Topographic Moisture Gradient Research Articles

Montane Meadow in the Siskiyou Mountains, Oregon, USA

Photo 1. Montane meadow on Grayback Mountain, Oregon, in Whittaker's study region, illustrating the high diversity of what he termed mesic sites along the topographic moisture gradient. Photo credit: Susan Harrison. This photograph illustrates the article “Plant community data collected by Robert H. Whittaker in the Siskiyou Mountains, Oregon and California, USA” by Robert H. Whittaker, Ellen I. Damschen, and Susan Harrison published in Ecology. https://doi.org/10.1002/ecy.3764.

Moisture availability limits subalpine tree establishment.

In the absence of broad-scale disturbance, many temperate coniferous forests experience successful seedling establishment only when abundant seed production coincides with favorable climate. Identifying the frequency of past establishment events and the climate conditions favorable for seedling establishment is essential to understanding how climate warming could affect the frequency of future tree establishment events and therefore future forest composition or even persistence of a forest cover. In the southern Rocky Mountains, USA, research on the sensitivity of establishment of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa)-two widely distributed, co-occurring conifers in North America-to climate variability has focused on the alpine treeline ecotone, leaving uncertainty about the sensitivity of these species across much of their elevation distribution. We compared annual germination dates for >450 Engelmann spruce and >500 subalpine fir seedlings collected across a complex topographic-moisture gradient to climate variability in the Colorado Front Range. We found that Engelmann spruce and subalpine fir established episodically with strong synchrony in establishment events across the study area. Broad-scale establishment events occurred in years of high soil moisture availability, which were characterized by above-average snowpack and/or cool and wet summer climatic conditions. In the recent half of the study period (1975-2010), a decrease in the number of fir and spruce establishment events across their distribution coincided with declining snowpack and a multi-decadal trend of rising summer temperature and increasing moisture deficits. Counter to expected and observed increases in tree establishment with climate warming in maritime subalpine forests, our results show that recruitment declines will likely occur across the core of moisture-limited subalpine tree ranges as warming drives increased moisture deficits.

Predicting Ecosystem Resilience to Fire from Tree Ring Analysis in Black Spruce Forests

Climate change has increased the occurrence, severity, and impact of disturbances on forested ecosystems worldwide, resulting in a need to identify factors that contribute to an ecosystem’s resilience or capacity to recover from disturbance. Forest resilience to disturbance may decline with climate change if mature trees are able to persist under stressful environmental conditions that do not permit successful recruitment and survival after a disturbance. In this study, we used the change in proportional representation of black spruce pre- to post-fire as a surrogate for resilience. We explored links between patterns of resilience and tree ring signals of drought stress across topographic moisture gradients within the boreal forest. We sampled 72 recently (2004) burned stands of black spruce in interior Alaska (USA); the relative dominance of black spruce after fire ranged from almost no change (high resilience) to a 90% decrease (low resilience). Variance partitioning analysis indicated that resilience was related to site environmental characteristics and climate–growth responses, with no unique contribution of pre-fire stand composition. The largest shifts in post-fire species composition occurred in sites that experienced the compounding effects of pre-fire drought stress and shallow post-fire organic layer thickness. These sites were generally located at warmer and drier landscape positions, suggesting they are less resilient to disturbance than sites in cool and moist locations. Climate–growth responses can provide an estimate of stand environmental stress to climate change and as such are a valuable tool for predicting landscape variations in forest ecosystem resilience.

Short‐term manipulation of precipitation in Mongolian steppe shows vegetation influenced more by timing than amount of rainfall

