On its way to flowering, a goldenrod plant grows upward. Sometimes they grow really tall, two meters or more, and sometimes they are fairly short. Height differs between species and between locations.
The apical meristem adds cells at the top of the growing plant. These cells become stem, leaf, and eventually, flower, seeds, and fruit. The apex also inhibits the growth of lateral meristems lower on the plant. The ideal growth direction is up. Only when it’s time to produce flowers might multiple stems be allowed to branch out and form the inflorescence (or not – many species have little or no branching to support their flowers).
But things can go wrong as the stem grows. If the apex is destroyed by a disease or herbivore or physical damage, then that avenue for growth is gone. But all is not lost, far from it. Those abundant lateral meristems, some not far below the apex, can take over once the inhibition of the apical meristem is gone.
For a species that has short branches in its inflorescence, as in Solidago puberula (downy goldenrod), a damaged apex leads to the production of branches far larger than normal, and the earlier the damage occurs, the longer those branches are. I know this because, many years ago, I removed the apical meristems from downy goldenrod plants at various times during the growing season and watched what happened. Apex-free plants looked quite different from intact plants. But as much as they grew, they couldn’t quite produce as many flowers as intact plants. They never caught up, though they certainly tried.
This June, there are already some stems that have lost apical dominance. Here are two Solidago arguta stems, one that has an intact central stem growing taller, and one that has lost its apex and started to grow lateral branches. This species normally produces a wide, branched inflorescence even when the apex remains intact. We’ll see how different they look by the time they flower.
Spring happens every year, and every year it is amazing. This is what happened to one small oak tree in 2021.
Why do things hang on when there is no hope? In the lines of Robert Frost: “The leaves are all dead on the ground/ Save those the oak is keeping” (the poem is “Reluctance”).
Usually, seasonally deciduous trees drop their leaves before the harsh season arrives – cold and/or dry – when the leaves cannot survive. The process of leaf fall is abscission, the thinning and breaking of cell walls where the leaf meets the stem.
But some leaves are marcescent, remaining on the tree after death. They have an abscission layer right were it is supposed to be, but it remains strong enough to keep the leaves on the tree. These are the leaves “the oak is keeping.”
The leaves leave eventually. They don’t stay on among the new green leaves. When do they fall? One source claims that the initiation of new leaves (the swelling of leaf buds) is the signal that sets the old leaves free. The authors suggest that some early-season hormone (probably from the growing buds) travels through the tree and stimulates the completion of abscission. Poof, no more brown leaves.
Their claim became the hypothesis for my observations in 2021. I expected the dead leaves to remain fast until the buds started to expand. Then the transition from brown to green would be quick, brown down, green up.
What actually happened?
There were quite a few leaves on a small red oak on February 28. I photographed the tree nearly every day until May 27 to see what happened. I also photographed some buds high on the tree (too high to measure directly), and measured the lengths of some buds on low branches that had no marcescent leaves.
By March 15 or so, half the leaves present at the end of February had fallen. In another two weeks, roughly a quarter of them remained. By April 10, it was down to about ten percent.
But it’s not quite that simple. Early on, the leaves broke off at the petiole (leaf stalk), not the abscission point where the petiole meets the stem. The petiole had weakened more than the abscission layer. On March 27, there were petiole stubs still on this stem, even though a large majority of the leaf blades had fallen. Technically, the leaves had not yet abscised.
But abscission was coming. Two days later, one petiole from the stem had cleanly abscised. By April 6, another was gone (the little wasp had nothing to do with it). Once the abscission layer had weakened sufficiently, the stub of the petiole could break free, leaving the typical leaf scar.
The weakening of the abscission layer is obvious in this April 14 photo
On April 16, we got several inches of wet snow. The next day, all the petioles were gone.
The late snow had accomplished what all the snows of winter had not. Only when the tree had weakened its abscission layers could the petioles fall off.
Several hormones affect abscission, some by inhibiting it, some by stimulating it. Petioles were breaking before the hormonal pathway had kicked in. But eventually, it did, and then the once stubborn stubs became stubborn no more.
What were the buds doing as the leaves were falling?
