Spatial scaling of the pollen-plant diversity relationship in landscapes with contrasting diversity patterns

We found a significant positive relationship between pollen and plant richness, regardless of differences in plant diversity, landscape structure, and environmental conditions between the two study regions. This discovery represents a major step towards more accurate paleoecological reconstructions of plant diversity in central temperate Europe, as previous studies on this topic have mainly been conducted in boreal and boreo-nemoral zones.8.11in high mountain habitatsten or in southern Europe9.12.

Methodological differences, for example, in diversity indices, data transformations or sample sizes used make comparison between studies difficult. Nevertheless, the strongest relationships seem to be found when habitats with contrasting patterns of plant diversity are compared, such as forests and alpine or forests, bogs and grasslands11. Also in our study, we found the strongest correlations when complete datasets combining forest and open habitats were analyzed together for both study regions. As it is well known that plant richness is generally lower in forests than in open landscapes in temperate and boreal regions28, this observation may seem rather trivial. However, it is important for paleoecological reconstruction because Holocene diversity changes in temperate regions were largely driven by changes in the relative abundance of major habitat types (such as forests, grasslands, wetlands and artificial habitats), and not only by changes in the species richness of these habitats5.6.

With respect to individual habitats, the pollen-vegetation diversity relationship is often quite strong and significant in grasslands and other open habitats8.11; for example, the open habitat subset of the WCM in this study. Open habitats are generally more species rich, thus offering a longer gradient of species richness compensating for the taxonomic imprecision of pollen analysis. In forest sites with fewer species, we found mostly insignificant relationships. In addition, two other factors may play a role.

First, the high pollen productivity of trees biases the diversity relationship according to studies from northern Europe16. However, a study of a height transect in southern Norway showed that the strongest representational bias occurs only in the boreal forest biome, which is dominated by large pollen producers.ten. Our dominant plant component, Spruce and Quercushave intermediate to high pollen productivity (2–2.5), whereas true high pollen producers such as Alnus and Betula (>3) are less abundant in our study area (Supplementary Fig. S2). Adjustment of pollen count by PPE led to a stronger relationship between pollen and floristic richness only in the WCM open habitat subset (Supplementary Fig. S4).

Second, interception of pollen by treetops29 and subsequent leaching of the forest floor affects the forest site diversity relationship more than pollen productivity. This noise also describes as a filtering of the vegetation30 can be illustrated in our dataset by long-distance transport pollen Ambrosia artemisiifolia-type, which has the nearest source populations ca. 50 km southeast of the WCM region31; or the pollen of Mugwort, growing in open habitats. Both pollen taxa are more abundant in forest than in open sites (Supplementary Fig. S3).

Regarding the application of these results for the interpretation of the fossil record, we suggest considering only marked changes in pollen richness in the past and avoiding overinterpretation of small differences, since insignificant relationships obtained in the two forest datasets suggest some limitations of the method.

We have shown that the pollen-plant diversity relationship can be at least partially unraveled by knowing the exact spatial position of plant species in the wider environment of pollen sampling sites. Changes in the relationship with spatial scale evolution are largely due to the number of newly emerging species as the radius of the surveyed area increases, especially as new habitats are added (Fig. 5, Supplementary Fig. S5). Remarkably, in the BMH region, it increases with distance, while the opposite trend was observed in the WCM region. This discrepancy can be explained by non-uniform richness patterns in different habitats and by different landscape structure (i.e. the spatial arrangement of different habitats) in the two study regions.

In open habitat sites in the WCM zone, most species generally appeared within the top 40 m. This observation is consistent with knowledge of extremely high fine-scale plant diversity in local steppe grasslands, where a substantial part of the species pool occurs at the scale of tens of square meters.32. Moreover, the grain size of the habitat mosaic in the WCM region is finer than in the BMH region. Therefore, the closest pollen-plant diversity relationship between habitats in the WCM region is obtained over shorter distances. Although habitats such as built-up areas and roads at distances greater than 40 m may be species-rich and different in composition from grasslands and forests, it appears that high plant diversity at small scales (in our case in the open habitat subset of the WCM) limits the influence of the surrounding landscape on pollen richness and reduces the source area of ​​pollen richness. Several studies of the relevant pollen source area report similar results.33,34,35. A weakening of the relationship between pollen diversity and plant diversity with distance has also been observed in the Mediterranean region9although their interpretations are limited by the methodology of the field surveys.

