Plant functional traits are used increasingly for linking environmental conditions, community structure and ecosystem functions. Traits associated with rapid resource capture may come at the expense of those related to stress resistance. In annual plants such tradeoff is reflected in patterns of plant size vs. biomass partitioning, as the former represents resource capture rate while the latter represents reorganization to cope with resource limitation. We investigated in a water-limited annual plant community: 1) the effects of water and nitrogen availability on plant size and biomass partitioning, i.e., root to shoot ratio (R/S), specific leaf area (SLA) and reproductive effort (R/V), 2) the relationship between plant size and biomass partitioning at the intra- and inter-specific levels, and 3) the relationships between plasticity in biomass partitioning and resource stress resistance. We hypothesized that plant size and biomass partitioning are mechanistically related, determining plant resistance to stress and shaping community structure under variable resource availability. In two controlled experiments (ten key species) species varying in their size range and functional characteristics were grown under variable water and nitrogen availabilities and their size and biomass partitioning monitored throughout growth and at fixed ontogenetic phases (flowering and seed set times). In legume species we examined also allocation to symbiotic nitrogen fixation and nitrogen partitioning.
Results/Conclusions
In all of the species, reduced water and/or nitrogen availability was associated with smaller size, increased R/S, and decreased SLA, while R/V remained constant. By considering ontogenetic effects, we revealed active changes ("true plasticity") in R/S (seven species) and SLA (three species) in response to soil resource availability, along the species growth course. At the intra-specific level, plant size at flowering time was negatively related to R/S and positively related to SLA. At the inter-species level, size was negatively related to stress resistance. However, no significant inter-species relationship was found between size or stress resistance and any of the biomass partitioning traits and their plasticity. Finally, three Legume species differed in biomass partitioning and in biomass:nitrogen-mass ratio, and both allocation indices corresponded to species N2 fixation strategies (facultative vs. obligatory). Our results show that species' resource capture rate is inversely related to stress resistance, but, this relationship is not simply mediated through a single resource partitioning strategy. We conclude that interspecific variation along the plant size to stress resistance tradeoff line is the outcome of a multi-trait matrix.