Donald B. Zobel

Relationships of Environment to Composition, Structure, and Diversity of Forest Communities of the Central Western Cascades of Oregon

Temperature and moisture stress of conifer saplings and needle nitrogen content of conifer saplings were measured at reference stands representing 16 forest communities in the central portion of the western Cascades province of Oregon. Most species occur over a wide range of temperature and moisture stress; many occupy a wider range of environments in the western Cascades than they do in the eastern Siskiyou Mountains of southwest Oregon. Differences between vegetation zones are reflected in a temperature index; within zones, communities are distinguished by moisture stress and, to a lesser extent, by temperature. In two cases vegetation differences appear to be related to low needle nitrogen contents. Use of complex gradients for vegetation ordination suggests certain environmental differences between communities which are contrary to the differences measured; therefore, we prefer the measured gradients over the complex gradients defined. Species diversity (the total number of vascular species) increases and dominance (Simpsons index) decreases away from moderate environmental conditions to warmer-drier and colder communities. Diversities of different strata are unrelated. Dominance is concentrated in fewer strata of the vegetation on the colder sites. However, discontinuities in the pattern of diversity with environment occur which are not related to major differences in our measured environmental indexes. Evergreenness of shrubs is highest in stands with the lowest foliar nitrogen levels.

Biomass, productivity, leaf longevity, and forest structure in the central Himalaya.

Patterns of leaf characteristics, forest structure, tree species diversity, bio- mass, and productivity across a gradient of 3300 m and 15.70C in mean annual temperature in Kumaun, in the Indian central Himalaya, were summarized and compared to values from other similar forests. Throughout the elevational gradient, the annual rainfall was high (100-300 cm), but not correlated with elevation. Evergreen species with a 1-yr leaf life-span dominated most of the elevational transect; above 1800 m, species with deciduous and multiyear evergreen leaves were also well represented. Although variability among sites within forest types was high, a number of consistent patterns were apparent. Forests of Pinus roxburghii and those at high elevations were most consistently different from other forest types. Leaf life-span was not strongly correlated with leaf mass, specific leaf mass, or leaf production efficiency (net primary productivity per unit leaf mass), contrary to relationships presented in the literature. Tree species richness and basal area were lower than for most similar types in Nepal. Biomass and productivity of the forests in Kumaun were relatively high, compared to mean values for similar forest types elsewhere. Measured values for most variables describing these forests (but not all) fell within the ranges for the variables in similar forests worldwide. The maximal values for forest biomass remained high, 500-600 Mg/ha, up to 2600 m elevation, but declined sharply in birch forest (_ 170 Mg/ha) above 3100 m. Net primary productivity (NPP) varied little (15-20 Mg* ha- I.yr- 1) below 2700 m, despite a 10WC gradient in mean annual temperature and marked changes in basal area, tree density, growth form, and leaf char- acters. The level of productivity appeared not to be limited by rainfall, forest structure, leaf type, or temperature above an annual mean of 1 PC. Leaf mass (LM) varied consid- erably among forest types, being 3.7-8.6 Mg/ha for deciduous species, 5.7-8.9 Mg/ha for P. roxburghii, and 10.0-28.2 Mg/ha for evergreen broad-leaved species. Leaf mass duration (leaf mass x months of the year with leaves present) was related directly to NPP and inversely to leaf production efficiency (NPP/LM). These data add substantially to the data base for forest properties, especially for broad-leaved evergreen forests.

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...

