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During the last deglaciation, atmospheric CO2 increased by about 75 ppm. The deep ocean is likely the dominant source of this atmospheric CO2 rise in the atmospheric pool; however, a consensus accounting for the entire 75 ppm remains elusive. Since the deep ocean cannot account for the entire 75 ppm, the terrestrial environment likely makes up the remainder. This paper provides a mechanism for an unaccounted-for portion of the source of this terrestrial carbon, that being soil organic carbon (SOC) from the tropical montane Andes, and with that, minimum constraints on the contribution of SOC to the total rise in atmospheric CO2 during the last deglaciation. Using numerical climate modeling input into an empirical model derived from tropical montane forests of the Andes Mountains in South America, this study finds that during the last deglaciation, the organic layer thickness was thinning from the Last Glacial Maximum (LGM) to the present (pre-industrial) in the tropical montane Andes. This overall warming and organic layer thickness thinning may have led to a loss of available carbon storage space, causing a leak of CO2 into the atmosphere over this time scale. This study finds an estimate for the contribution of global atmospheric CO2 from SOC in tropical montane Andean soils is likely at least ∼1.4 ppm CO2 since the LGM. [ABSTRACT FROM AUTHOR]

Plot-based monitoring has yielded much information on the taxonomic diversity and carbon (C) storage in tropical lowland forests of the Amazon basin. This has resulted in an improved understanding of the relationship between lowland forest biomass dynamics and global change drivers, such as climate change and atmospheric CO2 concentration. Much less attention has been paid to the mountain ecosystems of South America that comprise montane forests and alpine vegetation (páramo, puna, high Andean grasslands, wetlands, and alpine heath). This vegetation complex provides a variety of ecosystem services and forms a natural laboratory along various physiographic, geological and evolutionary history/biogeography, and land use history gradients. Here, we review existing empirical understanding and model-based approaches to quantify the contribution of mountain ecosystems to ecosystem service provision in the rapidly changing socioecological setting of the South American mountains. The objective of this paper is to outline a broad road map for the implementation of mountain vegetation into dynamic global vegetation models (DGVM) for use in Earth System Models (ESM), based on our current understanding of their structure and function and of their responsiveness to global change drivers. We also identify treeline processes, critical in mountain ecosystems, as key missing elements in DGVMs/ESMs, and thus explore in addition a treeline model. Stocktaking of the availability of empirical data was undertaken from eight research sites along the Andes and in south-eastern Brazil. Out of eight sites, two (one each in Venezuela and Brazil) had some climate, ecological and ecophysiological data potentially suitable to parametrise a DGVM. Tree biomass data were available for six sites. A preliminary assessment of the Joint UK Land Environment Simulator (JULES) DGVM was made to identify gaps in available data and their impacts on model parametrisation and calibration. Additionally, the potential climate-determined elevation of the treeline was modelled to check the DGVM for its ability to identify the transition between the montane forest and alpine vegetation. Outcomes of the evaluation of the JULES land surface model identified the following key processes in montane forests: temperature-related decrease in net primary production, respiration, and allocation to above-ground biomass and increase in soil C stocks with elevation. There was a variable agreement between simulated biomass and those derived from field measurements via allometric equations. We identified major gaps between data availability and the needs for process-based modelling of South American mountain vegetation and its dynamics in DGVMs. To bridge this gap, we propose a transdisciplinary network, composed of members of the theoretical/modelling and empirical scientific communities, to study the natural dynamics of mountain ecosystems and their responses to global change drivers locally, regionally and at the continental scale, within a social-ecological system framework. The work presented here forms the basis for the design of data collection from field measurements and instrumental monitoring stations to parametrise and verify DGVMs. The network is designed to collaborate with and complement existing long-term research initiatives in the region and will adopt existing standard field protocols. Complementary protocols will ensure compatibility between field data collection and data needed for process-based and empirical models. [ABSTRACT FROM AUTHOR]

Plants absorb nutrients and water through their roots, and modulate soil biogeochemical cycles. The mechanisms of water and nutrient uptake by plants depend on climatic and edaphic conditions, as well as the plant root system. Soil solution is the medium in which abiotic and biotic processes exchange nutrients, and nutrient concentrations vary with the abundance of reactive minerals and fluid residence times. High-altitude ecosystems of the tropical Andes are particularly interesting to study the association between vegetation patterns, soil hydrology and soil nutrient availability. The páramo landscape forms a vegetation mosaic of bunch-grasses, cushion-forming plants and forests. In the nutrient-depleted nonallophanic Andosols, the plant rooting depth varies with drainage and soil moisture conditions. Vegetation composition is a relevant indicator of rock-derived nutrient availability in soil solutions. The soil solute chemistry revealed patterns in plant available nutrients that were not mimicking the distribution of total rock-derived nutrients nor the exchangeable nutrient pool, but clearly resulted from strong biocycling of cations and removal of nutrients from the soil by plant uptake or deep leaching. Our findings have important implications for future management of Andean páramo ecosystems where vegetation type distributions are dynamically changing as a result of warming temperatures and anthropogenic disturbances. Such alterations may not only lead to changes in soil hydrology and solute geochemistry but also to complex changes in weathering rates and solute export downstream with effects on nutrient availability in Andean rivers and high-mountain lakes. [ABSTRACT FROM AUTHOR]

The role of topography in determining water transit times and pathways through catchments is unclear, especially in mountainous environments – yet these environments play central roles in global water, sediment, and biogeochemical fluxes. Since the vast majority of intensively monitored catchments are at northern latitudes, the interplay between water transit, topography, and other landscape and climatic characteristics is particularly underexplored in tropical environments. To address this gap, here we present the results of a multiyear hydrologic sampling campaign (twice-monthly and storm sampling) to quantify water transit in seven small catchments (<1.3 km 2 area) across the transition from the Andes mountains to the Amazon floodplain in southern Peru. We use the stable isotope composition of water (δ18 O) to calculate the fraction of streamflow comprised of recent precipitation ("young water fraction") for each of the seven small catchments. Flow-weighted young water fractions (Fyw) are 5 %–26 % in the high-elevation mountains, 22 %–52 % in the mid-elevation mountains, and 7 % in the foreland floodplain. Across these catchments, topography does not exert a clear control on water transit; instead, stream Fyw is apparently controlled by a combination of hydroclimate (precipitation regime) and bedrock permeability. Mid-elevation sites are posited to have the highest Fyw due to more frequent and intense rainfall; less permeable bedrock and poorly developed soils may also facilitate high Fyw at these sites. Lowland soils have low Fyw due to very low flow path gradients despite low permeability. The data presented here highlight the complexity of factors that determine water transit in tropical mountainous catchments, particularly highlighting the role of intense orographic precipitation at mountain fronts in driving rapid conveyance of water through catchments. These results have implications for the response of Earth's montane "water towers" to climate change and for water–rock reactions that control global biogeochemical cycles. [ABSTRACT FROM AUTHOR]