Key plant structural and allocation traits depend on relative age in the perennial herb Pimpinella saxifraga.

Abstract

Perennial plant formations always include a mixture of various-aged individuals of community-creating species, but the physiological and competitive potentials of plants of differing age and the importance on whole community functioning are still not entirely known. The current study tested the hypothesis that ontogenetically old plants have limited biomass investments in leaves and enhanced foliage support costs. Leaf structure, size and biomass allocation were studied in the perennial herb Pimpinella saxifraga during plant ontogeny from seedling to senile phases to determine age-dependent controls on key plant structural traits. The average duration of the full ontogenetic cycle is approx. 5-10 years in this species. Plants were sampled from shaded and open habitats. Leaflet dry mass per unit area (M(A)) increased, and the fraction of plant biomass in leaflets (F(L)) decreased with increasing age, leading to a 5- to 11-fold decrease in leaf area ratio (LAR = F(L)/M(A)) between seedlings and senescent plants. In contrast, the fraction of below-ground biomass increased with increasing age. Leaflet size and number per leaf increased with increasing age. This was not associated with enhanced support cost in older plants as age-dependent changes in leaf shape and increased foliage packing along the rachis compensated for an overall increase in leaf size. Age-dependent trends were the same in habitats with various irradiance, but the LAR of plants of varying age was approx. 1.5-fold larger in the shade due to lower M(A) and larger F(L). As plant light interception per unit total plant mass scales with LAR, these data demonstrate major age-dependent differences in plant light-harvesting efficiency that are further modified by site light availability. These ontogenetic changes reduce the differences among co-existing species in perennial communities, and therefore need consideration in our understanding of how herbaceous communities function.