LITTER TREATMENT EFFECTS ON BIOTIC FACTORS AT A MESOCOSM COMPLEX AND A PRAIRIE POTHOLE WETLAND
Introduction
Hybrid cattail (Typha x glauca), one of the most invasive and dominant plants in the prairie pothole region, can reduce the diversity of other wetland species because of its ability to form dense stands of living and dead biomass (Vaccaro et al. 2009). Since the growth of emergent wetland plants is largely a reflection of the growing conditions during the previous year, the impacts the environment during the year prior drives the abundance and diversity of vegetation observed during a growing season (Larkin et al. 2012, Farrer and Goldberg 2009).
When high amounts of cattail litter accumulate, it can prevent the growth of other species, decreasing species diversity (Xiong and Nilsson 1999, Vaccaro et al. 2009). Due to the high production of hybrid cattail and its slow litter decomposition (Davis and van der Valk 1978), the accumulation of its litter could make it easier for hybrid cattails to dominate wetlands that it has invaded. Vaccaro et al. (2009) stated “Increasing the cover of fallen cattail litter, without altering the density of live cattails, reduced species diversity. In plots where all litter was removed, juvenile plants of many species were observed, more seedlings survived, and species density increased.” The amount of litter present on a site can affect the diversity and production of the plants that grow there. Additionally, litter can inhibit growth of new species by its ability to physically prevent the establishment and growth of other species. Larger amounts of litter could reduce air and soil temperatures, increase the risk of obtaining a fungal disease, increase the chance of herbivory on seeds and seedlings, lower light penetration through the canopy, decrease the opportunity of seeds to reach the soil, and
chemically inhibit germination of native species by altering soil chemistry (increased NH4+) (Vaccaro et al. 2009). Cattails also have the ability to promote their own growth due to the persistence of their own litter. As litter accumulates, it will protect the live culms from water level fluctuations and physical breakdown. When water levels are stable, a positive feedback can be created that enables litter to continue to persist, adding to litter accumulation, which limits or prevents growth of other species, further increasing cattail abundance (Vaccaro et al. 2009).
Larkin et al. (2012) hypothesized that hybrid cattails could promote dominance through two pathways due to having high rates of production, dense growth, and high amount of litter production. The first pathway is when litter changes the conditions present within the habitat (such as decreased light penetration, temperature of the soil, and availability of
space). The second pathway is when living plants change the conditions of the habitat and/or outcompete other species for the same resources, such as nutrients, space, and light. They stated that the importance of these pathways shouldn’t be overlooked, since determining which pathway a hybrid cattail invasion may follow can and does have a large effect on how to respond using various management practices.
Litter accumulation can affect many wetland functions, including primary production, organic matter accumulation, nutrient availability and cycling, seed germination, and plant and animal biodiversity (Gingerich and Anderson 2011). If enough litter accumulates, a specialized community can develop within the litter area. For example, an increase in litter creates suitable habitat for species, such as voles, mice, and frogs. Litter can also supply nesting material for certain waterfowl species. When cattails are found in large,
rest of the wetland. This is due to changes in biotic conditions, including lower light levels, less dissolved oxygen in the water column, and lower water temperatures (Rose and
Crumpton 1996). Dense stands of litter can slow water currents through a wetland. Because of this decreased water velocity, suspended sediments and nutrients settle to the bottom of the wetland rather than continue on to the next connected water body (Atkinson and Cairns Jr. 2001).
