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Effects of pre-planting treatments on the initial establishment success of indigenous grass seedlings planted into a degraded Aristida junciformis-dominated grassland
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South African Journal of Botany 2002, 68: 362–369 Printed in South Africa — All rights reserved
SOUTH AFRICAN JOURNAL OF BOTANY ISSN 0254–6299
Effects of pre-planting treatments on the initial establishment success of indigenous grass seedlings planted into a degraded Aristida junciformisdominated grassland R Wiseman1, CD Morris2* and JE Granger1 1
School of Applied Environmental Sciences, University of Natal, Private Bag X01, Scottsville 3209, South Africa Range and Forage Institute, Agricultural Research Council, Private Bag X01, Scottsville 3209, South Africa * Corresponding author, e-mail: [email protected]
2
Received 15 December 2000, accepted in revised form 21 January 2001
The effects of various manipulations to facilitate the establishment of indigenous grasses planted into degraded Aristida junciformis subsp. junciformis-dominated grassland were investigated. Four treatments — complete clearing, a single spring burn, applications of Roundup®, and undisturbed (control) — were applied to replicated plots, with half of each plot mown before planting. Themeda triandra and Tristachya leucothrix seedlings were planted into each plot. Post-planting survival of the planted seedlings was uniformly high (>80%), except in the poisoned, unmown plots where residual herbicide appears to have significantly lowered seedling survival (<35%). Seedling mortality over the remainder of the growing season and through winter
was low (<10%). Seedlings planted into swards killed with herbicide had a greater basal area and more tillers than did those planted amongst living adult plants. However, basal cover of the sward in the poisoned plots a year after planting was significantly lower (<6%) than in other treatments (14–19%). The cost of implementing pre-planting treatments and planting the seedlings was high and the method is labour-intensive. It is therefore recommended that a pre-planting burn, which temporarily reduces the vigour of the surrounding sward and is economically viable, be applied before planting, and that the cost and effectiveness of strip-planting be investigated.
Introduction Large areas of mesic grassland in the south-eastern coastal hinterland and mistbelt of KwaZulu-Natal have been invaded by Aristida junciformis Trin et Rupr. subsp junciformis. This species usually enchroaches once populations of palatable perennial grasses, such as Themeda triandra Forssk and Tristachya leucothrix K. Schum., have been depleted or eliminated by selective grazing (Tainton 1972, Morris and Tainton 1993). The Natal Mistbelt and ’nGongoni grasslands have been particularly severely affected by A. junciformis invasion (Acocks 1988) and today less than three per cent of the Short Mistbelt Grassland remains in pristine condition (Granger and Bredenkamp 1996). Invasion by A. junciformis represents a severe form of rangeland degradation as this species is highly unpalatable, providing limited forage only after a burn (Theron and Booysen 1968, Venter 1968). Apart from considerably reducing livestock carrying capacity, encroachment of A. junciformis is often accompanied by a reduction in grass basal cover and increased soil erosion (Johnson 1989). Once A. junciformis has become established it is difficult to eradicate. The relative abundance of any extant palatable grasses (e.g. T. triandra) can be increased by resting and
burning, although this process can take more than ten years (Morris et al. 1992). Edwards et al. (1979) achieved some degree of control over the spread of A. junciformis by various grazing regimes. However, Johnson (1989) noted that no significant success in reversing the increase in A. junciformis dominance has been achieved by grazing or fire management alone. Various attempts to actively seed desirable grass species into degraded swards have resulted in limited success (Johnson 1989, Granger 1999). Poor survivorship, germination and emergence of surface-scattered T. triandra seeds have been attributed to high levels of innate dormancy and to depletion of viable seeds by desiccation, fire and predation by insects, termites, rodents and birds (Booysen 1981, Sindel et al. 1993, Everson 1994, O’Connor 1997, Granger 1999). Another disadvantage of overseeding is the wastage of seed which occurs when seeds get caught in existing vegetation. Edwards (1970) noted that up to 30% of seed broadcast over existing veld was caught by the vegetation, thus preventing germination. Pelleting seed with soil, compost, lime, flour or fertiliser before sowing to reduce depletion and increase germination increases cost and effort without sub-
South African Journal of Botany 2002, 68: 362–369
stantially enhancing seeding success (Edwards 1966, Booysen 1981). Similarly, feeding seed mixed with maize to cattle as a means of distributing seed (Edwards 1966) and attempts to create ‘safe’ germination microsites for seeds by disturbing the soil by cattle trampling prior to oversowing (Edwards 1970) have not proved effective enough for widespread application. The failure of attempts to rehabilitate Aristida junciformis-dominated rangeland by overseeding and sod-seeding palatable species has also been attributed to competitive exclusion of seedlings by A. junciformis tufts (Edwards et al. 1979). The use of seedlings of indigenous palatable perennial grasses, rather than their seeds, for rehabilitating degraded grassland is a recent innovation. Granger (1999) successfully established populations of T. triandra and Heteropogon contortus (L.) Roem. & Schult. in the Kamberg Nature Reserve (KwaZulu-Natal) by planting seedlings. These indigenous species were chosen because they are productive and palatable, and were deemed more suited to local conditions than many exotic species (Sindel et al. 1993). Further, it is unlikely that, once established, they would exclude other grass and herbaceous species, unlike the persistent monospecific stands of other species, e.g. Eragrostis curvula (Schrad.) Nees, used to revegetate disturbed areas in the uplands of KwaZulu-Natal (Granger 1999). Plant-plant interactions, chiefly competition, structure grasslands by influencing seedling recruitment, survival and growth of plants (Lauenroth and Aguilera 1998). The competitive advantage of extant plants over recruits is suggested to be the reason why seedling establishment in grassland is rare (Lauenroth and Aguilera 1998, O’Connor and Everson 1998) or completely absent without removal or disturbance of existing plants (Potvin 1993). In local grasslands, competition from existing vegetation is the main constraint to the survival and growth of seedlings (O’Connor and Everson 1998, Van Zyl 1998). Therefore, Van Zyl (1998) recommended that the canopies of adult plants need to be regularly severely reduced to allow seedlings to naturally establish. Similar treatment of the sward may be required when planting seedlings of preferred species into degraded grassland to ensure their establishment. This study investigated various methods of manipulating Aristida junciformis-dominated swards to reduce the possible competitive influence of adult plants on planted seedlings. Pre-planting manipulations of extant grasses ranged from (1) complete removal of all plants, (2) killing plants with herbicide to eliminate active competition for resources (nutrients, water), and (3) temporary reducing the vigour of the sward by spring burning (Tainton et al. 1977). Half of each treated sward was mown before planting to provide a favourable light regime for seedling growth. Survivorship and growth of seedlings during the first growing season, and basal cover in each treatment a year later, were measured to evaluate the success of pre-planting treatments. It was hypothesised that seedlings planted into swards manipulated in ways to reduce the competitive influence of individual plants and overall standing biomass (Foster 2000) would survive and grow better than those planted into untreated grassland. The relative cost and prac-
