Abstract

Pistacia atlantica Desf. subspecies atlantica (Atlas pistachio) is the only spontaneous tree species of the North-African steppe. Because individual trees can achieve great size and age, it is locally considered as fossil tree. Individual tree dimensions can exceed 25 m in height and 2 m in trunk circumference despite the very dry conditions of its growth. With the objective of determining the root architectures that optimize the survival in arid soils, root systems of seven Atlas pistachios were excavated and the number of superficial roots, deep roots and branch roots were determined and underlying soil for each tree was analyzed. Results show that young pistachio trees produce an initial orthogeotropic root which subsequently sends out several secondary ramifications. If soil is sandy, a shallow root network is developed to effectively mine soil moisture. On loamy or calcareous soils, extensive deep roots are developed to mine deeper water reserves typically protected against evaporation by upper soil levels. These deep roots also provide the required anchorage for the very large sizes that trees can achieve with age. Root architectures of Atlas pistachio thus differentially develop in response to physicochemical characteristics of underlying soils. Young trees rapidly develop an extensive shallow root which ensures a sufficient supply of water and minerals and subsequently developed a strong taproot to mine the deep ‘reservoir-levels’ sheltered from evaporation and to provide required anchorage.

Key words: Plant-soil interactions, aridity, Pistacia atlantica, drought-adaptation, root-architecture.