Parrotia subaequalis - it’s ecology and current status
While the future is uncertain for Chinese Ironwood in the wild, in gardens Parrotia is doing well.
Late in winter, before leaves have begun to emerge from their buds, small clusters of flowers open along the naked twigs of Chinese ironwood. The flowers have no petals but protruding from them are red anthers supporting yellow stamens which release their pollen to the wind. From a short distance away, this is easy to miss because of its small scale. Yet, when you step into the shadow of, right up into the branches, of Parrotia subaequalis, you can notice these flowers and appreciate their petite and delicate beauty.
It is known from only fourteen wild populations in Eastern China’s Jiangsu, Anhui, Henan, and Zhejiang provinces. It didn’t receive its name until 1992, though it was incorrectly assigned in 1960 to Hamamelis. (Li, 2008); (WeiLiGuang-FuZhang, 2015) (Lifang Zhang, 2018) While this history sounds long efforts have been slow to provide understanding for preservation and recovery. (Bing, 2016) In horticultural circles it has been approached as the new and sexy cousin of Persian ironwood, Parrotia persica, a commonly and long grown species from western Asia. But it deserves attention on its own merits.
It seems to be in the final paragraphs of a story which has spanned millions of years from the Tertiary period until now, when so few trees remain that are each one is counted and mapped.
The foliage of Parrotia subaequalis growing in the shade at Nanjing Botanical Garden at Nanjing Forestry University in April 2019.
The human population where Chinese ironwood lives is 224 million people. It is the densest population in densely populated eastern China. They exist on genetic islands, as old as the Pleistocene glaciers, but now are separated by vast areas of agricultural land. The fourteen relic populations are split into northern and southern groups by the Yangtze river, which helps to further isolate these trees. It isn’t only the isolation creating problems, however.
In addition to agriculture and forest exploitation bad site management has led to negative outcomes. At a few of the sites the underbrush has been cleared resulting in removal of natural seedlings. Also bamboo, of dubious nativity, has made things worse by making the forest floor an impossible place for seedling Parrotia to grow. (Qifang Geng, 2015). The entire population is very small but when sites are considered individually the danger is put into stark relief. In the Ningbo, and Jixi populations there are fewer than 20 plants each. (Lifang Zhang, 2018) It is easy to imagine a small lightening caused fire killing all the trees in one of these little forests.
The reproductive biology of this tree is complex and is in no small way contributing to its foundering. Flowering is irregular, as infrequent as only once every three years, during March and early April. As is common with other species lacking petals, Parrotia is wind pollinated. This is demonstrated by its feather-like styles which prepare to receive pollen from the air similar to pheromones to a moth’s antennae, and long stamens – a way to hold pollen out into more dynamic air currents. (Jannice Friedman, 2009) (Li, 2008) (Sork, 2002) It is genetically self-incompatible and it is very likely its pollination is genetically limited to certain subsets of the population, like is experienced in hazelnuts. (pers comm. Zhaung Xin, Michael Dirr, 2020) (Koen Kamalbeke, 2016) Often the weather is poor during the flowering period compounding low population density and depressing pollination. All living wild trees have been counted and are watched during their reproductive ardor. In 2015 of 100 sexually reproductive wild plants in eight of the populations, only ten trees produced seed (Bing, 2016) (Li Li, 2018).
After pollination the seed doesn’t mature for 18 months (Hu Yi Min, 2011). And, there are insurmountable obstacles for seed regeneration as the sites are managed with improperly executed site clearing or are overrun by bamboo which creates a shaded and inhospitable place for germination. Also there are insect and disease pests that reduce viable seed numbers. (Qifang Geng, 2015); (Hu Yi Min, 2011); (Bing, 2016).
At first the situation that Parrotia finds itself in seems dire. The natural regeneration is close to none, limited by natural biology, ecology, weather and geography. It is currently left with vegetative sprouts as one of its key holds on the environment. (Li Li, 2018) To call this its standard reproductive strategy is erroneous however. Resprouting is not ecologically viable for long unless the sprouts recover reproductive status. At best stump and root sprouting is a diminishing holding pattern for species whose normal reproductive strategy is via seed. (William J.Bond, 2001) In this case it has allowed us time to wake up to the problems facing the Parrotia. It is likely that many of the mature trees we see now are, in fact, sexually mature stump sprouts themselves. Chairman Mao instituted national programs of forest management beginning in the 1950s when nonproductive forest trees were removed. These occurred three times, are call the ‘Three Great Cuttings’, san da fa 三大伐. This is the case for trees on the western end of the Qinlin Dabie mountains near the family home of Zhaung Xin. (pers comm. Zhaung Xin, 2020)
Chinese ironwood growing in the shade at Nanjing Botanical Garden at Nanjing Forestry University in April 2019.
