Shoot-level biomass allocation is affected by shoot type in Fagus sylvatica

The present study aims (i) to examine if recently reported interspecific shoot-level biomass allocational trade-offs, i.e. isometric trade-offs between leaf mass (LM) and stem mass (SM) and between leaf size and leaf number, hold intraspecifically and (ii) to explore whether those scaling relationships are independent of shoot type (i.e. long vs. short shoots). In order to address our questions, we used Fagus sylvatica saplings growing under a broad light range that were sampled in the Western Carpathians Mountains (Slovakia). We found that: (i) intraspecific shoot-level biomass allocational trade-offs differ from those reported interspecifically and that (ii) long and short shoots differ in biomass allocation scaling coefficients. Allometric relationships with slopes statistically smaller than 1.0 or higher than -1.0, were found between SM and LM and between mean leafing intensity and individual leaf mass, respectively, in long shoots. In contrast, isometric scaling was found in short shoots. This suggests that leaf mass in short shoots is unaffected by shoot stem mass, in contrast to long shoots. Short shoots also had a larger fraction of biomass allocated to leaves. Beech shoots, as has been observed in other shoot dimorphic species, are specialized, with short shoots specializing in carbon gain and long shoots in space acquisition. A greater shift in LM than in SM among species during speciation shifting from allometric intraspecific relationships to an isometric interspecific scaling relationship between those traits could explain the discrepancies between the outputs of the present intraspecific study and others similar studies. This study draws attention to the importance of considering shoot types in future studies dealing with allocation rules in species with dimorphic shoots.