Abstract: A widely accepted perspective posits that an extension of the growing season enhances plant growth by increasing the duration of favorable environmental conditions under warming, which is described as an ecological effect. However, changes in growing season length can also influence plant functional traits and physiological processes, as suggested by the "leaf economics spectrum" theory, a physiological aspect frequently overlooked. Disentangling the ecological and physiological effects of growing season length on plant growth remains challenging due to their co-variation with climate factors. Here we explored the physiological effect through common garden experiments on the Qinghai−Tibetan Plateau. Our findings revealed a trade-off between growing season length and plant growth under controlled climatic conditions, a pattern further corroborated by satellite-based observations across most regions of the plateau. This trade-off was driven by a negative correlation between growing season length and photosynthetic efficiency, suggesting that an extended growing season does not necessarily translate into enhanced carbon assimilation. However, state-of-the-art dynamic global vegetation models failed to adequately capture this trade-off, underscoring the need to integrate the physiological effects of growing season length into these model frameworks for improved predictions of plant growth under climate change.

Key words: leaf economic spectrum, plant functional traits, phenology, common garden, Tibetan Plateau