Wood is anisotropic, meaning its strength properties vary with grain direction. Compressive strength is highest parallel to the grain and lowest perpendicular to the grain, while tensile strength is slightly lower than compressive strength parallel to the grain. Temperature increases and moisture content increases both decrease wood strength and toughness. Strength also varies significantly between wood species and grades, with lower grades containing more defects like knots that reduce strength values used for design compared to actual laboratory results.