When Trees Become Bare In Summer

WHEN TREES BECOME BARE IN SUMMER  

July/August, 2010 issue
  
 Moneymore  Road resident Lawrence Boyle had been looking forward to this day  for  some months. After a scheduled meeting of the Quinte Chapter of the  Eastern  Ontario Woodlot Association at a hall in Marlbank, he was  planning to showcase  his forest through a guided hike as the grand  finale of the day. He wasn’t  anticipating that unexpected guests would  arrive a few weeks earlier to his  beloved forest.
  
 He  was certain that we could cope with the bear he had encountered earlier   along the trail through his hardwood bush, if we remained vigilant  during our  walk. What he didn’t expect was the hungry invasion of  forest tent caterpillars  that descended on his forest, stripping it  clean. The once darkened forest was  now a stand of naked trees,  sunlight pouring onto the forest floor and  re-awakening plants that had  gone dormant in anticipation of a summer of subdued  light and humid  temperatures. 
  
 When  I was an interpretive naturalist at Sandbanks Provincial Park, we were   taught to seize the moment and use the unexpected appearance of nature  as an  interpretive tool. And that is exactly what we did, as we poked  our way along,  with lively discussions taking place about forest  inhabitants and how this  abrupt change in canopy might affect their  lives.  
  
 The  forest tent caterpillar invasion and the damage they had done was   actually the first I had ever witnessed. It is not a common occurrence,  and  cycles about every 15 years. Fortunately, there is only one  generation a year,  with the young caterpillars appearing just as the  leaves are beginning to  unfold. Their name is a misnomer for they  really don’t build a tent, like our  better known eastern tent  caterpillar, although we did come across one cluster  of leaves  containing a silken mat. It is here where some of the caterpillars may   rest or moult. This little congregation was on one of the lower  branches,  typical of the latter stages of the infestation. 
  
 In  some areas we walked, the defoliation was close to 90 per cent while   other trees fared better. There was concern from the woodlot owner about  the  future of his bush; however, Nature is very resilient, and it was  early enough  in the season for the trees to generate new leaves. This  type of defoliation can  probably take place for two years, even three,  without weakening the tree  substantially. Liken the forest tent  caterpillar moth – a rather nondescript  buffy moth – to flocks of   crossbills that follow pine cone harvests around in  the boreal forests,  here one year and gone the next. Forest tent caterpillar  moths are  also very nomadic, and the adults may not even be present next year in   any numbers to attack again.  On our first walk through the woodlot in  early  June, there was evidence that the outbreak was weakening, and we  came across one  caterpillar displaying classic signs of a virus that  usually breaks out in  massive invasions like this. In fact, when I  walked through the same bush two  weeks later with another group, much  of the foliage had already regenerated and  there was little to suggest  what had taken place earlier. We seldom have to  worry about predation  as Nature always has back up controls. 
  
 There  are several species of birds that feed upon these caterpillars, but  do  so cautiously, as the build-up of indigestible spines can puncture  their  stomachs, so they poke holes in the side of the caterpillar,  consume the  contents and leave the dangerous exoskeleton behind. Only  the black-billed and  yellow-billed cuckoos have evolved a way of  consuming the caterpillars in their  entirety. When their stomach  becomes congested with indigestible spines, the  bird simply throws up,  bringing up the spiny contents, stomach lining and all.  Think of it as a  miniature Glad Kitchen Catcher, but one that it must re-grow,  quickly,  before it can feast again. 
  
 Would  this caterpillar invasion affect the forest critters, many of which I   touched on in an earlier presentation for this same group in a small  hall in  Marlbank? Certainly there was no shortage of toads, wood frogs,  or millipedes as  we shuffled along, looking for stuff. Moonseed,  miterwort, wild lettuce, even  ostrich ferns, rattlesnake fern and  maidenhair fern were also found and  discussed. They may have even  benefitted from the open canopy and became more  luxuriant as a result.  Woodland birds would have established their nests well  before the  outbreak began a month ago, and simply adapted to the situation. We   heard several species including wood thrush, redstarts and red-eyed  vireos, and  several scarlet tanagers. Whether or not these nests were  all successful is  difficult to determine as the deeper forest interior  is what keeps aggressive  blue jays, grackles and cowbirds at bay. Would  they have taken advantage of the  now open forest and penetrated the  interior to invade a population of nesters  deeper in the forest? 
  
 Birds  and mammals and other forest inhabitants deal with these ups and  downs  in life every year. Their numbers rise and fall in direct response to  the  situation. The reason we see fewer evening grosbeaks now, for  example, is  because we haven’t had a major eruption of spruce budworm  since the 1970s.  Numbers of evening grosbeaks rise and fall as their  favourite food becomes  available; likewise with the Tennessee warbler,  also a spruce budworm  specialist. Bird banding records over the course  of many decades show a rhythmic  rise and fall on the graph in numbers  banded, probably reflecting a similar  interpretation of the overall  population from year to year.
  
 And  the cuckoos? Well, they likely fared well this year with all this manna   from the trees and stuffed themselves during the infestation. And  black bears,  we were told on one of our hikes, gorge on forest tent  caterpillars from any  lower branches that they can reach. Interesting  stuff, how a species will  exploit another when times are good.  It is  only during our bumbling efforts to  maintain control, that we actually  lose control. 
  
 For more information on birding and nature and guided hikes, check out the  NatureStuff website at www.naturestuff.net       
  
 Terry Sprague lives in Prince Edward County and is self-employed as a  professional interpretive naturalist.