Porcupine at Eye Level
Photo by Sylvia Kniss
Forest Ecology, Diversity, and Stability in Old Growth, Secondary Growth, and Recently Logged Forests
by: Sylvia Kniss
INTRODUCTION
Biodiversity is defined as the variation of life forms on Earth. Forest diversity is the variation of tree species in a particular area. Forest diversity is important because it stabilizes the environment where other organisms live and provide a larger variety of resources for animals and humans. Different types of trees can provide habitat or food sources for various species. The more diverse a forest is, the more stable it will be, making it more reliable for the organisms that depend on it. Diversity promotes growth for both plants and animals because if there is a greater variety of tree species, there will be a greater variety of resources for animals and plants to use to grow. This leads to more productive ecosystems in the sense that they generate more resources and grow faster. When an ecosystem is diverse, it is also more efficient in recovering from disturbances while still retaining the original species. If only one or two different species of trees encounter a change, they have a lower chance of being able to deal with it. However, if there are more species in that forest, the likelihood that the forest will be able to adapt increases. This also quickens the ability for forests and ecosystems to recover from disturbances because there are more than just a few species working to correct the problem.
Despite this potential for adaptability, there are many other threats to forest health and diversity from factors such as invasive species. The woolly adelgid, emerald ash borer beetle, and the spongy moth are just a few examples of species that are detrimental to forest health. The woolly adelgid is known for being harmful to hemlock trees in particular. These insects suck the sap out of hemlock needles, which kills the needles, shoots, and branches of the tree. The trees then turn a gray-green color instead of their normal yellow and dark green coloring. The woolly adelgid can cause the tree to die within 4-10 years (Michigan Invasive Species Program, 2022). The emerald ash borer beetle is dangerous for a variety of different species of ash trees. They have destroyed millions of trees in more than half the United States. The beetles cause the foliage of the tree to turn yellow and wilt, and leave behind D-shaped holes where they left the tree. There is currently no cure for the trees after the beetles have destroyed them, which makes them particularly dangerous (Animal and Plant Health Inspection Service, n.d.). The spongy moth is responsible for the destruction of millions of acres of forest since 1869 when they were first introduced. Their caterpillars feed on over 300 species of trees. The trees can generally survive this defoliation, but the caterpillars can cause damage to a large variety of species if there is an outbreak of the insect (Wisconsin Department of Natural Resources, n.d.). These three insects can affect forest health because they affect such a wide variety of species. They can be responsible for the death of trees and the reduction of diversity in a forest. These three examples are only a few invasive species that affect forests, so while these species have a substantial impact on their own, when combined with other species, that impact only increases. Invasive species and insects such as these can decrease forest health by causing trees to die or get sick. They can also affect diversity negatively because they can kill off certain species and decrease diversity. This could cause more species to die out as well if they depended on that original tree species and the cycle would just continue.
METHODS
To count the trees, a 256 m2 area was measured out with 16 by 16 m sides to create a square. Each tree that was larger than a wrist was identified and counted, while trees split above breast height were counted as two trees. The circumference of each tree at the same height was then recorded along with the name of the species in centimeters. To calculate the species richness for each forest, the number of different types of species in each plot was counted. The numbers for each plot were then averaged to find the species richness for the whole forest. The Shannon Weiner Diversity Index (SWDI) was calculated using the equation H’ = -Σ (pi ln pi). “i” is the species and “p” is the proportion of the population that that species represents. The SWDI values for each plot were then averaged to find the SDWI for the whole forest. The population density was calculated by dividing the number of trees in each plot by 256 m2 and then multiplying by 10,000. The population densities for each plot were then averaged to find the population density for the whole forest. A pivot table showing the count of each type of tree in the forest was used to determine how many different types of trees were in each plot to determine species richness. This table was also used to add up the number of trees in each plot to be used in the population density equation. A pivot table containing the fraction of each species out of the total number of trees in each plot was used to calculate the SWDI. The values from the table were used as pi in the equation.
RESULTS
Table 1: The average species richness, Shannon Weiner Diversity Index, and population density for each forest. Allows for comparison between the types of forest.
| Forest | Average Species Richness (Number of Species per Plot) | Average Shannon Weiner Diversity Index (H’) | Average Population Density (Number of Trees per Hectare) |
| Old Growth | 3 | 0.846022 | 1016 |
| Recent Logging | 2.5 | 0.785671 | 254 |
| Second Growth | 5.75 | 1.352883 | 1016 |
Figure 1: The average species richness for each type of forest. Allows for comparison between the different types of forest.
Figure 2: The average population density for each type of forest. Allows for comparison between the different types of forest.