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ticality of different planting methods were evaluated to provide recommendations for rehabilitation. Materials and Methods The study site, a four-hectare field dominated by A. junciformis (>30%) with lesser amounts of Trachypogon spicatus (L. F.) Kuntze, T. triandra, T. leucothrix, Eragrostis spp. and other grasses and forbs, was situated within Short Mistbelt Grassland (Acocks Veld Type 45, Granger and Bredenkamp 1996) on the Cedara Agricultural Development Institute Research Farm in KwaZulu-Natal (29°33’45” S, 30°15’08” E; 973m a.s.l.). Mean annual rainfall at Cedara is 865mm, of which approximately 84% falls in summer (October to April). Mean summer and winter temperatures at Cedara are 18.5°C and 4.6°C, respectively (Institute of Soil, Climate and Water, Cedara). The site, on a north north-west aspect, had a uniform slope of 5° with deep (>1m), acid (KCL pH = 4.34), infertile (5.67±1.03cmol l-1 total cations) Hutton soils (Soil Classification Working Group 1991). The area had been grazed intensively but sporadically by cattle and other herbivores (hares, grey duiker, common reedbuck) in the past. Four different (whole-plot) treatments were applied to twelve plots (30 x 30m), replicated three times in a completely randomised design: (1) no disturbance (control); (2) spring head-burn in early September 1998 (springburn); (3) three applications of Roundup®, (5% concentration at ca. 50lha-1) approximately two months, one month and five days prior to planting to kill all existing vegetation (poisoned); and (4) complete clearing of the plot using a tractor-drawn scraper followed by a rotovator to remove all vegetation previously poisoned with Roundup® (cleared). Except for the cleared plots, each plot was further divided into two sub-plots and a randomly-chosen sub-plot mown to ca. 100mm in early December 1998 and January 1999 (just prior to planting) with a sickle-bar mower and then raked to remove all cut herbage. Although the competitive pressure exerted by extant plants on seedlings was not directly measured, nor components of competition (aboveground competition for space and light, root competition for nutrients, water) elucidated, pre-planting manipulations provided a range of growing environments for planted seedlings: from those with many vigorous (control) or no adult neighbours (cleared) to those whose planted amongst grasses killed (poisoned) or just temporarily weakened by a spring burn. In early January 1999, approximately 13 000 T. triandra seedling plugs, including a small but indistinguishable proportion of T. leucothrix seedlings, were planted by hand into the sub-plots, at an approximate rate of 250 plants person-1 hour -1. A hand-held pick was used to make a shallow hole into which each seedling was planted. Seedlings were planted into the sward wherever a suitable-sized gap (approximately 100–150mm) in the sward existed. Depending on the basal cover of the original sward, between 95 and 190 seedlings were planted within the central 100m2 (10 x 10m) region of each whole plot (50m2 per sub-plot). In the completely cleared plots, seedlings were planted at
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150mm intervals to create a density commensurate with the total grass density of the surrounding veld. Most planted seedlings were not watered, although those in one of the cleared plots, planted at midday under conditions of high soil and air temperature, were watered once to decrease wilting which had begun to occur. Survivorship and performance of planted seedlings over the remaining six months of the growing season were assessed as follows. At the end of February 1999, postplanting survival of the planted seedlings was determined. Up to 100 seedlings (depending on survivorship) in each sub-plot were subsequently marked with wire pegs. Of these seedlings, 30 were randomly selected and labelled and their basal circumference, two height measures — the height of the tallest leaf, and the height below which 80% of the leaves occurred — and number of tillers (of twenty seedlings) were recorded at the start (February, t0) and end (June, t4) of the measuring period. Thirty seedlings, randomly selected from within the marked seedlings, were similarly measured at the end of March (t1), April (t2) and May (t3) 1999. Seedlings that were grazed by vagrant duiker and reedbuck were noted and no height measures were recorded for these individuals. The percentage survivorship of the marked seedlings was determined at times t1, t2, t3 and t4 and in August after the most severe frosts of winter. Basal cover of the sub-plots was determined at the height of the second growing season (December 1999/January 2000). The mean point-plant distance and the mean diameter of the tuft nearest to a number of points, systematically located with a metal spike, were recorded to calculate the total basal area (%) of grasses in each sub-plot (Hardy and Tainton 1993). The number of points per sub-plot varied from 50 to 100, depending on plant density. Analysis of variance was used to assess treatment effects on post-planting survival and end of season survivorship. A single error term was used in the analysis as the split plot treatment (mowing) was not applied to all whole plots. Counts of live plants were transformed with a double arcsine transformation (Freeman and Tukey 1950) to normalise the data and to stabilise variance for ANOVA (Zar 1996). Main and interaction effects of whole-plot and sub-plot treatments on the basal circumference of seedlings over time (month since first performance analysis) were investigated using Residual Maximum Likelihood (REML) (Genstat 1995), excluding the cleared treatment. Repeated measures analysis was not employed as a different random sample of tillers was measured each time. Treatment effects on seedling tiller number at the end of the growing season (t4) were assessed using analysis of covariance, with original tiller number (t0) as a covariate. Due to excessive grazing of some of the plots by vagrant antelope and lagomorphs, the height of some plants could not be measured and no statistical analyses were performed on the remaining height data. Treatment effects on basal cover and its components (mean tuft size, mean pointplant distance) were assessed with ANOVA. The costs per hectare of implementing the various treatments were calculated, including: (1) herbicide costs and application rates; (2) seedling and labour costs (TopCrop/Superlawn nursery); (3) tractor and machinery costs (Economics Department, Cedara Agricultural
Wiseman, Morris and Granger
Development Institute); and (4) burning costs (Economics Department, Cedara Agricultural Development Institute and Ukulinga Research Farm, University of Natal, Pietermaritzburg). All treatment costs were calculated using costs of machinery actually used in this trial, except for the mowing and raking which were done manually, but costed more practically for tractor-drawn machinery. Results Seedling survival immediately after planting was high (82–98%) (Figure 1), except for seedlings in the poisonedmown treatment which had a significantly lower (P<0.05) post-planting survivorship of 32±29.9%. After this initial period, mean survivorship of the seedlings remained high and fairly stable over the remainder of the growing season, decreasing gradually towards winter (Figure 2). Survivorship levelled off at between 96–98% of the original surviving cohort after six months. Those seedlings in the controlunmown plots, however, showed a sharper increase in mortality during winter, when survivorship fell from 96% to 91%. Seedlings in the poisoned-unmown treatment showed an anomalous increase in mean survival between June and August (Figure 2). There were, however, no significant differences between mean survivorship of seedlings under the different sub-plot treatments at the end of August (P=0.116). The effects of sward manipulations on seedling performance, as indexed by changes in tuft basal circumference over time, were highly variable (results not shown). All whole-plot and sub-plot main effects and first and second order interactions were significant (P<0.001), except for the sub-plot by time interaction (P>0.05). This temporal inconsistency in treatment effects is partly attributable to variable performance patterns: seedling size in some treatments remained constant (control), others increased gradually in girth after planting (e.g. poisoned), while the mean seedling circumference appeared to decline slightly with time in certain sub-plots (e.g. springburn). The only outstanding feature of these performance data is that seedlings that survived planting in plots where existing vegetation was removed or killed started off larger than seedlings in other treatments and maintained this size advantage (+60–100 mm) over the remainder of the growing season. The mean basal diameter of seedlings in all other treatments did not differ significantly (P>0.05) from those in the untreated sward. At the end of the growing season (June), treatments differed (P<0.001) in the mean number of tillers per plant. The initial number of tillers after planting, used as a covariate, had a highly significant influence (P<0.001) on the final number of tillers. The mean number of tillers in each sub-plot treatment at the end of June, adjusted for the initial tiller number, is presented in Figure 3. Seedlings in the poisoned and cleared treatments had more tillers (P<0.05) than those in the other treatments. Mowing did not appear to affect tillering in the control, poisoned or springburn plots. Seedlings in the springburn, control and poisonedunmown plots exhibited etiolated growth, resulting in tufts with the tallest leaves and greatest average plant height (Figure 4). The seedlings planted in the poisoned-mown and cleared plots showed the least vertical growth.
