Intraspecific trait plasticity to N and P of the wetland invader, Alternanthera philoxeroides under flooded conditions

Abstract

Interactions between invaders and resource availability may explain variation in their success or management efficacy. For widespread invaders, regional variation in plant response to nutrients can reflect phenotypic plasticity of the invader, genetic structure of invading populations, or a combination of the two. The wetland weed Alternanthera philoxeroides (alligatorweed) is established throughout the southeastern United States and California and has high genetic diversity despite primarily spreading clonally. Despite its history in the United States, the role of genetic variation for invasion and management success is only now being uncovered. To better understand how nutrients and genotype may influence A. philoxeroides invasion, we measured the response of plants from 26 A. philoxeroides populations (three cp haplotypes) to combinations of nitrogen (4 or 200 mg/L N) and phosphorus (0.4 or 40 mg/L P). We measured productivity (biomass accumulation and allocation), plant architecture (stem diameter and thickness, branching intensity), and foliar traits (toughness, dry matter content, percent N, and percent P). A short-term developmental assay was also conducted by feeding a subset of plants from the nutrient experiment to the biological control agent Agasicles hygrophila, to determine whether increased availability of N or P to its host influenced agent performance, as has been previously suggested. Alternanthera philoxeroides haplotype Ap1 was more plastic than other haplotypes in response to nutrient amendments, producing more than double the biomass from low to high N and 50%-68% higher shoot: root ratio than other haplotypes in the high N treatment. Alternanthera philoxeroides haplotypes differed in seven of 10 variables in response to increased N. We found no differences in short-term A. hygrophila development between haplotypes but mass was 23% greater in high than low N treatments. This study is the first to explore the interplay between nutrient availability, genetic variation, and phenotypic plasticity in invasive characteristics of the global invader, A. philoxeroides.

Keywords: adaptive evolution; biogeography; biological control; invasive species; latitudinal pattern; nutrient enrichment; phenotypic plasticity; wetland invader.

FIGURE 1

Influence of nutrients on biomass production (g DW; least squares mean ± SE) (a) and allocation (b, c) of A. philoxeroides haplotypes. These interactions were identified as influential based on model selection. Error bars obscured due to small size.

FIGURE 2

Influence of the nitrogen × phosphorus interaction (a) and the haplotype × nitrogen interaction (b) on branching intensity (least squares mean ± SE). These interactions were identified as influential based on model selection. Error bars obscured due to small size.

FIGURE 3

Influence of the haplotype х nitrogen interaction on A. philoxeroides stem diameter (a), stem wall thickness (b), and leaf toughness (least squares mean ± SE) (c). These interactions were identified as influential based on model selection. Error bars obscured due to small size.

FIGURE 4

Influence of nitrogen × phosphorus interaction (a), haplotype × nitrogen interaction (b) on A. philoxeroides leaf nitrogen and nitrogen × phosphorus interaction on leaf phosphorus (c) (least squares mean ± SE). These interactions were identified as influential based on model selection. Error bars obscured due to small size.

FIGURE 5

Influence of the nitrogen × phosphorus interaction on resistance to herbivory (A. hygrophila larval weight; least squares mean ± SE of untransformed data). The interaction was identified as influential based on model selection.

FIGURE 6

Plasticity (g; mean ± 95% CI) of A. philoxeroides haplotypes in response to nitrogen (black circle) or phosphorus (white circle) treatments.