Determining suitable time for exploitation/ grazing of rangeland plants using growing degree days (GDD) model

One of the key principles in rangeland management is determination of suitable times for exploitation and grazing of rangeland plants in summer and winter pasturelands. This determination of time not only prevents degradation of plant species, but also leads to optimal usage of plant yield. Multiple factors affect plant phenology among which temperature regime has the highest effect on plant development and consequently exploitation and grazing of the plant. In this study, we used growing degree days (GDD) model to assess the phenology of three key rangeland plants: Artemisia sieberi, Halocnemum strobilaceium and Poa bulbosa. Initially, rangeland types were identified and determined in each region, then phenology stage, plant height, cumulative temperature, soil moisture and precipitation were measured and collected. To pick the phenology stage data, 10 individuals were selected from each species in each region and field visits started from the early March. Result showed that the start and end of phenology periods were different between key plants in each region within different years. Plant phenology was affected by climatic condition particularly temperature and precipitation. According to the result, the beginning of vegetative period for A. sieberi and H. strobilaceium was from mid-March to the end of March and the beginning of the vegetative period for P. bulbosa was late-December. This variation in plants phenology lied behind the variation of average monthly temperature in different years. The result of current study is applicable in adjustment of exploitation and grazing periods from plant species in different regions as well as adjustment of pastoral calendar.