Lateral Snow Transport, Fire and Changing Treelines in Mount San Gorgonio, California, U.S.A.
The mountain tree line correlates with the limit of carbon gain in photosynthesis, a temperature dependent process linked to atmospheric lapse rates. Studies of tree lines often do not focus on specific mechanisms that limit their distributions, including lateral snow transport (LST) and snow drifting, as well as wildfires. At Mount San Gorgonio (elevation 3505 m) in the eastern San Bernardino Mountains, 130 km east of Los Angeles, the subalpine forest tree-line (Pinus flexilis, P. contorta) has been stable over the 20th century. Above 3000 m blowing snow and snow drifting associated with the jet stream, combined with topographic wind enhancement, including forests denuded by fires, can significantly affect timberline dynamics. The reduction of surface roughness due to tree removal accelerates LST and lowers the timberline on leeward slopes subject to increased snow deposition. This paper evaluates LST at three ridgeline timberline sites in the San Gorgonio Wilderness that were burned in 1863, 1869 and 1951. Post-fire tree growth response is measured by using 200 m long sampling quadrats that traversed from windward to leeward slopes. Metrics include tree cover, height and diameter data. The results indicate that the LST is enhanced by fire which increases wind speeds, windward slope snow ablation, and leeward slope snow deposition, slowing tree establishment and growth. In the krummholz zone near San Gorgonio summit, recurrent damage from wind, snow and ice produce shear injury to windward slope tree canopies downwind (northeastward), result in continuous LST. On leeward slopes, late summer or perennial snow cover, maintains bare zones (tree lines) due to tree mortality, the snow decrease the growing season and increase the probability of tree death. At global scales, the tree line is predicted to ascend mountain slopes with planetary warming because the upper elevation boundary of woody vegetation is assumed to represent temperature limitations to positive net photosynthesis. An essential issue in the growth of plant canopy is the separation of intrinsic productivity from physical dieback over ecological time scales: What is carbon gain in canopy versus the rate of canopy removal due to wind-driven disturbances and snow burial over the life of the tree that reduces photosynthetic capacities? Without mass loss from LST, a hypothetical increase in forest canopy would permit tree colonization to higher elevations than the modern empirical limit. These findings are applicable to other mountain ranges at temperate latitudes subject to jet stream induced LST, and must be accounted for in the observation and prediction of tree lines in response to global climate change.