AbstractQuestionsHow does plant productivity and community composition in the mountain steppe of northern Mongolia respond to increased precipitation? Do these changes differ with the timing of precipitation?LocationMountain steppe at ecotone between steppe and taiga, Dalbay Valley, Lake Hövsgöl International Long Term Ecological Research Site (ILTER), northern Mongolia (51°01.405′ N 100°45.600′ E).MethodsWe experimentally applied water, representing a 25% increase in growing season precipitation, to plots at the drier end of a topographic moisture gradient in mountain steppe vegetation. In one treatment, supplemental water was applied weekly for four growing seasons. A second treatment started in year 2, in which three times the weekly supplement of water was applied once every 3 wk for three growing seasons. Vascular plant species presence and percentage cover were used to assess water treatment effects on forb and graminoid composition. Plant species biomass and percentage cover in year 4 was used to determine treatment effects on total productivity, and on the abundances of legumes, non‐leguminous forbs and graminoids, separately. Lichen, litter and root biomass, plant available N, P and K, and seedling demography were also examined in year 4.ResultsWeekly watering changed the relative abundances of forbs and increased the total abundance of legumes and non‐leguminous forbs measured either as percentage cover or biomass. The 3‐wk watering treatment did not induce the same changes in forb composition or abundance of functional groups but produced less compositional change in graminoids compared to controls. Neither treatment increased total productivity. Three‐week watering increased plant available P and decreased litter. Seedling emergence, survival and size increased with one or both watering treatments.ConclusionIn mountain steppe, precipitation increases will produce immediate changes in composition and abundance of forbs, including legumes, but not in graminoids or total productivity. The steppe is more responsive to regular incremental increases in precipitation than to less frequent, larger storms whose effect would be timing‐dependent within the short, temperature‐limited growing season. Longer term investigations need to be conducted to explore if further lagged responses to water addition occur.

Changing fire regimes and old-growth forest succession along a topographic gradient in the Great Smoky Mountains

Patterns of past fire disturbance may be an important contributor to contemporary vegetation composition and structure in old-growth forests of the southern Appalachian Mountains. However, due to a lack of information on pre-suppression fire regimes, vegetation pattern in the region has been primarily attributed to variability in soils and climate. To assess the pre-suppression fire regime’s role in shaping vegetation pattern, we characterized temporal patterns of tree establishment in an old-growth forest that experienced two centuries of frequent fire, followed by a century of fire exclusion. Forest plots were inventoried and cored to characterize age structure and composition in yellow pine, chestnut oak, white pine-oak, and cove forest communities on the south-facing slope of an old-growth watershed in Great Smoky Mountains National Park, Tennessee USA. We compared the timing and composition of tree establishment from the following disturbance periods: (1) frequent fire 1700–1909; (2) post fire 1910–1949; and (3) mesophication 1950–2000. Non-metric multidimensional scaling characterized successional change between the three age classes. Multivariate dispersion, species richness, and beta diversity were calculated for establishment in each disturbance period. We found distinct peaks in tree establishment in the yellow pine, chestnut oak, and white pine-oak stand types that occurred soon (<40years) after fire cessation at the site. Xerophytic fire-tolerant species dominated establishment during the period of frequent fire; a mixture of xerophytic and mesophytic species established during the period immediately following the last major fire; and mesophytic, fire-intolerant species dominated establishment during the recent period of mesophication. Cohort recruitment was less clearly linked to fire suppression in the mesic cove stands; however “fire protected” cove stands exhibited different age structure and composition compared to cove stands adjacent to the frequently burned south-facing slope. Mean plot level species richness was greatest in the tree cohort that established soon after the last major fire; while beta diversity and multivariate dispersion were highest in the trees that had established during the frequent fire period. Tree establishment has generally shifted from shade-intolerant, drought-tolerant species to shade-tolerant, drought-intolerant species along the entire south-facing slope. Successional trajectory indicates a loss of yellow pine and chestnut oak communities as the xeric and sub-xeric sites convert to white pine and cove forest communities, which were formerly restricted to sub-mesic and mesic positions. Declines in beta diversity and multivariate dispersion within younger age classes indicate that in the absence of fire disturbance, community differentiation is declining along the topographic moisture gradient.

