Hydraulic limitation underlies the dieback of Populus pseudo-simonii trees in water-limited areas of northern China

Abstract

Populus trees play an important role in windbreak construction in northern China, but widespread decline and mortality of Populus windbreaks have occurred in recent years. The phenomenon of die-backs from the top of Populus trees downwards is commonly found in declining windbreaks preceding tree deaths, particularly in water-limited areas. To provide a mechanistic explanation for such top-down diebacks, we analyzed the variation in xylem hydraulics, leaf water relations, and tissue non-structural carbohydrate contents (NSC) along the vertical axes of Populus pseudo-simonii trees, i.e. one of the most commonly used tree species for creating windbreaks in northern China. Consistent changes were observed along the height gradient of the tree crown, i.e. with the height increase branches showed a clear trend of decrease in hydraulic efficiency and leaves showed symptoms of greater drought stress. The stem hydraulic conductivity standardized using whole cross-sectional wood area decreased from 4.66 at the bottom to 1.69 kg m−1 s−1 MPa−1 at the top of the tree crown. Such a change in hydraulic efficiency was accompanied by significant increase in the degree of xylem embolism (i.e. from < 10% at the bottom to about 25% at the top of the tree crown) and an overall reduction in hydraulic safety margin with branch height increase. Lower xylem hydraulic efficiency and symptoms of less favorable water status, however, did not result in reduced contents of tissue NSC in branches of higher positions. Rather, the stem and leaf soluble sugar contents both showed significant positive correlations (P < 0.01, linear regressions) with sampling heights and the starch contents largely remained stable along the height gradient. Our results highlight that greater risk of hydraulic failure rather than carbon depletion is underlying the commonly observed crown dieback in Populus windbreaks.