Field observations confirm that Australian alpine grassland species have limited long-distance seed dispersal capacity
By John W. Morgan, Lauren Szmalko, Iris Hickman, and Susanna E. Venn
Many estimates of seed dispersal use simple plant traits, such as dispersal mode. Yet, including hard traits, such as terminal velocity, allows for interpretation of the importance of updraft on dispersal. We improve current models of alpine grassland species dispersal in Australia and quantify dispersal distances in situ.
We build on published dispersal models of dispersal distance in alpine grassland species by including terminal velocity into models. Seed traps were deployed in the field over two dispersal seasons to quantify diaspore distance movements. Traps captured near-surface seed dispersal (45 cm above ground) and wind-column seed dispersal (150 cm above ground). Vertical wind speed was measured at trap sites, and the probability of dispersal by updraft was estimated.
Modelled diaspore dispersal suggests many alpine species have short-distance dispersal (<40 m), which was confirmed by field measurements. Diaspore trapping detected 31-33% of the total number of species flowering within 15 m of traps. The majority ofMost diaspores (64-77% across years) were trapped at 45 cm above the ground. Many species (64% of species across years) were captured at 1.5 m above the ground, including species with low terminal velocity (<1 m/s) that experience the greatest proportion of wind events that exceed their terminal velocity thresholds.
Alpine diaspore dispersal (modelled and observed) was short-distance for many species, constrained by the low release heights from adult plants and high terminal velocity. Two-thirds of seeding plants were not seen in seed traps across two survey years. Hence, the capacity of alpine species to track rapid climate change via dispersal over short timescales is potentially limited. However, seed trap results found several species capable of reaching height when dispersing, likely due to wind updraft events at speeds above their terminal velocity. The updraft of diaspores permits long(er) distance dispersal for species with low terminal velocity.