Signs of Winter 11: Oaks and Acorns

Photo by Jamaine Wikimedia Commons

Oak trees use their acorns to “fly” or “stride” (or hop or run) from one habitat to another. I now have a yard and field with twelve young oak trees (a mix of white, northern red, and scarlet oaks) growing in the places where eight of my forty year old spruces once stood (but which were, alas, knocked down in a June 2006 microburst event). The acorns that started these trees were undoubtedly dropped from the branches of the spruces when some careless blue jay or crow tried to talk with their beaks full. The oak seedlings were quite inconspicuous under the dense branches of the spruces but have now grown into solid looking pole trees that are twenty to twenty-five feet tall.

In 1996 my daughter had a fourth grade science project in which she had to identify the trees of her yard and neighborhood. She and I wandered about on our two acres and up and down our street and identified fifteen or twenty different tree species. We saw, though, no oaks at all! Most of the trees we identified were intentionally planted and placed as the surrounding houses were built in the 1950’s and 1960’s, and a few were orchard relics of an even older, farm-related past. Great swaths of lawn was the dominant habitat type all around us, and there were very few “wild” trees to be seen anywhere.

Twenty-one years later, though, the wild oaks abound on the edges of my property thanks to careless birds and also to my personal abhorrence of weed-whackers and any need for tidiness and control in my landscaping!

Photo by D. Mullen Flickr

Let’s think about oak tree flowers and acorn production (and I am going to focus on white oaks as my example oak species): white oaks flower in the spring right about the same time as they leaf out. Here, in Western Pennsylvania, that is mid-April or so. A given tree will have both yellow, staminate flowers arrayed in dense catkins and red, pistillate flowers that are usually singular or in pairs. The staminate flowers mature a week to ten days before the pistillate flowers (a good protection against self-pollination) and in the three days of pollen dissemination winds can carry the tiny pollen grains great distances. If a pistllate flower gets pollinated it will begin to form it acorn. If it does not get pollinated, it will senesce and drop off the tree. It is possible, then, to get a good idea of the success of a year’s pollination efforts by simply observing the abscission or persistence of the white oak’s pistillate flowers during this critical time period of the spring!

Weather conditions, though, greatly influence the timing and success of pollination. Wet weather slows down pollen release, and dry winds and freezing temperatures can greatly inhibit flower development. Ideal acorn development occurs, according to Sharp and Sprague (from their 1967 paper in Ecology), when the weather is warm for the ten days before flowering and then cool for the two or three weeks after flowering.

Thinking about the extremely variable weather of the month of April, these perfect conditions are not likely to occur very frequently! Interestingly, variability in the production of acorns may be an extremely important feature of an oak tree’s overall reproductive strategy!

Photo by Dcjsr Wikimedia Commons

A mature white oak (a tree between 50 and 200 years old) can produce up to 10,000 acorns a year! That same tree, though, may produce no acorns at all in any particular year, and somewhere in between zero and ten thousand in most of the intervening years. Acorn production (also called “mast production”), then, is quite unpredictable from year to year. On average, a good acorn year (also called a “mast year”) occurs only every four to ten years. These mast years, though, are only partially correlated with the weather and with some of those flower survival and pollen formation factors mentioned above. There seems to be some other, more innate biological process going on in the oak tree that helps to regulate the overall potential for the tree to make acorns.

Which gets me to the reason I am writing this post! Back in November, my good friend from California, Larry, sent me an article from the Wall Street Journal. The Journal is not a paper I regularly read, and it is definitely not a paper in which I would expect to find a very interesting article about oak tree reproduction! However, the November 6, 2016 issue had an article entitled “Boom or Bust Breeding Cycle that Helps the Mighty Oak Survive” (by J. C. McGinty) that beautifully described the evolutionary logic employed by oaks that necessitates great yearly fluctuations in acorn production.

D. Sillman

The idea is very straightforward: acorns (especially white oak acorns) are highly desirable food for many species of birds and mammals (in fact, more than 180 species of birds and mammals eat white oak acorns (everything from crows to turkeys to blue jays, and deer to raccoons to a variety of mice!). In non-mast years almost all of the white oak acorns produced are consumed before they can germinate. The only way that an oak tree can slip a few acorns through this consumption filter is to occasionally overwhelm the system with acorns. It is only in mast years, then, that substantial numbers of acorns survive to form oak seedlings!

Last Fall in Pennsylvania we had, according to Marc Abrams (a Penn State professor of forest ecology and physiology) a “super mast year!” Abrams reported a greater mass of acorns on the red oaks around State College than he had ever seen in his thirty years of observations. Deborah and I had noticed this, too, on our local hiking trails here in Western Pennsylvania. In places with abundant oak trees it was actually difficult to walk because of all of the rolling acorns underfoot! I had to go back in my hiking notes to 2006 (ten years ago! A typical mast-year cycle!) to find references to such acorn abundances.

Photo by D. Sillman

So, there will be a lot of winter food for white tailed deer and wild turkey this year! The Journal article also makes reference to another, less welcome species that is expected to benefit from the acorn abundance: the white footed mouse. White footed mice are one of the key biological reservoirs for the bacterium that causes Lyme disease. The mice pick up the bacterium from black legged tick bites and then pass it along to more ticks that subsequebtly feed on their blood. It is expected that with a booming population of white footed mice in our woods and fields, a higher percentage of black legged ticks will be exposed to the Lyme bacterium and thus potentially carry the disease to even more humans. Pennsylvania already leads the nation in the number of Lyme disease cases per year. This new, acorn powered mouse system, though, might drive our old infection numbers into even more astronomical levels!

So, oaks go boom and bust with their acorns and by feeding so many mammals and birds get the benefit of seed transport and dispersion! A couple of my young oaks actually made some acorns this past year! I wonder where they will end up?