[Spectral characteristics of dissolved organic matter and its environmental responses across successional stages in a glacier retreat area]. / å†°å·é€€ç¼©åŒºä¸åŒæ¼”æ›¿é˜¶æ®µåœŸå£¤æº¶è§£æ€§æœ‰æœºè´¨å ‰è°±ç‰¹å¾åŠçŽ¯å¢ƒå“åº”.

Dissolved organic matter (DOM), the most active type of soil organic matter, plays a key role in soil biogeochemical cycling. Therefore, exploring the source, composition, environmental response, and accumulation mechanism of DOM during vegetation succession has great significance for predicting soil carbon cycling. In this study, DOM was extracted from topsoil and subsoil at plots after 12, 30, 40, 50, 80, and 120 years of primary succession along the Hailuogou Glacier retreat area. The concentrations and spectral characteristics of DOM were analyzed via a combination of elemental analysis, ultraviolet-visible spectroscopy, and three-dimensional fluorescence excitation-emission matrix spectroscopy. The results showed that concentrations of soil dissolved organic carbon and dissolved organic nitrogen of both topsoil and subsoil increased significantly during vegetation succession. Along the chronosequence, the protein-like components and optical indices were significantly enhanced, humic-like components and the optical indices decreased, the aromaticity degree of DOM increased first and then decreased. Soil pH and NH4+-N content explained 62.2% of the total variation of surface soil DOM components, while soil moisture and pH explained 64.3% of that of subsurface soil DOM, indicating that environmental conditions were key factors affecting the concentrations and composition of soil DOM in the Hailuogou Glacier retreat area.

A new isotope framework to decipher leaf-root nitrogen allocation and assimilation among plants in a tropical invaded ecosystem.

Exotic plant invasion is an urgent issue occurring in the biosphere, which can be stimulated by environmental nitrogen (N) loading. However, the allocation and assimilation of soil N sources between leaves and roots remain unclear for plants in invaded ecosystems, which hampers the understanding of mechanisms behind the expansion of invasive plants and the co-existence of native plants. This work established a new framework to use N concentrations and isotopes of soils, roots, and leaves to quantitatively decipher intra-plant N allocation and assimilation among plant species under no invasion and under the invasion of Chromolaena odorata and Ageratina adenophora in a tropical ecosystem of SW China. We found that the assimilation of N derived from both soil ammonium (NH4+) and nitrate (NO3-) were higher in leaves than in roots for invasive plants, leading to higher leaf N levels than native plants. Compared with the same species under no invasion, most native plants under invasion showed higher N concentrations and NH4+ assimilations in both leaves and roots, and increases in leaf N were higher than in root N for native plants under invasion. These results inform that preferential N allocation, dominated by NH4+-derived N, to leaves over roots as an important N-use strategy for plant invasion and co-existence in the studied tropical ecosystem.

Photosynthetic regulation in response to fluctuating light conditions under temperature stress in three mosses with different light requirements.

Under natural field conditions, mosses experience fluctuating light intensities combined with temperature stress. Alternative electron flow mediated by flavodiiron proteins (FLVs) and cyclic electron flow (CEF) around photosystem I (PSI) allow mosses to growth under fluctuating light conditions. However, little is known about the roles of FLVs and CEF in the regulation of photosynthesis under temperature stress combined with fluctuating light. Here, we measured chlorophyll fluorescence and P700 redox state under fluctuating light conditions at 4 °C, 20 °C, and 35 °C in three mosses with different light requirements. Upon a sudden increase in light intensity, electron flow from photosystem II initially increased and then gradually decreased at 20 °C and 35 °C, indicating that the operation of FLV-dependent flow lasted much longer than previously thought. Furthermore, the absolute rates of FLV-dependent flow and CEF were enhanced under fluctuating light at 35 °C, pointing to their important roles in photoprotection when exposed to fluctuating light at moderate high temperature. Furthermore, the downregulation of FLV activity at 4 °C was partially compensated for by enhanced CEF activity. These results suggested the subtle coordination between FLV activity and CEF under fluctuating light and temperature stress. Racomitrium japonicum and Hypnum plumaeforme, which usually grow under relatively high light levels, exhibited higher FLV activity and CEF than the shade-grown moss Plagiomnium ellipticum. Based on our results, we conclude that photosynthetic acclimation to fluctuating light and temperature stress in different mosses is largely linked to the adjustment of FLV activity and CEF.

