Effects of spring and autumn prescribed fires on plant species diversity and ecological groups in a dry grassland

Abstract

Wildfire is of the major components of dry grasslands, which may significantly affect their composition and dynamics. However in the recent decades, it is intentionally being used for restoration of grazing lands e.g. for reducing thorny shrubs and bushes. Two prescribed fires were applied at spring and autumn 2015, in a dry grassland, Dehbar, Torghabeh, Northeast Iran. In each time, fire was applied on four plots (10 m2), which were randomly selected in a landscape. A control (unburnt) plot was considered close to the each burnt plot. Plant measurements were conducted at the time of full vegetation growth (June) in 2015 and 2016. Canopy cover, density and frequency were recorded for all plants, growing in the burnt and unburnt plots. Important value index (IVI) was calculated for the plant ecological groups, classified based on Raunkiar (1939). Spring burning (SB) increased species diversity and evenness but reduced species richness, in the first year. Contrasting results were found one year after SB, i.e. higher richness but lower diversity and evenness in the burnt as compared to the control plots. Autumn burning (AB) reduced species diversity, richness and evenness indices. IVI of hemicryptophytes were increased (33.3%), geophytes reduced (32%) and therophytes were unchanged, one year after SB. A drastic reduction (37%) was found in IVI of chamaephytes one month after SB, but they could recover to same level as unburnt site after a year. AB increased IVI of hemicryptophytes (10%) and therophytes (13%), reduced that of chamaephytes (21%), whereas geophytes were unchanged. In general, BS was advantageous because of increasing species richness and replacing perennial grasses and forbs in the expense of invasive and geophyte species such as Poa bulbosa and Rosa persica, but it was ineffective in reducing importance of thorny and aromatic shrubs. Autumn burning was more effective on reducing the invasive shrubs but it negatively increased therophytes and invasive geophyte species, also reduced species diversity indices.