Widespread negative correlations between black spruce growth and temperature across topographic moisture gradients in the boreal forest

The responses of tree growth to recent climate warming may signal changes in the susceptibility of forest communities to compositional change and consequently impact a wide range of ecosystem processes and services. Previous research in the boreal forest has largely documented negative growth responses to climate in forest species and habitats characteristic of drier conditions, emphasizing the sensitivity of drier or warmer landscape positions to climate warming. In this study, we explored relationships between climate and tree-ring growth of black spruce, a dominant tree species typical of cool and moist habitats in the boreal forests of North America. We assessed how these responses varied with stand characteristics and landscape position across four different regions in Alaska and Yukon Territory. Approximately half of the trees measured across regions and topographic gradients exhibited reduced radial growth in response to warm temperatures in the previous growing season and current spring, which we interpret as a signal of drought stress. Although we found considerable variation in the growth responses of individual trees within sites, landscape position and stand characteristics were weak predictors of this variability, explaining ≦̸12% of the variation in any region. Our results indicate that future warming, particularly in spring, is likely to result in drought stress and a reduction of black spruce radial growth independent of region, landscape position, or stand characteristics. The occurrence of negative growth responses to temperature, even in cool and moist habitats, suggests that drought stress limitations may be more widespread in the northern boreal forest than previously anticipated, indicating broad sensitivity of ecosystem processes and services to climate change across a diverse range of habitat types.

Tree growth response to fuel reduction treatments along a topographic moisture gradient in mixed-oak forests of Ohio, U.S.A.

This study examined the effect of the soil moisture gradient on tree growth response to prescribed fire and thinning in oak-dominated forests of Ohio. Six hundred and ninety-six increment cores (348 trees, five species) were collected from eighty 0.1 ha plots distributed across four treatments (control, thin, burn, thin + burn) in two sites. Ring widths were converted to basal area increments (BAIs). A water balance approach based on geographic information systems (GIS) was used to assess the potential evapotranspiration (PET) and moisture deficit for each tree, along with a long-term integrated moisture index, also based on GIS. The moisture gradients varied considerably across the landscape, with the highest PET and moisture deficit on ridges and south-facing slopes. This variation influenced the BAI of the studied species, but more strongly in the control stands than in the managed stands, where treatment effects became the main drivers of growth. Oaks exhibited greater BAI on sites with intermediate moisture demand or stress, whereas the non-oaks had greater BAI on more mesic sites. Moisture deficit and PET also interacted to influence BAI of yellow-poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.), particularly in the control. These results demonstrate the strong regulatory effect of the topographically controlled soil moisture gradient on tree growth in mixed-oak (Quercus spp.) forests, which can be explored to better understand community response to prescribed fire and thinning treatments.

Dendrochronological analysis of white oak growth patterns across a topographic moisture gradient in southern Ohio

Moisture availability is a key factor that influences white oak (Quercus alba L.) growth and wood production. In unglaciated eastern North America, available soil moisture varies greatly along topographic and edaphic gradients. This study was aimed at determining the effects of soil moisture variability and macroclimate on white oak growth in mixed-oak forests of southern Ohio. Using accurately dated and measured tree rings, we analyzed 119 white oaks growing across an integrated moisture index (IMI), a computer-generated GIS model that simultaneously combines topographic and edaphic features into a moisture index scale. Growth trends varied considerably across the IMI, with trees in mesic sites exhibiting patterns much different from those in either xeric or intermediate sites. BAI growth and biomass increments were higher for trees growing in the intermediate and mesic sites than those from the xeric sites. Correlation and response function analyses, and redundancy analysis revealed significant relations between ring-width indices and climate, with current year May–July PDSI, precipitation and temperature as the most important correlates of white oak growth. Additionally, climatic influences on growth rate were variable across the IMI; trees in xeric sites showed much greater coefficients relative to those from the intermediate and mesic sites. Despite these differences, xeric and intermediate trees exhibited similar growth patterns. The present results provide further evidence of the usefulness of the IMI for identifying and comparing white oak growth patterns across the complex, dissected landscape of southern Ohio.

Soil aeration in relation to soil physical properties, nitrogen availability, and root characteristics within an arctic watershed

