Higher association and integration among functional traits in small tree than shrub in resisting drought stress in an arid desert

Revealing plant adaptation strategies to drought has become one of the research hotspots in assessing the impact of global change on forest ecosystems. However, it is not clear whether the drought adaptation strategies are consistent across life forms. The scientific question of this study is whether small tree and shrub adopt different strategies to cope with drought stress. To answer it, we analyzed the differences of change pattern in four types of functional traits (leaf, photosynthetic, hydraulic and carbohydrate traits), as well as the asscoiation and integration among traits across a drought stress gradient between the most common small tree (Haloxylon ammodendron) and shrub (Alhagi sparsifolia Shap.) species in an arid desert of northwest China. Our results showed that the adaptation of photosynthetic and carbohydrate traits were the same in the two species, but leaf and hydraulic traits were different. Association and integration among traits were obviously higher in small tree than shrub because the former suffered more severe drought stress. All the functional traits of small tree can be integrated into two independent drought adaptation groups, which can be named drought resistance-nutrient accumulation group and hydraulic adaptation group, respectively. These two groups were all obviously related to drought adaptation, and can be considered as two adaptive strategies evolved by small tree to cope with drought stress. In contrast, all traits of shrub can be classified into four independent adaptive groups. However, it is difficult to associate all of them as drought adaptation strategies. In conclusion, shrub had a stronger drought tolerance, and showed an independent trait adaptability relative to small tree, with no close accociation between traits. Oppositely, the traits of small tree were closely related to each other. The trade-offs and compensations among traits greatly helpled small tree to reduce drought stress.