Belowground advantages in construction cost facilitate a cryptic plant invasion

Abstract

The energetic cost of plant organ construction is a functional trait that is useful for understanding carbon investment during growth (e.g. the resource acquisition vs. tissue longevity tradeoff), as well as in response to global change factors like elevated CO2 and N. Despite the enormous importance of roots and rhizomes in acquiring soil resources and responding to global change, construction costs have been studied almost exclusively in leaves. We sought to determine how construction costs of aboveground and belowground organs differed between native and introduced lineages of a geographically widely dispersed wetland plant species (Phragmites australis) under varying levels of CO2 and N. We grew plants under ambient and elevated atmospheric CO2, as well as under two levels of soil nitrogen. We determined construction costs for leaves, stems, rhizomes and roots, as well as for whole plants. Across all treatment conditions, the introduced lineage of Phragmites had a 4.3 % lower mean rhizome construction cost than the native. Whole-plant construction costs were also smaller for the introduced lineage, with the largest difference in sample means (3.3 %) occurring under ambient conditions. In having lower rhizome and plant-scale construction costs, the introduced lineage can recoup its investment in tissue construction more quickly, enabling it to generate additional biomass with the same energetic investment. Our results suggest that introduced Phragmites has had an advantageous tissue investment strategy under historic CO2 and N levels, which has facilitated key rhizome processes, such as clonal spread. We recommend that construction costs for multiple organ types be included in future studies of plant carbon economy, especially those investigating global change.

Keywords: Carbon dioxide; Phragmites; common reed; construction cost; eutrophication; intraspecific; invasion ecology; plant functional traits; rhizomes; wetlands..

Figure 1.

Organ-specific construction costs (CCs) for Phragmites lineages native to the North American North Atlantic coast (‘Native’) and introduced from Eurasia (‘Introduced’). Bar heights represent mean (±SE) CC for all plants grown in a combination of CO₂ and N fertilization treatments. Within each panel, lowercase letters above bars differ when Tukey's HSD tests for the best-fitting model identified statistically significant differences in means. Units are grams of glucose required per gram of biomass produced.

Figure 2.

Partitioning of (A) plant-level CCs and (B) plant biomass by organ type for native and introduced Phragmites under each of the environmental treatment combinations evaluated in this experiment. Mean values for the replicate individuals within each treatment are shown. The widths of bars in (B) are scaled to the total biomass produced across treatments.