MOWING AND HERBICIDE APPLICATION TO CONTROL Megathyrsus maximus: DAMAGE ON PRE-EXISTING VEGETATION IN A 20-YEAR REFORESTATION SITE

Abstract

A necessidade de controlar gram\uedneas ex\uf3ticas em \ue1reas de restaura\ue7\ue3o ecol\uf3gica por longos prazos pode acarretar elevados custos econ\uf4micos, danos colaterais \ue0 regenera\ue7\ue3o e danos menos \uf3bvios ao ecossistema. Assim, dimensionar estes fatores negativos como parte do esfor\ue7o para avaliar a raz\ue3o custo/benef\uedcio das t\ue9cnicas de controle dessas esp\ue9cies, se faz necess\ue1rio. Este estudo objetivou avaliar os danos causados por duas t\ue9cnicas de controle de capim-coloni\ue3o (Megathyrsus maximus (Jacq.) B. K. Simon & S. W. L. Jacobs), sobre a regenera\ue7\ue3o preexistente (custos ambientais), em um reflorestamento de 20 anos, e estimar os custos econ\uf4micos do emprego dessas t\ue9cnicas. Trinta parcelas de 10 x 20 m foram distribu\ueddas em tr\ueas grupos: (CON) controle, (ROC) ro\ue7agem com ro\ue7adeiras costais e (HERB) ro\ue7agem seguida da aplica\ue7\ue3o de herbicida \ue0 base de glifosato. Foi feito o invent\ue1rio permanente de todos os indiv\uedduos arb\uf3reos e arbustivos, os quais foram acompanhados durante o per\uedodo de um ano. N\ue3o houve diferen\ue7a na mortalidade imposta pela ro\ue7agem inicial, tampouco na mortalidade imposta exclusivamente pela ro\ue7agem ou pelo herbicida no HERB. Ap\uf3s um ano, n\ue3o houve diferen\ue7as entre a mortalidade e a riqueza de esp\ue9cies dos tr\ueas grupos. Quando os tr\ueas grupos foram comparados entre si ap\uf3s um ano, para as plantas > 1 m, houve redu\ue7\ue3o da abund\ue2ncia no CON e redu\ue7\ue3o da riqueza no HERB; j\ue1 para as plantas < 1 m, houve redu\ue7\ue3o da abund\ue2ncia no ROC, e redu\ue7\ue3o da abund\ue2ncia e riqueza no HERB. Portanto, tais t\ue9cnicas parecem afetar mais fortemente as plantas < 1 m. O custo econ\uf4mico final do ROC foi 34,1% maior do que o custo do HERB, e mesmo que o custo para se realizar novas interven\ue7\uf5es diminua para ambos, h\ue1 aumento dos custos quando ocorre necessidade de interven\ue7\uf5es esparsas em \ue1reas pequenas. Ambas as t\ue9cnicas s\ue3o indicadas para controlar o capim-coloni\ue3o, sendo que o uso de herbicida \ue0 base de glifosato deve ser restrito \ue0s \ue1reas com baixa cobertura de dossel, por demandarem mais manuten\ue7\uf5es, e por ser a t\ue9cnica mais prejudicial \ue0 regenera\ue7\ue3o, j\ue1 que houve alta mortalidade de indiv\uedduos de pequeno porte (73%) em \ue1reas com densa infesta\ue7\ue3o da gram\uednea (100%).The need to control exotic grasses in areas of ecological restoration for long periods is followed by high economical costs of application, collateral damage on regeneration and other less obvious environmental harms. Thus, it is necessary estimate such drawbacks in order to evaluate the cost/benefit rate of the application of these techniques. This study aimed to assess the damage caused by two control techniques of Guinea-grass (Megathyrsus maximus (Jacq.) B. K Simon & S. W. L. Jacobs), on pre-existing regeneration (environmental costs) in a 20-year reforestation site and estimate the economic costs of employing such techniques. Thirty 10 x 20m plots were divided into three groups: (CON) control, (ROC) mowing (using gas-powered brush cutters) and (HERB) mowing followed by the application of a glyphosate-based herbicide. We did a permanent inventory of all tree and shrub individuals, who were followed over a period of one year. No differences were found in mortality imposed by the initial mowing, neither in the mortality imposed exclusively by mowing nor in the mortality imposed exclusively by herbicide in HERB. After one year, there were no differences between mortality and species richness of the three groups. However, when the three groups were compared with themselves after one year, for plants > 1 m, we found a reduction on abundance in CON and reduction in species richness in HERB; whereas for plants < 1 m, ROC showed a reduction in abundance and HERB showed reduced abundance and species richness. Therefore, both techniques seem to affect more plants < 1 m (i.e., regeneration). The final economic cost of ROC was 34.1% bigger than the cost of HERB, and in spite of a likely reduction in costs to perform new interventions, proportional costs increased for scattered interventions over small areas. Both techniques were efficient to control Guinea-grass, but the use of glyphosate-based herbicide should be restricted to areas with low canopy cover, more maintenance-demanding sites, once the technique is more damaging on regeneration, since there was high mortality (73%) of individuals < 1 m in areas with dense grass infestation (100%)