ERV analysis was run separately on each of the three Cathedral Peak vegetation communities to estimate Relevant Source Area of Pollen (RSAP) for each community, and to calculate Pollen Productivity Estimate (PPE) values for dominant taxa from each community. ERV analysis was run using sub-models 1 and 2 only, as these models use pollen and vegetation percentage data (as was collected in this research), whereas sub-model 3 was not used as it requires absolute vegetation data (Gaillard et al., 2008). While RSAP values from sub-model 1 and 2 were of similar value, PPE values were markedly different, and the most appropriate model to the data was chosen according to which model showed PPE values with the closest fit to one another, i.e. PPE values closest together (Broström, 2002).
RSAP values appear to be reasonably uniform across all three communities, with estimates ranging between 100 – 150 m from the source: Themeda grassland RSAP was estimated at
86 150 m, Protea savanna RSAP at 150 m, and Leucosidea scrubland RSAP at 100 m. This implies that pollen coming from outside the RSAP is from the ‘background’ pollen component, and is assumed constant between sites – that is to say, the pollen signal of sample sites in this research are predominantly influenced by source vegetation abundance and distribution from within a 100 – 150 m radius (Sugita, 1994).
The concept of RSAP is useful in illustrating the influences of taxonomic dispersal differentials and basin size on the spatial scale represented in pollen archives that are collected. These influences theoretically include: i) the better dispersed the pollen types, the larger the RSAP, and ii) the larger the basin size, the larger the RSAP (Gaillard et al., 2008).
A low RSAP of 100 – 150 m for these Cathedral Peak vegetation communities could thus reflect the smaller and more confined sedimentary basin sizes that are a function of topography in montane regions. Bunting et. al. (2013) suggest that it is not uncommon in pollen modeling research for RSAP to reflect at the boundary between vegetation surveying strategies of zones B and C. It is therefore recommended that future research employ finer- grained vegetation surveys between 100-500 m to increase the precision on where RSAP occurs and decrease the influence of zone boundary changes.
While RSAP is useful in showing how a pollen signal is influenced by both pollen source plants abundance and geographical location relative to a sample point, inferences must also be made regarding regional versus local influences to pollen assemblages. Traditional fossil pollen work attempts to intuitively divide pollen sums into regional and local pollen so as to discern regional and local source vegetation influences respectively. It must be noted that whilst RSAPs for the three studied vegetation communities all lie within approximately 150 m of sample points, signifying an important local influence of source plants into the pollen sums, it is not being suggested that local influx is more important or influential than regional influx. More to the point, what RSAP is representing is a spatial context around a sample location from which researchers can deduce the influence of source vegetation abundance and location has on the pollen sum. That is to say, RSAP is not suggesting the majority of pollen comes from source plants within 150 m of the sample location, but rather within 150m of the sampling location (i.e. local pollen influence), source plant abundance and geographical location influences the pollen assemblage, and thus >150 m represents background pollen (i.e. regional pollen influence) where only source plant abundance is important. It is still important, however, to resolve the contributions and influences that local and regional pollen influxes have on a pollen assemblage.
87 PPE values were obtained for modelled taxa from three vegetation communities. PPE values are an important data for improving the reconstruction of past landscapes as they provide a means of quantifying past vegetation abundances of taxa in environments (Abrahams and Kozáková, 2012). While palaeoreconstruction was not the purpose here, PPE values provide important insights into pollen productivity abilities of taxa and their representation in the pollen archive from certain vegetation communities.
PPE values obtained from the Themeda grassland show that Asteraceae pollen (PPE = 0.00026) producing species are much less productive in terms of pollen production relative to Poaceae pollen (PPE = 1) producing species, whereas Pteridophyta (PPE = 3.14) are relatively higher palynomorph producers. Particularly, 1m2 of Asteraceae vegetation produces ≈ 3 800 times less pollen than 1m2 of Poaceae, and 1m2 of Pteridophyta vegetation produces ≈ 3 times more pollen than 1m2 of Poaceae (Chapter 4 – Table 4.3). The abovementioned PPE values further suggest that in a given pollen assemblage taken from the Themeda grassland, Asteraceae is under-represented as there is theoretically a high quantity of vegetation of this taxon in a landscape which reflects only in a low abundance in the pollen spectra, and Pteridophyta is over-represented as there is theoretically a low quantity of vegetation of this taxa and this reflects in a high abundance in the pollen spectra.
PPE values obtained from the Protea savanna show that Asteraceae and Proteaceae plants are less productive in terms of pollen production relative to Poaceae in a known area of vegetation, whereas Pteridophyta and Podocarpaceae are significantly higher pollen producers in a same sized area of vegetation. In addition, 1m2 of Asteraceae vegetation produces ≈ 2 600 times less pollen than 1m2 of Poaceae and 1m2 of Proteaceae vegetation produces ≈ 4.5 times less pollen than 1m2 of Poaceae, while Podocarpaceae produces ≈ 6.5 times more pollen than 1m2 of Poaceae and Pteridophyta produces ≈ 8.5 times more pollen than 1m2 of Poaceae (Chapter 4 – Table 4.4). These PPE values suggest that in a given pollen assemblage taken from the Protea savanna, Asteraceae and Proteaceae are under-represented as there is theoretically a high quantity of vegetation of this taxon in a landscape which reflects only in a low abundance in the pollen spectra, and Pteridophyta and Podocarpaceae are over-represented as there is theoretically a low quantity of vegetation of these taxa and this reflects in a high abundance in the pollen spectra.
PPE values obtained from the Leucosidea scrubland show that relative to Rosaceae, Poaceae and Ericaceae plants are less productive in terms of pollen production in a known area of
88 vegetation, whereas Pteridophyta are higher pollen producers in a same sized area of vegetation. Specifically, 1m2 of Poaceae produces ≈ 3.5 times less pollen than 1m2 of Rosaceae and 1m2 of Ericaceae vegetation produces ≈ 6 times less pollen than 1m2 of Rosaceae, and Pteridophyta produces ≈ 7 times more pollen than 1m2 of Rosaceae (Chapter 4 – Table 4.5). Moreover, PPE values suggest that in a given pollen assemblage taken from the Leucosidea scrubland, Poaceae and Ericaceae are under-represented as there is theoretically a high quantity of vegetation of this taxon in a landscape which reflects only in a low abundance in the pollen spectra, and Pteridophyta are over-represented as there is theoretically a low quantity of vegetation of this taxa and this reflects in a high abundance in the pollen spectra.
5.5.1. ERV analysis: limitations and recommendations
The number of samples collected for pollen modelling research compromises RSAP and PPE calculations when running ERV models and its subsequent ability to find solutions. The more pollen and vegetation data the models have to run, the more certainty can be placed in their outputs. A recommendation to improve RSAP and PPE calculations would therefore be to have as many samples from as many sites as is possible to enhance the dependability and certainty of ERV analysis results. Furthermore, a recommendation would be to use different distance weighting models to investigate confidence levels in the use of these models and their relevance to site-specific data sets.