Yes, its true of Europe, also true of the United States. The topic is more the domain of agronomists, ecoomists and geoscientists than historians, but we can say a few things:

There have been intentional reforestation programs, for example the Patrimonio Forestal del Estado in Franco's Spain, but these are less significant than the abandonment of low yielding agricultural uses.

Many low productivity fields that had once been worked, sometimes briefly as in North America, sometimes for centuries as in Europe, were abandoned when dramatic increases in agronomy in the 20th century made less productive farmlands economically obsolete and the rural population moved to more urban areas.

If you look at rural France for example, the population has fallen dramatically; this is a process that has been going on for more than a century. The same is true in many other European nations-- there simply aren't people in many formerly agricultural communities to work fields. Mechanization enables high productivity agriculture in fields with amenable terrain-- but rocky and hilly fields are often not worth the trouble.

On a similar note, in Eastern Europe many farms were abandoned after the collapse of communism. A number of factors were involved-- the farms themselves may not have been that productive, and the farm labor had better opportunities in the cities.

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See:

Fuchs, R. , Herold, M. , Verburg, P. H., Clevers, J. G. and Eberle, J. (2015), Gross changes in reconstructions of historic land cover/use for Europe between 1900 and 2010. Glob Change Biol, 21: 299-313. doi:10.1111/gcb.12714

Bauer, Rainer Lutz. “Economic Differentiation and the Divided Responses of Spanish Galician Farmers to Reforestation of the Commons under Franco.” Social Science History, vol. 29, no. 2, 2005, pp. 175–205.

Briner S, Elkin C, Huber R. 2013. Evaluating the relative impact of climate and economic changes on forest and agricultural ecosystem services in mountain regions. J. Environ. Manage. 129: 414-422

Thompson, John G. “Urbanization and Rural Depopulation in France.” Journal of Farm Economics, vol. 7, no. 1, 1925, pp. 145–151. [note the date, 1925 -- this issue was being discussed a century ago]

Kuemmerle, Tobias, et al. “Cross-Border Comparison of Postsocialist Farmland Abandonment in the Carpathians.” Ecosystems, vol. 11, no. 4, 2008, pp. 614–628.