South African Journal of Botany 2002, 68: 362–369
Unmown ab
80
ab a
ab ab
Poisoned, Unmown Control, Unmown Cleared Springburn, Mown
Mown b
Poisoned, Mown Control, Mown Springburn, Unmown
100
c
60
98 SURVIVAL (%)
SURVIVAL (%)
100
365
40 20
Poisoned
Cleared Springburn Control WHOLE-PLOT TREATMENT
96
94
92 Figure 1: Mean post-planting survival (±s.e.) of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute. Treatments with letters in common are not significantly different at the 5% level (LSD test)
By the middle of the second growing season, treated swards differed significantly in mean tuft size (F = 6.699, P<0.05), mean point-plant distance (F = 19.915, P<0.0001) and overall basal cover (F = 14.945, P<0.0001). The poisoned plots had a particularly sparse sward with mean pointplant distances larger than 80mm, in contrast to the dense swards of all other treatments where the mean size of the (bare) point-plant distance between tufts was less than 40 mm (Figure 5). The cover established by planting the cleared plots (14.4±0.18%) was commensurate with that of the undisturbed control (14.9±0.28%). Mean tuft diameter was notably large in those treatments where vegetation had been cleared (61±1.0mm) or plants poisoned then mown (51±2.9mm) (Figure 5). The cost of applying the different treatments to the original sward ranged from R124ha-1 for the mowing and raking to R4 810ha-1 for complete clearing (including initial poisoning with Roundup®) (Table 1). However, the total cost per hectare of applying pre-planting treatments, labour for planting and seedlings was high, ranging from R4 082ha-1 for planting seedlings into a mown and raked area to R61 690ha-1 for a completely cleared sward (assuming planting density of 36 seedlings m-2), of which >85% comprised seedling costs (R0.15 per seedling). Discussion Seedling establishment and performance On the whole, post-planting survival of T. triandra and T. leucothrix seedlings planted into A. junciformis-dominated grassland was good, except in swards treated with herbicide and left unmown. Grazing of newly-planted seedlings, especially in the cleared plots, by vagrant antelopes and lagomorphs (cattle were excluded from the site) could have contributed to
90
Feb.
Mar.
Apr.
May
Jun.
Aug.
MONTH Figure 2: Mean percentage survival of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute
the limited post-planting mortality as some seedlings were uprooted, presumably by herbivores, and some bare patches were created in the cleared plots apparently by the rolling of large grazing animals. Herbivory may therefore counteract the advantage that seedlings acquired by growing in areas relatively free of competition from the original sward species (Bergelson 1990a). Therefore, grazing animals should as far as possible be excluded from the planted area during the growing season after planting to allow seedlings to establish and build up tillers and energy reserves. The poor post-planting survival (32%) of seedlings planted into the poisoned, unmown plots could possibly be attributed to two causes. The most likely agent of mortality was residual herbicide still active on the original vegetation at the time of planting which could have washed off onto the newly planted seedlings during subsequent precipitation. Seedlings remaining alive in the poisoned-unmown treatment were generally found on the edges of the plots where a few other plants from the original sward were unaffected by the poison. Alternatively, standing dead material could have increased seedling mortality by reducing light for growth, increasing surface moisture and consequently fungal infections, and providing a physical barrier to growth (Goldberg and Werner 1983). Other studies have noted a similar negative effect of litter and leaf mulch on grass seedling survivorship (Bergelson 1990b, Fowler 1990, Van Zyl 1998). Irrespective of the explanation for high mortality in the poisoned-unmown treatment, it appears important to remove residual dead plant material and herbicide prior to planting to ensure seedling survival.
Wiseman, Morris and Granger
a
15
a
b
10
b
b
b
mo Sp rin wn gb urn ,u nm ow Sp n rin gb urn ,m ow n
Co ntr ol,
un mo wn
Co ntr ol,
Cle are d
5
TREATMENT Figure 3: Mean tiller number at the end of June of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute. Treatments with letters in common are not significantly different at the 5% level (LSD test)
MEANT POINT-PLANT DISTANCE (mm)
a
MEAN TUFT DIAMETER (mm)
20
Po iso ne d, un mo wn Po iso ne d, mo wn
MEAN NUMBER OF TILLERS
366
65
a)
55 45 35 25 15
120
b)
100 80 60 40 20
450
24
b)
BASAL COVER (%)
LEAF HEIGHT (mm)
350
250
150
Sp rin gb ur n, m ow n
Sp rin gb ur n, un m ow n
C on tro l, m ow n
C on tro l, un m ow n
C le ar ed
m ow n Po is on ed ,
Po is on ed ,
un m ow n
50
Figure 4: Mean height (±s.e.) in June of the tallest leaf (open bar) and mean tuft height (below which 80% of the leaves occur) (solid bar) of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute
20 16 12 8 4 0 SB/U
CLEAR CON/M POIS/M SB/M CON/U POIS/U
Figure 5: Mean tuft diameter (a), point-plant distance (b), and basal cover (c) of swards at Cedara Agricultural Development Institute into which seedlings of Themeda triandra and Tristachya leucothrix were planted after various sward treatments. SB/U = springburnunmown, SB/M = springburn-mown, CLEAR = cleared, CON/U = control-unmown, CONM = control-mown, POIS/U = poisonedunmown, POIS/M = poisoned-mown. (±s.e. within box, 95% confidence interval within range)
South African Journal of Botany 2002, 68: 362–369
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Table 1: Labour, time and cost of applying pre-planting and seedling planting treatments to an Aristida junciformis-dominated sward at Cedara Agricultural Development Institute, 1999. (Data supplied by the Economics Department, Cedara) Treatment
Treatment components
Number of labourers needed per ha
Poison
Roundup®
Variable – depends on labour available 8 (incl. 2 drivers)
Burn Clear
Mow & Rake
Roundup® Scrape Rotovate Mow Rake
1 1 1 1
(driver) (driver) (driver) (driver)
In montane grassland, survivorship of seedlings of T. triandra has been demonstrated to follow a Deevey Type IV survivorship curve, with a high morality in the juvenile stage and decreasing probability of mortality over time (Everson 1994). Similarly, in our trial, those seedlings that did not die immediately after planting were able to establish and largely survive (>90%) the growing season and subsequent winter. Winter is generally the period of greatest mortality of grass seedlings (Samuel and Hart 1992, O’Connor and Everson 1998). For example, the mortality risk for establishing T. triandra seedlings in montane grassland during the first winter following establishment was 60%, the high rate of death attributed to frost heave and desiccation (Everson 1994). In our trial, deaths during winter were minimal, except in the untreated sward where the greatest die-off occurred during winter, and where the number of surviving seedlings appeared to be declining generally at a faster rate than in other treatments. Aristida junciformis tufts are reported to strongly compete for resources in their immediate vicinity, thereby inhibiting seedling establishment (Van Zyl 1998) and growth of their neighbours (Morris and Tainton 1993). Similar asymmetrical competition between established tufts and emerging seedlings operates in pristine montane grassland, where T. triandra seedlings that established in grassland with few competitors showed increased growth, vegetative reproduction and survival (Everson 1994). There is some evidence from our trial that competition from existing vegetation does play a role in determining the establishment success of planted grass seedlings, and the hypothesis that competitive relationships between established plants and seedlings can be manipulated to favour seedling establishment is supported. However the exact nature of such competition, and mechanisms whereby it operates, have not been elucidated in this study. Removal or poisoning of existing plants enhanced seedling growth, resulting in bigger tufts with more tillers than those planted into undisturbed or burnt swards. This suggests that aboveground competition for space and belowground competition for nutrients and water could have operated to determine the fate and performance of introduced plants. Aboveground biomass (which is species and rainfall determined, and mediated by herbivory) is positively related to the inhibition ability of extant plants (Peart 1989, Morris and Tainton 1993, Silvertown et al.
