The Leaf Economics Spectrum Constrains Phenotypic Plasticity Across a Light Gradient

Coordination of water use strategies and leaf economic traits in coexisting exotic and native woody species from evergreen and deciduous broadleaf forests

The leaf economics spectrum (LES) describes the covariation of traits relevant for carbon and nutrient economy in different plant species. However, much less is known about the correlation of LES with leaf water economy, not only because some woody species do not follow the rules, but also because they are rarely tested on the widespread, non-native, fast-growing trees. We hypothesized that fast-growing exotic species that spread on the fast side of the LES coordinate their water-use strategies (WUS) to maintain rapid growth, and that the pattern of coordination differs between evergreen and deciduous forests. Using 4 exotic and 4 native species from evergreen and deciduous broadleaf forests in China, we measured 17 traits of LES and WUS and analyzed their functional roles in different species groups. Our results suggest that LES plays a more important role in the coexistence of species within a community, while WUS contributes more to the distribution of species across different regions. The multidimensional coordination of LES and WUS could better explain the growth and distribution of different plant species and shed light on the coexistence of species from different forest types, especially fast-growing woody exotics.

Respiration in light of evergreen and deciduous woody species and its links to the leaf economic spectrum

Leaf respiration in the light (Rlight) is crucial for understanding the net CO2 exchange of individual plants and entire ecosystems. However, Rlight is poorly quantified and rarely discussed in the context of the leaf economic spectrum (LES), especially among woody species differing in plant functional types (PFTs) (e.g., evergreen vs. deciduous species). To address this gap in our knowledge, Rlight, respiration in the dark (Rdark), light-saturated photosynthetic rates (Asat), leaf dry mass per unit area (LMA), leaf nitrogen (N) and phosphorus (P) concentrations, and maximum carboxylation (Vcmax) and electron transport rates (Jmax) of 54 representative subtropical woody evergreen and deciduous species were measured. With the exception of LMA, the parameters quantified in this study were significantly higher in deciduous species than in evergreen species. The degree of light inhibition did not significantly differ between evergreen (52%) and deciduous (50%) species. Rlight was significantly correlated with LES traits such as Asat, Rdark, LMA, N and P. The Rlight vs. Rdark and N relationships shared common slopes between evergreen and deciduous species, but significantly differed in their y-intercepts, in which the rates of Rlight were slower or faster for any given Rdark or N in deciduous species, respectively. A model for Rlight based on three traits (i.e., Rdark, LMA and P) had an explanatory power of 84.9%. These results show that there is a link between Rlight and the LES, and highlight that PFTs is an important factor in affecting Rlight and the relationships of Rlight with Rdark and N. Thus, this study provides information that can improve the next generation of terrestrial biosphere models (TBMs).

Temperature dependence of carbon metabolism in the leaves in sun and shade in a subtropical forest

Rising temperatures pose a threat to the stability of climate regulation by carbon metabolism in subtropical forests. Although the effects of temperature on leaf carbon metabolism traits in sun-exposed leaves are well understood, there is limited knowledge about its impacts on shade leaves and the implications for ecosystem-climate feedbacks. In this study, we measured temperature response curves of photosynthesis and respiration for 62 woody species in summer (including both evergreen and deciduous species) and 20 evergreen species in winter. The aim was to uncover the temperature dependence of carbon metabolism in both sun and shade leaves in subtropical forests. Our findings reveal that shade had no significant effects on the mean optimum photosynthetic temperatures (TOpt) or temperature range (T90). However, there were decreases observed in mean stomatal conductance, mean area-based photosynthetic rates at TOpt and 25 °C, as well as mean area-based dark respiration rates at 25 °C in both evergreen and deciduous species. Moreover, the respiration-temperature sensitivity (Q10) of sun leaves was higher than that of shade leaves in winter, with the reverse being true in summer. Leaf economics spectrum traits, such as leaf mass per area, and leaf concentration of nitrogen and phosphorus across species, proved to be good predictors of TOpt, T90, mass-based photosynthetic rate at TOpt, and mass-based photosynthetic and respiration rate at 25 °C. However, Q10 was poorly predicted by these leaf economics spectrum traits except for shade leaves in winter. Our results suggest that model estimates of carbon metabolism in multilayered subtropical forest canopies do not necessitate independent parameterization of T90 and TOpt temperature responses in sun and shade leaves. Nevertheless, a deeper understanding and quantification of canopy variations in Q10 responses to temperature are necessary to confirm the generality of temperature-carbon metabolism trait responses and enhance ecosystem model estimates of carbon dynamics under future climate warming.

