Architecture and growth patterns of Zelkova sicula (Ulmaceae) in South-East Sicily as a response to environmental conditions

Zelkova sicula Di Pasquale, Garfì et Quézel is a relic of the European Tertiary flora, presently found in sub-optimal environmental conditions. In order to elucidate the processes underlying the adaptive ability and the persistence of this species within the current habitat, architecture and growth patterns were investigated through observations and records of growth features. Stem analysis on 13 samples was performed to define height growth dynamics and growth form. Branching structure was also studied, based on some quantitative parameters of the first-order axes (e.g.: total number, age, length, etc.). Results suggest that the architectural model of Z. sicula can be referred to Troll's model. The great reiterative ability, which is a typical feature of this model, allows Zelkova to rapidly repair crown damage or stem decline following traumatic events such as severe water stress. Analysis of the height growth curve showed that under the present-day environmental conditions Z. sicula exhibits the growth form of a small shrub. This habit most likely reflects phenotypic plasticity resulting from an adaptive strategy to a high-stress environment, as it is often found in plants living out of optimal range limits. Anomalies in the morphometric structure of the crown were also observed. This probably depends on the repeated release of reiterative shoots following the loss of apical meristems, in response to even moderate water stress or persistent browsing disturbances. The different reiterative responses of Z. sicula surely reveal a notable adjustment ability, which has a basic significance in the point of view of its conservation in the present habitat. On the other hand, frequent recurrence of reiteration implies high energetic costs, negatively affecting global growth and/or phenological phases such as flowering.