Integrating novel chemical weapons and evolutionarily increased competitive ability in success of a tropical invader.

The evolution of increased competitive ability (EICA) hypothesis and the novel weapons hypothesis (NWH) are two non-mutually exclusive mechanisms for exotic plant invasions, but few studies have simultaneously tested these hypotheses. Here we aimed to integrate them in the context of Chromolaena odorata invasion. We conducted two common garden experiments in order to test the EICA hypothesis, and two laboratory experiments in order to test the NWH. In common conditions, C. odorata plants from the nonnative range were better competitors but not larger than plants from the native range, either with or without the experimental manipulation of consumers. Chromolaena odorata plants from the nonnative range were more poorly defended against aboveground herbivores but better defended against soil-borne enemies. Chromolaena odorata plants from the nonnative range produced more odoratin (Eupatorium) (a unique compound of C. odorata with both allelopathic and defensive activities) and elicited stronger allelopathic effects on species native to China, the nonnative range of the invader, than on natives of Mexico, the native range of the invader. Our results suggest that invasive plants may evolve increased competitive ability after being introduced by increasing the production of novel allelochemicals, potentially in response to naïve competitors and new enemy regimes.

Evolutionary increases in defense during a biological invasion.

Invasive plants generally escape from specialist herbivores of their native ranges but may experience serious damage from generalists. As a result, invasive plants may evolve increased resistance to generalists and tolerance to damage. To test these hypotheses, we carried out a common garden experiment comparing 15 invasive populations with 13 native populations of Chromolaena odorata, including putative source populations identified with molecular methods and binary choice feeding experiments using three generalist herbivores. Plants from invasive populations of C. odorata had both higher resistance to three generalists and higher tolerance to simulated herbivory (shoot removal) than plants from native populations. The higher resistance of plants from invasive populations was associated with higher leaf C content and densities of leaf trichomes and glandular scales, and lower leaf N and water contents. Growth costs were detected for tolerance but not for resistance, and plants from invasive populations of C. odorata showed lower growth costs of tolerance. Our results suggest that invasive plants may evolve to increase both resistance to generalists and tolerance to damage in introduced ranges, especially when the defense traits have low or no fitness costs. Greater defenses in invasive populations may facilitate invasion by C. odorata by reducing generalist impacts and increasing compensatory growth after damage has occurred.

Synergistic interactions of CO2 enrichment and nitrogen deposition promote growth and ecophysiological advantages of invading Eupatorium adenophorum in Southwest China.

Global environmental change and ongoing biological invasions are the two prominent ecological issues threatening biodiversity worldwide, and investigations of their interaction will aid to predict plant invasions and inform better management strategies in the future. In this study, invasive Eupatorium adenophorum and native congener E. stoechadosmum were compared at ambient and elevated atmospheric carbon dioxide (CO(2)) concentrations combined with three levels of nitrogen (N; reduced, control and increased) in terms of growth, energy gain, and cost. Compared with E. stoechadosmum, E. adenophorum adopted a quicker-return energy-use strategy, i.e. higher photosynthetic energy-use efficiency and shorter payback time. Lower leaf mass per area may be a pivotal trait for the invader, which contributed to an increased N allocation to Rubisco at the expense of cell walls and therefore to higher photosynthetic energy gain. CO(2) enrichment and N deposition synergistically promoted plant growth and influenced some related ecophysiological traits, and the synergistic effects were greater for the invader than for the native congener. Reducing N availability by applying sugar eliminated the advantages of the invader over its native congener at both CO(2) levels. Our results indicate that CO(2) enrichment and N deposition may exacerbate E. adenophorum's invasion in the future, and manipulating environmental resources such as N availability may be a feasible tool for managing invasion impacts of E. adenophorum.