Embracing the complexity of leaf shape: a commentary on 'Anatomical determinants of gas exchange and hydraulics vary with leaf shape in soybean'

This article comments on:

Bishal G. Tamang, Yanqun Zhang, Michelle A. Zambrano and Elizabeth A. Ainsworth Anatomical determinants of gas exchange and hydraulics vary with leaf shape in soybean, Annals of Botany, Volume 131, Issue 6, 9 May 2023, Pages 909–920, https://doi.org/10.1093/aob/mcac118

Leaf phenotypic plasticity coupled with integration facilitates the adaptation of plants to enhanced N deposition

The response of leaf functional traits can provide vital insight into the adaptive strategies of plants under global change. However, empirical knowledge on the acclimation of functional coordination between phenotypic plasticity and integration to increased nitrogen (N) deposition is still scarce. The variation of leaf functional traits of two dominant seedling species, Machilus gamblei and Neolitsea polycarpa, across four N deposition rates (0, 3, 6, and 12 kg N ha^-1yr^-1), along with the relationship between leaf phenotypic plasticity and integration were investigated in a subtropical montane forest. We found that enhanced N deposition promoted the development of seedling traits toward the direction of resource acquisition, including improved leaf N content, specific leaf area and photosynthetic performance. Appropriate N deposition (≤6 kg N ha^-1 yr^-1) might induce the optimization of leaf functional traits to promote the capability and efficiency of nutrient use and photosynthesis in seedlings. However, excessive N deposition (12 kg N ha^-1 yr^-1) would result in detrimental effects on leaf morphological and physiological traits, thus inhibiting the efficiency in resource acquisition. A positive relationship occurred between leaf phenotypic plasticity and integration in both seedling species, implied that higher plasticity of leaf functional traits likely led to better integration with other traits under N deposition. Overall, our study emphasized that leaf functional traits could rapidly respond to changes in N resource, while the coordination between leaf phenotypic plasticity and integration can facilitate the adaptation of tree seedlings in coping with enhanced N deposition. Further studies are still needed on the role of leaf phenotypic plasticity and integration in plant fitness for predicting ecosystem functioning and forest dynamics, especially in the context of future high N deposition.

Variation and association of leaf traits for desert plants in the arid area, northwest China

Characterizing variation and association of plant traits is critical for understanding plant adaptation strategies and community assembly mechanisms. However, little is known about the leaf trait variations of desert plants and their association with different life forms. We used principal component analysis, Pearson's correlation, phylogenetic independent contrasts, linear mixed model, and variance decomposition to explore the variation and association of 10 leaf traits in 22 desert plants in the arid area of northwest China. We found that: (1) the contribution of interspecific variation to the overall variation was greater than the intraspecific variation of all the studied leaf traits; (2) intraspecific and interspecific variation in leaf traits differed among life forms. Some leaf traits, such as tissue density of shrubs and specific leaf area of herbs, exhibited greater intraspecific than interspecific variation, while other traits exhibited the inverse; (3) desert shrubs corroborate the leaf economic spectrum hypothesis and had a fast acquisitive resource strategy, but herbs may not conform to this hypothesis; (4) there were trade‐offs between leaf traits, which were mediated by phylogeny. Overall, our results suggest that interspecific variation of leaf traits significantly contributes to the total leaf traits variation in desert plants. However, intraspecific variation should not be overlooked. There are contrasts in the resource acquisition strategies between plants life forms. Our results support understanding of the mechanisms underlying community assembly in arid regions and suggest that future works may focus on the variation and association of plant traits at both intra‐ and interspecific scales.

The fern economics spectrum is unaffected by the environment

The plant economics spectrum describes the trade-off between plant resource acquisition and storage, and sheds light on plant responses to environmental changes. However, the data used to construct the plant economics spectrum comes mainly from seed plants, thereby neglecting vascular non-seed plant lineages such as the ferns. To address this omission, we evaluated whether a fern economics spectrum exists using leaf and root traits of 23 fern species living under three subtropical forest conditions differing in light intensity and nutrient gradients. The fern leaf and root traits were found to be highly correlated and formed a plant economics spectrum. Specific leaf mass and root tissue density were found to be on one side of the spectrum (conservative strategy), whereas photosynthesis rate, specific root area, and specific root length were on the other side of the spectrum (acquisitive strategy). Ferns had higher photosynthesis and respiration rates, and photosynthetic nitrogen-use efficiency under high light conditions and higher specific root area and lower root tissue density in high nutrient environments. However, environmental changes did not significantly affect their resource acquisition strategies. Thus, the plant economics spectrum can be broadened to include ferns, which expands its phylogenetic and ecological implications and utility.

