Coordination between water transport capacity, biomass growth, metabolic scaling and species stature in co-occurring shrub and tree species

The significance of xylem function and metabolic scaling theory begins from the idea that water transport is strongly coupled to growth rate. At the same time, coordination of water transport and growth seemingly should differ between plant functional types. We evaluated the relationships between water transport, growth and species stature in six species of co-occurring trees and shrubs. Within species, a strong proportionality between plant hydraulic conductance (K), sap flow (Q) and shoot biomass growth (G) was generally supported. Across species, however, trees grew more for a given K or Q than shrubs, indicating greater growth-based water-use efficiency (WUE) in trees. Trees also showed slower decline in relative growth rate (RGR) than shrubs, equivalent to a steeper G by mass (M) scaling exponent in trees (0.77-0.98). The K and Q by M scaling exponents were common across all species (0.80, 0.82), suggesting that the steeper G scaling in trees reflects a size-dependent increase in their growth-based WUE. The common K and Q by M exponents were statistically consistent with the 0.75 of ideal scaling theory. A model based upon xylem anatomy and branching architecture consistently predicted the observed K by M scaling exponents but only when deviations from ideal symmetric branching were incorporated. We tested the often assumed but little tested idea that increasing whole plant sapflow or hydraulic conductance leads to greater biomass growth. We compared co-occurring shrub and tree species and generally found intraspecific isometry linking hydraulic conductance, sapflow and growth, although the two functional groups differed with trees tending to exhibit lower sapflow yet greater growth for a given hydraulic conductance. Additionally, relative growth rate tended to decline with size in all species but more steeply so in shrubs, apparently owing to subtle size-dependent changes in WUE instead of limitation by water transport. A model based on xylem anatomy predicted hydraulic conductances and suggested that species employ ontogenetic changes in branching architecture to achieve observed hydraulic scaling. Commentary: