Drought and high temperatures are main environmental stresses for noxious plants in the arid environments. Responses of arid land plants to drought are complicated and include different adaptive mechanisms in terms of physiological, morphological, and phenological responses. This research aimed at investigating phenological and/or morphological responses of two globally important noxious plant species, Centaurea virgata Lam, and Scariola orientalis -Boiss.- Soják, during growth season of a dry year. Materials & Methods The present experimental research site was conducted in Noh-Dareh Mountains, Mashhad, Iran. Weekly field visits were done during the growth season -March to September- in 2011. Understory soil moisture and air temperature were recorded together with some morphological plant traits of both species. Daily air temperature and sporadic rainfalls were recorded and their possible effects on changes in plant phenology were investigated. The data were analyzed by SPSS 22 software, using t-test to examine differences in the selected parameters between the two species at each phenological stage. Findings Both species produced temporal rosette leaves that favored the high soil moisture in early growth season -March- and led to rapid shoot -stems and cauline leaves- production in the late April. However, they showed contrasting growth strategies in response to rain pulses and summer drought. Conclusion C. virgata is a drought escaping and opportunistic plant that threats the native flora during spring of normal and wet years, whereas S. orientalis is a slow growing drought resistant species and can be a major treat both in dry and normal years.

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