Between 698 and 800, there were at least thirty-six years of plagues in Japan, or about one every three years. The most well-documented epidemic—and to judge by the mortality and its social, economic, and political effects, the most significant—was a smallpox outbreak during 735–737. It started in northern Kyushu, a certain sign of its foreign origin, but by 737 the virus had spread up the Inland Sea and on to eastern Honshu, aided, ironically enough, by the improved network of roads linking the capital and provinces. To its credit, the court tried to apply pragmatic principles to treat the symptoms of the disease, but to little effect. Statistics from various provinces scattered from northern Kyushu to eastern Honshu suggest that mortality was about twenty-five percent, meaning that a million or more persons may have succumbed. As a result of the depopulation, an entire layer of village administration was abolished. Another irony was that the death rate among the exalted aristocracy—living crowded together in the capital at Nara—was even higher, a full thirty-nine percent. At the end of 737, chroniclers wrote,”Through the summer and fall, people … from aristocrats on down have died one after another in countless numbers. In recent times, there has been nothing like this.” In the wake of the epidemic, government revenues plunged by more than twenty percent, even more draconian measures were implemented to stem cultivator flight from the land, and a guilt-ridden [Emperor] Shōmu approved large expenditures for Buddhist temples, statues, and other religious icons.
Epidemics certainly helped to reverse the long demographic expansion of the last several centuries, but two other factors contributed to population stasis. The first was crop failure and widespread famine, occurring about every third year between the late seventh and eighth centuries. Causes for bad harvests were complex, but various climate data indicate that the eighth century was one of the hottest and driest in Japanese history. In Western Europe, where there was a “medieval warm” at this time, the effect was to dry out water-logged soils and encourage the expansion of agriculture; in Japan, where farmers often depended upon rainfall as the only way to irrigate their paddies, the result was frequent crop failure and hunger. At ten to fifteen percent, mortality from a severe famine was lower than an epidemic, but, like pestilence, malnutrition also reduced fertility. Even in years when the harvest seemed adequate, the populace frequently went hungry in the spring when their supplies of grain were exhausted. More sophisticated means of watering rice paddies may have remedied the problem, but they were either unavailable or not applied.
A second factor leading to population stasis was the ecological degradation besetting the Kinai, the richest and most financially important region in the eighth century. Altogether, the government sponsored the construction of six capital cities and countless temples, shrines, and aristocratic mansions from 690 to 805. All these structures were built from timber harvested in the Kinai and adjacent provinces, and most had roof tiles requiring baking with charcoal in a kiln. During the second half of the eighth century, the shortage of lumber became so critical that planners began to recycle used timbers and roof tiles from older capitals, such as Fujiwara and Naniwa. When the court left Nara for Nagaoka in 784, for example, they used recycled lumber and tiles almost exclusively.
By the late eighth century, tile bakers were relying upon red pine to fire their kilns, a secondary forest cover that typically grows in nutrient-poor soil. Furthermore, the government began to note that the bald mountains in the Kinai and vicinity produced less rain and more erosion. In essence, the stripping of the forests throughout central Japan exacerbated the effects of the hot, dry climate and encouraged farmers to give up cropping altogether and flee to the seashores and mountains to forage as of old.