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 divergence

in the ability of plant species to cope with high concentrations of aluminum in the soil.

Adaptations for dealing with toxic metals include exclusion from uptake, inactivation in the root

stele and sequestration in above-ground plant parts. As one indication of tolerance, I compared

percent dry weight of aluminum in 1-2 year old leaves of mature A. myrtifolia and A. viscida plants. Tissue aluminum concentrations were not found to differ significantly either between the two species occurring in the same zone or for the same species among zones (Figure 8).

To determine if leaf aluminum concentrations in these species are affected differently by changing concentrations in the soil environment, I also compared leaf:soil aluminum ratios.

Linear regression shows no obvious divergence between A. myrtifolia and A. viscida in the

percent dry weight of aluminum in leaf tissues (Figure 9). Both species appear to respond

similarly to changes in the aluminum content of the substrate.

To further look at the direct effects of the Ione soils on the distribution of species in the Ione

chaparral, seeds of A. myrtifolia and A. viscida collected at Ione were sown in soils collected

from beneath stands dominated by A. myrtifolia, A. viscida, and Quercus wislizenii var. wislizenii

5 as well as in a commercial potting mix. Due to a lack of germination of a suitable number of these species, I used a closely allied species A. mariposa, collected from dissimilar soils in another county. I found a clear trend in decreasing root:shoot ratios for plants grown in soils along the vegetation gradient (Figure 10). While the high root:shoot ratio for plants grown in the

A. myrtifolia soils is an indication of general nutrient deficiency, the large amount of root biomass does not indicateinhibition of root growth or development in this presumably non- preadapted species.

In my attempt to isolate the specific edaphic conditions accounting for the isolated distribution of A. myrtifolia, I initially only considered soils beneath undisturbed vegetation.

However, fire is a normal part of the disturbance regime of the Ione chaparral and may alter the local environment in many ways. How the post-fire environment differs from that beneath undisturbed vegetation and whether or not differences in soil properties among zones are accentuated by the effects of fire are two important aspects to consider. While edaphic properties were shown to differ little among undisturbed vegetation zones, post-fire conditions affecting seedling establishment and stand regeneration may play a significant role in determining species distribution and maintenance of the vegetation pattern.

One aspect of the Ione vegetation that has received little attention is the presence of a dense and diverse layer of lichens and mosses. The presence of such a layer of cryptogams is unusual for chaparral and may affect vegetation dynamics in a variety of ways. A dominant lichen found at Ione is Cladonia cervicornis ssp. verticillata. This subspecies is restricted primarily to the mineral soils beneath A. myrtifolia and does not occur on the more organic soils covered with leaf litter found beneath the adjacent vegetation. Because many lichen species are known to accumulate metal ions from the environment, I analyzed thallus tissue of C. cervicornis ssp. verticillata for aluminum content. Aluminum levels in these tissues averaged nearly 0.22 percent dry weight (22 ppm) (Figure 11), demonstrating hyperaccumulation of this element.

6 While soil aluminum levels were not found to differ significantly among zones in

undisturbed stands, burning of the lichen cover may result in elevated soil aluminum levels

within pure stands of A. myrtifolia where the lichen cover is greatest. Although this effect is

considered transient, it may serve as an important ecological filter during the most critical

seedling establishment stage. And even though mature individuals of A. myrtifolia and A. viscida are not believed to possess divergent abilities to tolerate high concentrations of aluminum or low pH an low fertility, such a divergence may exist among seedlings.

Conclusions from this research include the following:

1. Adjacent vegetation zones associated with Arctostaphylos myrtifolia differ significantly

in cover and structure. Dormant seed banks reflect the distinct species composition

between zones.

2. PH, organic matter, and soil texture were found to vary significantly between zones.

3. While aluminum levels in the soil do not vary significantly between undisturbed zones,

marked differences after burning may result due to the accumulation of aluminum in

lichen tissues.

4. Changes in the vegetation coincide with overall changes in soil development. Increased

soil depth and nutrient availability appear to favor the establishment of the regional

chaparral vegetation.

5. There is no support for the hypothesis that A. myrtifolia and A. viscida possess divergent

mechanisms for avoiding aluminum toxicity, at least in mature plants. However,

physiological divergence may be responsible for differential seedling establishment.

6. A. myrtifolia demonstrates adaptive fine-tuning to especially shallow, acidic and nutrient-

poor soils.

7 Acknowledgements

I wish to express my gratitude to Dr. V.T. Parker, professor of ecology at San Francisco State

University for his assistance and guidance in conducting this research. Major funding was

provided by the California Endangered Species Tax Check-off Fund. Research grants were also

provided by the Hardman Foundation, the state office of the California Native Plant Society and

the East Bay Chapter of the California Native Plant Society.

Literature Cited

Aparicio, J. 1978. The plants of Ione. Fremontia 6:14-16. Bannister, P. 1978. Introduction to physiological plant ecology. Halsted Press, New York. California Department of Fish and Game (CDFG). 1990. Natural Diversity Data Base, Natural Communities. Natural Heritage Division. November. California Department of Fish and Game (CDFG). 1995. Special Plants List. Natural Heritage Division, Natural Diversity Data Base. June. Cate, R.B. and A.P. Sukhai. 1964. A study of aluminum in rice soils. Soil Sci. 98:85-93. Clarkson, D.T. 1965. Effect of aluminum on the uptake and metabolism of phosporus in barley seedlings. Pl. Physiol. 41:165-172. Gankin, R. 1957. The variation pattern and ecological restrictions of Arctostaphylos myrtifolia. Thesis. Univ. of California, Davis. 38 pp. Gankin, R. 1963. Notes on the geographic distribution of Arctostaphylos myrtifolia. Leaflt. West. Bot. 10:17-19. Gankin, R. and J. Major. 1964. Arctostaphylos myrtifolia, its biology and relationship to the problem of endemism. Ecology 45:792-808. Hackett, C. 1964. Ecological aspects of the nutrition of Deschampsia flexuosa (L.) Trin. I. The effect of aluminum, manganese and pH on germination. J. Ecol. 52:159-167. Marschner, H. 1986. Mineral nutrition in higher plants. Academic Press, San Diego. Pask, J. and M. Turner. 1952. Geology and ceramic properties of the Ione Formation. California Division of Mines, Special Report No. 19. Singer, M.J. 1978. The soils of Ione. Fremontia 6:11-13. Singer, M.J. and P. Nkedi-Kizza. 1980. Properties and history of an exhumed Tertiary oxisol in California. Soil Sci. Soc. Am. J. 44:587-590. Skinner, M.W. and B.M. Pavlik. 1994. Inventory of rare and endangered vascular plants of California. California Native Plant Society, Sacramento, California. Special Publication No. 1, fifth ed. 338 pp. Stebbins, G.L. 1978a. Why are there so many rare plants in California? I. Environmental factors. Fremontia 5:6-10. Stebbins, G.L. 1978a. Why are there so many rare plants in California? II. Youth and age of species. Fremontia 6:14-20. U.S. Fish and Wildlife Service (USFWS). 1994. Endangered and threatened wildlife and plants. 50 CFR 17.11 & 17.12. August 20. Wood, M. and V.T. Parker. 1988. Management of Arctostaphylos myrtifolia at the Apricum Hill Ecological Preserve. Unpubl. Tech. Report for the Calif. Dept. of Fish and Game, Region 2, Rancho Cordova, California.

8 Wood, M. 1989. Factors affecting the distribution of Arctostaphylos myrtifolia and A. viscida: the role of plant-plant and soil-plant interactions. Thesis. San Francisco State Univ., San Francisco. 147 pp.

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