Drought responses and their effects on radial stem growth of two co-occurring conifer species in the Mediterranean mountain range

KEY MESSAGE : Patterns of stem radial variations showed that Cedrus libani A. Rich. was less limited by summer drought than co-occurring Juniperus excelsa M. Bieb. Cedrus libani recovered faster from tree water deficit and showed significantly higher radial growth rates and annual stem increments than J. excelsa. However, the ability of J. excelsa to grow more hours per year may indicate a potential benefit in more extreme conditions. CONTEXT: Knowledge about species-specific drought responses is needed to manage productive forests in drought prone areas. Under water shortage, trees commonly show stem shrinkage, which is assumed to inhibit growth. AIMS: We investigated whether the two co-existing conifers Juniperus excelsa M. Bieb. and Cedrus libani A. Rich. (growing at the Taurus Mountains, SW-Turkey) show differences in water relations and stem growth in order to evaluate their respective drought tolerance. METHODS: Stem radius changes were hourly monitored over 2 years using high-resolution point dendrometers. Radial stem growth, tree water deficit-induced stem shrinkage, and maximum daily shrinkage were extracted from stem radius change measurements, investigated for their patterns, and related to environmental conditions. RESULTS: Cedrus libani recovered from tree water deficit under higher temperature and vapor pressure deficit than J. excelsa. The number of hours during which stem growth occurred was higher for J. excelsa; however, growth rates and annual increments were significantly lower than in C. libani. Both species showed highest maximum daily shrinkage during the driest months indicating the ability to maintain gas exchange all year round. CONCLUSION: Juniperus excelsa showed a more conservative growth strategy while C. libani was less limited by summer drought and showed more annual stem increment under the conditions investigated.