Shade-driven morpho-physiological plasticity buffers light–water co-limitation in Euterpe precatoria under rainfed field conditions

In seasonally humid Amazonia, rainfed plant establishment can be co-limited by high irradiance and seasonal water deficits. We tested whether temporary shade reduces leaf evaporative demand without reducing photosynthetic carbon assimilation during early field establishment of the Amazon palm Euterpe precatoria Mart. Seedlings were planted in Rio Branco, Acre, Brazil, and grown under five shade levels: 0, 18, 35, 50, and 65%. From 2 to 12 months after planting (MAP), we measured plant height, stem diameter, canopy diameter, and leaf number. At 6 MAP, we measured leaf gas exchange, chlorophyll index, and leaf microclimate. Antecedent climate was represented by 60-day mean air temperature and cumulative rainfall before each survey. Intermediate shade, especially near 50%, maximized net photosynthetic rate, chlorophyll index, and water-use efficiency, while reducing transpiration and leaf vapor pressure deficit. Growth increments were consistently associated with wetter 60-day windows, whereas temperature effects were not consistent after accounting for developmental stage and previous plant size. ANCOVA indicated that shade-related increases in photosynthesis were not fully explained by stomatal conductance alone, although this inference remains indirect because mesophyll conductance and A-Ci or A-PPFD curves were not measured. Integrated field indicators identified a Pareto-efficient shade window near 45-55%, combining rapid canopy development with lower thermal and evaporative risk. Moderate shade therefore provides a practical management window to improve rainfed plant establishment of E. precatoria in cultivation, agroforestry, and restoration systems.