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Soil seed bank dynamics and fertility on a seasonal wetland invaded by Lantana camara in a savanna ecosystem
South African Journal of Botany 100 (2015) 190–194
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South African Journal of Botany journal homepage: www.elsevier.com/locate/sajb
Soil seed bank dynamics and fertility on a seasonal wetland invaded by Lantana camara in a savanna ecosystem J. Muvengwi a,b,⁎, H.G.T. Ndagurwa c,d a
School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Department of Environmental Science, Bindura University of Science Education, Private Bag, 1020 Bindura, Zimbabwe Forest Ecology Laboratory, Faculty of Applied Sciences, National University of Science & Technology, P.O. Box AC 939, Ascot, Bulawayo, Zimbabwe d Department of Forest Resources and Wildlife Management, Faculty of Applied Sciences, National University of Science & Technology, P.O. Box AC 939, Ascot, Bulawayo, Zimbabwe b c
a r t i c l e
i n f o
Article history: Received 24 November 2014 Received in revised form 25 May 2015 Accepted 4 June 2015 Available online xxxx Edited by BS Ripley Keywords: Invasion Wetland Depth Lantana camara Soil nutrients Zimbabwe
a b s t r a c t Knowledge of seed bank status and dynamics is crucial for effective management of desirable and undesirable plant species in natural ecosystems. We studied the soil seed bank dynamics and soil nutrient concentrations in Lantana camara invaded and uninvaded patches at New Gada wetland in Harare, Zimbabwe. Soils were tested − for pH, ammonium (NH+ 4 ), nitrate (NO3 ), phosphorus (P), calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K). We also assessed the soil seed bank density to a depth of 15 cm over varied altitudinal zones. Soil nutrient concentrations increased by a factor of 2.5 for Na, 3.1 for NH+ 4 , 4.8 for Mg up to 8.5 for Ca with L. camara invasion. In contrast, L. camara invaded patches had a lower concentration of NO− 3 and P than uninvaded patches. Seed density significantly declined with both soil depth and slope with high seed density in the upper surface soil of the lower slopes of the wetland. The elevated soil nutrient concentrations along with a high soil seed bank density suggest that the wetland may still be susceptible to continued invasion by L. camara particularly on the lower slopes of the wetland. Thus, management and eradication efforts should focus on the areas that receive or trap the eroded soil seed bank. © 2015 SAAB. Published by Elsevier B.V. All rights reserved.
1. Introduction Invasive alien plants (IAPs) have drawn attention in plant ecology, as they have emerged as one of the biggest threats to global biodiversity and ecosystem stability (Drake et al., 1989; Holmes, 1990; Osunkoya and Perrett, 2011). Invasive alien plants can directly affect both the species composition and structure of ecosystems, water availability and alter soil quality (Vitousek, 1990; Wiser et al., 1998; Witkowski and Wilson, 2001; Booth et al., 2003; Hulme, 2003). Although some empirical studies have been carried out on the impact of invasive species on soil properties, majority lack comparison with native non-invaded plant communities, (see review Ehrenfeld, 2003). Lantana camara L. (Family Verbenaceae) is one of the known major problematic IAP species across the globe (Holmes, 1990; Osunkoya and Perrett, 2011). Although native to South America and West Indies, to date it has naturalized in at least 60 countries across the globe (Vivian-Smith and Panetta, 2009). Despite campaigns and efforts to control L. camara, it remains one of the biggest problem invaders worldwide. L. camara produces large quantities of seed, which are consumed ⁎ Corresponding author at: School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa. Tel.: + 27 749474218. E-mail address: [email protected] (J. Muvengwi).
http://dx.doi.org/10.1016/j.sajb.2015.06.007 0254-6299/© 2015 SAAB. Published by Elsevier B.V. All rights reserved.
