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Encroachment of Woody Species in a Coastal Plain Wetland
Peyton S. Harrelson, Department of Biology, East Carolina University
Abstract
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There are many possible driving f orces f or the encroachment of woody species on herbaceous landscapes. In this study, two-factor analysis of variance test was used to examine the ef fects of f ertilizer on the encroachment of woody species in mowed plots over time. Based on the oncoming data presented, there was a positive signif icant effect of f ertilizer and time on the encroachment of woody species in mowed plots. The encroachment of woody species in any landscape may alter plant communities, so it is important to understand how f ertilizer may escalate this ef fect.
Introduction
Many driving forces are possible f or the encroachment of woody species on herbaceous landscapes. The problem of this woody species' growth is not a problem that is limited to one area of the world or type of biome. The problem of encroachment has been studied on many continents f rom areas of grasslands to wetlands. Although there has not been much published on the “Although the problem of this type of intrusion may be inevitable, it still ef fect f ertilization has on the encroachment of woody species, there does not negate the fact that this still have been many published journal problem decreases the diversity of articles about the encroachment of herbaceous species. (Warren et al., woody species into wetlands. Although 2007).” the problem of this type of intrusion may be inevitable, it still does not negate the f act that this problem decreases the diversity of herbaceous species. (Warren et al., 2007). There have been many proposed mechanisms as to what can increase and help to contain the rate of encroachment. Some of these include soil composition, available nutrients, and disturbances (Mills et al., 2017). There has also been some interest in the increased carbon dioxide levels due to global warming f acilitating the incursion of woody species (Saintilan & Rogers, 2015). Given these f easible causes of encroachment, f ire suppression is named as the main f acto... Due to this f act, it is worth the investigation of what the true underlying cause of encroachment is and what other complicating
f actors are present. Encroachment cannot merely be explained by the absence of the disturbance of f ire.
This area of research is part of a larger, long-term study that f ocuses on the composition and changes that take place in a wetland due to mowing and f ertilization (Goodwillie et al., 2021). The wetland of interest in this study is located in the coastal plain of North Carolina and was originally burned to study the plant community after they were established during this secondary succession. This smaller experiment analyzed the percentages also known to be inf ertile, which is an important f actor to take into account due to the f ertilization that will be applied in this study aims to f ind the ef fect of f ertilizer on the encroachment of woody species over time in mowed plots. This study has been in progress f or almost two decades, and the encroachment of the woody species was studied over a seventeen-year long period, beginning with secondary succession that resulted f rom burning the landscape. In addition to the initial disturbance of burning, the land is also disturbed yearly by mowing. This mowing is like grazing or burning, which are some of the most common disturbances f ound amongst herbaceous landscapes. Alongside, wildf ires also previously were common among this landscape before human settlement. It is expected that f ertilizer will increase the encroachment of woody species, helping the woody species to outcompete the neighboring herbaceous species.
Methods
Study Site
The site of this experiment is in East Carolina’s West Research Campus and is considered to be in the wetlands of the Coastal Plain of eastern North Carolina. Some of the most dominant woody species in this plant community include the Rhus copal argutus. These two plant species effects are commonly known as winged sumac and sawtooth blackberry, respectively, and are both native to eastern North Carolina. This smaller study takes part at the site of a long-term study f ocusing on nutrient addition and mowing in the plant community (Goodwillie et al., 2021). The site is divided into eight blocks that are each divided equally into f our quadrants. Each block has quadrants that are f ertilized with a mowed and unmowed territory and unf ertilized with a mowed and unmowed territory. These f our treatments were randomly assigned to the quadrants in the plots f or replication purposes. This smaller study f ocused only on the unf ertilized mowed and f ertilized mowed quadrants of the plots. If you would like to f ind more details on the long-term study and sites visit Goodwillie et al. 2020 and Goodwillie and Franch 2006.
Experimental Design
This site was originally burned and tilled at the start of the long-term experiment. This experiment utilized mowing and f ertilization as variables to study their effects on the diversity of the wetland plant community, with mowing occurring once a year and f ertilization with 10-10-10 NPK f ertilizer three times a year. A drainage ditch was present along one side of the f ield, causing plots f arther away f rom the ditch to be more prone to f looding. Stem counts and percent covers of each species were recorded every
year f rom established square meter sections in all plots since the year 2004. This smaller experiment analyzed the percentage cover of woody species from the years 2004, 2013, and 2021 f rom the mowed/unfertilized and mowed/fertilized quadrants in all blocks. See Goodwillie et al. 2020 and Goodwillie and Franch 2006 f or more details on the long-term study and experimental design.
Statistical Analysis
A two-f actor analysis of variance test was used to test the ef fect of fertilizer on the encroachment of woody species in mowed plots. The independent variables consisted of lime and f ertilizer. Three years of percent cover data of woody species in the f ertilized and unf ertilized mowed plots were used to represent the initial, middle, and current points in time of the woody species growth including the years 2004, 2013, and 2021. The dependent variable was the total percent cover of all woody species f rom the corresponding years. The data set f rom the long-term experiment was reduced to the percent cover of each woody species in mowed plots only f rom the three chosen years. The percent covers of these woody species were then summed f or the f ertilized and unf ertilized quadrants, and then the ANOVA test was run.
Source of
Variation SS df MS F P-value F crit
Time 9612.633 2 4806.316 5.573993 0.0047 3.061716
Within 118994 138 862.2754
Total 140182.7 143
Results
There was a signif icant effect of f ertilizer on the encroachment of woody species in mowed plots. Fertilizer has a positive ef fect on the percent cover of woody species. The woody species had an average percent cover increase of 95% f rom the unf ertilized to f ertilized plots. Time had a signif icant effect on the encroachment of woody species in mowed plots. From 2004 to 2013, there was a 144% increase in the average percent cover of woody species in the mowed plots. From 2013 to 2021 there was an 8% increase in the average percent cover of woody species in the mowed plots.
