Pinus yunnanensis is an endemic tree species in southwest China that has high ecological and economic benefits. Nowadays, global climate change is remarkable, the frequency of drought is increasing day by day, the distribution of rainfall is unbalanced, and even the phenomenon of alternating drought and flood has appeared, which is unfavorable to the growth of P. yunnanensis. We set up four treatments, namely normal water (CK), light drought (LD), moderate drought (MD), and severe drought (SD), and water content was controlled by the weighing method. After continuous drought for 30 days, re-watering was performed for 7 days. The stoichiometric characteristics of non-structural carbohydrates (NSC), soluble sugars (SS), and starch (ST), as well as carbon (C), nitrogen (N), and phosphorus (P), in various organs of P. yunnanensis seedlings were measured. The results revealed significant effects of re-watering on NSC and its components in P. yunnanensis seedlings. The SS and NSC contents in the leaves of P. yunnanensis seedlings treated with SD were significantly higher than those of the control. The C content in the leaves and stems of P. yunnanensis seedlings recovered to the CK level after re-watering under different drought degrees. The contents of N in different organs and P in the fine roots of P. yunnanensis seedlings increased after re-watering with the LD, MD, and SD treatments, while the C/N ratio decreased. In summary, the recovery mechanism of P. yunnanensis seedlings to re-watering varied with the drought degree. The contents and ratios of NSC, C, N, and P in different organs of P. yunnanensis seedlings were significantly affected by re-watering. Combining the phenotypic plasticity index and PCA results, seedlings of P. yunnanensis adapted to drought re-watering by adjusting leaf NSC, leaf P, stem SS/ST, fine root ST, and fine root NSC.

The response of non-structural carbohydrates and stoichiometric characteristics of Ochroma lagopus to nitrogen addition is currently unclear. In this study, a 2-year-old O. lagopus was selected, and seven nitrogen addition treatments were set up to investigate the effects of nitrogen addition on the non-structural carbohydrates and stoichiometric characteristics. O. lagopus tree height and diameter increased by 2.00%–14.00% and 3.74%–16.93%, respectively. Nitrogen addition significantly increased leaf soluble sugar, starch, and NSC (non-structural carbohydrates) contents of O. lagopus forests. Their changing trends showed a first increasing and then decreasing trend with increasing fertiliser application in both urea and slow-release fertilisation treatments. Nitrogen addition significantly increased soil N content, leaf N content, and leaf N/P in O. lagopus forests, all of which increased with the increase in urea and slow-release fertiliser application; while leaf P content in O. lagopus forests was significantly lower in nitrogen addition treatments compared with CK (no fertiliser treatment), and decreased with the increase in urea and slow-release fertiliser application. O. lagopus leaf NSC had highly significant positive correlations with soil N, leaf N content, and N/P, and significant negative correlations with leaf P content and C/N. As a result, the application of both urea and slow-release fertiliser increased the soil N content, improved N uptake and utilisation, and promoted the growth of O. lagopus, with the application of slow-release fertiliser at 450 g/plant being the best treatment, and the slow-release fertiliser being superior to urea.

With global warming, the frequency and duration of drought is becoming longer and longer, which seriously affects the survival of trees. Light intensity control, such as shading, is an important measure in seedling nurseries. However, it is unclear whether early shading affects the drought tolerance of seedlings used in afforestation. We conducted a two-stage experiment on Pinus yunnanensis seedlings. First, three different shading treatments were set, namely HL (0% shading), ML (55% shading), and LL (80% shading). After 90 days of cultivation, the seedlings of each shading treatment were subjected to CK (water content of 90% ± 5%), LD (water content of 75% ± 5%), MD (water content of 60% ± 5%) and SD (water content of 45% ± 5%) continuous drought for 30 days. The contents of non-structural carbohydrates (NSCs), carbon (C), nitrogen (N), and phosphorus (P) and their ratios in the needles of P. yunnanensis seedlings were measured. Early shading affected the starch accumulation and the balance between C absorption and consumption in P. yunnanensis seedlings during drought. Early shading affected C consumption, P utilization efficiency, and N restriction under drought stress. The phenotypic plasticity index showed that the plasticity of P. yunnanensis seedlings under drought stress followed the order: LL > HL > ML. The results of principal component analysis showed that the performance under drought stress followed the order HL > LL > ML. These results indicated that early shading could affect the response of P. yunnanensis seedlings to drought. The P. yunnanensis seedlings grown under HL and LL were more resistant to drought stress than those grown under ML. It is suggested that 0% or 80% shading should be applied at seedling stage to improve the drought resistance of P. yunnanensis.

