Dry Heatwaves Alone Do Not Reduce Tree Resilience, but Their Compounding with Soil Drought Does

In this study, we examine tree resilience in response to compound atmospheric and soil droughts, using dendrometer-based stem diameter measurements in a semi-arid pine forest. Our main question is: what is the differential impact of atmospheric and soil drought on plant growth and resilience? By analyzing data from an irrigation experiment on mature pine trees, we developed new tools for characterizing heatwaves, and introduced novel resilience indices, especially designed for robust evaluation of recovery and resistance in trees experiencing short and intense heatwaves against the backdrop of a long soil drought. Our findings show that irrigation effectively shielded trees from the negative impacts of heatwaves. Non-irrigated trees exhibited a significant decline in resilience during the dry season, primarily during compound droughts, which irrigated trees did not experience. Following the beginning of the wet season, the resilience of non-irrigated trees increased rapidly, matching that of the irrigated trees, suggesting minimal compromise in hydraulic functioning. These findings have significant implications for understanding forest resilience in the face of escalating climate change and provide practical tools for real-time monitoring and assessment.