Emily Silver is a recent Masters graduate from the Natural Resources Science and Management program on the University of Minnesota’s Saint Paul campus. This month we feature an interview with Emily describing her research. More on Minnesota’s red pine forest type is here.
Your research investigated differences in stand structure between managed and natural old growth red pine stands in Minnesota. What were your research questions and how did you go about answering them?
I had three main research objectives. The first was to compare the variation in current structure between old growth, unmanaged second growth, and extended rotation red pine stands. The second was to contrast structural development through time for the old-growth and extended rotation stands, for which we had long-term data. Finally, I investigated the spatial pattern of tree mortality within the old growth stand.
My study sites were an 87-year historic dataset from Itasca State Park (the Allison plot dataset), a 61-year dataset from the Chippewa National Forest, and a red pine chronosequence dataset also from the Chippewa National Forest. These datasets contained long-term records of tree diameter, mortality, and ingrowth of new seedlings. I resampled the Itasca stand in 2010 to answer my question about the current structural variation and mapped the stems in the Itasca stand to investigate the spatial pattern of mortality.
So you mapped out every tree in these stands and tracked each tree’s development over almost a century?
Yes, it turned into a treasure hunt! For the Allison plot, we mapped all the living trees and did our best to recreate the standing location of all dead trees using a hand drawn map from 1952, a metal detector (to find the tree tags from 1923), and by looking for stumps or downed stems. The 87 years of data gave us a picture of when these trees fell. Attaching a current stem map to this dataset gave us a picture of the spatial pattern of these mortality events. It was painstaking, but there were moments of elation when we unearthed an old tree tag from 1923. We reached 90 years in the past and connected with John Allison’s research, further validating the work he did setting up this 5-acre plot.
In a nutshell, what did you find? How do managed red pine stands differ from natural stands?
We found that old-growth red pine stands were more structurally complex than unmanaged second-growth and extended rotation stands. This was mostly due to higher volumes of deadwood in the old growth stands and the random spatial patterns of tree mortality. However, the extended rotation managed stands accelerated the development of large diameter trees, indicating that extended rotation management could be used to incorporate more structural complexity and ecosystem diversity into these systems while still capturing economic value, if provisions are made for increased abundance of some coarse wood material.
Managed pine stands typically lack the coarse dead wood (often referred to as coarse woody debris) and large diameter trees found in natural stands, but this particular type of management has the potential to strike a balance between intensively managed and completely natural red pine systems.
The term “old growth” seems to mean different things to different people. As a silviculturist, how would you define it? What would a natural old growth Minnesota red pine stand look like?
Old growth looks different everywhere I study, travel, and live. As a silviculturist, my definition of “old growth” depends on the ecosystem or forest stand in question. Most broadly, I define old growth as forests of natural-origin with minimal human disturbance and canopy trees that are at least half of the maximum age for the dominant species. This definition allows for historic recreational use of forests, but does not include forests logged or otherwise disturbed on a stand or regional scale.
Today, a natural old growth Minnesota red pine stand will contain many large diameter trees ranging in age from 150-300 years, will likely have visible large diameter coarse woody debris on the forest floor, and will contain a variety of woody understory shrubs and/or advance regeneration of other tree species including birch, maple, spruce, and eastern hophornbeam. Many old growth red pine stands share canopy dominance with white pine and in rare cases, jack pine. These characteristics are unique to red pine forests; an old-growth maple-beech hardwood stand will look very different. The best known old growth red pine stands in Minnesota are in Itasca State Park, where well-used trails provide easy access.
Not much old growth remains in Minnesota. It takes a long time for natural old growth structure to develop. To what degree can management accelerate development of old growth conditions?
Certain management techniques can accelerate the growth of dominant trees to sizes found in old growth forests. For instance, thinnings (removing some trees, leaving more room for others to grow) can increase growth rates, producing larger trees faster. Variable density thinnings can produce a more random arrangement of remaining trees, with some clusters and other more open areas. This approach can promote the development of a wide range of tree diameters across forest stands. However, management is often designed to capture natural mortality and put that wood to use rather than letting it rot in the woods. For this reason, coarse woody debris is generally lower in a managed stand. If the management goal is to create old growth conditions, retaining more standing dead and fallen coarse wood should be part of the plan.
What barriers or downsides are there to this approach to red pine management?
As with any management approach, extended rotation forestry means waiting longer to capture economic value in a stand. In addition, although research shows that red pine stands do not fall apart before the extended rotation age, there is always a chance that valuable trees will be lost to mortality events prior to removal. Finally, this approach to red pine management still involves multiple stand entries, which might alter the stand environment more than is acceptable for the landowner’s ecological goals.
What role has wildfire played in the development of old growth red pine in Minnesota? In the era of fire suppression, does this create additional challenges in the restoration of red pine?
Fire has played a very important role in red pine development in Minnesota. Red pine seeds require exposed mineral soil for regeneration and are shade intolerant, implying that competition from understory vegetation will inhibit successful seedling growth. Fire is a natural way to expose mineral soil and reduce competition from understory vegetation. In addition, red pine has infrequent seed production (every 7-10 years), so surface fires need to occur fairly frequently to have a lasting impact on regeneration. Fire suppression has resulted in a lack of red pine regeneration in both second and old growth.
However, attempts to recreate historic fire regimes favorable to red pine have been met with only moderate success. In Itasca State Park (where our main study site is located), recent burns did little to control the understory competitors (mainly American and beaked hazelnut) and did not coincide with good seed years. In fact, the hazelnut was above my head in 2010 and kept our field crew awake with occasional slaps in the face as we worked. Given the early successional fire-dependent nature of red pine, strategies such as planting and mechanical removal of competing vegetation may be a more cost effective way to maintain a red pine component in sensitive recreational areas. However, as we saw with the recent Pagami creek fire, there are still areas where fire remains the dominant process for red pine regeneration.
For more on this topic, see Restoring Old Growth Characteristics (PDF) from the University of Massachusetts Extension.