The characteristics of C:N:P stoichiometry, nonstructural carbohydrate (NSC) content, and C stable isotopes and their relationships affect plant responses to environmental changes and are critical to understanding the ecosystem carbon and water cycles. We investigated the water use strategies and physiological changes of two pioneer tree species (Pinus armandii and Pinus yunnanensis) in response to seasonal drought in subtropical China. The seasonal variation in needle δ¹³C values, C:N:P stoichiometry, and NSC contents of the two tree species were studied in 25-year-old plantation in central Yunnan Province. The needle δ¹³C values of both species were highest in summer. Soluble sugars, starch and NSC content of the two tree species decreased from spring to winter, while there was no significant difference in the seasonal variation of soluble sugars/starch in P. armandii needles, the maximum soluble sugars/starch in P. yunnanensis needles was in autumn. In addition, the C, N, and P contents of the needles and the C:N and C:P ratios of the two species showed different seasonal fluctuations, whereas the N:P ratio decreased with the season. The C:N:P stoichiometry and NSC content of the needles showed significant correlations, whereas the needle δ¹³C was weakly correlated with C:N:P stoichiometry and NSC content. Phenotypic plasticity analysis and principal component analysis revealed that the needle nutrient characteristics (NSC and P contents and N:P ratio) and needle δ¹³C values were critical indicators of physiological adaptation strategies of P. armandii and P. yunnanensis for coping with seasonal variation. These results increase our understanding of the water-use characteristics of the two pioneer tree species and the dynamic balance between the NSC, C, N, and P contents of the needles.

Ochroma lagopus Swartz is a rapidly growing plant known for its lightweight wood; it is widely utilized for timber production and ecological restoration. We investigated the effects of different numbers of drought–rehydration cycles on O. lagopus seedlings cultivated at the Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences. The experiment comprised three treatments: normal watering (CK, 80–85% field capacity), one drought–rehydration cycle (D1, one rewatering), and three drought–rehydration cycles (D2, three rewaterings). We characterized the effects of these treatments on seedling growth, biomass allocation, non-structural carbohydrates (NSCs), malondialdehyde (MDA), catalase (CAT) activity, peroxidase (POD) activity, superoxide dismutase (SOD) activity, proline content, and soluble protein content. The number of drought–rehydration cycles had a significant effect on the growth characteristics and physiological and biochemical properties of leaves. As the number of drought–rehydration cycles increased, the height increased significantly (by 17.17% under D2). The leaf biomass ratio, soluble sugar content, and starch content decreased (15.05%, 15.79%, and 46.92% reductions under the D2 treatment); the stem biomass ratio and root biomass ratio increased; CAT activity increased and then decreased (it was highest at 343.67 mg·g⁻¹·min⁻¹ under D1); and the POD and SOD activities, the MDA content, the soluble protein content, and the soluble sugar/starch ratio increased significantly (395.42%, 461.82%, 74.72%, 191.07%, and 59.79% higher under D2). The plasticity of growth was much greater than that of physiological and biochemical traits. In summary, O. lagopus seedlings adapted to multiple drought–rehydration cycles by increasing the accumulation of soluble proteins (likely associated with osmotic protection), activating enzymes (POD and SOD), promoting the conversion of NSCs (increasing stored carbon consumption), and allocating more biomass to plant height growth than to diameter expansion. Under climate change scenarios with intensified drought frequency, elucidating the drought resistance mechanisms of O. lagopus is critical to silvicultural practices in tropical plantation.

Pinus yunnanensis has high economic and ecological value. The survival of P. yunnanensis has been greatly affected by global warming. This study examines the response of P. yunnanensis seedlings to drought stress and their recovery following rewatering, focusing on growth metrics and C:N:P stoichiometry. This experiment included the four following treatments: CK (water content was 90% ± 5%); LS (water content was 75% ± 5%); MS (water content was 60% ± 5%); and SS (water content was 45% ± 5%). This experiment also included a continuous drought period of 30 days followed by 7 days of rewatering. This study found that drought limited the growth of seedlings, and seedlings could recover rapidly from the damage caused by LS and MS treatments after rewatering, and the recovery of seedlings was greater following LS and MS treatments than following SS. After the drought, the C content in the leaves and stems of P. yunnanensis seedlings could recover to CK levels after rewatering. The N content of each organ and the P content of fine roots of P. yunnanensis seedlings showed an upward trend after rewatering. In summary, the growth of P. yunnanensis seedlings was significantly affected by drought stress, and the recovery mechanism of rewatering varied depending on the degree of drought. Combining the phenotypic plasticity index and principal component analysis, the stem biomass, thin root biomass, root/shoot ratio, leaf C:P, leaf N:P, leaf P, stem C:P, thin root N, thin root P, coarse root N, coarse root P, and coarse root C were the most important indexes for adaptation to drought and rewatering.