Adaptive Strategies Of Alternanthera Philoxeroides Clonal Fragments

Disturbance is common in nature and disturbance-caused fragmentation of clones happens frequently in clonal plants. Natural and human-caused disturbances can fragment the groups of connected ramets formed by clonal plants into smaller groups of one to several ramets. The capacity of these small fragments to disperse, survive, and grow is a major factor in the spread of clonal plants. Establishment of small fragments is particularly important in the spread of aquatic clonal species, including species of introduced, invasive plants in wetlands. Understanding what determines the survival and growth of small clonal fragments is thus of both scientific and practical interest. At both the single-ramet and whole fragment levels, we conducted a series of greenhouse experiments using the introduced, stoloniferous herb Alternanthera philoxeroides to test effects of fragmentation degree and environmental factors on survival and growth of fragments.These results show that (1) increasing internode length and presence of leaves significantly increased the survival rate and growth (biomass, leaf area, number of ramets, stolon length and number of leaves) of the A. philoxeroides plants. All growth measures of A. philoxeroides at harvest were larger when the ramets were attached with a distal internode than when they were attached with a proximal internode, but the survival rate was lower.(2) Survival and growth of the A. philoxeroides plants were greatest when fragments were positioned horizontally. Contrary to expectations, some of these effects of orientation were stronger when attached stolons were longer. Orientation had smaller effects than stolon length on the performance of fragments; survival of fragments was about60%with shorter stolons and90%with longer stolons.(3) Increasing burial depth significantly reduced survival of the A. philoxeroides plants and increased root to shoot ratio and total stolon length, but did not change growth. Increasing internode length significantly increased survival and growth, but there was no interaction effect with burial depth on any traits measured.(4) Shading severely inhibited growth of the A. philoxeroides plants in both the maternal environment and daughter’s environment. Maternal effect existed only in the daughter’s control treatment, in which fragments that did not experience shading in the maternal environment grew better than those that suffered shading. Strikingly, in the daughter’s control treatment, the demethylating agent5-azacytidine increased growth of fragments that experienced intermediate shading in the maternal environment.(5) Fragmentation and apex removal did not affect total growth of the A. philoxeroides plants, but more highly fragmented plants produced smaller but more ramets. The survival of fragments consisting of one original ramet was about85%, compared to100%survival of fragments with5original ramets. Fragments consisting of the youngest ramets along the original stolon grew more than fragments of the same size that consisted of older ramets.(6) Across connection treatments, water depth had a positive effect on most measures of growth, indicating that resource availability increased with water depth. Largely consistent with the conceptual model, restricting clonal integration by severing the apical and basal parts of an A. philoxeroides fragment had positive to neutral effects on total growth of fragments at0cm water depth, and neutral to negative effects at20and40cm. The effects of severance on the apical part were generally negative and greater at greater depth; effects of severance on the basal part were generally positive and smaller at greater depth.These results indicate that A. philoxeroides can response to clonal fragmentation via clonal storage, clonal integration and a trade-off between ramet number and ramet mass. Storage in stolons and leaves can contribute greatly to the survival and growth of A. philoxeroides, and detachment of small groups of distal ramets along stolons may be very effective in spreading the species. The trade-off between the number and the size of new ramets produced by fragments may represent an adaptive, plastic response to disturbance. Clonal integration also helps A. philoxeroides fragments become advantageous in homogeneous habitats where resource availability is high. Furthermore, environmental factors also influence the responses of A. philoxeroides to clonal fragmentation. Orientation will affect establishment of small clonal fragments, and such effects can be stronger in larger rather than smaller fragments. Regeneration capacity of fragments will be reduced by sand burial, but A. philoxeroides can maintain the same fitness of the surviving plants by changing biomass allocation and stolon length. A. philoxeroides can also response to shading by the maternal effect. Therefore, These results suggest that disturbance-based fragmentation of clones cannot control the growth rate of A. philaxeroides but can increase the spread rate of this species in disturbed habitats.