Abbreviations, periods, definitions, and sources of nature variables used in this paper.

aData exists for 1975–1981, but we were advised by BLM (T. McDonald, pers. comm.) that while 1982 and later data were based on reported use at fee sites and recreation concessions, data prior to 1982 was not and was much less reliable.bBLM data does not exist for the four years 1990 and 1993–1995: this is not our omission.cWeighted by survey firm to best estimate demographics of US population.a. 1975年至1981年存在相关数据集，但我们接收到美国土地管理局（Bureau of Land Management, BLM）（T. McDonald，私人通信）的告知：1982年及之后的数据基于收费场地与休闲特许经营点的上报使用量，而1982年之前的数据并非采用该统计方式，且可靠度大幅更低。 b. 1990年以及1993年至1995年这四年无美国土地管理局的相关数据，此并非我们的疏漏。 c. 由调研公司进行加权处理，以得到美国人口人口统计特征的最优估计值。