Factors Affecting the Distribution of Arctostaphylos myrtifolia (Ericaceae): the Role of Fire in the Maintenance of a Proposed Endangered Species and its Habitat Michael Wood Botanical Consulting Services Presented to the International Association of Wildland Fire Conference Fire Effects on Threatened and Endangered Species and Habitats November 13-16, 1995 INTRODUCTION Arctostaphylos myrtifolia (Ione manzanita; Ericaceae) is a highly restricted endemic plant found at low elevations of the central Sierra Nevada foothills of California. Its occurrence is limited to isolated populations on highly acidic, nutrient poor soils. Mining activities, land development, off-road vehicle use, and an as-yet unidentified pathogen currently pose great threats to the continued existence of this species. Because of its restricted distribution and perceived threats, A. myrtifolia is a first level (C-1)candidate for federal listing as Endangered by the U.S. Fish and Wildlife Service (USFWS, 1994). The USFWS is presently preparing a listing package for the species. It is considered a special plant by the California Department of Fish and Game (CDFG, 1995), a status affording it limited protection under the California Environmental Quality Act (CEQA). It is also considered a rare, threatened or endangered species by the California Native Plant Society (Skinner and Pavlik, 1994). The Ione chaparral vegetation type itself is considered to have the highest inventory priority by the CDFG (1990). Arctostaphylos myrtifolia is a semi-prostrate shrub 3-8 dm high with elliptic, acute leaves 5-15 mm long and having a rusty-green hue. It lacks a basal burl and reproduces after fire strictly by seed. Branches are dark red-brown with shreddy bark and they root readily at the nodes. Corollas are pinkish, 5-merous and up to 4 mm long. They develop from nascent inflorescences from November through February. The distribution of Arctostaphylos myrtifolia is limited principally to scattered populations over a 25-kilometer long stretch in Amador County, California at elevations between 60 and 140 meters (Gankin and Major, 1964; 1 California Natural Diversity Data Base) (Figure 1). A few scattered and disjunct populations also occur in Calaveras County at elevations up to 550 meters (Gankin, 1963). Stands of A. myrtifolia are remarkably pure, even for chaparral, with a relative cover close to 100 percent and as much as 50 percent bare ground (Wood, 1988). Stands are even- aged, dating back to the last fire. Populations are generally insular in nature and are surrounded by taller, denser mixed chaparral in more or less distinct concentric zones. Characteristic vegetation zones include pure A. myrtifolia, A. myrtifolia/A. viscida transition, pure A. viscida, mixed stands of A. viscida, Adenostoma fasciculatum (Rosaceae; chamise) and Quercus wislizenii var. frutescens (Fagaceae; interior live oak), and pure Quercus wislizenii var. wislizenii (Figure 2). This gradient in species composition and stand structure is repeated throughout the geographic range of A. myrtifolia. A prominent member of the vegetation adjacent to stands of A. myrtifolia is A. viscida (whiteleaf manzanita). Arctostaphylos viscida is a robust, profusely branching shrub 1-4 meters tall and with round, glaucous leaves 2.5-4 cm long. It occurs throughout much of California and southern Oregon at elevations from 150 to 1,500 meters above sea level. A unique feature of the Ione chaparral is the abrupt transition between stands of A. myrtifolia and A. viscida. This is especially pronounced by the very different growth habits of the two species. Edaphic factors are widely believed to account for the pattern in species composition in the Ione chaparral as well as the presence of many endemic and disjunct species (Gankin and Major, 1964; Stebbins, 1978a,b; Aparicio, 1978). These azonal soils frequently possess a lateritic crust, sometimes as much as six feet thick. The Ione soils are derived from the Ione Formation, an Eocene deltaic deposit believed to have formed during a tropical or subtropical climate (Pask and Turner, 1952). Classified as an exhumed oxisol, the Ione soils represent the only described oxisol in the continental U.S. (Singer, 1978; Singer and Nkedi-Kizza, 1980). Soils derived from the highly weathered Ione Formation are very acidic, have an extremely low 2 cation exchange capacity and exhibit high concentrations of exchangeable aluminum. Oxisols and lateritic soils like those found near Ione are usually associated with tropical regions of the world such as India, the West Indies, Africa and Australia. The unique properties of the Ione soils, alone or in combination, are widely regarded as being responsible for maintaining the vegetation pattern of the Ione chaparral. Gankin and