Abstract
Summary We compiled a global database for leaf, stem and root biomass representing c. 11 000 records for c. 1200 herbaceous and woody species grown under either controlled or field conditions. We used this data set to analyse allometric relationships and fractional biomass distribution to leaves, stems and roots.We tested whether allometric scaling exponents are generally constant across plant sizes as predicted by metabolic scaling theory, or whether instead they change dynamically with plant size. We also quantified interspecific variation in biomass distribution among plant families and functional groups.Across all species combined, leaf vs stem and leaf vs root scaling exponents decreased from c. 1.00 for small plants to c. 0.60 for the largest trees considered. Evergreens had substantially higher leaf mass fractions (LMFs) than deciduous species, whereas graminoids maintained higher root mass fractions (RMFs) than eudicotyledonous herbs.These patterns do not support the hypothesis of fixed allometric exponents. Rather, continuous shifts in allometric exponents with plant size during ontogeny and evolution are the norm. Across seed plants, variation in biomass distribution among species is related more to function than phylogeny. We propose that the higher LMF of evergreens at least partly compensates for their relatively low leaf area : leaf mass ratio.
Highlights
A plant’s organs serve multiple distinct functions
Straight lines fitted to the logtransformed leaf vs stem, leaf vs root and stem vs root biomass data had r2 values up to 0.988 (Table 2a; for separate regressions on herbaceous and woody species, see Table S4), examination of residuals (Fig. S1) showed clear patterns that reject the use of log-linear allometry to describe these data
We found a large contrast in biomass distribution within the herbaceous species between graminoids (Cyperaceae and Poaceae), which showed low pLMFs and high pRMFs, and herbaceous eudicots, which showed the reverse
Summary
A plant’s organs serve multiple distinct functions. For example, leaves provide sugars, stems and branches position the leaves in an advantageous light environment and transport water as well as nutrients, and roots acquire water and nutrients and anchor the plant. For a species to achieve optimal performance at the wholeplant level, there has to be a certain proportionality among these functions, as all are essential for growth and reproduction This proportionality depends in part on the relative amounts of mass present in these organs. Various terminology has been used (Reich, 2002), the generic term we will use throughout this paper to describe how the biomass of one organ relates to that of another or of the whole is ‘biomass distribution’. Note that this should not be confused with dynamic allocation of newly fixed photosynthates to different organ systems, as the realized biomass distribution at any moment is the cumulative result of dynamic carbon (C) allocation over time and loss rates of mass among organs throughout its life. We focus on the relationship of biomass distribution to plant size and its variation among species
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