This report summarizes research funded by the Joint Fire Science Program (JFSP Project Number 12-1-03-31) addressing needs for information regarding the effectiveness and longevity of fuels treatments. We investigated the longevity of effects associated with mastication treatments on variables including fuels, vegetation, and fire behavior in northern California and southern Oregon. We coordinated a synthesis of the effects of mastication on fire behavior; emergent findings and pressing needs were highlighted. In a large field study, we measured the effects of time-since-mastication on fuelbed properties. Masticated fuels decomposed and compressed over time across sites, with the greatest reductions in fine fuels (1-hr and 10-hr). In a subset of those plots, we measured laboratory fire behavior across a time-since-mastication chronosequence, highlighting reductions in fire intensity over time, but with increased smoldering combustion. We used the large plot network to evaluate shrub diversity, height, and cover responses to treatments. Vegetation height and cover was greatest in sites dominated by resprouting shrubs and trees. At two sites where we had longitudinal data on fuels and vegetation, we measured shrub and ladder fuel responses in relation to residual overstory produced through a range of fuels treatments. Treatment longevity was linked to supplemental treatments such as herbicide or prescribed fire. In those same sites, we measured overstory pine growth and longer-term mortality from treatments, including prescribed fire applied following mastication. Results highlight that of ponderosa pines that survive post-mastication prescribed fire, growth was best in cases with intermediate scorch. Collectively, the results generated in this study will enhance the information available to land mangers regarding the viability of masticating fuels across the region and more broadly how these fuels treatments maintain effectiveness and longevity.