Joseph A. Antos

SEX RATIO VARIATION IN THE DIOECIOUS SHRUB OEMLERIA CERASIFORMIS

Many examples of biased sex ratios are known in natural populations of plants. Proximal causes of these biases are gender diphasy (sex changing), differential mortality between male and female genets, differential rates of clonal growth (numbers of ramets per genet), and differential flowering (differences in flowering frequency or age to maturity). In the western North American shrub Oemleria cerasiformis we determined sex ratios for 60 natural populations and found an excess of males in 56 populations. The male bias was greatest in populations with little recent recruitment. Sampling of young and old plants indicated that males flowered at an earlier age than females, which led to a transient flowering bias in very young plants, and that the genet sex ratio was 1:1 in young mature plants but male biased in old plants, as a result of differential mortality. Examination of dead genets confirmed that mature females have higher mortality rates. Females also have greater reproductive effort and slower growth rates than males. The major cause of biased sex ratios in O. cerasiformis is greater mortality of female plants during their reproductive years, which probably arises directly or indirectly from their greater allocation to reproduction.

Relative reproductive effort in males and females of the dioecious shrub Oemleria cerasiformis

SummaryWe derived an index of reproductive effort (g reproductive tissue per g leaf) from data collected over two seasons on 28 males and 28 females of the dioecious shrub Oemleria cerasiformis. Males produced an average of three times as much flower and flower-stalk tissue as females, but because of their large fruits, females produced four times as much total reproductive biomass. Reproductive effort of both sexes was related to light. Fruit set in females (% carpels producing drupes) averaged 11.2% and was related to spring light levels. Male-biased sex ratios in this species may be related to the greater reproductive effort of females.

Correlations Between Forest Layers in the Swan Valley, Montana

Compositional patterns of forest layers (tree, shrub, herb, bryoid, and epiphyte) are weakly correlated in the Swan Valley. While one can roughly predict the composition of one layer based upon the composition of another layer, different strata do not change composition across environmental gradients at the same rate or in the same pattern. The relative position of two or more stands in species space is different for different layers. Evidence in support of this conclusion was derived from: (1) correlation of dissimilarity matrices, (2) correlation of stand placement on ordination axes, and (3) comparison of stand groups defined by cluster analysis. See full-text article at JSTOR

Ecological Implications of Belowground Morphology of Nine Coniferous Forest Herbs

The morphology of forest herbs was examined to determine how variation in growth form could relate to growth and survival in the forest. Five to 10 plants of nine herbaceous species were totally excavated in old-growth forests in the central Oregon Cascade Mountains. Underground parts were mapped, measured, oven-dried, and weighed. Additional information was derived from sites in the southern Washington Cascades. Achlys triphylla, Clintonia uniflora, and Smilacina stellata maintain extensive rhizome systems with both short and long shoots. This growth form allows these species flexibility in exploiting the forest environment. The three species differed in rate of extension growth and rooting depth. Arnica latifolia spreads by long rhizomes, which persist for only a few years; thus, extensive interconnected stem systems do not develop. Rubus lasiococcus and Linnaea borealis have extensive stolon systems with greater potential rates of spread than the three rhizomatous species. Although they expand rapidly under favorable conditions, they may be displaced by taller herbs. Rubus has larger and deeper roots than Linnaea. Tiarella trifoliata, Valeriana sitchensis, and Erythronium montanum have minimal vegetative spread. Plants of these three species often included the original seedling structure, indicating that seedling establishment is relatively frequent. On the six species with extensive vegetative spread, we never found a seedling source; genets are older than 5-36 yr, and new genet establishment appears to be uncommon. The differences in growth form among the species help to explain their ability to survive and coexist in the heterogeneous forest floor environment.

A DECADE OF RECOVERY OF UNDERSTORY VEGETATION BURIED BY VOLCANIC TEPHRA FROM MOUNT ST. HELENS

We examined changes in understory vegetation under an intact forest canopy during the first decade following the deposition of tephra (aerially transported volcanic ejecta) during the 1980 eruption of Mount St. Helens, Washington State, USA. Objectives were (1) to document vegetation response to a major disturbance that has received little attention but is widespread and relatively frequent in the northwestern United States, and (2) to analyze vegetation responses in terms of characteristics of the disturbance, responses of growth forms as well as those of species, components of vegetation change, and species autecology. We used permanent plots at four study sites, representing two tephra depths (≈4.5 and 15 cm), to examine understory vegetation change in old-growth, subalpine conifer forests. The two sites at each tephra depth differed in understory vegetation and amount of snowpack at the time of disturbance. At each site, plant cover and density were measured in 100 1-m2 plots with undisturbed tephra c...

