By Gabriel Stargardter EMILIANO ZAPATA Mexico (Reuters) – When foreign investors begin to pour into Mexico’s overhauled energy sector in the coming months, they will face a potent force well-known to miners: Mexico’s ejidos, or rural landowner groups. The product of revolutionary land reform – almost a century ago – that redistributed more than 100 million hectares from large landowners to small farming groups, the ejidos control surface rights to large swaths of Mexico. The ejidos are often poor but they can be powerful: machete-wielding landowners shuttered government plans for a new Mexico City airport in 2002. For years, foreign companies have owned concessions to mine metals in Mexico, leading them into delicate negotiations with the ejidos, who often block mines for months when they feel they are getting a raw deal.