Roukos et al. : The effect of altitudinal zone on soil properties, species composition and forage production in a subalpine grassland in northwest Abstract. The impact of altitudinal gradient on soil properties, species composition and forage production was assessed in the subalpine grassland of "Kostilata" in northwestern Greece. The area is important for endemic species and for its traditional transhumant livestock system. Soil properties, species composition and forage production were determined annually and monthly during three consecutive years (April 2013 -October 2015 from thirty experimental plots located in three altitudinal zones (i.e., lower, middle, and upper). Our results suggested that the altitudinal zo ne strongly affected soil physical and chemical properties, species composition and forage production. Indeed, altitude a.s.l. was positively correlated with soil sand content and negatively correlated with forage production. It is found that stocking rate exceeded the grazing capacity, which posed a hazard to grassland sustainability; a belief amplified by the high sand content in soil and a terrain with steep slopes, which would increase the risk of further soil erosion at all altitudinal zones.