Hornification is a well-known phenomenon describing what happens during the drying of lignocellulosic materials, often within and between cellulosic pulp fibers. For wood fibers used in papermaking, this phenomenon decreases fiber wall swelling, and internal and external fibrillation. It reduces flexibility of damp fibers, which leads to a diminished ability to form effective fiber networks, resulting in lower paper strength. This work investigates how drying temperature affects the changes in fiber morphology, connects this to the changes in sheet behavior, and proposes a combination of bonding mechanisms for hornification. Results show that hornification depends on drying temperature; higher temperature gives higher degrees of hornification with decreased WRV of about half the numerical value, from 1.5 g/g for never-dried pulp to 0.7 g/g for hardwood pulp samples. Higher temperatures, above 100°C, also change the pulp color, as measured by increased yellowness. Decreased swelling capacity and pulp yellowness are connected. This indicates parallel reactions, which both contribute to hornification. The mechanisms are proposed to be chains of hydrogen bonds, dominating at low temperatures and providing no color change, and dehydration reactions via pyrolysis, giving a yellow-to-brown color shift. Compression strength measurements show that major hornification adversely affects sheet strength due to poor network bonding. However, minor hornification can be beneficial for applications where compression strength is an important parameter.