intensive site preparation, including soil remediation, may be required.

Water Issues Related to Tree and Shrub Planting

The Colorado Front Range has been going through a drought for the past two to three years. Soil water levels are low. Many Front Range communities are dependent on Colorado River water diverted from the Western Slope of Colorado. Colorado River water is becoming scarce due to pervasive drought and reallocations. Groundwater supplies along the Colorado Front Range are not reliable in the long-term. New planting techniques are needed that do not require forever irrigation.

Carbon Credits Through Tree Planting

We should be reforesting floodplains and riparian areas primarily for ecological restoration, biodiversity, and climate change mitigation. It is important to note that an opportunity exists to help fund reforestation efforts through the voluntary carbon market program. Carbon is sequestered when trees take in carbon dioxide from the atmosphere and store the carbon molecules in the trees and soil. In this program, carbon credits are earned when trees are planted and maintained over time. Qualified tree-planting projects must be verified for emissions reduction to be claimed as a carbon offset. Furthermore, carbon benefits must be verified by an accredited and independent third-party.

It is time to start planting trees—lots and lots of trees—even in semiarid climates. We need to do it for the health of the planet. We owe it to our grandkids.

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Photo Credits: 
Image 1: This floodplain at South Platte River, Colorado, once forested, was over one-quarter mile pre-settlement. Photo credit: Bruce Van Haveren. 
Image 2: Floodplain forest at Bear Creek, Colorado in good ecological condition displaying good diversity of species and age classes, but in need of cottonwood regeneration. Photo credit: Bruce Van Haveren. 
Image 3: Depauperate floodplain forest at South Platte River, Colorado in need of cottonwood regeneration. Photo credit: Bruce Van Haveren. 

Bruce Van Haveren is a consulting forester and master arborist in Evergreen, Colorado.