The response of Piñon pine (Pinus edulis) to natural climatic variability

Background/Question/Methods Understanding the response of long-lived species at multiple scales, especially in the context of temporal variability, is a prerequisite to forecasting ecosystem responses to global climate change.  This study investigated the response of piñon pine (Pinus edulis) to natural climatic variability using information on physiology and growth recorded in leaves and tree-rings.  δ¹³C of annual leaf cohorts (δ¹³C_leaf) and tree-rings (δ¹³C_ring) were measured as an index for intrinsic water-use efficiency (iWUE) at two contrasting sites:  an ecotonal/xeric site and a mid-range/mesic site.  Ring width indices (RWI) were used to estimate annual growth of individual trees at both sites.  Relationships between climate and both δ¹³C and growth were also investigated at seasonal and annual timescales. 

Results/Conclusions Mean δ¹³C_ring was similar at both sites (20.66‰ ± 0.66‰ vs. -20.71‰ ± 0.75‰).  Comparing current year δ¹³C_ring with current year δ¹³C_leaf showed a strong correlation at the xeric site and a moderate correlation at the mesic site.  This suggests that current year δ¹³C_ring is either a function of stomatal regulation of all the leaf cohorts or is a function of current year δ¹³C_leaf photosynthate_.  Although the regressions for δ¹³C_ring and δ¹³C_leaf were significant in this study, δ¹³C_leaf  only accounted for 58% of the variation in δ¹³C_ring at Goat Draw and 29% of the variation in δ¹³C_ring at Cerro Montoso, suggesting that there may be a combination of current year photosynthate and stored carbon used in wood formation.  The δ¹³C-climate relationships were stronger for δ¹³C_leaf  than for δ¹³C_ring and this relationship was strongest at Goat Draw.  The mean monthly maximum summer temperatures over May through September (summer Tmax) had the strongest influence on δ¹³C_leaf .  Growth response did not vary between sites and there was a strong negative relationship between RWI with summer Tmax and a strong positive relationship between RWI with October to October precipitation (water year PPN).  This suggests that piñon pine populations could be vulnerable to decreased growth and, perhaps mortality, in response to warmer, drier conditions predicted by models of global climate change.