by a number of endozoochorous birds causing rapid and long distance spread (Daehler et al., 2001; Heleno et al., 2012). The seeds can stay in the soil for years and be able to germinate once conditions are suitable (Duggin and Gentle, 1998), an essential prelude to persistence (Garner and Witkowski, 1997). Drake et al. (1989) emphasized the need for additional research focused on the general effects of individual IAP species on ecosystems. Indeed, a lack of knowledge regarding seed banks in southern African savannas has been acknowledged (Witkowski and Garner, 2000, 2008). Although studies have looked at the influence of fire and endozoochory on L. camara seed viability and germination (Buddenhagen and Jewell, 2006; Raizada and Raghubanshi, 2010), little has been done on the soil seed bank dynamics. Understanding soil seed bank dynamics is important for IAP species management, because reinvasion of cleared areas has been observed to be largely from soil seed bank (Witkowski and Wilson, 2001; Witkowski and Garner, 2008). Knowledge of seed bank size before planning any management activities on invasive species facilitates effective proactive management of important ecosystems such as wetlands. Wetland ecosystems play several important functions in the ecosystem including water purification and control of flooding (Schuyt, 2005). In addition to occupying low-lying areas in the landscape, they are highly productive ecosystems due to an excess accumulation of resources. As a result, they are utilized extensively by both humans and animals making them susceptible to disturbance-
J. Muvengwi, H.G.T. Ndagurwa / South African Journal of Botany 100 (2015) 190–194
mediated invasions (Witkowski and O'Connor, 1996; Witkowski and Garner, 2008). In this regard, soil fertility and seed bank dynamics were investigated in a seasonal wetland invaded by L. camara. Specifically, the objectives of this study were to determine if (i) the abundance of L. camara seeds in the soil changes with increasing soil depth and (ii) soil nutrient levels are different under L. camara stands than in open areas. 2. Materials and methods 2.1. Study area The study was carried out at New Gada wetland located at 17° 53′ 24′′ S and 31° 8′ 51′′ E (Fig. 1). The wetland is approximately 15 km from the Harare, Zimbabwe. Common plant species on the wetland includes Eragrostis enamoena, Nymphea sp, Scirpus raynalii, Scirpus sinutus, Typha latifolia and Juncus sp. The average rainfall ranges between 650 and 850 mm/annum, and mean annual temperatures are 9 °C for winter and 40 °C for summer (Bulton, 1995). 2.2. Soil sampling and analyses The wetland was stratified into upper, middle and bottom based on altitude following water flow direction. Three L. camara patches, at least 400 m2 in area, canopy dominated by the target species were sampled in each stratum. Five, 5 m × 5 m sampling plots were marked, one at the centre of each sampling patch and the other four located, each at 5 m in the four cardinal points. A control plot 5 m × 5 m was randomly marked at least 20 m away of each L. camara patch in order to sample reference soil. A new location was only chosen if the first control location was falling in another L. camara infested patch or was not at least
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20 m from any nearby infested patch. Four 5 m × 5 m plots were marked around central control plot similar to the L. camara infested patch. Soil was collected at a depth of 15 cm using a soil auger from the centre and the four corners of the central plot only. The other four surrounding plots were used to sample soil seed bank. The samples were thoroughly mixed to obtain a composite sample and 500 g was taken from this mixture for analysis at the laboratory. A total of 18 soil samples were tested − for NH+ 4 , NO3 , Resin-extractable P, pH, exchangeable Ca, Mg, Na and K at the Department of Research and Specialist Services, Chemistry and Soil Research Institute in Harare, Zimbabwe. In the laboratory, soil samples were air dried at room temperature before analysis. Soil pH was obtained using the CaCl2 method (Thomas, 1996; Okalebo et al., 2002). Exchangeable Ca, Mg, K, and Na were extracted using the aqua regia digestion method (Anderson and Ingram, 1993). The resulting compound was then dissolved in concentrated HCl and filtered. The solution was diluted with distilled water. Using a spectrophotometer, total Ca and Mg were determined at 0.460 nm and 0.595 nm, respectively, and flame emission was used for K and Na. Total N was determined using a Kjeldahl method (Okalebo et al., 2002). Plant available phosphorus was determined using the molybdenum-blue calorimetric method (Sibbesen, 1978). 2.3. Soil seed bank assessment Data on soil seed bank were collected post-dehiscence in April. Soil seed bank samples were collected from four random positions in each 5 m × 5 m plot using a ridged steel quadrat, 30 cm × 30 cm in area, with lines curved every centimetre to indicate depths (Witkowski and Garner, 2000). Samples were air-dried, spread onto large trays and seeds picked-out using forceps. Firm seeds were counted and firmness determined by pinching with a forceps (Shaukat and Siddiqui, 2004).
Fig. 1. Location of New Gada wetland in Epworth, Zimbabwe and the experimental plots.
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450
Ninety eight percent of the samples collected deeper than 15 cm had no seed; therefore, we ended our assessment at 15 cm.
c
400 350
Data were tested for normality and homogeneity of variance and transformed as necessary. Independent t-test was used to compare soil chemical properties between L. camara infested plots and the reference soil. Data on seed bank densities from different soil depths and wetland strata (upper, middle and bottom) were analysed using a balanced three-way ANOVA. Data were presented as mean ± standard error (SE). Post-hoc multiple comparison of means was conducted using Tukey honest significance difference (Tukey HSD) to detect differences among slope positions and depths levels. All tests were significant at p b 0.05.
Seeds m-2
2.4. Data analysis
300 250
b a
200 150 100 50 0 upper
middle
bottom
Fig. 2. Mean ± SE seed density in the different sections of the wetland. Means not sharing a common letter are significantly different (Tukey HSD, p b 0.05).
