Tree growth and (delta)13C among populations of Pinus greggii Engelm. at two contrasting sites in central Mexico

Basal area growth and carbon isotopic composition ((delta)13C) of three Pinus greggii Engelm. populations were correlated to climate variables at two contrasting common garden sites. The populations were Molango (MO), Pinon (PI) and Zimapan (ZI), all native to the state of Hidalgo. The test sites included a low-precipitation (800 mm) and high-elevation site (2600 masl) at Metepec, Mexico (ME), and a high-precipitation (1833 mm) and low-elevation site (1440 masl) at Huauchinango, Puebla (HU). The MO population had the highest variation in productivity with 15 m^3 ha^-1 per year at the HU site and 7.7 m^3 ha^-1per year at the ME site. The PI population had the best tree growth at the ME site with an estimated production of 11 m^3 ha^-1 per year. Trees at the ME site showed an average elevated (delta)13C content of 2.67‰ compared to the HU site. Differences between sites in (delta)13C were explained by differences in precipitation and altitude, which influenced water status and gas exchange processes in the different populations. Changes in soil water potential during the dry season matched differences in (delta)13C observed at both test sites. At the site level, significant correlation between (delta) 13C and mean annual temperature were observed only at the HU plantation. Correlations for the HU site significantly improved when temperatures of the driest period of the year were considered. No significant correlation between (delta)13C and annual precipitation among populations was found at the two sites. Correlation between (delta)13C and basal area increment during a 5-year period was significant in MO and PI populations at the ME site, with correlations of r = -0.42 (P = 0.04) and r = -0.40 (P = 0.05), respectively. At the HU site only the PI population showed significant correlation between (delta)13C and basal increment (r = -0.43; P = 0.03). Differences in wood (delta)13C were consistently detected for the two contrasting environments. The results indicate that the wood 13C signature provides a good integral index for screening P. greggii Engelm. population adaptation for dry sites.