To uncover the internal mechanisms of various drought stress intensities affecting the soluble sugar content in organs and its regulation by endogenous abscisic acid (ABA), we selected the saplings of Platycladus orientalis, a typical tree species in the Beijing area, as our research subject. We investigated the correlation between tree soluble sugars and endogenous ABA in the organs (comprised of leaf, branch, stem, coarse root, and fine root) under two water treatments. One water treatment was defined as T1, which stopped watering until the potted soil volumetric water content (SWC) reached the wilting coefficient and then rewatered the sapling. The other water treatment, named T2, replenished 95% of the total water loss of one potted sapling every day and irrigated the above-mentioned sapling after its SWC reached the wilt coefficients. The results revealed that (1) the photosynthetic physiological parameters of P. orientalis were significantly reduced (p < 0.05) under fast and slow drought processes. The photosynthetic physiological parameters of P. orientalis in the fast drought–rehydration treatment group recovered faster relative to the slow drought–rehydration treatment group. (2) The fast and slow drought treatments significantly (p < 0.05) increased the ABA and soluble sugar contents in all organs. The roots of the P. orientalis exhibited higher sensitivity in ABA and soluble sugar content to changes in soil moisture dynamics compared to other organs. (3) ABA and soluble sugar content of P. orientalis showed a significant positive correlation (p < 0.05) under fast and slow drought conditions. During the rehydration stage, the two were significantly correlated in the T2 treatment (p < 0.05). In summary, soil drought rhythms significantly affected the photosynthetic parameters, organ ABA, and soluble sugar content of P. orientalis. This study elucidates the adaptive mechanisms of P. orientalis plants to drought and rehydration under the above-mentioned two water drought treatments, offering theoretical insights for selecting and cultivating drought-tolerant tree species.

Spring droughts, increasingly coinciding with canopy shade, interactively stress the growth of urban tree species and are poorly understood in Beijing. Three-year-old saplings of Pinus tabuliformis and Robinia pseudoacacia were subjected to comparative analysis under four drought–shade sequences, with a full-light, well-watered treatment serving as the control. During two periods encompassing the drought to wilting point and subsequent rewatering, we assessed leaf morphology, water status, photosynthetic gas exchange, and chlorophyll fluorescence. Both species exhibited losses in leaf water and carbon assimilation under drought, yet their adaptive strategies substantially differed. P. tabuliformis conserved water through the stable leaf anatomy and conservative stomatal control. In particular, P. tabuliformis under full-light and drought conditions decreased their specific leaf area (SLA) by 23%, as well as showing reductions in stomatal conductance (G_s) and transpiration rate (T_r) along with the drought duration (p < 0.01). As the duration of post-drought rewatering increased, the reductions in the net photosynthetic rates (P_n) of P. tabulaeformis showed that the shade condition intensified its photosynthetic limitation and slowed recovery after drought. Under low-light drought, R. pseudoacacia exhibited a 52% increase in SLA and a 77% decline in G_s; the latter was markedly smaller than the reduction observed under full-light drought. After rewatering, G_s displayed an overcompensation response. The rise in specific leaf area and the greater flexibility of stomatal regulation partly offset the adverse effects of drought. Nevertheless, post-drought P_n recovered to only 40%, significantly lower than the 61% recovery under full-light drought. Moreover, the negative correlation between SLA and P_n became significantly stronger, indicating that the “after-effects” of shade–drought hindered photosynthetic recovery once the stress was relieved. Drought duration eroded the phenotypic performance in both species, while the light environment during drought and subsequent rehydration determined the time trajectory and completeness of recovery. These results validate a trade-off between shade mitigation and drought legacy, and guide species selection: plant shade-tolerant R. pseudoacacia in light-limited urban pockets and reserve sun-dependent P. tabuliformis for open, high-light sites to enhance drought resilience of Beijing’s urban forests.

To explore whether there were differences among the patterns of response of grasslands with different levels of degradation to extreme drought events and nitrogen addition, three grasslands along a degradation gradient (extremely, moderately, and lightly degraded) were selected in the Bashang area of northern China using the human disturbance index (HDI). A field experiment with simulated extreme spring drought, nitrogen addition, and their interaction was conducted during the growing seasons of 2020 and 2021. The soil moisture, aboveground biomass, and composition of the plant community were measured. The primary results were as follows. (1) Drought treatment caused soil drought stress, with moderately degraded grassland being the most affected, which resulted in an 80% decrease in soil moisture and a 78% decrease in aboveground biomass. The addition of nitrogen did not mitigate the impact of drought. Moreover, the aboveground net primary production (ANPP) in 2021 was less sensitive to spring drought than in 2020. (2) The community composition changed after 2 years of drought treatment, particularly for the moderately degraded grasslands with annual forbs, such as Salsola collina, increasing significantly in biomass proportion, which led to a trend of exacerbated degradation (higher HDI). This degradation trend decreased under the addition of nitrogen. (3) The variation in drought sensitivities of the ANPP was primarily determined by the proportion of plants based on the classification of degradation indicators in the community, with higher proportions of intermediate degradation indicator species exhibiting more sensitivity to spring drought. These findings can help to provide scientific evidence for the governance and restoration of regional degraded grassland under frequent extreme weather conditions.