Hybrid cattail litter has strong effects on plant community dynamics (Facelli and Pickett 1991), as well as strongly influencing wetland ecosystem function and assembly (Weiher and Keddy 1995). Litter has also been shown to slow seed germination, decrease overall plant diversity, and decrease how well parent species reproduce (Haslam 1971; van der Valk 1986; van der Putten et al. 1997; Weltzin 2005). Litter accumulation rates are strongly tied to disturbances (Knapp and Seastedt 1986), such as fires or fluctuating water levels. These disturbances can impact the area conducive to plant growth, primary
production, and litter decomposition rates (van der Valk and Davis 1978; van der Valk 1986; van der Valk et al. 1991). If a disturbance doesn’t occur regularly, static conditions can persist, which may lead to steady growth of emergent plants within stable water
environments, which in turn may lead to large litter accumulations (Christensen 2007). Due to the fact that the amount of litter present in a wetland has such a profound effect on wetlands, I evaluated how the addition and removal of litter effects selected biotic factors. I hypothesized that when litter increases on a site, the number of cattail shoots and cattail heights will decrease. I used an experimental approach that utilized both a mesocosm complex, in which water levels are controlled, and a native prairie pothole wetland. In both
studies, litter level treatments were created: a low treatment by the removal of cattail litter, a high treatment by the addition of cattail litter, plus a control in which litter was undisturbed.
Methods
Mesocosms
The mesocosms were located on an Iowa State University farm, the Hinds Farm, near Ames, Iowa, USA. At this farm there is a mesocosm complex that was established in 1989 for a study on nitrate removal by wetlands. The mesocosm complex is now comprised of 12 mesocosms arranged in four blocks of three mesocosms (Figure 18 and Figure 19). These mesocosms are 3.35 meters in diameter, 0.91 meters deep, ultraviolet-stabilized polyethylene containment units that are buried part way in the ground to minimize temperature
fluctuations. At establishment, they were filled with wetland soil from a nearby restored prairie pothole and planted with hybrid cattail (Crumpton and van der Valk 1993).
Experiments ceased on these mesocosms in the late 1990s-early 2000s. Beginning in 2013, litter treatments were applied to 12 mesocosms. At this site, there are three litter treatments (control, low (standing litter removed), and high (standing litter added)) with four replicates of each treatment. Mesocosms in which litter was not manipulated were used as the controls. The low treatment involved the removal of all standing litter in early spring. The litter that was removed was placed onto the corresponding high treatment (i.e., the litter removed from the low treatment was placed in the high treatment in a block). Water levels in the
mesocosms were kept constant by supplying them with groundwater, as needed, by using a network of pipes that are gravity fed from a storage tank.
Figure 18. Aerial image of Mesocosm Complex (Google Earth (b)).
Figure 19. Mesocosm Complex.
Field Site
Anderson (Goose) Lake marsh is located one mile northeast of Jewell, Iowa, USA (Figure 20 and 21). This marsh is an enhanced prairie pothole. It has a maximum depth of 1.5 m and has a surface area of approximately 65 ha. It is a semi-permanent wetland that has its water level regulated by a dam on the southwestern end (Rose and Crumpton 1996).
Emergent vegetation consisting primarily of cattails is present around the perimeter of the wetland. The width of these perimeter cattail bands can reach >30 m. At this site, twelve 2m x 3m plots were set up in blocks of three plots. As in the mesocosm complex, in each replicate block, the standing litter from one plot was removed (low treatment) and put on another plot (high treatment), with the third plot in each block being used as a control without any litter manipulation. The litter at both sites was moved in early April 2015. Shoot Density and Height
Cattail shoots were counted at approximately two week intervals at each location during the growing season. To do this, a 1 m x 1 m PVC quadrat was placed in the center of each plot, and the number of shoots that grew within the quadrat was recorded. The heights of 10 randomly selected cattail shoots within the quadrat were also measured. This was done using a PVC pole that had measurements marked every 10cm on it. For each height
measurement, the PVC pole was pushed into the sediment until there was resistance, and then the tallest leaf of the cattail plant being measured was recorded. In September 2015 all
seedheads that were present within the 1 m x 1 m PVC quadrat were counted in each plot at each site.
Figure 20. Aerial image of Anderson Lake (Google Earth (a)). Study plot location indicated by white oval.
Figure 21. Anderson Lake, looking southwest from the northeast.