Buds low on the tree, where there were no marcescent leaves, remained essentially the same length through May. The fluctuations were mostly (entirely?) from my inconsistency in measurement, though there might have been some actual swelling and shrinking with humidity and internal moisture (if internal moisture changes, I don’t know). By the end of May, there was no sign that these buds were going to do anything at all. Was the tree sacrificing its lower branches? They had to go sometime, maybe this was it.
Buds higher on the tree, where the marcescent leaves were present, changed little through March, but began to swell in April. (I used a distinctive lenticel on the branch as a unit of measurement because the buds were out of my reach.)
But not all buds changed at the same rate. Some were only swelling while others had already burst with new tissues showing. These tissues turned out to be stems and leaves, no flowers. The buds that swelled only a little stopped swelling and remained intact by the end of May. Will they burst later? We’ll see.
Most marcescent leaf blades were gone before the buds began to swell. But true abscission happened later. The buds still hadn’t begun to lengthen when the first petioles fell, but the first sign of enlargement was evident shortly before the snow pulled off the last of the leaves. I suspect that hormones were stirring well before the buds started to expand, initiating the formation and growth of tissues at essentially the same time as the marcescent leaves were being set free.
The new leaves and stems were tinged with red, the brief pulse of color before they were fully expanded (see Robert Frost: “Nothing Gold Can Stay”). And they expanded really fast.
The new stem elongated impressively in ten days. The new leaves took on their distinctive oak shape and became bright green.
Some buds among the new leaves had still not burst. They had begun to swell with the others, but then stopped. What is their fate?
Buds low on the tree showed no sign of growth, and the branch they were on – a significant branch on this small tree – had drooped about 25 centimeters from March to May. What is going to happen to the buds and this branch?
There remain many things to observe.
A FEW MORE WORDS
Marcescent leaves are remnants of a bygone year, a season of productivity that went dormant for the winter. They have no reason to linger when the new season bursts forth, which is when they finally fall off. But perhaps we shouldn’t be surprised that there are a few that don’t play by what we think are the rules. Here are two brown leaves that were hanging on well into May.
Many arthropods (insects, arachnids, and others) live and hide among the tissues of trees. Some blend in so well that we miss them. I almost missed this spider among the buds in April. It was among these buds for three successive days, then was gone. Perhaps it is still somewhere on the tree. I don’t expect I’ll find it.
One more thought: This red oak tree, though small, has a lot going on: lingering leaves, buds that burst, buds that swell, buds that remain small, branches that burst forth with green, and branches that droop and remain gray. Such small trees might be useful – and accessible – as subjects for explorations of developmental physiology. If that is your field of interest and expertise (it is not mine), go for it.
Earl Berkley 1931. Marcescent leaves of certain species of Quercus. Botanical Gazette 92: 85-93.
Robert Hoshaw and Arthur Guard 1949. Abscission of marcescent leaves of Quercus palustris and Q. coccinia. Botanical Gazette 110: 587-593
The goldenrods are sending up their stems in spring. Or not.
This is a growing stem of early goldenrod, Solidago juncea, and it might get to be a meter tall.
This is also early goldenrod, but it won’t get tall. It will remain a rosette, a cluster of leaves close to the ground. The leaves are large, sometimes quite large, but the plant remains short.
This is another species, probably S. arguta, sharp-leaved goldenrod (I’ll check my identification later in the season). Sometimes they grow even taller than early goldenrod.
But sometimes they remain a rosette.
Goldenrods are perennials, but even so, a plant that produces a tall stem in one year might be a rosette the next year, and vice versa. Or the plant will be the same form this year as last. I don’t know what triggers a switch, or triggers a repeat, but the growth form is one or the other each year, not in between.
Other goldenrods nearly always produce all tall stems, almost never any rosettes. These clusters of goldenrods (probably S. rugosa and S. canadensis) are all growing tall. Next year, they will grow tall again. Some plants might not grow tall enough to flower, but they will have an elongated stem.
