This study assesses the impact of mid-21st century climate change on Chile's Mediterranean sclerophyllous forests, increasingly exposed to bioclimatic stress. A novel Bioclimatic Stress Index (BSI) was developed, using 1970-2000 as a baseline and 2050 as a future scenario. Trends in bioclimatic stress and vegetation vigor were analyzed from 1986 to 2024 using the Normalized Difference Vegetation Index (NDVI), a satellite-derived measure of vegetation health and productivity. The BSI shows a significant increasing trend (tau = 0.3968, p < 0.01), while NDVI exhibits a marked decline (tau = -0.433, p < 0.01), indicating worsening ecosystem conditions. Key stressors include rising maximum temperatures (+1.5 degrees C) and increasing water deficits (+100 mm/year), which reduce canopy density, reproductive capacity, and overall ecosystem resilience. Unlike traditional species distribution models, the BSI quantifies climatic stress by integrating thermal and water stress into a mechanistic framework. Identifying critical stress thresholds, it provides actionable insights for conservation planning, emphasizing the urgency of adaptive strategies. The BSI's ability to quantify stress intensity rather than just predicting habitat shifts makes it a valuable tool for resilience-building interventions. Incorporating bioclimatic modeling into conservation efforts is essential to mitigate degradation, safeguard biodiversity, and enhance ecosystem resilience under future climate conditions.