Relictual distribution reaches the top: Elevation constrains fertility and leaf longevity in Juniperus thurifera

Abstract

Juniperus thurifera populations are scattered throughout the western Mediterranean basin and are relictual from its Tertiary distribution due to progressive climatic warming since the last glacial period. To disentangle the factors responsible for its extremely low fertility we combined a microscale experimental design with a macroscale study. At the microscale we experimentally alleviated environmental stress by watering and fertilizing during two years a set of trees in one population. At macroscale we selected 11 populations across a geographical range and sampled them for three years. Macroscale patterns evidenced that both plant fertility and leaf longevity diminished with increasing elevation. Both microscale and macroscale illustrated the importance of water and nutrient availability on leaf growth and plant fertility: On the microscale experiments, regular supply of water and nutrients increased fruit-set by 300%. Macroscale showed that increases in resource availability (precipitation) resulted in reductions of seed abortion, although paralleled by increases in seed predation. Altogether, Our results indicate that fertility is constrained both by elevation and by resource limitation. Therefore any potential lift in the elevational distribution limits will result in synergistic fertility reductions due to harder physical conditions and lower water and nutrient availability. Both will compromise future regeneration of this relictual species, although population decline might be buffered temporary thanks to longevity of adult trees.