Data from: Does plant biomass partitioning reflect energetic investments in carbon and nutrient foraging?

1. Studies of plant resource-use strategies along environmental gradients
often assume that dry matter partitioning represents an individual’s
energy investment in foraging for above- versus belowground resources.
However, ecosystem-level studies of total belowground carbon allocation
(TBCA) in forests do not support the equivalency of energy (carbon) and
dry matter partitioning, in part because allocation of carbon to
belowground pools and fluxes that are not accounted for by root biomass
(e.g., mycorrhizal hyphae, rhizodeposition; root and soil respiration) can
be substantial. Here, we apply this reasoning to individual plants in
controlled environments and ask whether dry matter partitioning
belowground (root mass fraction, RMF) accurately reflects TBCA in studies
of optimal partitioning theory. 2. We quantified the relationship between
RMF and TBCA in individual plants, using 311 observations from 51 studies
that simultaneously measured both allocation variables. Our analysis
included tests of whether the RMF-TBCA relationship depended on mutualist
soil microbes, plant growth form, age, and study methodology including
isotopic pulse/chase duration. 3. We found that RMF was a poor proxy for
belowground energy investment. This disconnect of RMF and TBCA was driven
in part by plants of low RMF (<0.4) exhibiting significantly higher
rates of root and soil respiration per unit root mass than plants of high
RMF. Root colonization by mutualist microbes, including arbuscular
mycorrhizal fungi and nitrogen fixing bacteria, increased TBCA by 5-7%,
and TBCA was lower in grasses than other species by 9-16%. These patterns
were evident for relationships assessed both within and between species.
4. We conclude that optimal partitioning studies of plants along
environmental gradients are likely to underestimate plant energy
allocation belowground if the C costs of root and soil respiration are
ignored. Because energy rather than biomass better reflects how
assimilated C supports fitness, this omission of respired C suggests
existing studies misrepresent the significance of belowground processes to
plant function.