Figure 3: The average Shannon Weiner Diversity Index for each type of forest. Allows for comparison between the different types of forest.
DISCUSSION
The old growth forest was characterized by a species richness that was fairly low. This means that there is little diversity in the old growth forest. The lack of diversity could become problematic, as ecosystems with low diversity are less resistant to change and less stable over all. One of the main species in the old growth forest was the hemlock tree. This could become dangerous if an outbreak of woolly adelgid insects were to infest this forest because those insects would kill most of the trees. Forest diversity would then drop even lower, and the ecosystem would be even more vulnerable to disturbance. The old growth forest is dense, however, which means that it is more resilient to disturbances such as erosion or flooding. The trees do have more competition between one another for resources because they are closer together. The SWDI for the old growth forest is also relatively low due to there only being about three species of trees in the forest. This means that the forest has low diversity and is less resilient and stable. The low level and possible loss of producers like the trees could cause all the other levels of the food chain to fall. All organisms depend on the producers to survive and build up off of them. If the trees fall, the rest of the ecosystem will as well.
The secondary growth forest had the highest species richness out of all the different forests. This means that this type of forest has a very high forest diversity and is the most resistant to changes in the ecosystem. The forest had no hemlock trees or ash trees, so the woolly adelgid and emerald ash borer beetles would not be an issue for this forest, but the spongy moth would have an effect. While the trees were being measured, a spongy moth was even observed on one of the trees. The presence of spongy moths in the second growth forest could be an indication that the forest could defoliate and become warmer as more light is let through the trees. The higher temperature could make it more difficult for species to survive, decreasing the diversity of the forest. The secondary growth forest has the same average population density as the old growth forest, so this forest is resilient to disturbances like erosion as well and has high competition. The SWDI for this forest is high, which means the forest is diverse, as shown through the large number of species recorded in the area. A diverse forest is resilient and stable, so it is more likely to be able to recover from disturbances. The likelihood that the trophic levels will be lost in this forest is much lower because there are both a lot of trees and a wide variety of species. This means that the other, higher trophic levels will be able to thrive and grow in this forest system.
The recently logged forest has the lowest species richness out of all the forests which means that it is most vulnerable to change and the least likely to be able to recover from a disturbance. The low species richness and diversity in this ecosystem means it is unstable and not resilient to catastrophe. This forest did not have any hemlock trees or ash trees, meaning that the woolly adelgid and emerald ash borer would not be a threat to it. However, the spongy moth could still have a negative effect on the area, especially since it was very close to the second growth forest that is already infected with spongy moths. The spread of these insects would cause the forest to become even less diverse and less stable, meaning it would be even more vulnerable to change. The recently logged forest also had the lowest population density. This was easily observed when looking at the forest and was also shown in how tall the trees were due to them having little competition from each other. Nevertheless, the trees would be more vulnerable than the other forest types to disturbances like high winds, erosion, and flooding, which would decrease diversity to even more dangerous levels. The SWDI for this forest was about the same as the old growth forest, meaning that it has a lower diversity and is therefore less stable than forests such as the secondary growth forest. This forest is at risk of losing the producer trophic level because of the little stability and resilience it has. If the trees were to die out, the rest of the ecosystem in that area would fall because all higher trophic levels depend on producers.
In conclusion, the secondary growth forest is the most stable and resilient forest compared to the old growth and recently logged forests. It has the highest species richness and Shannon Weiner Diversity Index out of the three, which means it is the most diverse. It is still at risk from factors such as invasive species, but it is the most equipped to handle those disturbances. The recently logged forest is the least diverse and least stable forest ecosystem, meaning it is at the highest risk of not being able to adapt to disturbances. The old growth forest is only slightly more resilient than the recently logged forest, but it also has a very low diversity and is therefore not as stable. The stability and resilience of forest ecosystems depend on their diversity and in turn, organisms, including humans, depend on forest diversity for the necessary resources to survive.
Works Cited
Animal and Plant Health Inspection Service. (n.d.). Emerald ash borer beetle. Retrieved from https://www.aphis.usda.gov/aphis/resources/pests-diseases/hungry-pests/the-threat/emerald-ash-borer/emerald-ash-borer-beetle.
Michigan Invasive Species Program. (2022). Invasive species: Hemlock woolly adelgid. Retrieved from https://www.michigan.gov/invasives/id-report/insects/hemlock-woolly-adelgid.
Wisconsin Department of Natural Resources. (n.d.). Spongy moth (Lymantria Dispar). Retrieved from https://dnr.wisconsin.gov/topic/foresthealth/spongymoth.