The appearance of open habitats in the forests led to the increase in the number of species and to the local maxima of adjusted R2 in both regions. Whereas in the BMH forest the appearance of forest roads at about 70 m was crucial, grasslands and orchards at about 250 m played a similar role in the WCM forest subset. In the open habitat subset of the WCM, diversity patterns in the first tens of meters were crucial, while in the open habitat subset of the BMH, an increased correlation of floristic and pollen richness appeared only at 400 and 550 m; at this distance, many species have appeared due to the frequent transition from grassland complexes to shrubby habitats and built-up areas. Other studies of semi-open landscapes have also found a strong correlation between pollen richness and landscape openness.17,26,27.

Estimating the source area of ​​pollen variance as a regression of pollen and floristic variance implies that the resulting distance of 100–250 m represents all data sets. Although they differ in species richness, openness and habitats, the relationship between the variances is quite linear. The exception is the open habitat subset of the WCM, suggesting that the spatial scale at which pollen variance corresponds to floristic variance cannot be generalized.

The strong effect of high pollen richness in the open habitat subset of the WCM is also visible in the comparison of pollen and floristic variance. At 150 m, the WCM open habitat subset had much lower floristic variance than the other subsets. The floristic variance in this subset corresponding to the pollen variance and the pattern of the other datasets is at 6 m (Fig. 6b). Again, this may be caused by the large, small-scale diversity of grasslands, which includes most pollen types present in the surrounding landscape. Only a few new species appeared in the wider environment and at 150 m, the open habitats of the WCM are more similar than the other habitats analyzed. That extremely high alpha diversity is compensated by low beta diversity has already been reported in open habitats of the White Carpathians.36. The linearity and magnitude of the variance relationship in the rest of the datasets indicates the robustness and possible applicability to a variety of fossil records.

The mechanism for establishing the source zone of pollen variance was similar to that mentioned for the source zone of pollen richness. The appearance of new habitats with new species (Fig. 5) such as open habitat for forest sites (WCM forest subset) or built-up areas for open sites (BMH open habitat subset) , caused small to negligible increases in floristic variance. Moreover, the high but non-significant relationship of the variances at the distance between 250 and 600 m (Fig. 6a) corresponds to the distance of the second adjustment range between floristic and pollen richness (Fig. 4a).

Beta diversity, understood as directional turnover (temporal or spatial), is increasingly used in pollen analysis22.24 as beta diversity as non-directional variation. According to Nieto-Lugilde et al.25 renewal based on pollen correlates with renewal based on forest inventories. We extend this discovery from woody taxa to all species and from directional turnover to non-directional variance. Moreover, forest sites with high contribution to pollen beta diversity also show increased contribution to floristic beta diversity (Fig. 4b).

Baseline plant diversity data shows 1477 species in 15 map squares covered by our survey for the BMH region and 2045 species in 14 squares for the WCM region37. This means that we recorded 54.1 and 53.7% of the pool of known regional species in the two regions, respectively. We consider this to be a fairly good result and the close agreement in representativeness between the two regions testifies to the consistency of data quality between the datasets. We advise that future studies covering larger areas and various biomes preferably use high quality floristic data collected during targeted field surveys rather than database data or simplified field survey data. Only then can we understand pollen-plant diversity relationships more realistically and in a spatially explicit way.

In order to interpret fossil pollen richness in light of our current results, we need to consider landscape openness, which can be roughly inferred from the ratio of arboreal and non-arboreal pollens. The variation in pollen richness during the forest phases of the records should be interpreted with more caution, especially in cases of low variation. In all other cases, pollen richness is significantly linked to plant richness within a radius of ten to several hundred meters, depending on the distance from the expected species-rich plots.

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