We propose a new mechanism that could contribute to the observed decrease in the diurnal temperature range (DTR) over the last century: the physiological behavior of vegetation in response to climate. Using a physiologically based land surface model, we analyze the influence of vegetation on the response of the DTR to perturbations in the state of the climate and vegetation. Increasing down-welling long wave radiation and surface air temperature together, conditions that could occur as a result of doubling of atmospheric CO2, produced little change in the DTR. Changes in the state of the vegetation (i.e. amount, physiological capacity, stress) produce changes in the DTR of the order or larger than observed. Results emphasize that DTR modeling studies need to consider vegetation responses and suggest that recently reported increases in vegetation over the last decade could contribute to the observed decreases in the DTR. ¿ 2000 American Geophysical Union