Asymmetric response of forest and grassy biomes to climate variability across the African Humid Period : influenced by anthropogenic disturbance?

A comprehensive understanding of the relationship between land cover, climate change and disturbance dynamics is needed to inform scenarios of vegetation change on the African continent. Although significant advances have been made, large uncertainties exist in projections of future biodiversity and ecosystem change for the world's largest tropical landmass. To better illustrate the effects of climate–disturbance–ecosystem interactions on continental‐scale vegetation change, we apply a novel statistical multivariate envelope approach to subfossil pollen data and climate model outputs (TraCE‐21ka). We target paleoenvironmental records across continental Africa, from the African Humid Period (AHP: ca 14 700–5500 yr BP) – an interval of spatially and temporally variable hydroclimatic conditions – until recent times, to improve our understanding of overarching vegetation trends and to compare changes between forest and grassy biomes (savanna and grassland). Our results suggest that although climate variability was the dominant driver of change, forest and grassy biomes responded asymmetrically: 1) the climatic envelope of grassy biomes expanded, or persisted in increasingly diverse climatic conditions, during the second half of the AHP whilst that of forest did not; 2) forest retreat occurred much more slowly during the mid to late Holocene compared to the early AHP forest expansion; and 3) as forest and grassy biomes diverged during the second half of the AHP, their ecological relationship (envelope overlap) fundamentally changed. Based on these asymmetries and associated changes in human land use, we propose and discuss three hypotheses about the influence of anthropogenic disturbance on continental‐scale vegetation change

Pollen rain and pollen representation across a forest-páramo ecotone in northern Ecuador

Modern pollen spectra were studied in forest and páramo vegetation from the Guandera area, northern Ecuador. Pollen representation was estimated by comparing the presence of plant taxa from a recent vegetation survey with the pollen spectra in moss polsters and pollen traps. In total, 73 pollen taxa were identified in the pollen rain. Per relevé, moss polsters and pollen traps contained 21 pollen taxa on average. Redundancy analyses (RDA) of pollen rain spectra against vegetation types yielded similar results for moss polsters and pollen traps. Spectra from forest, páramo, and cultivated field/meadows were well separated along the principal RDA axes, showing the potential to distinguish these vegetation types in pollen records. The modern pollen spectrum from a patch of páramo vegetation located in Andean forest was similar to the spectra of the surrounding forests. Likewise, the pollen spectrum from a forest patch in páramo was similar to spectra from the nearby páramo matrix. The modern pollen spectra from cultivated field/meadows hardly contained pollen taxa typically found in páramo or forest. In both forest and páramo, pollen taxa with wind-dispersed pollination syndromes were overrepresented. Clusia, Ilex, and Weinmannia (only with high percentages) seemed best to infer local presence of forest from pollen records. Puya, Apiaceae, Poaceae (only with high percentages), and Cyperaceae came out as best candidates to infer the presence of páramo vegetation

