CONSTRAINED GROWTH, CAMBIAL MORTALITY, AND DENDROCHRONOLOGY OF ANCIENT THUJA-OCCIDENTALIS ON CLIFFS OF THE NIAGARA ESCARPMENT - AN EASTERN VERSION OF BRISTLECONE-PINE

Cliffs of the Niagara Escarpment, Ontario, Canada have recently been shown to support an ancient forest dominated by Thuja occidentalis. Most of the trees are stunted and deformed, but the processes responsible for this have not been investigated. To deal with this problem, cores, cross sections, and intact trees of Thuja occidentalis up to 1,032 yr old were examined at sites along the escarpment. Radial growth rates as low as 0.05 mm yr-1 were found, but greater rates were found in the very oldest stems, which were also characterized by extensive cambial dieback and strip-bark growth. Less than 2% of the potential cambial surface was present in some trees. Few specimens over 100 yr old were free of cambial disruption, and most had lost half of their cambium between the ages of 130 and 280 yr. Loss of cambium and morphological distortion appear to be the result of the accumulation of point sources of damage over time; exfoliation of rock from beneath trees is suspected to be the principal stimulus. These growth, developmental, and ecological proper-ties are similar to those described for Pinus longaeva. Both species show constrained growth, cambial mortality, and gross modifications to morphology. In addition, both species show great age only when cambial mortality is extensive. Despite the gross distortions of morphology, a 344-yr chronology was constructed from 28 cross-dated tree-ring series. Given the success to date in locating older individuals at many sites, the prospects are good for a millennia-long chronology for southern Ontario that can be used for climatic reconstruction.