Text by Giacomo Puglielli
Can we solve a long-standing debate in plant ecology?
Plants are known to allocate the greatest proportion of biomass to the organs involved in the acquisition of the most limiting resource for growth. For example, if light is limiting plant growth then plants are expected to allocate more biomass to aboveground parts (i.e. leaves and stems) and less to roots. Conversely, when soil water is the limiting factor, then biomass allocation to roots is expected to be greater compared to that allocated to aboveground parts. Such model is also known as Optimal Partitioning Theory (OPT), and its first formulation dates back to 1960s. OPT has been formulated and it is valid at the intraspecific level, when a single species is exposed to different environmental conditions.
Plant species inherently differ in their habitat affinity, so that there are species that consistently perform the best in shaded or in dry environments. A long-standing debate in plant ecology is if OPT predictions can equally apply when addressing interspecific differences in biomass allocation between species differing in their habitat affinity. Nevertheless, results are scattered and often not consistent across studies. The greatest limitations of previous studies are: i) the sample size (differences among few species from a single ecosystem type are evaluated); ii) the considered range of total plant biomass that strongly influences biomass allocation patterns. Also differences between plant functional types could have further blurred previous results.
We used the most extensive database of biomass allocation available for woody species (Poorter et al., 2015) spanning more than 10 orders of magnitude in plant size, and complemented it with information on species ecological tolerance of shade and drought (i.e. habitat affinity in response to light and water availability), and on plant functional type (deciduous and evergreen broad-leaf and evergreen needle-leaf). The final dataset included 7377 observations of biomass allocation to leaves, stems and roots spanning 604 species worldwide from tropical to boreal ecosystems. We used this dataset to test if OPT predictions are equally valid at the interspecific level independently of developmental stages and plant functional types.
The main and most novel result we obtained is that plant functional type is the major determinant of biomass allocation patterns independently of the considered tolerance. Ifanything, differences between tolerant and intolerant species often run in opposite directions compared to OPT predictions. Also the total plant size at which the comparison was made strongly influenced the observed differences. We conclude that the detection of a given difference between tolerant and intolerant species strongly depends on the size at which the comparison has been made within each plant functional type.
In the paper we also discuss other determinants of biomass allocation patterns between tolerant and intolerant species at the global scale, namely changes in organ morphology together with phenotypic plasticity and the effect of plant architecture on biomass allocation. Altogether, such factors allow tolerant and intolerant woody species to display multiple biomass allocation strategies in response to shade and drought.
While we partly solved a long-standing debate in plant ecology we could not challenge another one, biological patterns are not as easy as theory suggests….luckily for us!
Citation: Puglielli, G., Laanisto, L., Poorter, H., & Niinemets, Ü. (2020). Global patterns of biomass allocation in woody species with different tolerance of shade and drought: evidence for multiple strategies. New Phytologist, https://doi.org/10.1111/nph.16879
The optimal partitioning theory predicts that plants of a given species acclimate to different environments by allocating a larger proportion of biomass to the organs acquiring the most limiting resource. Are similar patterns found across species adapted to environments with contrasting levels of abiotic stress?
We tested the optimal partitioning theory by analysing how fractional biomass allocation to leaves, stems and roots differed between woody species with different tolerances of shade and drought in plants of different age and size (seedlings to mature trees) using a global dataset including 604 species.
No overarching biomass allocation patterns at different tolerance values across species were found. Biomass allocation varied among functional types as a result of phenological (deciduous vs evergreen broad‐leaved species) and broad phylogenetical (angiosperms vs gymnosperms) differences. Furthermore, the direction of biomass allocation responses between tolerant and intolerant species was often opposite to that predicted by the optimal partitioning theory.
We conclude that plant functional type is the major determinant of biomass allocation in woody species. We propose that interactions between plant functional type, ontogeny and species‐specific stress tolerance adaptations allow woody species with different shade and drought tolerances to display multiple biomass partitioning strategies.