Modern surface pollen assemblages from the Middle and High Atlas, Morocco: insights into pollen representation and transport

Abstract

33 modern surface samples were collected in the environmentally and climatologically contrasting regions of the Middle and High Atlas Mountains, Morocco. Samples representing forested and steppe montane environments (1935 m to 2760 m a.s.l.) are clustered around study sites at Lake Tislit (High Atlas, semi-arid oro-Mediterranean bioclime), and Lake Sidi Ali and Michliffen (Middle Atlas, sub-humid montane Mediterranean bioclime). Good discrimination between regional pollen spectra is evident, with Middle Atlas samples reflecting higher arboreal cover (Cedrus and evergreen Quercus) and High Atlas samples with high abundances of non-arboreal taxa, including Artemisia and Fabaceae. These four taxa (Cedrus, evergreen Quercus, Artemisia and Fabaceae) are furthermore shown to be reliable indicators of local source vegetation within a 100 m 2 quadrat, taking into account threshold abundances of 7%, 20%, 4% and 10%, respectively. Deciduous Quercus, Olea and Phillyrea show long-distance pollen dispersal across both regions, contributing to non-trivial arboreal pollen (AP) values of up to 35% (typically 20-30%) in the High Atlas spectra. In the Middle Atlas, AP values of 40-50% occur in open sampling locations and >60% under forest canopy cover. These insights should be taken into account when interpreting ancient pollen spectra from regional lakes and bogs for palaeoenvironmental reconstruction.

Environmental Drivers of Holocene Forest Development in the Middle Atlas, Morocco

“…AP percentages below 20% are consistent with frequencies observed in surface samples from non-forested areas of the Middle and High Atlas at present (Bell and Fletcher, 2016). Artemisiadominated vegetation does not have analogs in the Lake Sidi Ali area today, but is dominant in more arid environments with high continentality.…”

“…Furthermore, upslope transport of pollen from lower elevations is typical of montane locations (Markgraf, 1980). This phenomenon is evident in modern surface pollen samples from the Lake Sidi Ali catchment (Bell and Fletcher, 2016), which contain taxa such as Olea, Phillyrea, and deciduous Quercus that grow at lower elevations in thermo-, meso-, and supra/upper-mediterranean bioclimates, respectively (Figure 3). The interpretation therefore considers that pollen source areas will be very large and will integrate signals across several altitudinal levels.…”

“…Cedrus peaks exceed 15 and 20%, respectively, during these episodes. In the study of soil surface samples from the Sidi Ali catchment (Bell and Fletcher, 2016), Cedrus percentages exceeding 7% were generally observed only in the immediate vicinity of Cedrus trees. The inferred low pollen dispersal of Cedrus, similar to that reported for the eastern Mediterranean species Cedrus libani A.Rich (Hajar et al, 2008), supports the interpretation of local presence of Cedrus stands in the Sidi Ali catchment during these episodes.…”

A comparison of pollen extraction methods confirms dense‐media separation as a reliable method of pollen preparation

“…Previous researchers have used both techniques; most of the Moroccan vegetation studies prepared pollen samples using HF (Lamb et al . , , ; Lamb and Van der Kaars, ; McGregor et al , ; Rhoujjati et al , ; Amami et al , ; Muller et al , ), while several recent studies prepared samples using dense‐media separation protocols (Zapata et al , ; Nour el Bait et al, ; Bell and Fletcher, ; Fletcher and Hughes, ). In a region that is the focus of an increasing amount of interest, particularly regarding Holocene environmental change, the knowledge that these records are directly comparable should facilitate the accurate evaluation of biogeographical changes across the region, including the nature and timing of environmental and vegetation shifts at a regional scale.…”

Fables and foibles: A critical analysis of the Palaeoflora database and the Coexistence Approach for palaeoclimate reconstruction

“…(vii) A number of nearest-living relatives that can tolerate or prefer snow (CMT ≤ −3°C) and even polar climates (WMT ≤ 10°C) or desert-steppe environments (HMP ≤ 35 mm/month) are found in coexistence with a majority of taxa intolerant to such climate niches. The latter can also be observed in contemporary pollen spectra, due to mixing of pollen originating from different source regions (Hofmann, 2002;Bell and Fletcher, 2016). Because of the rare MAP, HMP and LMP combination, the reconstructed climate has no modern analogue, hence, represents an "extinct climate" (according Utescher et al, 2014, p. 68f).…”