Testing a mechanistic dispersal model against a dispersal experiment with a wind‐dispersed moss

Abstract

Wind is the main dispersal agent for a wide array of species and for these species the environmental conditions under which diaspores are released can potentially modify the dispersal kernel substantially. Little is known about how bryophytes regulate spore release, but conditions affecting peristome movements and vibration of the seta may be important. We modelled airborne spore dispersal of the bryophyte species Discelium nudum (spore diameter 25 mm), in four different release scenarios, using a Lagrangian stochastic dispersion model and meteorological data. We tested the model predictions against experimental data on colonization success at five distances (5, 10, 30, 50 and 100 m) and eight directions from a translocated point source during seven two-day periods. The model predictions were generally successful in describing the observed colonization patterns, especially beyond 10 m. In the laboratory we established spore release thresholds; horizontal wind speed sd  0.25 m s 1 induced the seta to vibrate and in relative humidity  75% the peristome was open. Our dispersal model predicts that the proportion of spores dispersing beyond 100 m is almost twice as large if the spores are released under turbulent conditions than under more stable conditions. However, including release thresholds improved the fit of the model to the colonization data only minimally, with roughly the same amount of variation explained by the most constrained scenario (assuming both vibration of the seta and an open peristome) and the scenario assuming random release. Model predictions under realised experimental conditions suggest that we had a low statistical power to rank the release scenarios due to the lack of measurements of the absolute rate of spore release. Our results hint at the importance of release conditions, but also highlight the challenges in dispersal experiments intended for validating mechanistic dispersal models.

Air humidity thresholds trigger active moss spore release to extend dispersal in space and time

“…To illustrate when spores are most likely to be released in nature, we use data from another study on moss dispersal (Lönnell et al . ). In that study, relative air humidity and wind speed were recorded at 20 cm above the ground during 15 days in the beginning of May 2011, in a relatively open habitat with scattered trees.…”

“…; Lönnell et al . ). We randomly chose one 10‐min period from each of the 15 days when the RH was in the same range as when the peristome started to open (RH; 75–89%) and the period having the highest wind speed for each day (i.e.…”

“…This maximum distance was chosen because (i) our model was parameterized based on the turbulence statistics in the ASL and therefore was not capable to predict the long‐distance dispersal depending on the development of the ABL state and larger scale meteorology; and (ii) we know that the model performs well up to 100 m, since model predictions have been validated by empirically observed colonization patterns of another ground living moss to that distance (Lönnell et al . ). The model is more thoroughly described in Appendix S3, and all the details on the modelling approach can be found in Lönnell et al .…”

“…The model is more thoroughly described in Appendix S3, and all the details on the modelling approach can be found in Lönnell et al . ().…”

“…Even if the density of deposited spores is highest within a few tens of metres from the source (Lönnell et al . ), several studies suggest that moss spores may be able to travel tens (Hutsemékers, Dopagne & Vanderpoorten ; Lönnell et al . ) or even hundreds of kilometres (Sundberg ).…”