The preferences for seasoning and cooking techniques in Chinese provinces depend on differences in social class, religion, historical background, and ethnic groups. Geographic features including mountains, rivers, forests, and deserts also have a strong effect on the locally available ingredients, considering that the climate of China varies from tropical in the south to subarctic in the northeast. Imperial royal and noble preferences also play a role in the change of Chinese cuisine. Because of imperial expansion, immigration, and trading, ingredients and cooking techniques from other cultures have been integrated into Chinese cuisines over time and Chinese culinary influences have spread worldwide.