The dominant conifers of the Pacific Northwest routinely attain 50+ m in height, which presents challenges in terms of maintaining the integrity of xylem water transport to their uppermost foliage. Gravity and frictional resistances associated with path-length combine to produce a gradient of increasing xylem tension with increasing height, which can steepen during warm, dry summers, especially as evaporative demand varies diurnally. Previous work conducted largely on tropical trees suggests that maintenance of hydraulic safety under dynamic conditions is achieved through combined reliance on hydraulic capacitance (C) to buffer diurnal fluctuations in xylem tension and xylem structural features that resist embolism formation. To gain insights into the mechanisms by which tall trees avoid catastrophic embolism in their upper branches, we measured xylem vulnerability to embolism, C, and seasonal variation in both minimum branch water potential (ψbr) and native embolism in four conifer species in an old-growth forest in southwestern Washington. Branch sapwood moisture release curves determined psychrometrically were used to evaluate the dependence of C on ψbr. Xylem pressures corresponding to 50% loss of hydraulic conductivity (P50) and the threshold at which embolism began to increase steeply (Pe) were used as indices of xylem vulnerability to embolism.
Results/Conclusions
Branch sapwood C and xylem vulnerability to embolism (Pe, P50) were positively correlated (p<0.02) and Pe and P50 increased by about 1.5 MPa over a range of C from about 200 to 400 kg m-3 MPa-1. Minimum ψbr measured during the driest part of the summer ranged from -2.5 MPa in Pseudotsuga menziesii, which had the lowest values of branch C, to -1.6 MPa in Thuja plicata, which had the highest values of branch C. Species-specific minimum ψbr measured in the field corresponded to the inflection points on sapwood moisture release curves at which C began to decrease sharply as water potential declined. The normal operating range of ψbr in situ coincided with the range over which the time constant for a transpiration-induced change in xylem tension asymptotically approached a minimum value, beyond which it would have increased sharply with the onset of embolism. The four tall conifer species studied attained adequate hydraulic safety under dynamic conditions by optimizing reliance on capacitance and xylem structural features to avoid excessive embolism. The behavior exhibited by these tall, tracheid-bearing trees appeared to be largely consistent with that previously observed by us in vessel-bearing tropical trees.