Effect of soil quality on determining the timberline at the Pico de Orizaba volcano

Abstract:

The inactive volcano Pico de Orizaba, located in the state of Veracruz, Mexico, has the highest treeline in the world at 4100 m above sea level, with the predominant tree species Pinus hartweggii. At this altitude trees are exposed to extreme conditions such as: low temperature (5°C), low humidity (<1%), UV radiation and a thinner atmosphere, among others factors. The study of soil factors is of paramount importance to understand the response of treeline to global warming. We have studied the physical and bioclimatic properties of the soils along altitudinal gradient (3500 a 4400 m) on the South face of Pico de Orizaba: organic matter, real and apparent density, texture and total amount of Va and Mo. Soil samples at treeline were also used to prepare thin sections to study the soil micromorphology. There is a disruption of values along the altitudinal gradient at treeline, between 4000 and 4100. The C/N ratio of soils below the treeline is typical of forest regions; however above 4100 m the value decreases to less than 1%. Also the quantity of interchangable cations is below 1cmol/kg above treeline. Analysis of thin soil sections show little weathering and compaction on regions above the treeline. We suggest that at least nine factors intervene in tree growth on the transition zone: soil temperature, organic matter, interchangable cations, clay content, compaction, porosity, C/N ratio, biological activity and soil structure. At the moment we cannot yet discern if some of them is preponderant as all of them are interlinked.