8 August 2025
Plant Ranges
Ranges are among the most diagnostic and ecologically salient features of plants. Frequently, the easiest way to establish the identity of a plant belonging to one of several closely related taxa is where you found it. Which species exists in this location? This may seem like cheating, but it’s not, and while it is risky to hang your determination on any single indicator, locality is often at least as good a diagnostic feature as any other.
The actual importance of range is much greater than fulfilling our need to pen a name on a specimen. Species, sub-species and ecotypes occupy their given ranges for a reason. For simplicity, let’s consider the ramifications of range with regard to individual species. A species’ geographic range, combined with environmental conditions within that range, encompass the ecological tolerances of the species, or perhaps more accurately, its ecological stronghold. We know from long experience that we can move plants to locations with drastically different conditions from their native ranges. While they may survive and grow, and perhaps even grow very well, it is frequently the case that these plants do not naturalize and spread. For such a plant, at some stage of its development, from gametes to zygote to seedling to mature plant, something is missing, or other plants simply perform better in this location at one or more critical stages of development. If we are outside the range of ecological strength for the species, it doesn’t thrive and expand.
A good example here in the Willamette Valley is Sequoiadendron, the giant sequoia, the natural range of which is limited to a small area in the southern Sierra Nevada, that among other differences, experiences tens of feet of snowfall in most years. This tree, when planted in the Willamette Valley, rapidly grows to enormous size and produces prodigious crops of viable seed. But in the wild and human-influenced environs of the Valley, I am unaware of any instance of this tree actually naturalizing. We can grow it in the nursery from this same local seed, plant out the seedlings and watch them grow to maturity. Why they fail to do all this on their own is unclear, but it probably has something to do with competition and growth during germination and early seedling development. No matter the reason, we are outside this species’ range of ecological strength.
Climate is not static, and therefore neither are species ranges. Plant population ranges are constantly following the shifting geographic outline of their zones of ecological strength, and have been since the Cambrian advent of embryophytes 500 million years ago. Many periods of rapid or even cataclysmic climate change have occurred in the intervening eons, some of these defining the end of one geologic era and the beginning of another. The flora of western North America has certainly been shaped by the more recent of these events, and species have been obliged to move great distances in order to avoid extinction as part or all of their current ranges suddenly have become inhospitable. The mechanisms of this mobility are exceedingly relevant to current discussions on “climate adapted plants” and they are the primary subject of my research. I will elaborate on this further in future posts.
Over the past few decades, summer temperatures in the Willamette Valley have increased about 1.5C while precipitation has decreased by 40%. These are now significant, long-term trends. As our climate is changing rapidly, we can expect most of our current flora – both native and non-native – to suffer declines or marked shifts in local abundance and distribution, as current distributions become increasingly decoupled from ecological strongholds. A few members of today’s local flora are likely to thrive and expand, but many niches that open in our new warm-mediterranean climate will be unoccupied until some new species arrives or we help it get here through assisted migration. This presents us with a window of opportunity, possibly, to influence what fills these niches. But they will only fleetingly be open, so there’s not a lot of time to think about it, unfortunately.
If we look at our situation realistically, we will see that we have already “assisted” at least 30,000 species of plants into the local area from all over the globe. Since there are only a few thousand proximate natives, it is likely that many or most of the new “winners” will not be from western North America. Under almost any scenario in the coming decades, our flora will undergo a radical transformation toward species from other parts of the world, and is in fact already on this path. In just the past 15 years, several non-local species have appeared at the farm, and some of these have exploded in frequency and cover. These exploding populations are indicative of what we might expect. One example among many is Melissa officinalis, or lemon balm, which I first noted about 2012 in a single location here at the farm. Lemon balm occurs naturally in warm-mediterranean regions of Eurasia, and as our local climate has veered rapidly from a temperate/cool mediterranean climate to a warm mediterranean one, we might expect such plants to thrive and increase in cover. Unsurprisingly, Melissa has come to dominate large areas of woodland understory and edge habitats at the farm and in surrounding forests, along with many other Old World weeds of warm mediterranean origin such as Hesperis and Geranium lucidum.
