Dominant Species in Subtropical Forests Could Decrease Photosynthetic N Allocation to Carboxylation and Bioenergetics and Enhance Leaf Construction Costs during Forest Succession

Nitrogen and phosphorus addition promote invasion success of invasive species via increased growth and nutrient accumulation under elevated CO2

In the context of the resource allocation hypothesis regarding the trade-off between growth and defence, compared with native species, invasive species generally allocate more energy to growth and less energy to defence. However, it remains unclear how global change and nutrient enrichment will influence the competition between invasive species and co-occurring native species. Here, we tested whether nitrogen (N) and phosphorus (P) addition under elevated CO2 causes invasive species (Mikania micrantha and Chromolaena odorata) to produce greater biomass, higher growth-related compounds and lower defence-related compounds than native plants (Paederia scandens and Eupatorium chinense). We grew these native and invasive species with similar morphology with the addition of N and P under elevated CO2 in open-top chambers. The addition of N alone increased the relative growth rate (RGR) by 5.4% in invasive species, and its combination with P addition or elevated CO2 significantly increased the RGR of invasive species by 7.5 or 8.1%, respectively, and to a level higher than that of native species (by 14.4%, P < 0.01). Combined N + P addition under elevated CO2 decreased the amount of defence-related compounds in the leaf, including lipids (by 17.7%) and total structural carbohydrates (by 29.0%), whereas it increased the growth-related compounds in the leaf, including proteins (by 75.7%), minerals (by 9.6%) and total non-structural carbohydrates (by 8.5%). The increased concentrations of growth-related compounds were possibly associated with the increase in ribulose 1,5-bisphosphate carboxylase oxygenase content and mineral nutrition (magnesium, iron and calcium), all of which were higher in the invasive species than in the native species. These results suggest that rising atmospheric CO2 concentration and N deposition combined with nutrient enrichment will increase the growth of invasive species more than that of native species. Our result also suggests that invasive species respond more readily to produce growth-related compounds under an increased soil nutrient availability and elevated CO2.

Plant Functional Groups Dominate Responses of Plant Adaptive Strategies to Urbanization

Urbanization causes alteration in atmospheric, soil, and hydrological factors and substantially affects a range of morphological and physiological plant traits. Correspondingly, plants might adopt different strategies to adapt to urbanization promotion or pressure. Understanding of plant traits responding to urbanization will reveal the capacity of plant adaptation and optimize the choice of plant species in urbanization green. In this study, four different functional groups (herbs, shrubs, subcanopies, and canopies, eight plant species totally) located in urban, suburban, and rural areas were selected and eight replicated plants were selected for each species at each site. Their physiological and photosynthetic properties and heavy metal concentrations were quantified to reveal plant adaptive strategies to urbanization. The herb and shrub species had significantly higher starch and soluble sugar contents in urban than in suburban areas. Urbanization decreased the maximum photosynthetic rates and total chlorophyll contents of the canopies (Engelhardtia roxburghiana and Schima superba). The herbs (Lophatherum gracile and Alpinia chinensis) and shrubs (Ardisia quinquegona and Psychotria rubra) species in urban areas had significantly lower nitrogen (N) allocated in the cell wall and leaf δ¹⁵N values but higher heavy metal concentrations than those in suburban areas. The canopy and subcanopy (Diospyros morrisiana and Cratoxylum cochinchinense) species adapt to the urbanization via reducing resource acquisition but improving defense capacity, while the herb and shrub species improve resource acquisition to adapt to the urbanization. Our current studies indicated that functional groups affected the responses of plant adaptive strategies to the urbanization.

Contrasting Dependencies of Photosynthetic Capacity on Leaf Nitrogen in Early- and Late-Successional Tropical Montane Tree Species

Differences in photosynthetic capacity among tree species and tree functional types are currently assumed to be largely driven by variation in leaf nutrient content, particularly nitrogen (N). However, recent studies indicate that leaf N content is often a poor predictor of variation in photosynthetic capacity in tropical trees. In this study, we explored the relative importance of area-based total leaf N content (N_tot) and within-leaf N allocation to photosynthetic capacity versus light-harvesting in controlling the variation in photosynthetic capacity (i.e. V _cmax, J _max) among mature trees of 12 species belonging to either early (ES) or late successional (LS) groups growing in a tropical montane rainforest in Rwanda, Central Africa. Photosynthetic capacity at a common leaf temperature of 25˚C (i.e. maximum rates of Rubisco carboxylation, V _cmax25 and of electron transport, J _max25) was higher in ES than in LS species (+ 58% and 68% for V _cmax25 and J _max25, respectively). While N_tot did not significantly differ between successional groups, the photosynthetic dependency on N_tot was markedly different. In ES species, V _cmax25 was strongly and positively related to N_tot but this was not the case in LS species. However, there was no significant trade-off between relative leaf N investments in compounds maximizing photosynthetic capacity versus compounds maximizing light harvesting. Both leaf dark respiration at 25˚C (+ 33%) and, more surprisingly, apparent photosynthetic quantum yield (+ 35%) was higher in ES than in LS species. Moreover, R_d25 was positively related to N_tot for both ES and LS species. Our results imply that efforts to quantify carbon fluxes of tropical montane rainforests would be improved if they considered contrasting within-leaf N allocation and photosynthetic N_tot dependencies between species with different successional strategies.