Population density influences tree responses to environmental stresses, such as drought and high temperature. Prolonged drought negatively affects the health of Mongolian pines in forests planted by the Three-North Shelter Forest Program in North China. To understand the relationship between stand density and drought-induced forest decline, and to generate information regarding the development of future management strategies, we analyzed the vulnerability to drought of planted Mongolian pines at three stand densities. A tree-ring width index for trees from each density was established from tree-ring data covering the period 1988–2018 and was compared for differences in radial growth. Resistance (R_t), recovery (R_c), resilience (R_s), and relative resilience (RR_s) in response to drought events were calculated from the smoothed basal area increment (BAI) curves. The high-density (HDT) group showed a consistently lower tree-ring width than the border trees (BT) and low-density (LDT) groups. The BAI curve of the HDT group started to decrease five years earlier than the LDT and BT groups. Pearson correlation analysis revealed that the radial growth of all of the groups was related to precipitation, relative humidity (RH), potential evapotranspiration (ET₀), and standardized precipitation evapotranspiration index (SPEI) in the previous October and the most recent July, indicating that Mongolian pine trees of different densities had similar growth–climate relationships. Over the three decades, the trees experienced three severe drought events, each causing reduced tree-ring width and BAI. All of the groups showed similar R_c to each drought event, but the HDT group exhibited significantly lower R_t, R_s, and RR_s than the BT group, suggesting that the HDT trees were more vulnerable to repeated drought stress. The RR_s of the HDT group decreased progressively after each drought event and attained <0 after the third event. All of the groups showed similar trends regarding water consumption under varying weather conditions, but the HDT group showed significantly reduced whole-tree hydraulic capability compared with the other two groups. From these results, HDT trees exhibit ecophysiological memory effects from successive droughts, including sap flux dysfunction and higher competition index, which may prevent recovery of pre-drought growth rates. HDT trees may be at greater risk of mortality under future drought disturbance.

Land ecological security (LES) and high-quality economic development (HED) are mutually influential. China has three world-class urban agglomerations; the Beijing–Tianjin–Hebei (BTH) region is one of them. It is an important platform for participating in international competition and cooperation. To promote regional sustainable development, it is critical to study the coupling coordination and interaction mechanism of LES–HED subsystems over time and space in the BTH region. This study involved the construction of an aggregated index system to obtain a comprehensive understanding of the subsystems, and then investigated the coupling coordination degree (CCD) of the LES–HED subsystems from 2007 to 2018 using a CCD model. Additionally, a panel vector autoregressive (PVAR) model was applied to analyze the interactive mechanism of the LES–HED subsystems. Finally, a model of the degree of relative development was used to classify the types of regional development. The results showed that the CCD of the LES–HED subsystems in the BTH region had significant spatial and temporal differences. The spatial distribution could be characterized as low in the central area and high in the peripheral area, and the evolutionary law of CCD in the system was from lower to higher levels over time. In addition, improvements in LES promoted HED, but the impact of HED on LES was limited. The development patterns within the cities of the study area included three development types, including an slightly lagging type of LES, a slightly lagging type of HED, and a significantly lagging type of HED. Given the spatial variability of the coupled and coordinated development of LES–HED subsystems, it is necessary to implement different development strategies. This study can inform decisions promoting the coordinated development of LES–HED subsystems for sustainable regional development.