Clinton Hart Merriam (1855˗1942) published on the concept of life zones over 100 years ago, beginning with 1892 and 1894 papers. But he was not the first to conceive of a spatially explicit relationship between the distribution of biota and climate. Möbius had coined the term 'biocenose' in 1877 to describe a community of living beings. Another early naturalist, Grisebach in 1838 introduced the term “formation” as a concept to embrace a large-scale plant community. Dialing even farther back, we find Alexander von Humbolt writing about “associations” in 1805.
The more recent versions of the life zones model cross-reference temperature (either as altitude or latitude) and moisture (as precipitation, humidity, and evapotranspiration). The results breakdown into life zones ranging from deserts to rain forests with all sorts of steppes, scrubs, woodlands, and forests.
Turning back to Merriam's work at the turn of the last century in the Southwest, we find him classifying NM's life zones as: Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic-Alpine. A 1898 review of Merriam's work (https://www.jstor.org/stable/pdf/1626459.pdf) points out the difficulties of working with these fairly simple models.
Today we see these life zones reworked for NM as: Desert, Grasslands/Woodlands, Transition, Coniferous Forest, Subalpine, and Alpine zones. For children and the young-at-heart, I recommend downloading this coloring book – https://www.wildlife.state.nm.us/download/education/conservation/coloring-books/Life-Zones-Coloring-Book.pdf.
Another more modern approach is the USDA's classification of landscapes by means of average annual lowest minimum winter temperature. That's a mouthful. Basically, they've mapped the US and color-coded it by how cold it might get in a typical year.
The USDA published these maps in 1990 and have revised them as of 2012 using 1976-2005 data. Here's the 1990 map for the north-central NM.
And here's the 2012 revision with its much higher resolution.
One important thing to notice is the obvious expansion of Zone 7b to now include most of metropolitan Albuquerque and Rio Rancho. One could be tempted to attribute this to climate change or maybe it's just better data.
While we're looking at patterns of vegetation and their relationship largely to minimum temperatures, it's informative to see what the historical cold spells have looked like.
You can definitely spot the cold air drainages in the state!
Now, just to provide for equal time for "fair and balanced," here's the USDA map of heat zones. These are the flip side of the cold-hardiness maps and show how many days an area is above 86°F.
But when one tries to map these layers onto the ecoregions or modern versions of Merriam's life zones, there is only superficial correspondence. That's because temperature, altitude, precipitation, and evapotranspiration are just a snapshot (more or less) of current conditions at a location. They don't capture the immense amount of geological and historical time that have had an impact on the plant communities, their soil, and co-evolving animal populations.
More to come...
