Tree leaf trade‐offs are stronger for sub‐canopy trees: leaf traits reveal little about growth rates in canopy trees

Abstract

Can morphological plant functional traits predict demographic rates (e.g., growth) within plant communities as diverse as tropical forests? This is one of the most important next-step questions in trait-based ecology and particularly for global reforestation efforts. Due to the diversity of tropical tree species and their longevity, it is difficult to predict their performance prior to reforestation efforts. In this study, we investigate if simple leaf traits are predictors of the more complex ecological process of plant growth in regenerating selectively logged natural forest within the Wet Tropics (WTs) bioregion of Australia. This study used a rich historical data set to quantify tree growth within plots located at Danbulla National Park and State Forest on the Atherton Tableland. Leaf traits were collected from trees that have exhibited fast or slow growth over the last ~50 yr of measurement. Leaf traits were found to be poor predictors of tree growth for trees that have entered the canopy; however, for sub-canopy trees, leaf traits had a stronger association with growth rates. Leaf phosphorus concentrations were the strongest predictor of Periodic Annual Increment (PAI) for trees growing within the sub-canopy, with trees with higher leaf phosphorus levels showing a higher PAI. Sub-canopy tree leaves also exhibited stronger trade-offs between leaf traits and adhere to theoretical predictions more so than for canopy trees. We suggest that, in order for leaf traits to be more applicable to reforestation, size dependence of traits and growth relationships need to be more carefully considered, particularly when reforestation practitioners assign mean trait values to tropical tree species from multiple canopy strata.

Leaf:wood allometry and functional traits together explain substantial growth rate variation in rainforest trees

“…Just as in the verbal models of the older literature, the Gibert/Falster theory can generate a scenario—concerning large trees—where growth rates (absolute or relative) and SLA may become negatively correlated. Indeed, this is what was found here ( r 2 = 0.21–0.34, depending which variant of GR was considered), in a recent study also concerning forest trees in the northern Queensland region (Wills et al 2018), and in an older study of Neotropical rainforest species (Poorter et al 2008). In the case of Poorter et al , who considered relative growth rate (RGR) rather than absolute growth rate, the authors questioned the validity of this negative relationship.…”

Disturbance history mediates climate change effects on subtropical forest biomass and dynamics

“…The increase of Pioneers and Large‐Seeded Pioneers we observed corresponds to increased regeneration opportunities in treefall gaps (Chami et al, ). It agrees with the well‐documented replacement of species groups during gap‐phase dynamics from stress‐tolerant species adapted to shade (high wood density, slow growth, large seeds, shallow crown) to competitive species adapted to resource‐rich environments (lightwood, fast‐growing, large leaves with high SLA, deep crowns, small seeds)(Wright et al, ; Grime & Pierce, ; Muscolo et al, ; Forgiarini, Souza, Longhi, & Oliveira, , but see Falster et al, ; Wills et al, ).…”

“…However, these forests did not show any changes in tree density, and their overall structure, including sapling and pole densities, approached that of unlogged stands (Souza et al, ). Their community‐weighted leaf length and crown depths approached the levels found in unlogged forests, while their reduced CWM seed size and height, and increased crown depth and SLA indicate functional relevance of fast‐growth resource‐acquiring strategies (Forgiarini et al, ; Grime & Pierce, ; Wright et al, ), although SLA may not be much influential for canopy trees (Falster et al, ; Wills et al, ). The logging of Wind Dispersed Large Trees and the long‐lived pioneers probably set subtropical Atlantic forests on a successional trajectory that proceeds without these functional groups (Edwards et al, ).…”

Successional habitat filtering of rainforest trees is explained by potential growth more than by functional traits

“…We here refer to growth rates in a common garden without light competition as potential growth, which are not necessarily maximum growth rates as factors other than light may be limiting. Interpretation of trait: growth relationships might differ substantially depending on the presence or absence of light competition and it has also been shown that the correlation is higher in saplings than in large trees (Wills et al, 2018;Wright et al, 2010).…”