Explaining within‐community variation in plant biomass allocation: a balance between organ biomass and morphology above vs below ground?

Questions It remains unresolved why, despite the obvious functional importance of leaves and roots, co‐existing plant species can display highly contrasting biomass distributions of these organs. Building on the ‘functional equilibrium’ hypothesis, we hypothesize that co‐existing species can each achieve balanced resource acquisition above vs below ground by trading off the biomass vs morphology of structures responsible for resource acquisition, i.e. leaves and fine roots. Methods We tested this hypothesis in a natural field setting by measuring plant above‐ and below‐ground biomass and morphological traits associated with resource uptake – specific leaf area ( SLA ) and specific root length ( SRL ) – of 18 dominant angiosperm species from a sub‐alpine plant community. Location New Zealand South Island. Results We found a significant negative relationship between the species leaf mass to fine root mass ratio and the SLA to SRL ratio when we considered eudicot species only. The SLA to SRL ratio and plant size explained 31% and 34% of the total variation in the species leaf to fine root mass ratio respectively, and 45% when used in combination ( P < 0.05 in all cases). Within a given plant size, 90% of the variation among species in total leaf area was due to differences in SLA , whereas variation in the fine root mass fraction was responsible for 71% of the variation among species in fine root length. Conclusions In support of our hypothesis, part of the difference between co‐occurring species in leaf and fine root biomass distribution could be explained by the variable morphologies of these organs as well as variation in plant size, independent of the plant economic strategy. We expect that this outcome may result from environmental and evolutionary constraints on plant species‐average traits, as well as plastic responses to local environmental conditions. These findings help explain why a diversity of strategies for achieving balanced resource acquisition can co‐exist within a single plant community.