Geographical and climatic limits of needle types of one‐ and two‐needled pinyon pines
Authors: Cole Kenneth L. , Fisher Jessica , Arundel Samantha T. , Cannella John , Swift Sandra 
Published: September 2007

Aim The geographical extent and climatic tolerances of one‐ and two‐needled pinyon pines (Pinussubsect.Cembroides) are the focus of questions in taxonomy, palaeoclimatology and modelling of future distributions. The identification of these pines, traditionally classified by one‐ versus two‐needled fascicles, is complicated by populations with both one‐ and two‐needled fascicles on the same tree, and the description of two more recently described one‐needled varieties: thefallax‐type andcaliforniarum‐type. Because previous studies have suggested correlations between needle anatomy and climate, including anatomical plasticity reflecting annual precipitation, we approached this study at the level of the anatomy of individual pine needles rather than species.

Location Western North America.

Methods We synthesized available and new data from field and herbarium collections of needles to compile maps of their current distributions across western North America. Annual frequencies of needle types were compared with local precipitation histories for some stands. Historical North American climates were modelled on ac. 1‐km grid using monthly temperature and precipitation values. A geospatial model (ClimLim), which analyses the effect of climate‐modulated physiological and ecosystem processes, was used to rank the importance of seasonal climate variables in limiting the distributions of anatomical needle types.

Results The pinyon needles were classified into four distinct types based upon the number of needles per fascicle, needle thickness and the number of stomatal rows and resin canals. The individual needles fit well into four categories of needle types, whereas some trees exhibit a mixture of two needle types. Trees from central Arizona containing a mixture ofPinus edulisandfallax‐type needles increased their percentage offallax‐type needles following dry years. All four needle types occupy broader geographical regions with distinctive precipitation regimes.Pinus monophyllaandcaliforniarum‐type needles occur in regions with high winter precipitation.Pinus edulisandfallax‐type needles are found in regions with high monsoon precipitation. Areas supportingcaliforniarum‐type andfallax‐type needle distributions are additionally characterized by a more extreme May–June drought.

Main conclusions These pinyon needle types seem to reflect the amount and seasonality of precipitation. The single needle fascicle characterizing thefallaxtype may be an adaptation to early summer or periodic drought, while the single needle ofPinus monophyllamay be an adaptation to summer–autumn drought. Although the needles fit into four distinct categories, the parent trees are sometimes less easily classified, especially near their ancestral Pleistocene ranges in the Mojave and northern Sonoran deserts. The abundance of trees with both one‐ and two‐needled fascicles in the zones betweenP. monophylla,P. edulisandfallax‐type populations suggest that needle fascicle number is an unreliable characteristic for species classification. Disregarding needle fascicle number, thefallax‐type needles are nearly identical toP. edulis, supportingLittle’s (1968)initial classification of these trees asP. edulisvar.fallax,while the californiarum‐type needles have a distinctive morphology supportingBailey’s (1987)classification of this tree asPinus californiarum.