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Resumen Objetivo. Se presenta un estudio de la dinámica del clima y la vegetación del humedal Medellincito (5° 19’ 022” N y 75° 54’ 515” W), localizado en el flanco occidental de la cordillera Occidental de Colombia a 2020 msnm. Alcance. El predominio del bosque Andino durante todo el registro indica que este creció en los alrededores inmediatos bajo condiciones casi permanentes de humedad. Sin embargo, eventos de menor precipitación se registraron alrededor de 6700, 5900, 5100, 4500, 4100-4300, 3200-1300, 1000-750 cal años AP, durante los cuales crecieron especies de vegetación abierta conformadas por Poaceae, Asteraceae, Ericaceae y Cyperaceae. La alta humedad del sitio puede deberse a la influencia constante de la corriente de chorro del Pacífico colombiano y de la Zona de Convergencia Intertropical (ZCIT). Aunque no hay evidencias directas de la acción del fenómeno El Niño de alto impacto en el occidente de Colombia, si puede especularse que los eventos más secos puedan tener alguna relación con este fenómeno. Resultados. Los registros de polen, esporas y diatomeas de los últimos ( 6700 años cal AP muestran que la vegetación del bosque Andino estuvo dominada por Cyatheaceae, Hedyosmum (Chloranthaceae), Solanaceae, Viburnum, Miconia, Weinmannia y Myrsine, y el bosque Subandino por Fabaceae, Alchornea (Euphorbiaceae) y Bignoniaceae. Conclusiones. En los últimos ( 150 cal años AP, el paisaje se ha modificado significativamente pasando de ser un bosque muy húmedo a bosque húmedo, reflejado en el drástico decrecimiento de Hedyosmum, Solanaceae y Cyatheaceae, y un incremento en la vegetación abierta, principalmente Poaceae junto a Cyperaceae en ambientes de pantano, debido a intervención antrópica (extracción selectiva de especies maderables y ganadería extensiva). Es posible que efectos negativos de advección de humedad por la corriente de Chorro hayan afectado la precipitación horizontal y con ello la disminución del bosque de niebla, característico de ecosistemas montanos bajos. Abstract Objective. Our research is focused on climate and vegetation dynamic recorded by the Medellincito wetland (5° 19’ 022” N and 75° 54’ 515” W), located on the western flank of the Western cordillera of Colombia at 2020 masl. Scope. A predominance of the Andean forest along the entire core suggests that the wetland was established under almost permanent humidity conditions. Nevertheless, low precipitation events are also recorded around 6700, 5900, 5100, 4500, 4100-4,300, 3200-1300, and 1000-750 cal years AP, when open vegetation such as Poaceae, Asteraceae, Ericaceae and Cyperaceae occurred. The high humidity conditions interpreted for the studied site may be indicating the constant influence of the Pacific jet stream and the Intertropical Convergence Zone (ITCZ). Although there is no direct evidence of El Niño activity, which has a high impact on the western Colombia, we speculate that drier events identified in the Medellincito wetland may be related to this phenomenon. Results. The pollen, spore, and diatom records reveal that during the last ( 6700 years Cal AP, the Andean forest vegetation was dominated by Cyatheaceae, Hedyosmum (Chloranthaceae), Solanaceae, Viburnum, Miconia, Weinmannia and Myrsine, and the Subandean forest by Fabaceae, Alchornea (Euphorbiaceae) and Bignoniaceae. Conclusions. In the last ( 150 years cal BP, the landscape has significantly changed from a very humid to a humid forest. This is evidenced by the drastic decrease of Hedyosmum, Cyatheaceae, and Solanaceae, and the increase of open vegetation, mainly Poaceae and Cyperaceae in swamp environments, due to anthropic intervention (selective extraction of timber species and extensive livestock). It is possible that negative effects of humidity advection by the Jet stream, have affected the precipitation and, consequently, decreased the cloud forest characteristic of low montane ecosystems.

A 17 kyr long pollen record from the unexplored Western Cordillera of Colombia (Paramo de Frontino; 3460 m elevation) shows vegetation change and inferred climate dynamics at ~ 125 yr resolution. The cold and wet Lateglacial showed well-defined stadials and interstadials. At the transition to the Holocene the upper forest line (UFL) shifted within 200 years 700-800 m upslope reflecting a temperature increase of ~ 4.5 °C. Dead wood in the forests may have caused frequent fires. Individual taxa show clear and discrete successive expansion events, such as Podocarpus around 15 cal ka, Quercus, Melastomataceae, Myrsine, Weinmannia, and Hesperomeles around 11.5 cal ka, and Alnus around 9 cal ka showing the floristic composition of the montane forest changed during its upslope migration. After forest had reached around 9 cal ka a quasi-stable altitudinal interval forest taxa continued to change proportions showing that upslope forest migration caused significant internal forest dynamics. Forest stability is not found suggesting that an equilibrium between forest composition and environmental variables was not reached. We hypothesize trees migrated faster upslope than the shrubs causing a temporarily loss of this shrub biome. During most of the Holocene the UFL varied between 3500 and 3700 m indicating warmer temperatures than today. High upper limits of subandean forest suggest that the lowermost level of significant night frost was at higher elevations than today. Driest pulses were registered ~ 9170 and ~ 8200 cal ka since 620 cal ka deforestation of Quercus, Podocarpus and Weinmannia in particular shows anthropogenic impact. Comparisons with other Andean climate records show a climatic asymmetry mainly related to migrations of the ITCZ. An implication of the present benchmark records of vegetation and climate variability and modeling is that new records should be analyzed at better than century resolution.