13 MEDITERRANEAN CALIFORNIA M.E. Fenn, E.B. Allen, L.H. Geiser 13.1 Ecoregion Description mixes depending on elevation and site conditions, but are typically characterized by dense mixed-aged stands Th e Mediterranean California ecoregion (CEC 1997; composed of coniferous (pine [Pinus spp.] and often fi r Fig 2.2) encompasses the greater Central Valley, [Abies spp.] and incense cedar [Calocedrus decurrrens]) Sierra foothills, and central coast ranges of California and broadleaved species (commonly oaks). south to Mexico and is bounded by the Pacifi c Ocean, Sierra Nevada Mountains and Mojave Desert. Th e 13.2 Coastal Sage Scrub ecoregion description is adapted from CEC (1997). It is distinguished by its warm, mild Mediterranean 13.2.1 Ecosystem Description climate, chaparral vegetation, agriculturally productive Coastal sage scrub is a semi-deciduous shrubland that valleys, and a large population (>30 million). Th e Coast occurs in the Mediterranean-type climate of southern Ranges crest at 600 to 1200 m. Th e broad, fl at, Central and central coastal California, extending southward to Valley is drained by the Sacramento and San Joaquin Baja California, Mexico. Th e United States portion of Rivers into the Sacramento Delta and San Francisco coastal sage scrub covers some 684,000 ha (CDFFP Bay. Rugged Transverse Ranges, peaking at 3506 m, 2007). Coastal sage scrub is very diverse. Unfortunately, border the Los Angeles basin. Soils are complex, mostly it is subject to extensive human impacts in rapidly dry, and weakly developed with high calcium (Ca) developing coastal California. Dominant species concentrations. Summers are hot and dry (>18 oC), throughout the range are coastal sagebrush (Artemisia winters are mild (>0 oC) with precipitation from winter californica), Eastern Mojave buckwheat (Eriogonum Pacifi c Ocean storms; coastal fog is common May fasciculatum), sage (Salvia spp.), brittlebush (Encelia through July. Annual precipitation (200 to 1000 mm) spp.), and other shrub species, with highest shrub is highly variable and droughts are common. Vegetation diversity occurring in the southern range. Th e forbs, is characterized by chaparral, patches of oak woodland, primarily annual, are especially diverse, with many grassland, and some coniferous forest on upper species of concern under the Endangered Species Act mountain slopes. Th e chaparral has thickened, hardened occurring throughout the range of coastal sage scrub in foliage resistant to water loss and forms a cover of closely California. spaced, mostly evergreen shrubs 1 to 4 m tall. Common shrubs include chamise (Adenostoma fasciculatum), 13.2.2 Ecosystem Responses to N Deposition buckbrush (Ceanothus cuneatus), and manzanita Ecosystem responses to elevated nitrogen (N) in (Arctostaphylos spp.). Coastal sagebrush (Artemisia coastal sage scrub include increases in exotic invasive californica) and summer-deciduous plants that tolerate annual grasses, loss of native shrub and forb cover, more xeric conditions are found at lower elevations. A reduced diversity of native annual forbs and arbuscular blue oak-California foothills pine (Quercus douglasii- mycorrhizal fungi, and elevated N mineralization. Pinus sabiniana) woodland community forms a ring Plants in this low-productivity ecosystem respond around the Central Valley, which once had extensive quickly to N; thus changes in species abundance grasslands and riparian forests. Th e southern oak are excellent indicators of ecosystem response to N woodland extends into the Transverse and Peninsular pollution. Th is is unlike ecosystems with long-lived Ranges and includes Southern California walnut (Juglans vegetation, such as forests or chaparral, where initial californica) and Engelmann oak (Quercus englemannii). ecosystem responses to N are measured by changes Mixed-conifer forests (Minnich 1999) occur at a higher in biogeocycling or nutrient runoff (Aber et al. 1989, elevation than chaparral and include a variety of species Section 13.3.3). Chapter 13—Mediterranean California GTR-NRS-80 143 13.2.3 Range of Ecosystem Responses Observed detected changes in soil N dynamics at a lower level of -1 -1 Studies along anthropogenic N deposition gradients N deposition (10 versus 4 kg N ha yr ) than the study -1 -1 and in experimental N-fertilized plots have shown by Sirulnik et al. (2007) (20.2 versus 6.6 kg N ha yr ), alterations in soil N cycling, plant cover and diversity, the former are used to report the N deposition response and arbuscular mycorrhizal fungi. Nitrogen cycling threshold for mineralization. studies include fi eld and laboratory N mineralization measurements that show higher turnover of inorganic Vegetation cover and species richness. Coastal sage scrub is -1 N under high N deposition or fertilization (Sirulnik et subject to levels of total N deposition up to 20 kg ha -1 al. 