The effect of defoliation environment on primary growth allocation and secondary tiller recruitment of two bunchgrasses

Abstract

Compositional shifts of bunchgrass species in the Southern Tall Grassveld in response to different combinations of non-selective mowing and burning appear to reflect differences in growth strategy among species, but there are few studies of South African grasses designed to test this. A descriptive study was undertaken both to observe how different species allocated resources in response to different defoliation treatments imposed at the start of spring, as it was thought that these responses might be critical for an understanding of tiller dynamics and compositional change in bunch grasslands. The study recorded the reproductive growth response of two spring-flowering species, Themeda triandra and Eragrostis curvula, subjected to three defoliation treatments: no defoliation (No Mow), defoliation during the previous summer (Summer Mow), and defoliation in early spring (Early Spring Mow). Plant growth response was separated into primary growth, measured as individual organ extension on individual phytomers, and secondary growth, measured as the number of secondary tillers recruited per live primary tiller. Phytomer growth depended on treatment and species. For either species, organ extension was greater for the No Mow treatment than for either of the two defoliated treatments. Tillers of E. curvula were substantially larger and grew faster than T. triandra tillers. In addition, apical meristems of E. curvula remained close to the ground until flowering. These combined differences are consistent with E. curvula being able to outcompete T. triandra in less-frequently defoliated swards. Secondary tiller recruitment was significantly related to defoliation treatment and species. Tiller recruitment was substantially lower on the No Mow treatment than for the two defoliation treatments, and tiller recruitment was greater for T. triandra plants than for E. curvula plants. Accumulated blade length was negatively related to tiller recruitment. Results suggest that tiller recruitment was limited by nutrient demand of primary structures, a consequence of which is that T. triandra will recruit more tillers than E. curvula when defoliation opens up space, accounting for T. triandra's dominance on more regularly-defoliated treatments.