Thermal niche for in situ seed germination by Mediterranean mountain streams: Model prediction and validation for Rhamnus persicifolia seeds

Background and Aims Mediterranean mountain species face exacting ecological conditions of rainy, cold winters and arid, hot summers, which affect seed germination phenology. In this study, a soil heat sum model was used to predict field emergence of Rhamnus persicifolia, an endemic tree species living at the edge of mountain streams of central eastern Sardinia. Methods Seeds were incubated in the light at a range of temperatures (10-25 and 25/10 8C) after different periods (up to 3 months) of cold stratification at 5 8C. Base temperatures (Tb), and thermal times for 50%germination (u50) were calculated. Seedswere also buried in the soil in two natural populations (RioCorreboi and Rio Olai), both underneath and outside the tree canopy, and exhumed at regular intervals. Soil temperatures were recorded using data loggers and soil heat sum (8Cd) was calculated on the basis of the estimated Tb and soil temperatures. Key Results Cold stratification released physiological dormancy (PD), increasing final germination and widening the range of germination temperatures, indicative of a Type 2 non-deep PD. Tb was reduced from 10.5 8C for nonstratified seeds to 2.7 8C for seeds cold stratified for 3 months. The best thermal time model was obtained by fitting probit germination against log 8Cd. u50 was 2.6 log 8Cd for untreated seeds and 2.17-2.19 log 8Cd for stratified seeds. Whenu50 values were integrated with soil heat sum estimates, field emergencewas predicted from March to April and confirmed through field observations. Conclusions Tb and u50 values facilitated model development of the thermal niche for in situ germination of R. persicifolia. These experimental approaches may be applied to model the natural regeneration patterns of other species growing on Mediterranean mountainwaterways and of physiologically dormant species, with overwintering cold stratification requirement and spring germination.