Forbs Rather Than Grasses as Key Factors Affecting Succession of Abandoned Fields - A Case Study from a Subalpine Region of the Eastern Tibet Plateau

An old-field chronosequence in the subalpine region of the Tibetan Plateau were used as a model system to test a hypothesis that forbs drive pathways of successional trajectories in earlier stages of succession and grasses drive the development of vegetation in later successional stages. All old fields were dominated by forbs, which accounted for 65-85% of species richness and abundance. Species richness and total plant abundance significantly increased with time since abandonment. This is in disagreement with ‘humped-back model’. Although no consistent changes in seed size in the different functional groups found over time, however, there was a significant decline for the forbs, legumes, and annuals, except for the 1-year old field. In this species-rich subalpine ecosystem, forbs rather than grasses and sedges were identified as key factors affecting community structure and plant assemblages, whenever in the earlier successional stages or in the later successional stages. These indicated that grassland managers and policy makers should recognize potential role of forbs in biodiversity conservation and ecosystem functioning.