5. MONITORING SERIAL CHANGES IN WOODY SPECIES COMPOSITIONS, GRASS COVER AND YIELD
5.4 Discussion
Table 5.6 Results of Particle Size and CNS analyses from the topsoil (150 mm) of coastal grasslands at the end of the study period
PARTICLE SIZE1 CNS2
Total Total 4max. avail. Total
Site Clay Silt Sand
C N 3N
S N:S
C:N kg-I ha-I N kg-I ha-I
% % % % % %
la 8 5 86 0.93 0.1 11.3 1025 51 0.01 7.45
1b 15 11 74 1.36 0.1 13 1300 65 0.02 6.93
2a 9 6 85 1.02 0.1 11.3 1125 56 0.01 6.92
2b 19 6 75 2.05 0.16 12.9 1987.5 99 0.03 6.36
3a 23 21 56 2.34 0.19 12.5 2337.5 117 0.02 8.13
3b 23 21 56 1.95 0.16 11.9 2037.5 102 0.02 9.06
3c 22 21 57 1.66 0.14 11.8 1762.5 88 0.02 7.83
3d 23 22 55 2.43 0.19 12.5 2425 121 0.02 8.08
4a 22 22 56 1.98 0.17 11.6 2137.5 107 0.03 6.84
4b 21 20 58 2.4 0.21 11.5 2612.5 131 0.03 8.36
1Determinedb~gipette method bMacVicar 1991). Texture classes: clay <0.002, silt 0.02-0. 0 and sand 2.0- .02
2Total C, N and S are analysed by the Automated Dumas dry combustion method using a LECO CNS 2000 (Leco Corporation, Michigan, USA).
3N kg-I ha-1
=
(Sample density x 106) x N % (MacVicaret al. 1991)4Maximum available N kg-I ha-1
=
N ha-1 x 0.05 (Wild 1988)Permanent grassland in temperate regions contain 2 to 6 t N ha-1(Wild 1988). In comparison, coastal grasslands corresponded to the lower N limit (Table 5.6). Approximately 5% (or less) of the N content is available to plants as nitrate and ammonium ions. Glenrosa samples (sites 3 and 4) had higher N levels than regic sands. Site 2b on regic sands and all sites on Glenrosa soils had reasonable levels of 'available' N, despite being burnt annually. Carbon to N ratios for all sites were within the limits between 10 and 14, a phenomena of most soils throughout the world. The N:S ratios were also in keeping within worldwide trends of between seven and eight.
effectively controlled moderate and dense infestations of chromolaena. Eighty percent mortality is acceptable for registering herbicides for the control of specific species in South Africa (E Wolmarans pers. comm., Registrar: Act No. 36 (1983), Private Bag X343, Pretoria 0001) and the results here are comparable. From the point of cost-effectiveness fire is free, and if mortality exceeds reinfestation rates, then the method is effective, no matter how low the degree of fire-induced mortality. Infestations with grass yields ofless than 1 t ha-! can be used effectively to reduce chromolaena, provided the weed is burnt when it is most
vulnerable at the height of the dry season.
Mortality in dense infestations after slashing and burning chromolaena was better than other cultural control experiments in fallow-cropping systems where the incidence of regrowth after fire was quite high (Slaats 1995). Chromolaena has a high seed production potential and survival is favoured by rapid growth, rapid net assimilation and allocation of many resources to the shoot system (Ramakrishnan& Vitousek 1989). Chromolaena is a C3species with large biomass contributions to secondary successional communities. The short agricultural cycles in fallow-cropping systems presents unique difficulties for suppressing chromolaena under conditions that attract early successional species monopolised by C3exotics. In systems where clearing of chromolaena is to promote long cycles or states, i.e. grassland, intense fires will impoverish the chromolaena seed bank and reduce reinfestation.
Secondary weed invasions are a problem after the removal of dense infestations. Bugweed and lantana were the most important alien species responsible for secondary invasions in coastal grasslands. Slash and bum practices must be integrated with chemical control to prevent seedling establishment in situations where chromolaena is dense and grass cover is nonexistent. Follow-up control of alien invasive species is one of the most important operations in integrated weed management (Goodall & Naude 1998). Routine burning as a control strategy will remove chromolaena infestations from grasslands and maintain them free of the weed indefinitely, but this depends on the density of the parent population and soil texture. Dense infestations other than on sandy or sandy loam soils will require extra effort.
Depending on the ecosystem, the presence or absence of fire causes ecological disturbances promoting chromolaena invasions. The presence of chromolaena in indigenous forest poses a
fire hazard during the dry season (Macdonald 1983). Veld fires incinerate infestations growing on forest margins, causing severe damage to fire-sensitive forest species. New opportunities are created for chromolaena to invade the margin and understory of fire- damaged forests. Repeated fire will not effectively control the chromolaena, however, the forest ecosystem will be destroyed. Fire is therefore not an appropriate control method in indigenous forests. Coastal grasslands are seral to weedy scrub in the absence of fire.
Implementation of burning regimes will end woody plant succession and promote the establishment of grass. Fire is therefore a disturbance event in forest but a determinant of grassland. The absence of fire in grassland is seen as a process-interruption facilitating undesirable woody plant establishment.
Regic sands appeared stable and resilient to woody invasion but the window of observation was extremely narrow. Site 2 showed strong signs of forest development over a 30 year period without fire (Plate 1, c and d). Bush clumps were initiated byS. cordatum,
presumably by growing away from its dwarf shrub habit under irregular burning regimes.
