Multi-scale temporal variability of water relations in pinon pine (Pinus edulis)

This study characterizes the response of pinon pine ( Pinus edulis) to natural climatic variability by examining water relations at multiple scales, a prerequisite to forecasting the response of pinon-juniper ecosystems to global climate change. Stomatal gas exchange was measured with respect to soil moisture and atmospheric conditions over a period of two years. Inter-annual physiological responses were investigated at leaf level and whole tree level using carbon isotopes in leaves (delta 13Cleaf) and tree-rings (delta13C ring) as an index of integrated water use efficiency ( iWUE). Ring width indices (RWI) were used to estimate annual growth of individual trees. This study was conducted at two contrasting sites: an ecotonal/xeric site and a mid-elevation/mesic site.;The results of this study showed that pinon limits stomatal conductance (gs) in order to maintain mid-day water potentials above -2.6 MPa indicative of isohydric regulation of transpiration. Between site differences were most evident during the drought year when the xeric site had significantly higher water use efficiency and delta13Cleaf. Pre-dawn water potential was a strong determinant of the time of stomatal closure thereby limiting carbon gain in pinon. Evaporative demand also limited carbon gain through decreases in gs at higher VPD's, however, this varied with site and year.;Analysis of inter-annual variability in delta13C leaf indicated that pinons at the xeric site exhibit greater adjustments in annual iWUE in response to climatic variability than the pinon population at the mesic site. Analysis of inter-tree differences revealed that variability of delta13Cleaf is greater between individuals at the mesic site suggesting that environmental selection factors would favor only those trees with a higher iWUE at the xeric site.;The delta13C--climate relationships were stronger for delta13Cleaf than for delta13C ring, and these relationships were strongest at Goat Draw. The mean monthly maximum summer temperatures over May through September (summer Tmax) had the strongest influence on delta13Cleaf. 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 pinon pine populations could be vulnerable to decreased growth and, perhaps mortality, in response to warmer, drier conditions predicted by models of global climate change.