Ricardo D. Valdez-Cepeda
Universidad Autónoma Chapingo, Centro Regional Universitario Centro Norte, Apdo., Postal 196, CP 98001, Zacatecas, Zac., México
Blanca Mendoza
Universidad Nacional Autónoma de México, Instituto de Geofísica, Departamento de Investigaciones Solares y Planetarias, CP 04510, Ciudad Universitaria, Coyoacan, Méx., D. F., México
Rosa Díaz-Sandoval
Universidad Nacional Autónoma de México, Instituto de Geofísica, Departamento de Investigaciones Solares y Planetarias, CP 04510, Ciudad Universitaria, Coyoacan, Méx., D. F., México
José Valdés-Galicia
Universidad Nacional Autónoma de México, Instituto de Geofísica, Departamento de Investigaciones Solares y Planetarias, CP 04510, Ciudad Universitaria, Coyoacan, Méx., D. F., México
José D. López-Martínez
Universidad Juárez del Estado de Durango, Facultad de Agricultura y Zootecnia, División de Postgrado, Apdo. Postal 142, CP 35000, Gómez Palacio, Dgo., México
Enrique Martínez-Rubín de Celis
Instituto Agropecuario La Laguna, Apdo. Postal 3F, CP 27000, Torreón, Coah., México

Received 12 November 2002
Accepted 30 December 2002

Advanced statistical approaches have been applied to determine the temporal scaling-invariance laws in annual mean yields registered in Mexico during the period 1897–1999. When the maize, bean, wheat and rice series are treated as self-affine profiles through power spectrum analysis, the temporal fluctuation f^-β is detected in each case for time scales of 2 to 103 years. β values (p < 0.0001) of -1.84 ± 0.15, -1.71 ± 0.218, -1.62 ± 0.194 and -1.7 ± 0.139 for maize, bean, wheat and rice profiles, respectively, suggest that short-term variation is important and are influencing the dynamic evolution of the yearly mean yields. Thus, in addition to the linear trends driven by endogenous factors of agricultural systems, grain yield variation may be due to external forcings such as climatic oscillations and/or solar signals. Through the spectral analysis using the maximum entropy method, the influence of external forcings on the behavior of the annual mean yields in all four crops was evidenced. The detected main external forcings are: a 6- to 8-year political oscillation, the magnetic and sunspot cycles of the Sun, and El Niño Southern Oscillation (ENSO). Taking into account the number of significant periodicities, the order of annual mean yield series' sensitivity to external forcings influencing their short-term variation is as follows: maize < wheat < rice < bean.

Self-affinity; fractal dimension; oscillation; ENSO; maize; bean; wheat; rice