Academic journal article The American Midland Naturalist

Secondary Plant Compounds in Seedling and Mature Aspen (Populus Tremuloides) in Yellowstone National Park, Wyoming

Academic journal article The American Midland Naturalist

Secondary Plant Compounds in Seedling and Mature Aspen (Populus Tremuloides) in Yellowstone National Park, Wyoming

Article excerpt

ABSTRACT.-Widespread establishment of seedling aspen (Populus tremuloides) occurred in Yellowstone National Park (YNP) following the extensive1988 fires. Aspen stands occupy -2% of YNP and aspen stems are intensively browsed by native ungulates. Chemical composition, especially secondary compounds, may influence levels of herbivory and, hence, survival of aspen, but concentrations of such compounds in aspen in the northern Rocky Mountains are not known. Quantitative profiles of foliar nitrogen and secondary compounds (condensed tannins and the phenolic glycosides, salicortin and tremulacin) in aspen were assessed to address the following questions: (1) Do concentrations of secondary compounds differ between seedling and mature aspen stands? (2) Do concentrations of secondary compounds in seedling aspen differ between unbrowsed and artificially browsed seedlings? (3) Among mature aspen stands, do concentrations of secondary plant compounds differ among, (a) burned and browsed, (b) unburned and browsed and (c) unburned and unbrowsed stands? Concentrations of phenolic glycosides were significantly higher in seedlings than in mature stands, although condensed tannin concentrations and leaf nitrogen were higher in mature stands. Concentrations of leaf nitrogen and all secondary compounds were greater in unbrowsed seedlings than in seedlings subjected to simulated browsing. Concentrations of secondary compounds did not differ between mature aspen stands that were unburned regardless of whether they were browsed; however, burned stands (all of which were browsed) had significantly greater concentrations of secondary compounds and leaf nitrogen than the unburned stands. Results from this research suggest that foliar phenolic glycosides and tannins are not active defenses induced in response to browsing by large mammals. Rather, variation in levels between juvenile and mature ramets represents ontogenetic shifts in expression of defense, whereas variation between clipped and unclipped seedlings results from shifts in carbon/nutrient availability.

INTRODUCTION

Extensive crown fires in 1988 affected ~45% (400,000 ha) of Yellowstone National Park (YNP) and surrounding lands, producing long-lasting effects on the park's flora and fauna (Christensen et aL, 1989; Knight and Wallace, 1989; Turner and Romme, 1994; Turner et al., 1994a, b; Turner et al., 1997). The fires resulted from an unusually prolonged drought and strong persistent winds (Christensen et al., 1989; Renkin and Despain, 1992). Such fires are major, but infrequent, natural disturbances in the YNP area, occurring at 100 to 300-y intervals (Romme, 1982; Romme and Despain, 1989; Millspaugh and Whitlock, 1995). The fires created a striking mosaic of burned and unburned forest across the landscape, and postfire succession has proven to be complex (Turner et al., 1997; Turner et al., 1998).

A surprising result of the 1988 fires was the unexpected recruitment of aspen (Populus tremuloides) seedlings. Aspen seedlings established within burned areas that were previously dominated by lodgepole pine (Pinus contorta var. latifolia) and in places well beyond the prefire distribution of aspen (Kay, 1993; Tuskan et al., 1996; Romme et al., 1997). Aspen is the only deciduous forest type in YNP and it comprises only ~2% of the forested area of YNP (Despain, 1990). In the Rocky Mountains aspen is primarily a clonal species which reproduces almost exclusively via root sprouting and produces large stands composed of genetically identical stems (Barnes, 1966; McDonough, 1985). Most Rocky Mountain aspen genets are thought to be hundreds or thousands of years old (Baker, 1925; Barnes, 1966), although genetic analyses indicate infrequent episodes of new genet establishment (Jelinski and Cheliak, 1992; Jelinski, 1993; Tuskan et al., 1996). A rare aspen seedling recruitment event followed the 1988 fires which created extensive areas suitable for germination and were followed by several relatively moist summers. …

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