Clipping frequency but not nutrients affect the architecture and non-structural carbohydrates of a browsing lawn

Abstract

“Grazing lawns” are well-known effects of large mammalian herbivores on a short grass sward that is maintained through a positive feedback effect; plants are re-grazed due to their increased palatability. However, there are few studies that have addressed the notion of “browsing lawns.” I simulated browsing and added nutrients to Acacia nigrescens, a palatable and dominant African tree, to examine the effects on the architecture of these trees in a greenhouse. I recorded investments in non-structural carbohydrates and crude protein between roots and stems as a possible mechanism behind such architectural changes. There was a significant decrease in mean branch internode length and an increase in the branching ratio with clipping; the last mentioned indicated that there was more extensive tree branching. There were no significant effects on plant architecture of nutrient addition. There were changes in non-structural carbohydrates and crude protein between the roots and stems due to clipping but not to nutrient addition. Non-structural carbohydrate concentrations decreased with clipping in the roots and increased in the stems. Crude protein decreased in the stems with clipping frequency but not in the roots. There was an increase in root:shoot ratio of crude protein with increasing clipping frequency. A. nigrescens performs like a browsing lawn because of the changes in tree architecture (mean branch internode length and branching ratio) that are consistent with clipping. Clipping but not nutrient addition was important for these trees. Clipping frequency was important for non-structural carbohydrate redistribution from roots to stems. A positive feedback could have long-term effects on the palatability of these trees which should cause large mammalian herbivores to consistently favor A. nigrescens.