The role of fire and fire-related factors on germination and growth of grassland species.

Abstract

Fire, natural or of anthropologic origin is a recurrent phenomenon in South African mesic grasslands. The species composition of these grasslands is sensitive to fire frequency and there exists a permanent fire-species relationship syndrome. The shift follows a general trend where, in the absence of fire for longer periods, the native grass species, possibly the most economically important grass species, give space to relatively less desirable and invasive species accompanied by significant decline in basal cover. Though much is known as to how the floristic composition of the grassland changes in response to fire, the underlying mechanisms responsible for changes in plant vigour and species composition are not fully understood. There exists limited information on germination requirements of the local flora and how recruitment of the species is influenced by fire, fire-related factors and the interaction of these factors among themselves is virtually unknown. Since the discovery by DE LANGE and BOUCHER (1990), plant-derived smoke is being widely studied as an important germination cue and the theme has been the subject of intense experimental research and theoretical work. Germination studies conducted on South African Fynbos, Californian Chaparral, and Australian species have illustrated the widespread ability of plant-derived smoke in promoting germination of species from both fire and non-fire prone habitats. Studies have also showed that plant-derived smoke plays a significant role in vegetative growth of many plants including horticultural and agricultural crops, though mechanisms of smoke action in enhancing germination and promoting plant growth are still under active research. This study sought to gain insight into the role of fire and fire-related cues and other related factors on germination and seedling growth of key grass species from fire-prone grassland in South Africa. In this study, various investigations were conducted on different aspects related to smoke-induced seed germination. Furthermore, in depth examinations were conducted on the effects of fire-related factors such as smoke, heat, soil and nutrients on seed germination and seedling growth of representative key mesic grassland species from South Africa. Laboratory-based germination and vigour experiments were conducted using smoke solutions. The aim was to examine if the strong fire-species relationship syndrome we encounter in post-fire mesic grassland in South Africa is related to the effect of plant-derived smoke on germination and seedling growth. In addition, the interaction of plant-derived smoke solutions with temperature was examined by incubating seeds at a range of temperatures. Treating seeds with smoke-water and butenolide, the germination rate (GR) and final germination percentage (FGP) were greater in 3 of the 6 species. Themeda triandra Forssk. and Tristachya leucothrix Trin. ex Nees showed the greatest response, with final germination increased from 43% to 67% and 35% to 63% respectively. These smoke solutions have also significantly enhanced germination in Eragrostis tef (Zucc.) Trotter from 62% to 80%. The degree of responsiveness varied from species to species and across different temperatures. Findings from this study suggested that plant-derived smoke and its interaction with temperature significantly influence the germination and seedling growth of the South African mesic grassland species, which can further impact on the grassland composition. To characterise the role of plant-derived smoke on certain economically important seed traits, the effect of smoke-water and a smoke-isolated butenolide on seed germination and seedling growth of Eragrostis tef (grass species which has moderately good tolerance to east African drought) was investigated at a range of temperatures, light conditions and osmotic potentials. Smoke-water (1:500 v/v) and butenolide (10ˉ⁸ M) treatment of the seeds increased percentage germination, seedling vigour and imbibition from high to low osmotic potential. These results suggested that smoke solutions have characteristics with overriding effects toppling stresses exerted from high temperature and low osmotic potential, thereby widening the temperature and moisture zone over which germination can occur. Furthermore, these findings suggested that the mode of smoke action in promoting seed germination is likely to be linked to the role of smoke in facilitating water uptake by seeds (imbibition). A simple and rapid bioassay was implemented to detect the germination activity of extracts from soils in pre/post-burn status. Soil samples taken from burnt, unburnt and adjacent plots at depths of 0-2, 2-4, 4-6 and 6-8 cm before and after burning mesic grassland in South Africa were analysed for germination activity over an eight-week period. Soil samples were extracted using dichloromethane and bioassayed using Grand Rapids lettuce (Lactuca sativa L.) achenes. The Grand Rapids lettuce seeds exhibited several-fold greater germination percentages when treated with extracts from burnt soil compared to the other plots. The magnitude of such an activity declined through time since burn. The Grand Rapids lettuce seeds also showed significantly greater germination percentage when treated with unburnt soil extracts compared to the control (distilled water) which indicates the existence of other factors controlling germination in unburnt soil. Germination percentage was negatively influenced in the adjacent plots which reflected the inhibitory role of smoke on adjacent plots neighbouring the burn. Findings from this study suggested that the germination activity of a burning plant-material (smoke) indeed diffuses into the soil and its persistence declines with time. Furthermore, the findings implied that fire and smoke may significantly influence the germination of the soil seed bank of mesic grassland in South Africa. The effect of smoke solutions of various concentrations and their interaction with soil macronutrients NPK in influencing seedling growth of three selected grass species namely, Eragrostis curvula, Panicum maximum and Themeda triandra was investigated. These grass species were selected on the basis of the contrasting response they previously showed to smoke treatments. Results showed that the interaction between species, treatment and nutrients for seedling vigour index was highly significant (P < 0.001). In the absence of NPK elements, the smoke solutions generally enhanced seedling growth in Themeda triandra, while suppressing seedling vigour indices of Eragrostis curvula and Panicum maximum. It is ecologically significant that not only smoke but also its interaction with soil macronutrients appears to be important in structuring the post-fire regeneration and colonisation processes. To examine the effect of fire-associated cues of smoke and high temperature on germination and seedling emergence from the mesic grassland soil seed bank in South Africa, and disentangle responsible factors, the effect of heat and smoke-water on seedling emergence and the resultant effect on biomass production was examined in a greenhouse tray experiment over 90 days. Soil seed bank samples obtained from 0-5 cm top soil were treated with smoke-water solution (SW 1:500 v/v), heat (H = 100 °C for 15 min) and heat + smoke-water (H = 100 °C for 15 min + SW = 1:500 v/v) combined. Significantly, the highest number of seedlings emerged from soil seed bank samples treated with heat + smoke-water combined followed by smoke-water alone. Heat + smoke-water treatment of soil seed bank samples also significantly enhanced the rate of seedling emergence and total dry biomass production. Heat treatment by itself was not statistically different from the control in many respects. Generally, these results suggest that both heat and smoke are important germination cues and play a curial role on germination and seedling emergence from the mesic grassland soil seed bank. A greater number of fire-loving Themeda triandra seedlings emerged from soils treated with smoke, though this was not statistically significant. Seedling emergence indicators showed that seedling emergence was favoured more by smoke and smoke + heat treatments compared to the control. Seedling growth indicators also showed that soil samples treated with smoke or smoke + heat combined produced significantly higher biomass than untreated samples. In general, findings from this comprehensive study suggest that plant-derived smoke and its interaction with temperature (heat) and soil nutrients significantly influences seed germination and seedling growth of South African mesic grassland species differently, this can further influence the grassland composition. The association of certain fire-climax species with the re-occurrence of fire can partly be explained due to the association of such species with fire-induced changes in the habitat. Post-fire conditions of low nutrient (NPK) availability, presence of high heat and smoke cues in the habitat may favour the competitive ability of native short and smoke-responsive species such as Themeda triandra and Tristachya leucothrix over those characteristic species of fertile habitats such as Eragrostis curvula and other non-smoke-responsive species.