3. Results 3.1. Soil chemical properties Soil from L. camara invaded patches had significantly higher levels of Ca, Mg, Na, and NH4+ compared to the reference soil on the wetland by factors of 8.5, 4.8, 2.5 and 3.1, respectively (Table 1). However, reference soil had superior concentration of NO− 3 and available P compared to L. camara invaded patches by factors of 3.7 and 1.6, respectively (Table 1). There was no significant difference (p N 0.05) in K concentration between L. camara invaded patches and reference sites on the wetland. Soil pH was significantly lower (p b 0.001) in L. camara infestations compared to reference soil. 3.2. Seed density and vertical distribution A total of 3576 seeds were sampled on the wetland and of these 26%, 32% and 42% were sampled in the upper, middle and bottom sections of the wetland, respectively. Consequently, seed density significantly differed with slope position (F2, 126 = 84.11, p b 0.001) with greater densities at the bottom slope, followed by the middle slope and least at the upper slope (Fig. 2). A similar pattern was observed at each soil depth (Table 2). As a result, there was a significant interaction (F 2, 126 = 16.86, p b 0.001) between slope position and depth on seed density. Depth had a significant influence (F2, 126 = 1175.21, p b 0.001) on seed density, with a consistent decline in seed density with increasing depth across all sections of the wetland (Table 2). Most of the seeds were located in the top 5 cm of the soil profile, declining to almost zero at depths of 10 to 15 cm with 98% (44/45) of the samples collected below 15 cm containing no seed. 4. Discussion 4.1. Soil nutrient content Invasive alien plants have been shown to alter soil nutrient pools, salinity, moisture, and pH in many ecosystems (Ehrenfeld, 2003; Osunkoya and Perrett, 2011). Consistent to the general trend of increased nutrient pools in topsoil in response to invasion reported in other studies (Hobbs, 1992; Christian and Wilson, 1999), soil nutrient
concentrations increased by a factor of 2.5 for Na, 3.1 for NH+ 4 , 4.8 for Mg up to 8.5 for Ca. These changes in the concentration of base cations may alter the distribution and concentration of base cations at soil exchange surfaces potentially affecting cation exchange processes, soil pH, and soil organisms (Reich et al., 2005). In contrast, the concentration of NO− 3 and available P was lower on L. camara invaded patches than on un-invaded patches, which suggests that there is likely a higher plant uptake of these elements owing to the high plant biomass on L. camara invaded patches than on un-invaded patches. Additionally, invasive alien plants are also associated with reduced soil moisture content (Ehrenfeld, 2003). Thus, considering that reduced soil moisture content can suppress nitrification rates (Paul and Clark, 1989), it is possible that the low soil NO− 3 concentration on L. camara invaded patches is likely due to soil moisture suppression on nitrification. However, we did not examine this possibility, which suggests future research directions. The high concentration of NO− 3 and available P on un-invaded patches suggests that there is high likelihood that L. camara will proliferate on the wetland since systems with abundant resources particularly those that limit plant growth are susceptible to invasion (Davis et al., 2000). The concentration of K was similar between L. camara invaded patches and reference sites implying that invasion did not affect the prior concentration of K on these soils. The soil pH was significantly lower (p b 0.001) in L. camara infestations compared to un-invaded patches contrary to findings of other studies (Gentle and Duggin, 1997). This change in pH may have important implications for many soil properties since soil pH is central to many soil processes (e.g., decomposition Stursova et al., 2006; nitrification Seymour et al., 2014). Small changes in pH have been shown to result in large effects on soil properties (e.g., denitrification Stevens et al., 1998; Šimek and Cooper, 2002; sulphate adsorption of mineral soil Nodvin et al., 1986; fungi/bacteria ratio Baath and Anderson, 2003). Thus, the effect of pH on soil properties may be considered, even if the actual pH difference is rather small. This suggests that differences in soil nutrient concentrations observed between L. camara invaded patches and un-invaded patches were probably due to differences in pH between treatments. For example, the low NO− 3 recorded on L. camara invaded patches could be attributed to slowed nitrification due to a decline in pH (Stevens et al., 1998; Šimek and Cooper, 2002; Seymour et al., 2014). The concentration of base cations of most soils decreases when soil pH decreases (Zhang et al., 2013). In contrast, we
Table 1 Mean ± SE concentration of soil chemical variables measured on L. camara invaded patches and reference soil.