The seasonal change in soil oxygen availability was determined in several habitats along a topographic moisture gradient in an arctic watershed. The effect of changes in soil aeration on soil chemical and plant properties was examined by comparison of the driest (tussocks) and wettest (wet sedge tundra) sites along this gradient. Spatial variability and seasonal change in soil oxygen availability was closely linked to the hydrologic regime and the thickness of the organic soil horizon. The greatest extension of the aerobic soil layer was found beneath well-drained tussocks, while less than 10% of the unfrozen soil layer is aerated in flooded wet sedge tundra. Intertussock areas and watertracks (channels of water drainage) have intermediate levels of aeration. In tussock tundra, soil oxygen diffusion is restricted in the mineral soil layer below the organic horizon due to reduced pore space. Organic matter constituents and their change with depth were similar beneath tussocks and in wet sedge tundra, indicating that factors other than soil aeration (e.g. low soil temperatures, short growing season) are the primary controls on decomposition in these two arctic tundra systems. NH4+, the dominant form of inorganic N, was more available in wet sedge tundra than in tussock tundra. At both sites, extractable and soil solution NO3- concentrations increased 4 to 8 fold in the second part of the growing season, indicating increased nitrifier activity with improved soil oxygen availability. Although soils thawed as deep as 60 cm, approx. 90% of the root biomass was concentrated within 20 cm of the surface. Despite the anaerobic soil environment in wet sedge tundra, the dominant species there, Eriophorum angustifolium, reached slightly greater rooting depths than E. vaginatum, whose roots grow in the elevated, aerobic portion of tussocks. E. angustifolium had a root porosity of 31%, within the range found for wetland species, while roots of E. vaginatum had a porosity close to 12%. Rhizome porosity were low in both species (11%).

Middle and high elevation coniferous forest communities of the North Rim region of Grand Canyon National Park, Arizona, USA

We examined the composition and structure of forest communities in a 3700 ha watershed in relation to environmental gradients and changes in land management practices. We identified four mixed-conifer forest types dominated by different combinations of Abies concolor, Picea pungens, Pinus ponderosa, Populus tremuloides, and Pseudotsuga menziesii, and a spruce-fir type dominated by Picea engelmannii and Abies lasiocarpa. The forest types occur in a complex pattern related to elevation and topographicmoisture gradients and variations in past fire regimes. However, widespread regeneration of A. concolor following possible changes in the fire regime in the late 19th century and continuing with institution of a fire suppression policy early in the 20th century is producing a more homogenous mixed-conifer forest with greater horizontal and vertical continuity of fuel. This shift toward landscape homogeneity not only may adversely affect biodiversity, but also may be perpetuated as the probability of large, high-severity fires increases with continued fire suppression.

Within-community vegetation structure in the conifer woodlands of the Siskiyou Mountains, Oregon

The conifer woodlands found on serpentine-derived soils in southwest Oregon, USA, are characterized by a debse but discontinuous shrub stratum and a species-rich herbaceous stratum. Quantitative analysis of shrub and herb distributions within 10 sites showed that small-scale patterns within the conifer woodland sites are as important as landscape-scale patterns in community organization. Gradient analysis was used to describe the distribution of herbaceous species with respect to gradients of shrub influence (shading, decreased soil temperature, increased soil moisture, increased litter depth) within sites and topographic moisture among sites. Regression analysis of the resulting species distributions and calculations of alpha and beta diversity showed that (a) the shrub-influence microgradient significantly affected distributions for 15 out of 20 major herbaceous species, and the topographic-moisture gradient influenced 13 out of the 20 species, (b) species richness was higher under intermediate conditions along both gradients, and (c) beta diversity within communities was 2.5 to 3.8 times the beta diversity of the site-to-site topographic-moisture gradient.

Vegetation Patterns and Structure of an Old-Growth Forest in Southeastern Ohio

MCCARTHY, B. C. (Dept. of Biological Sciences, Rutgers University, P.O. Box 1059, Piscataway, NJ 08855), C. A. HAMMER, G. L. KAUFFMAN AND P. D. CANTINO (Dept. of Botany, Ohio University, Athens, OH 45701). Vegetation patterns and structure of an old-growth forest in southeastern Ohio. Bull. Torrey Bot. Club 114:33-45. 1987.-A large, minimally disturbed, and floristically diverse old-growth forest (Hawk Woods) on the Allegheny Plateau in southeastern Ohio was studied to compare woody vegetation structure, composition, and soils both within and among communities. Five vegetation types were initially identified based on visual estimates of dominance and indicator species: mixed mesophytic, mixed oak-tulip poplar, white oak, oak-hickory, and oak-heath. Principal components ordination did not reveal distinct groupings of stands within and among vegetation types. Further quantitative analysis suggested that fewer ecological groupings were sufficient to describe vegetation types. Cluster analysis supported three ecological groupings apparently related to a topographic-moisture continuum. Multivariate analysis of variance of soil variables indicated significant differences among stands. Linear correlation of soil variables with principal components suggested the presence of a soil nutrient gradient associated with the topographic-moisture gradient. Edaphic parameters were not useful for discriminating community types but did however covary with the vegetation continuum. Size-class analysis of vegetation types indicated similarities and differences both among communities within Hawk Woods and among other forests in the Plateau region. Diameter distribution curves for vegetation types were observed to fit the negative power function more closely than the negative exponential function. Size-class distributions of individual species varied, apparently as a function of broad-scale ecological characteristics (e.g., shade tolerance). Quercus spp. exhibited marked underrepresentation in the smaller size-classes, suggesting an inability to self-perpetuate.

