I think this is referring to the bioclimatic zone, as in this text below (FAO document for ecuador):

Los pisos altitudinales en que se ha dividido el país, son similares a las regiones
latitudinales que se distribuyen en el globo terrestre. De este modo, la clasificación
bioclimática del Ecuador, en función de gradiente térmica, tiene las siguientes
categorías:
Piso Tropical (para la vertiente Occidental de la cordillera fluctúa de
0 a 300 msnm y para la Oriental de 0 a 600 msnm);
Piso Subtropical (de 300 a
2 000 msnm para la vertiente Occidental y de 600 a 2 000 msnm para la Orien-
tal);
Piso Temperado (de 2 000 a 3 000 msnm para las dos vertientes); Piso Sub-
temperado (de 3 000 a 4 000 msnm); y, Páramo (sobre los 4 000 msnm)

http://74.6.239.67/search/cache?ei=UTF-8&p="Piso tropical" e...




This document about Nepal discusses the same sort of breakdown by altitude, calling the regions "belts" or "zones":

Phytogeographical studies of Nepalese flowering plants indicated that the lower sub-tropical and tropical bioclimatic belts under 1,000m altitude are floristically related to south-east Asia-Malaysian and Indian floristic regions. The middle belt composed of upper subtropical and temperate bioclimatic zones from 1,500m to 3,000m altitudes are floristically related to Sino-Japanese floristic region, and the area above this belt is composed of the subalpine and alpine zones are floristically related to the central Asiatic floristic region. The floristic relations of Nepal with Saharo Zambian and Irano-Turanian floristic regions are feeble in magnitude in the western parts of Nepal.