Thursday, October 27, 2011

End of the Season comes to Colorado and many parts of the Northern US

It is always fun to try to predict when the growing season will end. In some places Halloween seems like a common time for the first big storm, in others right around Labor Day is when the first killing frost may come. And sometimes it comes early. The extremes of weather are usually quite important in determining what plants and animals can survive in a given place. Now is a great time to learn about how this works, and it is yet another great reason for the value of making regular walks outside and keeping track of what is going on outside, even this late in the season.

Yesterday our first winter storm hit hard. The day before it was nearly 80 degrees, today it is 11° here! So there has been a lot of frenzied activity all around town, everybody was blowing out their lawn sprinkler systems, putting up storm windows, and harvesting, covering up, or putting to bed, their veggie gardens, and digging out the snow shovel. Today there is a steady drone of chainsaws dealing with piles of twisted branches, broken fences, and blocked driveways and even streets. What a mess!

This has been quite a disaster for towns in the northern Front Range of Colorado because of the combination of a very late fall with leaves still being on trees, and a very heavy wet snow of up to 10" accumulation. Its a good example of how phenology can help explain why storms can sometimes be especially destructive to trees and our cities or towns. It also explains why scientists are so worried about the implications of climate change, since one of the predictions is an increase in wild weather events as we have seen this week.

One of the great pleasures in being more closely connected to the patterns in nature is appreciating how complex and intricate these seasonal changes can actually be, and having greater respect for these plants that have to take whatever comes! There is a real beauty and elegance in how plants and animals have adapted over the millennia to be ready for these often violent swings of weather that can come in the fall and spring. Of course all do not make it, but the native species usually come through pretty well.

Cities and other human designed enterprises are always trying to optimize efficiency by planning long ahead, including preparing for seasonal changes and also being ready to adapt suddenly if they need to. There are dates selected long in advance for when we should be buying or changing our wardrobe, getting the snow plows ready, winterizing our sprinklers and gardens. But nature often has different ideas. This year for example, as was mentioned in the blog last month, it has been quite a warm and extended fall. But it can all end quite abruptly!

As we have seen this year when you get warm weather, especially warmer night temperatures this can create a lot of havoc in how quickly we see fall colors and full hardening off of plants for winter. In New England, and many other parts of the eastern part of the country an epic season of rains and cloudy weather has drawn out the fall season resulting in either late colors or fewer bright colors this year. Here in the Rocky Mountain region we are getting reports from Montana down to Colorado and New Mexico of a very late fall season as well. In our case this appears to be due to an unseasonably warm fall as well.

Just before this storm there were still wildflowers in bloom. Golden aster (Heterotheca villosa), goldenrod (Solidago spp.), bush sunflower (Helianthus pumilus) and a couple of species of white aster (mostly Virgulus falcatum), and some weeds like mallow (Malva neglecta) were all open. Our annual Cosmos plants were still in full bloom in our garden too! These are all very hardy plants, so they should survive fine (or at least the seeds in the case of annuals) even with this sudden hard freeze. The critical tissues and resources for perennials growing next year are safely contained in roots or rhizomes deep underground. But it is still always fascinating to me that you can have plants in flower at 5,000' in Colorado at the end of October! I saw the same thing when I lived in Montana. One of our worst weeds, spotted knapweed (Centaurea stoebe), always had open flowers until very late in the fall or until the soil froze. This plant is very well adapted to cold northern climates!

Looking at how plants prepare for winter is a great way to better appreciate the richness and diversity of adaptation of the plant species that we have here. Some plants end the season very early, especially those that are sensitive to drought. This year for example wax currants (Ribes cereus) were dropping leaves in the middle of the summer. Several other shrubs native to our dry foothills areas do this. Sometimes they put out new leaves in September if there is enough rain, then drop them again sometime in October. Here as well in their native habitats back east, American linden (Tilia americana) and red maple are usually the first to lose their leaves, and oaks are generally the last. Everything else is somewhere in-between.

Right now green ash (Fraxinus pennsylvanica), chokecherry and plains cottonwood have been in full rich color for a few weeks, and leaves are beginning to fall. The trees in my yard just reached the 50% leaf fall stage the day before the storm. The late fall species such as northern red oak, burr oak, norway maple, mountain ash and others are just starting to change colors. It is likely they will drop as brown leaves this year since they will get a hard frost before they reach full color. So each of these plants has a slightly different cue as to when it is time to quit. Part of this also has to do with the structure of the leaves. Oaks for example have tough leathery leaves, so are more resilient to fall weather than silver maple or lindens. It is all a question of tradeoffs between growing a little longer and risking frost damage.

Almost two months ago many alpine plants ended their growing season here. A sudden frost and snowfall in late August took care of that. Just like here in the lowlands in late October, up there only a few hardy flowers were left at that time. One spot I was studying up on Niwot Ridge at 12,500' elevation, a major center for alpine research near Boulder, apparently had about a two week growing season! The late spring and heavy snowfall from last winter and spring resulted in very slow snowmelt on north facing slopes, and then an early snowstorm hit. It was just enough time for a few Parry's primroses (Primula parryi) and sedges to put out leaves and flowers before they got frozen again. So depending on where you are it was either a short or long growing season, and there are plants that can more or less adapt to this amazing range of conditions. The big question for scientists is to learn more about how well plants can adapt to even greater changes in weather and climate over the years as the climate changes. Our data from Budburst should be critical in figuring out which species will be most resilient or most stressed by these changes, and how this will change depending on where you are.

Stay tuned on what happens next. Maybe we will have another warm spell and some plants may put out new flowers. There is always something new to see out there. I look forward to hearing what you all discover this fall and winter. Please let us know on our Facebook page or leave comments to this blog.

Paul Alaback

All photos taken the the past few weeks around the Colorado Front Range by Paul Alaback.

1. Top: photos of Massachusetts Avenue in Berthoud, CO before and after our recent storm!

2. Norway maple illustrates what happens when trees with leaves get a wet heavy snow! Many branches were broken and the tree will probably have a new shape like many others here in town, especially the ornamental or exotic species which did not evolve in this weird climate.

3. Hiking along trail on Kenosha Pass, just south of Denver when quaking aspen was in full color.

4. Goathead (Tribulus terrestris) a terrible weed that punctures tires and flowers until the first deep freeze occurs.

5. Colorful view of mountains near Kenosha Pass taken a few weeks ago. Notice that aspen varies from green to yellow in most groves, showing a very slow transition to fall colors this year.

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