Relative growth rate (RGR) is an important trait distinguishing plant species' strategies. Plant RGR depends on biomass allocation to leaves (leaf mass fraction, LMF), efficient construction of leaf surface area (specific leaf area, SLA) and biomass growth per unit leaf area (net assimilation rate, NAR). If species of different functional groups differ in one or more of these components, then (1) the functional groups may differ in mean RGR, and (2) RGR variation across species within each group may be driven by different traits.

  Dr. Kyle W. Tomlinson of Xishuangbanna Tropical Botanical Garden (XTBG) and his colleagues from Wageningen University evaluated RGR in relation to biomass allocation, leaf morphology and root carbohydrate content using data they had accumulated on 51 tropical savanna tree species differing in leaf habit. They predicted that: (1) mean RGR of deciduous species and evergreen species did not differ significantly because they did not differ in LAR; (2) RGR variation was predominantly driven by LMF variation among deciduous species and by SLA or NAR variation among evergreen species; and (3) RGR and LMF were negatively related to root carbohydrate storage among deciduous species but not among evergreen species.

They compared RGR and traits related to RGR of seedlings of savanna tree species of different leaf habit taken from three continents. Surprisingly, RGR was greater among evergreen species than among deciduous species. Furthermore, they observed that the contribution of drivers of RGR variation, LMF, SLA and NAR, differed between deciduous and evergreen species: RGR variation among evergreen species was mainly driven by SLA variation whereas RGR variation among deciduous species was more equally driven by SLA and LMF.

The results suggested that differences in drivers of RGR occur among savanna species because they have different selected strategies for coping with fire disturbance in savannas.

The study entitled “Relative growth rate variation of evergreen and deciduous savanna tree species is driven by different traits” has been published online in Annals of Botany.