Combined use of leaf size and economics traits allows direct comparison of hydrophyte and terrestrial herbaceous adaptive strategies

Abstract

Background and aims: Hydrophytes generally exhibit highly acquisitive leaf economics. However, a range of growth forms is evident, from small, free-floating and rapidly growing Lemniden to large, broad-leaved Nymphaeiden, denoting variability in adaptive strategies. Traits used to classify adaptive strategies in terrestrial species, such as canopy height, are not applicable to hydrophytes. We hypothesize that hydrophyte leaf size traits and economics exhibit sufficient overlap with terrestrial species to allow a common classification of plant functional types, sensu Grime's CSR theory.

Methods: Leaf morpho-functional traits were measured for 61 species from 47 water bodies in lowland continental, sub-alpine and alpine bioclimatic zones in southern Europe and compared against the full leaf economics spectrum and leaf size range of terrestrial herbs, and between hydrophyte growth forms.

Key results: Hydrophytes differed in the ranges and mean values of traits compared with herbs, but principal components analysis (PCA) demonstrated that both groups shared axes of trait variability: PCA1 encompassed size variation (area and mass), and PCA2 ranged from relatively dense, carbon-rich leaves to nitrogen-rich leaves of high specific leaf area (SLA). Most growth forms exhibited trait syndromes directly equivalent to herbs classified as R adapted, although Nymphaeiden ranged between C and SR adaptation.

Conclusions: Our findings support the hypothesis that hydrophyte adaptive strategy variation reflects fundamental trade-offs in economics and size that govern all plants, and that hydrophyte adaptive strategies can be directly compared with terrestrial species by combining leaf economics and size traits.

Fig. 1.

Comparison of leaf economics traits (LCC, leaf carbon concentration; LDMC, leaf dry matter content; LNC, leaf nitrogen concentration; SLA, specific leaf area) and leaf size traits (LA, leaf area; LDW, leaf dry weight; LFW, leaf fresh weight) between terrestrial herbs (n = 506) and aquatic species (n = 61). Data represent the mean of ten replicates, and means of the two groups are compared by Student's t-test, following normalization for each trait as detailed in the text.

Fig. 2.

Principal components analysis (PCA) biplot showing the first two principal axes of variation in mean leaf trait data for 506 herbaceous (grey circles) and 61 aquatic (black circles) plant species from alpine, sub-alpine and lowland continent bioclimatic zones of northern Italy. PCA axis 1 and axis 2 together account for 72·1 % of variability in the data set. Significant correlations between trait scores and PCA axes were determined using Spearman's correlation coefficient (ρ), where *** denotes a significant correlation at the P ≤ 0·001 level. Traits are: LA, leaf area; LCC, leaf carbon content; LDW, leaf dry weight; LDMC, leaf dry matter content; LFW, leaf fresh weight; LNC, leaf nitrogen content; SLA, specific leaf area.

Fig. 3.

A comparison of the PCA ordinations of eight of the most frequently represented hydrophyte growth forms (black circles) within the context of terrestrial herbaceous plant trait variation (grey circles). Line drawings are copyright-free material made available by the USDA-NRCS PLANTS Database (http://plants.usda.gov), originally by Britton and Brown (1913).