The dendroecological potential of shrubs in north Iranian semi-deserts

In deserts and semi-deserts such as in the Irano-Turanian region in northern Iran, forest vegetation is scarce but shrubs are dominant. For this floristic province, placed in a biodiversity hotspot with a cold and dry climate, we provide the first climate-growth study on shrubs. From stems of three wide-spread shrubs (Astragalus, Rhamnus and Ephedra species) annual rings were identified and their widths measured.On average, around 40-year long annual-ring series per stem were obtained, cross-dated and related to meteorological variables. Astragalus and Rhamnus reflected a clear regional climate signal in their ring widths whereas Ephedra showed an only weak association with climate variables. While above-average air humidity in combination with low temperatures in spring and summer favored shrub growth, precipitation had surprisingly only a weak effect on growth. From the abundance of fog events in this area, we concluded that the extreme moisture dependency of the shrubs before and throughout the growing season may have been relieved by the uptake of fog drip through the foliage.As projected by climate models, the deficit in humidity will intensify and temperature will continue to rise in this region. So, the ability of the Irano-Turanian endemic shrubs to infiltrate into neighboring regions could become limited and their current distribution range may be confined to higher elevations which provide a moister and cooler environment.