Sunlight is essential for all plants, and in my previous post, I talked about species of Solidago that do best in full sunlight, a habitat type that is rare in regions dominated by forest. In the middle of the continent, of course, sunlight is easy to find. But there is more to a species’ habitat than sunlight.
The sunny habitats span an enormous range of temperature and moisture across the North American continent. Canada goldenrod (S. canadensis) grows in open places in nearly every state and province north of Mexico. In Alberta, summer day length peaks near 18 hours, while in Texas, it is a little over 14 hours. Temperatures differ accordingly. In Ohio, mean annual rainfall is roughly a meter. In New Mexico, it is one third of that.
Canada goldenrod’s temperature tolerance, growth rate, flowering time and drought resistance vary enormously, perhaps because of phenotypic plasticity, or because of local adaptation, or both. In any one location, of course, it will occur in locations that have what it needs (for example, relatively dry soil farther east, relatively wet soil farther west). But even with some microhabitat preferences, this species is one of many goldenrods, and many plants in general, that have extraordinarily wide environmental tolerance.
But most goldenrods live in fewer places and in a narrower range of conditions.
Some species of goldenrods do best in shade. Solidago caesia (blue-stemmed goldenrod) and S. flexicaulis (zigzag goldenrod) are two of those in eastern North America. They are not huge plants, but they can be quite abundant, nearly always beneath a canopy of trees. They bloom in summer and autumn, so they are not spring ephemerals that bask in sunlight before the trees leaf out. No, these are shade-loving plants, in stark contrast to many of their congeners.
Some other shade-dwelling species of note live in eastern mountain ranges. Solidago ouachitensis, as the name implies, is found only in the Ouachita Mountains of Oklahoma and Arkansas, preferring forested areas on north-facing slopes. Not to be outdone, gorge goldenrod, S. faucibus, lives up to its name by living down in the gorges of the middle and southern Appalachian Mountains. Probably the most extreme example is S. albopilosa, whitehair goldenrod, found only in the Red River Gorge of eastern Kentucky, only under overhanging rock ledges, and only in sandy soil that is shaded, but not deeply shaded. Perhaps it is no surprise that this species is the most endangered of all the goldenrods in North America.
Some of the sun-loving goldenrods can grow also in the woods. Susan Beatty and her students often found S. juncea (early goldenrod) and S. rugosa (wrinkleleaf goldenrod) under tree canopies in upstate New York. These two species are common in neighboring old fields, and there seeds drifted in among the trees and managed to take root. In Gray’s Manual of Botany, Fernald notes that S. ludoviciana (Louisiana goldenrod) is another species that can grow in “open woods,” though when they do, the plants are “mostly sterile.” But even if unable to flower, goldenrods can usually spread vegetatively (a topic for a later post).
The term “open woods” is used often to describe goldenrod habitats, but it covers an enormous range of plant communities. At one extreme, the openings are like those mentioned in my previous posts, gaps in the canopy resulting from the death of one to many trees. Those gaps are doomed to close, but for a while, there is enough sunlight for non-forest plants to grow. Solidago rugosa seems particularly good at finding such gaps.
In other places, “open woods” means that the trees are spaced more widely than in a typical forest, forming a woodland without a completely closed canopy. Pines and oaks are common trees in such woodlands, and goldenrods do well in the many spaces between the trees. As the distance between trees becomes even greater, woodlands becomes savannas, and there is far more open space than shade. Then sunlight is rarely limiting.
While some goldenrods seem to care hardly at all about the quality of the soil, others are quite picky.
Fernald (Gray’s Manual of Botany) notes that several species are found in “rich” soil, though I’m not sure precisely what that means. A soil can be rich and dry, so richness is not moisture. My best guess is that he was referring to soil depth, especially depth of the organic layer. If I am right, then talus slopes would be the least rich, with far more rock than soil. Only the crevices between the rocks provide access to soil, so the plants are inevitably sparse. Wright’s goldenrod (S. wrightii) often grows on talus slopes, as do many of the high-elevation goldenrods (see below).
Soil pH is another soil characteristic that is important for several species. Houghton’s goldenrod (S. houghtonii), Ohio goldenrod (S. ohioensis), and Nevada goldenrod (S. spectabilis) all occur in wetlands that are alkaline. Western rough goldenrod (S. radula) and Gattinger’s goldenrod (S. gattingeri) grow in limestone soils, but not in wetlands. In contrast, bog goldenrod (S. uliginosa) prefers stereotypically acidic bogs.
Gorge goldenrod, mentioned earlier, often grows beneath hemlock trees (Tsuga canadensis). Not only do these conifers produce deep shade, they also alter the soil chemistry (as do essentially all plants, but hemlock is more significant than most). Susan Beatty (also mentioned earlier) found that the forest understory community below hemlock tended to be measurably different from that under other species of trees. Something about hemlock-modified habitats seems to suit gorge goldenrod.
In Oklahoma and Texas, high plains goldenrod (S. altiplanites) can grow on gypsum soils, high in calcium sulfate but otherwise nutrient-poor. Farther west, Guirado goldonrod (S. guiadonis) grows in wetlands on serpentine soils in southern California. These soils are low in nutrients, but also high in heavy metals, a seriously stressful combination of conditions. These two species are outliers among goldenrods, but a clear testament to the potential for evolutionary adaptability in Solidago.
North American goldenrods can be found from Alaska and Nunavut in the north to at least the Isthmus of Tehuantepec in the south. But no one species lives everywhere, and some have distinct temperature preferences.
Solidago leiocarpa and S. spithamaea live at high elevations, the former in the northeast, the latter in the Great Smoky Mountains. Another species (S. multiradiata) has at least three common names, all of them suggesting cold: alpine goldenrod, northern goldenrod, and Rocky Mountain goldenrod. The distribution map is impressive – all across Canada (plus Maine), then south into the Rockies, Cascades, Sierras, and some sky islands in the southwest, but always at high elevations. None of these species live where there are prolonged high temperatures.
There are no true desert goldenrods, but some of them can tolerate intense sunlight at subtropical latitudes. None has evolved succulent tissues anything like those in drought-adapted Senecio, but they are certainly tough. Within temperate latitudes, goldenrods can be found in all but the hottest places.
Water is supremely important for all plants, and goldenrods get their water from the soil in which they grow. But it’s not just soil moisture that matters for some species. I will devote another post to goldenrods and water.