Friday, November 18, 2011

Life of an Oak Part 2: from acorn to sprout

These charred branches are evidence of ground fires in this Oak Savanna.
Fire and the Young Oak
Just prior to and after winter, oak woodlands historically experienced another form of harsh natural selection.  Just when life appears to be as hard as it gets, fire enters the picture.  In the fall before snow fall and early spring after snow melt, the ground is covered with a large supply of flammable dead vegetation.  Oak trees themselves contribute significantly to this fuel with their crunchy leaves covering the ground.  This dead vegetation historically has supported low intensity ground fires across the prairies and bordering oak savannas and woodlands.  Amazingly, most oaks are highly adapted to this type of fire, and anywhere you find fire disturbance you will typically find oaks.  Native Americans and many early European settlers throughout the eastern United States liberally utilized fire for a number of purposes.  In-fact, prior to any European impact Native Americans utilized fire so frequently and on such a large scale that essentially none of the Eastern or Midwestern landscape can be considered without human disturbance.  The general attitude seems to have been when in doubt, burn it.  The thinking went something like this: I want an open forest, lets burn this area.  I want more berry and fruit production here, lets burn it.  I want to create a fire break for the village, lets burn the area surrounding the village.  It would be easier to gather nuts or wild ground bird eggs if there wasn't so much vegetation, lets burn it so we can find these easier.  Prairie and forest floor fires had the added benefit of cooking the eggs also, making them ready to eat.  I want to create better habitat for game animals, burn it.  We need to herd some animals to make the hunting easier, lets round them up by burning them out.  Lets clear the land so we can plant a garden, better burn it.  There are too many snakes and ticks in this area, lets burn it.  And the list goes on.  Scientists have found that fire returned to areas anywhere from every two to 11 years.  Most of this burning was human started for the purpose of benefiting the landscape.  Very few of these fires were "natural" lightning started fires.
A recently burned oak savanna in Eastern Iowa.
Fire was amazingly effective in accomplishing all kinds of positive effects on the prairies, woodlands,and savannas; and oaks always followed right behind these fires.  As fire goes, so goes the oak, or any other tree for that mater.  While most trees are killed by fire, many upland oaks greatly benefit from ground fires.  Shortly after fall fires, Bluejays will begin caching acorns from nearby forests in newly burned areas.  Even if caches are made just before an area burns, low intensity ground fires cause very little harm to these acorns being they are protected underground.  Insect predators, such as Acorn Moths and weevils, are nearly eliminated in burn areas, greatly increasing acorn viability.  Fires also clear thick overhead vegetation, decrease surface soil moisture, and kill seeds deposited on the surface.  This allows for the full sun oaks require for establishment as well as lack of competition they thrive on.  Other tree seedlings typically require higher soil surface moisture for establishment or are killed by fire while oak acorns are safely buried just under the surface, and have access to soil moisture.  Even with very little soil moisture, the large acorn seed is capable of sprouting and sending out a thick strong taproot to search for moisture.  Very few plant seeds have this ability to germinate and survive such low moisture conditions.
This young oak tree has four stems coming from one original stem that was burned in a fire during a previous year.  The four stems showing in this picture were killed by a surface fire this fall but the root remains alive and will sprout next spring.
The cache is an ideal acorn hiding spot where they can snugly wait the winter out.  A few acorns, such as those from White Oaks, will sprout within weeks of falling of the tree in the fall and occasionally even sprouting on the tree.  This gives them a head start come spring and helps them avoid seed predators.  Bluejays or squirrels returning to a cache for a dinner of White Oak acorns, will likely be very disappointed when they find unappetizing sprouting acorns.  Most acorns however wait for snow melt and spring temperatures before germination, requiring exposure to cold before they are even able to germinate.  Once germination starts the seedling immediately puts all its energy into producing a thick taproot that can grow to six inches or more before any greenery sprouts above ground.  Establishing a healthy root system helps ensure the longevity of the plant as well as prepares it for future fires.  When fire does burn through an area, established seedlings may be top killed but thick healthy roots remain safe underground and can quickly sprout new stems.  In general, the thicker and larger a root system the more likely it will survive a top killing fire.  Quick recovery and growth after fire allows oak seedlings to out compete other plants, ensuring survival.
Fall in an eastern oak woodland.

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