Aridity and drought: distinct concepts often conflated in ecological research

As well-established drivers of ecological pattern and process, aridity and drought are linked by a common focus on water limitation - with both aridity and especially drought projected to increase in extent and severity with climate change. There is a long history of assessing plant adaptations to drought through the lens of aridity. Consistent with this history, trait-based studies of drought typically frame their hypotheses around how plants, communities, and ecosystems vary along spatial aridity gradients. However, aridity, as a long-term, water-limited climatic state, differs fundamentally from drought, which is an anomalous but discrete period of severe water limitation. Here, we ask whether patterns observed along aridity gradients should conceptually underpin how plants are hypothesized to respond to drought. We briefly review literature that either supports or challenges the use of traits that vary along aridity gradients to predict responses to drought. We then highlight reasons why plants and ecosystems might be expected to respond differently to aridity and drought. We conclude that the conflation of aridity with drought may hamper progress in understanding the ecological consequences of globally increasing water limitation. Advancing the field of drought ecology would be better served by testing hypotheses arising from alternative frameworks.