Tree rings are useful to study changes if forest productivity, human-induced changes in atmospheric composition and physiological responses of trees to climate variability. This presentation will show particular experiences about the response of some Mexican forest species to changes in the chemistry of the atmosphere. We used isotopic measurements (¹³C, ¹⁸O, and ¹⁵N) and dendrochronological techniques to deduce these responses.
Study 1:
Indicates that Abies religiosa (Ar) and Pinus hartwegii (Ph) trees, in central Mexico, have increased their intrinsic water use efficiency (iWUE) with not increments in basal area (BAI), which suggest no CO₂ fertilization effects. Possibly, other factors like progressive nutrient limitation could have contributed to BAI decline.
Study 2:
Here we use a dual isotope approach (¹³C and ¹⁸O) in tree ring of Pseudotsuga menziesii (Douglas fir), to understand how this species coped with extreme climate events over the past century. Three primary forest sites, spanning ~5° of north latitude, in northwestern Mexico were analyzed to determine the effect of atypically dry and wet periods on carbon assimilation and intrinsic water use efficiency (iWUE). Results show that trees that resisted extreme dry events during the last 150 years did so by reducing BAI (>50%), requiring 6–10 years to return to original growth rates under favorable conditions. Extreme wet events, on the other hand, resulted in about 17% increase of BAI, a beneficial effect that lagged for ~3 years under average precipitation. Physiological changes, reflected by shifts in atmosphere to wood ¹³C discrimination and tree-ring ¹⁸O enrichment, appear to have been induced by changes in atmospheric CO₂ levels, but varied significantly between dry and wet periods. Under wet conditions increases in iWUE favoured carbon assimilation (A), but in dry years led to growth decline with lower stomatal conductance (gs) and associated declines in water loss trough transpiration.
Study 3.
We studied the combined effect of atmospheric CO₂ and nitrogen deposition on conifer (Pinus hartwegii – Ph; Abies religiosa - Ar) dominated forests near Mexico City. We show that species composition is a critical factor in predicting the impact of atmospheric changes on forest ecosystems. Ar trees promptly responded to atmospheric N deposition showing altered leaf nutritional status and significant growth decline. The response of Ph trees was delayed by decades until a similar decline in growth was observed. Multivariate analyses of plant nutritional status indicate that growth decline can be attributed to a concurrent shift in the availability of macro (P, K, Ca) and micro (Cu, Fe, Zn, Mn) nutrients in polluted N-rich sites. However, both leaf nutrient levels and growth response were species dependent, with fir trees better representing N saturation than pine trees due to intrinsic differences in canopy assimilation.