Data Analysis
Seedhead counts, cattail heights, and the number of cattail shoots were analyzed using a Kruskal-Wallis test in Microsoft Excel. A p-value of less than or equal to 0.05 was
considered significant. Sites were analyzed separately because they were spatially separated and are comprised of different regulatory systems.
Results
Shoot Density
At the mesocosm complex, the number of shoots in the low treatment was
significantly different from both the high and control at each date (Table 14). Averaged over the entire growing season, there were significant differences between the high and control treatments (difference of 7 cattail shoots) (p=4.6E-4), high and low treatments (difference of 34.5 cattail shoots) (p=5.8E-12), and control and low treatments (difference of 27.5 cattail shoots) (p=6.1E-12). These results were similar to the previous year (2014) where the overall average number of shoots for the high treatment was 33 (SD=4.31), for low treatment it was 44 (SD=6.53), and for the control it was 48 (SD=13.6). The high and control
treatments (difference of 10.9 cattail shoots) (p=7.9E-5) and the high and low treatments (difference of 14.5 cattail shoots) (p=5.0E-5) were significantly different, but the low and control treatments (difference of 3.6 cattail shoots) were not significantly different (Table 15).
Table 14. Mean number of cattail shoots per m2 by date and treatment at mesocosm complex during the 2015 growing season. H, L, and C represent the high, low, and control treatments, respectively.*
Date High Control Low
6/1 23.0a 28.5a 62.5b 6/8 33.6a 35.8a 61.8b 6/27 28.6a 36.8a 62.8b 7/7 30.5a 35.8a 62.5b 7/22 29.3a 35.8a 63.3b 8/6 27.8a 37.3b 63.8c 8/19 28.0a 35.3a 63.0b 9/5 23.8a 36.5a 61.3b Mean 28.0a 35.0b 62.5c St. Dev. 3.3 2.7 0.8
*Values in the same row followed by the same letter were not significantly different (p>0.05). No letters are shown where there were no significant differences.
At Anderson Lake, there were no significant differences in the number of shoots between the high, control, or low treatments at any given date (Table 16). Averaged over the entire growing season there was a significant difference between the high and low treatments (difference of 6.5 cattail shoots) (p=0.01), but no significant difference between the high and control or low and control treatments.
Table 15. Mean number of cattail shoots per m2 by date and treatment at mesocosm complex during the 2014 growing season. H, L, and C represent the high, low, and control treatments, respectively.*
Date High Control Low
5/30 30.9a 46.8b 36.3ab 6/3 31.5a 46.9b 38.4ab 6/6 31.6a 47.0b 40.8ab 6/11 31.8a 46.5b 42.4ab 6/13 31.8a 46.5b 42.4ab 6/18 31.8a 46.5b 42.4ac 6/25 31.8a 46.5b 43.1ab 7/2 40.8a 44.8ac 55.8b 7/16 34.0a 39.0ac 54.8b 7/30 35.0a 39.0ac 58.3b 8/13 34.8a 39.0ac 59.0b 8/27 32.5a 40.5ac 58.8b Mean 33.2a 44.1b 47.7b St. Dev. 2.7 3.6 8.8
*Values in the same row followed by the same letter were not significantly different (p>0.05). No letters are shown where there were no significant differences.
Table 16. Mean number of cattail shoots per m2 by date and treatment at Anderson Lake. H, L, and C represent the high, low, and control treatments, respectively.*
Date High Control Low
6/2 54.8 62.3 62.5 6/17 61.8 64.3 66.5 7/1 58.8 62.3 64.3 7/19 57.3 63.5 62.5 8/12 55.0 59.3 63.0 8/29 54.8 60.8 62.8 9/13 54.0 54.3 60.5 Mean 56.5a 61.0ab 63.0b St. Dev. 2.8 3.4 1.8
*Values in the same row followed by the same letter were not significantly different (p>0.05). No letters are shown where there were no significant differences.
Shoot Height
For the mesocosm complex, the average cattail shoot height for each treatment at a given date can be seen in Table 17. Significant differences existed throughout June, with the low treatments having a greater mean height.