Labour hours per ha
18.5 8.0 18.5 1.0 5.5 1.0 1.0
Total treatment cost per ha (incl. labour) (Rands) 3 335
Total cost per ha, (incl. seedlings and planting labour) (Rands) 7 293
136 3 335 1 475
4 094 61 690
124
4 082
1994). Root competition is, however, generally reported to have a greater influence on seedling dynamics than aboveground competition for space and light (e.g. Samuel and Hart 1992, Peltzer et al. 1998, Van Zyl 1998). Planted seedlings grew most vigorously in swards where root competition was absent (cleared) or minimal (poisoned), suggesting that belowground competition has an important influence on seedling dynamics in Mistbelt grassland. The effects of competition for nutrient and water may, however, be confounded with competition for light and density dependent factors. Plant density, rather than plant size, is the main determinant of the basal cover of tufted grassland (Hardy and Tainton 1993). Good basal cover increases water infiltration into the soil, and consequently plant production (O’Connor et al. 2001), and reduces surface runoff and soil erosion (Snyman 1999). Therefore, the use of herbicides before planting to favour seedling establishment (Masters et al. 2001) is detrimental in the long-term as it resulted in a sparsely vegetated sward with large bare areas between plants which could be subject to weed (and A. junciformis) invasion and soil erosion. Economic and practical considerations As noted by Granger (1999), the cost of rehabilitation using seedlings rather than seeds is high. The need for some form of pre-planting treatment to facilitate establishment adds further expense. Full removal of existing vegetation before planting, although providing ideal conditions for seedling establishment and growth, is not a practical means of rehabilitating degraded grassland because of the high mechanical and labour inputs required. Even if planted at a lower plant density (12–16 seedlings m-2 recommended), and with a discount for bulk purchases of seedlings, costs for this treatment are prohibitive. Although spring burning does not appear to facilitate seedling growth to the extent that full clearing or poisoning does, it is the cheapest, practical method for preparing the sward for planting. In tallgrass prairie, burning before planting or interseeding tallgrass prairie is recommended to reduce excessive shading of emerging seedlings (Packard and Mutel 1997). Following establishment, regular burning will be required to maintain palatable species in the sward, and such burning can favour
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palatable perennial grasses (such as T. triandra and H. contortus) over A. junciformis in the long-term (Morris et al. 1992). An alternative to planting an area completely with indigenous grass seedlings or to dispersing the planting throughout the sward, is to plant only in strips or circles within the sward. Concentrated planting in cleared strips or small areas reduces herbicide, labour and seedling costs (
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Duncan P (2000) An Experimental Approach to Improving Grassland Habitat for Wildlife in the Karkloof, KwaZulu-Natal. BSc(Agric) Research Project, University of Natal, Pietermaritzburg, South Africa Edwards PJ (1966) Veld replacement by improved grasslands in Natal. Proceedings of the Grassland Society of Southern Africa 1: 63–67 Edwards PJ (1970) Radical Veld Improvement in South Africa with Special Reference to the Highland Sourveld of Natal. PhD thesis, University of Natal, Pietermaritzburg, South Africa Edwards PJ, Jones RI, Tainton NM (1979) Aristida junciformis Trin et Rupr.: a weed of the veld. In: Neser S, Cairns ALP (eds) Proceedings of the Third National Weeds Conference of South Africa. Balkema, Cape Town, South Africa, pp 25–32. ISBN 0–86961–118–6 Everson TM (1994) Seedling Establishment of Themeda triandra Forssk. in the Montane Grasslands of Natal. PhD thesis, University of Natal, Pietermaritzburg, South Africa Everson CS, Everson TM, Tainton NM (1985) The dynamics of Themeda triandra tillers in relation to burning in the Natal Drakensberg. Journal of the Grassland Society of Southern Africa 2: 18–25 Foster BL (2000) Competition at the population level along a standing crop gradient: a field experiment in successional grassland. Plant Ecology 151: 171–180 Fowler NL (1990) The effects of competition and environmental heterogeneity on three coexisting grasses. Journal of Ecology 78: 389–402 Freeman MF, Tukey JW (1950) Transformations related to the angular and the square root. Annals of Mathematics and Statistics 21: 607–611 Genstat 5 (1995) Lawes Agricultural Trust, Rothamsted Experimental Station Goldberg DE, Werner PA (1983) The effects of size of opening in vegetation and litter cover on seedling establishment of goldenrods (Solidago spp.). Oecologia 60: 149–155 Granger JE (1999) Artificial establishment of indigenous bunch grasses from plugs. In: Eldridge D, Freudenberger D (eds) People and Rangelands: Proceedings of the VIth International Rangeland Congress. International Rangeland Congress Inc., Aithkenvale, Qld., pp 317–319. ISBN 0–9577394–1–9 Granger JE, Bredenkamp G (1996) Short Mistbelt Grassland. In: Low AG, Rebelo AG (eds) Vegetation of South Africa, Lesotho and Swaziland: A Companion to the Vegetation Map of South Africa, Lesotho and Swaziland. Department of Environmental Affairs and Tourism, Pretoria, South Africa, 50pp. ISBN 0–621–17316–9 Hardy MB, Tainton NM (1993) Towards a technique for determining basal cover in tufted grasslands. African Journal of Range and Forage Science 10: 77–81 Johnson PA (1989) Veld Degeneration Associated with an Increase in Aristida junciformis Trin et Rupr. Dominant Communities. Investigational Report No. 39, Institute of Natural Resources, University of Natal, Pietermaritzburg, South Africa Lauenroth WK, Aguilera MO (1998) Plant-plant interaction in grasses and grasslands. In: Cheplick GP (ed) Population Biology of Grasses. Cambridge University Press, Cambridge, pp 209–230. ISBN: 0–521–57205–3 Masters RA, Beran DD, Gaussoin RE (2001) Restoring tallgrass prairie species mixtures on leafy spurge-infested rangeland. Journal of Range Management 54: 362–369 Miller TE, Werner PA (1987) Competitive effects and responses between plant species in a first-year old-field community. Ecology 68: 1201–1210 Morris CD, Tainton NM (1993) The effect of defoliation and competition on the regrowth of Themeda triandra and Aristida junci-
