Ecological theory is built on trade-offs, where trait differences among species evolved as adaptations to different environments. Trade-offs are often assumed to be bidirectional, where opposite ends of a gradient in trait values confer advantages in different environments. However, unidirectional benefits could be widespread if extreme trait values confer advantages at one end of an environmental gradient, whereas a wide range of trait values are equally beneficial at the other end. Here, we show that root traits explain species occurrences along broad gradients of temperature and water availability, but model predictions only resembled trade-offs in two out of 24 models. Forest species with low specific root length and high root tissue density (RTD) were more likely to occur in warm climates but species with high specific root length and low RTD were more likely to occur in cold climates. Unidirectional benefits were more prevalent than trade-offs: for example, species with large-diameter roots and high RTD were more commonly associated with dry climates, but species with the opposite trait values were not associated with wet climates. Directional selection for traits consistently occurred in cold or dry climates, whereas a diversity of root trait values were equally viable in warm or wet climates. Explicit integration of unidirectional benefits into ecological theory is needed to advance our understanding of the consequences of trait variation on species responses to environmental change.

生态学理论建立在权衡取舍的基础上，其中物种之间的特征差异随着对不同环境的适应而演变。权衡通常被认为是双向的，特征值梯度的两端在不同环境中赋予优势。然而，如果极端特征值在环境梯度的一端带来优势，而范围广泛的特征值在另一端同样有益，那么单向效益可能会很普遍。在这里，我们表明根性状可以解释沿温度和水可用性的广泛梯度的物种出现，但模型预测仅类似于 24 个模型中的两个模型的权衡。具有低比根长和高根组织密度 (RTD) 的森林物种更有可能出现在温暖气候中，而具有高比根长和低 RTD 的森林物种更有可能出现在寒冷气候中。单向收益比权衡取舍更普遍：例如，具有大直径根和高 RTD 的物种通常与干燥气候相关，但具有相反特征值的物种与潮湿气候无关。性状的定向选择始终发生在寒冷或干燥的气候中，而根性状值的多样性在温暖或潮湿的气候中同样可行。需要将单向效益明确整合到生态理论中，以增进我们对性状变异对物种对环境变化反应的影响的理解。