Major (1964) were the first to look at the possible factors accounting for the restricted occurrence of A. myrtifolia. In their classic paper "A. myrtifolia, its biology and relationship to the problem of endemism", they suggested that A. myrtifolia is competitively inferior on zonal soils capable of supporting the regional vegetation. Because A. myrtifolia cannot persist in the understory of the surrounding vegetation, it appears to be restricted to only those sites that inhibit the establishment of the taller, regional chaparral species. Although A. myrtifolia may be regarded as being competitively inferior on the better developed regional soils, given the extremely inhospitable nature of the Ione soils, it could also be regarded as being competitively superior. Gankin and Major (1964) hypothesized that unique soil properties prevent invasion by adjacent species and that A. myrtifolia is restricted to sites where competition is eliminated or reduced. While Gankin and Major (1964) proposed some factors responsible for the restricted distribution of A. myrtifolia, no attempt has been made to quantify the edaphic environment supporting the Ione chaparral. For my research, I proposed the following null hypotheses: 1) edaphic factors do not vary significantly between vegetation zones and do not correlate with the vegetational gradient; 2) A. myrtifolia and A. viscida do not possess divergent mechanisms for nutrient uptake or the avoidance of aluminum toxicity; 3) species composition does not represent a consistent pattern in the Ione chaparral. Transitions in the vegetation are the result of chance dispersal. 3 In quantifying the Ione chaparral, I stratified the vegetation into four zones and collected cover data using randomly placed quadrats. Three zones, pure A. myrtifolia, pure A. viscida and pure Quercus wislizenii var. wislizenii are composed almost exclusively of these species while the mixed zone is made up of virtually equal numbers of Adenostoma fasciculatum, A. viscida and Quercus wislizenii var. frutescens (Wood, 1989) (Figure 3). Because the above ground vegetation in chaparral represents only part of the actual species composition of the vegetation at any one time, I also compared the dormant seed banks occurring beneath the pure A. myrtifolia, transition and pure A. viscida zones. Significant differences in both the average number of species (Figure 4) and seedling density among zones (Figure 5) reflect the composition of the mature vegetation. Highly weathered, acidic tropical soils like those derived from the Ione Formation frequently exhibit deficiencies in many essential elements and toxicities in others. A high concentration of exchangeable aluminum is a common characteristic of tropical soils and is considered the most universally toxic element (Bannister, 1978). More than 70 percent of acid soils under cultivation in tropical America aluminum toxicity problems (Marschner, 1986). Aluminum has been shown to inhibit cell elongation and division (Clarkson, 1965) and seed germination in some grasses (Hackett, 1964). As little as 1-2 ppm aluminum has also been shown to inhibit root growth in rice (Cate and Sukhai, 1964). In assessing the effect of soil aluminum on the distribution of species in the Ione chaparral, I collected and analyzed soils from four vegetation zones at 15 different locations and at two separate depth ranges. Overall, I found a high degree of variability in plant available aluminum between sites and among vegetation zones. Although differences in soil aluminum content between zones were not found to be significant, a subtle trend in the aluminum concentration appears to correspond with the vegetation gradient (Figure 6). Lacking any significant differences in soil aluminum between stands of A. myrtifolia and the 4 adjacent vegetation, I looked at other soil features. In examining pH, I found that pH consistently becomes more neutral progressing away from the pure A. myrtifolia zone and that differences are significant at opposite ends of the vegetation gradient (Figure 7). A similar trend was also found for organic matter content and concentrations of calcium, magnesium, nitrogen, phosphorous, potassium, and iron. The subtle changes in these elements demonstrate an overall amelioration of the edaphic environment as one moves from pure stands of A. myrtifolia toward pure stands of Quercus wislizenii var. wislizenii. Because aluminum is widely believed to be toxic to plants in acidic environments and due to its abundance in the Ione soils, I focussed on the role of aluminum in the distribution of A. myrtifolia. Gankin and Major's hypothesis that competition is responsible for the maintenance of A. myrtifolia in pure, isolated stands requires that there be some sort of physiological