Human disturbance amplifies Amazonian El Niño-Southern Oscillation signal

Fire activity and deforestation in Remote Oceanian islands caused by anthropogenic and climate interactions

Remote islands in the Pacific Ocean (Oceania) experienced dramatic environmental transformations after initial human settlement in the past 3,000 yr. Here, human causality of this environmental degradation has been unquestioned and viewed as evidence of the inherent destructive tendencies of human societies in both archaeological and popular discourse. We use charcoal and stable carbon isotopes from deep soil cores to reconstruct the dynamics of fire activity and deforestation across the Sigatoka River valley on the leeward (dry) side of Viti Levu, Fiji. Fires and pyrogenic patches of grassland predated human settlement by millennia, but the magnitude of fire activity and landscape transformation accelerated with the establishment and expansion of swidden agriculture. Regional comparisons with previous studies in Fiji and elsewhere in Remote Oceania settled between 3,200 and 2,900 yr BP reveal a similar pattern of pre- and post-settlement fire activity and landscape change. Pre-settlement fires generally corresponded to droughts, probably driven by El Niño, often correlating with drought-driven wildfires elsewhere in the region. Post-settlement, charcoal and C4 grasses increased dramatically, but nearly all major peaks in charcoal and grasses corresponded to increased El Niño activity. This indicates that fire activity and deforestation were a product of the interaction between swidden agriculture and climate rather than land use alone.

Human practices behind the aquatic and terrestrial ecological decoupling to climate change in the tropical Andes

Anthropogenic climate change and landscape alteration are two of the most important threats to the terrestrial and aquatic ecosystems of the tropical Americas, thus jeopardizing water and soil resources for millions of people in the Andean nations. Understanding how aquatic ecosystems will respond to anthropogenic stressors and accelerated warming requires shifting from short-term and static to long-term, dynamic characterizations of human-terrestrial-aquatic relationships. Here we use sediment records from Lake Llaviucu, a tropical mountain Andean lake long accessed by Indigenous and post-European societies, and hypothesize that under natural historical conditions (i.e., low human pressure) vegetation and aquatic ecosystems' responses to change are coupled through indirect climate influences-that is, past climate-driven vegetation changes dictated limnological trajectories. We used a multi-proxy paleoecological approach including drivers of terrestrial vegetation change (pollen), soil erosion (Titanium), human activity (agropastoralism indicators), and aquatic responses (diatoms) to estimate assemblage-wide rates of change and model their synchronous and asynchronous (lagged) relationships using Generalized Additive Models. Assemblage-wide rate of change results showed that between ca. 3000 and 400 calibrated years before present (cal years BP) terrestrial vegetation, agropastoralism and diatoms fluctuated along their mean regimes of rate of change without consistent periods of synchronous rapid change. In contrast, positive lagged relationships (i.e., asynchrony) between climate-driven terrestrial pollen changes and diatom responses (i.e., asynchrony) were in operation until ca. 750 cal years BP. Thereafter, positive lagged relationships between agropastoralism and diatom rates of changes dictated the lake trajectory, reflecting the primary control of human practices over the aquatic ecosystem prior European occupation. We interpret that shifts in Indigenous practices (e.g., valley terracing) curtailed nutrient inputs into the lake decoupling the links between climate-driven vegetation changes and the aquatic community. Our results demonstrate how rates of change of anthropogenic and climatic influences can guide dynamic ecological baselines for managing water ecosystem services in the Andes.

Analysis of how the spatial and temporal patterns of fire and their bioclimatic and anthropogenic drivers vary across the Amazon rainforest in El Niño and non-El Niño years

In the past two decades, Amazon rainforest countries (Brazil, Bolivia, Colombia, Ecuador, Guyana, Peru and Venezuela) have experienced a substantial increase in fire frequency due to the changes in the patterns of different anthropogenic and climatic drivers. This study examines how both fire dynamics and bioclimatic factors varied based on the season (wet season and dry season) El Niño years across the different countries and ecosystems within the Amazon rainforest. Data from publicly available databases on forest fires (Global Fire Atlas) and bioclimatic, topographic and anthropogenic variables were employed in the analysis. Linear mixed-effect models discovered that year type (El Niño vs. non-El Niño), seasonality (dry vs. wet), land cover and forest strata (in terms of canopy cover and intactness) and their interactions varied across the Amazonian countries (and the different ecosystems) under consideration. A machine learning model, Multivariate Adaptive Regression Spline (MARS), was utilized to determine the relative importance of climatic, topographic, forest structure and human modification variables on fire dynamics across wet and dry seasons, both in El Niño and non-El Niño years. The findings of this study make clear that declining precipitation and increased temperatures have strong impact on fire dynamics (size, duration, expansion and speed) for El Niño years. El Niño years also saw greater fire sizes and speeds as compared to non-El Niño years. Dense and relatively undisturbed forests were found to have the lowest fire activity and increased human impact on a landscape was associated with exacerbated fire dynamics, especially in the El Niño years. Additionally, the presence of grass-dominated ecosystems such as grasslands also acted as a driver of fire in both El Niño and non-El Niño years. Hence, from a conservation perspective, increased interventions during the El Niño periods should be considered.