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formis subsp. junciformis. African Journal of Range and Forage Science 10: 124–128 Morris CD, Tainton NM, Hardy MB (1992) Plant species dynamics in the Southern Tall Grassveld under grazing, resting and fire. Journal of the Grassland Society of Southern Africa 9: 90–95 O’Connor TG (1997) Micro-site influence on seed longevity and seedling emergence of a bunchgrass (Themeda triandra) in a semi-arid savanna. African Journal of Range and Forage Science 14: 7–11 O’Connor TG, Everson TM (1998) Population dynamics of perennial grasses in African savanna and grassland. In: Cheplick GP (ed) Population Biology of Grasses. Cambridge University Press, Cambridge, UK, pp 333–365. ISBN: 0–521–57205–3 O’Connor TG, Haines LM, Snyman HA (2001) Influence of precipitation and species composition on phytomass of a semi-arid African grassland. Journal of Ecology 89: 850–860 O’Connor TG, Morris CD, Marriott DJ (in press) Change in land use and botanical composition of KwaZulu-Natal’s grasslands over the past fifty years: Acocks’s sites revisited. South African Journal of Botany Packard S, Mutel CF (eds) (1997) The Tallgrass Restoration Handbook: For Prairies, Savannas and Woodlands. Island Press, Washington DC, USA, ISBN 1–559–63320–4 Peart DR (1989) Species interactions in a successional grassland. II. Colonization of vegetated sites. Journal of Ecology 77: 252–266 Peltzer DA, Wilson SD, Gerry AK (1998) Competition intensity along a productivity gradient in low-diversity grassland. American Naturalist 151: 465–476 Potvin MA (1993) Establishment of native grass seedlings along a topographic/moisture gradient in the Nebraska Sandhills. American Midland Naturalist 130: 248–261 Samuel MJ, Hart RR (1992) Survival and growth of blue grama seedlings in competition with western wheatgrass. Journal of Range Management 45: 444–448 Schramm P (1997) Hand-planted prairies. In: Packard S, Mutel CF (eds) The Tallgrass Restoration Handbook: For Prairies, Savannas and Woodlands. Island Press, Washington DC, USA, pp 217–220. ISBN 1–559–63320–4
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Silvertown J, Dodd ME, McConway K, Potts J, Crawley M (1994) Rainfall, biomass variation, and community composition in the Park Grass Experiment. Ecology 75: 2430–2437 Sindel BM, Davidson SJ, Kilby MJ, Groves RH (1993) Germination and establishment of Themeda triandra (Kangaroo Grass) as affected by soil and seed characteristics. Australian Journal of Botany 41: 105–117 Snyman HA (1999) Soil erosion and conservation. In: Tainton NM (ed) Veld Management in South Africa. University of Natal Press, Pietermaritzburg, South Africa, pp 355–380. ISBN 0–86980–974–4 Soil Classification Working Group (1991) Soil Classification: A Taxonomic System for South Africa. Department of Agriculture and Development, Pretoria, South Africa. ISBN 0–621–10784–0 Tainton NM (1972) The relative contribution of overstocking and selective grazing to the degeneration of tall grassveld of Natal. Proceedings of the Grassland Society of Southern Africa 7: 39–43 Tainton NM, Groves RH, Nash RC (1977) Time of mowing and burning veld: short term effects on production and tiller development. Proceedings of the Grassland Society of Southern Africa 12: 59–64 Theron EP, Booysen P de V (1968) The tensile properties of ten indigenous grasses. Proceedings of the Grassland Society of Southern Africa 3: 57–61 Van Zyl DD (1998) Aspects of the Invasion of Southern Tall Grassveld by Aristida junciformis subsp. junciformis Trin. et Rupr. MSc thesis, University of Natal, Pietermaritzburg, South Africa Venter AD (1968) The problems of Aristida junciformis encroachment into the veld of Natal. Proceedings of the Grassland Society of Southern Africa 3: 163–165 Zar JH (1996) Biostatistical Analysis, 3rd ed. Prentice-Hall, Upper Saddle River, N.J. ISBN 0–13–084542–6
South African Journal of Botany 2002, 68: 362–369 Printed in South Africa — All rights reserved
SOUTH AFRICAN JOURNAL OF BOTANY ISSN 0254–6299
Effects of pre-planting treatments on the initial establishment success of indigenous grass seedlings planted into a degraded Aristida junciformisdominated grassland R Wiseman1, CD Morris2* and JE Granger1 1
School of Applied Environmental Sciences, University of Natal, Private Bag X01, Scottsville 3209, South Africa Range and Forage Institute, Agricultural Research Council, Private Bag X01, Scottsville 3209, South Africa * Corresponding author, e-mail: [email protected]
2
Received 15 December 2000, accepted in revised form 21 January 2001
The effects of various manipulations to facilitate the establishment of indigenous grasses planted into degraded Aristida junciformis subsp. junciformis-dominated grassland were investigated. Four treatments — complete clearing, a single spring burn, applications of Roundup®, and undisturbed (control) — were applied to replicated plots, with half of each plot mown before planting. Themeda triandra and Tristachya leucothrix seedlings were planted into each plot. Post-planting survival of the planted seedlings was uniformly high (>80%), except in the poisoned, unmown plots where residual herbicide appears to have significantly lowered seedling survival (<35%). Seedling mortality over the remainder of the growing season and through winter
was low (<10%). Seedlings planted into swards killed with herbicide had a greater basal area and more tillers than did those planted amongst living adult plants. However, basal cover of the sward in the poisoned plots a year after planting was significantly lower (<6%) than in other treatments (14–19%). The cost of implementing pre-planting treatments and planting the seedlings was high and the method is labour-intensive. It is therefore recommended that a pre-planting burn, which temporarily reduces the vigour of the surrounding sward and is economically viable, be applied before planting, and that the cost and effectiveness of strip-planting be investigated.
