For a recent lecture in Rangeland Grasses, I was hoping to include a slide or two showing the decrease in grassland at the Jornada (Jornada Experimental Range and Chihuahuan Desert Rangeland Research Center). A recent paper documents this decrease:
Gibbens, R.P., R.P. McNeely, K.M. Havstad, R.F. Beck, and B. Nolen, 2005. Vegetation changes in the Jornada Basin from 1858 to 1998. Journal of Arid Environments 61: 651-668.
So, I thought, I’ll just pull the maps from there. Unfortunately, it is not that simple. One problem is common to almost all published vegetation maps: you can’t just look at the map and say, “OK, all this is grassland, and here it is quite extensive in 1918, and then there’s not much of it in 1998,” because the color system used is fairly confusing and you’d have to spend a minute pointing out that yellow, light pink, dark tan, etc., are grassland while light green, light blue, light tan, etc., are shrubland. There has to be a better way to do this! Here’s the figure (just for the JER; there’s a separate figure for the CDRRC), see how long it takes you to figure out the extent of grasslands at any point in time:
Looking at the map more closely I realized that, in this case, there are further difficulties. The map legend does not match the colors used. Here’s a comparison of the colors used in the legend and in each map:
That dark pink for “other grasses” never occurs in the maps; two different browns not in the legend are used. Presumably those browns mean “other grasses”, but I don’t know. It is odd that most, but not all of the areas of Scleropogon brevifolius (burrograss; light pink) in the 1918-19 map become brown in the 1928-29 and 1998 maps. It’s hard to tell if this is supposed to indicate a change in vegetation, or is simply an error in producing the map. I assume the latter, because similar maps were produced again, in:
Havstad, K.M, L.F. Huenneke, and W.H. Schlesinger (editors), 2006. Structure and Function of a Chihuahuan Desert Ecosystem. Oxford University Press, New York, NY.
Here, that difficulty seems to be solved. So far as I can tell, the legend matches the maps, and all those areas of Scleropogon brevifolius in 1918-19 are still Scleropogon brevifolius in 1928-29 and 1998 (as they continue to be today). However, this map is more precise about what the dominant species is (e.g., listing Sporobolus airoides, Sporobolus flexuosus, Sporobolus nealleyi and Sporobolus spp. instead of just “Sporobolus spp.”), which is good, but exacerbates the problem of colors that are difficult to distinguish to the point that much of the map is unusable. Worse, the maps are small and resolution is poor; and I have a physical copy of the map rather than a .pdf. There is no way, AFAICT, to use these as workable images in a powerpoint presentation. Luckily the GIS layers are available on the Jornada website, so I can make my own maps (which I may do as time goes by).
So, that’s frustrating. Here’s another difficulty: “Plant nomenclature follows Allred (2003).” Except, it doesn’t. For instance, Allred recognizes Gutierrezia microcephala and Gutierrezia sarothrae, while the Jornada literature uniformly lumps both as Gutierrezia sarothrae. And then there are the big questions that remain even if we leave this niggling behind us:
1) How exactly do we determine what the dominant species is in the first place? The standard has always been that you look out at a landscape and guess (and the methods of the 2005 paper are consistent with this). In clear cases, this should work. In other cases, it won’t, and you’ll get different answers depending on who’s doing the fieldwork (or, for that matter, what time of year, how wet the year is, etc.). An objective measure, and some account of seasonal / yearly variation as well, is needed.
2) How much does that tell you, anyways? If we want to understand the distribution of vegetation, don’t we want to know more than just one (or two–the 2006 book includes separate maps for the second most abundant species) of the species at a site? This is a constant frustration of mine with ecologists and land management agencies. Interest is almost always focused on the most abundant species and, due to the Endangered Species Act, the least abundant species. The other 95% of the biodiversity is uniformly overlooked.
Lest you think I am singling out the Jornada for criticism, I also searched for vegetation mapping information for the Sevilleta LTER. Results were worse. A vegetation map was created, but never published. At one point it was online, but at present it is not available through the Sevilleta website; you can find a fairly illegible jpeg through archive.org, but that’s it. Despite 25 years of research and three long-term NSF grants at the Sevilleta LTER, so far as I can tell there simply is no published account of the vegetation, nor one available online. This is disappointing. Poking around online also led me to this article:
Weiss, J.L., D.S. Gutzler, J.E. Allred Coonrod, C.N. Dahm, 2004. Long-term vegetation monitoring with NDVI in a diverse semi-arid setting, central New Mexico, USA. Journal of Arid Environments 58: 249-272.
From which I quote:
“The objective of this study is to examine 11 years (1990-2000) of seasonal and inter-annual variability of NDVI in a diverse semi-arid setting in central New Mexico, USA, that includes six different vegetation communities: Great Plains / desert grassland (GPGrslnd), Chihuahuan Desert (ChiDes), piñon-juniper woodland (PJWdlnd), juniper savanna (JunSav), Colorado Plateau shrub-steppe (CPShbStp), and Colorado Plateau grassland (CPGrslnd) (Moore, 1989-2001).”
OK, we have six different kinds of vegetation. You might ask: How are they different, exactly? How do we know that they’re different? And just how much of the variation in vegetation at these sites is captured by those designations (e.g., does “juniper savanna” mean just any kind of juniper with any kind of grass, or is it always Juniperus monosperma with Bouteloua gracilis; and what about the other 95% of the plants in that habitat)? In particular, I was curious what exactly the difference between Great Plains grassland and Colorado Plateau grassland is at the Sevilleta. However, in the paper there is no explanation of any of these points, nor any citation indicating how you could find that information (the Moore citation is for meteorological data). Near as I can tell, “Great Plains grassland” must refer to communities dominated by Bouteloua gracilis east of the Rio Grande, and “Colorado Plateau grassland” must refer to communities dominated by Bouteloua gracilis west of the Rio Grande, but is that a meaningful distinction? Are these actually distinct plant communities or just different names for the same thing? I don’t know, but it’s the kind of basic knowledge I can’t imagine conducting ecological research that relies on vegetation classifications (nevermind running an LTER site) without. And yet… well, maybe it’s all in that unpublished vegetation study.