Analysis Of The Paper Ecosystem Succession And Nutrient Retention: Vitousek And Reiners’ Hypothesis
Theory, Hypothesis & Predictions
Vitousek & Reiners (1975) tested the hypothesis that an ecosystem’s ability to conserve nutrients increases as it matures, as originally hypothesized by Odum in his paper “The Strategy of Ecosystem Development” (1969). They predicted that an ecosystems ability to conserve nutrients would be maximized at an intermediate stage of development. An ecosystem consists of inputs, outputs, and internal recycling.
Systems at the start of succession, such as an empty sand dune, would have inputs and outputs that match since plants would not be present to provide a means of conserving nutrients. Inputs through precipitation would escape through hydrologic outputs. As development progresses through succession, plants and necessary nutrients will accumulate providing a means of conserving nutrients. This pattern follows Odum’s hypothesis, but biomass cannot continue to increase forever. Odum also suggested that eventually production and respiration balance making net production equal to zero. Therefore, once the ecosystem reaches this stage of succession, inputs and outputs must again match. They predicted that the rate nutrients assimilate into the system will increase quickly in early succession and eventually reach the maximum, and will decrease until it reaches the steady state where net production is equal to zero.
Their logic behind the predictions seams accurate and the effect of dominant plants on soils during succession was also tested in the Netherlands in nutrient poor systems which supports their predictions. They also stated that the pattern of nutrient retention over the stages of succession should apply to all terrestrial ecosystems.
Craft, 1996 tested the same hypothesis as it applies to wetland ecosystem succession and found that phosphorus cycling differs in wetlands than other terrestrial ecosystems, in that Phosphorus retention is greatest at early stages of succession rather than intermediate or mature stages. They found that phosphorus is regulated by processes outside of succession, whereas nitrogen concentrations, as tested by Vitousek and Reiners (1975) are controlled by biological processes.
Experimental Design
Vitousek and Reiners (1975) tested their hypothesis on Mt. Moosilauke of New Hampshire where there are watershed ecosystems that varied in plants from alpine tundra (higher elevations), to spruce fir forests (intermediate elevations), and northern hardwood forests (lower elevations). The intermediate and higher elevations had little human disturbance, whereas the northern hardwoods had been significantly logged. Two ridges were used in this study that contained watersheds with comparable bedrock. One watershed above 750 meters had no record of being logged, therefore, if logging occurred, it would have occurred over 100 years ago. The other ridge experienced logging between 1946 and 1948. Seven watersheds on the west facing slope were tested on each ridge between 750-1200 meters, all within four kilometers of each other. The recently logged ledge contained two watersheds that were not logged and the other five were in intermediate stages of succession. Therefore, the sample locations included mature ecosystems, and intermediate aged ecosystems. During the growing season, the streams in all watersheds were sampled biweekly and in winter, they were sampled every 3 to 4 weeks. Each sample was tested for ammonium, nitrate, metallic cations, and chloride concentrations .
Statistical Analysis
Vitousek and Reiners (1975) tested each water sample for ammonium, nitrate, metallic cations, and chloride concentrations. They averaged the stream water concentrations during growing season (June 1 to September 30, 1973, 1974) to determine the average nutrient loss in stream drainage of the old aged ecosystems and the intermediate age ecosystems. Variations in the mean calculations were represented as the standard deviations. They stated that the difference between the watersheds were significant at p<0.01, but they did not explicitly discuss the test used to get this value. They also presented the average concentration of nitrate from 5 intermediate aged forests and 9 old aged forests in a graph representing change over time to illustrate seasonal trends. There were a few assumptions that had to be met to complete the analysis for their comparison, but based on the close proximity of the sampling locations (within 4km of each other), similarity in bedrock, and similarity in hydrologic regimes and precipitation chemistry, all assumptions appear to have been met.
Presentation of the Data
Vitousek and Reiners’ (1975) graph illustrating the seasonal change in nitrate concentrations in the streams draining the old age ecosystem and the intermediate age ecosystem is convincing. The graph clearly shows that nitrate concentrations remained fairly high and constant in the mature ecosystem output, whereas the nitrate concentration showed a cyclical pattern with season in the intermediate age ecosystem. This was consistent with their hypothesis that plant uptake regulates nitrogen losses, indicating that concentrations should be higher in stream drainage in winter seasons when root uptake is not occurring. They also present a table with the concentrations of essential elements from the stream water which clearly shows that the intermediate stage ecosystems retain magnesium, potassium, and calcium (essential plant nutrients) more effectively since the concentrations in the stream drainage is lower than the mature ecosystem. Since the visuals were convincing, I don’t think a different approach would have been better. Grimm (1987) tested the same hypotheses and found a similar pattern in a desert stream ecosystem which supports their statement that the pattern of nutrient retention should apply to other terrestrial ecosystems with slight variations.
Interpretation of the Data
Their interpretation of the data seems accurate and unbiased. They were able to explain the results and compare them to other studies that tested the same hypothesis on different locations. They did explain some of the alternative explanations. For example, they used their data to explain why some older ecosystems my still accumulate more nitrogen through precipitation than is lost in streamflow. If the mature ecosystem exists in a location that has frequent disturbances, such as frequent fires, the patch turnover allows for more nutrient retention. Therefore, this study may have relevance in studying the intermediate disturbance hypothesis, which states that species richness will be highest at intermediate levels of disturbances .
Summary
This paper was convincing because they compared their results to many other studies that supported their hypothesis as well as more studies I found online, and they explained some of the alternative explanations that might occur in other terrestrial systems. The data is also clearly represented in the paper and their interpretation appears unbiased making their results and discussion easy to follow. They followed most of the steps as outlined in the textbook, but they mentioned the p-value was <0.001 when comparing the mean concentrations, but didn’t state what test they used to get to the p-value.
