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205 



Voss, E.G. 1972. Michigan flora: a guide to Ihe identifica- 

 tion and occurrence of the native and naturalized seed 

 plants of the state, 2 vols. Cranbrook Institute of Science. 

 Bloomfield Hills. MI. 



Wagner. W.L.. D.R. Herbst. and S.H. Sohmer. 1990. Manual 

 of the flowering plants of Hawaii. 2 vols. University of 

 Hawaii Press and Bishop Museum Press. Honolulu. 



199.^. Ptendophytcs. 

 1:247-266. Oxford 



Wagner. W.H., Jr.. and A.R. Smiih. 



Flora of North America. Vol. 



University Press, New York. 

 Welsh. S. L. 199.^. A Utah flora. Brigham Young Universilv. 



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For further information: 



Nancy Morin 



Missouri Botanical Garden 



PO Box 299 



St. Louis. MO 63 1 66 



Many of the best-known cases of cata- 

 strophic decline in trees are hnked to 

 introduced pathogens that circumvent the 

 natural defenses of their adopted host, leav- 

 ing it vulnerable to attack. Notable examples 

 of such declines include Dutch elm disease 

 and the chestnut blight. Similarly, numerous 

 studies have linked environmental degrada- 

 tion (e.g.. acid rain, ozone depletion, and 

 global warming I to altered interactions 

 among species. In the case of plants and 

 their pathogens, environmental degradation 

 may result in increased disease susceptibili- 

 ty and mortality as is true for the general for- 

 est declines in Europe and the widespread 

 decline of red spruce {Picea nihens) in the 

 northeastern United States. Identifying the 

 specific mechanisms for increased mortality 

 in nonspecific tree declines is often very dif- 

 ficult, and debate ensues as to which sources 

 of mortality are primary disease agents and 

 which are merely opportunistic. 



Both introduced pathogens and altered 

 environmental conditions have been hypoth- 

 esized as contributing to the decline of 

 Torreya taxifclki. a narrowly restricted 

 endemic conifer The range of the Florida 

 torreya spans an area of less than 400 km- 

 (154 mi-) along the Apalachicola River in 

 northern Florida and adjacent Georgia. In 

 the 1950's this mid-sized tree species was 

 struck by a catastrophic decline that has left 

 it on the verge of extinction in the wild. High 

 mortality is reducing the population by an 

 estimated 5% per year. Formerly a common 

 tree within its range, there are fewer than 

 1.500 trees left in the wild. 



The average height of a Florida torreya is 

 currently less than 1 m (3.3 ft). The average 

 age of the oldest stem on trees is less than 15 

 years. While a handful of trees produces 

 pollen, there have been no sexually mature 



Environmental Change 

 and the Florida Torreya 



by 



Mark W. Schwartz 



University of California-Davis 



Sharon M. Hermann 

 Tall Timbers Research Station 



females observed in the wild for at least 15 

 years. Syinptoms of disease include needle 

 spots, needle necrosis, and stem cankers. 

 Primary stem mortality has reduced the aver- 

 age height of trees by 10 cm (4 in) during the 

 past 3 years. Thus, the Florida torreya has 

 shown no sign of recovery or stabilization 

 during the 35 years subsequent to the onset 

 of the species" decline. If current patterns 

 persist, the Florida torreya is destined for 

 extinction in the wild. 



The search for a cause for the decline of 

 the Florida torreya began in the 1960"s when 

 a team of pathologists studying the case 

 could find no introduced fungal pathogens. 

 Pathologists studying the problem during the 

 1990's have shown thai (1) there does not 

 appear to be any viral or bacterial pathogens 

 associated with T. taxifoUa; (2) a very com- 

 mon native fungal endophyte {Peslalotia 

 milans). often pathogenic in other plants, 

 does not appear virulent on T. taxifoUa; and 

 (3) the less common Scytalidium sp., not 

 typically noted for its pathogenicity, pro- 

 duces pathogenic symptoms on T. taxifoUa 

 and was likely introduced to the region dur- 

 ing the late 1950"s, when slash pine planta- 

 tions were planted from nursery stock. 

 Finally, growth experiments have suggested 

 that environmental stress triggers episodes 

 of mortality in the trees. Greenhouse experi- 

 ments on Florida torreya trees derived from 



cuttings also suggest the likelihood that 

 structural changes in the slope forests along 

 the Apalachicola that have resulted in lower 

 light levels have also stressed wild popula- 

 tions of Florida torreya. 



The current hypothesis is that the decline 

 of Florida torreya is a result of facultative 

 [see glossary) pathogens attacking trees 

 under conditions of increased environmental 

 stress. Several potential stress factors, 

 including fire suppression, climatic changes 

 such as temperature extremes and drought, 

 and altered hydrologic regimes in ravine 

 forests and resultant changes in nutrient flow 

 have also been hypothesized as contributing 

 to the species' decline. 



Despite extensive research to find a link 

 between disease agents and environmental 

 stress, the mechanisms for forest decline 

 remain rather speculative. Torreya taxifoUa 

 has such a narrow distribution that a decline 

 in the populations in the Apalachicola basin 

 has brought the species to near extinction. 

 With the increasing magnitude of abiotic 

 environmental changes, we may expect 

 more cases that are similar to the decline of 

 T. tcLxifoUa. Unfortunately, the lack of iden- 

 tification of specific disease agents and spe- 

 cific mechanisms has hindered action to cor- 

 rect potential problems that cause forest 

 declines. Given the difficulty in delineating 

 mechanisms for declines, we typically can- 

 not ascertain exact mechanisms until it may 

 be too late. Waiting to be absolutely certain 

 of the triggers for particular forest declines 

 before corrective action is taken is likely to 

 be a costly strategy. 



For further information: 



Mark W. Schwartz 

 Center for Population Biology 

 University of Califomia-Davis 



Davis, CA 95616 



Most of the familiar flora of the American 

 landscape, such as trees, shrubs, herbs, 

 vines, grasses, and ferns, are known as vascular 

 plants. These plants have systems for transport- 

 ing water and photosynthetic products and are 

 differentiated into stems, leaves, and roots. 

 Nonvascular plants — the algae, fungi, and 

 mosses and lichens — are considered in other 

 articles in this volume. Except in Arctic and 

 alpine areas, vascular plants dominate nearly all 



of Notlh America's natural plant communities. 

 About 17,000 species of vascular plants are 

 native to one or more of the 50 U.S. states. 

 along with several thousand additional native 

 subspecies, varieties, and named natural hybrids 

 (Kartesz 1994). 



Human activities have expanded the geo- 

 graphical distributions of many plant species, 

 particularly fann crops, timber trees, garden 

 plants, and weeds. When a non-native plant 



Native 



Vascular 



Plants 



