Five studies were conducted to address some of the gaps in our current knowledge of hybrid poplar production. The first was a study of an alleycropping system in which winter triticale was grown as a source of early income between rows of hybrid aspen trees during the first three years of establishment, with the objective of evaluating whether the productivity of the system would be affected by topographic position and fertilizer rate. The results demonstrated that total aboveground biomass productivity was not significantly affected by topographic position, with the exception of the floodplain in the first year, where weed pressure was high. Fertilizer placed in the planting hole had positive and statistically significant effects; the trees receiving the highest fertilizer rate had nearly twice as much biomass as those receiving none. Fertilizer rate and age were found to be relatively strong predictors for total aboveground biomass, but the branch fraction of total aboveground biomass was better predicted by tree size.

The objectives of the second study were to quantify the regeneration of hybrid aspen arising from roots sprouts following the harvest of a mature plantation, and to devise an equation to aid in row thinning the root sprouts to a density suitable for another short rotation. Using variable radius plot sampling, similar estimates of root sprout density and harvestable biomass were attained using basal area factors (BAFs) of 1.56 and 2.78 m² ha^&minus1. The latter did so with roughly half as many measurement trees, but also had a larger confidence interval; thus, a trade-off exists between reducing sampling time and obtaining more precise estimates. Using a BAF of 6.22 m² ha^&minus1 resulted in even wider confidence intervals, and lower estimates of root sprout density. The row thinning equation developed in the study was effective for predicting the size of the largest gap in the row based on the number of sprouts in the row, which (in conjunction with inventory data for sprout density) dictates the appropriate width of the unharvested row for a desired maximum gap size.

The objectives of the third study were to use previously published productivity data to calibrate and validate a tree growth model (3PG) for hybrid cottonwood in the North Central U.S., and to use the validated model to map predicted yields for the region. The results suggest that 3PG is well suited for modeling hybrid poplar aboveground biomass productivity. Linear regression of actual versus predicted total aboveground biomass for the validation dataset demonstrated a strong fit (R² = 0.89, RMSE = 8.1 Mg ha^&minus1 ). When used to map mean annual biomass productivity (total aboveground dry biomass divided by age), predicted values ranged from 4.4 to 13.0 Mg ha^&minus1 yr^&minus1 across Minnesota and Wisconsin, with the highest productivity mainly concentrated in the area stretching from south&ndashcentral Minnesota across southern Wisconsin.

The objective of the fourth study was to evaluate the feasibility of using fly ash (produced from a biomass boiler at an ethanol facility) as a foliar fertilizer for hybrid aspen trees in both greenhouse and field settings. The results demonstrated that several nutrients in the fly ash were absorbed by hybrid aspen both in the greenhouse and in the field; however, this absorption did not significantly affect tree growth in either setting. The ash appeared to be compatible with foliar nitrogen fertilizer, as inclusion of the fly ash did not significantly alter the effects of the nitrogen fertilizer on tree growth. Additional research should be done with crops known to benefit from foliar application of the nutrients found in the ash, particularly the nutrients shown in this study to be available for uptake by plants.

The objective of the fifth study was to evaluate the feasibility of using biochar (produced by a fast pyrolysis bio&ndashoil reactor) as a substitute for vermiculite to grow hybrid poplar in the greenhouse. The results suggest that biochar can be an effective substitute for vermiculite when substituted on a volume basis. The biochar and vermiculite mixes similarly produced higher shoot and total biomass than the peat moss control; vector analysis indicates that this was primarily due to superior availability of K. The increased availability of K may be related to the elevated initial levels of K detected in the biochar and vermiculite mixes, rather than a superior ability of the mixes to adsorb cations from the soil solution. Thus, future studies should attempt to separate such pre&ndashloading effects from CEC effects, as well as test a wider selection of crops and biochars. In addition, the biochar mix was associated with lower root biomass than the vermiculite mix. Additional research is needed to determine the cause of this response, and whether it affects long term growth and survival of the trees.