An Analysis of the Forests of the Bristol Hills of New York

A detrended correspondence analysis ordination display of 433 20 x 50 m forested plots and 11 100-m line-intercepts from the Bristol Hills of New York indicated that the vegetation was responding to a topographic-moisture gradient and a successional gradient. Comparison of plots according to aspect showed that a portion of the moisture gradient was related to differences in slope aspect. The moisture gradient by aspect went from xeric S to W to ridge-top to Nto E-facing mesic slopes. Percent similarities analysis showed the vegetational aspect differences to be due to differences in relative importance values of the species and not to large differences in species presence and absence. Sugar maple and beech extend further upslope on Eand N-facing slopes than on drier W slopes. Red oak becomes a leading dominant at a lower elevation on W and S slopes than on Nand E-facing slopes. Chestnut oak is not a very important species on E slopes but has an importance value of 10-23% on other aspects. Vegetational differences along the topographic-moisture gradient are the result of the beech-maple forest being at lower elevations and the hemlock-white pine-northern hardwood forest occupying the higher elevations.

Vegetation analysis, primary production and selected nutrient budgets for a southern Appalachian oak forest: A synthesis of IBP studies at Coweeta

The vegetation of lower slope (700–1000 m) positions in the southern Appalachians is dominated by several species of oaks. Based in basal area these forests have attained pre-chestnut blight levels; however, when age structures are examined one still finds evidence of disturbance (chestnut blight, logging and drought) in the form of the more individuals that became established between 1918–1938. The forests are composed of about 40 tree species whose spatial distribution patterns on the slopes seem to follow complex topographic moisture gradients. A group of mesic species (tulip poplar, yellow birch, hemlock, witch hazel, dogwood and rhododendron) are positively correlated with distance from the water divide and aspect, and are negatively correlated with distance from the stream channel and elevation. A second group of more xeric species (chestnut oak, scarlet oak, mountain laurel, sourwood, red maple, black gum and pignut hickory) are positively correlated with distance from the stream channel and elevation, and are negatively correlated with distance from the water divide. Total standing crop of biomass was estimated to be 191,332 kg ha −1 (roots 27%, stems 70%, leaves 3%). Even though these are considered to be deciduous forests, evergreen species contribute 30–35% to leaf biomass. Root biomass is equally divided between roots < 2.5 cm diameter (27,216 kg ha −1) and those > 2.5 cm diameter (24,216 kg ha −1). Ninety-five of the roots are in the upper 60 cm of soil. Total NPP was estimated at 14,754 kg ha −1year −1 (6000 kg roots, 4195 kg leaves, 2907 kg wood, 680 kg bark, 424 kg reproductive tissue, 365 kg loss to consumers, 184 kg twigs) or 7.7% of the total standing crop. From a mineral cycling point of view some tree species are more important than one would predict based purely on density. There are several reasons: (1) higher nutrient concentrations in certain tissues of some species; (2) proportion of nutrient rich tissues varies with species; and (3) during any one year allocation of NPP to various tissues varies with species.

