Different approaches have been used so far to define STZ, such as the ecoregional approach which involves the overlap of different layers (climate, geology, vegetation, soils, etc.) to find similar ecological conditions; the genetic approach, to study the level of population differentiation; and the adaptive approach using common-garden experiments to find the best adaptive plant population for a specific restoration goal (relevant publications: Vander Mijnsbrugge et al. 2010; Durka et al. 2016; Jørgensen et al. 2016).

Even if we know that "Nature ignores borders", it may be practical to define STZ at specific geographical-administrative levels.

What's the best approach to define STZ and what's the best level at which they should be defined, so as to ensure that movement of seeds for ecological restoration does not cause outbreeding in local populations?