In the key for the identification of goldenrods in Gray’s Manual of Botany, Merritt Fernald split major groups of goldenrods according to how their leaves and stems grow. On some, the basal leaves are the largest, and the leaves decrease in size dramatically going up the stem – if there is an elongated stem – or remain as a rosette.
In contrast, the leaves on other goldenrods are similar in size up the stem, and the plants hardly ever form rosettes of leaves.
What’s going on in each growth form? Defining a few terms will help:
What’s a node? That’s a location on a stem where one or more leaves are attached.
What’s an internode? The stem between nodes.
What’s a rosette? A stem that didn’t elongate its internodes.
What makes a stem tall? One or more elongated internodes.
Somewhere in the ancestry of goldenrods, one or more lineages went all in on elongated internodes, and others retained internode plasticity, able to elongate them or not. I don’t know the underlying mechanisms of internode development, but I am pretty sure hormones are involved, probably gibberellins. In some, they are always on (expressed). In others, they can be turned on or off, apparently early in the growth of any one stem.
I think this difference is impressive, one more aspect of the genus Solidago that intrigues me.
This is the only website I’ve yet found that points out this developmental contrast. From my perspective, its inclusion speaks well of the authors of the website.
It’s time to kill the lie: goldenrods never have and never will cause hay fever.
Here’s why not:
Hay fever, and any seasonal allergy, is caused by pollen from plants that are pollinated by the wind, or by spores from plants and fungi that release spores into the air. In late summer, the main wind pollinated species are ragweed and perhaps some grasses.
Goldenrods are not pollinated by the wind.
Goldenrods are pollinated by insects. Their pollen grains are heavy and somewhat adhesive so that they stick to the bodies of bugs.
Goldenrod pollen grains are too heavy and too sticky to blow around.
Goldenrod gets blamed for hay fever because it blooms at the same time as ragweed. Ragweed has green, inconspicuous flowers that most people miss.
Goldenrod has bright yellow or white flowers that are conspicuous and hard to miss.
Ok, have we got it now?
Repeat after me:
Goldenrods do not cause hay fever.
GOLDENRODS DO NOT CAUSE HAY FEVER.
GOLDENRODS DO NOT CAUSE HAY FEVER.
If you don’t believe me, use any search engine to check goldenrod and hay fever.
Or just take a pleasant walk with Matt Candeias to see several species of goldenrods, including a reinforcement of the point of this post: goldenrods do not cause hay fever.
More sunlight, warmer air, longer days, it’s springtime. Having waited underground all winter, it is time to grow.
Early goldenrods (Solidago juncea) are named for their early flowering, but some could also be named for their early emergence. They have begun to grow along our road, down toward the highway where there is more exposure to sunlight. A few old stems from last year mark the patch, but marked or not, up they come.
It’s April, and really too soon to see the stems elongating from the rosettes of leaves. But it won’t be long.
Perhaps I shouldn’t be surprised, but already, there are small holes and notches chewed into a few of the leaves. The insects are emerging, too.
I’m also watching the patch of Canada goldenrod (S. canadensis) to see if any stems are evident among the early leaves. Not yet. But it won’t be long.
Growth of plants in the Northern Hemisphere will, from April to August, pull so much carbon dioxide out of the air that the concentration measured at Mauna Loa Observatory in Hawai’i will decrease. It decreases every summer, only to be driven back up ever higher when the growing season is over.
Much of the annual growth of goldenrod will die and decay, returning carbon dioxide to the atmosphere. But some will remain underground in the roots and rhizomes and soil organic matter. I don’t know what the net result is for a field of goldenrods, but I suspect that there is some sequestration of carbon in the soil before they are overtopped by trees. In grasslands, they just kept storing it for thousands of years until humans broke the sod for their farms. Never underestimate a goldenrod.
How could I have forgotten one of the most conspicuous winter features of dead goldenrod stems: galls of the goldenrod gall fly? The answer: easily, because even though they are dirt common in New York and Pennsylvania (and probably elsewhere), they are not common in central Massachusetts. I had forgotten about them until I stumbled across a place where they were abundant enough for me to notice. So it’s time to talk about old galls.