Lantana Reference t-Value p-Value
pH
K (me %)
Ca (me %)
Mg (me %)
Na (me %)
P (mg/kg)
NH+ 4 (ppm)
NO− 3 (ppm)
6.1 ± 0.1 7.0 ± 0.09 −5.9 0.001
0.4 ± 0.08 0.4 ± 0.07 0.3 0.745
11.9 ± 2.1 1.4 ± 0.2 4.8 0.0001
0.4 ± 0.07 1.9 ± 0.3 5.5 0.0001
0.2 ± 0.02 0.08 ± 0.007 4.8 0.01
6.4 ± 0.6 10.3 ± 0.5 −1.5 0.01
11.6 ± 0.7 8.2 ± 0.6 0.6 0.01
11.9 ± 1 43.9 ± 0.7 5.2 0.0001
J. Muvengwi, H.G.T. Ndagurwa / South African Journal of Botany 100 (2015) 190–194 Table 2 Mean ± SE of seed density (m−2) recorded at three depths on three slope positions of the wetland. Different superscript letters in the the same column (a, b and c) and same row (A, B and C) indicate significant differences between means at p b 0.05, based on Tukey HSD test. Depth
0–5 cm 5–10 cm 10–15 cm p-Value
Slope position Upper
Middle
Bottom
p-Value
432.6 ± 14.6aA 260.0 ± 9.3bA 2.2 ± 1.2cA b0.001
523.7 ± 22.8aB 308.1 ± 16.0bB 6.7 ± 1.8cB b0.001
657.0 ± 18.5aC 438.5 ± 14.0bC 20.0 ± 4.1cC b0.001
b0.001 b0.001 b0.001
found that the concentration of base cations (Ca, Na and Mg) increased with invasion suggesting that other factors such as litterfall, nutrient uptake and soil moisture availability which change with invasion could explain the observed differences in soil nutrient concentrations (Ehrenfeld, 2003). Additionally, animals and frugivorous birds visiting fruiting L. camara plants may concentrate nutrients through faecal droppings, which may also lead to elevated nutrients in invaded patches. While changes in soil nutrient concentrations occur with exotic invasions, the general direction of change is unpredictable because findings have been varied among studies (Ehrenfeld, 2003). Similarly, in this study, there was an increase (Ca, Mg, Na and NH+ 4 ), a decrease (NO− 3 and P) or there was no change (K) in concentration of soil nutrients with L. camara invasion. This suggests that effects of IAP on soil properties depend on the element under examination. Alternatively, it could be because soil cores, as used in this study, are point samples that miss nutrient dynamics over time. Thus, further research is required to establish the temporal variation in soil nutrient concentrations with invasion. 4.2. Soil seed bank Seed densities reported here, ranged from 2 to 657 seeds m−2, are greater than those reported by Vivian-Smith et al. (2006). The high seed quantity recorded in this study can be attributed to L. camara fruiting at least twice in a year due to availability of resources on the wetland such as moisture, light and soil nutrients (Duggin and Gentle, 1998; Day et al., 2003). Consistent with findings of other studies (García-Fayos and Cerdà, 1997; Jones and Esler, 2004), we found that seed density increased down gradient on the wetland. It is likely that the soil seed bank is eroded down gradient from upper sections of the wetland and end up concentrating in the lower sections of the wetland. As a result, soil seed bank density was higher in lower slope microsites that trap eroded soil than in the upper slopes. This may limit L. camara recolonization on the upper slopes since seed loss is a limiting factor on the revegetation of eroded slopes (García-Fayos and Cerdà, 1997; Jones and Esler, 2004). In this regard, management and eradication efforts to control L. camara invasion should focus on the lower slopes of the wetland. Seed density decreased with depth across all sections of the wetland consistent with findings of other studies (Witkowski and Wilson, 2001; Wilson and Witkowski, 2003; Sandrine et al., 2006). As a result, most of the seeds were located in the top 5 cm of the soil profile. We attribute the decline in seed density with soil depth to very little vertical dispersion of seed in the soil-by-soil transport or soil organisms. Alternatively, the fine texture of the soil likely reduced seed movement through the soil profile by reducing the action of percolating water or did not facilitate seed penetration into the soil profile as also suggested by Hopkins and Graham (1983). High seed density in the 0 to 5 cm has important implications for the spread of L. camara in the wetland since the buried seeds are likely to be brought to the surface with minimal levels of disturbance. Disturbances lead to increased germination of L. camara seeds from soil seed bank because disturbance increases resource availability such as light and nutrients (Duggin and Gentle, 1998). Additionally,
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although seed density declined with soil depth, the presence of seed at depths below 10 cm suggests that L. camara may have a persistent seed bank in this wetland. A persistent seed bank together with the high concentration of nutrients is also likely to predispose the wetland to continued invasion as well as suggest that L. camara may be difficult to eradicate on this wetland. However, buried seeds face several mortality factors including decay caused by bacterial and fungal microorganisms (Baskin and Baskin, 1998; Wagner and Mitschunas, 2007). Thus, the potential to germinate depends on the viability of the seed, which was not tested, and suggest future research directions. In conclusion, L. camara invasion altered the concentration of soil nutrients but the direction of change was dependent on the nutrient being examined with an increase (Ca, Mg, Na and NH+ 4 ), a decrease (NO− 3 and P) and no change (K) recorded in the concentration of soil nutrients with L. camara invasion. 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South African Journal of Botany journal homepage: www.elsevier.com/locate/sajb
