Evolution of clonal growth forms in angiosperms

Clonal growth of plants is attained by a number of morphologically different organs (e.g. stolons, rhizomes, and roots), which are not functionally equivalent. Consequently, these clonal growth organ (CGO) types can determine functional traits that are associated with clonality, although little is known about their evolutionary flexibility or the constraining role they play on clonal traits. We investigated the rates of evolutionary change by which individual CGOs are acquired and lost using a set of 2652 species of Central European flora. Furthermore, we asked how these individual CGOs constrain functionally relevant clonal traits, such as lateral spread, number of offspring, and persistence of connections. We show that plants can easily switch in evolution among individual types of CGO and between clonal and nonclonal habits. However, not all these transitions are equally probable. Namely, stem-based clonal growth and root-based clonal growth constitute evolutionarily separate forms of clonal growth. Clonal traits are strongly constrained by individual CGO types. Specifically, fast lateral spread is attained by stolons or hypogeogenous rhizomes, and persistent connections are attained by all rhizome types. However, the ease with which clonal organs appear and disappear in evolution implies that plants can overcome these constraints by adjusting their morphologies.

Invasive alien plants benefit more from clonal integration in heterogeneous environments than natives

What confers invasive alien plants a competitive advantage over native plants remains open to debate. Many of the world's worst invasive alien plants are clonal and able to share resources within clones (clonal integration), particularly in heterogeneous environments. Here, we tested the hypothesis that clonal integration benefits invasive clonal plants more than natives and thus confers invasives a competitive advantage. We selected five congeneric and naturally co-occurring pairs of invasive alien and native clonal plants in China, and grew pairs of connected and disconnected ramets under heterogeneous light, soil nutrient and water conditions that are commonly encountered by alien plants during their invasion into new areas. Clonal integration increased biomass of all plants in all three heterogeneous resource environments. However, invasive plants benefited more from clonal integration than natives. Consequently, invasive plants produced more biomass than natives. Our results indicate that clonal integration may confer invasive alien clonal plants a competitive advantage over natives. Therefore, differences in the ability of clonal integration could potentially explain, at least partly, the invasion success of alien clonal plants in areas where resources are heterogeneously distributed.

Differences in below-ground bud bank density and composition along a climatic gradient in the temperate steppe of northern China

BACKGROUND AND AIMS

Understanding the changes in below-ground bud bank density and composition along a climatic gradient is essential for the exploration of species distribution pattern and vegetation composition in response to climatic changes. Nevertheless, investigations on bud banks along climatic gradients are still scarce. The below-ground bud bank is expected to be reduced in size in arid conditions, and costly, bud-bearing organs with long spacers would be replaced by more compact forms with buds that are better protected than those found in moist conditions.

METHODS

How total bud density and composition (different bud bank types) change with aridity (calculated value 0·43-0·91), mean annual precipitation (MAP; 93-420 mm) and mean annual temperature (MAT; -1·51 to 6·93 °C) was tested at 21 sites along a 2500-km climatic gradient in the temperate steppe of northern China.

CONCLUSIONS

Belowground bud bank density decreases towards the dry, hot end of the climatic gradient. Based on the distribution of bud types along the climatic gradient, bulb buds and tiller buds of tussock grasses seem to be more resistant to environmental stress than rhizome buds. The dominance of annual species and smaller bud banks in arid region implies that plant reproductive strategies and vegetation composition will be shifted in scenarios of increased drought under future climate change.

Polyploid species rely on vegetative reproduction more than diploids: a re-examination of the old hypothesis

Background and Aims

Polyploidy is arguably the single most important genetic mechanism in plant speciation and diversification. It has been repeatedly suggested that polyploids show higher vegetative reproduction than diploids (to by-pass low fertility after the polyploidization), but there are no rigorous tests of it.

Methods

Data were analysed by phylogenetic regressions of clonal growth parameters, and vegetative reproduction in culture on the ploidy status of a large set of species (approx. 900) from the Central European Angiosperm flora. Further, correlated evolution of ploidy and clonal traits was examined to determine whether or not polyploidy precedes vegetative reproduction.

Key Results

The analyses showed that polyploidy is strongly associated with vegetative reproduction, whereas diploids rely more on seed reproduction. The rate of polyploid speciation is strongly enhanced by the existence of vegetative reproduction (namely extensive lateral spread), whereas the converse is not true.

Conclusions

These findings confirm the old hypothesis that polyploids can rely on vegetative reproduction which thus may save many incipient polyploids from extinction. A closer analysis also shows that the sequence of events begins with development of vegetative reproduction, which is then followed by polyploidy. Vegetative reproduction is thus likely to play an important role in polyploid speciation.