Results from experiments measuring the contribution of the surface layer to the optical seeing at the Observatorio Astron omico Nacional at San Pedro M artir (OAN{SPM) are reported. Microthermal sensors placed at 7 levels on a 15-m-high instrumented mast were used to measure the structure constant of the refractive index C 2 n . The integrated seeing parameter was measured with a Dieren tial Image Motion Monitor during 23 nights. Log{normal statistics were found for the seeing with mean of 0.98 00 and median value 0.84 00 . The contribution of the surface layer (2.3 to 15 m) to the total optical turbulence has a mean value of 16%, which corresponds to a degradation of 10% of the total seeing. These values are similar to those found in other observatories around the world, suggesting that the presence of trees in the OAN{SPM does not have a signican t eect on the surface layer seeing. Further studies should provide a conrmation of this tendency.