HIMALAYAN FORESTS AND ECOLOGICAL GENERALIZATIONS : FORESTS IN THE HIMALAYADIFFER SIGNIFICANTLY FROM BOTH TROPICAL AND TEMPERATE FORESTS

E cological measurements, as traditionally made, represent small areas. However, in response to worldwide concern about environmental quality, ecologists have expanded their focus to encompass the earth (e.g., Field et al. 1995, Frank and Inouye 1994, Neilson and Marks 1994, Wessman 1992). Analyzing and predicting processes at the scale of the biosphere require that information-including data about geographic regions and biological systems that are not well represented in widely available data basesbe integrated at the appropriate scale. To obtain the information that is required to draw conclusions at the scale of the biosphere, ecologists must make new measurements at broader scales and in regions for which data are currently sparse, must extrapolate from the existing data, or must search for less well known data that are reliable and appropriate. The first option-large-scale collection of field data-is expensive

ECOLOGICAL ASPECTS OF NITROGEN FIXATION BY PURSHIA TRIDENTATA

SummaryThis study examines several aspects of nitrogen fixation by Purshia tridentata (Pursh) D.C., a rosaceous shrub widespread in the Central Oregon pumice region, especially as an understory species in Pinus ponderosa and Pinus contorta forests. Acetylene reduction was used to assay nodule activity in both field and greenhouse plants. The maximum rates were observed at 20°C, although summer soil temperatures were frequently around 15°C, at which a much lower rate was observed. Acetylene reduction by excised nodules was linear for 5 h and then slowly declined, finally ceasing after 19 h. Nodule activity declined in water stressed plants, essentially ceasing in plants with xylem pressure potentials below −25 bars.Field studies at five sites revealed that nodule activity began in mid-May or early June when soil temperature at 20 cm increased to above 10°C. Activity began later and remained lower until July 20 in plants located under Pinus contorta, probably because of the cooler temperatures at this site. Nodule activity at all sites was maximum in June and July. In late July, nodule activity declined sharply, corresponding with moisture stress readings in the −25 bar range. Acetylene reduction rates declined sharply during the night; this decline was even more severe late in the season.Only 46 per cent of Purshia plants were nodulated. Several possible explanations for this low nodulation are discussed, but the primary reasons appear to be low soil temperature and unfavorable moisture conditions. Previous speculations that Purshia may contribute significant amounts of nitrogen to the ecosystems in which it occurs are disputed using estimates based on seasonal acetylene reduction rates and a determination of nodule biomass at one site. The estimated nitrogen accretion rate was only 0.057 kg N/ha.yr. re]19760310

Recovery of forest understories buried by tephra from Mount St. Helens

To determine the effects of tephra (volcanic aerial ejecta) on forest understory plants, six sites were chosen along a tephra depth gradient (23 to 150 mm) northeast of Mount St. Helens, USA. All sites were in old forests beyond the limits of direct blast damage from the volcanic eruption. At each site, 150 one m2 plots were permanently marked; all tephra was removed from 50 of these in 1980. Cover and density of plant species were recorded during 1980, 1981, and 1982. Tephra 23 mm deep had almost no effect on cover and density of vascular plants, and reduced bryophyte cover for only two years. Tephra 45 mm deep destroyed almost all bryophytes. Although damaged by 45 mm tephra, deciduous herbs recovered by 1982, but some evergreen species did not. Tephra 75 mm deep reduced herb cover in 1982 to 32% and density to 26% of that in cleared plots. At two sites with an average tephra depth of 150 mm, almost all herbs were eliminated except in microsites where tephra was thin, but shrub abundance was greatly reduced only where snow had been present during tephra deposition. Almost all cover was contributed by plants established previous to the eruption; seedling cover never exceeded 0.2%. Refugia with thin tephra, resulting from erosion, were vital to the survival of many species, especially bryophytes.

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

Growth and development of natural seedlings of Abies and Tsuga in old-growth forest

One- to six-year-old natural seedlings of Abies amabilis, Tsuga heterophylla and T. mertensiana were collected from 15-cm-deep volcanic tephra deposited beneath a surviving tree canopy during the 1980 eruption of Mount St Helens, Washington, U.S.A. Seedling dimensions and architecture were measured and dry weights of leaves, stem and roots determined as loss-on-ignition. Abies seedlings were heavier than Tsuga initially, with deeper and more-complex roots; however, biomasses of Abies and Tsuga seedlings became more similar with time. Height above the cotyledons was similar in older seedlings of the two genera, but branching began earlier in Tsuga than in Abies (...)

Survival of Plants Buried for Eight Growing Seasons by Volcanic Tephra

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