Thermal niche for germination and early seedling establishment at the leading edge of two pine species, under a changing climate

Abstract

Pines are the most popular species for reforestation in Mexico, however reforestation programs have little success due to a high mortality of pine seedlings. Shifts in the distribution range due to climate change has been predicted for many species specially at their distribution borders. Understanding the thermal limits of germination and early seedling growth at the leading edge population in current and future climates could enable the development of suitable propagation practices and conservation strategies. Seeds of Pinus douglasiana and Pinus maximinoi, from their northern distribution range in a temperate sub humid region, were germinated and seedlings were grown across a range of temperatures from 10 to 35 °C. A cardinal temperature model was then employed to obtain the thermal coefficients for seed germination, seedling growth and survival. Projected temperature increase by 2090 according to IPCC scenarios RCP 2.6 and RCP 8.5 were used to predict the germination and early establishment behaviour of both species. Thermal requirements were similar between species and in different developmental stages showing base temperatures of 9.4−10 °C for germination, 9.8 °C for root growth, 9.6 °C for shoot appearance and 8.4 °C for shoot growth; and thermal time requirements of 65−69 °C d for germination, 107 °C d for root growth, 103−107 °C d for shoot appearance and 140−141 °Cd for shoot growth. However, seedling survival was higher at 20 and 25 °C. Different sensitivities to low temperatures for germination and seedling establishment could be factors that drive ecological divergence between these species and explain their altitudinal tolerances. As timing of germination and seedling growth are expected to be accelerated under climate change scenarios, seedling establishment and survival in both species could be altered .