Characteristic straight-chain lipid ratios as a quick method to assess past forest–páramo transitions in the Ecuadorian Andes

Abstract

To assist sustainable reforestation efforts in the Andes in Northern Ecuador, the position of the natural upper forest line (UFL) has to be assessed. To this end, new vegetation reconstruction techniques to complement traditional reconstruction methods such as pollen analysis are being developed. One such technique is the biomarker ratio approach that uses changes in characteristic ratios of extractable lipids, such as n-alkanes and n-alcohols for a quick assessment at high spatial resolution of abrupt past vegetation changes. In the present study we assessed the applicability of this approach as indicator of past vegetation shifts from forest to páramo or vice versa in Northern Ecuador. We looked at the n-alkane and n-alcohol signals of two soil monoliths, one taken in a forest patch located 100 m above the current UFL, and the other at the same altitude in the surrounding páramo. The monoliths were sampled to a depth corresponding to at least 1500 cal yr BC. Links were sought between biomarker ratio signals in soil organic matter (SOM) and those found in plants from the vicinity during previous research. Based on the outcome an attempt was made at determining whether the forest patch had been part of the continuous forest further down slope during the last 3500 cal years. Results show that specific ratios of extractable straight-chain lipids can be used to get a quick indication of whether an area was forested or not in the past. In SOM the n-alkane relationship C 29/C 31 > 1 combined with the n-alcohol relationship C 22/C 24 < 1 was found to be characteristic for dominant forest vegetation, while the opposite relationships were indicative for past páramo vegetation. The n-alkane relationship could be explained very well by the lipid ratios of the current forest and páramo vegetation in the area. While agreeing well with the n-alkane signal, the n-alcohol signal was less robust, probably partially due to degradation of wax esters in the soil. Contrary to expectations, we conclude that the investigated forest patch was never part of the continuous forest in the vicinity in the past 3500 cal years. Instead our biomarker ratio approach suggests that the forest patch soil was once covered by páramo vegetation. Overall, this study shows that the biomarker ratio approach in volcanic ash soils can serve as a quick method for an assessment of abrupt vegetation changes such as associated with past UFL shifts at a high spatial resolution and at the location where the shift occurred. As such it is highly complementary to traditional proxies such as pollen analysis.