Research Report #39 — Flowering Dogwood: Decline in the Shawangunks
For many years scientists and naturalists have been studying and observing the flora and fauna of the Shawangunk Ridge. Foremost among them was Daniel Smiley, for whom Mohonk Preserve’s Daniel Smiley Research Center is named. Dan wrote numerous reports summarizing his observations on various topics. This regularly occurring series will feature some of these reports; some hold tremendous scientific value today and just await an interested researcher to follow up, others showcase a quirky sense of humor or highlight an oddity of nature.
Read the Report: Flowering Dogwood: Decline in the Shawangunks. May, 1985. Paul C. Huth and Daniel Smiley.
A Note from Paul C. Huth, Director of Research Emeritus: Most of us have a species of plant whose flowers we watch for each spring season with great anticipation and pleasure, knowing that once found, the season has arrived. We at Mohonk Preserve’s Daniel Smiley Research Center are fortunate in that we watch multiple plant species each spring in places that Daniel Smiley went to, beginning in some cases more than 80 years ago. This annual record of observations of such things as first plant bloom and bird arrivals, first woodchuck and first fawn seen, and first emergence of insects like honeybees and mourning cloak butterflies, make up the study of natural events called phenology. Each year we record phenological events for over 250 species! When these long-term observations are compared with our 123 years of weather recording at the Mohonk Lake Cooperative Weather Station (NWS), we can clearly detect definite changes over time. As one of the old climate scientists told me decades ago, if you determine that the climate is changing, “what are the species doing”? We can, as at few other places nationwide, make those comparisons here at the Mohonk Preserve. In my four-plus decades of research here, I think it also keeps us closer to the land.
Since the early days of Mohonk, especially after major carriage drives were constructed, like Oakwood Drive in 1898, guests would go on “nature-oriented” carriage “excursions” to seasonal destinations reached eventually by some “Seventy Miles of Drives.” Dan recalled that one such annual spring carriage “excursion,” likely using the Mohonk Tally Ho or Roof Seat Break Carriage, about the last week of May was out along Oakwood Drive to see the display of full bloom of the native Flowering Dogwood. This carriage drive runs with the slope about a hundred yards above a plateau where the understory dogwood trees were abundant and which allowed guests to “view the concentrated impact of this species in bloom in the woodland.” Of course, what the guests were actually seeing was the display of the four large white showy bracts that surround the small central “button-like” yellowish flower clusters. The ripe pollen of bloom occurs about when the bracts are fully open to the flat or slightly downward position. Ripe fruit in late August was also of interest to us, especially for its attraction to many birds like the Robin, Catbird, Wood Thrush, Bluebirds and Flickers, to name a few.
The first mention of Flowering Dogwood at Mohonk was found in a letter from Alfred H. Smiley to Albert K. Smiley, dated 4 May 1878, where it was noted “in bloom.” Our regular annual observations, starting with Dan Smiley, now span nearly 90 years. Many of the observations of dates of bloom were made along the historic route of Oakwood Drive, which before the carriage drive had been built in the late 1890s, had been roughly cleared as a result of extensive ridge-wide slope forest cutting for charcoal production and for agricultural applications.
On 25 May 1979, staff noted it as a “very good year for bloom” of Flowering Dogwood along Oakwood Drive. A month later, passing by, Dan was surprised by what he saw and recorded “defoliation noticeable from road. Leaves shriveled and dropped.” What was happening? A year later, on 16 May 1980, Dan, checking for bloom on the stand, recorded “trees seem mostly dead with few leaves showing. Cause?” Many people at a distance from Mohonk were also noticing the problem, even occurring widespread in ornamental plantings. Of most concern to us was the loss “across all age classes, not just mature trees, but seedlings and saplings” too. Causes were attributed to lots of things, including “two unusually cold winters, (no), two wet weeks in August” (preceding the decline-no), potential damage from stem borers (a possible but unproven contribution), and maximum maturity of trees (generally 35–40 years-no). None of these fit our situation.
From 1981 to 1983, we inspected Flowering Dogwood stands in various locations and habitats on both the shale and conglomerate bedrock. We were saddened to see some stands reduced to a point “where dogwoods had dominated the understory, the subcanopy was becoming very open due to the absence of (dogwood) foliage.” We also found in 1983 that an old Flowering Dogwood border in the Mohonk Garden along Huguenot Drive, planted there according to Ruth H. Smiley in the “mid-1940s, was dying and dead.” Its origin, then, was likely as saplings transplanted from the stand along Oakwood Drive. In May and June 1984, we made a survey to observe the extent and intensity of the decline. We were saddened that in almost every stand we visited, “every tree, both large and small, was weakened and dying. We saw no seedling or sprout reproduction.” In July, reaching out to experts in the horticultural field, we finally learned that the cause of the major decline was, in fact, an anthracnose fungus. Both the white and the pink ornamental form were heavily impacted. We felt it interesting that the non-native planted Kousa Dogwood showed little or no symptoms. Our question then, and now, was why did the anthracnose strike so strongly, out of the blue, in June 1979? Was it something new? Was the fungus always here and had somehow become more virulent, or did the Flowering Dogwood become more susceptible, for a variety of reasons. On 9 May 1985, we again checked the stand on Oakwood Drive, and found “only 5 trees with a few flowers at the top most branches, rest dead.”