Age-Structure Relationships of Trees Species in an Appalachian Oak Forest in Southwest Virginia

Ross, M. S., T. L. SHARIK, and D. WM. SMITH. (Dept. Forestry, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061, U.S.A.). Age structure relationships of tree species in an Appalachian oak forest in southwest Virginia. Bull. Torrey Bot. Club 109: 287-298. 1982.-The complete age and diameter structures were determined for all tree species in seven Appalachian oak stands arranged along a topographic moisture gradient in southwestern Virginia. All stands but one exhibited a prominent gap in the age distribution in the 20-40 year age classes. No gap was evident in the corresponding diameter distributions. Total and juvenile age distributions were described for selected species. Age structures differed among species, and reflected the different spatial and temporal niches which they occupy. It was hypothesized that the shape of the total age distribution in individual stands is related to the disturbance sequence, and to site through its influence on the species present. Coupling of age and diameter distributions of individual populations provided insights into the functional roles of species along the moisture gradient.

Drought tolerance and water use by plants along an alpine topographic gradient.

In the alpine zones of western North American mountains, topographic-moisture gradients are the results of winter winds blowing snow from the upper windward slopes and ridgetops into snowdrifts on the lee slopes. Wet meadows at the foot of the lee slope are the result of summer snowmelt. Such gradients are repeated many times in a single mountain range. They are useful units for studies of the effects of drought and water use on patterning of alpine vegetation.The research was done through an entire growing season along a topographic gradient at 3,300 m in the Medicine Bow Mountains, Wyoming. Plant water potentials were measured on 29 species (pre-dawn to sunset) at weekly intervals. Simultaneously, leaf conductances were measured on 16 species of these; the remainder had leaves too small for the porometer. Leaf water potentials were generally lowest on the ridgetop and highest in the wet meadow. Highest mean leaf conductances were in the wet meadow plants; the lowest occurred on plants on the upper windward slope. None of the plants on the ridgetop had leaves large enough for the porometer. Plants of most species at all sites but the wet meadow showed sharply reduced leaf conductance or leaf death at plant water potentials below-1.5 MPa. Deep-rooted species such as Trifolium parryi showed maximal conductance at water potentials as low as-1.7 MPa and little reduction in conductance even at lower water potentials. Plant water potentials and leaf conductances showed close relationships with rooting depth, length of dry periods, and position on the gradient.

Forest vegetation of the Colorado Front Range

The forest vegetation of the northern Colorado Front Range was studied using a combination of gradient analysis and classification methods. A graphical model of forest composition based on elevation and topographic-moisture gradients was constructed using 305 0.1 ha samples. To derive the topographic moisture gradient, stands were stratified into eight 200 m elevation belts, and then ordinated by correspondence analysis using understory (<1 m) data. Each of the resultant gradients was scaled against a standard site moisture scalar derived from incident solar radiation and topographic position. Except for krummholz sites, the vegetation defined gradients fit the moisture scalar closely. Once scaled, these gradients were stacked vertically, sandwich-style, to create the graphical representation shown in Figure 5.

Structure of coniferous forest communities in Western Washington: Diversity and ecotope properties

Vegetation patterns were examined in three regions of coniferous forest, arrayed along a gradient of decreasing maritime influence. The following responses to decreasing maritime influence were noted: all aspects of species diversity increase; growth-form complexity increases; habitat heterogeneity increases; species habitat breadths decrease; and habitat overlaps decrease. These observations result from studies of both individual plots and data aggregated into community types. The contrasts between the maritime western study region and the more continental eastern study region imply that ecotope differentiation increases from west to east. The enhanced structural complexity of the eastern region results from climatical limitation of the dominant tree species. Reduced canopy dominance produces greater within-habitat heterogeneity with respect to moisture, light, and soil properties. Furthermore, contrasts between the extremes of the topographic-moisture gradient increase toward the east. These factors combine to increase alpha, beta, and gamma diversities in the eastern study region above those of the western study region. Corresponding to these diversity trends are decreasing mean relative niche, habitat, and ecotope breadths of species toward the east

Effects of Crude and Diesel Oil Spills on Plant Communities at Prudhoe Bay, Alaska, and the Derivation of Oil Spill Sensitivity Maps