Soil seed bank dynamics and fertility on a seasonal wetland invaded by Lantana camara in a savanna ecosystem J. Muvengwi a,b,⁎, H.G.T. Ndagurwa c,d a
School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa Department of Environmental Science, Bindura University of Science Education, Private Bag, 1020 Bindura, Zimbabwe Forest Ecology Laboratory, Faculty of Applied Sciences, National University of Science & Technology, P.O. Box AC 939, Ascot, Bulawayo, Zimbabwe d Department of Forest Resources and Wildlife Management, Faculty of Applied Sciences, National University of Science & Technology, P.O. Box AC 939, Ascot, Bulawayo, Zimbabwe b c
a r t i c l e
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Article history: Received 24 November 2014 Received in revised form 25 May 2015 Accepted 4 June 2015 Available online xxxx Edited by BS Ripley Keywords: Invasion Wetland Depth Lantana camara Soil nutrients Zimbabwe
a b s t r a c t Knowledge of seed bank status and dynamics is crucial for effective management of desirable and undesirable plant species in natural ecosystems. We studied the soil seed bank dynamics and soil nutrient concentrations in Lantana camara invaded and uninvaded patches at New Gada wetland in Harare, Zimbabwe. Soils were tested − for pH, ammonium (NH+ 4 ), nitrate (NO3 ), phosphorus (P), calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K). We also assessed the soil seed bank density to a depth of 15 cm over varied altitudinal zones. Soil nutrient concentrations increased by a factor of 2.5 for Na, 3.1 for NH+ 4 , 4.8 for Mg up to 8.5 for Ca with L. camara invasion. In contrast, L. camara invaded patches had a lower concentration of NO− 3 and P than uninvaded patches. Seed density significantly declined with both soil depth and slope with high seed density in the upper surface soil of the lower slopes of the wetland. The elevated soil nutrient concentrations along with a high soil seed bank density suggest that the wetland may still be susceptible to continued invasion by L. camara particularly on the lower slopes of the wetland. Thus, management and eradication efforts should focus on the areas that receive or trap the eroded soil seed bank. © 2015 SAAB. Published by Elsevier B.V. All rights reserved.
1. Introduction Invasive alien plants (IAPs) have drawn attention in plant ecology, as they have emerged as one of the biggest threats to global biodiversity and ecosystem stability (Drake et al., 1989; Holmes, 1990; Osunkoya and Perrett, 2011). Invasive alien plants can directly affect both the species composition and structure of ecosystems, water availability and alter soil quality (Vitousek, 1990; Wiser et al., 1998; Witkowski and Wilson, 2001; Booth et al., 2003; Hulme, 2003). Although some empirical studies have been carried out on the impact of invasive species on soil properties, majority lack comparison with native non-invaded plant communities, (see review Ehrenfeld, 2003). Lantana camara L. (Family Verbenaceae) is one of the known major problematic IAP species across the globe (Holmes, 1990; Osunkoya and Perrett, 2011). Although native to South America and West Indies, to date it has naturalized in at least 60 countries across the globe (Vivian-Smith and Panetta, 2009). Despite campaigns and efforts to control L. camara, it remains one of the biggest problem invaders worldwide. L. camara produces large quantities of seed, which are consumed ⁎ Corresponding author at: School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa. Tel.: + 27 749474218. E-mail address: [email protected] (J. Muvengwi).
http://dx.doi.org/10.1016/j.sajb.2015.06.007 0254-6299/© 2015 SAAB. Published by Elsevier B.V. All rights reserved.
by a number of endozoochorous birds causing rapid and long distance spread (Daehler et al., 2001; Heleno et al., 2012). The seeds can stay in the soil for years and be able to germinate once conditions are suitable (Duggin and Gentle, 1998), an essential prelude to persistence (Garner and Witkowski, 1997). Drake et al. (1989) emphasized the need for additional research focused on the general effects of individual IAP species on ecosystems. Indeed, a lack of knowledge regarding seed banks in southern African savannas has been acknowledged (Witkowski and Garner, 2000, 2008). Although studies have looked at the influence of fire and endozoochory on L. camara seed viability and germination (Buddenhagen and Jewell, 2006; Raizada and Raghubanshi, 2010), little has been done on the soil seed bank dynamics. Understanding soil seed bank dynamics is important for IAP species management, because reinvasion of cleared areas has been observed to be largely from soil seed bank (Witkowski and Wilson, 2001; Witkowski and Garner, 2008). Knowledge of seed bank size before planning any management activities on invasive species facilitates effective proactive management of important ecosystems such as wetlands. Wetland ecosystems play several important functions in the ecosystem including water purification and control of flooding (Schuyt, 2005). In addition to occupying low-lying areas in the landscape, they are highly productive ecosystems due to an excess accumulation of resources. As a result, they are utilized extensively by both humans and animals making them susceptible to disturbance-