Introduction Large areas of mesic grassland in the south-eastern coastal hinterland and mistbelt of KwaZulu-Natal have been invaded by Aristida junciformis Trin et Rupr. subsp junciformis. This species usually enchroaches once populations of palatable perennial grasses, such as Themeda triandra Forssk and Tristachya leucothrix K. Schum., have been depleted or eliminated by selective grazing (Tainton 1972, Morris and Tainton 1993). The Natal Mistbelt and ’nGongoni grasslands have been particularly severely affected by A. junciformis invasion (Acocks 1988) and today less than three per cent of the Short Mistbelt Grassland remains in pristine condition (Granger and Bredenkamp 1996). Invasion by A. junciformis represents a severe form of rangeland degradation as this species is highly unpalatable, providing limited forage only after a burn (Theron and Booysen 1968, Venter 1968). Apart from considerably reducing livestock carrying capacity, encroachment of A. junciformis is often accompanied by a reduction in grass basal cover and increased soil erosion (Johnson 1989). Once A. junciformis has become established it is difficult to eradicate. The relative abundance of any extant palatable grasses (e.g. T. triandra) can be increased by resting and
burning, although this process can take more than ten years (Morris et al. 1992). Edwards et al. (1979) achieved some degree of control over the spread of A. junciformis by various grazing regimes. However, Johnson (1989) noted that no significant success in reversing the increase in A. junciformis dominance has been achieved by grazing or fire management alone. Various attempts to actively seed desirable grass species into degraded swards have resulted in limited success (Johnson 1989, Granger 1999). Poor survivorship, germination and emergence of surface-scattered T. triandra seeds have been attributed to high levels of innate dormancy and to depletion of viable seeds by desiccation, fire and predation by insects, termites, rodents and birds (Booysen 1981, Sindel et al. 1993, Everson 1994, O’Connor 1997, Granger 1999). Another disadvantage of overseeding is the wastage of seed which occurs when seeds get caught in existing vegetation. Edwards (1970) noted that up to 30% of seed broadcast over existing veld was caught by the vegetation, thus preventing germination. Pelleting seed with soil, compost, lime, flour or fertiliser before sowing to reduce depletion and increase germination increases cost and effort without sub-
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stantially enhancing seeding success (Edwards 1966, Booysen 1981). Similarly, feeding seed mixed with maize to cattle as a means of distributing seed (Edwards 1966) and attempts to create ‘safe’ germination microsites for seeds by disturbing the soil by cattle trampling prior to oversowing (Edwards 1970) have not proved effective enough for widespread application. The failure of attempts to rehabilitate Aristida junciformis-dominated rangeland by overseeding and sod-seeding palatable species has also been attributed to competitive exclusion of seedlings by A. junciformis tufts (Edwards et al. 1979). The use of seedlings of indigenous palatable perennial grasses, rather than their seeds, for rehabilitating degraded grassland is a recent innovation. Granger (1999) successfully established populations of T. triandra and Heteropogon contortus (L.) Roem. & Schult. in the Kamberg Nature Reserve (KwaZulu-Natal) by planting seedlings. These indigenous species were chosen because they are productive and palatable, and were deemed more suited to local conditions than many exotic species (Sindel et al. 1993). Further, it is unlikely that, once established, they would exclude other grass and herbaceous species, unlike the persistent monospecific stands of other species, e.g. Eragrostis curvula (Schrad.) Nees, used to revegetate disturbed areas in the uplands of KwaZulu-Natal (Granger 1999). Plant-plant interactions, chiefly competition, structure grasslands by influencing seedling recruitment, survival and growth of plants (Lauenroth and Aguilera 1998). The competitive advantage of extant plants over recruits is suggested to be the reason why seedling establishment in grassland is rare (Lauenroth and Aguilera 1998, O’Connor and Everson 1998) or completely absent without removal or disturbance of existing plants (Potvin 1993). In local grasslands, competition from existing vegetation is the main constraint to the survival and growth of seedlings (O’Connor and Everson 1998, Van Zyl 1998). Therefore, Van Zyl (1998) recommended that the canopies of adult plants need to be regularly severely reduced to allow seedlings to naturally establish. Similar treatment of the sward may be required when planting seedlings of preferred species into degraded grassland to ensure their establishment. This study investigated various methods of manipulating Aristida junciformis-dominated swards to reduce the possible competitive influence of adult plants on planted seedlings. Pre-planting manipulations of extant grasses ranged from (1) complete removal of all plants, (2) killing plants with herbicide to eliminate active competition for resources (nutrients, water), and (3) temporary reducing the vigour of the sward by spring burning (Tainton et al. 1977). Half of each treated sward was mown before planting to provide a favourable light regime for seedling growth. Survivorship and growth of seedlings during the first growing season, and basal cover in each treatment a year later, were measured to evaluate the success of pre-planting treatments. It was hypothesised that seedlings planted into swards manipulated in ways to reduce the competitive influence of individual plants and overall standing biomass (Foster 2000) would survive and grow better than those planted into untreated grassland. The relative cost and prac-
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ticality of different planting methods were evaluated to provide recommendations for rehabilitation. Materials and Methods The study site, a four-hectare field dominated by A. junciformis (>30%) with lesser amounts of Trachypogon spicatus (L. F.) Kuntze, T. triandra, T. leucothrix, Eragrostis spp. and other grasses and forbs, was situated within Short Mistbelt Grassland (Acocks Veld Type 45, Granger and Bredenkamp 1996) on the Cedara Agricultural Development Institute Research Farm in KwaZulu-Natal (29°33’45” S, 30°15’08” E; 973m a.s.l.). Mean annual rainfall at Cedara is 865mm, of which approximately 84% falls in summer (October to April). Mean summer and winter temperatures at Cedara are 18.5°C and 4.6°C, respectively (Institute of Soil, Climate and Water, Cedara). The site, on a north north-west aspect, had a uniform slope of 5° with deep (>1m), acid (KCL pH = 4.34), infertile (5.67±1.03cmol l-1 total cations) Hutton soils (Soil Classification Working Group 1991). The area had been grazed intensively but sporadically by cattle and other herbivores (hares, grey duiker, common reedbuck) in the past. Four different (whole-plot) treatments were applied to twelve plots (30 x 30m), replicated three times in a completely randomised design: (1) no disturbance (control); (2) spring head-burn in early September 1998 (springburn); (3) three applications of Roundup®, (5% concentration at ca. 50lha-1) approximately two months, one month and five days prior to planting to kill all existing vegetation (poisoned); and (4) complete clearing of the plot using a tractor-drawn scraper followed by a rotovator to remove all vegetation previously poisoned with Roundup® (cleared). Except for the cleared plots, each plot was further divided into two sub-plots and a randomly-chosen sub-plot mown to ca. 100mm in early December 1998 and January 1999 (just prior to planting) with a sickle-bar mower and then raked to remove all cut herbage. Although the competitive pressure exerted by extant plants on seedlings was not directly measured, nor components of competition (aboveground competition for space and light, root competition for nutrients, water) elucidated, pre-planting manipulations provided a range of growing environments for planted seedlings: from those with many vigorous (control) or no adult neighbours (cleared) to those whose planted amongst grasses killed (poisoned) or just temporarily weakened by a spring burn. In early January 1999, approximately 13 000 T. triandra seedling plugs, including a small but indistinguishable proportion of T. leucothrix seedlings, were planted by hand into the sub-plots, at an approximate rate of 250 plants person-1 hour -1. A hand-held pick was used to make a shallow hole into which each seedling was planted. Seedlings were planted into the sward wherever a suitable-sized gap (approximately 100–150mm) in the sward existed. Depending on the basal cover of the original sward, between 95 and 190 seedlings were planted within the central 100m2 (10 x 10m) region of each whole plot (50m2 per sub-plot). In the completely cleared plots, seedlings were planted at
