Stem diameter growth rates in a fire‐prone savanna correlate with photosynthetic rate and branch‐scale biomass allocation,but not specific leaf area

Plant growth rates strongly determine ecosystem productivity and are a central element of plant ecological strategies. For laboratory and glasshouse‐grown seedlings, specific leaf area (SLA; ratio of leaf area to mass) is a key driver of interspecific variation in growth rate (GR). Consequently, SLA is often assumed to drive GR variation in field‐grown adult plants. However, there is an increasing evidence that this is not the general case. This suggests that GR – SLA relationships (and perhaps those for other traits) may vary depending on the age or size of the plants being studied. Here we investigated GR – trait relationships and their size dependence among 17 woody species from an open‐canopy, fire‐prone savanna in northern Australia. We tested the predictions that SLA and stem diameter growth rate would be positively correlated in saplings but unrelated in adults while, in both age classes, faster‐GR species would have higher light‐saturated photosynthetic rate (A_sat), higher leaf nutrient concentrations, higher branch‐scale biomass allocation to leaf versus stem tissues and lower wood density (WD). SLA showed no relationship to stem diameter GR, even in saplings, and the same was true of leaf N and P concentrations, and WD. However, branch‐scale leaf:stem allocation was strongly related to GR in both age groups, as was A_sat. Together, these two traits accounted for up to 80% of interspecific variation in adult GR, and 41% of sapling GR. A_sat is rarely measured in field‐based GR studies, and this is the first report of branch‐scale leaf:stem allocation (analogous to a benefit:cost ratio) in relation to plant growth rate. Our results suggest that we may yet find general trait‐drivers of field growth rates, but SLA will not be one.     

Linking plant functional traits with post‐fire sprouting vigour in woody species in central Argentina

Abstract Sprouting vigour is determined by the plant amount of reserves and intrinsic growth rate of plants. While the first factor has been well studied, the second is far less understood. Although a higher growth rate would imply a higher sprouting vigour, fast‐growing species may have less below‐ground reserves, and thus, a lower sprouting potential. The relative importance of both opposite effects was little explored in the literature. To analyse the influence of growth rate on sprouting vigour, one growth season after a fire we measured plant height of the old (pre‐fire) and new (post‐fire) tissue in 194 individuals of 14 woody species from a woodland in central Argentina. We calculated a mean value of pre‐ and post‐fire height for each species, and obtained from a database potential height at maturity, wood density (WD) and specific leaf area (SLA), as surrogates of intrinsic growth rate. We performed a forward stepwise multiple regression using WD and SLA, together with mean pre‐fire height or potential height as independent variables, and mean post‐fire height (as an indicator of resprout vigour) as the dependent variable. Interactions were also tested. Pre‐fire height, WD and their interaction term were the variables that best explained post‐fire height. We also analysed the relationship between pre‐ and post‐fire size for each species independently by fitting hyperbolic functions. Then we correlated both parameters of the functions to species characteristics (WD, SLA, potential height and mean pre‐fire height). Both parameters of the hyperbolic functions were significantly correlated only with WD, but not with the other species characteristics. All results together indicate that species with low WD (i.e. high potential growth rate) regrow more vigorously than species with high WD when pre‐fire individuals were tall. In contrast, when pre‐fire individuals were small, WD had no influence on sprout vigour. A trade‐off between allocation of biomass to underground reserves and shoot growth seems to be responsible for the patterns obtained. For small individuals, below‐ground reserves seem to play a more important role than inherent growth rate (here measured through WD) in determining the sprouting vigour, while for large individuals, growth rate seems more important than reserves.     

Linking plant functional traits with post‐fire sprouting vigour in woody species in central Argentina

Abstract Sprouting vigour is determined by the plant amount of reserves and intrinsic growth rate of plants. While the first factor has been well studied, the second is far less understood. Although a higher growth rate would imply a higher sprouting vigour, fast‐growing species may have less below‐ground reserves, and thus, a lower sprouting potential. The relative importance of both opposite effects was little explored in the literature. To analyse the influence of growth rate on sprouting vigour, one growth season after a fire we measured plant height of the old (pre‐fire) and new (post‐fire) tissue in 194 individuals of 14 woody species from a woodland in central Argentina. We calculated a mean value of pre‐ and post‐fire height for each species, and obtained from a data‐base potential height at maturity, wood density (WD) and specific leaf area (SLA), as surrogates of intrinsic growth rate. We performed a forward stepwise multiple regression using WD and SLA, together with mean pre‐fire height or potential height as independent variables, and mean post‐fire height (as an indicator of resprout vigour) as the dependent variable. Interactions were also tested. Pre‐fire height, WD and their interaction term were the variables that best explained post‐fire height. We also analysed the relationship between pre‐ and post‐fire size for each species independently by fitting hyperbolic functions. Then we correlated both parameters of the functions to species characteristics (WD, SLA, potential height and mean pre‐fire height). Both parameters of the hyperbolic functions were significantly correlated only with WD, but not with the other species characteristics. All results together indicate that species with low WD (i.e. high potential growth rate) regrow more vigorously than species with high WD when pre‐fire individuals were tall. In contrast, when pre‐fire individuals were small, WD had no influence on sprout vigour. A trade‐off between allocation of biomass to underground reserves and shoot growth seems to be responsible for the patterns obtained. For small individuals, below‐ground reserves seem to play a more important role than inherent growth rate (here measured through WD) in determining the sprouting vigour, while for large individuals, growth rate seems more important than reserves.     

Manipulation of the hypocotyl sink activity by reciprocal grafting of two Raphanus sativus varieties: its effects on morphological and physiological traits of source leaves and whole‐plant growth

To reveal whether hypocotyl sink activities are regulated by the aboveground parts, and whether physiology and morphology of source leaves are affected by the hypocotyl sink activities, we conducted grafting experiments using two Raphanus sativus varieties with different hypocotyl sink activities. Comet (C) and Leafy (L) varieties with high and low hypocotyl sink activities were reciprocally grafted and resultant plants were called by their scion and stock such as CC, LC, CL and LL. Growth, leaf mass per area (LMA), total non‐structural carbohydrates (TNCs) and photosynthetic characteristics were compared among them. Comet hypocotyls in CC and LC grew well regardless of the scions, whereas Leafy hypocotyls in CL and LL did not. Relative growth rate was highest in LL and lowest in CC. Photosynthetic capacity was correlated with Rubisco (ribulose 1·5‐bisphosphate carboxylase/oxygenase) content but unaffected by TNC. High C/N ratio and accumulation of TNC led to high LMA and structural LMA. These results showed that the hypocotyl sink activity was autonomously regulated by hypocotyl and that the down‐regulation of photosynthesis was not induced by TNC. We conclude that the change in the sink activity alters whole‐plant growth through the changes in both biomass allocation and leaf morphological characteristics in R. sativus.