Vegetation that becomes overtopped usually experiences a decrease in abundance or species richness. When an overtopping plant alters the physiognomy of the existing vegetation (e.g., trees invading a shrubland), ecosystem processes can also be dramatically altered. Worldwide, Monterey pine (Pinus radiata) cultivars have been planted in Mediterranean-like climates and are known to invade surrounding natural communities. Ecological impacts resulting from these invasions have been widely investigated; however, the effects from solitary pine trees on the vegetation they overtop are lacking. Furthermore, studies on the impact of P. radiata cultivars from the California floristic province, where P. radiata is native, do not exist. In coastal California, north of the present-day range of native P. radiata stands, cultivars of this species have invaded northern coastal scrub vegetation. To determine the impact of pine invasion on species richness and structure in this habitat, floristic surveys were conducted in 20 blocks that consisted of invaded and uninvaded plots. An invaded plot contained two subplots located under the canopy of an isolated pine tree, whereas a paired, uninvaded plot contained two subplots located in coastal scrub adjacent to each pine. Pine trees selected ranged in size from 2.8 to 119 cm (1.1 to 46.9 in) basal diameter. Our results demonstrate that understory native cover and species richness are negatively correlated with tree size. Understory exotic plant cover and richness of species other than P. radiata did not show any correlation with tree size, mainly because exotic plants had a very low abundance overall.

The land-ocean interface along the central coast of California is one of the most diverse biogeographic regions of the state. This area is composed of a species-rich mosaic of coastal grassland, shrubland, and forest vegetation types. An acceleration of conifer encroachment into shrublands and shrub encroachment into grasslands along the coast has been recently documented. These vegetation changes are believed to be driven primarily by fire suppression and changing grazing patterns. Climatic variables such as precipitation, evapotranspiration, fog, cloud cover, and temperature also play an important role in vegetation succession. Our study area includes the central California coast, which is characterized by a precipitation gradient from the relatively wetter and cooler north to the drier and warmer south. The primary objective of this study was to determine where vegetation has undergone succession and to identify what kind of succession has taken place. Furthermore, we sought to identify climatic variables that contribute to succession, specifically the transition among grassland, shrubland, and forested vegetation types. To identify vegetation types and locations where succession has taken place, we classified two Landsat-5 TM scenes from 1985 to 2010 with a resulting overall accuracy of 83.3%. Our results show that forested vegetation was more abundant in the north while shrublands were most abundant in the south. Forests were associated with high mean annual precipitation and mean July plus August evapotranspiration. Grasslands were associated with low mean July and August evapotranspiration and places with the greatest extremes in mean annual maximum and minimum temperatures (e.g. inland areas). Shrublands were not strongly associated with any climatic variables. Throughout the entire study area, approximately 19% of the vegetation has converted from one vegetation type to another between 1985 and 2010, with greater type-conversion north of the San Francisco Bay than in the south. Forests and shrublands have expanded in places with relatively high annual precipitation and July plus August evapotranspiration. Forests have expanded more than any other broadly classified vegetation type and mostly at the expense of shrublands. This study suggests that climatic variables exert a strong level of influence on vegetation and that future climate change could profoundly alter vegetation patterns and processes.

Limited information exists regarding the impact of fire on annual plant composition in creosote bush scrub vegetation.The impact of recurrent fires on annual plants is even less understood.To investigate this matter, annual vegetation was sampled in a stand of creosote bush scrub in western Coachella Valley, California that had recently experienced two wildfires.The wildfires fragmented the once contiguous shrubland into three sections: unburned, once-burned, and twice-burned stands, all of which were separated by fuel breaks that contained each fire.For all three stands, annual plant cover and species richness were determined in the field, soil seed bank samples were collected and assayed in a glasshouse, and soil chemistry and physical properties were measured.We found that invasive annual grass cover was highest in the twice-burned stand and native annual plant cover was greatest in the unburned stand.Native annual species richness significantly decreased each time a stand burned resulting in low native annual plant diversity.Seed bank assays revealed that invasive annual grass germinants were orders of magnitude greater in the twice-burned stand compared with the other two stands.Lastly, soil total N, C, and soil pH were elevated in both burned stands.Overall, we found that recurrent fire can result in strong impacts to annual vegetation; however, the twiceburned stand was sampled only three years after burning while the once-burned stand was sampled 20 years after burning.Thus, longer-term fire effect studies plus replication with additional study sites are still needed to improve our understanding of how recurrent fire impacts annual plants.