Table 1. Two-factor analysis of variance test with replication.
Figure 1. Average of total percent cover summed across all woody species from three checkpoints in time.
There was no signif icant interaction between f ertilizer and time of the encroachment of woody species in the mowed quadrant. There was a similar pattern through time that both the f ertilized and unf ertilized woody species followed, which can be seen in Figure 1. This pattern f eatures a steep increase initially f rom 2004 to 2013 and then a plateau between 2013 and 2021 in both f ertilized and unf ertilized quadrants. When the average total percent cover of all woody species f rom 2004, 2013, and 2021 was summed, Rhus copallinum and Rubus arbutus was f ound to have the highest percent cover total. These two species were f ound to have a dramatically higher percent cover in f ertilized quadrants in comparison to all other woody species, which can be seen in Figure 2.
Figure 2. Sum of all woody species average percent cover in mowed plots from years 2004, 2013, and 2021.
Discussion
Fertilizer has had a signif icant positive effect on the encroachment of woody species. The results of previous studies on the ef fect of fertilization on encroachment have varied. One study used ammonium sulf ate to f ertilize their encroaching landscape but f ound that this type of f ertilization had negligible ef fects on the encroachment (Mills et al. 2017). Their study also stated that this was likely due to the f ertilizer affecting soil components like pH and acidity. The mentioned study was done in the savanna biome, while this study took place in a coastal wetland region. Differences in the soil composition, precipitation, and f ertilizer used were likely the cause of the dif fering results.
Time also had a signif icant effect on the encroachment of woody species. This is likely due to the architectural dif ferences between the herbaceous and woody species. Woody species are of ten able to dominate herbaceous species because they have advantages like a greater height that can be more competitive for sunlight (Warren et al. 2007). Another major advantage that the woody species have, is that an established shape of a shrub of ten uses its broad area to catch and direct precipitation to its base, which is an advantage that thin grasses do not have (Saintilan & Rogers, 2015). Woody species are known to be larger and stronger than herbaceous species, which also helps the woody species take up the space and nutrients it needs f or further competition. Time also
increases the ability of the woody species to compete, even though in our study all species are mowed over each year. The woody species may have to regrow in height to outcompete herbaceous species, but its root system underground remains intact and competitive. The woody species in our study were disturbed by mowing every year, while in most studies, and nature, woody species are allowed to establish themselves over many years. This may help to f urther solidif y the results of the study because encroachment is seen to be increased with f ertilization even when the new growth of the woody species is eradicated each year. This also raises an interesting question as to what rate would the woody species in undisturbed areas encroach. Although this data was not part of this particular study, it is still up f or interpretation. It was seen that woody species dominated the unmowed plots of the f ertilized and unf ertilized regions in the larger study but still needs f urther analysis to hold statistical signif icance. There was no signif icant interaction between the f ertilization and time of the woody species' encroachment because there was a similar pattern through time that both f ertilized and unf ertilized woody species f ollowed. This pattern of dramatic increase and then a plateau is the same f or the f ertilized and unf ertilized plots, but the percent cover of the f ertilized plots is still seen to be higher at all times. This may be explained by the beginning of the establishment of the woody species after burning. Due to competition, the plants took over the land quickly, then the percent cover did not increase as rapidly. Looking into the f uture, the composition of this wetland may continue to change if the woody species become more established of the annual disturbance of mowing t, but in undisturbed areas, there is hope f or herbaceous species to later increase in diversity (Warren et al. 2007). Once the woody species become f ar established and herbaceous species have diminished, there will be room f or a later increase in shade-tolerance f or herbaceous species.
References
Goodwillie, C., & Franch, W. R. (2006). An experimental study of nutrient addition and mowing ef fects on a ditched wetland plant community: results f rom the f irst year. Journal of the North Carolina Academy of Sciences, 122(3), 106–117. Goodwillie, C., McCroy, M. W., & Peralta, A. L. (2021). Long‐term nutrient enrichment, mowing, and ditch drainage interact in the dynamics of a Wetland Plant Community. The Bulletin of the Ecological Society of America, 102(1). https://doi.org/10.1002/bes2.1815 Massi, K. G., Eugênio, C. U., Franco, A. C., & Hof fmann, W. A. (2021). The ef fects of tree cover and soil nutrient addition on native herbaceous richness in a Neotropical savanna. Biotropica, 53(3), 888–895. https://doi.org/10.1111/btp.12940
Mills, A. J., Milewski, A. V., Snyman, D., & Jordaan, J. J. (2017). Ef fects of anabolic and catabolic nutrients on woody plant encroachment af ter long-term experimental f ertilization in a South Af rican savanna. PLOS ONE, 12(6). https://doi.org/10.1371/journal.pone.0179848 Saintilan, N., & Rogers, K. (2015). Woody plant encroachment of grasslands: A comparison of terrestrial and wetland settings. New Phytologist, 205(3), 1062–1070. https://doi.org/10.1111/nph.13147 Warren, R. J., Rossell, I., Moorhead, K. K., & Pittillo, J. D. (2007). The inf luence of woody encroachment upon herbaceous vegetation in a southern Appalachian wetland complex. The American Midland Naturalist, 157(1), 39–51. https://doi.org/10.1674/0003-0031(2007)157[39:tioweu]2.0.co;2
