Old tree morphology in singleleaf pinyon pine (Pinus monophylla)

Singleleaf pinyon pine (Pinus monophylla) is a long-lived tree species that dominates montane plant communities over large areas of the arid and semi-arid Intermountain West, USA. Although old-growth forests are widely valued by society, old trees in pinyon-dominated woodlands may be threatened by active management against woodland expansion on rangelands, particularly in the absence of knowledge concerning old tree morphology and crown architecture. This study, which took place within a 15-km² area in central Nevada, used detailed field observations of tree morphology and dendrochronological measurements of tree age, in combination with principal components analysis and multiple linear regression, to identify the distinctive attributes of old P. monophylla trees that require long time periods to develop. P. monophylla tree age was most parsimoniously quantified using three linear combinations of field-measured variables, representing overall tree size, crown diminishment with age, and “stubbiness” (i.e. the combination of short stature and wide girth). These composite variables were derived from just five types of tree measurements: stem diameter, height, diameter of lowest branch, crown area, and an index of bark texture. The three composite variables have intuitive meaning and do not require precise quantification to aid researchers and natural resource managers in developing more accurate visual estimations of approximate tree ages in P. monophylla or other cembroid pines characteristic of arid lands. Old tree attributes identified in this study are likely characteristic of long-lived conifers growing in arid and semi-arid regions, and are expected to provide important elements of habitat structure and complexity for diverse taxa.