The goldenrod gall fly, Eurosta solidaginis, produces one generation per year, at least in the northeastern parts of the United States where I’ve seen it. Right now (April), the pupae are living inside of the galls that the larvae produced last summer. The stems were alive and growing when the eggs were laid, and the larvae induced the stem tissue of the host goldenrod to swell, forming the walls of the gall with a hollow chamber inside where the larvae live and feed. When the stems died, the larvae did not – the hardened gall protected them pretty well from physical harm. The galls and flies got cold in the winter, which didn’t bother the galls at all, since they were dead. It didn’t bother the larvae, either, because they have an array of internal antifreezes (complex alcohols) and well-placed water-transport proteins that protect the cells from freezing (some fluids of the larvae might actually freeze, but in locations that don’t matter). At some point, the larvae pupated inside the gall, and waited.
The pupa is inside the reddish brown puparium. It’s surrounded by fungi, and has a groove on it. Is it viable? Only time will tell.
This gall was on a stem that was bent but not fallen in April. The hole shows that something has exited the gall. What was it? The hole is neat, so it’s not the result of birds pecking away at the gall to get to the insect inside, or of predatory beetle larvae boring in to find a meal. Was it an adult fly? Or an adult parasitoid that killed the fly? If we look at the pupa inside the gall, we might be able to tell. If the exit from the puparium is neat, the fly did it. If not, an enemy did it.
Whatever came out of this gall, the gall has been through two winters. That’s a tough gall. If it had fallen to the ground before the fly emerged, decay could have been a problem. I wonder whether the presence of a gall is associated with a tougher stem so that the gall stays above ground long enough for the fly to emerge. Do gall-inhabited stems remain upright more often than gall-free stems? If so, then I also wonder whether the larva stimulates not only the swelling of the gall but also the hardening of the stem?
The exit hole is neat and circular because the larva, before it pupated, bored a tunnel nearly all the way out of the gall. The pupa can’t even move, much less bore its way out, and the adult has weak mouthparts (some would say none at all). The newly emerged fly has to swell its head to push its way through the last thin wall of the tunnel provided by the larva. If all is well, it will find a mate, and then females will find growing stems of Canada goldenrod, tall goldenrod or giant goldenrod on which to lay their eggs.
There are many good sources information about Eurosta solidaginis, so you should have no trouble learning more about them. I will revisit these flies when new galls form this summer.
Warren Abrahamson and Arthur Weis. 1997. Evolutionary Ecology across Three Tropic Levels: Goldenrods, Gallmakers, and Natural Enemies. Princeton University Press.
(If you plug this into Google Scholar, you get a link to all the works that cited this seminal book; some of them involve Solidago)
Of course all plants need water to grow, and goldenrods are no exception. But goldenrods vary widely in their demand for water, and some are only found near particular types of water.
At one extreme, there are goldenrods that thrive in or near wetlands (see previous post on Soil). Some prefer alkaline environments, others prefer acidic, but all require wet soil.
Toward the other extreme, prairie plants grow in full sun with a modest amount of rainfall. But their exposure (i.e., absence of a tree canopy) ensures that atmospheric moisture condenses during radiative cooling at night. Much (most?) of the dew evaporates as the morning warms up, but some of it drips or runs into the soil and helps the plants endure the times between rains.
In these upland environments, there is variation in tolerance and/or preference for levels of soil moisture. Patricia Werner and Robert Platt examined five species of goldenrods in a prairie preserve in Iowa (a sixth species, grass-leaved goldenrod, is now in the genus Euthamia). The species had broad overlap across a range of soil moisture, but each had a distinct peak where they were most abundant (see the graph). Would each species shift its peak if one or more of the other species were absent?
Giant goldenrod, Solidago gigantea, (#5 on the graph) is on the wetter end of the moisture gradient, which is consistent with my experience in upstate New York. Giant goldenrod was in nearly every old field I explored, but nearly always in parts of that field that tended to be wetter than the rest. When the soil elsewhere was firm, the soil around giant goldenrod was softer, or if it was also firm, it was darker from the higher moisture content. Throughout its extensive range, giant goldenrod occurs where the soil holds sufficient water for its growth (but not too much). Solidago canadensis (#4) and S. speciosa (#3) overlapped with giant goldenrod, but grew well in drier conditions.
