Responses of aboveground production to changes in water availability: Understanding the meristem density limitation

L. G. REICHMANN; O. E. SALA; D. PETERS

Estes Park, Colorado, USA

Congreso; LTER all scientists meeting; 2006

Aboveground net primary production (ANPP) and its variability have been topics of interest to ecologists for a long time. In arid ecosystems, ANPP increases linearly along spatial precipitation gradients, but ANPP variability is largest in sites with intermediate mean annual precipitation (MAP)(Knapp and Smith 2001). Bud density and dynamics are thought to be one of the principal constraints, after annual precipitation, of ANPP to changes in water availability in water-limited ecosystems. Recent studies have tested the meristem limitation hypothesis using a space-for-time substitution sampling. Under this hypothesis, plant meristem density could be a proxy of production potential and can explain a high amount of the observed variability in ANPP among years. For example, in a study of meristem density (MD) and ANPP along a precipitation gradient in the southern Patagonian steppe, we found that MD and MAP effects combined explained 73% and 62% of the ANPP variability for shrubs and grasses respectively (Reichmann and Sala 2006). In a similar way, Dalgleish and Hartnett (2006) found that belowground bud bank increased linearly with MAP along a precipitation gradient in the grasslands of central North America. The first objective of this work is to test the meristem-limitation hypothesis using the vegetation community at Jornada LTER site as a model of arid ecosystems. With a three-year long rainfall manipulation experiment, we will reproduce a precipitation gradient that will allow us to create a temporal model of the ANPP response. In addition, we are going to test if meristem density is responsible for the observed ANPP hysteresis in several arid ecosystems. If meristem density varies among years in response to abiotic and biotic variables, then we expect that productivity on a year following a wet year will be less constrained by meristem density than productivity on a year that follows a drought, creating an asymmetry in the ANPP response to inter annual precipitation variability. Understanding these mechanisms will improve ANPP forecast at a temporal scale, and may be useful to project ecosystem responses under different climate scenarios.