Crude oil was spilled on six of the major Prudhoe Bay plant communities at an intensity of 12 l/m². The communities occurred along a topographic-moisture gradient. The reaction of the major species of the various communities was recorded one year following the spills. Sedges and willows showed substantial recovery from crude oil spills. Mosses, lichens, and most dicotyledons showed little or no recovery. On a very wet plot with standing water, the vegetation showed total recovery one year following the spill. Dry plots, on the other hand, showed very poor recovery. Dryas integrifolia M. Vahl, the most important vascular species on dry sites, was killed. Identical experiments using diesel oil rather than crude oil showed all species except an aquatic moss to be killed. A sensitivity index for the communities was calculated on the basis of the percentage cover of the resistant species divided by the original total plant cover of the community. With this information an oil spill sensitivity map for an area of Prudhoe Bay was constructed using a vegetation map as a base. Using the crude oil data from Prudhoe Bay together with some from the literature, a predictive sensitivity map was also constructed for an accidental crude oil spill at nearby Franklin Bluffs. In this example all the community types are considered to have moderate to excellent recovery potential. Implications of the experiments and the mapping exercises for oil spill contingency planning are discussed.

Vegetation of the Santa Catalina Mountains, Arizona: A Gradient Analysis of the South Slope

Vegetation of the southwest slope of the Santa Catalina Mountains of southeastern Arizona was sampled and transects prepared for 1,000—ft (305 m) elevation belts on granite and gneiss soils from the summit forests (2,440—2,750 m) to the base of the mountains (900 m). Transects also represented subalpine forests above 2,750 m in the Pinaleno Mts. and vegetation of the valley plain or bajada below the mountains, and samples were taken from volcanic soils below 900 m in the Tucson Mts. Principal community—types from high elevations to low are: subalpine forest (Picca engelmanni in the Pinaleno Mts. and Abies lasiocarpa), montane fir forest (Abies concolor, Pseudotsuga menziesii), pine forests (Pinus ponderosa, P. strobiformis), pine—oak forests (P. ponderosa, Quercus hypoleucoides), pine—oak woodlands (P. ponderosa, P. chihuahuana, Q. hypoleucoides, Q. arizonica), pygmy conifer—oak scrub (Pinus cembroides, Juniperus deppeana, Q. arizonica, Q. emoryi, Arctostaphylos pringlei, A. pungens, monocot shrubs), open oak woodland (Q. emoryi, Q. oblongifolia, Vauquelinia californica, monocot shrubs, and grasses), desert—grassland (Agave schottii, Haplopappus laricifolius, and grasses), Sonora desert of mountain slopes (north—slope shrub phase, and south—slope spinose—suffrutescent phase), upper bajada desert (Cercidium microphyllum, Franseria deltoidea), and lower bajada desert (Larrea tridentata). Forests of canyons and arroyos are also described. Relations of communities to elevation and topograhic moisture gradients are represented in a mosaic chart. Physiognomic relations of communities are represented in charts of growth—form coverage in relation to elevation and topographic moisture gradients. Growth—form diversity increases from high—elevation forests strongly dominated by evergreen—needleleaf trees to desert of lower mountain slopes in which pinnate leguminous trees, spinose shrubs, suffrutescent semi—shrubs, and stem—succulents share dominance. Among Raunkiaer life—forms hemicryptophyte species are most numerous at middle and higher elevations, phanerophyte species at lower elevations. In open oak woodlands and desert grasslands phanerophytes, hemicryptophytes, and suffrutescent chamaephytes each make up about one—third of the perennial flora. Desert floras of mountain slopes are characterized by predominance of suffrutescent chamaephytes over both phanerophytes and hemicryptophytes, and large numbers of therophyte species. Analysis in terms of geographic areas of species shows decreasing numbers of Rocky Mountain, Western, and Northern species from high—elevation forests downward, increasing numbers of Southwestern and Latin American species at lower elevations. Madrean species of the Mexican Plateau and Southwestern species predomonate in pine—oak forests and woodlands and pygmy conifer—oak scrub. Sonoran, Chichuahuan, and Latin American species predominate in the desert of lower mountain slopes, and widely distributed Southwestern species in the Larrea desert. Flora of the Catalina Mountains is rich and community species diversities are high. Species diversities increase toward lower elevations desert—grasslands and deserts of lower mountain slopes are among the riches communities in the United States. Floristic diversity is higher in continental than maritime climates, as indicated by comparison of species diversities and community differentiation along topographic moisture gradients in the Catalinas and in California mountains.