Answer:
Sexual reproduction
Explanation:
Sexual reproduction is defined as the process in which male gamete combine with female gamete to form a new organism.
The given example is a type of sexual reproduction as it involves two different  sexes a male poplar tree and a female poplar tree, and pollens from male poplar tree transfers to the sex cells on a female poplar tree result in the process of fertilization.
Hence, the correct answer is "Sexual reproduction".