Drought memory expression varies across ecologically contrasting forest tree species

Trees may exhibit long-lasting morpho-physiological acclimation in response to drought (i.e. drought memory) throughout their extensive life cycles. This acclimation might be particularly crucial for seedlings and saplings due to their limited access to water. We studied the development of drought stress memory in seedlings of black walnut (Juglans nigra L.), western larch (Larix occidentalis Nutt.), and Douglas-fir ((Pseudotsuga menziesii var. menziesii (Mirb.) Franco) in response to controlled drought exposure during their germination year (drought priming). We evaluated the effects of drought priming under a second-year drought, focusing on changes in water uptake capacity and transpiration demand, biomass allocation to new roots and foliage, root architecture, and photosynthesis. Drought priming led to significant morpho-physiological responses in the new leaves and roots developed during the subsequent growing season drought. Western larch showed increased biomass allocation to roots, higher specific root length and root tips, and enhanced water uptake, while Douglas-fir exhibited earlier bud break, greater net photosynthesis, and increased foliage growth. In contrast, black walnut seedlings displayed no notable changes in biomass allocation or physiology. Our results also show that biomass allocation to new roots plays a crucial role in enhancing water uptake capacity and gas exchange during seedling establishment. These findings underscore the importance of drought memory for stress resistance in trees, influencing the capacity of forests to regenerate and respond to recurrent droughts and climate change. The formation and expression of drought memory, however, varied across species, highlighting the complexity of adaptive responses across different forest ecosystems.