Hydraulic connectivity from roots to branches depicted through sap flow: analysis on a Quercus suber tree.

The topology of the xylem network is likely to affect the transport of water, propagation of embolism and plant survival and growth. Few studies have been conducted on the hydraulics of the entire water pathway in trees. We evaluated the hydraulic connections from roots to branches in a mature Quercus suber L. tree, through sap flow responses upon branch severing. Sap flow was recorded in branches, stem and roots by the heat field deformation (HFD) method. Results showed that roots, except for the taproot, were hydraulically connected to all branches, but the rest of the tree (stem, branches and taproot) was highly sectored. In the large roots that showed an integrated response to branch severing, the outer xylem was preferentially connected to the same side branch and the inner xylem to the opposite branch. The hydraulic sectoriality in branches, stem and taproot may be regarded as an adaptive trait to water stress. The integrated hydraulic structure of roots is advantageous under patchy soil conditions, but may allow the spread of root diseases. The HFD sap flow method proved extremely useful to calculate xylem flux connectivity between different organs of a large tree, providing a comprehensive picture of its hydraulic architecture.

Importance of short-term dynamics in carbon isotope ratios of ecosystem respiration (delta13C(R)) in a Mediterranean oak woodland and linkage to environmental factors.

Temporal dynamics in carbon isotope ratios of ecosystem respiration (delta13C(R)) were evaluated on hourly, daily and annual timescales in a Mediterranean woodland. Emphasis was given to the periods of transition from wet to dry season and vice versa, when the system turns from a net carbon sink to a source. The constancy of nocturnal delta13C(R) was tested. The relationship between delta13C(R) (determined through Keeling plots) and environmental factors was evaluated through time-lag analysis. Delta13C(R) exhibited high annual variation (> 7). During the transition periods, delta13C(R) correlated significantly with factors influencing photosynthetic discrimination, soil respiration, and whole-canopy conductance. Time-lags differed between below- and above-ground variables, and between seasons. A shift in regression parameters with environmental factors indicated seasonal differences in ecosystem responsiveness (e.g. temperature acclimation). Delta13C(R) exhibited substantial nocturnal enrichment (> 4) from dusk to dawn. These data indicate pronounced short-term dynamics in delta13C(R) at hourly to daily timescales and a modulated response to environmental drivers. Substantial short-term changes in nocturnal delta13C(R) may have important implications for the sampling protocols of nocturnal Keeling plots.