The ecology of encephalartos lanatus in Middelburg district, Mpumalanga Province (South Africa).

Abstract

Cycads are long-lived, slow-growing gymnosperms that were once widely distributed but today are limited to the tropical and subtropical regions of the world. Cycads are of conservation importance because of their unique ecology. First, they are the only gymnosperms that provide an important ecosystem service of nutrient cycling, such as nitrogen fixation, through their association with nitrogen-fixing bacteria present in specialized coralloid roots that are similar to root nodules in legumes. Second, they have unique life histories based on their growth. They are a group of dioecious gymnosperms that occur in various life stages within a population. Third, they are the most threatened plant group on Earth, with over 50 % of the more than 300 species facing a high risk of extinction. The extinction is well noted for the African Encephalartos species, which are threatened by habitat loss through transformation, frequent fires, over-exploitation for landscaping and medicinal purposes, reproduction failures and other natural courses like herbivory and poor dispersal, which all affect the population health and structure. There is little knowledge on the ecology of Encephalartos cycads, and their extinction will lead to a decline in ecosystem services, and also. information about the diversity of nitrogen-fixing bacteria and other associated microbes in the roots and soils of Encephalartos species, and knowledge of their contribution to soil nutrient improvement is limited. This study, therefore, aimed to investigate the effects of disturbances/threats to E. lanatus health and population structure. Additionally, the study investigates the associated roots and soil microbes and the effects of E. lanatus on soil nutrient status. Encephalartos lanatus in a protected area in Botshabelo, Mpumalanga, is prone to fire, herbivory, and habitat conversion, and therefore serves as a model species for the aim of the study. The effects of fire and herbivory on E. lanatus health and population structure were assessed by counting and measuring all plants (seedlings, juveniles, adults, coning) in burnt and unburnt plots in Botshabelo. The plants were sexed based on the presence of live cones, remnants of cones from a previous season, or seedlings under the adult plant. Cone damage was assessed by counting and recording cones that were completely or partially damaged by fire and/or herbivores. For soil nutrient status, soil samples were collected from the rhizosphere of a total of 20 adult E. lanatus plants (10 each from burnt and unburnt plots), and 20 control samples were collected 5 m away from each target plant, and analyzed for soil nutrients (nitrogen [N], phosphorus [P], potassium [K], calcium [Ca], magnesium [Mg], manganese [Mn], copper [Cu], zinc [Zn], pH, acid saturation, total cation exchange, exchangeable acidity, and organic carbon [C]), enzyme activities using ꞵ-(D)-glucosaminidase, ꞵ-glucosidase, and phosphatase (alkaline & acid). Furthermore, the rhizosphere and non-rhizosphere soils, together with coralloid roots, were analyzed for N-fixing, N-cycling, and P-solubilizing bacteria and other associated microbes. The data obtained for the effect of fire and herbivory on population structure was analyzed using analysis of covariance (ANCOVA) and analysis of variance (ANOVA) in RStudio. A two-sample t-test in Statistix 10 software was used to analyse the differences in nutrient concentration and enzyme activities in the rhizosphere and non-rhizosphere soils. The coralloid roots, rhizosphere and non-rhizosphere soils were analysed for their microbial composition. The study revealed that the population follows a “J” structure with a greater (P < 0.01) number of adults compared to any other life stage, and E. lanatus populations that were affected by fire produced a higher number of cones and were more prone to baboon damage than the cones in the unburnt sections. Rhizosphere soils had a significantly higher concentration (P < 0.05) of Mg and Mn than non-rhizosphere soils. In addition, both the rhizosphere and non-rhizosphere soils had significantly similar concentrations (P > 0.05) of N, P, K, Ca, Cu, Zn, pH, acid saturation, total cation exchange, exchangeable acidity, and organic C. Specifically, ten bacteria families were identified in the E. lanatus rhizosphere, non-rhizosphere soils, and coralloid roots. The Burkholderiaceae and Rhizobiaceae were the most dominant microbial families in both soils. The enzymes ꞵ-(D)-glucosaminidase and alkaline phosphatase were higher in the rhizosphere than in the non-rhizosphere soils but not significant (P > 0.05). In contrast, acid phosphatase, nitrate reductase and beta-glucosidase were significantly higher (P < 0.05) in the rhizosphere than in the non-rhizosphere soils. In conclusion, the study provides evidence that E. lanatus face challenges in their natural habitat. However, E. lanatus have developed coping mechanisms to withstand the harsh environment and host N-fixing, N-cycling and P-solubilizing microbes that assist the plants to thrive in nutrient-poor conditions. Generally, the current findings show that the identified N-fixing and nutrient-cycling bacteria, and their associated enzymes in E. lanatus coralloid roots, rhizosphere, and non-rhizosphere soils account for soil nutrient status improvement in the E. lanatus nutrient-poor ecosystem.