Our street trees are dying. How should we replace them?

Experts estimate that 60 percent of Santa Rosa's street trees are at risk due to climate change. In the broader Pacific Northwest, 30% of native tree species are vulnerable and may face range contractions.

The crucial role of trees in urban areas

Trees play a vital role in carbon sequestration, absorbing CO2 and releasing oxygen. Their loss would exacerbate CO2 levels and could speed up global warming. In urban areas, street trees are essential for providing shade and mitigating the urban heat island effect. They also support urban birds and enhance the aesthetic and psychological well-being of people.

Economic and environmental benefits of healthy urban forests

The health and coverage of tree canopies are crucial in reducing urban heat effects and improving air quality. Healthy urban forests contribute to higher property values, lower crime rates, manage storm water, and reduce runoff and erosion.

Drought and hydraulic deterioration

Urban environments, particularly those with extensive impervious surfaces like pavements, can exacerbate water stress in trees, increasing their vulnerability to drought and hydraulic failure. These conditions may lead to higher mortality rates for street trees during climatic extremes.

Heat stress and species suitability:

Increasing temperatures due to climate change may require the reassessment of suitable tree species. Studies have shown that many current species might become unsuitable under future warmer conditions , necessitating the introduction of more resilient species.

Pests and diseases:

Climate change is expected to alter the prevalence and impact of pests and diseases on urban trees, with warmer temperatures potentially increasing the susceptibility of trees to these threats.

Ecosystem services and urban heat:

Street trees provide crucial ecosystem services, such as reducing urban heat island effects and improving air quality. However, the effectiveness of these benefitsdepends on the health and coverage of the tree canopy, which is threatened by climate change.

The need for a holistic approach to urban tree management

A holistic approach is needed to understand the full impact of street trees on urban ecosystems and to guide decisions for creating resilient cities . This includes reassessing suitable tree species, as many current species could become unsuitable under warmer conditions.

Prioritizing resilient tree species

A key priority is the planting of tree species that are resilient, require low water, and can thrive in urban settings. Programs like California ReLeaf are actively providing grants to local groups to plant and care for trees, promoting volunteerism and community engagement in urban forestry.

To adapt to forecasted climate conditions, the city of Santa Rosa and Sonoma County Master Gardeners are teaming up to identify appropriate trees to plant. We compiled our own list for Gazette readers. Here’s our top ten, for broadwer Sonoma County environments.

- Quercus agrifolia (Coast Live Oak) — Superior in drought tolerance and wildlife support.

- Quercus douglasii (Blue Oak) — Excellent for hot, dry conditions with minimal water needs.

- Chilopsis linearis (Desert Willow) — Ideal for full sun and high heat, pest-resistant.

- Heteromeles arbutifolia (Toyon) — Drought-tolerant and supports various wildlife.

- Cercis occidentalis (Western Redbud) — Adaptable to different light conditions and requires minimal irrigation.

- Arbutus menziesii (Madrone) — Thrives in Mediterranean climates and is drought-tolerant.

- Populus fremontii (Arizona Cypress) — Known for drought tolerance and heat resistance.

- Platanus racemosa (California Sycamore) — Provides generous shade and some drought resistance.

- Quercus lobata (Valley Oak) — Large canopy cover with minimal watering needs.

- Parkinsonia florida (Palo Verde) — Recognized for its green bark and high drought tolerance.

Challenges with nonnative species

While nonnative species like the autumn blaze maple and tulip poplar are being considered to combat warming effects, they require careful selection to avoid issues such as exacerbating local pest problems. Some popular plants harbor pests like the glassy-winged sharpshooter, threatening vineyards by spreading Pierce's disease . It’s critical to avoid planting known pest hosts like citrus trees and oleanders near vineyards.

By selecting tree species that can withstand future climates, we’re creating urban forests that are more adaptable to increasing temperatures and water scarcity. Go plant a tree!