Table 17. Mean heights (m) of cattail shoots per m2 by date and treatment at mesocosm complex during the 2015 growing season. H, L, and C represent the high, low, and control treatments, respectively.*
Date High Control Low
6/1 0.79a 1.01b 1.25c 6/8 1.33a 1.43b 1.59b 6/27 1.68a 1.80ab 1.88b 7/7 2.14 2.12 2.09 7/22 2.37a 2.38a 2.17b 8/6 2.45a 2.44a 2.24b 8/19 2.43 2.33 2.30 9/5 2.46 2.41 2.43
*Values in the same row followed by the same letter were not significantly different (p>0.05). No letters are shown where there were no significant differences.
For Anderson Lake, the average cattail shoot height for each treatment at a given date can be seen in Table 18. There were no significant differences in the early part of the
growing season, but significant differences did exist between the high and control treatments and high and low treatments towards the end of the growing season.
Table 18. Mean heights (m) of cattail shoots per m2 by date and treatment at Anderson Lake. H, L, and C represent the high, low, and control treatments, respectively.*
Date High Control Low
6/2 1.76 1.71 1.81 6/17 2.24 2.23 2.26 7/1 2.66a 2.51b 2.48b 7/19 2.75a 2.63b 2.70ab 8/12 2.78a 2.64b 2.68ab 8/29 2.73a 2.81ab 2.85b 9/13 2.80 2.82 2.82
*Values in the same row followed by the same letter were not significantly different (p>0.05). No letters are shown where there were no significant differences.
Height data for the Mesocosm Complex was also recorded during the 2014 growing season (Table 19). The only timeframe that contained significant differences was throughout the month of June, when the control treatments were significantly lower than either the high or control treatments.
Table 19. Mean heights (m) of cattail shoots per m2 by date and treatment at mesocosm complex during 2014 growing season. H, L, and C represent the high, low, and control treatments, respectively.*
Date High Control Low
5/16 0.32 0.40 0.34 5/19 0.35 0.37 0.45 5/21 0.38 0.46 0.49 5/23 0.52 0.54 0.59 5/30 0.64 0.67 0.71 6/3 0.82 0.80 0.89 6/6 1.03b 0.93a 1.13b 6/11 1.42b 1.18a 1.45b 6/13 1.50b 1.31a 1.53b 6/18 1.72b 1.43a 1.71b 6/25 1.86b 1.68a 1.78b 7/2 1.87b 1.69a 1.85b 7/16 2.00 1.96 2.03 7/30 2.10 2.09 2.07 8/13 2.11 2.12 2.11 8/27 2.11 2.15 2.13
*Values in the same row followed by the same letter were not significantly different (p>0.05). No letters are shown where there were no significant differences.
For Anderson Lake, the average number of seedheads per m2 was 2.25 in the high treatment, 4.75 in the low treatment, and 1.75 in the control treatment, with no significance between treatments (p=0.237). For the mesocosm complex, the average number of seedheads was 1.50 in the high treatment, 13.25 in the low treatment, and 2.00 in the control treatment, with no significance between treatments (p=0.052).
Discussion
Shoot Density
I hypothesized that the number of cattail shoots would decrease with increasing litter. My hypothesis was supported, with the number of shoots in the low treatments at the
mesocosm complex being significantly higher than either the low or the control treatments; however, Anderson Lake provided inconclusive results. The treatments at the mesocosm complex have been applied annually beginning in 2013. The treatments at Anderson Lake were applied in early spring of 2015. Because the mesocosm complex has had these
treatments applied for a longer period, it was expected that the plots there would exhibit more discernable differences than at Anderson Lake. Additionally, the cattail shoots that were measured and counted in 2015 were largely the product of the previous years’ growing conditions. Therefore, the litter treatments that were applied in spring of 2015 will likely have more effect on the cattail shoots present in the spring of 2016. The values at Anderson Lake are suggestive of the trend that was clearly evident at the mesocosm complex. These results were expected due to the fact that low treatments allow for greater sunlight
penetration, which aids in warming of the water and sediment (Weltzin et al. 2005). Also, there was no obstruction that shoots growing in the low treatment had to grow through, compared to the control and high treatments where litter was present and could have inhibited cattail shoot emergence. Had the plots still been established at Anderson Lake in summer of 2016, we would hypothesize that they would begin to show a similar trait in terms of shoot abundance as was found at the mesocosm complex.
