Saturday, May 9, 2015

Is over-reliance of using herbarium specimens for taxonomic studies leading us to underestimate Southeastern plant diversity? Part 1 of 2.

Most of our current knowledge of Southeastern plant biodiversity comes from botanists who have worked on bits and pieces over the past 250 years. Prior to the early to mid-1800s most of the botanists dealing with the Southeastern flora, even the American flora for that matter, were Europeans who had never or only rarely set foot on American soil. Afterward, in the late 19th and up through the mid-20th centuries the majority of botanists working on the eastern U.S. flora were affiliated with "Northern" academic institutions like Columbia, Harvard, and Cornell universities. With a few notable exceptions, most of these botanists rarely traveled to the Southeast. This meant that they rarely got into the field to observe firsthand the South's exceptional biodiversity. Instead, botanists like Asa Gray, Merritt Fernald, and Arthur Cronquist mostly relied on dried, pressed herbarium specimens sent to them for study by Southern botanists who lacked the prestige and reputation of their Northern counterparts. In many cases, these Northern botanists would look at specimens sent to them that resembled species common in the Northern states and they would often identify the Southern example as being the same as the northern species - lumping them together.

One notable exception to the Northern lumping botanists was John Kunkell Small of Columbia University, who was affiliated with the New York Botanical Garden. Small studied the Southeastern flora extensively, especially that of Florida and adjacent states. Many of his Northern counterparts felt he was too "splitty" in his classification of Southern plants. After he published his Flora of the Southeastern U.S in 1933 it was only a matter of time before Northern botanists began to lump his Southern species into Northern ones, usually doing so at whim and without the backing of scientific evidence. Many of Small's species that he recognized in 1933, which were subsequently disregarded by many later botanists, are now being upheld by science as good species and are being resurrected.

The study of plant species, their characteristics, and how they are classified is known as plant taxonomy. It is usually cost prohibitive for botanists to drive around the country to study hundreds of populations of a particular group of plants just to determine whether the group is just one widespread species with lots of variation or whether the group consists of multiple similar but distinct species. Imagine with today's gas prices how much it would cost to study a species that has a range that covers most of the eastern U.S.! 

Approximate geographic distribution of Wild Ginger (Asarum 
canadense). The red, yellow, and blue polygons represent morpho-
logical extremes that some botanists have recognized as distinct 
species or varieties. The most recent consensus is that they
represent a single highly variable species whose morphological 
variability lacks geographic correlation. Most studies of wild ginger 
variation have relied heavily on examination of herbarium 
specimens and little on examination of living populations. 
Attempting to study live populations across such a vast range is 
difficult due to high travel costs. Map from Estes (unpubl. data).
So, since we can't afford to cover so much ground, like our botanical forefathers before us, we must rely heavily on the study of pressed, dried, plant specimens stored in museums called herbaria. To thoroughly understand the taxonomy of a particular group of plants may require examining thousands of specimens collected by dozens of botanists over hundreds of years and stored in many different herbaria. From the study of these specimens, we usually can identify a specimen to a given species. We can even use the specimens to ask questions about whether something that has always been called one species should be divided into two or more species. The scientific method can then be employed to answer these questions.

Botanists must be careful however not to rely too much on the study of herbarium specimens alone. For one, we are often only examining a part of a much larger plant that can't be pressed on a piece of herbarium paper that is typically 18 x 11.5 " long. For example, with trees we tend to press a section of a twig but we can't press the entire tree. Consequently there are features we miss. Other features like floral scent, bark texture, and colors may not preserve on herbarium specimens and they often aren't recorded by the botanists who collected them. Fortunately, herbarium specimens seem to allow most trees, shrubs, small herbs, ferns, grasses, sedges, and rushes to be studied and differentiated rather easily.

Some plant groups present great challenges to botanists who rely solely or primarily on herbarium specimens to interpret patterns of variation and species boundaries. For example, palm trees are difficult to study because their leaves and other parts are enormous and impossible to fit on a single herbarium specimen. 

Other plant groups are what I call "architecturally complex." These are groups that have complicated flower shapes or leaves whose potentially important taxonomic characters become destroyed upon pressing. For example, think about the complexity of most orchid flowers and imagine smashing that flower and drying it for days and then trying to compare the fine details of the smashed flower of your specimen to hundreds of other smashed flowers from other orchid collections. Preserving the flowers in some sort of liquid preservative would prevent destroying taxonomically important features but this is rarely done. 

Herbarium specimens of wild ginger showing variation in flower size and shape when pressed and dried. These flowers are  highly three-dimensional and their original dimensions, orientation, color, and scent are lost when pressed and dried. 

Blackberries are another architecturally complex group. They are difficult to study since no one likes to collect them because of their prickles and because they are hard to press. Typically one must cut a whole blackberry bush into numerous sections, pressing each section carefully. The stems may have certain growth patterns that can't be observed on a herbarium specimen, like whether they grow appressed to the ground or form an arching shrub. in order to really have a specimen worth studying later, nearly complete specimens are needed. Such complete collections typically don't exist in most herbaria. 

In the Southeastern flora there are many groups which are architecturally complex, including wild gingers (Asarum), jack-in-the-pulpit (Arisaema), various orchids, blackberries, pitcher plants (Sarracenia), and leather flowers (Clematis). This architectural complexity has made these groups difficult to study using herbarium specimens. Once their parts become smashed and dried the various species often look alike and differences that seem to exist when viewing living plants side-by-side in the field or garden seem to break down in the herbarium. This has led many botanists to do a lot of "lumping" in some of these groups. 

Wild ginger (Asarum canadense)  has been classically treated as a single widespread and variable species by most botanists. These two flowers were photographed in Maryland where two strikingly different morphological "forms" co-occurred in the same forest. Plants with flowers like those on the left have been treated as Asarum reflexum while those on the right are best treated as typical Asarum canadense. At this site the plants generally occupied different microhabitats with one occurring on a floodplain terrace and the other on adjacent toe slopes.

Photographs showing "architecturally complex" flowers of wild gingers. Recent research by Estes (unpubl. data) suggests that what we currently call one species (Asarum canadense) is best considered three distinct species, A. canadense, A. reflexum, and A. acuminatum. Reliance on herbarium specimens by many has led to lumping these three as one species in the past. Their distinctiveness is getting clearer but only after logging several thousand miles across eastern North America to study and examine living populations. Estes still needs to examine populations in portions of the Midwest and Great Lakes states to finish the study. 

Recently, evidence has been mounting that suggests relying too much on herbarium specimens for interpreting the taxonomy of architecturally complex plant groups may lead to a serious underestimation of biodiversity. As such we may be failing to protect critically endangered species that we don't realize exist. 

In the second part of this series, I will discuss how we have seriously underestimated biodiversity in the viny leather flowers of the southeastern U.S. and in doing so have overlooked nearly a dozen undescribed species.