Importance of whole-plant biomass allocation and reproductive timing to habitat differentiation across the North American sunflowers

Abstract

Background and aims: Trait-based plant ecology attempts to use small numbers of functional traits to predict plant ecological strategies. However, a major gap exists between our understanding of organ-level ecophysiological traits and our understanding of whole-plant fitness and environmental adaptation. In this gap lie whole-plant organizational traits, including those that describe how plant biomass is allocated among organs and the timing of plant reproduction. This study explores the role of whole-plant organizational traits in adaptation to diverse environments in the context of life history, growth form and leaf economic strategy in a well-studied herbaceous system.

Methods: A phylogenetic comparative approach was used in conjunction with common garden phenotyping to assess the evolution of biomass allocation and reproductive timing across 83 populations of 27 species of the diverse genus Helianthus (the sunflowers).

Key results: Broad diversity exists among species in both relative biomass allocation and reproductive timing. Early reproduction is strongly associated with resource-acquisitive leaf economic strategy, while biomass allocation is less integrated with either reproductive timing or leaf economics. Both biomass allocation and reproductive timing are strongly related to source site environmental characteristics, including length of the growing season, temperature, precipitation and soil fertility.

Conclusions: Herbaceous taxa can adapt to diverse environments in many ways, including modulation of phenology, plant architecture and organ-level ecophysiology. Although leaf economic strategy captures one key aspect of plant physiology, on their own leaf traits are not particularly predictive of ecological strategies in Helianthus outside of the context of growth form, life history and whole-plant organization. These results highlight the importance of including data on whole-plant organization alongside organ-level ecophysiological traits when attempting to bridge the gap between functional traits and plant fitness and environmental adaptation.

Keywords: Biomass; Helianthus; bud; climate; daylength; flower; growing season; leaf; life history; root; soil fertility; stem.

Fig. 1.

Variation in biomass allocation and total plant size across Helianthus. (A) Species mean total plant biomass at harvest. Error bars reflect standard errors of species means calculated from population means. (B) Relative biomass allocation among plant organs represented as species mean below-ground, stem, leaf and reproductive mass fractions. Note that infraspecific epithets are omitted for space.

Fig. 2.

Differences in plant traits among species based on growth form and life history using phylogenetic ANOVA (Garland et al., 1993) in the R package ‘phytools’ (Revell, 2012). Bars that do not share letters are significantly different by the Holm post-hoc test.

Fig. 3.

Variation in reproductive timing across Helianthus. (A) Population mean daylength at first flower in Athens, Georgia common gardens (black square) plotted on population source sites across North America. (B) Species mean daylength at first bud and (C) first flower in Athens, Georgia common gardens.