What is a biotic factor which would affect the net primary production of a plant?
Several biotic factor can affect the primary prodction of a plant, may it be an antagosnistic action that would stress out the plant and decreas it's primary production or on the contrary a positiv interaction that would increas it.
The primary production of plants (a.k.a it's production of biomass through growth) may be altered or boosted true a wide range of biotic factors ( a.k.a interaction with other living being).
Some good example of decreasing factor could be parazitisme. Indeed, insects like several true bugs ( order: hemiptera) feed on plants. This negative interaction could stress the plant and slowdown the plant's growth.
Herbivory ni some contexte could be a disminishing factor of growth. Lamarre et al. in 2012 shown that in a poor environnement where growth isn't easy to acheive, a high herbivory rate would favour a strategy of defences instead of a strategy of high growth that would compensate herbivory.
On the other hand, positive interaction could help plant's growth. For instance the mycorrhiza symbiosis between plant's roots and ground fungus. This association where the plant feed the fungus and the fungus help the root system to be more effectif. This biotic interaction can help the plant to find enought nutrient to increase it's growth (Mosse 1973).
There are many more exemple of factor that could affect the primary production of a plants, competition, commensalism and every other ecological interaction thaht could happen in a community and that can be seen as a biotic factor.
Lamarre G. P. A., Baraloto C., Fortunel C., Dávila N., Mesones I., Rios J. G., Rios M., Valderrama E., Pilco M. V., Fine P. V. A. (2012). Herbivory, growth rates, and habitat specialization in tropical tree lineages: implications for Amazonian betadiversity. Ecology, 93(8) : 195-210
Mosse, B. (1973). PLANT GROWTH RESPONSES TO VESICULAR‐ARBUSCULAR MYCORRHIZA. New Phytologist, 72(1), 127-136.