Transpiration causes daytime trunk shrinkage via water loss

Steps:

• Transpiration pulls water from roots to leaves, reducing trunk turgor and diameter.
• Key drivers: light opens stomata; temperature accelerates evaporation; wind removes boundary layer; low humidity steepens vapor gradient.
• Daytime peaks in light, temperature, wind, and low humidity maximize water loss.
• Excluding humidity from factors allows unmitigated promotion of transpiration for largest shrinkage.

Why D is correct:

• Transpiration rate follows Penman-Monteith equation, where light, temperature, and wind directly increase flux, while low humidity (excluded here) amplifies without opposition.

Why the others are wrong:

• A: Humidity inclusion allows high levels to reduce vapor gradient, limiting shrinkage.
• B: Humidity dampens evaporation gradient despite light and temperature.
• C: Humidity counters wind and light effects on transpiration.

Final answer: D