Plant functional traits and functional diversity play a key role in forest ecosystems. Altitudinal patterns of plant functional diversity are poorly investigated in the Hyrcanian relict forest, northern Iran. In this study, differences in trait composition between lowland and montane forest types were analyzed. Variation of plant functional traits along a 2400 m altitudinal gradient was examined in 67 vegetation sampling plots established at different elevations along the gradient. Eight plant functional traits related to plant physiognomy, competitive ability and dispersal were considered for 174 recorded vascular plants. We used generalized linear model, principal coordinate analysis and functional diversity metrics to investigate functional changes along the gradient. The importance of different traits such as the chamaephyte life form, tiny leaves, dry indehiscent fruits and small seeds increase with altitude. In contrast, other traits including large leaves, maximum height of plants, fleshy fruits, anemophily as well as medium to large seeds decrease with increasing elevation. Our findings support the existence of strong functional differences between lowland and montane forest vegetation types. Trait associations, different environmental parameters, disturbances and especially elevation were discussed as important predictors of functional changes. Vegetation type is a key factor affecting functional diversity variation in temperate forests. Therefore, we strongly recommend exploring and considering functional diversity in forest management practices.

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