Holocene environmental change at the upper forest line in northern Ecuador

We have reconstructed the altitudinal position of the upper forest line (UFL) during the last 6000 years. This boundary between montane forest and páramo (tropical alpine grasslands) has important ecological and societal relevance. It is suggested that human-induced fires and deforestation during the long occupation history of the Central Valley of Ecuador have caused a downslope shift of the UFL and have given way to a downslope expansion of páramo vegetation. More recently, montane forests and lower páramo have been replaced to a large extent by agricultural land. Pollen analysis of a 90 cm long sediment core G15-II from a small mire at 3400 m elevation, 200 m below the actual UFL in Guandera Biological Reserve (0°36'N, 77°42'E), shows the altitudinal position of the UFL during seven discrete intervals: (1) from 7150 to 6240 cal. yr BP the UFL was at c. 3100—3200 m and climatic conditions were cool; (2) from 6240 to 5320 cal. yr BP the UFL shifted to c. 3600 m and upper montane rainforest (UMRF) surrounded the mire; (3) from 5320 to 2160 cal. yr BP the UFL was at 3600—3650 m elevation and montane forest consisted mainly of Hedyosmum, Weinmannia , Melastomataceae, Ilex, Scrophulariaceae and Symplocos; (4) from 2160 to 910 cal. yr BP the UFL shifted downslope to 3350 m and the mire was located in the lowermost páramo; (5) from 910 to 520 cal. yr BP cooler climatic conditions prevailed and the UFL was at 3250—3300 m; (6) since c. 520 cal. yr BP the UFL has shifted upslope to 3600 m. During this period presence of agricultural weeds (Rumex) and evidence of draining and disturbance of the mire indicate that agricultural activities expanded and almost reached the reserve area; (7) during the last 150 cal. yr disturbance increased. We conclude that during the last 6000 years the UFL reached a maximum altitude of 3650 to 3700 m, indicating that páramo grasslands above this elevation represent a natural ecosystem. Under the Kyoto Protocol-driven reforestation activities, trees should be planted up to a maximum of 3700 m. Planting trees (exotic species in particular) above 3700 m would contribute to the degradation of the natural ecosystem

Reconstruction of late Holocene forest dynamics in northern Ecuador from biomarkers and pollen in soil cores

Centuries of human interference have led to large scale reduction of montane forests in the northern Ecuadorian Andes. As a result the natural position of the upper forest line (UFL) in the area is now subject of scientific debate, which is hindering sustainable reforestation efforts. Uncertainty is fuelled by insufficient precision of fossil pollen spectra to reconstruct the natural UFL position. Here we tried to resolve this issue by using biomarkers, i.e. plant species specific patterns of n-alkanes and n-alcohols, preserved in soils in the northern Ecuadorian Andes as additional proxy to reconstruct the natural UFL position. To unravel preserved biomarker patterns we used the recently developed VERHIB model, and for the first time assessed its applicability in soil archives. Changes in Holocene biomarker-based vegetation composition were directly compared to changes in pollen-based vegetation composition from the same soil profiles. Both proxies proved to be complementary and a combined application allowed for a more accurate reconstruction of past vegetation than with pollen analysis alone. We found that the present-day UFL in the study area has not been significantly depressed by human interference and was at 3650 m maximally during late Holocene times. For the moment of post-glacial forest development we found a migration lag between pollen (earlier) and biomarkers (later). This reflects the difference between the non-transported biomarker signal showing spot-dating (thus in paleoecological studies functionally equalling the information from plant macro-remains in peat bogs), and the upslope wind-blown pollen signal showing an upslope forest expansion up to over a millennium ahead. The combined pollen-biomarker approach in soil cores shows great potential for vegetation reconstruction. However, more research of biomarker consistency and preservation is needed before application in other environments

The dynamic history of the upper forest line ecotone in the northern Andes

In the Andean cordilleras very conspicuous ecotones can be found. The transition from continuous upper montane forest to treeless herbaceous vegetation, regionally known as "páramo" (Cleef 1981; Luteyn 1999) is known as the "upper forest line" (UFL) or "timber line" (Holtmeier 2009). Above the UFL trees may occur forming small patches with diameters of ten to several hundreds of metres. The elevation where individual trees find their altitudinal limits is at significantly higher altitudes and this limit reflects the "upper tree line". In the Colombian Andes the upper tree line, most formed by dwarf trees of Polylepis, may be up to 800 m above the UFL. Therefore, it is relevant to differentiate between both ecotones. Across the Andes the altitudinal position of the UFL varies much depending latitude (Schmithüsen 1976) but in the area under consideration located between 0 and 11°N the altitudinal position of the UFL mostly varies between 3,000 and 3,800 m. In this paper we focus on the tropical Andes of northern Ecuador and Colombia. The spatial and temporal dynamics of the UFL ecotone, which has scientific and economic relevance is explored and discussed