The comparative phenotypic plasticity of 16 species of tropical rainforest shrubs (genus Psychotria, Rubiaceae) was investigated by growing plants in three light environments on Barro Colorado Island (BCI, Panama). The three light environments gave daily photon flux densities (PPFD) similar to the natural light gradient from shaded forest understory to small and large canopy gaps. Six of the species are principally found in gaps or forest edge environments, whereas the other ten species are principally found in shaded understories. Interactions between light treatment and species resulted in unpredictable mean phenotypic expression across light treatments. Shoot relative growth rates (RGR) were similar for understory and gap species in the low light treatment. Gap species had significantly greater shoot RGR in the intermediate light treatment than in the high light treatment. Mean plasticity was significantly lower for morphological variables when com- pared to physiological variables, while variation in plasticity was significantly greater for structural variables. Significant differences between gap and understory species were found in the plasticity of six out of the seven variables. The mean phenotypic plasticity of the seven variables was significantly greater for gap than for understory species. The high plasticity of gap species was consistent with the hypothesis that specialization in a more favorable environment increases plasticity. The species exhibited a wide range of leaf longevities, from four to 29 months, with gap species having, on average, shorter leaf life- span than understory species. Mean phenotypic plasticity decreased with increasing leaf longevity. Selection for greater plasticity may be stronger in the gap species because gaps exhibit a relatively predictable decrease in PPFD for which plasticity could be adaptive. While we have found a significant correlation between phenotypic plasticity and habitat affiliation, phylogeny (subgenus ascription) was not correlated with plasticity or with plant performance in any given PPFD treatment, reinforcing the hypothesis that phenotypic plas- ticity has evolved through natural selection in this diverse genus.

/pdf/plastic-phenotypic-response-to-light-of-16-congeneric-shrubs-sxgz758f12.pdf

Plastic Phenotypic Response to Light of 16 Congeneric Shrubs From a Panamanian Rainforest

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.

An attempt is made to review the whole range of plants able to cope with shortages of mineral nutrients, water or light for at least a substantial part of their lives. In the past insufficient attention has been paid to quantitative measurements of the tolerance of various species for lack of resources. Growth rate at a low level of supply and mere survival are the variables to be considered. Problems in defining tolerance arise where a collection of species shows a negative correlation between survival at a low level of supply and growth rate under these conditions; in at least some circumstances a balance between survival and growth rate is important. Plants coping with lack of major resources display one of three strategies: ‘low-flexibility’, ‘switching’ or ‘gearing-down’. The essential features of the low-flexibility strategy are long-lived leaves, low maximum relative growth rates, and inflexibility of form and of gas exchange rates when resource-shortage is relieved, both in seedlings and in adults. Plants which show the switching strategy display the low-flexibility strategy as young seedlings, but are flexibile in form as older plants, and commonly have high relative growth rates then. The gearing-down strategy is based on an ability to reduce strongly the respiration rate when resources are in short supply, both as seedlings and as adult. In some cases this involves shedding of parts which would otherwise consume respiratory substrate. Plants showing this strategy have some characteristics which are the opposite of those shown by plants with the low-flexibility strategy: short-lived leaves, and high flexibility in form and in rates of gas exchange. All three strategies are represented among plants tolerant of the most extreme shortages of nutrients, water and light. It is concluded that while ecologists should attempt to reduce the complexities of nature to an oligo-dimensional framework of generalizing ideas, it is not reasonable to expect that any very simple scheme – such as might be represented by three or four reference points in one plane – will have generality on a world scale.

A reassessment of the strategies of plants which cope with shortages of resources

The Ecology of Trees in the Tropical Rain Forest

Summary 1 We conducted a field experiment to test whether aggregated spatial distributions were related to soil variation in locally sympatric tree species in the rain forests of Sarawak, Malaysia. Dryobalanops aromatica , Shorea laxa , and Swintonia schwenkii are naturally aggregated on low-fertility humult ultisols, Dryobalanops lanceolata and Hopea dryobalanoides on moderate-fertility udult ultisols and Shorea balanocarpoides is found on both soil types. 2 Seedlings of all six species were grown in a nested-factorial experiment for 20 months in humult and udult soils in gaps and in the understorey to test for soil-specific differences in performance. Phosphorus addition was used to test for effects due to P-limitation. 3 Four species showed significantly higher growth on their natural soils, but one humultsoil species ( D. aromatica ) and the broadly distributed species were not significantly affected by soil type. 4 One udult-soil species, D. lanceolata , had both lower relative growth rate and lower mycorrhizal colonization on humult soil. However, humult soils also had lower levels of Ca, Mg, K, N and probably water availability. 5 The overall ranking of growth rates among species was similar on the two soils. Growth rates were strongly positively correlated with leaf area ratio and specific leaf area among species in both soils. With the exception of D. aromatica , species of the higher-nutrient soils had higher growth rates on both soils. 6 Although P addition led to elevated soil-P concentrations, elevated root- and leaf-tissue P concentrations on both soils, there was no significant growth enhancement and therefore no evidence that P availability limits the growth or constrains the distribution of any of the six species in the field. Differences in soil water availability between soils may be more important. 7 Our results suggest that habitat-mediated differences in seedling performance strongly influence the spatial distributions of tropical trees and are therefore likely to play a key role in structuring tropical rain forest communities.

