Plant life-form and germination in a Mexican inter-tropical desert: effects of soil water potential and temperature

We investigated the effects of soil water potential (SWP)and temperature on seed germination of six coexisting species of an inter-tropical desert. These species include three life-forms; the shrubs Cercidium praecox and Prosopis laevigata; the columnar succulents Neobuxbaumia tetetzo and Pachycereus hollianus; and the arborescent semi-succulents Beaucarnea gracilis and Yucca periculosa. In the six species germinability increased and germination time and speed of germination (t50)decreased as SWP decreased. The SWP treatments were 0 MPa,!0)12 MPa,!0)2 MPa,!0)41 MPa and!0)66 MPa. There was, however, a SWP threshold below which germination time and t50 increased and germinability decreased. The shrubs had the highest germinability whereas the columnar succulents had the lowest. The shrubs also had shorter germination time and t50 than arborescent semi-succulents whereas seeds of the columnar succulents were indeterminate. In all species except P. laevigata germinability increased and the germination time and t50 decreased as temperature increased. The temperature treatments were 123C, 203C and 263C. The shrubs had the shortest t50 and germination time and the highest germinability at all temperatures. Arborescent semi-succulents had the lowest germinability and longest germination time and t50 at the three temperatures treatments. Our results support the hypothesis that in desert environments different plant life-forms utilise different germination strategies to persist.