Pollen-vegetation richness and diversity relationships in the tropics

Understanding Spatiotemporal Variation in Richness and Rate of Within-Site Turnover for Vegetation Communities in Western Eurasia over the Last 4000 Years

Vegetation communities are intricate networks of co-occurring species. Logistical challenges in collecting primary data means research often utilises short-term data from restricted geographical areas. In this study, we examine spatiotemporal change in richness and turnover of vascular plants and bryophytes over the last 4000 years at 23 sites in western Eurasia using high-resolution palaeoecological data. We find support for the Latitudinal Diversity Gradient and Altitudinal Diversity Gradient in both the overall vegetation community (arboreal and non-arboreal species) and the shrub and herb sub-community (non-arboreal species only), as well as a significant temporal increase in the gradient of both relationships. There was a temporal increase in (alpha) richness; the rate of turnover was high but temporally consistent for the overall vegetation community and high but decreasing over time for the shrub and herb sub-community. The rate of change in turnover was affected by latitude (steeper negative relationship at higher latitudes) and altitude (steeper negative relationship at lower altitudes). The Diversity-Stability Hypothesis was supported: vegetation communities changed from “lower richness, higher turnover” historically to “higher richness, lower turnover” more recently. Causal mechanisms for these complex interlinked biogeographical patterns remain ambiguous, but likely include climate change, non-native introductions, increasing homogenisation of generalist taxa, landscape simplification, and anthropogenic disturbance. Further research into drivers of the spatiotemporal patterns revealed here is a research priority, which is especially important in the context of biodiversity decline and climate change.

Long-Term Vegetation Dynamics in a Megadiverse Hotspot: The Ice-Age Record of a Pre-montane Forest of Central Ecuador

Tropical ecosystems play a key role in many aspects of Earth system dynamics currently of global concern, including carbon sequestration and biodiversity. To accurately understand complex tropical systems it is necessary to parameterise key ecological aspects, such as rates of change (RoC), species turnover, dynamism, resilience, or stability. To obtain a long-term (>50 years) perspective on these ecological aspects we must turn to the fossil record. However, compared to temperate zones, collecting continuous sedimentary archives in the lowland tropics is often difficult due to the active landscape processes, with potentially frequent volcanic, tectonic, and/or fluvial events confounding sediment deposition, preservation, and recovery. Consequently, the nature, and drivers, of vegetation dynamics during the last glacial are barely known from many non-montane tropical landscapes. One of the first lowland Amazonian locations from which palaeoecological data were obtained was an outcrop near Mera (Ecuador). Mera was discovered, and analysed, by Paul Colinvaux in the 1980s, but his interpretation of the data as indicative of a forested glacial period were criticised based on the ecology and age control. Here we present new palaeoecological data from a lake located less than 10 km away from Mera. Sediment cores raised from Laguna Pindo (1250 masl; 1°27'S, 78°05'W) have been shown to span the late last glacial period [50-13 cal kyr BP (calibrated kiloyears before present)]. The palaeoecological information obtained from Laguna Pindo indicate that the region was characterised by a relatively stable plant community, formed by taxa nowadays common at both mid and high elevations. Miconia was the dominant taxon until around 30 cal kyr BP, when it was replaced by Hedyosmum, Asteraceae and Ilex among other taxa. Heat intolerant taxa including Podocarpus, Alnus, and Myrica peaked around the onset of the Last Glacial Maximum (c. 21 cal kyr BP). The results obtained from Laguna Pindo support Colinvaux's hypothesis that glacial cooling resulted in a reshuffling of taxa in the region but did not lead to a loss of the forest structure. Wide tolerances of the plant species occurring to glacial temperature range and cloud formation have been suggested to explain Pindo forest stability. This scenario is radically different than the present situation, so vulnerability of the tropical pre-montane forest is highlighted to be increased in the next decades.