Estimating forest-grassland dynamics using soil phytolith assemblages and δ¹³C of soil organic matter

Our objectives were to examine the ralationship between contemporary vegetation and surface soil phytolith assemblages, and use phytoliths and 8¹³ of soil organic matter (SOM) to explore forest-grassland vegetation dynamics. We established plots within three canopy types (open, old-growth and dense young pine) with different grass species compositions in a Pinus ponderosa forest in northern Arizona and collected vegetation data and surface (0-2 cm) and subsurface (2-7 cm) mineral soil samples. Surface soil phytolith assemblages strongly reflected vegetation at the site scale (within several km²). Local vegetation patterns (< 50 m²) associated with overstory canopy types were weakly detected. Significantly fewer C₄ grass and ponderosa pine phytoliths were found in subsurface compared to surface soils. Surface 8¹³C values showed no difference among canopy types. Subsurface 8¹³C values were significantly (+ 0.83 %) more enriched in ¹³C than surface values. Phytolith assemblages and 8¹³C of SOM reflect long-term accumulation of organic matter in soils and may not mirror contemporary vegetation for many reasons, including spatial shifts in species distribution and productivity. Considering all our phytolith and 8¹³C evidence, we suggest that C₄ grasses were more widely distributed but less abundant, grasses were more spatially continuous, total grass productivity was greater, and species in the genus Kneleria and Bromus were more common in the past.