J. Muvengwi, H.G.T. Ndagurwa / South African Journal of Botany 100 (2015) 190–194
mediated invasions (Witkowski and O'Connor, 1996; Witkowski and Garner, 2008). In this regard, soil fertility and seed bank dynamics were investigated in a seasonal wetland invaded by L. camara. Specifically, the objectives of this study were to determine if (i) the abundance of L. camara seeds in the soil changes with increasing soil depth and (ii) soil nutrient levels are different under L. camara stands than in open areas. 2. Materials and methods 2.1. Study area The study was carried out at New Gada wetland located at 17° 53′ 24′′ S and 31° 8′ 51′′ E (Fig. 1). The wetland is approximately 15 km from the Harare, Zimbabwe. Common plant species on the wetland includes Eragrostis enamoena, Nymphea sp, Scirpus raynalii, Scirpus sinutus, Typha latifolia and Juncus sp. The average rainfall ranges between 650 and 850 mm/annum, and mean annual temperatures are 9 °C for winter and 40 °C for summer (Bulton, 1995). 2.2. Soil sampling and analyses The wetland was stratified into upper, middle and bottom based on altitude following water flow direction. Three L. camara patches, at least 400 m2 in area, canopy dominated by the target species were sampled in each stratum. Five, 5 m × 5 m sampling plots were marked, one at the centre of each sampling patch and the other four located, each at 5 m in the four cardinal points. A control plot 5 m × 5 m was randomly marked at least 20 m away of each L. camara patch in order to sample reference soil. A new location was only chosen if the first control location was falling in another L. camara infested patch or was not at least
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20 m from any nearby infested patch. Four 5 m × 5 m plots were marked around central control plot similar to the L. camara infested patch. Soil was collected at a depth of 15 cm using a soil auger from the centre and the four corners of the central plot only. The other four surrounding plots were used to sample soil seed bank. The samples were thoroughly mixed to obtain a composite sample and 500 g was taken from this mixture for analysis at the laboratory. A total of 18 soil samples were tested − for NH+ 4 , NO3 , Resin-extractable P, pH, exchangeable Ca, Mg, Na and K at the Department of Research and Specialist Services, Chemistry and Soil Research Institute in Harare, Zimbabwe. In the laboratory, soil samples were air dried at room temperature before analysis. Soil pH was obtained using the CaCl2 method (Thomas, 1996; Okalebo et al., 2002). Exchangeable Ca, Mg, K, and Na were extracted using the aqua regia digestion method (Anderson and Ingram, 1993). The resulting compound was then dissolved in concentrated HCl and filtered. The solution was diluted with distilled water. Using a spectrophotometer, total Ca and Mg were determined at 0.460 nm and 0.595 nm, respectively, and flame emission was used for K and Na. Total N was determined using a Kjeldahl method (Okalebo et al., 2002). Plant available phosphorus was determined using the molybdenum-blue calorimetric method (Sibbesen, 1978). 2.3. Soil seed bank assessment Data on soil seed bank were collected post-dehiscence in April. Soil seed bank samples were collected from four random positions in each 5 m × 5 m plot using a ridged steel quadrat, 30 cm × 30 cm in area, with lines curved every centimetre to indicate depths (Witkowski and Garner, 2000). Samples were air-dried, spread onto large trays and seeds picked-out using forceps. Firm seeds were counted and firmness determined by pinching with a forceps (Shaukat and Siddiqui, 2004).
Fig. 1. Location of New Gada wetland in Epworth, Zimbabwe and the experimental plots.
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450
Ninety eight percent of the samples collected deeper than 15 cm had no seed; therefore, we ended our assessment at 15 cm.
c
400 350
Data were tested for normality and homogeneity of variance and transformed as necessary. Independent t-test was used to compare soil chemical properties between L. camara infested plots and the reference soil. Data on seed bank densities from different soil depths and wetland strata (upper, middle and bottom) were analysed using a balanced three-way ANOVA. Data were presented as mean ± standard error (SE). Post-hoc multiple comparison of means was conducted using Tukey honest significance difference (Tukey HSD) to detect differences among slope positions and depths levels. All tests were significant at p b 0.05.
Seeds m-2
2.4. Data analysis
300 250
b a
200 150 100 50 0 upper
middle
bottom
Fig. 2. Mean ± SE seed density in the different sections of the wetland. Means not sharing a common letter are significantly different (Tukey HSD, p b 0.05).