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150mm intervals to create a density commensurate with the total grass density of the surrounding veld. Most planted seedlings were not watered, although those in one of the cleared plots, planted at midday under conditions of high soil and air temperature, were watered once to decrease wilting which had begun to occur. Survivorship and performance of planted seedlings over the remaining six months of the growing season were assessed as follows. At the end of February 1999, postplanting survival of the planted seedlings was determined. Up to 100 seedlings (depending on survivorship) in each sub-plot were subsequently marked with wire pegs. Of these seedlings, 30 were randomly selected and labelled and their basal circumference, two height measures — the height of the tallest leaf, and the height below which 80% of the leaves occurred — and number of tillers (of twenty seedlings) were recorded at the start (February, t0) and end (June, t4) of the measuring period. Thirty seedlings, randomly selected from within the marked seedlings, were similarly measured at the end of March (t1), April (t2) and May (t3) 1999. Seedlings that were grazed by vagrant duiker and reedbuck were noted and no height measures were recorded for these individuals. The percentage survivorship of the marked seedlings was determined at times t1, t2, t3 and t4 and in August after the most severe frosts of winter. Basal cover of the sub-plots was determined at the height of the second growing season (December 1999/January 2000). The mean point-plant distance and the mean diameter of the tuft nearest to a number of points, systematically located with a metal spike, were recorded to calculate the total basal area (%) of grasses in each sub-plot (Hardy and Tainton 1993). The number of points per sub-plot varied from 50 to 100, depending on plant density. Analysis of variance was used to assess treatment effects on post-planting survival and end of season survivorship. A single error term was used in the analysis as the split plot treatment (mowing) was not applied to all whole plots. Counts of live plants were transformed with a double arcsine transformation (Freeman and Tukey 1950) to normalise the data and to stabilise variance for ANOVA (Zar 1996). Main and interaction effects of whole-plot and sub-plot treatments on the basal circumference of seedlings over time (month since first performance analysis) were investigated using Residual Maximum Likelihood (REML) (Genstat 1995), excluding the cleared treatment. Repeated measures analysis was not employed as a different random sample of tillers was measured each time. Treatment effects on seedling tiller number at the end of the growing season (t4) were assessed using analysis of covariance, with original tiller number (t0) as a covariate. Due to excessive grazing of some of the plots by vagrant antelope and lagomorphs, the height of some plants could not be measured and no statistical analyses were performed on the remaining height data. Treatment effects on basal cover and its components (mean tuft size, mean pointplant distance) were assessed with ANOVA. The costs per hectare of implementing the various treatments were calculated, including: (1) herbicide costs and application rates; (2) seedling and labour costs (TopCrop/Superlawn nursery); (3) tractor and machinery costs (Economics Department, Cedara Agricultural
Wiseman, Morris and Granger
Development Institute); and (4) burning costs (Economics Department, Cedara Agricultural Development Institute and Ukulinga Research Farm, University of Natal, Pietermaritzburg). All treatment costs were calculated using costs of machinery actually used in this trial, except for the mowing and raking which were done manually, but costed more practically for tractor-drawn machinery. Results Seedling survival immediately after planting was high (82–98%) (Figure 1), except for seedlings in the poisonedmown treatment which had a significantly lower (P<0.05) post-planting survivorship of 32±29.9%. After this initial period, mean survivorship of the seedlings remained high and fairly stable over the remainder of the growing season, decreasing gradually towards winter (Figure 2). Survivorship levelled off at between 96–98% of the original surviving cohort after six months. Those seedlings in the controlunmown plots, however, showed a sharper increase in mortality during winter, when survivorship fell from 96% to 91%. Seedlings in the poisoned-unmown treatment showed an anomalous increase in mean survival between June and August (Figure 2). There were, however, no significant differences between mean survivorship of seedlings under the different sub-plot treatments at the end of August (P=0.116). The effects of sward manipulations on seedling performance, as indexed by changes in tuft basal circumference over time, were highly variable (results not shown). All whole-plot and sub-plot main effects and first and second order interactions were significant (P<0.001), except for the sub-plot by time interaction (P>0.05). This temporal inconsistency in treatment effects is partly attributable to variable performance patterns: seedling size in some treatments remained constant (control), others increased gradually in girth after planting (e.g. poisoned), while the mean seedling circumference appeared to decline slightly with time in certain sub-plots (e.g. springburn). The only outstanding feature of these performance data is that seedlings that survived planting in plots where existing vegetation was removed or killed started off larger than seedlings in other treatments and maintained this size advantage (+60–100 mm) over the remainder of the growing season. The mean basal diameter of seedlings in all other treatments did not differ significantly (P>0.05) from those in the untreated sward. At the end of the growing season (June), treatments differed (P<0.001) in the mean number of tillers per plant. The initial number of tillers after planting, used as a covariate, had a highly significant influence (P<0.001) on the final number of tillers. The mean number of tillers in each sub-plot treatment at the end of June, adjusted for the initial tiller number, is presented in Figure 3. Seedlings in the poisoned and cleared treatments had more tillers (P<0.05) than those in the other treatments. Mowing did not appear to affect tillering in the control, poisoned or springburn plots. Seedlings in the springburn, control and poisonedunmown plots exhibited etiolated growth, resulting in tufts with the tallest leaves and greatest average plant height (Figure 4). The seedlings planted in the poisoned-mown and cleared plots showed the least vertical growth.
South African Journal of Botany 2002, 68: 362–369
Unmown ab
80
ab a
ab ab
Poisoned, Unmown Control, Unmown Cleared Springburn, Mown
Mown b
Poisoned, Mown Control, Mown Springburn, Unmown
100
c
60
98 SURVIVAL (%)
SURVIVAL (%)
100
365
40 20
Poisoned
Cleared Springburn Control WHOLE-PLOT TREATMENT
96
94
92 Figure 1: Mean post-planting survival (±s.e.) of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute. Treatments with letters in common are not significantly different at the 5% level (LSD test)
By the middle of the second growing season, treated swards differed significantly in mean tuft size (F = 6.699, P<0.05), mean point-plant distance (F = 19.915, P<0.0001) and overall basal cover (F = 14.945, P<0.0001). The poisoned plots had a particularly sparse sward with mean pointplant distances larger than 80mm, in contrast to the dense swards of all other treatments where the mean size of the (bare) point-plant distance between tufts was less than 40 mm (Figure 5). The cover established by planting the cleared plots (14.4±0.18%) was commensurate with that of the undisturbed control (14.9±0.28%). Mean tuft diameter was notably large in those treatments where vegetation had been cleared (61±1.0mm) or plants poisoned then mown (51±2.9mm) (Figure 5). The cost of applying the different treatments to the original sward ranged from R124ha-1 for the mowing and raking to R4 810ha-1 for complete clearing (including initial poisoning with Roundup®) (Table 1). However, the total cost per hectare of applying pre-planting treatments, labour for planting and seedlings was high, ranging from R4 082ha-1 for planting seedlings into a mown and raked area to R61 690ha-1 for a completely cleared sward (assuming planting density of 36 seedlings m-2), of which >85% comprised seedling costs (R0.15 per seedling). Discussion Seedling establishment and performance On the whole, post-planting survival of T. triandra and T. leucothrix seedlings planted into A. junciformis-dominated grassland was good, except in swards treated with herbicide and left unmown. Grazing of newly-planted seedlings, especially in the cleared plots, by vagrant antelopes and lagomorphs (cattle were excluded from the site) could have contributed to
90
Feb.
Mar.
Apr.
May
Jun.
Aug.
MONTH Figure 2: Mean percentage survival of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute
the limited post-planting mortality as some seedlings were uprooted, presumably by herbivores, and some bare patches were created in the cleared plots apparently by the rolling of large grazing animals. Herbivory may therefore counteract the advantage that seedlings acquired by growing in areas relatively free of competition from the original sward species (Bergelson 1990a). Therefore, grazing animals should as far as possible be excluded from the planted area during the growing season after planting to allow seedlings to establish and build up tillers and energy reserves. The poor post-planting survival (32%) of seedlings planted into the poisoned, unmown plots could possibly be attributed to two causes. The most likely agent of mortality was residual herbicide still active on the original vegetation at the time of planting which could have washed off onto the newly planted seedlings during subsequent precipitation. Seedlings remaining alive in the poisoned-unmown treatment were generally found on the edges of the plots where a few other plants from the original sward were unaffected by the poison. Alternatively, standing dead material could have increased seedling mortality by reducing light for growth, increasing surface moisture and consequently fungal infections, and providing a physical barrier to growth (Goldberg and Werner 1983). Other studies have noted a similar negative effect of litter and leaf mulch on grass seedling survivorship (Bergelson 1990b, Fowler 1990, Van Zyl 1998). Irrespective of the explanation for high mortality in the poisoned-unmown treatment, it appears important to remove residual dead plant material and herbicide prior to planting to ensure seedling survival.
