Walter E. Westman

Managing for Biodiversity

A discussion is presented of efficient strategies for species preservationin spite of continued human alteration of the environment. Current policy and unresolved questions are included in the discussion. Incentives to maintain seminatural areas as a conservation strategy are recommended: planting of hedgerows or windbreaks to provide corridors for migration of species during climate change; purchase of development rights of natural and seminatural land for conversion to park reserves when climate stabilizes; use of intercropping, traditional forest gardens and crop plantings in the tropics; and maintenance of seminatural habitats on public and private lands.

Diversity Relations and Succession in Californian Coastal Sage Scrub

The facultatively drought-deciduous shrublands of coastal California and Baja Cali- fornia are lowest in species richness of the four Mediterranean-climate regions where this physiog- nomic type occurs. Alpha richness in the North American coastal sage scrub varies primarily with the abundance of herbaceous annual species. Herb levels in turn vary with differences in levels of precipitation, favorableness of temperature during the winter and spring growing season, shading by shrubs, soil nitrogen, and air pollution. Levels of herbaceous annuals are highest in the growing season following fire, and show a second pulse of abundance in stands 15-25 yr old. Mature stands of coastal sage scrub are typically low in species equitability, due at least in part to the shade- intolerance of the herbaceous understory and to reduced levels of soil nitrogen. Symbiotic nitrogen- fixing organisms are virtually absent from stands which have not burned in 20 yr or more. The pattern of postfire succession varies markedly with fire intensity; dominant shrubs sprout abundantly from root crowns only following less intense fires. While there are -50 widespread sage scrub species, more than half of the 375 species encountered in the present study of the sage scrub flora are rare in occurrence within the habitat range. In view of the reduction of the area of coastal sage scrub in California to 10-15% of its former extent and the limited extent of preserves, measures to conserve the diversity of the flora are needed.

Measures of resilience: the response of coastal sage scrub to fire

Measures of four components of resilience are developed and used to quantify the response of coastal sage scrub to fire in southern California: (1) elasticity (rate of recovery following disturbance), (2) amplitude (threshold of disturbance beyond which recovery to the original state no longer occurs), (3) malleability (extent of alteration of the new stable-state from the original) and (4) damping (extent and duration of oscillation in an ecosystem parameter following disturbance). Vegetation and soil properties measured before fire, and for the first 5–6 yr after fire on four coastal (Venturan association) and four inland (Riversidian association) sites of coastal sage were used to follow changes. In addition, results from a simulation model of post-fire succession in Venturan coastal sage scrub (the FINICS model of Malanson) were used to examine resilience behavior over a 200 yr period. Resilience behavior of coastal sage scrub is critically influenced by the presence of a competitive mix of inherently strongly and weakly resprouting species. Sites dominated by weak resprouters exhibit lower elasticity and less damping of year-to-year fluctuations in composition in the early post-fire years. Sites with a mixture of weak and strong resprouters have a lower threshold of disturbance (amplitude) before species extirpation occurs, a result intensified by a higher frequency of disturbance. Malleability is also greater in these systems under higher disturbance frequency.

Xeric Mediterranean-type shrubland associations of Alta and Baja California and the community/continuum debate

A survey of the xeric shrublands of Pacific coastal North America from San Francisco to El Rosario (Mexico), including the inner Channel Islands, was conducted using 99 sample sites of 0.063 ha size. TWINSPAN classification and DECORANA ordination confirmed the existence of two plant formations, distinguishable physiognomically: coastal sage scrub and coastal succulent scrub. Within coastal sage scrub, four floristic associations were recognized: Diablan, Venturan, Riversidian and Diegan. Within coastal succulent scrub, two floristic associations were defined: Martirian and Vizcainan. These associations occur in distinct geographical regions following the coastline, with the Riversidian association occurring in the basin inland from Venturan and Diegan regions. Their locations are strongly correlated with differences in evapotranspirative stress regimes. The Channel Island sites show affinities to several of the mainland associations. The Venturan association can be further subdivided floristically into two subassociations, dominated by Salvia mellifera and S. leucophylla respectively. These subassociations which are coextensive geographically at a regional scale, typically do not intermingle at a local scale but often meet along sharp boundaries in the landscape. The dominant species segregate by moisture preference, S. mellifera preferring coarser-texture soils and more southerly aspects than S. leucophylla. Richness and equitability of these sites are depressed relative to other xeric shrubland sites, reflecting the fact that the two subassociations partition the Venturan flora into substantially non-overlapping subsets of species. This segregation of associates between the two Salvia dominance types strongly suggests biotic influence of the dominants on subordinate species, perhaps mediated by allelopathy. This biotic interaction, leading to relatively strong floristic subassociations segregating independently in the landscape, would provide an example of the holistic community structure referred to by Clements and his followers, embedded within a larger pattern of continuity in species distributions.

Oxidant Effects on Californian Coastal Sage Scrub

Causes for the reduced cover of native species of coastal sage scrub in certain southern Californian sites wvere sought among 43 habitat variables. The mean annual concentration of oxidants (which averaged 18 parts per 100 million on the 11 most polluted sites) is statistically indicated as the most likely causal factor. Sites of high oxidant levels in the region are also characterized by declining species richness and equitability.

