D³ database

Terminal Velocity

Summary

 

The TERMINAL VELOCITY is the maximum speed (in m/s) of a falling diaspore in still air.

 

 

Trait description

 

TERMINAL VELOCITY (column code: VTERM) is a continuous trait that describes the maximum speed (in m/s) of a falling diaspore in still air.

Diaspores (as any falling object) accelerate during the free fall and approximate TERMINAL VELOCITY only after a well-defined time period, which depends mainly on its mass and drag coefficient. Original measurements thus generally do NOT refer to the TERMINAL VELOCITY but to an average velocity measured during the initial acceleration phase. The difference between the measured and the 'real' TERMINAL VELOCITY will often be significant: For medium-sized spherical seeds the measured velocity will often be 1.5 to 2 times smaller than the real TERMINAL VELOCITY, for heavy diaspores the difference might be considerably higher, depending mainly on the drop height. TERMINAL VELOCITY in our database is therefore corrected for this initial acceleration wherever possible. The correction factor is derived from a discretised simulation of free fall of the diaspores with drag based on the laws of physics following Schäfer (2002).

If more than one measurement was available, generally only the mean value is presented. Raw data are available on request.

 

 

Data sources

 

The original measurements were mostly made with a custom made terminal velocity-meter, which is a further development of the apparatus presented by Askew et al. (1997). TERMINAL VELOCITY of large diaspores or diaspores with dynamic flight behaviour (e.g. Acer spec.) was measured manually. TERMINAL VELOCITY mostly comprises original measurements of diaspores, which were corrected for the effect of initial acceleration. Some datasets from published literature are also included, if the measurement method was not likely to underestimate terminal velocity significantly. In contrast, larger third party datasets (e.g. from LEDA) are not included here.

 

 

Literature

 

Askew, A.P., Corker, D., Hodkinson, D.J. & Thompson, K. (1997): A new apparatus to measure the rate of fall of seeds. Functional Ecology 11, 121-125.

 

Jongejans, E. & Schippers, P. (1999): Modeling seed dispersal by wind in herbaceous species. Oikos 87, 362-372.

 

Luftensteiner, H.W. (1982): Untersuchungen zur Verbreitungsbiologie von Pflanzengemeinschaften an vier Standorten in Niederösterreich. Bibliotheka Botanica. Schweizerbartsche Buchhandlung, Stuttgart.

 

Schäfer, M. (2002): Beobachtung, Analyse und Beschreibung von realen Fallbewegungen. DLR, Göttingen.

 

Tackenberg, O. (2001): Methoden zur Bewertung gradueller Unterschiede des Ausbreitungspotentials von Pflanzenarten. - Modellierung des Windausbreitungspotentials und regelbasierte Ableitung des Fernausbreitungspotentials. Cramer, Berlin.

 

Thompson, K. (2005): Terminal Velocity. In: Knevel, I.C., Bekker, R.M., Kunzmann, D., Stadler, M., Thompson, K. (eds.): The LEDA traitbase - Collecting and measuring standards of life-history traits of the Northwest European Flora.