ECOLOGICAL PERSISTENCE IN A CLOSED FOREST ECOSYSTEM FROM THE EARLY NEOGENE MUSH VALLEY LAGERSTÄTTE, NORTH-CENTRAL ETHIOPIA

Lacustrine sediments in the Mush Valley, north-central Ethiopia Highlands, provide a high resolution record of community composition and dominance-diversity patterns over several thousand years in an early Miocene closed canopy, mixed-moist semi-evergreen forest. Exquisite fossil leaves are preserved in a seven-meter thick section dominated by carbonaceous shales. Interspersed among the shale beds are 19 thin (1-3 cm) ash beds and a larger ash bed 10 cm thick. We censused over 2400 fossil leaves from six stratigraphic levels: two carbonaceous shales below the thicker ash bed (Levels A and B), the ash bed itself (Level C), and three carbonaceous shales above the ash bed (Levels D-F). Paleoprecipitation estimates derived from leaf area measurements per taxon conducted at each census level reveal no significant changes.

The census includes 49 leaf morphotypes. Species richness at 300 leaves varies from 20.4 species at F to 30.9 at D. Species richness was additively partitioned following Lande’s (1996) recommendations. Alpha richness captures 42% of total species richness, beta within a stratigraphic level 18%, and beta between stratigraphic levels 40%. Simpson’s D, which emphasizes abundant taxa over rare taxa, was also partitioned into alpha (>95%) and beta (<5%) levels. The same legume morphotype dominates all sites, and at least three of the five most abundant morphotypes at each level are legumes. Floral composition is very similar at levels B, D, E, and F, as demonstrated by non-metric multidimensional scaling, which operates on a matrix of species abundance data, and Jaccard similarity analyses, which utilize species presence-absence data. Level C is most distinct, with higher than average abundances of a Euphorbiaceous morphotype and Tacca umerii, an understorey taxon usually associated with secondary forests. Our data support ecological persistence of a forest ecosystem, with relatively minor compositional variance, during an interval of periodic volcanism.