All forests change over time, but the changes can be slow and not immediately apparent, unless there is a major disturbance like a windstorm or a very hot bushfire.
These photos of Ponderosa Pine (Pinus ponderosa) are from the Bitterroot Mountains in Western Montana. They were taken by the US Forest Service and follow the subtle changes to the structure of the forests from 1897 through to 2015. (AKA – Forest Succession).
Before European settlement of the Pacific Northwest, the forests had repeatedly experienced low-intensity fires which burned, on average, every seven years.
The fires may have started by lightning or were deliberately lit by native Americans who burned a lot.
The frequent burning created a complex mosaic of open forest with a grassy understory that carried low-intensity ground fires which cleaned out the dead wood on the forest floor as well as the undergrowth of shrubs and saplings, removing the elevated ladder fuels and leaving only the big, thick-barked trees standing.
Because of the historic fire regime, these ponderosa pine forests were once dominated by scattered old trees with about 50 large stems per acre, but with patches of younger trees in the bigger gaps.
This sequence of amazing photos was taken from a fixed vantage point and clearly illustrates the loss of openness and thickening of the understory.
Also, in the absence of fire to clear out the undergrowth, Douglas Fir (Pseudotsuga menziesii) which is an evergreen conifer, has begun moving onto these sites and threatening the once pure stands of Ponderosa Pine. In wildfire conditions the thick regrowth of Douglas Fir forms very flammable ladder fuels.
US Forest Service ecologist, Professor Paul Hessburg convincingly argues that because of the absence of frequent low-intensity fires, together with the removal of larger trees through selective logging over the last century, the forests have now become overgrown, uniform aged and dominated by younger trees and understory shrubs.
They also now have tonnes of woody debris on the forest floor which presents an increased risk of dangerous mega-fires.
And furthermore, Hessburg says it was the previous openness and patchiness that broke up the continuous fuels and provided some inherent protection from the large fires that the US now commonly experiences.
The debates about the future of native eucalypt forests, fire management and low intensity burning, and how to protect communities, are strongly echoed in Australia.
I’m not aware of any long-term photo study plots in Victorian native forests but I sense the changes would be similar.
https://research.fs.usda.gov/treesearch/56719
Victoria’s Forests & Bushfire Heritage 