Studies of the Pinus rigida-Serotina Complex I. A Study of Geographic Variation

Abstract

Pinus rigida and P. serotina, two closely related taxa which inhabit the eastern United States, have been described historically as varieties and subspecies of P. rigida and as separate species.These taxa are allopatric except for a narrow contact zone coincident with the Delmarva Peninsula and southeastern New Jersey.In this contact zone intermediate specimens have been encountered, and these have been interpreted both as belonging to clinally intermediate populations of a single continuum and as natural hybrids between two sympatric taxa.The present study was undertaken to provide additional information about the relationship of P. rigida and P. serotina and the composition of populations in the contact zone.A number of populations were sampled from the natural ranges of the two taxa and from the transition zone.Bud and seed characteristics, along with cone length, were found to be quite similar in the two taxa.Other features, such as needle length, peduncle length, cone diameter, and serotiny differed.All of these characters varied clinally within each taxon, and these clinal patterns converged toward the transition zone.The natural populations of this zone were shown to possess characteristics of a single taxon that were morphologically intermediate between nearby populations of the two taxa.They were interpreted as being intermediate populations of a single geographically variable complex.Evidence was detected of a local steepening of the morphological gradation through the transition zone, and this evidence was interpreted as supporting a system of secondary intergradation, rather than primary radiation.The authors follow Clausen (1939) in denoting the taxa as subspecies: P. rigida subsp.rigida and P. rigida subsp.serotina, because they fail to maintain their separate morphological and reproductive integrities in the contact zone, where they coalesce into a single entity.Pitch pine (Pinus rigida Mill.) and pond pine (P.serotina Michx.) are two closely related taxa of the genus Pinus L. subsection Australes Loud, which have long been the objects of taxonomic uncertainty.They have been described as distinct species (Shaw, 1914;Coker & Totten, 1945;Little, 1953), as varieties of P. rigida (Louden, 1838;Sargent, 1926; Sudworth, 1927;Harrar & Harrar, 1962), and as subspecies of P. rigida ( Clausen, 1939 ) .The most commonly cited diagnostic features for these taxa are needle length, cone shape, size and persistence of prickles, and degree of cone serotiny.Clausen (1939) described a dine in needle length which extended from pitch pine in Massachusetts to pond pine in Florida and which passed through a limited contact zone coincident with the Delmarva Peninsula and southeastern New Jersey.He also observed that the size and persistence of prickles were quite variable within each taxon, and that serotiny was not entirely reliable as a