The fate of Chinese understory species in coniferous forests of the Pacific Northwest: The role of light

Molly Muir Brooke

An effective method to predict invasive plant species could prevent annually billions of dollars of environmental damage in the U.S. In this study, I propose such a method based on the identification of likely sources of future invaders and factors that could determine the fate of these introductions. Many ornamental species from temperate China may hold a risk of invasion for the Pacific Northwest because of the climatic similarities between temperate coniferous forest regions of China and the Pacific Northwest. Further, these two regions are floristically related and contain many congeneric species that occupy similar ecological niches. Such similarly adapted species could potentially displace their congeners in the introduced range. However, if the non-native species cannot tolerate the shade levels in these forests, they pose no risk of invasion. I experimentally subjected four Chinese understory species, three native congeners for comparison and one bamboo species to two shade levels commonly found in Pacific Northwest forests and monitored their growth rates, leaf morphologies and chlorophyll contents. The Chinese Allium tuberosum demonstrated greater leaf morphology plasticity and relative growth rates than the native A. cernuum but little difference in chlorophyll acclimation. The Chinese Philadelphus kansuensis displayed greater leaf morphology plasticity and more rapid branch elongation than the native P. lewisii but little difference in chlorophyll acclimation. The native Spiraea douglasii appeared light-limited in both shade treatments and the non-native S. japonica and S. wilsonii exhibited no greater abilities to acclimate to shade than the native. The Chinese Fargesia rufa survived and grew in both shade treatments, but the lack of a native bamboo prohibits further comparisons. I predict that A. tuberosum and P. kansuensis are more likely to be able to naturalize in these shade regimes in the Pacific Northwest than are S. japonica or S. wilsonii. The fate of F. rufa in these shade conditions cannot be predicted without further trials. This study illustrates one step in a practical, field-based protocol for evaluating potential plant invaders into intact forest ecosystems where shade may limit invasion success. Horticultural introductions, particularly those from temperate China, may well contribute future plant invaders to the Pacific Northwest.

The fate of Chinese understory species in coniferous forests of the Pacific Northwest: The role of light

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