We concluded our Research Report with a “Prognosis.” We said then that “from extensive field observations in the Shawangunks….nowhere did we find a healthy unaffected individual. In most cases the large formerly showy stands have been decimated. Currently, no small seedlings have been found in the Shawangunk forest….we feel little will be left in five years or less.”
In 1989, I had a serendipitous contact with researchers at the Brooklyn Botanic Garden Research Center in Ossining. Cooperative research topics included old field allelopathy with practical applications for rights-of-way vegetation management, Pink and Yellow Lady-slipper decline research concentrating on the role of mycorrhizae, and pathology of American Chestnut blight and Flowering Dogwood anthracnose. In regard to the latter disease, we were fortunate to have Dr. Craig R. Hibben, Research Plant Pathologist, leading the research at the Mohonk Preserve. Craig’s Research Associate Project included setting up study plots in areas where the anthracnose was severe, recording conditions of the Flowering Dogwood, including individuals that “appear relatively unaffected by the disease….which might be sources of seed and/or cuttings for propagation and subsequent testing for resistance,” (and a) search for “seedlings, if any, as a measure of reestablishment of the woodlands by this species.”
An article in the New York Times on February 14, 1989, gave the sense of magnitude of the anthracnose infection from New England down the spine of the Appalachians to Georgia. US Department of Agriculture experiments with range-wide seed collection from 17 states, germination at the National Arboretum, and planting in natural settings in the mountains of Maryland, showed all seedlings susceptible with “the potential for genetic resistance….relatively nil.”
The anthracnose pathogen was identified as a fungus, Discula destructiva. In wet spring conditions in shady dogwood stands it generally causes leaf spot which spreads to the whole leaf surface. It also affects the stem and trunk cambium forming cankers which kill the lower branches first, symptoms we observed and described in our Research Report, and then traveling up the tree. Some dead leaves “held over winter.” One disease characteristic of note, which we also saw and described, was the evidence of clusters of water sprouts on the lower limbs and trunks of heavily infected trees. Dr. Hibben noted that from his research over several years, “anthracnose fungi….do not require a weakened host to become established. Therefore, predisposition of dogwood by some stress seems a less likely explanation for the current epidemic.” Further research with colleagues at Cornell University and the University of Washington found that a similar disease occurring in the range of the Pacific or Western Flowering Dogwood, Cornus nuttallii, was in fact the same disease. Craig noted that “it is intriguing that the disease was reported at about the same time on both coasts.” He also wondered if changes in spring and early summer climate could play a roll and suggested an analysis of records. Craig’s 30 years of research experience also led him to an additional possible connection. That of “the early reports of dogwood anthracnose near ports of entry on both coasts raise the possibility of Discula sp. being an introduced pathogen, much like the importation of the Dutch elm disease into the U.S. in the 1930’s.” But, he felt as a careful scientist, that while it is possible, “the sudden and widespread distribution of anthracnose….renders this explanation less tenable.” Also of interest to us at Mohonk was Research that showed “evidence that acid rain makes Dogwood foliage more susceptible to invasion by Discula.” Recent 2016 research into the origin of the fungus has used herbarium specimens collected in Asia, prior to the disease’s eruption in North America that were positive for Discula, supporting the hypothesis that the pathogen was imported “with host plants from Asia.”
In 2002, Craig Hibben reported to me that he had “wound down (his) program looking for exploitable resistance among survivors (of Flowering Dogwood) at Mohonk primarily because of the dramatic decrease in the severity of the anthracnose disease in this region.” In fact, over time in the field, we were seeing a stabilization of the species and a slight increase in vigor of individual trees, many with a relatively more showy bloom. So what was happening? Craig went on to explain to me that research was showing that the “Discula fungus was becoming hypovirulent (virus infected)….explaining the sudden decrease in severity of the dogwood anthracnose in our area.” While the disease is still present, “it apparently remains at a low level despite periodic conditions favorable for the disease.” So in a way, a virus has apparently saved our native Flowering Dogwood! And, more questions, like where did this virus come from?
Observations will continue in the Shawangunks, at the old historic stands and along our Citizen Science Phenology Trail, to see and record what the status of Flowering Dogwood becomes in the future. Closeness to the land and detailed records are the key to assess species change over time.
Read the Report: Flowering Dogwood: Decline in the Shawangunks. May, 1985. Paul C. Huth and Daniel Smiley.
