Is treeline moving up? Evidence from Colorado bristlecone pine (Pinus aristata)

Background/Question/Methods

Treeline shifts have been reported for the Alps, the Arctic and at several North American locales.  In most cases trees colonize alpine or arctic shrub or grassland just beyond current treeline.   To contrast the number of trees surviving above and below treeline since the 1950’s we measured bristlecone pine (Pinus aristata) stands with several surveys.  Our primary sample sites straddled treeline at 3660 m extending 50 m above and below treeline on the east aspect of Mount Goliath, 35 km west of Denver, CO.  Bristlecone tree density, height, and age were determined as well as branch and needle length for the past two years. 
Results/Conclusions

We found greater bristlecone density below treeline but since the late 1980’s trees have established at an equal rate above treeline. Bristlecone growing above treeline are not growing faster (cm/yr) nor are they significantly taller overall than those of the same age below tree line. Needles produced in 2007 are significantly longer above treeline (U = 367; p < 0.05), however, needle length for 2006 and branch length for both years show no significant difference.  During the growing season, soils above treeline wereare significantly warmer by 2.1^oC (U = 1575, p < 0.001) but have slightly less soil moisture than below treeline (U= 2447, p < 0.05).  Taken together, these data indicate that bristlecone pine has become established above present day treeline and is growing just as well (or better than) newly recruited trees in the same cohort below treeline.  In addition, bristlecone pine have established as high as 3713 m which is very close to the summit of Mt Goliath (3723 m).   We propose that bristlecone pine are successfully at colonizing within the alpine and that these trees are responding to changes in ambient conditions at the forest-alpine ecotone.