2007, Vourlitis et al. 2007a, Vourlitis et al. 2007b). yr , as estimated by the U.S. Environmental Protection Plant cover and diversity studies were done along an Agency’s Community Multiscale Air Quality (CMAQ) N deposition gradient and show threshold losses of model (Tonnesen et al. 2007) in inland Riverside and native annual forb species of 45 percent (a decrease from San Bernardino Counties, an area that has been rapidly 67 to 37 species per site) under 10 kg N ha-1 yr-1 of converted to exotic annual grassland in the past 30 to 40 modeled and 7.8 kg ha-1 yr-1 measured N deposition.15 years (Allen et al. 1998, Talluto and Suding 2008). Th e A study of arbuscular mycorrhizal fungi was made on conversion to grassland is likely caused by a combination the same gradient and showed similar threshold losses of of elevated N deposition that promotes increased grass mycorrhizal root infection and diversity at many of the biomass and frequent fi re that prevents establishment of same sites showing plant loss under 9 kg N ha-1 yr-1 of native shrubs (Allen et al. 1998, Minnich and Dezzani modeled N deposition (Egerton-Warburton and Allen 1998, Talluto and Suding 2008). 2000), as described below. Vegetation-type conversion studies in coastal sage scrub Nitrogen mineralization and nitrifi cation. Several have examined patterns of vegetation change, but have fertilization studies conducted in coastal sage scrub not determined the N deposition response threshold. used high levels of N fertilizer, 50 to 60 kg N ha-1 yr-1 Research in the adjacent North American Deserts (Allen et al. 1998, Vourlitis et al. 2007a). However, two ecoregion related N deposition, exotic grass biomass, studies on N mineralization (production of ammonium and fuel for fi re to determine the N deposition response + - threshold (Fenn et al. 2010, Rao et al. 2010, Chapter (NH4 ) and nitrate (NO3 )) along an N deposition gradient in coastal sage scrub showed increased rates 12). of N mineralization in soils receiving 10 compared 15 to 4 kg N ha-1 yr-1 (Vourlitis et al. 2007b, Vourlitis A fi eld survey in 2003 determined the eff ects of N and Zorba 2007). Another fi eld experiment (Sirulnik on the native and exotic vegetation along a gradient of et al. 2007) tested the eff ects of elevated N on net N seven sites that ranged in N deposition from 8.7 to 19 -1 -1 mineralization and nitrifi cation in coastal sage scrub kg N ha yr (Fenn et al. 2010; Table 13.1). Nitrogen in sites with modeled (14.7 and 8.7, as modeled by deposition values reported are modeled wet plus dry Tonnesen et al. [2007]) and inferential (20.2 vs. 6.6 kg deposition on a 4-km grid (Tonnesen et al. 2007). In N ha-1 yr-1; Table 13.3) N deposition rates. Few changes addition, inferential calculations of N deposition are in N mineralization were observed, but nitrifi cation reported for three sites (Table 13.1). Th e inferential - method is used to calculate dry deposition to surfaces (production of NO3 ) was signifi cantly higher in high deposition than control plots on at least one date in all (plant canopies and soil) based on atmospheric 3 years (Sirulnik et al. 2007). Because the studies by concentrations of N pollutants, the deposition velocities Vourlitis et al. (2007b) and Vourlitis and Zorba (2007) of each pollutant, surface areas, and other factors (Fenn et al. 2009). Measurements of air quality, soil N, and 15 short-term deposition to leaf surfaces (Padgett et al. Allen, E.B. Unpublished data. Professor and natural resources extension specialist, Department of Botany 1999) confi rm this air pollution gradient. Extractable - + -1 and Plant Sciences and Center for Conservation Biology, soil N (NO3 plus NH4 ) ranged from 10 μg g at University of California, Riverside, CA 92521. the low end of the N gradient to 38 μg g-1 at the high 144 Chapter 13—Mediterranean California GTR-NRS-80 Table 13.1—Percent cover and richness (per 3 ha) of plant groups along an N deposition gradient in western Riverside County, California. Sites are arranged from north to south along an urban to rural gradient. Forb species richness in bold shows a rapid drop in richness, suggesting a critical load of 10 kg N ha-1 yr-1 by the CMAQ model (between 9.0 and 11.1 kg N ha-1 yr-1), and 7.8 kg N ha-1 yr-1 by the inferential method (between 6.6 and 8.9 kg N ha-1 yr-1). Site Exotic grass Native forb Shrub Native forbs soil N N deposition % cover % cover % cover no. of species μg g-1 kg N ha-1 yr-1 per 3 ha CMAQa Inferentialb Jurupa Hills 63.5 4.0 2.2 16 37.7 19.6 Box Springs 69.2 18.5 2.4 31 32.6 14.7 20.2 Botanic Garden 36.0 25.4 0.2 20 28.9 13.4 Lake Perris 0.5 26.1 2.8 30 20.3 11.1 Mott Reserve 6.7 14.3 11.2 37 30.6 11.1 8.9 Lopez Canyon 11.1 19.6 19.3 67 9.6 9.0 6.6 Tucalota Hills 1.5 35.7 35.0 50 10.5 8.7 aN deposition using the CMAQ model is reported as modeled wet plus dry deposition (Tonnesen et al.