Differential hydric deficit responses of Robinia pseudoacacia and Platycladus orientalis in pure and mixed stands in northern China and the species interactions under drought

The differential responses to water deficit would be the key factors to cause black locust decline and to drive a change in the forest composition in the mixed forest. This study was conducted in arborvitae (Platycladus orientalis)-black locust (Robinia pseudoacacia) mixedwood plantations with various compositions ranging from pure black locust to pure arborvitae to mixtures of the two located in semi-arid rocky hill area in northern China. In current years, black locust tends to decline and the forest composition is changing in favor to arborvitae. A dendroecological investigation and delta C-13 measurement of tree rings were applied to assess the relationships between water relations and growth in response to water deficits. Black locust in mixed forest had smaller tree-ring growth than that in pure forest in recent years, while arborvitae trees in mixed forest had bigger tree-ring width than those in pure stand. The delta C-13 values suggested that black locust in mixed forest had severer drought stress than that in pure forest, and arborvitae showed an opposite scenario. In early growth season, the soil was the driest in pure arborvitae, followed by mixed and then black locust forest. Black locust was more sensitive to drought than arborvitae with quickly closing stomata and reducing photosynthesis. Thus, the soil water regime in mixed forest was worse than that in pure forest for black locust, and was opposite for arborvitae. As a result, black locust in mixed forest had lower stomatal conductance and photosynthesis than that in pure black locust forest, whereas arborvitae had an opposite scenario. In summary, the results suggest that the differential responses to water deficit and the species interactions would be the key factors to cause black locust decline.