Environmental control of canopy dynamics and photosynthetic rate in the evergreen tussock grass Stipa tenacissima

Seasonal changes in leaf demography and gas exchange physiology in the tall evergreen tussock grass Stipa tenacissima, one of the few dominant plant s pecies in the driest vegetation of Europe, were monitored over a period of two years at a field site in semi-arid south-eastern Spain. Three age-class es of leaves - young, mature and senescent - were distinguished in the gree n canopy. Production of new leaves and extension growth of older leaves occ urred exclusively from October-November to May-June. The rate of extension was significantly correlated with gravimetric soil water content. Leaf grow th ceased after gravimetric soil water content fell below 0.015 g g(-1) at the beginning of the dry season which corresponded to pre-dawn leaf water p otentials of -3.0 MPa. Leaf senescence and desiccation reduced green leaf a rea by 43-49% during the dry season. Diurnal changes in the net photosynthe tic rate of all three cohorts of leaves were bimodal with an early morning maximum, a pronounced midday depression and a small recovery late in the af ternoon. Maximum photosynthetic rates of 10-16 mu mol CO2 m(-2) s(-1) were attained from November 1993 to early May 1994 in young and mature leaves. P hotosynthetic rate declined strongly during the dry season and was at or be low compensation in September 1994. Gas exchange variables of young and mat ure leaves were not significantly different, but photosynthetic rate and di ffusive conductance to water vapour of senescing leaves were significantly lower than in the two younger cohorts. Leaf nitrogen content of mature leav es varied seasonally between 2.9 and 5.2 g m(-2) (based on projected area o f folded leaves), but was poorly correlated with maxima of the photosynthet ic rate. There was a stronger linear relationship between the daily maxima of leaf conductance and pre-dawn leaf water potential than with atmospheric water vapour saturation deficit. Seasonal and between-year variation in da ily carbon assimilation were caused mainly by differences in climatic condi tions and canopy size whereas the effect of age structure of canopies was n egligible. Since water is the most important limiting factor for growth and reproduction of S. tenacissima, any future rise in mean temperature, which might increase evapotranspiration, or decrease in rainfall, may considerab ly reduce the productivity of the grasslands, particularly at the drier end of their geographical distribution.