Light is widely considered to be the most important factor limiting the performance of plants on the floors of forests and woodlands, but the roles of nutrient availability and water supply remain poorly defined. We seek to predict the types of forest in which root competition affects seedling performance, and the types of plants that respond most strongly to release from root competition. We then test our predictions by reviewing experiments in which tree seedlings and forest herbs are released from belowground competition, usually by cutting trenches to sever the roots of surrounding trees. First, we provide a worldwide review of changes in canopy form and fine-root mass along gradients of soil fertility and seasonal drought, keeping in mind the stages of forest development. Our review shows that penetration of light is least in forests on moist soils providing large amounts of major nutrients. The changes are far more complex than those considered by allocation models. Dry woodlands typically allow 20 ...

Impacts of root competition in forests and woodlands: a theoretical framework and review of experiments

Estimates of the extent of the discrimination against13CO2 during photosynthesis (ΔA) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate ΔA. Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated ΔA for C3 biomes ranged from 12.9‰ for xerophytic woods and shrub to 19.6‰ for cool/cold deciduous forest, with an average value from C3 plants of 17.8‰. This is slightly less than the commonly used values of 18–20‰. For C4 plants the average modelled discrimination was 3.6‰, again slightly less than would be calculated from C4 plant dry matter carbon isotopic composition (yielding around 5‰). From our model we estimate that, on a global basis, 21% of GPP is by C4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8‰. There are large variations in ΔA across the globe, the largest of which are associated with the precence or absence of C4 plants. Due to longitudinal variations in ΔA, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.

13C discrimination during CO2 assimilation by the terrestrial biosphere

Internal conductances to CO2 transfer from the stomatal cavity to sites of carboxylation (gi) in hypostomatous sun-and shade-grown leaves of citrus, peach and Macadamia trees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m−2) for all leaves, including sclerophyllous Macadamia, whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high gi, the chloro-plast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a), the lowest tissue density (Td) and the highest chloro-plast surface area (Sc) exposed to ias. There were negative correlations between gi and leaf thickness or D/a, but positive correlations between gi and Sc or Sc/Td.

We developed a one-dimensional diffusion model which partitioned gi into a gaseous diffusion conductance through the ias (gias) plus a liquid-phase conductance through mesophyll cell walls (gcw). The model accounted for a significant amount of variation (r2=0.80) in measured gi by incorporating both components. The gias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The gcw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between gi and leaf thickness or D/a related to gias whereas positive correlations between gi and Sc or Sc/Td, related to gcw. The gcw was consistently lower than gias, and thus represented a greater constraint on CO2 diffusion in the mesophylls of these hypostomatous species.

On the relationship between leaf anatomy and CO2 diffusion through the mesophyll of hypostomatous leaves

By means of meta-analyses we determined how 70 traits related to plant anatomy, morphology, chemistry, physiology, growth and reproduction are affected by daily light integral (DLI; mol photons m-2 d-1 ). A large database including 500 experiments with 760 plant species enabled us to determine generalized dose-response curves. Many traits increase with DLI in a saturating fashion. Some showed a more than 10-fold increase over the DLI range of 1-50 mol m-2 d-1 , such as the number of seeds produced per plant and the actual rate of photosynthesis. Strong decreases with DLI (up to three-fold) were observed for leaf area ratio and leaf payback time. Plasticity differences among species groups were generally small compared with the overall responses to DLI. However, for a number of traits, including photosynthetic capacity and realized growth, we found woody and shade-tolerant species to have lower plasticity. We further conclude that the direction and degree of trait changes adheres with responses to plant density and to vertical light gradients within plant canopies. This synthesis provides a strong quantitative basis for understanding plant acclimation to light, from molecular to whole plant responses, but also identifies the variables that currently form weak spots in our knowledge, such as respiration and reproductive characteristics.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.15754

A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance.

Trade-offs between acquisition capacities for aboveground and belowground resources were investigated by studying the phenotypic plasticity of leaf and root traits in response to different irradiance levels at low nutrient supply. Two congeneric grasses with contrasting light requirements, Dactylis glomerata and D. polygama, were used. The aim was to analyze phenotypic covariation in components of leaf area and root length in response to above- and belowground resource limitation and the consequences of this variation for resource acquisition and plant growth. At intermediate shading (30 and 20% of full sunlight) the plants were able to maintain their total root length, despite a strongly increased total leaf area and a reduced biomass allocation to roots. This was associated with an unaltered or slightly increased nutrient uptake and growth. At 5.5% relative irradiance, growth was severely reduced, especially in the shade-tolerant D. polygama. The results show that constraints on acquisition capacities for aboveground and belowground resources, caused by biomass allocation, may be alleviated by plasticity in other traits such as tissue-mass density and thickness of roots and leaves. The results also suggest different adaptive constraints for phenotypic plasticity and for genetically determined interspecific variation. Phenotypic plasticity tends to maximize resource acquisition and growth rate in the short term, whereas the higher tissue-mass density and the longer leaf life-span of shade-tolerant species indicate reduced loss rates as a more advantageous speciesspecific adaptation to shade in the long term. In most environments plants compete both for aboveground and belowground resources. The capacity to acquire aboveground resources is associated with leaf area (Lambers and Poorter, 1992), and the capacity to acquire belowground resources is associated with root length (Ryser, 1998). These traits are phenotypically plastic and

https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.2307/2656636

Consequences of phenotypic plasticity vs. interspecific differences in leaf and root traits for acquisition of aboveground and belowground resources

Seedling trees of Argyrodendron sp., A. trifoliolaturn, Flindersia brayleyana and Toona australis were grown for c. 180 days under one of three light regimes with either of two nutrient levels (6 treatments in all). Light regimes spanned the range of environmental conditions which these species would normally experience in northern Queensland rainforest: deep shade (1.3 mol quanta m-2 day-1, equivalent to forest floor), moderate light (5.6 mol quanta m-2 day-1, comparable to midcanopy), and strong light (23 mol quanta m-2 day-1, matching daily irradiance of exposed crowns). Long-term shade tolerance in Argyrodendron sp. and A. trifoliolaturn was associated with limited responses in growth and leaf anatomy to low light and nutrients. Starch accumulation in leaves under all treatments, and especially low nutrients, implied that supply of photoassimilate exceeded demand. Such a conservative carbon economy, plus the accumulation of stem P reserves, even in a weak light environment, is consistent with a protracted existence as part of a forest floor community. By contrast, shade-intolerant Toona is an early successional species and lacks such adaptive features. Instead, light and nutrients had a strong interactive effect on growth. Flindersia, with a broad tolerance to sun and shade, was intermediate in growth response and leaf adjustment, which is consistent with its success across a wide size range of forest gaps.

WA Thompson

Papers

Photosynthetic Response to Light and Nutrients in Sun-Tolerant and Shade-Tolerant Rainforest Trees. I. Growth, Leaf Anatomy and Nutrient Content.