3. Results 3.1. Soil chemical properties Soil from L. camara invaded patches had significantly higher levels of Ca, Mg, Na, and NH4+ compared to the reference soil on the wetland by factors of 8.5, 4.8, 2.5 and 3.1, respectively (Table 1). However, reference soil had superior concentration of NO− 3 and available P compared to L. camara invaded patches by factors of 3.7 and 1.6, respectively (Table 1). There was no significant difference (p N 0.05) in K concentration between L. camara invaded patches and reference sites on the wetland. Soil pH was significantly lower (p b 0.001) in L. camara infestations compared to reference soil. 3.2. Seed density and vertical distribution A total of 3576 seeds were sampled on the wetland and of these 26%, 32% and 42% were sampled in the upper, middle and bottom sections of the wetland, respectively. Consequently, seed density significantly differed with slope position (F2, 126 = 84.11, p b 0.001) with greater densities at the bottom slope, followed by the middle slope and least at the upper slope (Fig. 2). A similar pattern was observed at each soil depth (Table 2). As a result, there was a significant interaction (F 2, 126 = 16.86, p b 0.001) between slope position and depth on seed density. Depth had a significant influence (F2, 126 = 1175.21, p b 0.001) on seed density, with a consistent decline in seed density with increasing depth across all sections of the wetland (Table 2). Most of the seeds were located in the top 5 cm of the soil profile, declining to almost zero at depths of 10 to 15 cm with 98% (44/45) of the samples collected below 15 cm containing no seed. 4. Discussion 4.1. Soil nutrient content Invasive alien plants have been shown to alter soil nutrient pools, salinity, moisture, and pH in many ecosystems (Ehrenfeld, 2003; Osunkoya and Perrett, 2011). Consistent to the general trend of increased nutrient pools in topsoil in response to invasion reported in other studies (Hobbs, 1992; Christian and Wilson, 1999), soil nutrient
concentrations increased by a factor of 2.5 for Na, 3.1 for NH+ 4 , 4.8 for Mg up to 8.5 for Ca. These changes in the concentration of base cations may alter the distribution and concentration of base cations at soil exchange surfaces potentially affecting cation exchange processes, soil pH, and soil organisms (Reich et al., 2005). In contrast, the concentration of NO− 3 and available P was lower on L. camara invaded patches than on un-invaded patches, which suggests that there is likely a higher plant uptake of these elements owing to the high plant biomass on L. camara invaded patches than on un-invaded patches. Additionally, invasive alien plants are also associated with reduced soil moisture content (Ehrenfeld, 2003). Thus, considering that reduced soil moisture content can suppress nitrification rates (Paul and Clark, 1989), it is possible that the low soil NO− 3 concentration on L. camara invaded patches is likely due to soil moisture suppression on nitrification. However, we did not examine this possibility, which suggests future research directions. The high concentration of NO− 3 and available P on un-invaded patches suggests that there is high likelihood that L. camara will proliferate on the wetland since systems with abundant resources particularly those that limit plant growth are susceptible to invasion (Davis et al., 2000). The concentration of K was similar between L. camara invaded patches and reference sites implying that invasion did not affect the prior concentration of K on these soils. The soil pH was significantly lower (p b 0.001) in L. camara infestations compared to un-invaded patches contrary to findings of other studies (Gentle and Duggin, 1997). This change in pH may have important implications for many soil properties since soil pH is central to many soil processes (e.g., decomposition Stursova et al., 2006; nitrification Seymour et al., 2014). Small changes in pH have been shown to result in large effects on soil properties (e.g., denitrification Stevens et al., 1998; Šimek and Cooper, 2002; sulphate adsorption of mineral soil Nodvin et al., 1986; fungi/bacteria ratio Baath and Anderson, 2003). Thus, the effect of pH on soil properties may be considered, even if the actual pH difference is rather small. This suggests that differences in soil nutrient concentrations observed between L. camara invaded patches and un-invaded patches were probably due to differences in pH between treatments. For example, the low NO− 3 recorded on L. camara invaded patches could be attributed to slowed nitrification due to a decline in pH (Stevens et al., 1998; Šimek and Cooper, 2002; Seymour et al., 2014). The concentration of base cations of most soils decreases when soil pH decreases (Zhang et al., 2013). In contrast, we
Table 1 Mean ± SE concentration of soil chemical variables measured on L. camara invaded patches and reference soil.