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a
15
a
b
10
b
b
b
mo Sp rin wn gb urn ,u nm ow Sp n rin gb urn ,m ow n
Co ntr ol,
un mo wn
Co ntr ol,
Cle are d
5
TREATMENT Figure 3: Mean tiller number at the end of June of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute. Treatments with letters in common are not significantly different at the 5% level (LSD test)
MEANT POINT-PLANT DISTANCE (mm)
a
MEAN TUFT DIAMETER (mm)
20
Po iso ne d, un mo wn Po iso ne d, mo wn
MEAN NUMBER OF TILLERS
366
65
a)
55 45 35 25 15
120
b)
100 80 60 40 20
450
24
b)
BASAL COVER (%)
LEAF HEIGHT (mm)
350
250
150
Sp rin gb ur n, m ow n
Sp rin gb ur n, un m ow n
C on tro l, m ow n
C on tro l, un m ow n
C le ar ed
m ow n Po is on ed ,
Po is on ed ,
un m ow n
50
Figure 4: Mean height (±s.e.) in June of the tallest leaf (open bar) and mean tuft height (below which 80% of the leaves occur) (solid bar) of Themeda triandra and Tristachya leucothrix seedlings planted into plots with different sward treatments at Cedara Agricultural Development Institute
20 16 12 8 4 0 SB/U
CLEAR CON/M POIS/M SB/M CON/U POIS/U
Figure 5: Mean tuft diameter (a), point-plant distance (b), and basal cover (c) of swards at Cedara Agricultural Development Institute into which seedlings of Themeda triandra and Tristachya leucothrix were planted after various sward treatments. SB/U = springburnunmown, SB/M = springburn-mown, CLEAR = cleared, CON/U = control-unmown, CONM = control-mown, POIS/U = poisonedunmown, POIS/M = poisoned-mown. (±s.e. within box, 95% confidence interval within range)
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Table 1: Labour, time and cost of applying pre-planting and seedling planting treatments to an Aristida junciformis-dominated sward at Cedara Agricultural Development Institute, 1999. (Data supplied by the Economics Department, Cedara) Treatment
Treatment components
Number of labourers needed per ha
Poison
Roundup®
Variable – depends on labour available 8 (incl. 2 drivers)
Burn Clear
Mow & Rake
Roundup® Scrape Rotovate Mow Rake
1 1 1 1
(driver) (driver) (driver) (driver)
In montane grassland, survivorship of seedlings of T. triandra has been demonstrated to follow a Deevey Type IV survivorship curve, with a high morality in the juvenile stage and decreasing probability of mortality over time (Everson 1994). Similarly, in our trial, those seedlings that did not die immediately after planting were able to establish and largely survive (>90%) the growing season and subsequent winter. Winter is generally the period of greatest mortality of grass seedlings (Samuel and Hart 1992, O’Connor and Everson 1998). For example, the mortality risk for establishing T. triandra seedlings in montane grassland during the first winter following establishment was 60%, the high rate of death attributed to frost heave and desiccation (Everson 1994). In our trial, deaths during winter were minimal, except in the untreated sward where the greatest die-off occurred during winter, and where the number of surviving seedlings appeared to be declining generally at a faster rate than in other treatments. Aristida junciformis tufts are reported to strongly compete for resources in their immediate vicinity, thereby inhibiting seedling establishment (Van Zyl 1998) and growth of their neighbours (Morris and Tainton 1993). Similar asymmetrical competition between established tufts and emerging seedlings operates in pristine montane grassland, where T. triandra seedlings that established in grassland with few competitors showed increased growth, vegetative reproduction and survival (Everson 1994). There is some evidence from our trial that competition from existing vegetation does play a role in determining the establishment success of planted grass seedlings, and the hypothesis that competitive relationships between established plants and seedlings can be manipulated to favour seedling establishment is supported. However the exact nature of such competition, and mechanisms whereby it operates, have not been elucidated in this study. Removal or poisoning of existing plants enhanced seedling growth, resulting in bigger tufts with more tillers than those planted into undisturbed or burnt swards. This suggests that aboveground competition for space and belowground competition for nutrients and water could have operated to determine the fate and performance of introduced plants. Aboveground biomass (which is species and rainfall determined, and mediated by herbivory) is positively related to the inhibition ability of extant plants (Peart 1989, Morris and Tainton 1993, Silvertown et al.
Labour hours per ha
18.5 8.0 18.5 1.0 5.5 1.0 1.0
Total treatment cost per ha (incl. labour) (Rands) 3 335
Total cost per ha, (incl. seedlings and planting labour) (Rands) 7 293
136 3 335 1 475
4 094 61 690
124
4 082
1994). Root competition is, however, generally reported to have a greater influence on seedling dynamics than aboveground competition for space and light (e.g. Samuel and Hart 1992, Peltzer et al. 1998, Van Zyl 1998). Planted seedlings grew most vigorously in swards where root competition was absent (cleared) or minimal (poisoned), suggesting that belowground competition has an important influence on seedling dynamics in Mistbelt grassland. The effects of competition for nutrient and water may, however, be confounded with competition for light and density dependent factors. Plant density, rather than plant size, is the main determinant of the basal cover of tufted grassland (Hardy and Tainton 1993). Good basal cover increases water infiltration into the soil, and consequently plant production (O’Connor et al. 2001), and reduces surface runoff and soil erosion (Snyman 1999). Therefore, the use of herbicides before planting to favour seedling establishment (Masters et al. 2001) is detrimental in the long-term as it resulted in a sparsely vegetated sward with large bare areas between plants which could be subject to weed (and A. junciformis) invasion and soil erosion. Economic and practical considerations As noted by Granger (1999), the cost of rehabilitation using seedlings rather than seeds is high. The need for some form of pre-planting treatment to facilitate establishment adds further expense. Full removal of existing vegetation before planting, although providing ideal conditions for seedling establishment and growth, is not a practical means of rehabilitating degraded grassland because of the high mechanical and labour inputs required. Even if planted at a lower plant density (12–16 seedlings m-2 recommended), and with a discount for bulk purchases of seedlings, costs for this treatment are prohibitive. Although spring burning does not appear to facilitate seedling growth to the extent that full clearing or poisoning does, it is the cheapest, practical method for preparing the sward for planting. In tallgrass prairie, burning before planting or interseeding tallgrass prairie is recommended to reduce excessive shading of emerging seedlings (Packard and Mutel 1997). Following establishment, regular burning will be required to maintain palatable species in the sward, and such burning can favour
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palatable perennial grasses (such as T. triandra and H. contortus) over A. junciformis in the long-term (Morris et al. 1992). An alternative to planting an area completely with indigenous grass seedlings or to dispersing the planting throughout the sward, is to plant only in strips or circles within the sward. Concentrated planting in cleared strips or small areas reduces herbicide, labour and seedling costs (
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