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We tested the null hypothesis that diameter class transitions over a 40-year period (1940–1980) in an even-aged mixed Pinusresinosa Ait. – Pinusstrobus L. stand represent a stationary Markov process. Transition probabilities were constructed from growth data on 202 trees ≥ 5.1 cm DBH in a 0.4-ha permanent plot. For each species, a diameter distribution in 1980 was predicted with the stationary Markov model using the 1940–1950 transition probability matrix. The predicted and observed 1980 distributions were significantly different for P. resinosa (χ² = 31.67, p < 0.01), but not P. strobus (χ² = 7.86, not significant). In P. resinosa, growth rates of trees in the same diameter class at different points in time declined with increasing stand age. This violates the assumption of stationarity. Growth did not decline in most diameter classes in P. strobus. The model also assumes that differences in the competitive …