Abstract
The Alborz mountains in N-Iran at 36° N rise from the Caspian Sea to 5671 m a.s.l., with warm-temperate, winter-deciduous forests in the lower montane belt in northern slopes, and vast treeless terrain at higher elevation. A lack of rainfall (ca. 550 mm at high elevations) cannot explain the absence of trees. Hence, it is an open question, which parts of these mountains belong to the alpine belt. Here we use bioclimatic data to estimate the position of the potential climatic treeline, and thus, define bioclimatologically, what is alpine and what is not. We employed the same miniature data loggers and protocol that had been applied in a Europe-wide assessment of alpine climates and a global survey of treeline temperatures. The data suggest a potential treeline position at ca. 3300 m a.s.l., that is ca. 900 m above the upper edge of the current oak forest, or 450 m above its highest outposts. The alpine terrain above the climatic treeline position shows a temperature regime comparable to sites in the European Alps. At the upper limit of angiosperm life, at 4850 m a.s.l., the growing season lasted 63 days with a seasonal mean root zone temperature of 4.5 °C. We conclude that (1) the absence of trees below 2850 m a.s.l. is clearly due to millennia of land use. The absence of trees between 2850 and 3300 m a.s.l. is either due to the absence of suitable tree taxa, or the only potential regional taxon for those elevations, Juniperus excelsa, had been eradicated by land use as well. (2) These continental mountains provide thermal life conditions in the alpine belt similar to other temperate mountains. (3) Topography and snow melt regimes play a significant role for the structure of the alpine vegetation mosaics.
Highlights
High elevation ecosystems have to be viewed from a climatological, rather than an elevation perspective, because the driver of life is the microhabitat’s climate rather than meters above sea level (Körner 2011)
The length of the growing season as defined by soil temperature at 10 cm depth varies from nearly 8 months at the oaks limit to 2 months at the high elevation angiosperm limit, with mean growing season soil temperatures in low stature vegetation declining from 14.5 °C to 4.5 °C
The temperatures at the regional upper limit of the Hyrcanian forest is far away from what is known to be critical for treeline globally, suggesting that this is a tree species limit, with trees missing at the potential climatic treeline
Summary
High elevation ecosystems have to be viewed from a climatological, rather than an elevation perspective, because the driver of life is the microhabitat’s climate rather than meters above sea level (Körner 2011). To capture the life conditions in such thermal niches, miniature data loggers have been successfully employed (Körner et al 2003) When such devices are buried in deep shade under high elevation trees or shrubs at 10 cm depth (no radiation driven soil heat flux), convective exchange between air and soil cause such data to mirror weekly means of air temperature as they act on tree crowns (Körner and Paulsen 2004). With such sensor placement, one can capture atmospheric conditions without installing a weather station
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