PATTERNS OF STEM PHOTOSYNTHESIS IN 2 INVASIVE LEGUMES (SPARTIUM-JUNCEUM, CYTISUS-SCOPARIUS) OF THE CALIFORNIA COASTAL REGION

Stem assimilation of Spartium junceum and Cytisus scoparius was measur ed over diurnal cycles during four time periods of 1992 in California. Spartium junceum plants were growing at an inland, coastal mountain s ite while the C scoparius plants were growing at a coastal bluff habit at. Both species had positive stem assimilation that resulted in appro ximately 200 mmol m-2 day-1 carbon dioxide accumulation. Daily carbon gain decreased from spring to fall for S. junceum due to a decrease in shoot water potential. Although C scoparius had the same relationship between assimilation and shoot water potential as that of S. junceum, carbon gain and water potential did not decrease during the year for C. scoparius. The major limitation for carbon gain of C scoparius was light intensity because the coastal site was characterized by lower te mperature, higher humidity, and more fog than the inland site. Althoug h these species grew in habitats that had a different vapor pressure a nd temperature, the assimilation response to vapor pressure was simila r between the species. Water use efficiency was higher and intercellul ar carbon dioxide was lower for S. junceum compared to C. scoparius. A lthough the leaves of both species are ephemeral, the canopy has a con stant positive carbon balance because of stem assimilation. The consta nt carbon gain throughout the year, from stem assimilation, may enhanc e the growth capacity of these invasive species in disturbed habitats.