Xylem vessel structure is linked to functional shifts between post-fire resprouting and unburned chaparral shrubs

Background/Question/Methods

Post-fire chaparral shrub resprouts have stems that are generally more vulnerable to water stress induced xylem cavitation when compared to co-occurring unburned plants.  Resprouts also have lower xylem density and increased hydraulic efficiency.  The structural changes that drive these functional shifts are unknown.  The purpose of the present study was to examine xylem structural changes that may be linked to the observed shift in cavitation resistance and hydraulic efficiency of resprouting plants as compared to unburned plants of the same species.  Measured traits included vessel diameter, vessel to vessel contact fraction, connectivity, and (t/b)_h².  These traits were measured in one year old resprouting stems and in same-sized stems from unburned, approximately 30 year old plants, in 8 chaparral shrub species.

Results/Conclusions

All species displayed differences in vessel structure when one year old resprouting plants were compared to unburned plants.  In most species, vessel diameter significantly increased in resprouts, such as in Rhus ovata (mean ±SE vessel diameters in resprouts of 40.7±0.9 microns compared to 31.2±2.0).  However, in some species, there was no change in vessel diameter, suggesting that shifts in vessel diameter alone were not driving observed xylem functional shifts.  Changes in connectivity and contact fraction may account for some of the observed changes in efficiency and cavitation resistance, although shifts in these traits varied by species.  For stance, connectivity was higher in resprouting Ceanothus leucodermis compared to unburned plants (1.51±0.04 compared to 1.09±0.12), lower in Rhus ovata (0.96±0.06 compared to 1.19±0.11), and not different in Quercus wislizeni (0.09±0.03 compared to 0.10±0.10).  Consistent with changes in cavitation resistance, (t/b)_h² was consistently lower in resprouts.  These results support the link between vessel structure and function, while also demonstrating that structural traits can vary independently in different species.  Plants have many different ways that they can vary xylem structure to achieve the same overall functional effect and this explains, in part, the dramatic xylem anatomical diversity found among woody plants.