The mountain pine beetle killed a number of trees in the Greater Yellowstone Ecosystem from the late 1960s through the mid-1980s. This created a heterogeneous forest in which old surviving trees were mixed with younger trees of various ages. This mixed stand structure may have provided ladder fuels that contributed to the fires. In addition, the winter season of 1987–1988 was drier than usual, with the Greater Yellowstone Ecosystem receiving only 31% of the normal snowpack. However, April and May 1988 were very wet and the abundant moisture greatly aided grass and understory development. By June, the rainfall stopped and little was recorded in the entire Greater Yellowstone Ecosystem for the next four months. By July, the worst drought in the history of Yellowstone National Park began. Grasses and plants which grew well in the early summer soon turned to dry tinder. Relative humidity levels fell further, desiccating the forest. Fuel moisture content in dead and fallen timber was recorded as low as 5%. By mid-August, humidity levels were averaging below 20% and were recorded as low as 6% on one occasion. To compound the lack of rainfall, the majority of Yellowstone's soils are rhyolitic volcanic rocks and soils which have poor moisture retention. A series of strong but dry storm fronts also led to the rapid spread of a number of the largest fires.