Non-structural carbohydrates (NSC), including soluble sugars (SS) and starch (ST), are vital for plant metabolism and stress resilience. However, how the allocation of NSC and their components varies diurnally between C3 trees and shrubs in arid regions, and their respective roles in drought response, has received limited attention. This study examines the diurnal dynamics of NSC in leaves of woody species to elucidate growth-form-specific carbon storage strategies. In August 2023, we measured SS and ST concentrations in leaves of 16 C3 common species (11 trees, 5 shrubs) in Xinjiang, comparing daytime and nighttime levels. We used a two-way ANOVA to assess the effects of life form (tree/shrub) and time (day/night) on NSC, SS, ST, and SS:ST. Trees had significantly higher NSC and SS concentrations than shrubs. Life form and time jointly influenced NSC, ST, and SS:ST ratios, while SS concentration varied only with life form. Starch accumulates during the day and decreases at night, indicating it is a temporary carbon reserve that is converted to sugars for nighttime metabolism. The findings highlight divergent carbon allocation strategies between the studied C3 trees and shrubs, with trees maintaining higher NSC pools. Diurnal starch turnover underscores its importance in balancing carbon supply under arid conditions. These insights advance our understanding of growth-form-specific adaptations in carbon allocation within water-limited ecosystems.