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All Things Warm and Small
Article #344 March 2023
By Bill Cook

A casual walk in the woods might reward one with forensic clues about the previous night’s activity or, perhaps, a sighting of something not yet seen or noticed.  Given the structure and complexity of a forest, the degree of collective bounty of little things can knock your socks off.   

    Microclimates can be very micro.  I mean, really, really small.  Take, for instance, the “climate” around a fallen alder conelet on the snow, or that of an oak leaf.  On a sunny day, even during bitter cold, the dark surface absorbs heat and gradually sinks into the snow.  On warmer days, maybe when the temperatures are around the mid-thirties, these dark heat-absorbers almost plummet through the snowpack. 
    These micro/mini warm spaces also occur on the bark of trees.  Tiny heat sensors emplaced by researchers record temperatures as high 75 degrees on days where the ambient temperatures are below the freezing mark.  How can this be, you might think?  Well, the answer lies in the difference between heat and temperature.  Without going into a lot of hard science detail, the dark surfaces absorb more energy from the sun than does air.  With more heat packed into less space, bark surface temperatures can rise surprisingly high. 
    What does this matter?  Hmmm.  I can think of at least several ways, off the top of my head. 
    First, if you’re lichen or moss growing on a tree trunk, the warmth will initialize metabolic activity that leads to photosynthesis.  That process is, of course, rather important.  More interestingly, to me at least, is that microscopic water in the cambium of a tree (the living tissue under the bark) might begin to transmit liquid water.  But, trees don’t have leaves during the winter?  Oops, not so.  Think of all the conifers!  With water able to move to the needles, these trees can also photosynthesize.  This gives conifers a marked competitive advantage over deciduous trees.  But wait again!
    Some deciduous trees have thin bark capable of photosynthesis.  Leaves are not the only place that this magic chemistry happens.  Aspens, especially bigtooth aspen, can utilize those warmer micro-temperatures to make sugar.  But, there’s risk in this!
    Some thin-barked trees, often reported with much chagrin on younger apple trees, begin to move water up from the roots, to those toasty cambium layers, only to have the sun migrate behind a building or other barrier.  With this event, bark temperatures dramatically drop to ambient temperatures, the liquid water under the bark freezes, as the tree doesn’t have enough time to re-acclimate, and the expanding ice cracks the bark.  Should this happen regularly enough during the winter, especially late winter, the trunk will eventually display mystifying cracks and sloughing bark over the dead cambium underneath. 
    For those that have been around the tree-growing block for a while know that by simply wrapping the lower four feet, or so, of the apple tree trunk in burlap prevents frost cracking.  Easy peasy. 
    Now, the tubular micro-structure of conifers differs from that of deciduous trees.  Most conifers have adapted their plumbing to tolerate a daily or regular freeze-thaw cycle.  This ability digs deep into plant anatomy, physiology, and the bizarre physical properties of water.  Armed with this amazing accoutrement of characteristics, conifers can frequently photosynthesize during winter, albeit at lower rates.  But still, it’s a cool ability to possess! 
    A footnote to this coniferous charisma is the requirement that the roots not be entirely frozen.  Deep snow goes a long way to insulate the ground and some of the root systems to remain unfrozen and actively able to move water.  Without enough snow, or no snow, cold northern winters push the frost frontier deep into the soil.  Under these conditions, water cannot move.  Unfortunately, the needles don’t know this and slowly photosynthesize under the conditions just described.  Photosynthesis requires water.  If water cannot travel up the plumbing system, due to frozen roots, the nearby molecular water is soon exhausted and tissues can die.  When this happens, we call this “winter burn”, which is actually a weird term.  Burning implies heat, but “winter” suggests cold.  And really, burning has nothing to do with it.  Rather, it’s all about the needles drying-out to the point of death. 
    Normally, this desiccation cannot be observed at the time when it happens, during the winter.  The needles appear happily green, although they’re already dead.  Maybe they just don’t know it, yet.  However, come spring, sometimes months later, the needles turn brown.  If you own a Christmas tree farm this can be quite disappointing.  The ruined trees might be blamed on a mysterious disease or an intractable soil deficiency.  And while such things do, indeed, happen, it may simply be winter burn that occurred back in February. 
    All this said, the forest is chock-full of a nearly endless number of microclimates that matter to critters from big black bears to gazillions of living things that only a microscope can see.  Small holes.  Large logs.  Spaces behind flaky bark.  The fuzz on a leaf bud.  There are millions of miniscule dramatic scenes occurring all around us, in all seasons, and most of us remain happily unaware of them.  Biochemistry.  It can blow your mind!  We are large creatures that lumber around usually just minding our own business.  However, in reality, most of the planet’s life is tiny, most far tinier than anything we can easily see, for those of us that try to.  Life is mostly a teeming mass of squirrely little creepy-crawlies. 
    I suppose, somewhere in there, exists a life lesson? 

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With tiny warm microclimates,conifers can photosynthesize during the winter.

TRAILER- This website was created by a consortium of forestry groups to help streamline information about forestry and coordinate forestry activities designed to benefit the family forest owner and various publics that make up our Michigan citizenry.  This website is maintained by Bill Cook, Retired Michigan State Extension Forester/Biologist.  Direct comments to cookwi@msu.edu.