Widespread reforestation before European influence on Amazonia

An estimated 90 to 95% of Indigenous people in Amazonia died after European contact. This population collapse is postulated to have caused decreases in atmospheric carbon dioxide concentrations at around 1610 CE, as a result of a wave of land abandonment in the wake of disease, slavery, and warfare, whereby the attendant reversion to forest substantially increased terrestrial carbon sequestration. On the basis of 39 Amazonian fossil pollen records, we show that there was no synchronous reforestation event associated with such an atmospheric carbon dioxide response after European arrival in Amazonia. Instead, we find that, at most sites, land abandonment and forest regrowth began about 300 to 600 years before European arrival. Pre-European pandemics, social strife, or environmental change may have contributed to these early site abandonments and ecological shifts.

Facing Change through Diversity: Resilience and Diversification of Plant Management Strategies during the Mid to Late Holocene Transition at the Monte Castelo Shellmound, SW Amazonia

Recent advances in the archaeology of lowland South America are furthering our understanding of the Holocene development of plant cultivation and domestication, cultural niche construction, and relationships between environmental changes and cultural strategies of food production. This article offers new data on plant and landscape management and mobility in Southwestern Amazonia during a period of environmental change at the Middle to Late Holocene transition, based on archaeobotanical analysis of the Monte Castelo shellmound, occupied between 6000 and 650 yr BP and located in a modern, seasonally flooded savanna–forest mosaic. Through diachronic comparisons of carbonized plant remains, phytoliths, and starch grains, we construct an ecology of resource use and explore its implications for the long-term history of landscape formation, resource management practices, and mobility. We show how, despite important changes visible in the archaeological record of the shellmound during this period, there persisted an ancient, local, and resilient pattern of plant management which implies a degree of stability in both subsistence and settlement patterns over the last 6000 years. This pattern is characterized by management practices that relied on increasingly diversified, rather than intensive, food production systems. Our findings have important implications in debates regarding the history of settlement permanence, population growth, and carrying capacity in the Amazon basin.

2,100 years of human adaptation to climate change in the High Andes

Humid montane forests are challenging environments for human habitation. We used high-resolution fossil pollen, charcoal, diatom and sediment chemistry data from the iconic archaeological setting of Laguna de los Condores, Peru to reconstruct changing land uses and climates in a forested Andean valley. Forest clearance and maize cultivation were initiated during periods of drought, with periods of forest recovery occurring during wetter conditions. Between AD 800 and 1000 forest regrowth was evident, but this trend was reversed between AD 1000 and 1200 as drier conditions coincided with renewed land clearance, the establishment of a permanent village and the use of cliffs overlooking the lake as a burial site. By AD 1230 forests had regrown in the valley and maize cultivation was greatly reduced. An elevational transect investigating regional patterns showed a parallel, but earlier, history of reduced maize cultivation and forest regeneration at mid-elevation. However, a lowland site showed continuous maize agriculture until European conquest but very little subsequent change in forest cover. Divergent, climate-sensitive landscape histories do not support categorical assessments that forest regrowth and peak carbon sequestration coincided with European arrival.

A 7000-year history of changing plant trait composition in an Amazonian landscape; the role of humans and climate

Tropical forests are shifting in species and trait composition, but the main underlying causes remain unclear because of the short temporal scales of most studies. Here, we develop a novel approach by linking functional trait data with 7000 years of forest dynamics from a fossil pollen record of Lake Sauce in the Peruvian Amazon. We evaluate how climate and human disturbances affect community trait composition. We found weak relationships between environmental conditions and traits at the taxon level, but strong effects for community‐mean traits. Overall, community‐mean traits were more responsive to human disturbances than to climate change; human‐induced erosion increased the dominance of dense‐wooded, non‐zoochorous species with compound leaves, and human‐induced fire increased the dominance of tall, zoochorous taxa with large seeds and simple leaves. This information can help to enhance our understanding of forest responses to past environmental changes, and improve predictions of future changes in tropical forest composition.

The deep human prehistory of global tropical forests and its relevance for modern conservation

Significant human impacts on tropical forests have been considered the preserve of recent societies, linked to large-scale deforestation, extensive and intensive agriculture, resource mining, livestock grazing and urban settlement. Cumulative archaeological evidence now demonstrates, however, that Homo sapiens has actively manipulated tropical forest ecologies for at least 45,000 years. It is clear that these millennia of impacts need to be taken into account when studying and conserving tropical forest ecosystems today. Nevertheless, archaeology has so far provided only limited practical insight into contemporary human-tropical forest interactions. Here, we review significant archaeological evidence for the impacts of past hunter-gatherers, agriculturalists and urban settlements on global tropical forests. We compare the challenges faced, as well as the solutions adopted, by these groups with those confronting present-day societies, which also rely on tropical forests for a variety of ecosystem services. We emphasize archaeology's importance not only in promoting natural and cultural heritage in tropical forests, but also in taking an active role to inform modern conservation and policy-making.