Final answer:
Water is the most important factor influencing Net Primary Productivity (NPP) because it is vital for photosynthesis, cell growth, and nutrient transport, which are necessary for plant growth and biomass production.
Step-by-step explanation:
The most important factor that affects Net Primary Productivity (NPP) is water. Water is essential not only as a reactant in photosynthesis but also as a major component of new cells and as the transportation medium for nutrients within the plant. The presence of water enables the stomata in plant leaves to remain open to absorb carbon dioxide (CO₂) without losing too much moisture, which is crucial for photosynthesis and, therefore, biomass production.
Environments that are warm and wet provide conditions that optimize photosynthesis, enzyme efficiency, and stomatal function, leading to high levels of NPP and subsequently, a greater amount of biomass. On the other hand, environments that are dry and cold result in lower photosynthetic rates and reduced biomass production, affecting the availability of food and habitat for the animal communities living there. Given this importance, any changes in the availability of water due to climatic fluctuations or other factors can have significant impacts on NPP in terrestrial ecosystems.