This tree, a Tertiary relic surviving the devastations of the Pleistocene glaciation, is barely hanging on. However, we can save the Parrrotia in our gardens because it is easy to propagate from cuttings. Growth and survivability of the propagules is good allowing wide horticultural use and this is to our benefit because it is a highly ornamental tree.
The mountains where these remaining populations grow are around 1,500 meters tall and they occur so far south and east that they don’t experience much depth of cold. The populations are at elevations from 192m to 988m and grow in mixed broadleaf evergreen forests and are associated with other known garden denizens, such as Ligustrum lucidum, Quercus accutissima, Pterostyrax corymbossum, Cornus controversa, and Stewartia rostrata and sinensis. It is a sub-tropical climate. (Lifang Zhang, 2018) (Andrews, 2007) However, the Chinese ironwood is far more hardy than this information would suggest. With extensive trials in the much colder parts of the USA Parrotia has been found fully hardy down into USDA zone 4, that is -25 to -35F (-31 to -37 C). (Dirr, 2018) These temperatures are not reached in its native range and serve to demonstrate why it is believed to have moved south in advance of the Pleistocene glaciers and found refugia here along the Yangtze.
While the flowers need special attention to be noticed by the casual observer the tree itself has clearly obvious merits. Widely tolerant of full sun or shade it will be broadly applicable in planted sites. It does best with moist soil. Grown in full sun it is an upright and full small tree. It grows with a single stem with a little attention. Late autumn brings deep purple color to the foliage and it is for this trait it is most regarded. Over time, as the tree gains size, its dark trunk will begin to exfoliate revealing attractive patches of lighter colored bark. Its wood is dense, very hard and durable, thus the common name ironwood. It is a long-lived modestly sized tree.
Epilogue
The CoVid19 pandemic and the ensuing pandemonium in China has prevented me from traveling to some of these sites to see and photographer Parrotia in 2020.
Works Cited
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Bing, Z. (2016). Living Conditions of and Conservation Strategies for Parrotia subaequalis in Huangwei Township, Yuexi County 岳西县黄尾镇银缕梅的生存状况与保护对策. Anhui Forestry Technology, 42(3), 51-53.
Dirr, M. (2018, March 06). Parrotia subaequalis. Nursery Management. Retrieved from https://www.nurserymag.com/article/parrotia-subaequalis/
Eric E. Knapp, M. A. (2001). Pollen-limited reproduction in blue oak: implications for wind pollination in fragmented populations. Oecologia, 128, 48-55. doi:DOI 10.1007/s004420000623
Hu Yi Min, F. G. (2011). Status of Parrotia subaequalis in Taxonomy, Reasons for its Endangerment and Protective Measures. 安徽林业科技, Anhui Forestry Science and Technology, 37 (2), 46-48. Retrieved from http://en.cnki.com.cn/Article_en/CJFDTOTAL-AHLY201102013.htm
Jannice Friedman, S. C. (2009, February 14). Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants. 103, 1515–1527. doi:10.1093/aob/mcp035
Koen Kamalbeke, A. A. (2016). Tree of the Year: Corylus Fargesii. International Dendrological Society Yearbook 2016, 12-31.
Li Li, Z. G. (2018, August). Structure, spacial pattern and regeneration of Parrotia subaequalis population in Liyan mountainous area of Jiangsu Province. Journal of Forestry Science & Technology, 45(4).
Li, J. a. (2008). The Chinese Parrotia: A Sibling Species of the Persian Parrotia. Arnolida, 66, 6-9. Retrieved from http://arnoldia.arboretum.harvard.edu/pdf/articles/1902.pdf
Lifang Zhang, J. Y. (2018, May 4). Leaf Venation Variation and Phenotypic Plasticity in Response to Environmental Heterogeneity in Parrotia subaequalis (H. T. Chang) R. M. Hao et H. T. Wei, An Endemic and Endangered Tree Species from China. Forests.
Qifang Geng, Y. Z. (2015). Effect of Yangtze River on population genetic structure of the relict plant Parrotia subaequalis in eastern China. Ecology and Evolution, 5(20). Retrieved from https://doi.org/10.1002/ece3.1734
Sork, V. L. (2002, September 03). Pollen movement in declining populations of California Valley oak, Quercus lobata: where have all the fathers gone? Molecular Ecology. doi:https://doi.org/10.1046/j.1365-294X.2002.01574.x
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William J.Bond, J. J. (2001, January 1). Ecology of sprouting in woody plants: the persistence niche. Trends in Ecology & Evolution, 16, 45-51. doi:https://doi.org/10.1016/S0169-5347(00)02033-4
Zhang Yun-Yan, S. E.-P.-F.-S. (2018, March 01). Development and Application of Genomic Resources in an Endangered Palaeoendemic Tree, Parrotia subaequalis (Hamamelidaceae) From Eastern China. (R. Zhou, Ed.) Frontiers in Plant Science.