Tree invasion into a mountain-top meadow in the Oregon Coast Range, USA

The grassy meadow on the top of Marys Peak, in the Oregon Coast Range, is being invaded, primarily along the margins, by Abies procera. To examine vegetation and envi- ronmental changes across the forest-meadow transition and to evaluate factors affecting tree invasion, belt transects were established at 10 sites. Correlations of environmental vari- ables with ordination axes from Detrended Correspondence Analysis suggest that the sites were distributed along a mois- ture gradient. Although the sites varied considerably in spe- cies composition and environment, tree invasion of the meadow was occurring at all sites. Reduction in abundance of the dense herbaceous vegetation of the meadow is required before the small seedlings of Abies procera can become established. Almost all tree invasion of the meadow occurs in the narrow forest-meadow ecocline because trees at the edge of the forest reduce the cover of herbaceous plants in the adjacent meadow. On some sites, above average snow accumulations further reduce vigor of meadow vegetation and increase tree estab- lishment. On the driest sites wet summers facilitate tree estab- lishment. Infrequent fires, which remove the fire sensitive Abies procera, are probably required to reverse the slow but persistent trend of forest expansion and ultimately maintain the meadow.

The Effect of Fire on an Ungrazed Western Montana Grassland

On 28 June 1977 a hot wildfire burned part of a species-rich foothills grassland dominated by Festuca scabrella, F. idahoensis and Agropyron spicatum. We sampled plant species cover in paired stands, on and off the burn, during 1977 and 1978 to determine the initial impact of the fire, and during 1980 to document recovery after 3 years. Grass cover decreased on the burn due primarily to a sharp drop in the cover of the two dominant fescues. Forb cover increased. Moss and lichen cover dropped greatly. Much of the difference in species response was related to plant growth form and phenology. Large bunches of F. scabrella were more seriously damaged than small ones, indicating long intervals between fires are detrimental to the species when fire does return. At burned sites, afternoon soil temperatures were higher and near-surface soil moisture was lower. Three years after the fire, cover of most vascular species was similar in burned and unburned stands. Festuca idahoensis had recovered completely. Festuca scabrella had increased but was still well below unburned levels. Total moss cover on the burn was nearly that on unburned sites, but the species composition was greatly altered. Lichens were still reduced. The stability of the grassland in relation to fire is discussed.

SPECIES REPLACEMENT DURING EARLY SECONDARY SUCCESSION: THE ABRUPT DECLINE OF A WINTER ANNUAL

The factors that contribute to species establishment and decline determine the rate and pattern of successional change. We tested a commonly held assumption that competitive displacement is responsible for the loss of species during succession. Manip- ulative field experiments were used to examine the effects of interspecific competition on the population dynamics of Senecio sylvaticus, a winter annual that briefly dominates post- harvest sites in the western Cascade Range of Oregon. Senecio increased in density 400- fold from the first to the second growing season after disturbance but decreased precipitously in year 3 to 10% of the density and 0.5% of the biomass per plot of the previous year. Although interspecific competition reduced the cover and biomass of Senecio during its peak year, it had little or no effect on either the population increase or decline; the pattern of change was-similar among all treatments. These counterintuitive results underscore the importance of testing, not simply assuming, that interspecific competition is responsible for the replacement of a species during succession.

