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

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. Seeds were incubated in the light at a range of temperatures (1025 and 25/10 C) after different periods (up to 3 months) of cold stratification at 5 C. Base temperatures (T-b), and thermal times for 50 germination (THETA(50)) were calculated. Seeds were also buried in the soil in two natural populations (Rio Correboi 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 (Cd) was calculated on the basis of the estimated T-b and soil temperatures. Cold stratification released physiological dormancy (PD), increasing final germination and widening the range of germination temperatures, indicative of a Type 2 non-deep PD. T-b was reduced from 105 C for non-stratified seeds to 27 C for seeds cold stratified for 3 months. The best thermal time model was obtained by fitting probit germination against log Cd. THETA(50) was 26 log Cd for untreated seeds and 217219 log Cd for stratified seeds. When THETA(50) values were integrated with soil heat sum estimates, field emergence was predicted from March to April and confirmed through field observations. T-b and THETA(50) 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 mountain waterways and of physiologically dormant species, with overwintering cold stratification requirement and spring germination.