The following information is derived largely from monitoring records of the Illinois Department of Natural Resources but some may come from the literature and other sources.
Species: Cladrastis kentuckea, Yellowwood.
Compiler: John Schwegman, Illinois Department of Natural Resources.
Date of Most Recent Update: January 21, 1992
Location of Monitored Populations: Wolf Creek Botanical Area, Shawnee National Forest, Alexander County, IL.
Dates of Monitoring and Sample Size: A 9 acre stand of up to 76 stems were marked and monitored from May 1987 thru May 1990. The period included a severe drought in the summer of 1988 and an unusual cold temperature of -20 F in mid December of 1989.
Range of Natural Communities: Mesic Upland Forest.
Range of Plant Communities: Beech-Sugar Maple-Red Oak.
Range of Soil and Substrate: Thin loess to no loess on cherty limestone.
Range of Slope and Aspect: 325 degrees (northwest) for entire slope, but individual trees on small north to west-facing slopes.
Range of Shading or Crown Cover: 98%.
Flowering Dates and Data: May 14 and May 19 are observed dates but each was late in the flowering season. Estimate most flowering from May 1 to May 20. No plants flowered in 1990 after a low temperature of -20 F in min December of 1989 and a late freeze in the spring of 1990. Flowering plants numbered 1 in 1987, 6 in 1988 and 5 in 1989. Only larger plants flowered, with the smallest flowering stem being 6.7 cm DBH. One 28 cm DBH plant bloomed 3 consecutive years.
Fruiting Dates and Data: Seed was already shed by July 27, 1989. 1988 was a good seed year, and seedlings found in 1990 were probably from these seeds.
Known Phenotypic Variation: None noted.
Known Pollinators in Illinois: Bees and occasionally black swallowtail butterflies, but thesewere not identified due to the height of flowers above the ground. Pollinators are abundant and do not seem to be a problem for the species.
Reproductive Mode: Stump sprouts from disease killed stems and seed.
Known Conditions for Sexual and Asexual Reproduction: Stump or basal sprouts are most common after death of a sexually mature stem but are also seen on stumps or old root crowns that have a healthy larger stem. Seedlings were observed in dense leaf litter indicating that bare mineral soil is not necessary for seedling establishment and that leaf litter may be necessary mulch for seedlings. Seedlings found in 1990 may have come from the good seed crop of 1988. All seedlings discovered were in close proximity to trees we had observed in flower.
Known Diseases: The fungal pathogen Botryosphaeria dothidea was confirmed by a USFS pathologist as the disease that kills many stems (both large and small) by girdling. This is a widespread opportunistic disease known to attack stressed plants. Many larger trees in this stand have scars and hollows on the upslope side interpreted as fire scars. It was thought that possibly the pathogen infected trees after physical injury such as fire and that young uninjured seedlings would be free of disease. In 1989 this disease was noted on several 2 to 3 year old seedlings implying that injury is not required for infection. A 10 cm DBH tree that was killed by this disease during the monitoring, died in 1988. It was in the process of dying (had small atypical leaves) in May before the severe drought set in but may have been influenced by dry conditions in 1987. It was on a small west-facing slope that was steep and rocky. It had a large fire scar on its upslope side and may have had other diseases like heart rot. Many smaller stems also died during the monitoring. Since yellowwood is at the very edge of its range in Illinois, climate stress is a logical suspect. The one tree that died was on a drier west-facing slope indicating moisture stress may be a factor. No increase in disease mortality following the severe cold of 1989 indicates that cold stress may not be the culprit although we did not monitor into 1991 when increased disease could have been more apparent. Stress related to physical injury is also probably a factor. It seems unlikely that shade stress would be a factor in a subcanopy species adapted to filtered light. Healthy populations in deep shade in other states like Indiana argue against shade as a problem. These healthy Indiana trees, farther north than Illinois' populations, argue for moisture stress as opposed to temperature as the stress factor. Air pollution may also play a role in stress.
Known Grazers and Parasites: A few holes were observed in canopy leaves, indicating insect grazing or a leaf disease. On April 23 of 1981, the monitored site suffered near total defoliation of all species except yellowwood by a severe infestation of smooth green caterpillars. It appears likely that yellowwood contains a chemical insect defense lacked by most deciduous trees in the area. An alternate explanation is that the caterpillars were just adapted to oak, hickory and beech and did not like yellowwood's taste.
Known Problems with Exotic Species: None noted.
Known Response to Fire: This thin-barked species is easily injured by fire as evidenced by the numerous fire scars on the bases of the larger trees. These injuries seem to contribute tomortality.
Known Mortality Factors: The fungal pathogen Botryosphaeria dothidea was responsible for all mortality noted. Moisture stress and physical injury from fire seems to contribute to disease onset and severity.
Known or Suspected Responses to Environmental Stress: Moisture stressed plants seem more vulnerable to disease. Extreme cold temperatures and a late freeze prohibited all flowering in 1990. Air pollution may foster disease and ultimately death.
Summary of Apparent Factors Regulation Size and Health of Populations:
The fungal pathogen Botryosphaeria dothidea kills both large and small stems. It infects plants free of injury as well as fire scarred ones. Plants stressed by low moisture seem to be most vulnerable to mortality due to this disease. Past fires injured many of the larger trees, but current fire protection favors the species through reduced injury stress and provision of proper conditions for seedling establishment. Air pollution may affect disease rates. A possible chemical defense in the leaves of yellowwood protect it from insect grazing that can severely effect the other trees it grows with in Illinois.