Proteas as ‘tumbleseeds’: Wind dispersal through the air and over soil

Abstract

The fruits of the genus Protea have long stiff trichomes forming a pappus-like structure. In serotinous species the achenes (seeds) are released only after fire when vegetative barriers to dispersal are minimized. The dispersal of seed by free-fall from the cones to the ground (phase I) was compared with subsequent dispersal by rolling over the substrate (phase II). Seed shadows observed in the field as well as seed release under controlled conditions suggest that phase I dispersal is seldom more than 30 m. However phase II dispersal distances measured both from seed shadows and controlled-release experiments were much greater, frequently exceeding 50 m with a maximum over 500 m. The most important biological determinant of both phase I and phase II dispersal was the size of the tuft of hairs and wing loading of the seed. Substrate roughness was a major physical determinant of phase II dispersal distance. In rocky areas, phase II dispersal can be effectively discounted. In smooth areas, previous estimates of Protea migration rates may be an order of magnitude too low. Other Cape Proteaceae with hairy seeds and serotinous cones occur in Aulax and a few species of Leucadendron. This convergence suggests that long-distance phase II transport may have adaptive value. Die sade van die genus Protea is bedek met lang trigome (hare). Die saad van saadhoudende spesies word eers na vuur vrygestel en versprei wanneer die plantaardige versperrings minimaal is. Die verspreiding van die saad direk uit die keels grondwaarts (fase I), is met die daaropvolgende verspreiding, die rolbewegings oor die grond (fase II), vergelyk. Die verspreiding van saad om die moederplant (saadskaduwee) soos in die veld waargeneem en saad-vrystelling onder beheerde toestande dui daarop dat fase I verspreiding selde meer as 30 m is. Fase II verspreidingsafstande soos gemeet deur beide die saadskaduwee en beheerde saadvrystellingseksperimente was egter baie verder en dikwels meer as 50 m met ’n maksimum van meer as 500 m. Die mees bepalende biologiese faktore in beide fase I en fase II verspreiding was die lengte van die trigome (hare) en die gevolglike vlerklading van die saad. Vir die fase II verspreidingsafstande was die bepalende fisiese faktor die ruheid van die substraat. In rotsagtige gebiede kan fase II verspreiding geignoreer word. Dit blyk dat in areas met gladde oppervlaktes, Protea-migrasie-afstande vantevore onderskat is. Verdere voordele van spesies met saadhoudende keels in die Kaapse Proteaceae word gevind in die genera Aulax en Leucadendron. Hierdie samelopende ontwikkeling dui daarop dat langafstand-fase II verspreiding aanpasbaarneidswaarde mag hê.