Germination responses of two key mountain tree species to single and combined fire-related stresses: does elevational origin matter?

Abstract

Since climate change is expected to increase fire activity and drought stress in forests across the globe, studying seed resistance to heat stress and drought is imperative to predict future mountain forest regeneration scenarios. Here, stress co-occurrence and among population variation in stress resistance are key elements to consider. We hypothesized that i) increasing heat/osmotic stress intensity decreases germination performance; ii) seeds from higher elevations (i.e. cooler and wetter climatic conditions) exhibit lower resistance to heat/osmotic stress showing a stronger decline in germination performance as compared to seeds from lower elevations (i.e. warmer and drier climatic conditions); and that iii) elevational differences in seed stress resistance vary depending on whether stresses are applied alone or in combination. We exposed seeds from Polylepis australis and Escallonia cordobensis from low (1200 – 1400 m), mid (1650 – 1800 m) and high (2000 – 2200 m) elevational origins to different heat (control, 75, 105 °C) and osmotic (0, -0.3, -0.6 MPa polyethylene glycol) stress intensities. Germination performance decreased with increasing heat/osmotic stress intensity and clear differences among elevational origins were observed. In both species low-mid elevation seeds showed lowest performances but exhibited slightly higher stress resistance than high elevation seeds; yet the nature of these responses was species-specific. Our results demonstrate that germination response to a combination of heat stress and drought is different from the response to each of these stresses applied individually. Therefore, understanding how interactions of climate change with fire events can influence germination timing and success in mountain trees requires considering the effect of multiple stressors on seed physiology. Our findings also highlight the potential population-specific and species-specific responses, which could be used as guidance for ongoing restoration plans.