| The serious soil erosion on the Loess Plateau cause the deep cut gullies, fragmentation oflandform and form a complicated landform, thus result in spatial heterogeneity of water andheat. Then, there are series of adjustments occur on plant community structure, reproductivestrategies, leaf morphological and physiological characteristics to adapt different soilenvironments. In this study, the community structures in five soil erosion environments onslope-gully system (sunny gully slope, sunny hilly slope, hilltop, shady hilly slope, and shadygully slope) on three vegetation zones (forest zone, forest-steppezone, and steppe zone) on thehilly-gully region of the Loess Plateau were examined. The species niche breadth andimportant value in community were analysed to identify dominant species at different soilerosion environments. The reprodution, morphology, anatomy, and physiology of seclectednine dominant species (Artemisia scoparia, Artemisia gmelinii, Artemisia giraldii,Bothriochloa ischcemum, Stipa bungeana, Phragmites australis, Lespedeza davurica,Periploca sepium and Sophora davidii) were studied. And, four typical shoot architecturespecies (A. gmelinii, with dense shoot architecture; A. giraldii, with expanding shootarchitecture; B. ischaemum, with a tussock-formingshoot architecture; and S. viciifolia, with amain-stem shoot architecture) was selected to analysis the shape of phytogenic mounds andits influence of mirco-environmnent on slope. The main results are as follows:(1) Fom forest to steppe zone, plant community structures become simple, speciesdiversity decreased, and the number of species showed a slight increased with mesophytesand phanerophytes decreased, while xerophytes and cryptophytes increased. At different soilerosion environments on slope-gully system, the variation of species evenness was notsignificant. On shady slope, the species diversity and richness were relative larger than sunnyslope. The xerophyte and cryptophytes increased at sunny gully slope. Hilltop had a higher therophyte proportion than the other erosion environments. From sunny to shady gully slope,the species niche and important value in community decreased with the decreasing of erosiondegree. Xerophyte and xero-mesophytes have a wide range of distribution on the hilly-gullyregion of the Loess Plateau.(2) For same species, single dry weight of inflorescence (flower) and seed mass showedcertain genetic stability, and no significant difference existed in different soil erosionenvironments on slope-gully system. At the sunny gully slope and shady gully slope, plantreproductive investment was low, but the high proportion of cryptophytes on theseenvironments made vegetation regeneration relying mainly on bud bank. The reproductiveinvestment of species on hilltop and sunny hilly slope were larger than the other three erosionenvironments which result in a large number of seeds and thus was beneficial to the expansionof seed dispersal plants. L. davurica had a larger reproductive allocation, while two shrubs (P.sepium and S. davidii) were relatively less. In vegetation turning-green season, the largerperennial bud bank ensure a rapid vegetation renewal. The larger seasonal bud bank onfruit-setting season ensure the accumulation of photosynthate.(3) The leaf morphological and anatomical characteristics at same slope-gully erosionenvironment on the three vegetation zones had a slight changes, but these characteristicsshowed significant changes at different erosion environments on slope-gully system. With theincreasing of soil erosion degree, leaf thickness, dry matter content, cuticle thickness, palisadetissue thickness, xylem area/phloem area increased, while leaf area, epidermal cell size,spongy tissue thickness, intercellular space decreased. In those leaf morphological andanatomical indices, xylem area, which mediate plant water transportation, had the closestrelationship with plant environmental adaptability. Different plants have its special structureto adapt the semi-arid climate on the Loess Plateau: water storage parenchyma of A. scoparia,total-palisade tissue of A. gmelinii, bi-palisade tissue and big bundle sheath cells of A. giraldiiand S. davidii, mucilage cell of L. daurica; special bulliform cells of B. ischcemum and P.australis; multilayer sclerenchyma of S. bungeana; thick cuticle of P. sepium. From leafmorphological and anatomical characteristics, comprehensive analysis of the subordinatefunction showed that S. bungeana, which has a well developed protective tissues, had thestrongest environmental adaptability.(4) Similar to plant morphological characteristics, plant physiological indices at sameslope-gully erosion environment on the three vegetation zones showed a slight changes, butthose characteristics presented significant changes at different erosion environments on slope-gully system. From sunny to shady gully slope，with the increasing of soil erosiondegree, the degree of cell damage and amount of chlorophyll destruction increased. But plantresistance to environmental stress by increasing leaf water-holding ability, osmotic adjustmentsubstances, stress-resistance enzymes, non-enzymatic antioxidants, meanwhile by decreasingleaf relative water content and free water/bound water. This processes are at the expense ofplant growth rate to deal with the environmental stress. Among the21measured physiologicalindices, ascorbic acid oxidase (APX) had the closest relationship with plant environmentaladaptability. From plant physiological indices, comprehensive analysis of the subordinatefunction showed that two testing shrubs, S. davidii and P. sepium, had a strong environmentaladaptability.(5) Different shoot architecture species have different ability of preventing soil erosionand retaining sediment, thus the shape of phytogenic mound changes with slope gradientincreasing. Mound shapes changed from symmetrical mound-type structures intoasymmetrical terrace-type structures as slope gradient increased, but the change at0-15°forS. viciifolia, at26-35°for A. gmelinii and B. ischaemum, at>35°for A. giraldii. Among theplant shoot architecture parameters, plant basal diameter along the slope was significantlycorrelated with mound height, while the plant basal diameter perpendicular to the slope andtotal cross-sectional area of the stem at the plant base were associated with mound area.Mound height increased with slope for all species, while mound area showed oppositetendency. The phytogenic mounds can act as ‘resource islands’, thus improved speciesrichness and stress resistance of plant community on slope. A. giraldii with expanding shootarchitecture and S. viciifolia with its large canopy can create relatively large mounds.(6) In reproductive strategy, K-reproductive strategyare adopted by A. scoparia, A.gmelinii, A.giraldii, B. ischcemum and P. sepium; and r-reproductive strategy by S. bungeana,L. davurica and S. davidii. According to the palnt anatomical and physiological characteristicsof environmental adaptation, the testing plants were divided into three types: I, stressavoidance, including A. scoparia, S. bungeana and P. australis; II, strain repair, including A.giraldii, B. ischcemum and L. davurica; III, strain avoidance, A. gmelinii, P. sepium and S.davidii. Integrative subordinative value was calculated based on reproductive, morphological,anatomical and physiological characteristics of plant. The integrative subordinative values oftesting plants on different soil erosion environments of slope-gully system were positivelycorrelated with the species important value in community. From the results of comprehensiveanalysis of the subordinate function, two shrubs, S. davidii and P. sepium, showed higher environmental adaptability than another testing plants, followed by widespread plants (A.gmelinii, L. davurica, S. bungeana), the environmental adaptability of B. ischcemum, A.giraldii, P. australis, A. scoparia which only distributed on certain erosion environments,were relatively weak among testing plants. |
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