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& Key message Defoliation modality significantly affected leaf traits and carbon allocation especially in the early recovery period of two Leguminosae woody species. Robinia pseudoacacia L. recovered more quickly following defoliation than Amorpha fruticosa L. & Context Defoliation resulting from anthropogenic disturbance, insects, and grazing animals affects plant growth and vegetation dynamics. Robinia pseudoacacia L. and Amorpha fruticosa L. with different susceptibility to biotic attack are commonly used for vegetation restoration in North China. However, plant responses to defoliation were not fully understood in the two Leguminosae woody species. & Aims We selected R. pseudoacacia and A. fruticosa to determine the effects of defoliation on individual plant growth, leaf traits, and carbohydrate allocation and compared the two species in terms of recovery strategies following defoliation for vegetation restoration. & Methods In a greenhouse experiment, A. fruticosa and R. pseudoacacia seedlings were subjected to three defoliation treatments: defoliation of the top down (TD) or bottom up (BU) 50% of the crown, and a control without defoliation (CK). The physiological and growth traits of seedlings were determined at 1, 3, 14, 30, and 60 days following defoliation.

Plant size influences plant responses to combined environmental factors under climate change. However, their roles in plant ecophysiological responses are not fully understood. Two rapidly growing Leguminosae species (Robinia pseudoacacia and Amorpha fruticosa) were used to examine plant responses to combined drought and defoliation treatments (two levels of both treatments). Both 1.5 month-old seedlings and 3 month-old seedlings were grown in a greenhouse, and seedling growth, leaf gas exchanges, stem hydraulics, and concentrations of non-structural carbohydrates were determined after 60 days of treatment. Our results indicated defoliation had no significant effect on plant height, basal diameter, and total biomass whatever plant sizes and species. Under the low water availability treatment, the defoliated seedlings significantly increased by 24% in stem water potential compared with non-defoliated seedlings in large R. pseudoacacia. Compared with the high water availability in large non-defoliated R. pseudoacacia seedlings, the low water availability significantly reduced by 26% in stem starch concentration to maintain the stem soluble sugar concentration stable, but not in small R. pseudoacacia seedlings. We also found a negative correlation between leaf and root soluble sugar concentration under low water availability in A. fruticosa. The results demonstrate defoliation could relieve the effect of low water availability in large seedlings. Large seedlings had more compensatory mechanisms in response to defoliation and drought treatments than small seedlings, thus species with large carbon reserves are more recommended for vegetation restoration under combined drought and defoliation conditions. Future studies with more species are crucial for obtaining more rigorous conclusions.

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