Environment & Climate 7 min read 1h ago

Colorado Trees Face Climate Heat

Speakers at the Eighth Annual Tree Diversity Conference described Front Range growing conditions as 'harsh' and said climate models point to temperatures rising faster than
Over-reliance on ash trees and other narrow species selections has created monoculture urban forests that are highly vulnerable to Emerald Ash Borer and changing climate
Four Colorado tree specialists recommend prioritising drought- and heat-tolerant species — particularly oaks, Hackberry, Kentucky Coffee Tree, and Caddo Maple — alongside greater

Colorado's Front Range cities are facing a deepening crisis in their urban forests: a combination of harsh growing conditions, advancing tree disease, and accelerating climate change is placing the region's tree canopy under acute stress — and experts warn that decades of over-reliance on a narrow selection of species has left cities dangerously exposed.

Harsh conditions, harder to come

The Front Range presents what specialists at the Eighth Annual Tree Diversity Conference, held on 4 March, described bluntly as "harsh" conditions for tree growth. The region endures hot summers, cold winters, low rainfall, low humidity, strongly alkaline soils, late spring and early autumn frosts, persistent high winds, and a growing season roughly a month shorter than those of the Midwest or the eastern United States. Climate models presented at the conference, which speakers said they regarded with confidence, project temperatures rising more rapidly than in the past — with higher elevations warming fastest, since heat rises. As extreme heat events intensify across the northern hemisphere, the pressures on already-stressed urban tree populations are expected to compound.

Monocultures and the ash borer threat

A structural weakness underpins the vulnerability: urban forests across the Front Range have coalesced around a small number of tried-and-tested species, creating what experts characterise as de facto monocultures. The ash tree has been particularly heavily relied upon, and its dominance is now a liability. The Emerald Ash Borer — an invasive beetle capable of killing ash trees within a few years of infestation — is advancing through Colorado, and specialists say large numbers of ash trees, cottonwoods, and already-weakened specimens are likely to disappear in the coming years. Their loss will worsen the urban heat island effect, as roads, car parks and buildings absorb solar radiation where trees once provided cooling shade. Higher-density construction is also reducing the ground available for tree roots, further constraining what can be planted and sustained.

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Four practitioners with long experience of planting, propagating, and studying trees across Colorado shared their assessments and recommendations. Scott Skogerboe, propagator for Fort Collins Wholesale Nursery and a key figure behind the resilient tree collection at the Cheyenne Horticultural Research Station, argues that the region should look southward — to Texas, Oklahoma, and southern Nebraska — for seed sources suited to a warming climate, rather than defaulting to cold-tolerant northern selections alone. Among his specific recommendations are the hardy Arizona Cypress, Caddo Maples from Oklahoma (including the John Pair selection), a non-suckering hybrid chokecherry called Sucker Punch, oaks — which he considers the most valuable for ecosystem services — and a Gambel Oak selection from western New Mexico known as Gila Monster, rated cold-tolerant to -30°F and highly drought resistant. He also endorses Hackberry, the male Boxelder selection Sensation, Kentucky Coffee Tree in its male Espresso form, Catalpa, and Russian Hawthorn.

Tim Buchanan, who served as Fort Collins city forester for 41 years and oversaw the trialling and planting of numerous unusual varieties, cautions that Fort Collins presents particular challenges: its proximity to Wyoming makes it colder than Denver or Boulder, and its soils register a pH of around 8, making them quite alkaline. He has found that popular choices such as Autumn Blaze Maple, Silver Maple, Norway Maple, and Sugar Maple perform poorly there. He endorses Caddo Maple, Black Maple for its heat and alkaline tolerance, Texas Red Oak for its reliable autumn colour, Kentucky Coffee Tree, and a range of conifers including Blue Spruce, Douglas Fir, Concolor Fir, and Swiss Stone Pine. He and Skogerboe have together trialled a hardy Northern Pecan which has, by his account, developed into notably fine trees.

Panayoti Kelaidis, Senior Curator and Director of Outreach at Denver Botanic Gardens, draws on a long-term survey of Denver's unusual trees — first recorded in 1968 by landscape designer Al Rollinger — to identify the strongest performers over decades: Bur Oak, Kentucky Coffee Tree, Chinkapin Oak, Texas Red Oak, and Yellow Buckeye. He is critical of the widespread practice of clonal propagation — grafting from a single variety to produce genetically identical individuals — describing it as a "disaster in the making" that prioritises commercial convenience over the genetic diversity urban forests need to survive future shocks. He also notes that western soils (the Pedocal type, rich in calcium carbonate and low in organic matter) are fundamentally different from the darker, more fertile and acidic Pedalfer soils of the Midwest and East, which helps explain why many otherwise hardy eastern species fail to establish on the Front Range unless the soil has been amended over many years.

Sonia John, curator of the Regis University Arboretum and the founder of the Annual Tree Diversity Conference in 2014, shares enthusiasm for oaks — particularly Bur Oak hybrids with Gambel Oak — noting that they leaf out late and appear resistant to Front Range freezes. She also recommends American Smoketree, Yellowhorn for its drought tolerance and flowers, Soapberry, Hickory, and Northern Pecan. She suggests that oaks, because they hybridise readily, could potentially be selectively bred for greater tolerance of heat, cold, and drought — an avenue she considers underexplored.

Practical steps for a more resilient canopy

Beyond species selection, practitioners point to several cultivation practices that may improve establishment and long-term resilience. Inoculating tree roots with mycorrhizal fungi — including fungi sourced from thriving native communities for species such as oaks that require specific strains — is cited as beneficial. Incorporating 20 to 30 per cent compost at planting helps retain moisture and support soil biology. Root-pruning containers can prevent the girdling roots that can eventually kill a tree. The author, writing with 35 years of experience as an arborist, expresses the view that trees are healthier when planted outside irrigated lawn areas, since systems designed for frequent, shallow lawn watering can deprive tree roots of oxygen; most trees, in this assessment, fare better with deep, infrequent watering. Tolerating minor insect damage — allowing caterpillars to consume up to roughly 10 per cent of leaf area before intervening — is also recommended, on the basis that limited predation stimulates the tree's immune response and sustains the bird populations that depend on insect larvae.

As drought forces growers across the northern hemisphere to rethink how they manage trees and crops, the message from Colorado's specialists is consistent: diversity, genetic variety, and a willingness to trial unfamiliar species are the foundations on which a climate-resilient urban forest must be built. Without them, the next pest, the next drought, or the next heatwave could strip entire city canopies with little warning.

Topics

Climate ResilienceUrban ForestryInvasive Species

People

Panayoti KelaidisScott SkogerboeSonia John

Organizations

Denver Botanic Gardens

Places

Colorado Front Range

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