Post-Fire Succession in Californian Coastal Sage Scrub: the Role of Continual Basal Sprouting.

Dominant shrub species of coastal sage scrub in coastal southern California are able to produce shoots from their base on a continual basis in the absence of fire or other major defoliation. As a result, each shrub becomes a population of mixed-aged branches (ramets) and extends its duration in the canopy beyond the age of any of its above-ground phytomass, reduces the incidence of senescence, and permits an individual (genet) to survive during long fire-free intervals. A computer simulation of succession in coastal sage scrub under differing fire intervals shows that continual basal sprouting may be significant in influencing the long-term composition of the vegetation.

Seasonal dimorphism of foliage in Californian coastal sage scrub

SummaryA pattern of seasonal variation in leaf size and position relative to the stem is reported for twelve mesophyllous shrubs in the coastal sage scrub of Mediterranean-climate areas of southern California. Leaves of reduced size, produced on axillary short shoots late in the growing season, persist through the dry season while many of the larger leaves on main stems are abscissed. The short-shoot leaves of Salivia mellifera have the same density thickness as those of the main stem, leaving open the question of whether the smaller summer leaves have physiological mechanisms for drought tolerance beyond a reduction in total transpiring surface. In Californian coastal shrublands seasonal dimorphism of leaves appears limited to mesophyllous shrubs. It is also found in analogues in comparable climatic regions of the Mediterranean, Chile and South Africa.

Air pollution injury to coastal sage scrub in the Santa Monica Mountains, Southern California

A field survey observed 26 types of foliar damage symptoms on seven species of coastal sage scrub in the Santa Monica Mountains National Recreation Area near Los Angeles, California. Of these, 6 symptoms were likely related to herbivory. Forty percent of visual injury symptoms in the field matched symptoms observed on these species exposed for 10 weeks to controlled fumigation with 0.1, 0.2 and 0.4 ppm of O3 and 0.05, 0.2 and 0.5 PPM of SO2 or combinations of these. Based on a comparison of field and chamber symptoms, it appeared that both 03 and SO2 were responsible for field injury symptoms, with 03 being the more frequent cause. Symptoms also appeared to be produced at lower concentrations in the field than in chambers. Using the foliar damage symptom present in greatest intensity on a 3-point scale, species were generally found to exhibit increased intensity of this symptom on sites estimated to experience higher levels of 03, SO2 and NO2. Pollution levels at sites were estimated from seven surrounding air quality monitoring stations. One- or two-month-old leaves exhibit sufficient intensity of symptom damage that not all observed injury can be attributed to senescence. Brachyblast leaves of Salvia mellifera exhibited more severe damage symptoms than dolichoblasts, confirming fumigation chamber results; for three other seasonally dimorphic species, brachyblasts were not more sensitive. Fasciated stems were found both in fumigation chambers and in the field on Artemisia californica, Lotus scoparius and Eriogonum fasciculatum, but sample sizes were not sufficient to distinguish among alternative potential causes of this phenomenon.

Plant Community Structure — Spatial Partitioning of Resources

The spatial partitioning of phytomass is considered in this chapter at three levels of community organization: the supra-organismal (patterning among suites of species, associations and “microcommunities”), the organismal (spatial patterning of a species colony or population) and the suborganismal (vertical and horizontal arrangement of leaves, stems and roots). The evergreen and facultatively deciduous shrublands of the five mediterranean-climate regions are considered, recognizing that the distinctive soil fertility levels and longer evolutionary histories of the Australian and South African heathlands reduce their comparability with other mediterranean-climate shrublands (Specht 1979a; Naveh and Whittaker 1980).

Patterns of nutrient flow in the pygmy forest region of northern California

Vegetation of the Mendocino coastal region forms a gradient from coastal redwood (Sequoia sempervirens) stands, through Bishop pine (Pinus muricata), to endemic pygmy conifers (Pinus contorta ssp.bolanderi, Cupresses pygmaca) on terrace flat/podzols. Along this gradient nutrient stocks and net uptake decrease, and strategies for rapid nutrient circulation within the ecosystems are increasingly in evidence. Within the pygmy conifer forest, the dominant plant species exhibit differing characteristics in their pattern of net uptake, partition and turnover or recirculation of each essential element. ‘The pygmy forest nutrient pool has apparently been built up over centuries by soil weathering, N-fixation by lichens, and inputs from precipitation. Such an ecosystem would be likely to show weak’ resilience to a perturbation that removed standing vegetattion or the litter layer. Across a wide range of terrestrial ecosystems, the ratio of plant nutrient stocks to phytomass tends to decrease with increase in phytomass and net primary productivity. This trend is reflected on a smaller scale by the decrease in nutrient phytomass ratio from pygmy forest, through Bishop pine, to redwood stands (0.013, 0.009, 0.006 respectively). Nevertheless, terrestrial communities in the temperate and subtropical regions of moderate rainfall and non-saline soils appear to share a relatively constant nutrient/phytomass concentration (mean ±s.e.=0.101±0.0003; n=9).