The advantage of low leafing intensity under water limitation: An illustrative study of Populus euphratica in hyper-arid climates

The trade-off between leaf size and leafing intensity (i.e., the number of leaves per unit stem size) is a key axis of trait covariation across the diversity of plant foliage deployment. However, the functional significance of leafing intensity and its possible combinations with leaf size in dealing with water limitation remains unclear. Using <i>Populus euphratica</i> as an illustrative tree species growing in hyper-arid climates, we investigated how leaf size and leafing intensity co-varied under varying water stresses. In the Ebinor lowlands and the upper reaches of the Tarim River (NW China), we sampled > 1800 current-year twigs from 505 trees across 14 sites along a climatic gradient characterized by precipitation, potential evapotranspiration and vapor pressure deficit. Leafing intensity based on stem mass (LIM) decreased with climatic aridity, primarily due to greater stem mass, but not fewer leaves. This indicates a higher investment in structural support for leaf attachment under water stress. Both leaf area and mass decreased with LIM at a lower-than-proportional rate, with the decrease in leaf size being more pronounced under drier climates. This suggests that higher LIM incurs a high cost of reducing leaf size in water-limited habitats. These findings challenge the assumption that higher leafing intensity always confers an advantage ready for environmental stresses due to higher developmental flexibility offered by more axillary buds. Rather, we propose that a strategy of lower leafing intensity, with greater structural support for leaf attachment and less compromise in leaf size, can be advantageous under water limitation.