Relative pollen productivity estimates of savanna taxa from southern Africa and their application to reconstruct shrub encroachment during the last century

Abstract

To understand the resilience of African savannas to global change, quantitative information on the long-term dynamics of vegetation is required. Past dynamics can be reconstructed with the REVEALS model, which requires pollen productivity estimates (PPE) that are calibrated using surface pollen and vegetation data. Here we calculated PPE values for five savanna taxa using the extended R-value (ERV) model and two pollen dispersal options: the Gaussian plume model (GPM) and the Lagrangian stochastic model (LSM). The ERV calculations failed to produce a reliable PPE for Poaceae. We therefore used Combretaceae as the reference taxon – although values obtained with Poaceae as the reference taxon are presented in the supplement. Our results indicate that Combretaceae is the taxon with the highest pollen productivity and Grewia the taxon with the lowest productivity. Acacia and Dichrostachys are intermediate pollen producers. We find no clear indication of whether the GPM PPEs or the LSM PPEs are more realistic, but the differences between these values confirmed that the pollen fall speed has a greater effect in the modelling of GPM than in the LSM. We also applied REVEALS to the pollen record of Lake Otjikoto (northern Namibia) and obtained the first quantitative reconstruction of the last 130 years of vegetation history in the region. Cover estimates for Poaceae indicate the predominance of a semi-open landscape throughout the 20th century, while cover values below 50% since the 21st century correspond to a thick savanna. This change in grass cover is associated with the spread of Vachellia, Senegalia and Grewia reflecting an encroached state.

Melissopalynological investigations of seasonal honey samples from the Greater Kruger National Park, Savanna biome of South Africa

“…The focus was on several specific species that were commonly (including very frequent, frequent, and isolated pollen) found in pollen samples (> 1% average abundance). Selected plants with a high relative abundance of associated pollen in the honey samples were identified along two 250 m transects from North to South and from West to East across the centre of the location of all the beehives (adapted from Tabares et al 2021), since the hives where all within 150 m range of each other.…”

Relative Pollen Productivity Estimates for Major Plant Taxa in Middle Subtropical China

“…This has aided large-scale quantitative estimates of Holocene land-cover changes which in turn provides important information to assess the interactions between land-cover changes and climate in the past (e.g., [15,16]). However, attempts to estimate RPPs in tropical and subtropical regions of the world are rare, including only several studies from China and South Africa [17][18][19][20][21][22]. The difficulty of such attempts includes a wide variety of pollen productivity and dispersal mechanisms of plant species in the tropics with extremely high biodiversity [17].…”

Surface pollen quantification and floristic survey at Shaheed Chandra Shekhar Azad (SCSA) Bird Sanctuary, Central Ganga Plain, India: a pilot study for the palaeoecological implications

“…In concurrence with these studies, Gillson & Duffin (2007) used process-based model to translate tree pollen percentages into past woody plant cover in savannah habitat in southern Africa to inform the identification of thresholds of concern for conservation monitoring in the region. Recently, some studies on quantitative pollen-based Holocene reconstruction have estimated Relative Pollen Productivity (RPP) of key taxa from tropical and subtropical regions of the world (Jiang et al, 2020;Wan et al, 2020), including South Africa (Hill et al, 2021), Cameroon highlands (Gaillard et al, 2021) and Namibia (Tabares et al, 2021).…”