Ontogenetic shift in the scaling of dark respiration with whole-plant mass in seven shrub species

Summary: 1. Recent evidence suggests that metabolic scaling exponents should shift from ≈ 1 to ∼3/4 as plants grow from seedlings to mature trees. This study examines whether there is an ontogenetic shift in the scaling exponent of dark respiration with whole-plant biomass and whether the allometry of dark respiration vs. plant total nitrogen content is consistent with these metabolic scaling relationships. 2. We examined seedling mass, dark respiration, biomass allocation, water and nitrogen contents, and relative growth rates for 351 seedlings of seven shrub species that span three orders of magnitude in total plant mass (i.e. <1 mg to >1 g). 3. Dark respiration increased with increasing seedling mass in each of three growth stages for each species. The scaling exponents for dark respiration vs. plant mass were c. 1·0 during the earliest stage of seedling development (<60 days after germination) but declined in the later two growth stages (>90 and <120 days after seed germination) for five of the seven species studied. However, the metabolic scaling relationship was isometric when the data were pooled for each species across all size classes. 4. With increasing plant size, the biomass allocated by each species to the metabolically active organ (leaf) generally scaled against whole plant biomass with a slope smaller than 1·0 but significantly larger than 3/4. 5. Water and nitrogen contents declined with increasing seedling mass and whole plant dark respiration increased disproportionally (i.e. scaling slope >1) with increasing total plant water or nitrogen content, possibly because small seedlings have a lower water and nutrient-use efficiency compared to larger ones in water- and nutrient- unlimited habitats. 6. The change in relative growth rate paralleled the ontogenetic shift in the dark respiration vs. mass relationship, which may explain the isometric metabolic scaling relationship if growth efficiency is inversely proportional to seedling mass. 7. We therefore demonstrate an ontogenetic shift in the metabolic scaling during plant ontogeny. Although the proximate mechanisms underlying the shift are not fully understood, we speculate that a contributing factor is the accrual of less metabolically active tissues or organs with increasing total body mass in the form of secondary tissues with high cell wall volume fractions (e.g. wood and phellum).