Holocene changes of Andean alder (Alnus acuminata) in highland Ecuador and Peru

Abstract

Fossil pollen records from the high Andes of Ecuador and northern Peru show that Alnus increased in abundance at most sites following warming associated with the onset of the Holocene. The peak of Alnus pollen abundance occurred between $ 9000 and 5000 yr BP. However, between 6500 and 4500 yr BP, the abundance of Alnus pollen declined markedly in all of the available records, including sites both within and above its modern distribution range. Even though the beginning of this decline may have been time-transgressive, the greatest reduction of Alnus abundance is consistent between sites, occurring at ca. 4500 yr BP. This synchrony suggests that one or more factors affected Alnus populations across the whole region. A cooler climate, droughts and anthropogenic activity are discussed as possible causes on such a large spatial scale. A scenario is suggested in which the early-mid Holocene increase in Alnus populations was curtailed by droughts, and then further impacted by major regional cooling from ca. 5000 yr BP onwards. Human agricultural activities in this wetter interval may also have delayed Alnus recovery. A resurgence of Alnus populations at 2000-1000 yr BP may have been a response to regional warming coupled with human land management practices.

A Holocene environmental record reflecting vegetation, climate, and fire variability at the Páramo of Quimsacocha, southwestern Ecuadorian Andes

“…However, all these taxa can either point to drier conditions or to disturbance (Ayers 1997;Weng et al 2004;Brunschön and Behling 2009). Taxa of the upper mountain rainforest also increase in abundance in this zone, and are thus indicating an upward shift of the upper forestline or a change towards windier conditions.…”

High-resolution studies on vegetation succession, hydrological variations, anthropogenic impact and genesis of a subrecent lake in southern Ecuador

“…Cyperaceae grew up in wet locations such as small ground depressions and watersides. At the same time pioneer trees of Alnus, here Alnus acuminata, expanded on moist and probably disturbed sites (Weng et al 2004b) and their pollen reached its highest proportion of 24% in an early successional stage. Some Alnus pollen could have originated at lower elevations and might have been dispersed to higher altitudes, but as pollen proportions never fell below 5% it probably reflects local occurrence (Weng et al 2004a).…”

Human settlements and climatic changes in arid southern Peru over the last 4 000 years around the Nevado Coropuna volcano

“…La forte diminution de l'influx et de la concentration polliniques avant cette période nous permet de déduire une période de forte sécheresse qui pourrait être à l'origine de la diminution des forêts de Polylepis. Hansen et al, 1984 ;Hansen & Rodbell, 1995 ;Chepstow-Lusty et al, 1998 ;Weng et al, 2004) Thompson et al, 1985 ;Abbott et al, 1997 ;Binford et al, 1997 ;Sterken et al, 2006). Au cours de l'anomalie climatique médiévale (ACM), l'augmentation de l'irradiation solaire et/ou une activité volcanique faible seraient à l'origine de l'augmentation des températures et de la diminution des précipitations (Mann et al, 2005).…”