Limits to water transport in Juniperus osteosperma and Pinus edulis: implications for drought tolerance and regulation of transpiration

1. An air-injection method was used to study loss of water transport capaci ty caused by xylem cavitation in roots and branches of Pinus edulis (Colora do Pinyon) and Juniperus osteosperma (Utah Juniper). These two species char acterize the Pinyon-Juniper communities of the high deserts of the western United States. Juniperus osteosperma can grow in drier sites than P. edulis and is considered the more drought tolerant. 2. Juniperus osteosperma was more resistant to xylem cavitation than P. edu lis in both branches and roots. Within a species, branches were more resist ant to cavitation than roots for P. edulis but no difference was seen betwe en the two organs for J. osteosperma. There was also no difference between juveniles and adults in J. osteosperma; this comparison was not made for P. edulis. 3. Tracheid diameter was positively correlated with xylem cavitation pressu re across roots and stems of both species. This relation suggests a trade-o ff between xylem conductance and resistance to xylem cavitation in these sp ecies. 4. During summer drought, P. edulis maintained higher predawn xylem pressur es and showed much greater stomatal restriction of transpiration, consisten t with its greater vulnerability to cavitation, than J. osteosperma. 5. These results suggest that the relative drought tolerance of P. edulis a nd J. osteosperma results in part from difference in their vulnerability to xylem cavitation.