Abstract:

Background and Aims Biological invasions by non-native species have become a major environmental problem and a focus of ecological research. Relatively few studies have focused on invasibility and invasivness among microhabitats within communities. We compared the abundance and performance of non-native Alternanthera philoxeroides (alligator weed) and its co-occurring native congener, Alternanthera sessilis (sessile joyweed), in a wetland community along a riparian zone in southeast China to test the hypotheses that: 1) the degree of invasion differs between different types of microhabitats within the community; 2) microhabitat types that differ in invasion also differ in soil resource availability or in sediment characteristics likely to affect resource availability; and 3) phenotypic plasticity must have played a key role in adaptation to diverse habitats for A. philoxeroides because of its extremely low genetic diversity throughout China.

Methods Field surveys of natural distribution and performance of the two varieties on adjacent and contrasting microhabitat types (abandoned field, swamp, marsh dunes and gravel dunes) along a riparian zone were conducted in the autumn of 2003.

Key Results Soil organic matter, total N, available P and K were significantly higher and pH was lower in dryland and swamp than in dunes. Total vegetation coverage was significantly higher in dryland and swamp than in dunes. The relative coverage of A. philoxeroides was much higher than that of A. sessilis in more productive habitats (i.e. dry land and swamps), but this pattern reversed in less productive habitats (i.e. marsh and gravel dunes). A. philoxeroides showed greater morphological plasticity in response to habitat variation. Especially, the CV (coefficient of variation) of leaf area and branch angle in A. philoxeroides was 70 and 83 times greater than those in A. sessilis, respectively. Several morphological traits related to light foraging increased significantly from marsh and gravel dunes to more productive habitats (dry land and swamps). These traits included stem length, internode length, the number of nodes, and leaf length and width. Moreover, the leaf-bearing stems grew more vertically in dry land and swamps. Meanwhile, these traits in A. sessilis had no obvious variations among microhabitats.

Conclusions These results suggest that high plasticity in vertical growth and occupancy of soil microhabitat of rich nutrient resources may facilitate the invasions of A. philoxeroides.

Key words: Alternanthera philoxeroides, Alternanthera sessilis, Biological invasions, Light competition, Invasiveness, Resource ratios model