Water relations and carbon gain are closely related to cushion size in themoss Grimmia pulvinata

The present study of structural and physiological changes during the develo pment of the cushion moss, Grimmia pulvinata, quantifies the size-dependenc e of various parameters of water relations such as changes in surface:volum e ratio (S/V) or water loss rates, and also measures net CO2 gas exchange i n the light and the dark. Larger cushions had lower S/V values than smaller ones and featured lower rates of area-based evapotranspiration, owing to h igher boundary-layer resistance, but did not differ in relative water stora ge capacity (expressed as a percentage of d. wt). In combination, this lead s to considerably longer hydration periods in larger cushions. By contrast, CO2 gas-exchange parameters were negatively correlated with size: larger c ushions showed significantly lower (mass-based) rates of net photosynthesis and dark respiration. Using these data, we estimated carbon budgets during a drying cycle as a function of cushion size. When including alternations of dark and light periods, the relationship proved to be rather complicated . Depending on the time of hydration, net carbon budgets not only varied qu antitatively with size but sometimes took on both positive and negative val ues depending on cushion size. We conclude that neglecting plant size can l ead to unrepeatable or even misleading results in comparative ecophysiologi cal studies, and therefore urge for adequate attention to be paid to size i n these studies.