Soil‐related habitat specialization in dipterocarp rain forest tree species in Borneo

1 We report growth and photosynthetic characteristics of seedlings of two contrasting climax tree species (Shorea leprosula Miq. and Dryobalanops lanceolata Burck., Dipterocarpaceae) growing in three differing forest light environments (ranging from 2.4-21.8 mol photon m-2 day-') in lowland rain forest in Sabah, Malaysian Borneo. 2 Our aim was to determine whether interspecific differences in seedling ecology reflected differences in the ability of the photosynthetic apparatus to acclimate to light (assessed by measuring photosynthetic light response curves in the field). 3 S. leprosula has been described as a fast growing light-demander whose seedlings will not survive in deep shade whereas D. lanceolata is slower growing with seedlings that can survive a range of light conditions from deep shade to large canopy gaps. 4 Light environments were characterized by analysis of hemispherical photographs taken in the position of each leaf used for photosynthesis measurements, such that we could analyse both within and between site variation. Growth, seedling architecture and mortality were recorded. 5 D. lanceolata showed a greater ability to acclimate to shade, where it had the greater net carbon assimilation rate of the two species at low instantaneous photon flux densities (PFD). Conversely, S. leprosula had a greater ability to acclimate to high light and had greater assimilation rates at high instantaneous PFDs in all environments. 6 Photosynthetic performance, growth and light availability showed strong correlation across environments. Although there were small interspecific differences in photosynthesis within each forest environment, differences in growth and survival were more clearly related to the differing carbon allocation patterns of the two species. 7 D. lanceolata showed an increasing allometric shift in low light in favour of the growth of long branches which are likely to increase sunfleck interception. S. leprosula maximizes height growth and thus has a competitive advantage in more open sites. 8 Although photosynthetic performance paralleled that of growth, we conclude that rates of carbon dioxide fixation alone cannot define the seedling ecology of a species in the absence of information on photosynthate allocation.

Photosynthesis in relation to growth and seedling ecology of two dipterocarp rain forest tree species

The light environment within tropical rain forests varies considerably both spatially and temporally, and photon flux density (PFD) is considered to be an important factor determining the growth and survival of rain forest tree seedlings. In this paper we examine the ability of four ecologically contrasting dipterocarps (Dryobalanops lanceolata, Shorea leprosula, Hopea nervosa and Vatica oblongifolia) to utilise and dissipate light energy when grown in different light environments in lowland dipterocarp rain forest in the Danum Valley Conservation Area, Sabah, East Malaysia. Specifically we report (i) photosynthetic light response curves and associated fluorescence characteristics, including quantum yield (ΦPSII) and non-photochemical quenching (qN) and (ii) the extent to which photoinhibition occurs when plants grown in either high or low light are exposed to short bursts of high PFD. When grown in low light (artificial or forest shade) all four species had low light saturated rates of photosynthesis which were achieved at low PFDs. In addition, values of ΦPSII and qN were similar over a range of measurement PFDs. D. lanceolata and S. leprosula were also grown at high PFD and showed marked differences in their responses. S. leprosula demonstrated an ability to increase its rate of photosynthesis and there was a small increase in capacity to dissipate excess light energy non-photochemically at high PFDs. Partitioning of this qN into its fast, photo-protective (qE) and slow, photoinhibitory (qI) components indicated that there was an increase in qE quenching. In contrast, although D. lanceolata survived in the high light environment, greater rates of photosynthesis were not observed and the plants showed a greater capacity to dissipate energy non-photochemically. Partitioning of qN revealed that the majority of this increase was attributable to the slower relaxing phases.

Differences in light energy utilisation and dissipation between dipterocarp rain forest tree seedlings.

1 This paper reports on changes in photosynthetic induction and stomatal conductance (g,) in response to (i) continuous saturating light and (ii) darkness, for seedlings of two climax tree species (Shorea leprosula and Dryobalanops lanceolata, Dipterocarpaceae) growing in three differing rain forest light environments (ranging from 2 4 to 21 8 mol photon m-2 day-') in Malaysian Borneo. 2 For some species in understorey environments, a high proportion of diurnal carbon gain is attained during sunflecks (transient periods of high light), and D. lanceolata is slower growing than S. leprosula but its seedlings can survive under deeper shade conditions. Our aim was to determine whether interspecific differences in dynamic photosynthetic responses to light could further explain observed differences in seedling ecology. 3 In contrast to the expected trend for shade-tolerant species, D. lanceolata showed faster induction in higher light environments and S. leprosula showed no relationship between light environment and induction rate. However, both species showed greater potential sunfleck utilization efficiency in low light environments through slower rates of both induction loss and stomatal closure. 4 Shorea leprosula attained greater rates of maximal photosynthesis and gs, had faster rates of induction and retained a higher level of induction in prolonged darkness (> 30 min) than D. lanceolata in all light environments. However, S. keprosula showed faster induction loss and stomatal closure in the short term (10 min) than D. lanceolata, which, together with a potentially negative carbon balance between sunflecks, may limit its distribution to microsites of higher sunfleck frequency. 5 The balance between photosynthesis (A) and gs during induction resulted in a constant intercellular CO2 concentration of c. 270 p.p.m. after c. 11 min, which may represent a physiological optimum for both species. 6 In some circumstances transient peaks in A and ga were observed during induction, rather than a rise to a stable maximum, which we attribute to overcompensation of the stomatal response to light. In some cases for S. leprosula this initiated synchronized damped oscillations in gs and A that continued for c. 1 h in both continuous and discontinuous (80-s light/80-s dark) light. 7 Shorea leprosula plants with the ability to 'peak' or oscillate had the potential to increase both the rate and magnitude of response to sunflecks in comparison with simple sigmoidal induction.

https://www.jstor.org/stable/pdfplus/2960572.pdf

Photosynthetic Induction and Stomatal Oscillations in Relation to the Light Environment of Two Dipterocarp Rain Forest Tree Species

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Photosynthesis in relation to growth and seedling ecology of two dipterocarp rain forest tree species

Differences in light energy utilisation and dissipation between dipterocarp rain forest tree seedlings.

Photosynthetic Induction and Stomatal Oscillations in Relation to the Light Environment of Two Dipterocarp Rain Forest Tree Species