There are a few western native migrants as well. One good example is Psilocarphus tenellus, which is interesting enough on its own to warrant a future post. More on this later.
Meanwhile, many native and some non-native species locally are in various stages of decline. Native plants in decline locally include western redcedar, grand fir, red alder, red elderberry, salmonberry, several woodland herbs, various species of huckleberry and many others. Some non-native plants are also in decline, including temperate north Eurasian grasses such as Alopecurus pratensis and Phalaris arundinacea. In fact, many C3 grasses are in decline while some C4 grasses are now thriving in the Willamette Valley. Predominantly C4 genera such as Paspalum, Eragrostis, Digitaria and others are increasing in frequency and cover. Meanwhile, genera encompassing both C3 and C4 species, such as Leersia, may afford opportunities for hybridizing species to exchange favorable metabolic traits as a means to adapt to changing climate. No one has looked broadly at Leersia oryzoides, which seems to be thriving and increasing locally, to see if such an exchange might be occurring. This would be a great masters thesis if anyone is looking for a project.
The challenge in front of us now is how to use knowledge of local and regional floristics, current species ranges, past and predicted future climate profiles, plant population structure, modes of gene flow and other information to anticipate what might happen, and to suggest what we might do to push the needle in ways that benefit, or at least reduce damage to, local ecosystem functions.
For instance, salmonberry is an extraordinarily important shrub in coastal portions of the Northwest, blanketing hundreds of thousands of acres of riparian forests and mountain slopes. The flowers, fruit and vegetation of this plant support an array of birds, insects and other wildlife, and the expansive dominance of this plant means that any major change in its health and cover will have equally large impacts on ecosystem function.
Generally, plants with distributions similar to salmonberry are in decline locally. Range maps for western redcedar, western hemlock, red alder and several other critically important species look remarkably similar, and all of these that I have observed appear in decline here in our local area. Species with this particular range deserve special attention, both to conserve their current genetics and to anticipate their possible decline. How are we going to fill the gaps in the landscape that these dying plants leave behind? Are we going to explore the flora to our south and consider species that may be good surrogates or replacements? Or are we simply going to allow the tens of thousands of non-native introduced plant species – plants like Daphne laureola and Torilis nodosa (both Mediterranean basin natives) – to occupy newly emerging niches and establish local strongholds unopposed by proximate western North American species?
This is the choice we face, and our choice so far has been to focus primarily on conserving the local flora. Most local agencies have opted to utilize our current native flora and local genetics only, in the hopes that we can select from existing populations individuals that appear robust to the new climate order. Other agencies have been open to the possibility, at least, that we might incorporate genetics from the south and east to recruit individuals adapted to hotter and drier conditions, but only from species native to the local area. Very few agencies have been open to the possibility of moving species beyond their pre-settlement ranges, even those that have previously been introduced here and are naturalizing on their own, such as Calocedrus and Umbellularia.
Under the scenarios that are causing all of this worry in the first place, much of our current flora will be displaced. It is nearly undeniable that much of the territory abandonned by the departing taxa will be occupied by species from far away, mostly Eurasia. If we are going to intervene in this process at all, it seems reasonable, then, to do all three: select healthy local plants, add putatively appropriate genetics to local gene pools and promote the migration of West Coast natives. Migration has to take place anyway, and besides, much of this is moot because the ornamental plant trade has already begun the process for many important species.
At Scholls, we have been anticipating this choice for decades and have presented several options, including focusing seed collection on healthy individuals from local populations, augmenting genetics of local populations with southern Oregon seed, and introducing new species that are native to our immediate south. This is a long-term process, and we are only just beginning to approach it in a thoughtful manner, over fifty years after the general scientific consensus on the likelihood of the climate shift that is now well underway. We might as well take advantage of the advanced work initiated by the ornamental nursery trade. We can observe and learn from patterns of naturalization in species such as Calocedrus decurrens, Umbellularia californica and Juglans hindsii X nigra, and where appropriate, we can promote these newcomers from our south and east. They are here anyway, doing well, and they aren’t going away any time soon.