Ecosystems Growth & Development

Are a few, large seeds or many, small seeds a better strategy for a plant?

For the first time, estimates of comparative seed yield and number for over 800 annuals and their predictor equations have been produced and the ecological importance of these regenerative traits has been illustrated.

There is a three-way trade-off in adaptive strategies for plants, explained by the CSR (competitor, stress tolerant, ruderal) triangle theory. The triangle represents different trait combinations arising under different levels of competition, abiotic limitation to growth (stress) or periodic biomass destruction (disturbance). Some plants might be better competitors than tolerating disturbance and have different traits.

Hodgson and colleagues from the UK, Spain, Slovakia, Morocco, and Mexico analysed 19 plant traits and CSR strategy of over 800 annual plants in Europe. The researchers found that seed mass yield per plant increased with plant size and competitive ability, and was largely independent of seed size. Understanding whether plants partition their seed yield into few large seeds, or many small ones can help conservationists predict which species can establish under different conditions.

The CSR strategy triangle and the position of the highly competitive angelica (Angelica archangelica), which tolerates mild abiotic stress but does not tolerate disturbance and heather (Calluna vulgaris), which is not competitive but tolerates high disturbance. Sources Robert Flogaus-Faust/WikimediaCommons and Peter O’Connor/Flickr.

The researchers used data from vegetation surveys in England, Tortosa (Spain) and Zaragoza (Spain) which constituted a gradient from a cool wet (‘Atlantic’) to a ‘Mediterranean’ semi-arid climate. Nineteen plant traits were estimated based on multiple plant trait datasets relating to plant canopy, leaves, and seed mass for over 868 annual plant species.

The researchers investigated if the seed yield (seed mass and seed number) per plant and seed yield per m2 could be predicted based on how a plant grows (e.g. plant height, leaf size, canopy size) from previous datasets from the UK and USA. They also explored which plant species were increasing and declining in abundance in the UK.

The scientists carried out a Principal Component Analysis for examining the CSR strategy based on four functional traits (leaf dry matter content, leaf width, leaf mass per node and canopy size). Contours were added to visualise seed mass per plant and leaf nitrogen content to locate putative positions of the CSR strategy types.

The Principal Component Analysis (PCA) of 868 annual plant species. The dots represent plant species, the highlighted text suggest the CSR strategy (e.g. Ann_S represents a cluster of species which are stress tolerant), the blue contour represents seed mass per plant, the red contour represents leaf nitrogen content whilst the yellow dotted lines represent the proportion of ruderal species per vegetative sample. Source Hodgson et al. 2020

Overall, there was a greater than 8-, 250-, 1000- and 10000-fold variation in leaf structure, plant size, leaf size and seed mass amongst 886 annual plant species. Habitat range and even life history differed markedly between Spain and UK for some species.

Many species which were previously identified as R-strategists were classified as of intermediate strategy (e.g. Poa annua, Senecio vulgaris) according to the PCA. Statistical analysis identified strategy as the more important determinant of seed size than local habitat.

Greater plant and leaf size correlated with greater seed mass and seed number per plant. Seed mass could be ordered as R < S < C based on strategies and as Central England < North-east Spain < North central Spain based on location.

Species around urban landscapes and with wasteland and spoil habitats with C and CR strategy increased in abundance which also tended to produce more seeds but at the same weight. In comparison, plants with more “stressed” strategies have declined on pastural (upland) landscapes, wetland and arable habitats.

“[O]n the basis of data from our study area, we predict that, there is likely to be selection for larger seeds and potentially increased seedling survivorship in populations of rare species confined to small isolated patches of habitat. In contrast, the production of a larger number of smaller widely-dispersed seeds may facilitate the spread of increasing species,” Hodgson and colleagues wrote.

Some predicted changes in trait by annuals in response to changing land use in the United Kingdom. Source Hodgson et al. 2020

“[T]rait‑based ecology is some way from this synthesis but we are making significant progress. Importantly, our new regenerative dimension to trait-based plant ecology is already addressing key but poorly understood ecological processes, the establishment of ‘winners’ by seed, and the associated extinction of ‘losers’, during episodes of vegetation change. We invite others to explore the potential of the appended database, to improve the methodology and to take ‘regenerative functional plant ecology’ to the next level.”