Variations in the ¹³C/¹²C ratio in trees are examined in the light of a simple expression relating the relative isotope composition of plant material delta¹³<sup^b>p ^to ^<delta><s13 u^{b>a, the} ^atmospheric ^{isot ope} ^val<sup^e, ^c<s<sup^{b>a, the} _p measurements where independent information on c_l exists, such as seasonal growth, growth low in the canopy, and in conditions of low humidity. The expression provides possible explanations for two previously unexplained phenomena: the absence of anticipated changes due to fossil fuel-induced changes in delta₁₃_a, and regional differences in delta₁₃_p trends. -- AATA