Análisis Documental: Resolución 4434 de 8 de Agosto de 2006

The investigations reported in this thesis were intended to determine what the most effective treatments were for utilizing the ‘spray and hay’ method for replacing N. trichotoma with T. triandra during revegetation of Western (basalt) plains grassland. The reasons why some treatments involving herbicides, type

of removal of T. triandra hay and timing of accession of treatments were more successful than others was not a key focus for study. However, the outcomes of the investigations do lead to some reflections as to why particular treatments may have been more successful than others, which may form a basis to guide future investigations aimed at further developing and refining the ‘spray and hay’ method. In brief, these reflections are as follows:

Seedlings of both T. triandra and N. trichotoma can begin to grow on

revegetation sites cleared of vegetation (above ground and below ground) in October. However, the pattern of seedling establishment of the two species is different. Germination and establishment of T. triandra seedlings can take place over late spring (December) and even into January and February, depending on how much water is available in the soil over summer, whereas germination and

establishment of N. trichotoma seedlings is mostly complete by November. A

small percentage of N. trichotoma seedlings are also able to establish in the

autumn following revegetation.

Other weeds which are capable of establishing on revegetation plots (thistles, other broad leaf weeds and annual grasses) begin germination and establishment in the early spring and are mostly finished establishing by November or December. There is an additional short period of germination and establishment for some of these species in the autumn following revegetation.

The application of atrazine during ‘spray and hay’ revegetation, at label strength and backpack rates, in any of the possible accession times, ensures that there is sufficient residual atrazine in the soil to prevent N. trichotoma seedling establishment during the optimum period for this species (October – December). Also there is sufficient atrazine residuality in the autumn following

revegetation to minimize N. trichotoma seedling establishment during February

– May. Residual levels of atrazine are also effective in preventing the establishment of other weed species in spring, and in minimizing establishment of these species in the autumn following revegetation.

Whether hay is removed from revegetation plots by burning or physical

removal affects the pattern of T. triandra seedling establishment, but not the

final outcome. Burning of the T. triandra hay also removes dead N. trichotoma

stubble and results in rapid establishment of T. triandra seedlings, whereas

physical removal of hay leaves some dead N. trichotoma stubble in place, and

hence partial shading of the ground. The effect of this is a slower, more

progressive germination and establishment of T. triandra seedlings over late

spring and summer, and may leave T. triandra seedling establishment more

susceptible to the effects of summer drought.

Application of glyphosate as part of ‘spray and hay’ revegetation kills the mature N. trichotoma on site, but does not provide residual protection. The time at which the ‘spay and hay’ revegetation steps take place have an important bearing on the outcomes. If the T. triandra hay (and hence protective cover) is removed from revegetation sites during early or mid-spring, the opportunity arises for N. trichotoma and other weed seedlings to establish at high densities.

If protective hay cover is retained until October, then establishment of N.

trichotoma and other weeds is lessened. However, the lower level establishment of weeds in autumn is not avoided.

Burning of T. triandra hay in July on glyphosate treated plots does not have a

major effect on the establishment of annual grasses and broadleaf weeds, because the main time for seedling establishment of these weeds is after this time. However, burning hay in October or December reduces the level of spring establishment of annual grasses and broadleaf weeds, presumably because by this stage some seedlings are beginning to germinate under the thatch, and burning kills these seedlings.

However, when hay is burnt on glyphosate treated plots in July or December,

the establishment of N. trichotoma seedlings is promoted more than on physical

hay removal treatments, presumably because the fire removes the dead N.

trichotoma stubble on the revegetation sites, and exposes the seeds to light and higher soil temperatures, both of which would promote seed germination and seedling establishment.

It is also apparent that there is a more complex set of interactions between plant species growing on revegetation sites after ‘spray and hay’ treatments (especially for glyphosate treatments), which affect the outcomes of revegetation. Two examples of this are: (1) N. trichotoma seedling establishment on glyphosate, burn, May – October succession treatments is probably kept low

because the treatment also results in high persistent T. triandra seedling

densities over the spring and autumn periods of optimum establishment for N.

trichotoma. The high densities of T. triandra seedling would most likely offer

vigourous competition for resources which would limit N. trichotoma seedling

establishment; and (2) the removal of the dead N. trichotoma stubble by burning can also advantage the establishment of high densities of T. triandra seedlings, but only if the burning takes place immediately prior to the optimum time for

T. triandra seedling establishment (late spring to mid summer). Burning at other

times (e.g. July) can advantage the establishment of N. trichotoma and other

weed seedlings over T. triandra seedling establishment. If significant weed

numbers are on revegetation sites at the optimum time for T. triandra seedling

establishment, the cover of these weeds and their utilization of underground resources (water, nutrients, etc.) will probably have an inhibitory effect on T. triandra seedling establishment.

6. General conclusions and recommendations