Aboveground respiratory CO2 effluxes from olive trees (Olea europaea L.)

The accurate assessment of respiration by woody vegetation, still a challenge in plant productivity models, is generally a problem of correctly scaling-up the process from organs to the whole plant. We used a large (41.6 m(3)), canopy chamber to enclose mature olive trees and to measure aboveground respiration (R (ag) ) under natural environmental conditions in an irrigated olive orchard in Crdoba (Spain). The 3-year study assessed nocturnal and seasonal R (ag) patterns in terms of temperature (T), plant dry matter composition, and phenology. The relative contributions of maintenance and growth respiration to R (ag) were determined empirically via an independent experiment. Although short-term variations in R (ag) rates were explained mainly by T variations, over seasonal time-scales this relationship was modulated by the vegetative composition of the olive trees and the contribution of growth respiration to R (ag) when the plants, in different seasons, allocated most of the new assimilates to actively growing shoots, flowers or fruits. Leaf mass and fruit load were the main determinants of R (ag) , which was weakly affected by differences in woody biomass since woody tissue respiration accounted for just 15 % of R (ag) . Respiration in olive trees during fruit setting periods is composed of approximately 30 % growth and 70 % maintenance. This study provides an independent evaluation of how, and to what degree, seasonally varying plant organ composition determines total respiration. Improved modelling of ecosystem respiration can be achieved by accounting for plant biological patterns characterising energy-requiring growth and maintenance processes, since biochemical kinetics alone cannot explain the observed seasonal variability.