Allometric patterns of below-ground biomass in Mediterranean grasslands

Abstract

Below-ground biomass might play an important role in the persistence of grasslands. The summertime water stress of Mediterranean grasslands very much accounts for annual plants in their communities. These plants die off in summer, and the soil seed bank ensures the grasslands' recovery every autumn. Temperature decreases with altitude, together with the proportion of annual and perennial herbs. This paper explores the vertical profile of this biomass in grasslands on the Iberian Península along an altitudinal gradient and two geomorphological positions in each site. Both factors were considered to determine the development of below-ground biomass. We also took Ínto consideration the experimental exclusion of livestock, considering that the presence or absence of herbivores may infiuence the aerial and underground structure of the grasslands. Using an allometric approach, we examined the relative distribution ofbelow-ground biomass with depth and its temporal variation ("construction"). We found that there is a general allometric pattern in the vertical profile ofvariation of below-ground biomass in which density is inversely proportional to the depth of the stratum raised ro· the power of 1.8 (at high elevations) and 2.5 (low elevations). The construction process of these grasslands exhibits slight variations within a common pattern. Growth of the plant canopy can be temporally limited by the action of herbivores, a more complex underground structure thereby developing, with more strata and a more even biomass distribution. An adaptive reaction to the notable Mediterranean seasonal change appears to occur but, surprisingly, may depend upon altitude rather than upon the infiuence of herbivory.