Quantifying past forest cover and biomass changes in the Ecuadorian Amazon

Abstract

Here, we developed and applied models to quantitatively reconstruct forest cover and biomass changes at three lakes in northwestern Amazonia over the past > 1500 yr. We used remotely sensed data and a modern dataset of 50 Amazonian lakes to develop generalized linear models that predict aboveground biomass, using phytolith morphotypes and forest cover as predictor variables. Also, we applied a published beta regression model to predict forest cover within 200 m of each lake, using Poaceae phytoliths. Charcoal and maize phytoliths were analysed to identify past land use. Results showed forest cover and biomass changes at our study sites ranged between 48-84% and 142-438 Mg ha^-1, respectively. Human occupation was discontinuous, with major changes in forest cover and biomass coinciding with periods of land use. Forest cover and biomass decreased notably after fire (at all sites) or cultivation events (Lakes Zancudococha, Kumpak^a). The timing and ecological impact of past land use were spatially and temporally variable. Our results suggest past human impact was small-scaled and heterogenous in northwestern Amazonia, with a significant impact of fire on forest cover and biomass changes.

Keywords: Amazon; Ecuador; biomass; carbon dynamics; forest cover; palaeoecology; phytoliths.

Fig. 1

The left panel shows Ecuador in South America, the mean annual precipitation, our study sites in white circles (Lakes Añangucocha, Zancudococha, and Kumpak^a), nearby Lake Ayauchⁱ (as a white circle), and in white triangles Caves Pumacocha, Palestina and El Tigre Perdido (van Breukelen et al., ; Bird et al., ; Apaéstegui et al., ; Åkesson et al., 2023). The right panels show current forest cover around Lakes Añangucocha, Zancudococha and Kumpak^a, and the black lines represent the 200 m and 1 km buffers that were used in the quantitative estimations of forest cover and biomass.

Fig. 2

Estimated biomass (Mg ha⁻¹) and forest cover (%) changes through time (BCE to CE), along with Poaceae percentages (orange), charcoal (in scaled volumes) and maize presence for Lakes Añangucocha, Zancudococha and Kumpak^a. Black dots indicate a trace presence of charcoal. For Lake Kumpak^a, previously published forest cover estimates within a 1 km buffer based on Poaceae pollen are also shown (Blaus et al., 2023). Grey areas indicate the confidence intervals of the forest cover and biomass predictions for the three lakes.

Fig. 3

Oxygen isotope records (blue) that reflect drier and wetter conditions during the late Holocene (van Breukelen et al., ; Bird et al., ; Apaéstegui et al., 2014). These climatic reconstructions are compared with estimates of biomass (Mg ha⁻¹) and forest cover (%) changes through time (BCE to CE) for Lakes Añangucocha, Zancudococha and Kumpak^a. The climatic reconstructions are > 350 km from the lake study sites (see Fig. 1). Grey areas indicate the confidence intervals of the forest cover and biomass predictions for the three lakes.