Print PageThis study investigates the causes for, and distribution of, unimodal versus bimodal seasonal cycle of vegetation greenness in the Southwest United States using extensive site observations, climate data, satellite data, and the Lund-Potsdam-Jena (LPJ) vegetation model.
Results/Conclusions
A bimodal seasonal cycle of vegetation greenness is evident in satellite data and LPJ simulations across eastern Arizona and western New Mexico, characterized by peaks during late spring-early summer and late summer-early autumn. This bimodal green-up remains a pressing paradox for which many competing hypotheses exist. The mechanism for this seasonal pattern is demonstrated using LPJ and observational data and is found to deviate from the traditional pulse-reserve paradigm. This paradigm states that rainfall events in aridlands produce nearly immediate pulses of vegetation growth and accumulation of reserves but does not consider cold dormancy, time-lagged vegetation responses, or rainfall seasonality.
The following soil-moisture based mechanism for bimodal greening is proposed. The initial peak in vegetation greenness during late spring-early summer results from a break in cold dormancy and benefits from the gradual winter-long accumulation of deep soil moisture from weak synoptic rain events and snow melt in colder regions. Limited precipitation and ongoing transpiration, from the initial vegetation greening, trigger a mid-summer drying of the soil and a consequential minimum in vegetation activity. Later, pulses of monsoon rainfall in late summer-early autumn support the secondary greening, although significant runoff of brief, intense rainstorms and substantial soil evaporation limit moisture to the upper soil layers.
