A major goal in ecology is to make generalizable predictions of organism responses to environmental variation based on their traits. However, straightforward relationships between traits and fitness are rare and likely to vary with environmental context. Characterizing how traits mediate demographic responses to the environment may enhance the predictions of organism responses to global change. We synthesized 15 years of demographic data and species-level traits in a shortgrass steppe to determine whether the effects of leaf and root traits on growth and survival depended on seasonal water availability. We predicted that (1) species with drought-tolerant traits, such as lower leaf turgor loss point (TLP) and higher leaf and root dry matter content (LDMC and RDMC), would be more likely to survive and grow in drier years due to higher wilting resistance, (2) these traits would not predict fitness in wetter years, and (3) traits that more directly measure physiological mechanisms of water use such as TLP would best predict demographic responses. We found that graminoids with more negative TLP and higher LDMC and RDMC had higher survival rates in drier years. Forbs demonstrated similar yet more variable responses. Graminoids grew larger in wetter years, regardless of traits. However, in both wet and dry years, graminoids with more negative TLP and higher LDMC and RDMC grew larger than less negative TLP and low LDMC and RDMC species. Traits significantly mediated the impact of drought on survival, but not growth, suggesting that survival could be a stronger driver of species' drought response in this system. TLP predicted survival in drier years, but easier to measure LDMC and RDMC were equal or better predictors. These results advance our understanding of the mechanisms by which drought drives population dynamics, and show that abiotic context determines how traits drive fitness.

中文翻译：

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水的可用性决定了植物性状如何预测人口统计率

生态学的一个主要目标是根据生物体的特征对生物体对环境变化的反应进行普遍预测。然而，特质和适应性之间的直接关系很少见，而且可能会随着环境背景而变化。表征特征如何调节人口对环境的反应可能会增强对生物体对全球变化反应的预测。我们综合了短草草原 15 年的人口统计数据和物种水平性状，以确定叶片和根性状对生长和存活的影响是否取决于季节性可用水量。我们预测 (1) 具有耐旱性状的物种，例如较低的叶膨压损失点 (TLP) 和较高的叶和根干物质含量（LDMC 和 RDMC），由于更高的抗枯萎性，在干旱年份更有可能存活和生长，(2) 这些性状不能预测湿润年份的适应性，(3) 更直接测量水分利用生理机制的性状（如 TLP）最能预测人口反应。我们发现具有更多负 TLP 和更高 LDMC 和 RDMC 的禾本科植物在干旱年份具有更高的存活率。福布斯表现出类似但更多变的反应。无论性状如何，禾本科植物在潮湿的年份都会长得更大。然而，在潮湿和干燥的年份，具有更多负 TLP 和更高 LDMC 和 RDMC 的禾本科植物比负 TLP 更少和低 LDMC 和 RDMC 的物种长得更大。性状显着调节了干旱对生存的影响，但不影响生长，这表明生存可能是该系统中物种干旱反应的更强驱动力。TLP 预测干旱年份的存活率，但更容易测量的 LDMC 和 RDMC 是相等或更好的预测因子。这些结果促进了我们对干旱驱动种群动态机制的理解，并表明非生物环境决定了性状如何驱动健康。