By Julia Kast & Em Roberts

Arbor Day provides us an excellent opportunity to celebrate trees, our urban forests, and remind ourselves of the importance of taking care of the impressive mature trees throughout Washington. This year’s Washington Arbor Day Celebration will be held April 26 starting at 10:30 am at 11th Avenue SW across from the GA Building on the Capitol Campus. 

Many communities celebrate this event by planting a tree, just as the State Capitol Campus has done each year for nearly a decade. This year on Capitol Campus, we will be planting three trees! Over the years, trees on Capitol Campus have been planted with thought, care, and preparation in mind. This year, our Arbor Day theme is Thriving Urban Forests: Tree Species Diversity.

Planting a variety of tree species and creating diversity throughout your urban forest is crucial to maintaining health and forest resilience. Creating diversity throughout your urban forest can provide countless benefits including pest and disease resistance, resilience to climate change, increased wildlife habitat, and more.

Washington State’s Capitol Campus in Olympia is an impressive example demonstrating species diversity. There are currently 105 genera represented on campus, with a goal to have approximately 125 represented. If you do have a chance to visit, don’t forget to check out the map below and take a self-guided tour to visit a handful of Arbor Day trees!

As you may remember from last year’s theme, Care of Trees in the Urban Landscape, planting is just the beginning. After planting, care extends into trees’ establishment periods. Throughout a lifetime of maintenance of individual trees, through ensuring the urban forest has not only species diversity, but also age and size diversity; many trees on the Capitol Campus live on in the form of benches and other repurposed wood memorials.

The following trees were planted on our State Capitol Campus over the past nine years. Let’s walk through the care and purpose that was considered with each planting, including this year’s three new trees to emphasize the importance of tree species diversity.

Tree 1:  Cloud 9 dogwood (Cornus florida ‘Cloud 9’)

Thriving through Community – the plant community, that is. While planting individual trees in a yard with mulch around them is perfectly acceptable site selection, it may be healthier for the new tree planting to be integrated into a new or existing landscape bed so the soil, mycological network, hydrology, and nutrient cycling are all connected and supported by the community network. This tree is an example of that integration.

Tree 2:  Western redcedar (Thuja plicata)

Planning Ahead – all trees come to the end of their life cycle at some point whether senescence happens naturally through old age, or tree removal is planned for the purpose of safety mitigation.  No matter the reason, a well-trained eye can often see the end nearing some years before death occurs. This tree was planted in preparation for the removal of a tree of the same species located just 20’ away.  Replacement happened before removal – which is an example of the proactive mindset that is critical to successful urban forest management.

Tree 3:  saucer magnolia (Magnolia X soulangeana)

Planting for Purpose – when selecting tree species to plant, we should consider what attributes our selections will provide throughout the seasons. This tree was planted for the seasonal timing of its bloom to bring the joy of its lovely and unique flowers to the Legislators to signify the ending of the Legislative session on the State Capitol Campus

Tree 4:  red oak (Quercus rubra)

Structural Pruning of Young Trees – Trees may come from the nursery “headed off” to encourage a structure that resembles a mature tree, but at a very young age unnaturally. This can unfortunately discourage the central leader growth form natural to species like Q. rubra. Trees can develop growth patterns that are going to lead to structural instabilities in the future for many reasons. Structural pruning programs for young trees can help avoid large costs to financial budgets and to tree resource allocation of compartmentalization of decay in trees (CODIT) from large pruning cuts later in the tree’s life.

Tree 5:  American hornbeam (Carpinus caroliniana)

Mulching – The benefits of proper mulching techniques are numerous and include reducing vegetative competition, cycling nutrients back into the soil, building and improving soil structure, assisting with moisture retention by reducing soil moisture evaporation, providing an insulating layer which protects roots from extreme heat and cold, and reducing the impacts of damage from lawn care operations.

Tree 6:  bur oak (Quercus macrocarpa) – 2023 Arbor Day tree

Tree-Site Compatibility – Trees mature to all shapes and sizes. It is important to plan for the future when selecting sites for new trees in the landscape. This species of oak may reach a mature size of 80’ in height as well as 80’ in width, or more. There is plenty of open space with no overhead power lines for this oak tree to grow to its full maturity without causing any conflicts in the landscape or creating the need for pruning.

Tree 7:  dove tree (Davidia involucrate)

Trees as Memorials – This tree represents cultural and historic significance. It was the 100th tree planted on campus to celebrate the 100th anniversary of the National Association of State Foresters. The common name of dove tree also symbolizes the importance of peace when considering the damages of war.

Trees 8-10:  Kousa dogwood (Cornus kousa)

Right Tree, Right Place – These trees represent the proven success of appropriate tree species selection through common challenges of the urban landscape.

  • Acceptable maximum height at maturity for placement below power lines
  • Trees along streets require appropriate growth form and hardiness
  • No irrigation was available in area, so these trees were watered using slow-release watering bags during their establishment period

Trees 10-13: hardy pecan (Carya illinoinensis), Chinese pistache (Pistacia chinensis), and sassafrass (Sassafras albidum) – 2024 Arbor Day Trees

Tree Species Diversity -this year, three different tree species, each from a different genus, were selected for the 2024 Arbor Day plantings. Each tree’s native growth range is unique from one another, but also similar enough to our climate to grow successfully here. Our urban forests are made up of many species from all over the world. These trees also represent different size classifications, shapes, and characteristics within the forest. The sassafras brings the ultimate diverse characteristic to this year’s planting palette, being one of just a few tree species that has three different leaf shapes (leaf morphologies) on the same tree. We are excited to continue to add to the tree species diversity on the State Capitol Campus.