I hypothesized that cattail shoot heights would decrease with increasing litter. My hypothesis was supported, as the mesocosm complex had clear trends in cattail height, displaying significant differences between litter treatments, especially early in the growing season. While significant differences were present at Anderson Lake, the growth habits of cattails (as previously discussed) should be taken into consideration when evaluating the validity of these differences. At the mesocosm complex, by June 1there were already significant differences between each treatment type, with the low treatment producing the tallest shoots on average and the high treatment producing the shortest. This trend continued until mid-July when the average height of the high treatments overtook the height of the control and low treatment, before numbers began to stabilize again near the end of the growing season. The reasoning for higher initial growth in the low treatment can be attributed to having unobstructed room for primary growth, while the high and control treatment plots had litter that kept temperatures cool longer and made it more challenging for new cattail shoots to penetrate through. However, as the growing season progressed, the high treatment mean surpassed both the control and low treatment means. This is likely due to etiolation. The decreased light in the control and high plots meant that the shoots initially were growing in low light under the litter layers. Once these shoots reached the top of the litter layer, they were able to grow quickly, catching and surpassing the mean height of the low litter treatment.
The low litter treatment plots allowed for greater light penetration which warmed the water and sediment faster than the high or control treatments. The higher temperatures in the low treatments enabled cattail shoots to start growing sooner. By mid-summer the
Mean cattail heights were greater for each treatment type at Anderson Lake than at the mesocosm complex. This can be attributed to the differences in growing conditions (such as etiolation), especially water depths. The cattails at the mesocosm complex were growing in very shallow water while those at Anderson Lake were growing in much deeper water. Anderson Lake is also located in a watershed that is dominated by agricultural production and has nutrient-rich runoff, whereas the mesocosm complex is gravity fed by a series of pipes from a storage tank that is filled by a pump supplied with groundwater. These two different water sources could contain different amounts of nutrients that could also affect the rate of growth as well as overall cattail production, which may be reflected in the seedhead count data.
Past studies by van der Valk and Davis 1978, van der Valk 1986, Bohlen 1991, van der Putten et al. 1997, Meyerson et al. 2000, and Weltzin et al. 2005 have shown that litter can both directly or indirectly impact wetland vegetation by influencing seed germination and primary production. Farrer and Goldberg (2009) found that cattail litter had a significant effect on Typha max height, native vegetation max height, species diversity, stem diversity, light, N mineralization, and NH4+. Their litter plots reduced total culm density by 75% compared to the no-litter plots. Litter did not have a significant effect on the density of other Typha culms, with no-litter treatments averaging 23.8 +/- 3.1 and litter treatments averaging 26.8 +/- 1.2 culms. Those results contradict the results from this study, in which both the mesocosm complex and Anderson Lake indicated a significant difference in shoot density between the high and low treatments.
Farrer and Goldberg (2014) found that when Typha litter was added (high treatment), it had no effect on the density of Typha x glauca, but limited the density in removal (low)
treatments and produced taller plants when litter was added. Their results indicated that cattails growing in the plots that contained added litter were significantly taller than either the control or low treatments at both sites during the middle of the field season.
Litter accumulation is greater in the interior of a cattail stand compared to the edge of the same stand that is closer to open, deeper water (Christensen 2007). Phragmites stands that were closer to the deeper, open water areas of a marsh had less litter accumulation than stands that were located in the shallow interior portions of the marsh (Clevering 1998). The