COTTONWOOD REFORESTATION

Background

Black cottonwood is a foundational tree species in western rivers, including the Yakima River basin.  In recent years, concern has been growing about the health, extent, and old age of cottonwood forests along the Yakima River.  Mid-Columbia Fisheries and our partners have been working to assess the size, condition, and age of cottonwood forests in the region. An assessment completed in 2022 documented that cottonwood forests in the Kittitas reach of the Yakima River are regenerating at an unnaturally slow rate.  This means that overtime there will be significantly less cottonwood forests along the river – a change that has a negative impact on fish habitat.  The loss of cottonwood forests and their slow regeneration rates are caused by:

• Clearing of riparian areas

• Infrastructure that changes and narrows the river’s natural channel migration zone, and

• Alteration of the river’s natural hydrology for irrigation.

Similar changes have been documented along other managed rivers in the west.  The decline of cottonwood forests has a number of adverse impacts on fish habitat, including:

• Reducing shade that keeps water temperatures cool,

• Reducing leaf litter that supports aquatic insects, which are foundational to the food web,

• Increasing bank erosion,

• Reducing large wood available to fall into the river, which improves in-river fish habitat. 

The life history of black cottonwood is entwined with river processes.  Cottonwood forests are generated by a complex combination of river processes, and the biologic process of seed maturity and dispersal.  River processes create potential seedbeds along shorelines.  Seedbeds are bars of gravels, sand or cobbles aggraded during flood events with adequate power to erode, move and deposit sediments.  During the spring freshet, water levels are high and these river bars are saturated or underwater.  In most years, the recession of the spring freshet, which exposes moist sediments of bars, coincides with cottonwood seed dispersal.

Cottonwood trees produce seed in capsules that burst open as temperatures warm.  A period of peak seed release occurs regionally, with peak release in the Kittitas valley (Thorp to the Yakima Canyon) occurring within the timespan of a few weeks.  In 2021, peak cottonwood seed release occurred between May 27 and June 24.  As capsules open, seed is dispersed by wind.  Seed that is deposited on bare, moist soil – including bars – will germinate within 24 hours.  Seed landing on the water surface of rivers and streams may be deposited on a hospitable location, producing narrow bands of seedlings along a bar or channel margin.  Moistened seed dispersed on waterbodies has a shortened lifespan, losing viability within 48 hours.

Once germinated, developing seedling roots must keep pace with the rate of decline of the water table.  On a natural river system, this translates to a gradual flow recession which equates to a drop in soil moisture of less than 5 cm (2 in) per day.  Literature suggests that maintaining a moist surface for the first three weeks following germination, or by a slower initial flow recession, is beneficial to seedling survival.