Thursday, May 29, 2014

Legacy Trees (Burr Oak)

One of the appeals of planting oak trees is that there is a very real possibility that a tree you plant could still be giving pleasure to people two or even three hundred years from now.

Some genus are more suited for "legacy trees" than others.  Oak are a great choice because they are long lived trees, can grow to impressive size and offer benefits beyond simply producing shade.

I am not an unbiased observer.  I think that Burr Oak (Quercus macrocarpa) is one of the finest species within the least in the Northeast United States.  Here are my reasons:

Legacy trees must be able to fend for themselves.  There is no guarantee that all of the property owners over the next three hundred years will be "tree huggers"  Burr Oak are probably the most fire resistant tree in the Northeast United States.  Burr Oak are the oak trees of the tall grass prairie Oak Savannahs. Those savannahs were maintained by regular burns that killed off the Box Elder, Red Maple and other invading species.

If you suspect that your planting may be exposed to "burns", then it makes sense to select seedlings with "corky" 2nd year bark.  Cull the rest or give them to friends.

The more I look at individual oak trees the harder it gets for me to pigeon hole them into a species.  This tree has characteristics of both Swamp White Oak (Q. bicolor) and Burr Oak.  Growing in a sedge marsh with cattails, sedge and willow.  Growing beside Canfield Rd, Eaton County, Michigan.

Burr Oak in a flood plain beside Nye Hwy.  Other side of road is willow, cottonwood, elderberry and Reed Canary grass.  Understory here is primarily Giant Ragweed.

Burr Oak are moderately-to-highly tolerant of flooding.  My personal belief is that different land races of Burr Oak have varying degrees of tolerance to flooding.  Flooding depletes the soil of oxygen.  Paving and compaction have a similar effect.

Young Burr Oak showing persistent "whiskers".  Beside Gunnell Rd, Eaton County, Michigan
Older Burr Oak showing persistent "whiskers".  5 Point Hwy, Eaton County, Michigan.

Burr Oak have an abundance of small, persistent side branches.  These small branches lower the value of the tree for lumber and make it much more difficult to split for firewood.  Those kinds of "blemishes" reduce the likelihood that the tree will be "poached" because of its lesser value or because it is simply more work to burn.

There is much genetic diversity within the species.  It ranges from Maine to Montana, Alabama (nice specimen next to the College of Veterinary Medicine at Auburn) and southern Texas to Manitoba.

The abundance small, persistent side branches also make the tree more likely to survive a lightening strike.  The tree may be killed down one side or may be killed to within 15 feet of the ground, but those persistent side branches means that the tree will recover.  In addition to the persistent side branches, the heart wood is quite rot resistant which allows the tree to heal over the injuries. Rot resistant wood also makes hollow Burr Oak a rarity...helping them resist wind storms even in their dottage.

I know I am fixating on a "Legacy tree's" need to shrug off insults and adversity.  A good primer for the kinds of challenges that "Legacy trees" must endure can be gleaned by studying The Endicott Pear, one of this nation's best documented trees.

Most Burr Oak have a classic, forest tree form.  Many "Champion" trees are grotesque, warped trees that scored well.  Many, many Burr Oak look like the McBaine Oak, a former National Champion.

The downside

Many Burr Oak retain their leaves into the winter which makes them more vulnerable to ice damage.  This trait is most pronounced when the tree is juvenile and abates, somewhat, as the tree matures.

Of the oaks in the White Oak group, Burr Oak is reputed to be the most susceptible to Oak Wilt.  It is still more resistant than any of the species in the Red Oak group.

Many Burr Oak produce exceptional amounts of acorns.  This is a great thing if you are interested in oak as a food crop or if you want to feed wildlife.  The downside is that the energy that could be going into growing height and girth...outcompeting its neighbors is diverted into producing acorns.  How many of you have stumbled across an ancient apple orchard overgrown by elm, ash, wild cherry and Red Oak?  The weight of the fruit bends the branches down.  The apples simply cannot grow tall enough to fend for themselves if the orchard is left unattended for even thirty years.  Some Burr Oak selections also seem to be vulnerable to having their branches dragged down by bumper crops.

Bottom Line

If you are planting trees to establish a legacy you could do far worse than to plant a grove of Burr Oak (Q. macrocarpa) or its hybrids.  You can improve the odds of the trees you planted surviving many hundreds of years if you select seeds from waterlogged sites, grow them for three years and select for corky bark (a desirable trait).  Selecting for earliest winter leaf fall within the seedling population will decrease vulnerability to ice damage and is somewhat correlated with greater winter hardiness.  Rogue out all seedlings that show leaf diseases like powdery mildew.

Choose a site with suitable fertility.  Inter-plant the Burr Oak with other, non-oak species to achieve a total plant density of about 400-to-500 trees per acre (about 10'-by-10' or 3m-by-3m spacing).  The goal is to reduce the density of trees that are vulnerable to Oak Wilt.  A reasonable starting point is to start with 25% of the trees (100 per Burr Oak per acre) as Burr Oak with a mind to ending up with a final population of 20-to-30 Burr Oak per acre.

One source proposes some of the following species as "pioneer" species.  These might be good plants to use as some of the "non-oak species" because they will self extinguish and some of them will add nitrogen, attract wildlife or have other advantages.
  • Alder (fixes nitrogen)
  • Birch (live hard, die young, leave a good looking corpse)
  • Apples and Crabapples
  • Pears
  • Mulberry
  • Chokecherry
  • Sumac
  • Locust (Black Locust fixes nitrogen and is good firewood and fence posts)
  • Willow. Many species available.  S. alba, S. fragilis and S. matsudana become large trees.  S. nigra and S. amygdaloides tend to be somewhat smaller. Most other willow species are 20 feet tall.
  • Box Elder (or the more ornamental Asian equivalents A. ginnala, A. buergerianum)
  • Tree dogwoods (C. florida, C. mas, C. kousa)
  • Aspen
  • Yaupon

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