Effects Of Habitat Heterogeneity On Ecological Traits Of Aquatic Macrophytes

Habitat heterogeneity, which is structured by different substrates and disturbances, plays crucial roles in influencing the plant performance. As a major nutrient source for plants, substrate affects the growth of plants. Disturbance affects the plant performance by regulating the resource availability. Plant traits including growth traits and phenotypic plasticity reflect the strategy of plants to cope with environmental change. Researches on invasion ecology have proved that the trait advantages of invasive plants facilitate their invasion success. The present study used macrophytes as a model to explore the effects of substrate and disturbance on the traits of macrophytes. The theme was divided into three parts:"the effects of substrate heterogeneity and disturbance factors on the clonal growth of macrophytes", "the trait comparison among phylogenetically relevant exotic invasive, exotic non-invasive and native macrophytes under different substrate and disturbance conditions", and "the trait comparison among exotic invasive, exotic non-invasive and native counterparts under different substrate and disturbance conditions".Three experiments were deployed to discuss the first part. In "The foraging behavior of Potamogeton maackianus A. Benn. in variable environments of substrate type and light condition", the ramets of P. maackianus were planted into the different heterogeneous substrates constituted by clay and sand patches under heterogeneous light conditions. The idiosyncratic behavior of "sidewalk" was observed in P. maackianus-the lineage distribution of stolon in sand patch along the border of sand patch and clay patch. Potamogeton maackianus invested 95% of root mass in sand patch. In addition, specific root length and specific root area increased in heterogeneous substrates compared to those in homogenous substrate. The branching angle of ramets was lower in half-shaded treatments than in non-shaded treatments. And this caused the increased shoot biomass investment in light-rich zone rather than in light-poor zone in half-shaded treatments. Thus, the chlorophyll content of apical leaves was homogenized due to the variance of shoot investment. The increase of the maximal fluorescence in all the half-shaded treatments compensated the light shortage. The "non-clonal" plastic responses of the modules led to the performance maintenance of P. maackianus in the measurement of total biomass.In "The response of Vallisneria spiralis L. to different water depths and heterogeneous substrates" and "The response of Scirpus yagara Ohwi. to different water levels and heterogeneous substrates", similar manipulation of substrate type and water level was exerted on two submerged and emergent clonal plant species with creeping stems. In both two experiments, we found that the effects of substrate heterogeneity and the interaction of substrate heterogeneity and water level on clonal performance (e.g. ramet number, ramet height, stolon length, length and thickness of spacer and biomass) were insignificant. And water level significantly influenced the clonal performance of both species. However, the effects of substrate heterogeneity and the interaction of the two environmental factors on clonal performance were stronger in S. yagara than in V. spiralis. It was probably a consequence of the distinction of creeping stems between the two species-the rhizome of S. yagara can assimilate and store nutrients besides the functions of linking the ramets and transporting resources which are also possessed by stoloniferous V. spiralis.Two experiments were deployed to discuss the second part. In "The comparative study of confamilial exotic invasive Eichhornia crassipes (Mart.) Solms, exotic non-invasive Pontederia cordata L. and native Monochoria vaginalis (Burm. f.) Presl ex Kunth in three different riparian soils", three species were separated planted into three representative riparian soils-red soil, yellow soil and lake sediment in littoral zone of Liangzi Lake. After harvest, morphological and physiological functional traits including specific leaf area, chlorophyll content, leaf nitrogen content, leaf carbon:nitrogen ratio, leaf construction cost, root:leaf mass ratio, specific root length and specific root area were measured and the plasticity index was calculated. We found that E. crassipes took advantage in trait values and trait plasticity of root-level traits in comparison with the other two non-invasive confamilials. Hence, we concluded that the advantages in trait values and trait plasticity of root-level traits likely play a crucial role in the invasion success of E. crassipes.In "Responses of invasive Alternanthera philoxeroides (Mart.) Griseb. and its native congener A. sessilis (L.) R. Br. to different riparian soil environments and nutrient adding conditions", we explored the trait performance of A. philoxeroides and A. sessilis in different treatments of riparian soil type and cow manure fertilization. We found that disturbance of cow manure fertilization improved the trait performance of A. philoxeroides soil type-independently. Alternanthera philoxeroides and A. sessilis shared similar responses in growth traits including internode length, rooted node number, specific leaf area, leaf mass ratio, the maximal fluorescence, leaf construction cost and non-structural carbohydrate content to different soil types and nutrient addings. However, A. philoxeroides took advantage in several traits related to resource use including chlorophyll content, leaf nitrogen content and leaf carbon:nitrogen under the most eutrophic conditions, which is likely an invasion mechanism for A. philoxeroides.Two experiments were deployed to discuss the third part. In "The morphological performance of four riparian stoloniferous species-invasive A. philoxeroides, native congener A. sessilis, exotic non-invasive Myriophyllum aquaticum (Veil.) Verdcourt and native Jussiaea repens L." in heterogeneous substrates", we compared plant traits of four stoloniferous species in heterogeneous substrates. We found that A. philoxeroides was superior in relative growth rate, stolon length, biomass and biomass allocation to roots in comparison with A. sessilis. Moreover, the advantages of biomass allocation to roots were shown in A. philoxeroides rather than in M. aquaticum and J. repens. In contrast with empirical thoughts, increasing substrate heterogeneity did not promote the biomass accumulation of the four stoloniferous clonal plants. The biomass of A. philoxeroides and A. sessilis maintained, and the biomass of M. aquaticum and J. repens decreased with the increase of heterogeneity. We summarized that A. philoxeroides took advantage in root foraging and vegetative growth compared to A. sessilis. Compared to M. aquaticum and J. repens, A. philoxeroides took advantage in root foraging and alleviating heterogeneity. These advantages likely facilitate the invasion success of A. philoxeroides in heterogeneous mesic habitats.In "The comparative study of the populations of four riparian species-invasive A. philoxeroides, native congener A. sessilis, exotic non-invasive M. aquaticum and native J. repens in water level-fluctuated environments", the performances of invasive A. philoxeroides and its co-occurring counterparts under different conditions of disturbance of water level fluctuation were compared. Based on the variation levels of traits including ramet number, stolon length, stolon diameter, specific leaf area, biomass allocation, root-shoot ratio, the maximal fluorescence, chlorophyll content, leaf nitrogen content and non-structural carbohydrate content in each species, we found that the mature clonal population of A. philoxeroides did not outperform A. sessilis and M. aquaticum in resisting water level fluctuation in general. However, A. philoxeroides outcompeted J. repens in resisting water level fluctuation. Jussiaea repens is a dominated native species in the wetlands of sub-tropical and tropical China. The disturbance of water level fluctuation likely facilitates the successful invasion of A. philoxeroides into local communities.The present study revealed that the substrate and disturbance influence the growth of macrophytes by regulating the resource availability. The interactive effects of substrate heterogeneity and disturbance factors on the growth of macrophytes are not significant. The disturbance factors exert more significant effects on the growth of macrophytes. Under different substrate and disturbance conditions, the trait advantages of invasive macrophytes over non-invasive macrophytes likely facilitate their invasion success.