Lantana Reference t-Value p-Value
pH
K (me %)
Ca (me %)
Mg (me %)
Na (me %)
P (mg/kg)
NH+ 4 (ppm)
NO− 3 (ppm)
6.1 ± 0.1 7.0 ± 0.09 −5.9 0.001
0.4 ± 0.08 0.4 ± 0.07 0.3 0.745
11.9 ± 2.1 1.4 ± 0.2 4.8 0.0001
0.4 ± 0.07 1.9 ± 0.3 5.5 0.0001
0.2 ± 0.02 0.08 ± 0.007 4.8 0.01
6.4 ± 0.6 10.3 ± 0.5 −1.5 0.01
11.6 ± 0.7 8.2 ± 0.6 0.6 0.01
11.9 ± 1 43.9 ± 0.7 5.2 0.0001
J. Muvengwi, H.G.T. Ndagurwa / South African Journal of Botany 100 (2015) 190–194 Table 2 Mean ± SE of seed density (m−2) recorded at three depths on three slope positions of the wetland. Different superscript letters in the the same column (a, b and c) and same row (A, B and C) indicate significant differences between means at p b 0.05, based on Tukey HSD test. Depth
0–5 cm 5–10 cm 10–15 cm p-Value
Slope position Upper
Middle
Bottom
p-Value
432.6 ± 14.6aA 260.0 ± 9.3bA 2.2 ± 1.2cA b0.001
523.7 ± 22.8aB 308.1 ± 16.0bB 6.7 ± 1.8cB b0.001
657.0 ± 18.5aC 438.5 ± 14.0bC 20.0 ± 4.1cC b0.001
b0.001 b0.001 b0.001
found that the concentration of base cations (Ca, Na and Mg) increased with invasion suggesting that other factors such as litterfall, nutrient uptake and soil moisture availability which change with invasion could explain the observed differences in soil nutrient concentrations (Ehrenfeld, 2003). Additionally, animals and frugivorous birds visiting fruiting L. camara plants may concentrate nutrients through faecal droppings, which may also lead to elevated nutrients in invaded patches. While changes in soil nutrient concentrations occur with exotic invasions, the general direction of change is unpredictable because findings have been varied among studies (Ehrenfeld, 2003). Similarly, in this study, there was an increase (Ca, Mg, Na and NH+ 4 ), a decrease (NO− 3 and P) or there was no change (K) in concentration of soil nutrients with L. camara invasion. This suggests that effects of IAP on soil properties depend on the element under examination. Alternatively, it could be because soil cores, as used in this study, are point samples that miss nutrient dynamics over time. Thus, further research is required to establish the temporal variation in soil nutrient concentrations with invasion. 4.2. Soil seed bank Seed densities reported here, ranged from 2 to 657 seeds m−2, are greater than those reported by Vivian-Smith et al. (2006). The high seed quantity recorded in this study can be attributed to L. camara fruiting at least twice in a year due to availability of resources on the wetland such as moisture, light and soil nutrients (Duggin and Gentle, 1998; Day et al., 2003). Consistent with findings of other studies (García-Fayos and Cerdà, 1997; Jones and Esler, 2004), we found that seed density increased down gradient on the wetland. It is likely that the soil seed bank is eroded down gradient from upper sections of the wetland and end up concentrating in the lower sections of the wetland. As a result, soil seed bank density was higher in lower slope microsites that trap eroded soil than in the upper slopes. This may limit L. camara recolonization on the upper slopes since seed loss is a limiting factor on the revegetation of eroded slopes (García-Fayos and Cerdà, 1997; Jones and Esler, 2004). In this regard, management and eradication efforts to control L. camara invasion should focus on the lower slopes of the wetland. Seed density decreased with depth across all sections of the wetland consistent with findings of other studies (Witkowski and Wilson, 2001; Wilson and Witkowski, 2003; Sandrine et al., 2006). As a result, most of the seeds were located in the top 5 cm of the soil profile. We attribute the decline in seed density with soil depth to very little vertical dispersion of seed in the soil-by-soil transport or soil organisms. Alternatively, the fine texture of the soil likely reduced seed movement through the soil profile by reducing the action of percolating water or did not facilitate seed penetration into the soil profile as also suggested by Hopkins and Graham (1983). High seed density in the 0 to 5 cm has important implications for the spread of L. camara in the wetland since the buried seeds are likely to be brought to the surface with minimal levels of disturbance. Disturbances lead to increased germination of L. camara seeds from soil seed bank because disturbance increases resource availability such as light and nutrients (Duggin and Gentle, 1998). Additionally,
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although seed density declined with soil depth, the presence of seed at depths below 10 cm suggests that L. camara may have a persistent seed bank in this wetland. A persistent seed bank together with the high concentration of nutrients is also likely to predispose the wetland to continued invasion as well as suggest that L. camara may be difficult to eradicate on this wetland. However, buried seeds face several mortality factors including decay caused by bacterial and fungal microorganisms (Baskin and Baskin, 1998; Wagner and Mitschunas, 2007). Thus, the potential to germinate depends on the viability of the seed, which was not tested, and suggest future research directions. In conclusion, L. camara invasion altered the concentration of soil nutrients but the direction of change was dependent on the nutrient being examined with an increase (Ca, Mg, Na and NH+ 4 ), a decrease (NO− 3 and P) and no change (K) recorded in the concentration of soil nutrients with L. camara invasion. Seed density decreased with soil depth and altitudinal gradient, which suggested a surface-soil persistent seed bank on the lower slopes of the wetland. Collectively, these results suggest that the wetland may still be predisposed to continued invasion by L. camara particularly on the lower slopes of the wetland, and thus these areas should be the focus of management and eradication efforts. Acknowledgements The authors are indebted to the reviewers for their time and effort that allowed us to increase the clarity and quality of our work. References Anderson, J.M., Ingram, J.S.I., 1993. Tropical soil biology and fertility: a handbook of methods, second ed. CAB International. Wallingford, UK, pp. 221. Baath, E.T., Anderson, H., 2003. Comparison of soil fungal/bacterial ratios in a pH gradient using physiological and PLFA-based techniques. Soil Biology and Biochemistry 35, 955–963. Baskin, C.C., Baskin, J.M., 1998. Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination. 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