Do trait–growth relationships vary with plant age in fire-prone heathland shrubs?

1. Growth rate is a central element of a plant's ecological strategy in its competition with other plants. It is well known that traits give rise to differences in growth between species, but there is limited understanding of how trait–growth relationships change with age and whether these relationships vary depending on how growth is measured. 

2. Field growth rates were measured at different plant ages for 14 shrub species in an eastern Australian heathland. Ages were identifiable from local fire history because all species regenerate from seed, post-fire. One year of growth was tracked on plants at six ages: 1.4, 2.4, 5, 7, 9 and 32 years old. A novel sampling protocol allowed us to express annual growth rate increments of (i) stem diameter, (ii) total leaf area, (iii) height, (iv) above-ground standing biomass, and (v) total above-ground biomass including losses to tissue turnover. 

3. Overall, wood density (WD) and leaf mass fraction (LMF, ratio of total leaf to above-ground plant dry mass) emerged as the two traits most clearly influencing growth rates, higher WD corresponding to slower growth and higher LMF to faster growth. Higher leaf mass per area (LMA) also corresponded to faster growth rates; however, this was seemingly a secondary correlation originating via a negative relationship between WD and LMA. More weakly and less consistently, higher leaf N and (less so) higher leaf P per unit leaf area corresponded to faster growth. In general, trait–growth relationships were weaker in the younger age classes (1.4 and 2.4 years) and strongest at ages 5, 7 and 9 years. WD and LMA effects on growth were markedly heterogeneous among ages; effects of LMF and leaf nutrients were more consistent. Although our five growth metrics each measure distinct aspects of changes in plant size, their correlations with traits showed considerable similarity. 

4. Synthesis. We demonstrate that key functional traits undergo shifts in their relationship with growth as plants mature. Therefore, it will be valuable to shift our understanding of plant strategies away from the notion that traits influence growth rates in a fixed manner across plant sizes and ages.