The carbon economics of vegetative phase change

Across plant species and biomes, a conserved set of leaf traits govern the economic strategy used to assimilate and invest carbon. As plants age, they face new challenges that may require shifts in this leaf economic strategy. In this study, we investigate the role of the developmental transition, vegetative phase change (VPC), in altering carbon economics as plants age. We used overexpression of microRNA 156 (miR156), the master regulator of VPC, to modulate the timing of VPC in Populus tremula x alba, Arabidopsis thaliana and Zea mays to understand the impact of this transition on leaf economic traits, including construction cost, payback time and return on investment. Here, we find that VPC causes a shift from a low-cost, quick return juvenile strategy to a high-cost, high-return adult strategy. The juvenile strategy is advantageous in light-limited conditions, whereas the adult strategy provides greater returns in high light. The transition between these strategies is correlated with the developmental decline in the level of miR156, suggesting that is regulated by the miR156/SPL pathway. Our results provide an ecophysiological explanation for the existence of juvenile and adult leaf types and suggest that natural selection for these alternative economic strategies could be an important factor in plant evolution.

Rapid estimation of photosynthetic leaf traits of tropical plants in diverse environmental conditions using reflectance spectroscopy

Tropical forests are one of the main carbon sinks on Earth, but the magnitude of CO₂ absorbed by tropical vegetation remains uncertain. Terrestrial biosphere models (TBMs) are commonly used to estimate the CO₂ absorbed by forests, but their performance is highly sensitive to the parameterization of processes that control leaf-level CO₂ exchange. Direct measurements of leaf respiratory and photosynthetic traits that determine vegetation CO₂ fluxes are critical, but traditional approaches are time-consuming. Reflectance spectroscopy can be a viable alternative for the estimation of these traits and, because data collection is markedly quicker than traditional gas exchange, the approach can enable the rapid assembly of large datasets. However, the application of spectroscopy to estimate photosynthetic traits across a wide range of tropical species, leaf ages and light environments has not been extensively studied. Here, we used leaf reflectance spectroscopy together with partial least-squares regression (PLSR) modeling to estimate leaf respiration (R_dark25), the maximum rate of carboxylation by the enzyme Rubisco (V_cmax25), the maximum rate of electron transport (J_max25), and the triose phosphate utilization rate (T_p25), all normalized to 25°C. We collected data from three tropical forest sites and included leaves from fifty-three species sampled at different leaf phenological stages and different leaf light environments. Our resulting spectra-trait models validated on randomly sampled data showed good predictive performance for V_cmax25, J_max25, T_p25 and R_dark25 (RMSE of 13, 20, 1.5 and 0.3 μmol m^-2 s^-1, and R² of 0.74, 0.73, 0.64 and 0.58, respectively). The models showed similar performance when applied to leaves of species not included in the training dataset, illustrating that the approach is robust for capturing the main axes of trait variation in tropical species. We discuss the utility of the spectra-trait and traditional gas exchange approaches for enhancing tropical plant trait studies and improving the parameterization of TBMs.

Prediction of photosynthetic light-response curves using traits of the leaf economics spectrum for 75 woody species: effects of leaf habit and sun-shade dichotomy

PREMISE

Photosynthetic light-response (PLR) curves for leaves are important components of models related to carbon fixation in forest ecosystems, linking the Mitscherlich equation and Michaelis-Menten equation to traits of the leaf economics spectrum (LES). However, models do not consider changes in leaf habits (i.e., evergreen and deciduous) and within-canopy shading variation in these PLR curves.

METHODS

Here, we measured the PLR curves in sun and shade leaves of 44 evergreen and 31 deciduous species to examine the relationships between variables of the Mitscherlich equation and Michaelis-Menten equation, leaf nitrogen (N) and phosphorus (P) content, and leaf mass per area (LMA).

RESULTS

Small changes were caused by different leaf habits and shade variations in relationships linking variables of the two equations to leaf N and P content and LMA. Values of the scaling exponents for PLR curve parameters did not differ regardless of canopy position and leaf habit (P > 0.05). The PLR curves in species with different leaf habits (i.e., evergreen and deciduous) at different canopy positions could be predicted using the general allometric relations between leaf traits and PLR parameters in the two equations. For photosynthetic photon flux densities from 0 to 2000 μmol m^-2 s^-1 , approximately 71% (Mitscherlich equation) and 70% (Michaelis-Menten equation) of the net assimilation rates could be predicted.

CONCLUSIONS

These findings indicate that leaf net assimilation rates can be predicted through the large available data for LES traits. Incorporation of values for these traits available in the LES databases into ecosystem models of forest productivity and carbon fixation warrants further investigation.