Plant Responses in Forests of the Tephra-Fall Zone

Tephra fall is the most widespread disturbance resulting from volcanic activity (del Moral and Grishin 1999), including the 1980 eruption of Mount St. Helens (Sarna-Wojcicki et al. 1981). Tephra is rock debris ejected from a volcano that is transported through the air some distance from the vent that produced it. Fine-textured tephra (less than 2 mm in diameter) is referred to as volcanic ash. Tephra may be transported far from a volcano and affect vegetation over thousands of square kilometers, well beyond the influence of other types of volcanic ejecta. Individual tephra deposits from volcanoes in the Cascade Range have been traced east into the Great Plains, and others cover much of the Pacific Northwest (Shipley and SarnaWojcicki 1983). Mount St. Helens has been the most frequent source of tephra in the Cascades for 40,000 years, producing dozens of tephra layers equal to or larger than the 1980 eruption, three experienced by trees alive in 1980 (1480, 1800, and 1980; Mullineaux 1996). The likely extent and magnitude of past volcanic eruptions are apparent in Cascade Range soils near or downwind from major volcanoes, soils that are largely formed from tephra (Franklin and Dyrness 1973), and in the large amounts of tephra in soils far east of the Cascade Range (Smith et al. 1968). Tephra has had major effects on plants in many parts of the world (Table 1.1). Some trees may survive burial by tephra 2 m deep, but smaller plants are killed by much thinner deposits (Antos and Zobel 1987). Thin layers (a few millimeters thick) are likely to have little effect on plants. Between these extremes, various combinations of depth, texture, and frequency of deposition produce a wide range of plant responses (Antos and Zobel 1987). Effects of tephra on plants may include direct damage from impact, alteration of leaf gas and energy exchange by tephra adhering to foliage, modification of the soil environment, and burial of small plants and seed banks. Leachate from tephra may contain toxic elements that damage root systems. Conversely, tephra can be a source of plant nutrients, although it lacks nitrogen and most of its phosphorus is not easily leached (Hinkley 1987). Fine-textured tephra may harden after wetting to produce a surface crust with poor permeability to water. Such a crust is often dense and strong enough to restrict plant growth. For smaller plants, burial is the most important effect of tephra, and the ability to grow through the deposit is a key to survival (Griggs 1919, 1922; Antos and Zobel 1987). From 1980 to 2000, we have studied the effects of Mount St. Helens tephra on understory plants in old-growth conifer forests with trees more than 500 years old, using two sites at each of two tephra depths, 4.5 and 15 cm, located 22 and 58 km northeast of the crater (Table 4.1). The sites are on flat topography at elevations between 1160 and 1290 m, in the Abies amabilis (Pacific silver fir) vegetation zone of Franklin and Dyrness (1973). Large conifer trees, including Tsuga heterophylla (western hemlock), T. mertensiana (mountain hemlock), Pacific silver fir, Pseudotsuga menziesii (Douglas-fir), and Chamaecyparis nootkatensis (Alaska cedar), dominate the sites, with a patchy distribution of smaller trees, primarily Pacific silver fir and Tsuga spp. (hemlocks). Before the eruption, understories contained ericaceous shrub layers 1 to 1.5 m tall with 17% to 45% cover, primarily Vaccinium membranaceum (big huckleberry) and V. ovalifolium (ovalleaf huckleberry). Herbaceous layers varied considerably among sites in cover (6% to 35%) and diversity (Table 4.1), but included a variety of growth forms. Bryophyte layers (9% to 36% cover) were dominated by Dicranum spp. (broom mosses) and Rhytidiopsis robusta (pipecleaner moss) (Zobel and Antos 1997). Wood more than 5 cm in diameter covered 3% to 11% of the preeruption surface. For 3 years after the eruption, from 1980 to 1983, we also sampled sites with 2and 7.5-cm tephra at 550 and 880 m in elevation, respectively, in the western hemlock zone (Antos and Zobel 1985b, 1986). Our intent was to study long-term effects of tephra deposits on understory plants; thus, our intensive study sites have gentle slopes, although most Cascade Range topography in the tephra-fall zone is steep. On some very steep slopes, erosion was extensive, producing much greater understory cover. However, on many steep

Age structure and growth of the tree-seedling bank in subalpine spruce-fir forests of south-central British Columbia.

Abstract We examined the dynamics of the tree-seedling bank in old-growth, subalpine Picea engelmannii—Abies lasiocarpa forests in south-central British Columbia by collecting all individuals <1.3 m tall within belt transects at two sites (n = ca. 500 per site) and determining their height, basal diameter and age. Seedlings were up to 149 y old. Regressions showed that spruce 1-m tall averaged 65-y old and fir 92-y old. Age structures indicated that recruitment into the seedling bank was variable with time, but that individuals of both species occurred in almost all 5-y age classes. Regressions of height and diameter vs. age had high predictability and showed that spruce grew more rapidly than fir. Allometric (height/diameter) relationships differed significantly between species; spruce had a much greater height/diameter ratio than fir. Seedlings accumulate gradually and persist for long periods in the seedling bank, thus forming a large pool of individuals that can potentially respond to opening of the canopy.