Many forests surveyed in the Midwest and northeast United States, since the 1950s, have reported shifts tree species composition. Data from these studies have consistently documented decreases in representation of white oak, Quercus alba. Sugar maple, Acer saccharum, has shown the largest increase in number in stands where the presence of Q. alba has decreased.
Oak regeneration and recruitment
Regeneration is the process that begins after a disturbance that allows for seedlings and saplings to compete for resources available and ends when growth of the saplings prevent any new sources of reproduction to enter the ground layer.
The saplings then compete with one another resulting in loss of saplings and stem-thinning, in a process known as stem exclusion.
As the process of stem exclusion ends and mature trees establish an overstory canopy, the recruitment stage begins. In this stage, the oak saplings in the understory respond to disturbances by competing for light gaps and working their way through the stem exclusion process to become the new canopy layer in a process known as understory reinitiation.
The problem
Recently, the successful establishment of Q. alba into the canopy layer has been at risk because of a decrease in disturbances (which allows regeneration and recruitment of new oaks to replenish the canopy layer). This decrease has caused the understory to become increasingly dominated by shade-tolerant species like A. saccharum at the expense of light-deprived oak saplings and young trees, thus preventing the understory reniniation process for white oak.
Causes
Traditional plant ecology places primary emphasis on the topography of the terrain in setting the distribution patterns of individual tree species on a site. The topography of the terrain generates variability in site quality through impacts on soil abiotic factors such as pH, moisture, temperature, and light intensity. And so, some people might argue that the presence or absence of mature and young oaks are simply due to topographic and soil variability. Although these factors might play might have a considerable impact on the performance of shade-tolerant species and hence the decrease in oak regeneration, studies have suggested that other influences such as fire suppression, influence of deer, and climate change may be at work in tandem against oak regeneration and recruitment.
Fire suppression
As oaks are fire tolerant species, fires would help clear out an understory to allow for new stand initiation through regeneration. Fire suppression decreases one source of disturbance that would otherwise confer an advantage to oak seedlings.
Influence of deer
The removal of keystone species due to urbanization and limited hunting due to commercialization of the food industry have led to high populations of deer. This has had negative impacts on oak regenerations as dense deer populations remove acorns from viable sites for consumption.
Climate Change
This global phenomenon has caused changes in weather patterns, including the increase in precipitation. The increase in precipitation might cause increase in soil moisture. Although the ridge tops will still be drier than the ravines at the bottom due to topographic variation, if the overall moisture itself is higher, then it would not necessarily mean that the ridge tops and southwest facing slopes are dry enough to provide a competitive edge for Q. alba over A. saccharum.
Therefore, this research was conducted to question whether the same phenomenon might be happening at Palisades-Kepler State Park, IA. And if so, what might be the potential causes.
Here are some research papers for more information and reference (please click on the image to follow the link to the article):
Oaks at Palisades 