Shoot characteristics of Acacia tortilis (Forsk.) in wildlife and rangeland habitats of Botswana

Abstract

Length, branch production, orientation, spinescence and biomass were measured for current shoots of Acacia tortilis shrubs in a range of habitats in Botswana. All shoot characteristics varied between individuals and between habitats with longer, less spinescent shoots produced in the habitat recently protected from wildlife. Biomass allocated to woody material increased with shoot length at the expense of leaf mass, whilst investment in straight prickles was as much as 6% of shoot dry mass. Shoot length had significant effects on total current shoot mass per shrub which was greatest in the recently protected area but the mean number of shoots per shrub also varied between habitats. Age and history of disturbance, including herbivory, appear to be important determinants of morphology and growth pattern of individual shrubs. Fitness of A. tortilis individuals and the consequence of flexibility in growth responses for vegetation structure and community organization are discussed. RbumCOn a mesure la longueur, la production de branches, l'orientation, l'epinosite et la biomasse des pousses rkcentes des arbustes d' Acacia tortilis dans diffkrents habitats du Botswana. Toutes les caractkristiques des pousses variaient selon les individus et les habitats, avec des pousses plus longues et moins Cpineuses dans l'habitat protege depuis peu contre la faune. La biomasse de la matiere ligneuse augmentait avec la longueur des pousses au detriment de la biomasse des feuilles, tandis que l'investissement dans les raides epines pouvait atteindre 6% du poids sec des pousses. La longueur des pousses avait un impact significatif sur la rnasse totale des pousses par arbuste, qui Ctait la plus ClevCe dans la zone recemment protegee, mais le nombre moyen de pousses par arbuste variait aussi selon I'habitat. L'gge et le taux de derangement, comme par exemple par les herbivores, semblent etre des facteurs determinants dans la morphologie et la croissance de chaque arbuste. La sante de chaque Acacia tortilis et la consequence de la flexibilitk des reponses de croissance sur la structure de la vkgitation et I'organisation des communautks sont discutees.

Plant responses and herbivory following simulated browsing and stem cutting of Combretum apiculatum

“…The browsing on clipped woody species followed patterns reported in earlier studies in the temperate zone (Machida 1979;Danell et al 1985) and in tropical/ subtropical areas Dangerfield et al 1996) with an increased browsing on already browsed trees ('feeding loop'). This resource regulation may benefit the large herbivores although it is doubtful whether this is an evolved strategy (Gordon & Lindsay 1990).…”

Acacia karroo responses to early dormant season defoliation and debarking by goats in a semi-arid subtropical savanna

“…The absence of an effect on spinescence supported similar observations of A. karroo (Scogings 1998a) and A. drepanolobium (Gadd et al 2001), but contradicted observations of increased spinescence among African Acacia species (Milton 1988;Dangerfield et al 1996;Midgley and Ward 1996;Young and Okello 1998). However, studies that detected increased spinescence were of trees under repeated or prolonged browsing.…”

“…Whereas spinescence may or may not be induced by short-term browsing, depending on the season, it appears to be consistently induced by long-term or repeated browsing (Milton 1988;Dangerfield et al 1996;Midgley and Ward 1996;Young and Okello 1998). This response cannot be explained simply in terms of increased growth, because long-term browsing of African Acacia species has generally had either no effect or a negative effect on growth Dangerfield et al 1996;Scogings 1998a).…”

“…This response cannot be explained simply in terms of increased growth, because long-term browsing of African Acacia species has generally had either no effect or a negative effect on growth Dangerfield et al 1996;Scogings 1998a). Therefore, the response must be explained in terms of morphological differences between shoots before and after browsing, such as shorter internodes (leading to more spines per unit shoot) or longer spines on new shoots (Midgley and Ward 1996).…”

Spines protect plants against browsing by small climbing mammals

“…This variation is usually attributed to individual plant histories of environmental stresses (Grubb 1992;Danger®eld et al 1996;Gowda 1996a), and particularly to browsing by large mammals (Young 1987;Milewski et al 1991). Spines are known to protect plants by limiting the rate at which large browsers can remove leaves and shoots (Cooper and Owen-Smith 1986;Belovsky et al 1991;Gowda 1996b).…”