shade tree production crop

Plant Health: When Do Shade Trees Do What They Do?

Brian Hill

A Phenology Study is underway at NWREC!

The pretty flowers of spring, shade providing leaves of summer, and fire like colors of fall help us know when the seasons are changing. We use calendars to plan everything in our lives. Nature does the same but not in the same way.

Progression of seasonal changes in shade tree crown and color
Shade tree seasonal progression from spring (top left) to fall (bottom right).

What causes trees to change?

We can all remember early spring weather that was warm and dry as well as those years when we hoped our 4th of July BBQ would not get rained out. These weather differences from year to year influence events in nature that are crucial for species to survive. Plants and insects go dormant over the cold winter and begin growing in the spring and do it without a single calendar. They use day length and temperature to schedule their life events. The day length in the Willamette valley changes from 8 hours and 46 minutes in winter to 15 hours and 36 minutes in summer. These changes are predictable because they are cause by the tilt of the Earth, which doesn’t change. Our calendars align with these dates (winter solstice and summer solstice).Temperature, however, is unpredictable because there are a vast number of factors that influence it.

What changes are we monitoring?

A large part of the nursery industry in Oregon is dedicated to growing shade trees. Best management practices require monitoring for signs of event changes throughout the tree’s life cycle. This is known as Phenology, defined as the study of cyclic and seasonal natural phenomena, especially in relation to climate and plant and animal life. The Nackley Lab has a small in ground tree nursery that we use for experiments. For the past 3 years we have been conducting a Phenology Study where we have tracked the dates our tree events happened.

Monitored events in our Phenology study

  1. Bud Break: Date when the protective scale coating is shed from the bud exposing the tender new growth shoot
  2. First leaf: Date the first leaves are completely unfolded on at least 3 branches
  3. All Leaves Unfolded: Date when 90% of buds have reached first leaf
  4. First Flower: Date the first flowers are opened and stamens are visible on at least 3 branches.
  5. Full Flower: Date when half or more of the flowers are fully open
  6. First Ripe Fruit: date when the first fruits become fully ripe or seeds drop naturally
  7. Full Fruiting: Date when half or more branches have fully ripe fruit or have dropped seeds
  8. 50% Color: Date when half or more of the branches have leaves that have started to change color
  9. 50% Leaf Fall: Date when half or more of the leaves have fallen off the tree
image shows the phenology stages of flowering trees
Phenological stages of flowering trees from bud break (top left) to full flower (bottom right).

How can this information be useful?

As we collect data in year 3 (2022) of this study we are excited for how this data may be used in the future. Temperature data can be used to make degree day models which are based on heat units. The number of heat units per day are added together in a running total. This information is much better at predicting events in nature when compared to calendars. When growing shade trees in a production nursery setting, defending the crop from disease and predators is essential. Spraying a tree with a fungicide at bud break keeps them growing healthy. Spraying pesticides at first flower protects trees from insect attacks. By creating degree day models, growers can predict when to apply chemical protection for trees, eliminating double applications caused by calendar reliance.

Growers know the uncertainty caused by a changing climate impacts tree growth events. It’s hard not to trust the calendar dates which we plan everything else in our lives by. Future projects include modeling the events recorded over the last 3 years, and seeing how they align with degree day accumulation. The end goal is to use what we’ve learned to help keep the labor and pesticide costs down for the local nurseries while the produce the beautiful tree’s we all depend on.

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