Divergent foliar trait responses to elevation in sympatric Mexican alpine pines

Abstract

This study examines functional trait variation in two sympatric Mexican alpine pines (Pinus culminicola and Pinus hartwegii) along an elevational gradient (2900–3700 m a.s.l.) on Cerro El Potosí, Nuevo León, Mexico. Five foliar traits (thickness, area, dry matter content, succulence, and specific leaf area) were measured in 25 Pinus hartwegii individuals (five elevations) and 15 Pinus culminicola individuals (three elevations) following standardized protocols. P. culminicola exhibited coordinated trait variation with elevation: needle thickness increased 23%, succulence increased 168%, while specific leaf area decreased 14% and dry matter content decreased 33%. Intra-specific variability decreased at higher elevations for most traits. In contrast, P. hartwegii maintained stable trait values across elevations for four of five traits, with only needle area showing a quadratic response (peak at mid-elevations, -32% at 3700 m). P. hartwegii exhibited needles 4.6 times larger and 18% higher SLA, while P. culminicola showed 39% higher dry matter content. We tested whether these differences reflect lineage-level patterns (Diploxylon vs. Haploxylon). Global analysis of 10 montane pine species revealed no consistent lineage-level patterns in trait-elevation relationships, with within-species variation often exceeding between-lineage differences. These contrasting responses to elevation—phenotypic stability in the widespread P. hartwegii versus coordinated trait changes in the endemic P. culminicola—illustrate the complexity of functional trait evolution in alpine environments.