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How soil rebounds from disturbance

In this piece MA student Natalie Bray sums up a new peer reviewed MS which she found interesting. Take it away Natalie….

 

Soil is something that has been fascinating me for the last five years. From the animals that live in it to the gases that are respired from it, I’ve worked on many different aspects of this vast topic. Though my main research focus is now soil animals, last semester I worked on a semester long project involving soil respiration. A recent paper published by a former student, Dr. Jennifer Levy-Varon of my advisor Dr. Kevin Griffin, looks at the changes in soil respiration at Black Rock Forest in the tree girdling experiment, in her second paper on this topic, Rapid rebound of soil respiration following partial stand disturbance by tree girdling in a temperate deciduous forest, published in December 2013 in Oecologia.

 

Oak Tree at Black Rock Forest

Oak Tree at Black Rock Forest

 

I realize that there is some jargon and unfamiliar concepts in my post today so I’m going to clarify a few concepts and processes before explaining the findings of this paper. First, what is soil? Most people know what dirt looks like, what dirt feels like and what dirt smells like but most do not know what is actually in it. Soil is a mix of organic matter, air, water and weathered rock and is most often covered by a layer of leaf litter and of course, contains large and small organisms. The release of carbon dioxide (CO2) from soils, or soil respiration, is a major component of the global carbon cycle. The CO2 emitted from the soil is a combination of CO2 release from the roots, the organic matter in the soil and the leaves on top of the soil. With recent concerns about climate change and carbon storage, it is important to measure soil respiration and its potentially changing contribution to the global carbon cycle.

Equipment to measure soil respiration

Equipment to measure soil respiration

Fig. 2 from Levy-Varon et al. (2013)

Fig. 2 from Levy-Varon et al. (2013)

 

Because of recent forest disturbances such as tree removal, disease outbreaks and other natural and anthropogenic perturbations, it is important to look at the changes in forest carbon uptake and release in response to these disturbances. Recently, the loss of oak, Quercus sp., is a major concern in the Eastern United States. With many biological and physical threats, the regeneration of oaks is threatened and could have significant effects in Northeastern forests where it is a dominant species. A large-scale oak girdling experiment was performed in a section of Black Rock Forest (Cornwall, NY) during the summer of 2008 and since has been a major site looking at the effects of girdling. Girdling is an effective way of killing a tree that simulates the natural death of the tree, as the tree is still standing in the forest. A chainsaw is used to cut a ring around the tree so that there is no longer any water or nutrient flow.

Levy-Varon et al. (2013) wanted to look at the changes of soil respiration after for a given time period after the girdling and determine if disturbances like tree death had a short- or long-term impact on the CO2 emissions from the soil. This study did comprehensive measures on CO2 emissions, weather patterns over three years, and even estimated the amount of living aboveground biomass, which is essentially determining how much tree material and mass there was in the particular section of the forest. The data collected for this study illustrates the interconnectedness of the forest ecosystem and how many measures are required to get an accurate assessment of the changes in the forest due to girdling. Figure 2 from the paper, is a great illustration of the linked changes in soil respiration, soil temperature and soil water content throughout the course of the experiment. The four sets of data points in each graph represent the different treatments in the study: control, non-oaks girdled, 50% oaks girdled and 100% oaks girdled. The top three graphs show soil respiration, which is high immediately after the girdling and decreases over time and in two later summers, there are an increasing then decreasing trend, which correlates with peak summer temperatures in the second row of graphs. Generally, the control points are higher than the girdled points, which is expected as the dead trees contribute less to the overall CO2 flux initially but over time, the difference between the two plots decreases. The term “rebound” is applicable because there is an obvious perturbation right after the girdling during the first year of measurements but the second and third year of measurements show less dramatic changes and more of a consistent pattern.

 

Overall, the changes in soil respiration were short (less than three years). The fact that the respiration rate increased, decreased then returned to a somewhat stable level indicated that this forest and the soils were able to rebound from this major ecological change and can be characterized as resilient to disturbances. The important nuance highlighted in this paper is that the soil respiration patterns after the disturbances were not the same as the patterns before the disturbances but there was evidence of a recovery period, then an establishment of a new regime. These findings were very interesting as it showed the importance of the timeline of recovery from disturbance and how even though these forests are facing many obstacles and modifications, they are still capable of bouncing back.

Black Rock Forest

Black Rock Forest

 

Forests are incredibly important for some many different reasons including biodiversity, ecosystem services, and global carbon regulation. Soils are often overlooked though they are a critical component of forest ecosystems. Thinking about how disturbances interact with multiple components of forests like trees and soils will allow us to gain a better understanding of how changes, whether they be natural or anthropogenic, will effect this vital part of our planet.

 

 

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