Few, if any, studies have been carried out that examine and relate in vivo pollen germination following stimulated wet deposition of acidity and copper with various fruit and seed set parameters of trees. The methods used with inflorescences of a wind-pollinated tree, Populus tremuloides Michx., are reported here. The responses of components of the reproductive system to rain simulants indicated significant (P < 005) overall response of in vivo pollen germination to pH. The combined data demonstrated a significant (P < 005) negative correlation between germination and pH. However, the response in the absence of copper showed no linear correlation because of a recovery at the lowest pH used (26). The effect of simulated rain at the time of pollination was shown to increase fruit abortion significantly. In addition, an overall significant negative correlation was found between fruit abortion and pH, but again in the absence of copper, fruit abortion recovered at the lowest pH (2–6). Seeds per fruit and percentage of placenta with full seeds were significantly reduced at pH 2–6 in the presence of 0–05 mg 1−1 Cu compared with the control. These two seed set parameters were also significantly intercorrelated (P < 0.01) with fruit abortion suggesting that lack of seed set may be the cause of the abortion. The significant correlation of both in vivo pollen germination and fruit abortion with pH, especially in the presence of copper, supports the view that inhibition of the pollen on the stigma by the simulated rain is responsible for the lack of seed set above the required threshold that prevents fruit abortion.