Vegetation response to climatic changes in western Amazonia over the last 7,600 years

Abstract

Aim Ongoing and future anthropogenic climate change poses one of the greatest threats to biodiversity, affecting species distributions and ecological interactions. In the Amazon, climatic changes are expected to induce warming, disrupt precipitation patterns and of particular concern, to increase the intensity and frequency of droughts. Yet the response of ecosystems to intense warm, dry events is not well understood. In the Andes the mid‐Holocene dry event (MHDE), c. 9,000 to 4,000 years ago, was the warmest and driest period of the last 100,000 years which coincided with changes in evaporation and precipitation that caused lake levels to drop over most of tropical South America. This event probably approximates our near‐climatic future, and a critical question is: How much did vegetation change in response to this forcing? Location Lake Pata, Brazilian Western Amazonia. Taxon Terrestrial and aquatic plants. Methods We used pollen, charcoal, total organic carbon (TOC), total nitrogen (TN), δ13C and δ15N data from a new high‐resolution core that spans the last c. 7,600 years history of Lake Pata. Results We found that in the wettest section of Amazonia changes associated with the MHDE were detected in the geochemistry analysis but that vegetation changed very little in response to drought during the Holocene. This is the first high‐resolution core without apparent hiatuses that spans most of the Holocene (last 7,600 cal yr bp) from Lake Pata, Brazil. Changes in the organic geochemistry of sediments indicated that between c. 6,500 and 3,600 cal yr bp lake levels dropped. Vegetation, however, showed little change as near‐modern forests were seen throughout the record, evidencing the substantial resilience of this system. Only a few species replacements and minor fluctuations in abundance were observed in the pollen record. Main conclusions The mid‐Holocene warming and reduced precipitation had a limited impact on western Amazonian forests. We attribute much of the resilience to a lack of fire in this system, and that if human‐set fires were to be introduced, the forest destruction from that cause would override that induced by climate alone.

The resilience of Amazon tree cover to past and present drying

“…Building on this extensive modern work, this study aims to characterize the resilience of central and western Amazon tree cover to drier, mid-Holocene (∼ 6 ka) conditions [21,22,23,24,25,26,27,28], and compare mid-Holocene tree cover resilience to the present. We stress that this approach cannot account for other dimensions of rainforest resilience, like species composition or diversity.…”

Widespread reforestation before European influence on Amazonia

“…A high-resolution pollen analysis of Lake Pata revealed no evidence of human occupancy and forest pollen percentages were >95% throughout the last 2000 years (Fig. 3) (29).…”

On the scaling and standardization of charcoal data in paleofire reconstructions

“…We used the frequency metric because the variance in catchment size and measurement metrics makes scaling by means, medians or percentiles impossible. Lake sediments containing a low frequency of samples containing charcoal (e.g., Lake Werth) typically also have low abundance measurements (Bush et al 2007b;Åkesson et al in revision, Bush et al 2007a, Nascimento et al 2019. These low-abundance, low-frequency charcoal measurements highlight the need for down-weighting in inter-site comparisons (Fig.…”

“…In the wet tropical forests of the Amazon, many lake sediment records contain frequent or abundant charcoal (e.g., Bush & Silman 2007, Bush et al 2016, while others do not (Bush et al 2007b, Schiferl et al 2017, Nascimento et al 2019, Åkesson et al 2020). Here we show how both the z-score and relative scaling approaches can be problematic when interpreting datasets with very little charcoal.…”