Gray goldenrod, S. nemoralis, (#1 on the graph) grew on the driest soils in Werner and Platt’s study, which is pretty impressive. I often find gray goldenrod in New England in the driest soils near roads. They are always short, but they still manage to flower and produce seeds.
Missouri goldenrod, S. missouriensis, (#2 on the graph) requires a bit more water than gray goldenrod, but also has a reputation for being tolerant of drought. In the Dust Bowl of the 1930s, when many grassland plants were dying for lack of water, Missouri goldenrod was said to survive well, and even spread into areas left bare after the demise of other plants. Or maybe it was just more conspicuous when other plants were gone. Whether it spread or merely survived, it depended on its deep roots (over two meters down) to harvest whatever moisture was available deep below the surface. It’s an impressive contrast to those species that are restricted to wetland habitats.
Proximity to water
Some goldenrods are most abundant near the seacoast, perhaps benefitting from climate moderation. Seaside goldenrod, S sempervirens, ranges from the Canadian Maritimes into the states along the Gulf of Mexico. Its leaves tend to be a bit leathery, and it almost certainly has some degree of salt tolerance from the sea spray blown inland by storms. It also does well along the shores of lakes, though it seems not to have gotten to these shores on its own. People have planted it far from the ocean, perhaps for its long-lasting flowers.
Wand goldenrod (S. stricta) is another coastal species, found from New Jersey through the Gulf of Mexico, growing in wet sandy soil. Pine barren goldenrod (S. fistulosa) grows in similar habitats as far south as Louisiana. Carolina goldenrod (S. pulchra) is also in similar habitats, but only in three counties of North Carolina (who knows why?). On the face of it, sandy soil shouldn’t be wet, but there are plenty of places where the water table is near the surface (coastal sands can be excellent aquifers) or where there is something impervious below the sand.
Elliott’s goldenrod (S. latissimifolia) is yet another coastal species, growing from Nova Scotia to Alabama in wetlands, some of which are brackish, a clear sign of salt tolerance. None of the habitat descriptions mentions sand, though, so its specific habitat seems to differ from some of the other coastal species.
Small’s goldenrod (S. pinetorum) and twisted-leaf goldenrod (S. tortifolia) grow on sandy soils, but farther inland where the sand is a good deal drier than the soils that wand and pine barren goldenrods need.
Hairy-seeded goldenrod (S. villosicarpa) always grows near estuaries, only in North Carolina, especially in places where the trees have been felled by hurricanes, leaving a lot of sunlight until the trees grow back. This combination of conditions occurs in just a handful of locations.
Solidago spathulata has somehow earned the common name of coast goldenrod, perhaps because it occurs only on dunes and headlands in a few places along the coast of California and Oregon. Like its eastern congeners, Small’s and twisted-leaf goldenrod, it manages to survive in dry sandy soils.
Solidago simplex, sticky goldenrod, grows in a wide range of harsh habitats, but one variety (gillmanii) is found only on the dunes around Lake Michigan and Lake Huron. We’ve seen this combination before: sand and large bodies of water. But the water here is entirely fresh, just to add to the variety of habitats exploited by goldenrods.
But wait, there’s more. At least two other species live in close proximity to water, but only water that is flowing in streams. They are riparian. One is rock goldenrod, S. rupestris, scattered in river valleys from Tennessee to Pennsylvania. The common name suggests that it prefers places where the river has washed away most of the small particles like soil (!). Such places are not great for other plants, so these goldenrods might benefit from the absence of competitors.
The other species, plumed goldenrod (S. plumosa), probably depends on riverbanks scoured by floods where few other species can survive. I say “probably” because there are hardly any plants of this species still alive, and those are found only along the Yadkin River in (you guessed it) North Carolina, where dams might be reducing or eliminating the scouring floods on which it depends.
