PHYSIOLOGICAL RESPONSE OF Taxodium mucronatum Ten. TO THE INCREASES OF ATMOSPHERIC CO2 AND TEMPERATURE IN THE LAST CENTURY

The increase of atmospheric CO2 can influence the growth of vegetation causing stomatal closure, changes in the water-use efficiency, or a fertilization effect that leads to greater growth. However, the response of vegetation depends on the environmental conditions and physiology of each species. The objective of this study was to evaluate the composition of carbon and oxygen isotopes (delta C-13 and delta O-18), the net discrimination of C-13 (Delta C-13), the growth rate in basal area (IAB) and the intrinsic water-use efficiency (iWUE) of Taxodium mucronatum Ten. (Montezuma baldcypress) over the last 100 years in three regions of central Mexico. The hypotheses were: 1) the physiological response of T. mucronatum Ten. to climatic variation is different among regions; 2) changes in the growth rate in basal area and physiological variables of the Montezuma baldcypress tree suggest that fertilization effects took place in the last century. The regions under study were: Estado de Mexico (MEX), Queretaro (QRO), and Morelos (MOR), with 600, 750, and 1038 mm rainfall. Nine trees per site were sampled to build a common chronology. In the growth rings, delta C-13 and delta O-18 were analyzed to estimate discrimination (Delta C-13), internal CO2 concentration (C-i), and iWUE in the last century. The data were analyzed as samples repeated over time with variable spacing. The difference between sites was tested with a multivariate analysis of variance (MANOVA) and contrasts tests. The delta C-13 on wood and C-i varied according to the moisture regime in MEX and MOR. In the past five decades, delta O-18 increased by 4 parts per thousand and the average annual temperature (AAT) increased from 1.2 to 1.4 degrees C. Since 1950, Delta C-13 decreased by 1.1 parts per thousand and iWUE increased by 50 %. The increase in Ca, iWUE, and AAT was related to the IAB decrease in MEX site, and a slight IAB increase in QRO and MOR, indicating a different CO2 fertilization effect depending on the moisture gradient.