Drought effects on tree growth, water use efficiency, vulnerability and canopy health of Quercus variabilis-Robinia pseudoacacia mixed plantation.

Drought-induced forest canopy die-back and tree mortality have been commonly recorded in the lithoid mountainous regions of northern China. However, the capacity of trees to regulate their carbon and water balance in response to drought remains inadequately understood. We measured tree growth, intrinsic water use efficiency (iWUE), vulnerability, and canopy health during drought events using dendrochronology, C isotope measurements, and a tree canopy health survey in a mixed plantation of Quercus variabilis and Robinia pseudoacacia. Resistance (Rt), recovery (Rc), resilience (Rs), and increased amplitude in iWUE compared to the indices 3 years before drought (iWUEr) were calculated for each species across the dominant tree (D), co-dominant tree (CD), and suppressed tree (S). Our results revealed that D and CD showed lower Rt, higher Rc, and higher iWUEr than S. After exposure to multiple sequential drought events, Q. variabilis showed an increasing trend in Rt, and R. pseudoacacia showed a decreasing trend in Rc. R. pseudoacacia exhibited a more conservative strategy towards drought, resulting in a negative SRt-iWUEr (slope of the linear model fitted to capture the trend between Rt and iWUEr) during drought events than Q. variabilis. For individual trees, lower Rc or positive SRt-iWUEr Q. variabilis and negative SRt-iWUEr R. pseudoacacia were more susceptible to canopy die-back. In conclusion, our study offers a new perspective for improved management practices in the design of silvicultural actions for forestry plantations in lithoid mountainous areas with increasing drought risk.

[Growth response to climatic factors and drought events in Quercus variabilis trees of different diameter classes at south aspect of Taihang Mountains, China]. / å¤ªè¡Œå±±å—éº“ä¸åŒå¾„çº§æ “çš®æ Žç”Ÿé•¿å¯¹æ°”å€™è¦ç´ åŠå¹²æ—±äº‹ä»¶çš„å“åº”.

To explore the effects of diameter class on the climate-growth relationship of trees, the width chronologies of ringwood, earlywood, and latewood of Quercus variabilis were established in the low altitude area of the south aspect of the Taihang Mountains. We estimated the sensitivity of radial growth of two diameter classes of Q. variabilis plantation to climate. The effects of drought events on Q. variabilis plantation was quantified by superimposed time analysis. Under climate change, we aimed to provide reference data for sustainable management of Q. variabilis. Our results showed that the mean sensitivity of ringwood and latewood of large diameter classes were higher than small diameter classes. The mean sensitivities of earlywood of small diameter classes were higher than large diameter classes. The latewood was the most sensitive component of tree-ring. The responses of the standard chronological indices of two diameter classes to climate factors were similar. The growth of ringwood and latewood of small diameter classes were more sensitive to the climatic factors from June to August. The earlywood was more sensitive to the climatic factors before the growing season (from January to February). Furthermore, the earlywood of large diameter classes was more sensitive to the climatic factors of the last year, with a stronger hysteresis effect. It suggested that Q. variabilis was sensitive to drought. The radial growth of Q. variabilis of ringwood and latewood in drought year was significantly lower than that of last year. The reduction of Q. variabilis with large diameters was higher than that of small diameters, but with higher growth recovery degree after drought.