Comparisons of plastic responses to irradiance and physiological traits by invasive Eupatorium adenophorum and its native congeners.

To explore the traits contributing to invasiveness of Eupatorium adenophorum and to test the relationship between plasticity of these traits and invasiveness, we compared E. adenophorum with its two native congeners at four irradiances (10%, 23%, 40%, and 100%). The invader showed constantly higher performance (relative growth rate and total biomass) across irradiances than its native congeners. Higher light-saturated photosynthetic rate (P(max)), respiration efficiency (RE), and nitrogen (PNUE) and water (WUE, at 40% and 100% irradiances only) use efficiencies contributed directly to the higher performance of the invader. Higher nitrogen allocation to, stomatal conductance, and the higher contents of leaf nitrogen and pigments contributed to the higher performance of the invader indirectly through increasing P(max), RE, PNUE and WUE. The invader had consistently higher plasticity only in carotenoid content than its native congeners in ranges of low (10-40%), high (40-100%) and total (10-100%) irradiances, contributing to invasion success in high irradiance by photo protection. In the range of low irradiances, the invader had higher plasticity in some physiological traits (leaf nitrogen content, nitrogen contents in bioenergetics, carboxylation and in light-harvesting components, and contents of leaf chlorophylls and carotenoids) but not in performance, while in the ranges of high or total irradiances, the invader did not show higher plasticity in any variable (except Car). The results indicated that the relationship between invasiveness and plasticity of a specific trait was complex, and that a universal generalization about the relationship might be too simplistic.

Invasive Eupatorium adenophorum has ecophysiological advantages over native congeners but similar responses to CO(2) enrichment.

Both global change and biological invasions threaten biodiversity worldwide. However, their interactions and related mechanisms are still not well elucidated. To elucidate potential traits contributing to invasiveness and whether ongoing increase in CO(2) aggravates invasions, noxious invasive Eupatorium adenophorum and native E. japonicum and E. chinensis were compared under ambient and doubled atmospheric CO(2) concentrations in terms of growth, biomass allocation, morphology, and physiology. The invader had consistently higher leaf mass fraction and specific leaf area than the natives, contributing to a higher leaf area ratio, and therefore to faster growth and invasiveness. The higher leaf mass fraction of the invader was associated with lower total root mass fraction. The invader allocated a higher fraction of leaf nitrogen (N) to photosynthesis, contributing to higher area-based N content in photosynthetic apparatus, photosynthetic rate, nitrogen- and water-use efficiencies, and invasiveness. CO(2) enrichment increased growth of all studied plants by increasing actual photosynthesis, although it decreased photosynthetic capacities due to decreased area-based leaf and photosynthetic N contents. Responses of the invasive and native plants to elevated CO(2) were not significantly different, indicating that the ongoing increase in CO(2) may not exacerbate biological invasions, inconsistent with the prevailing results in references. The difference may be associated with the fact that almost all previous studies compared phylogenetically unrelated invasive and native plants. More comparative studies of sympatric, related invasive and native plants are needed to elucidate whether CO(2) enrichment facilitates invasions.

Evolutionary tradeoffs for nitrogen allocation to photosynthesis versus cell walls in an invasive plant.

Many studies have shown that individuals from invasive populations of many different plant species grow larger than individuals from native populations and that this difference has a genetic basis. This increased vigor in invasive populations is thought to be due to life history tradeoffs, in which selection favors the loss of costly defense traits, thereby freeing resources that can be devoted to increased growth or fecundity. Despite the theoretical importance of such allocation shifts for invasions, there have been no efforts to understand apparent evolutionary shifts in defense-growth allocation mechanistically. Reallocation of nitrogen (N) to photosynthesis is likely to play a crucial role in any growth increase; however, no study has been conducted to explore potential evolutionary changes in N allocation of introduced plants. Here, we show that introduced Ageratina adenophora, a noxious invasive plant throughout the subtropics, appears to have evolved increased N allocation to photosynthesis (growth) and reduced allocation to cell walls, resulting in poorer structural defenses. Our results provide a potential mechanism behind the commonly observed and genetically based increase in plant growth and vigor when they are introduced to new ranges.