These are some really good questions, ones that really drive at the core fundamentals of forest ecology in the northern Midwest. Hopefully I can at least begin to answer them succinctly in a manner that is understandable.
My understanding regarding Bronze birch borer is that it is a native pest, and really is only able to damage native birches in the forest when they are stressed / damaged or on the decline due to old age. As you bring up, there was pretty high mortality of birch back in the 80s, in which the borer played a big role. However, it was more the secondary result of so many of the area’s birches being on the declining end of their lifespan and therefore more susceptible to infestation, than it was the borer suddenly “taking over.”
A key concept in forest ecology is that different tree species have different “shade tolerances.” “Pioneer species” such as paper birch and aspen are the two most common “shade intolerant” deciduous species in the northern Midwest. These shade intolerant species basically need full sunlight (no forest canopy cover) in order to thrive once established. Over the course of the 1800s / early 1900s, nearly this entire region was completely cut over, in which these faster growing, shade intolerant species could establish and thrive in the wake.
As shade intolerant species grow to maturity, more shade tolerant species (yellow birch, maples, etc) begin to establish as seedlings and grow up beneath the mature birch/aspen – a concept known as “forest succession.” Even though paper birch were the tree species to first establish after an area was cut over (heavily disturbed), they are not able to regenerate without another disturbance causing large openings in the forest canopy. Pioneer species aspen and paper birch are short lived species (70-100 years), which fits the timeline and narrative of what has occurred the last 100+ years. As these over-mature paper birch die, the shade tolerant species which have established below them grow into the canopy to take their place.
At this point, much of the U.P. / northern WI is in the stage of “secondary succession.” The pioneer species have or are dying out, with more shade tolerant species such as sugar maple, red maple, yellow birch, basswood, and northern red oak taking their place as being most commonly seen in Northern hardwood forests.
In today’s forest management, the most common active management technique in Northern hardwood forests is known as “single tree selection.” This management technique is foresters going into a stand (area) and mimicking the most common natural disturbance which occurs in these forests – single tree mortality due to factors such as wind, disease, old age, etc. A (good) forester will go and mark out the trees of lesser quality and vigor (the ones you would expect to die first naturally), leaving the higher quality trees to grow and create room for continued growth from below. However, in this management, creating a gap by removing a single tree does not create enough resources (in the terms of this discussion, sunlight) for shade intolerant species to establish and thrive. Instead, more shade tolerant species, which are probably already established as seedlings/saplings, will grow into that space. In order to have shade intolerant species grow, much larger gaps are needed, along with an already present seed source.
In general, today’s public perception of “clear cuts” is that they are all bad and damaging to the environment. However, it is proven that shade intolerant species need much larger areas of disturbance than just a single tree being removed, which is what clear cuts and other similar harvest types provide. Due to this perception, “single tree selection” is by far the more common and accepted management technique in northern hardwoods. We have reached a point in which there are actually less areas of “early successional” forests(aspen / P. birch dominated) on today’s landscape than there were pre-European arrival in the 1800s.
The best way to promote or encourage the persistence of paper birch on a property which has it is to implement a harvest which provides suitable growing conditions. First, there needs to be paper birch already in the area as a source of regeneration, either to provide seed or to allow for stump sprouting once cut. Clearcutting is the most common management for promoting paper birch regeneration, but smaller types of clearcuts are considered in areas where moisture is limiting.
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