An old tree and its many‐shaped leaves

Abstract

The desert poplar (Populus euphratica) is an ancient and protected species, and forms riparian forests in the deserts of mid- and west Asia, northern Africa, and southern Europe. More than half of all desert poplar forests are found along the Tarim River, within the Taklamakan, a desert in northwest China. The Taklamakan is in the rain shadow of the Himalayas and is the world's second largest shifting sand desert. Although forest extent has been greatly reduced, mostly due to flow diversion of the river for agriculture, many old trees persist. This tree, photographed in November 2013, is the oldest recorded living desert poplar. The innermost ring dates back to 1709 or earlier. Its leaves vary widely in shape; some smooth-edged, others serrated, some narrow, others broad, some shaped like lance heads while others are oval. Drought is a dominant stressor in this hyperarid environment, with highly variable temperatures and soil salinity. Leaf shape may relate to trade-offs among water conservation, thermoregulation, and growth rate. Are these large variations in leaf shape on a single tree an adaptation to wide environmental fluctuations? Or is leaf polymorphism a neutral and therefore non-adaptive consequence of variable gene expressions related to developmental stages? Answers to these questions could enrich the ecological and evolutionary understanding of trees and their responses to stressors.