Limited evidence for a consistent seed mass‐dispersal trade‐off in wind‐dispersed pines

Abstract

Much of the variation in dispersal potential among seeds of different plant species has been related to a putative trade‐off between dispersal potential and seed mass. Yet to date there are few taxonomically controlled comparisons that explore the generality of this relationship across and within multiple species. We measured the seed mass and dispersal traits across multiple pine species that encompassed a 100‐fold range in seed mass. We expected that for the larger seeded species, the resource investment required in the cone to house seeds and their wings may constrain dispersal potential. We examined the relative roles of samara and cone traits in determining the terminal velocity, and investigated how the relative investment of resources in cone structure related to seed mass and samara dispersal traits. Finally, we examined the partitioning of variation in seed mass and terminal velocity within and among individuals of each species. We found a strong positive relationship between seed mass and samara terminal velocity across all 12 species; however, this seed mass‐dispersal trade‐off was only significant within two of our species. For most species, wing loading was the most important trait determining samara terminal velocity. Across species, increases in cone investment were associated with heavier seeds with longer wings but higher terminal velocities, while within‐species increases in cone investment were associated with decreases in terminal velocity. The majority of the variation in terminal velocity occurred within individual trees, while seed mass variation mostly occurred among trees and may therefore be subject to stronger selection pressure. Synthesis. We have found strong support for a trade‐off between seed mass and dispersal potential across a wide range of wind‐dispersed Pinus species. Importantly, we show that this trade‐off is largely absent within species, potentially reducing selective pressure. This may be largely due to the relationships between cone investment and samara traits, whereby increases in cone investment result in a proportionally greater increase in wing size than seed mass. Where resources are limited, investment in seed mass may be prioritized over dispersal.

Dispersal potential rather than risk assessment scores predict the spread rate of non‐native pines across New Zealand

“…The Z-score integrates seed mass, the length of the juvenile period and the interval between major seed crops (Rejmánek & Richardson, 1996;Rejmánek et al, 2005). The Z-score therefore captures information on fecundity, seed size (correlated with seed terminal velocity across pine species; Wyse & Hulme, 2021) and the likelihood of seedling establishment, and is also strongly correlated with seedling relative growth rate (RGR) of pines (Grotkopp et al, 2002). Relative growth rate was not found to be correlated with the spread rates of naturalised Pinaceae in New Zealand (Bellingham et al, 2004), although this may relate to the age of the individuals measured (11-18 months) which does not encompass the period of maximum seedling growth rate (Grotkopp & Rejmánek, 2007).…”

“…Such an approach could quickly adapt to incorporate new species of interest, once trait data were obtained, and would provide consistent and unbiased results among species. For example, modelled dispersal distances for P. canariensis and P. coulteri utilising terminal velocity data (Wyse & Hulme, 2021) and their maximum heights (Richardson & Rundel, 1998) suggest these species have comparatively low dispersal potential (median dispersal potentials of 71 and 44 m, respectively, from forest trees), which combined with their low frequency of planting results in these species being found in only a few regions in New Zealand (Howell, 2019).…”

“…Seed terminal velocity data (Table 1) for each species were extracted from Wyse and Hulme (2021), who determined the terminal velocities from New Zealand populations of the 10 pine species in still air using the device and methodology described by Wyse et al (2019).…”

“…) according to the updated equation provided byRejmánek et al (2005), using species mean seed mass fromWyse and Hulme (2021) and the juvenile period and interval between large seed crops…”

Seed morphological traits as a tool to quantify variation maintained inex situcollections: a case study inPinus torreyana(Parry)

“…For Torrey pine, increased seed size on the island may have evolved to limit potential seed losses via wind-dispersal, as seed mass negatively correlates with dispersal distance in pines (Greene and Johnson 1993; Debain et al . 2003, but see Wyse and Hulme 2020). Nonetheless, Torrey pine seeds possess degenerated wings (Ledig and Conkle 1983), suggesting that other mechanisms likely contribute to seed dispersal in this species.…”