Habitat, plant communities and threats to the Robertson Granite Renosterveld

Abstract

The Robertson Granite Renosterveld (FRg3) vegetation type is found north of La Colline in the Breede River Valley near Robertson at altitudes of between 250 – 850 m a.s.l. This vegetation type is identified as understudied because of the isolated character of its granite pluton, which made it virtually unknown and led to it being identified as a priority for scientific research. As a result, the current study aims to improve the current understanding of plant communities, important habitat characteristics, threats, and identify dominant and endemic species in the FRg3. To investigate the species composition and communities within the FRg3, 53, 10m x 5m plots were sampled across the vegetation type for cover abundance of all the plant species coupled with environmental variables (altitude, and the ground covers of herbaceous vegetation, litter, bare ground, and rock). In addition, soils (~0-10 cm depth) were tested for pH (KCl), total organic carbon (C), soil texture (sand, silt, and clay), exchangeable phosphorus (P), and extractables of boron (B), sodium (Na), magnesium (Mg), potassium (K), and calcium (Ca). Fire is an important ecological disturbance in the Fynbos biome; therefore, Google Earth Engine (GEE) was explored to assess the fire history (2000-2023) of the FRg3 and map the outputs using image collections from MODIS/061/MCD64A1. The plant communities were classified using dissimilarity hierarchical cluster analysis, which showed that the FRg3 comprises five major plant communities, the following four of these communities are found in fire affected areas: Dodonaea viscosa – Euryops tenuissimus; Dicerothamnus rhinocerotis - Dodonaea viscosa, Passerina obtusifolia–Restio capensis, and Muraltia heisteria – Restio capensis; and the fifth community (Pteronia paniculata–Dicerothamnus rhinocerotis) is found in areas that have not burnt since 2000. The FRg3 had three major fires (2000, 2006, and 2017) over 24 years (2000-2023). In 2000, approximately 35% of the FRg3 burnt, while in 2006 and 2017 the fire burnt approximately 24% and 53% respectively. There have been overlaps in areas burned by multiple fires (2000 & 2017 = 23%; 2006 & 2017 = 22%), and almost half (47%) of the vegetation type did not burn at all over the 24 years. The soil analysis showed significant differences (P ≤ 0.05) in pH, Na, and texture (sand and clay) across the five communities. The FRg3 still retained most (99%) of its natural vegetation in 2023; however, there are disturbances and threats to the vegetation type, such as land clearing for agricultural purposes, and alien invasive plants such as Hakea sericea were observed within the vegetation type in 2023. The 24- year (2000-2023) changes in vegetation structure of the FRg3 were investigated using the Normalized Difference Vegetation Index (NDVI), which was computed from Landsat 5 and 8 over five different time periods (2000 – 2006, 2006 – 2013, 2013 – 2017, 2017 – 2023, and 2000 – 2023). This analysis showed an increase in FRg3 biomass (taller vegetation) between 2000 and 2006, largely because the area that burnt in 2000 had recovered despite the fire in 2006 which only burnt the small northern portion of the FRg3. The FRg3 lost most of its above ground biomass in 2017 due to a fire that burnt over half of it. The FRg3 had the most biomass in 2013 and 2023, likely due to the lack of natural or anthropogenic disturbances.