Relationship between stem diameter and whole-tree transpiration across young, mature and old-growth ponderosa pine forests under wet and dry soil conditions

Previous studies have reported cross-site relationships between stem diameter at breast height (DBH) and whole-tree transpiration (Q(t)) across a variety of species and locations. These relationships enable us to estimate canopy transpiration rates using DBH data from forest inventory networks worldwide. However, transpiration responses to environmental conditions such as soil moisture often depend on tree species and age, suggesting that the DBH-Q(t) relationship varies with soil moisture conditions. In the present study, we collated Q(t) of 41 ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) trees in young (23-year-old), mature (90-year-old) and old-growth (250-year-old) forests using the sap flow technique. Sap flux density considering the radial decline (u) was three times higher in young trees than mature and old-growth trees under wet soil conditions. The largest declines in u were observed in young trees and then mature trees when soil moisture condition changed from wet to dry. Decreases in u were smaller for old-growth trees than for young and mature trees. The DBH-Q(t) relationship differed with soil moisture conditions, showing a strong linear correlation under wet condition across all four age groups but a non-linear correlation under dry condition. u was overestimated by up to 50%, particularly in the young trees when neglecting differences in the response of u to soil water deficit among different ages. These results suggest that differences in u and its responses to soil water deficit among stand ages should be considered when applying the DBH-Q(t) relationship for other tree species.