The bush clump in Site 2 (Plate 5) in 1990 contained large Bridelia micrantha trees killed by fire in 1991, leaving a community of fire resistant species(s. cOl-datum, Phoenix reclinata and Strelitzia nicolai). These findings concur with von Maltitz et al. (1996) who showed that sandy ThemedalAristida coastal grasslands are seral to forest. In addition, forest occurs almost continuously along the KZN coastal sand dunes (Tinley 1985) where it has not been interrupted by cane or timber plantations.
Routine fires in coastal grasslands on Glenrosa soils killed forest precursor species but did not kill savanna species. Studies on tree survival strategies in West African savannas show that bark thickness is the main explanation for survival in intense fires (Gignoux et al. 1997;
Hochberg etal. 1994). Tree recruitment rather than adult survival, stem profile and growth rate of young trees between successive fires were the main reasons for survival in systems with annual but low intensity fires.
Simulated savanna tree dynamics in control and bum plots showed a doubling rate of tree populations every six years under fire exclusion (Hochberg et al. 1994). Annual fires merely prolonged doubling to around 30 years. Mortalities from fire had no impact on equilibrium
density. Tree survival strategies in the form of fire resistance (bark properties), or the ability to rebuild aboveground structures quickly, provided windows of opportunity for trees to shade-out and displace grass. The probability of annual fires occurring in fragmented coastal grasslands are remote. This study showed that once savanna trees established in grassland, fire alone was unable to restore the former state. Fire favoured the development of grassland vegetation but trees and shrubs that established during fire-free windows suppressed grass (cf.
Trollope 1984).
Authors in the past have intuitively argued about the role chromolaena plays in woody succession in KZN, some advocating facilitation (Cooper 1977; le Roux & le Roux 1991), while others suggest inhibition (Wilson 1995; Goodall& Erasmus 1996). Facilitation requires that chromolaena dies out once its niche is replaced by higher life forms, i.e. tree species (Connell & Slatyer 1977). Seedlings and saplings of indigenous species growing with chromolaena (Appendix C) did not prove that the alien weed promoted succession to forest. Chromolaena was persistent in 30 year successions without being replaced by other speCIes.
Inhibitor species under the Connell and Slatyer (1977) model tolerate individuals of other species that have lower competitive thresholds. Competitive exclusion would have caused mortalities of indigenous species of similar size but this was not observed prior to the first bum. Self-thinning was only observedinchromolaena. The inhibition model does not work either because native species were well represented at seedling and adult stages in 1990 (Appendix C2 - C4). Seedlings were skewed in the direction of juveniles but this may be more a factor of young woody successions in unbumt grassland than a process of inhibition.
Earlier hypotheses that the chromolaena inhibits forest regeneration (Goodall& Erasmus 1996) are possibly incorrect.
The divergence model (Huston& Smith 1987) manifests the arrival of a 'super-species' in multispecies succession. Inthis model competitive abilities are equal in early stand
development when resources are not scarce. This phase is followed by one species displaying a competitive advantage for many reasons, e.g. regenerative ability, growth rate, size,
morphology, resistance to herbivory by insects and other invertebrates. The super-species in this case is chromolaena.
Huston and Smith (1987) simulations on the removal of an earlier colonist in a successional sequence showed that species replacements are different had the colonist not been removed.
The trend was observed in sites 4a and 4b when seedlings of chromolaena were removed by hand in Site 4a and left in Site 4b. This single action, in combination with fires prescribed to both sites, resulted in 4a becoming open woodland while 4b reverted to alien thicket.
The response in Site 4a was also conditional on the form of the next dominant. In the simulations of Huston and Smith (1987), removal of the earlier dominant caused dramatic increases in the next dominant. In weed ecology this model predicts the arrival of another alien 'super-species'. In Site 4a bugweed emerged as the second dominant that was more resistant to fire than chromolaena. InSite 4b lantana, more resistant to fire than both bugweed and chromolaena, emerged as the second dominant after the initial treatment. In this case an interesting picture emerged, also predicted from the simulation models.
Although chromolaena was replaced by lantana and bugweed, the thicket-forming process resisted fires and indirectly facilitated conditions for chromolaena to reestablish and become dominant again.
The role of chromolaena in forest establishment remains a 'chicken or egg' scenario. The entire process from the immigration of species to its hypothetical disclimax or equilibrium would have to be measured in order to do so. Control plots cannot be used to evaluate treatment effects in experiments sampled only at the end (O'Connor 1985). Baseline data cannot be used to draw conclusions about succession in long term studies that do not maintain a subset of baseline information in an untreated state.
Alien species play a pivotal role in present day ecology, particularly in the southern African eastern coastal zone. Many of these species possess abilities to invade natural vegetation and alter existing states, arresting the processes determining stability and resilience of intact ecosystems. Woody plant communities where unstable but resilient in open grassland under conditions of no burning. Savanna succession was replaced by forest species and
chromoleana. Although savanna seedlings were arrested under chromolaena thickets the domain dominated by woody vegetation persisted. Fire eliminated forest and chromolena successions but was only partially effective with savanna species. Coastal grasslands are non-equilibrium communities that function in multiple states whether fires are excluded or are a part of vegetation management.
Often overlooked, is the impact natural enemies (insects, pathogens and viruses) have on regulating species populations and most noticeable in biological control projects in alien dominated vegetation. At some stage biological control of chromolaena may reverse the dominance of a Huston and Smith (1987) 'super-species'. The possibility of another scenario emerging is a distinct possibility, namely a 'super-species' niche may be replaced by another
'super-species' if biological control provides an advantage for competitive displacement.
Displacement was observed between chromolaena and bugweed when fire was introduced as a management strategy. Fire can therefore be used as a cost-effective control option for reducing the distribution and local density ofthe chromolaena in open and wooded
grasslands, but must be integrated with chemical control should hardier alien species become secondary invaders.
6. MONITORING SERIAL CHANGES IN HERBACEOUS SPECIES