So what can we conclude about water and goldenrods? Some need a lot, some can survive on a little, and many are in between. Some need to be near large bodies of water, perhaps for the reduction in seasonal temperature fluctuation, perhaps for the humidity, perhaps even for a bit of salt. Some depend on disturbance, perhaps to open up the canopy and let in sunlight, perhaps to keep the number of competitive roots down to a tolerable level.
Goldenrods are definitely diverse. Since they shared common ancestry in the distant past, they have diverged to live in different places, under different conditions, and with different requirements. Sure, there are other groups of plants with an even wider diversity of habitats and adaptations, but goldenrods are pretty impressive. Before I began posting about them, I had no idea how varied they were. I already liked goldenrods. Now I like them even more.
Patricia A. Werner and William J. Platt. Ecological Relationships of Co-Occurring Goldenrods (Solidago: Compositae). The American Naturalist, Vol. 110, No. 976 (Nov. – Dec., 1976), pp. 959-971
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.
The most conspicuous goldenrods in many parts of North America are those growing in fields that once were used for farming, so-called old fields (Catherine Keever was one of the first ecologists to carefully document old-field succession). Some of them (perhaps most) grew well in the grasslands and prairies that preceded European-style agriculture in the middle of the continent, places with lots of sunlight because woody vegetation was grazed and/or burned away, or couldn’t grow in the dry conditions. With plenty of light, rainfall, and good soil, some goldenrods can grow two meters tall, while other species are shorter.
When Europeans began to colonize the east coast of North America, most of the land they encountered was forested. As they spread agriculture and cities westward, up to 90% of the forest was cut in any one location. We can see the results of deforestation in the sediments of lakes and other bodies of freshwater. Most trees and many weeds are pollinated by wind, not animals, and their pollen blows all over the place, including into the water. The outer covering of the pollen grains is highly resistant to decay, and the grains remain identifiable (to some degree) for thousands of years. Researchers (Margaret Davis being one of the most notable) can extract, identify and determine the age of the pollen laid down in these sediments. What do they find? When settlers removed the trees, tree pollen decreased precipitously. At the same time, ragweed pollen shot up like a rocket, so much so it the Ambrosia pulse (Ambrosia is the genus of ragweeds) is now used to mark the arrival of European-style agriculture in any part of eastern North America. Ragweed thrived in the sunlight and exposed soil of farms. Before that, it was extremely rare.
Goldenrod pollen is heavy and sticky, great for glomming onto passing insects, but terrible for blowing in the wind. There is no pollen record for goldenrod. But the sun-loving goldenrods might have behaved much like ragweed, their sun-loving herbaceous neighbors. Or not, because their seeds disperse differently. Goldenrod seeds (fruits, technically) disperse in the wind, while ragweed fruits have no hairs (pappus, technically) but its fruits contain some oils that can attract birds.
But where were these sun-loving species before the land was cleared, before there was abundant available sunlight? We don’t find them today in the deep shade of the forest (with few exceptions), nor was there more than a trace of ragweed pollen for most of the time the forest was present after the ice age. These weeds must have had difficulty finding places to live when the forest was intact.
But their lives were not impossible. There are always some places where trees die and light reaches the forest floor for a few years until other trees fill in the gap. Sometimes, the gaps are no larger than a single tree. Other times, they are huge when a storm is intense (a hurricane, or a tornado, or just a nasty regular storm). Or maybe a pest (disease, insect, etc.) had its way with some trees, maybe a lot of trees.
One of these enemies might have been beavers. These busy rodents build dams where there isn’t already a pond, and the resulting flood kills trees that were living near the stream. Beavers also gnaw down trees to reach the twigs that they store for winter. In short, beavers can make major modifications of the landscape.
After some number of years, beavers are likely to deplete the supply of preferred trees within easy access of the pond. Then they move. Or they might starve in a severe winter, or some predator might manage to catch them. However it happens, when the beavers are gone, the dam will fail, the pond will drain, and the land will revert to meadow.
I’m just speculating here, but could the areas thinned by beavers, and/or the meadows that grow when beavers were gone, have been habitat for those goldenrods that thrive in lots of sunlight? There is no pollen record to test this hypothesis, so for the moment, I haven’t been proven wrong.