Prolonged periods without precipitation in spring prevent timely and rapid germination of pioneer tree seeds and could lead to an accelerated loss of germination capacity (reduced lifespan). To get knowledge about the shortening of seed lifespan and, thus, the shortening of natural regeneration windows under climate change, an experiment was conducted. Seeds of Alnus glutinosa, Betula pendula, Populus tremula, Salix caprea, Larix decidua, Pinus sylvestris, and Picea abies were exposed to temperatures of 15 °C, 25 °C (past or present climate), and 40 °C (future climate) with low or high humidity for a period of 3 months. Regardless of air humidity, the initial germination percentage of Alnus glutinosa, Betula pendula, Larix decidua, and Pinus sylvestris seeds decreased only slightly by 5–15% over the 91-day period when stored at 15 °C and 25 °C. For Populus tremula and Salix caprea, time windows of maximum 14–49 days and 42–91 days were identified, respectively. However, as climate change progresses with rising temperatures and increasing absolute air humidity values, the window of opportunity for successful germination will shorten for all studied tree species. In the moist air humidity variant of 40 °C, the germination percentage dropped to 0% after 42 days for Alnus glutinosa, Betula pendula, and Larix decidua. The natural regeneration window will be reduced by half from more than three months to about three weeks under climate change. The lifespan of Populus tremula and Salix caprea seeds will be shortened to 1 week. For Picea abies, the time window will shorten to a maximum of 28 days.