The impacts of fire on creosote bush scrub vegetation have received attention recently as fire has become locally common throughout the Mojave and Sonoran deserts. One area of particular concern is western Coachella Valley, which forms the northwestern extent of the Colorado Desert. This is a major wildland-urban interface area that has been significantly impacted by atmospheric nitrogen deposition concomitant with fuel alterations from invasive annual grasses and increased ignition frequencies from human activities. Creosote bush scrub takes much longer than more mesic vegetation types to re-establish after fire, and the majority of desert species lack traits associated with resiliency to fire disturbance. Previous research in this area has only investigated once-burned stands for up to five years since fire. This study documents perennial vegetation from seven sites that represent a 2- to 28-year-old fire chronosequence. Our surveys revealed that fire significantly reduced shrub richness and diversity regardless of time since fire. Total shrub cover and density returned to or exceeded unburned levels at least 20 years after fire, although species composition was almost entirely brittlebush (Encelia farinosa), a shortlived shrub. Longer-lived shrubs indicative of unburned vegetation, such as creosote (Larrea tridentata), white bursage (Ambrosia dumosa), and white ratany (Krameria grayi), failed to recover. Shrub plus cacti richness and diversity were lower in burned stands regardless of time since fire. Encelia shrublands may form an alternate stable state following fire in this region.

The Society for Range Management (SRM) hosted a special conference to organize information required to manage the challenges posed by invasive plants and wildfi re. This special feature of Rangeland Ecology & Management contains fi ve papers from this conference, which provide a detailed synthesis of the scientifi c literature on the interactions and impacts of wildfi re and invasive plants on North American deserts. We hope that this information provides solutions to more effectively assess current impacts of wildfi re and invasive plants, and offers management strategies to minimize future impacts.

The land-ocean interface along the central coast of California is one of the most diverse biogeographic regions of the state. This area is composed of a species-rich mosaic of coastal grassland, shrubland, and forest vegetation types. An acceleration of conifer encroachment into shrublands and shrub encroachment into grasslands along the coast has been recently documented. These vegetation changes are believed to be driven primarily by fire suppression and changing grazing patterns. Climatic variables such as precipitation, fog, cloud cover, temperature, slope, and elevation also play an important role in vegetation succession. Our study area is located along the central California coast, which is characterized by a precipitation gradient from the relatively wetter and cooler north to the drier and warmer south. Some studies indicate changing fog patterns along this coast, which may greatly impact vegetation. A decrease in water availability could slow succession processes. The primary objective of this project is to determine if vegetation succession rates are changing for the study area and to identify climate and ecosystem variables which contribute to succession, specifically the transition among grassland, shrubland, and forest. To identify vegetation types and rates of succession, we classified two Landsat TM 5 scenes from 1985 to 2010 with a resulting overall accuracy of 82.4%. Vegetation succession was correlated to changes in maximum and minimum temperatures, precipitation, and elevation for each sub-region of the study area. Fog frequency was then compared between the northern and southern regions of the study area for determining the spatial relation between fog frequency and the percent of vegetation change.

A review of literature shows that both fire and invasive species may cause changes in biological, chemical, and physical properties of desert soils. Although soil may recover from the impacts of fire during succession, these changes are permanent under persistent invasive species. The most severe effects of fire occur under high temperatures with high fuel buildup and soil moisture that conducts heat downward. Deserts typically have low fuel mass and low soil moisture, both conditions that would contribute to lower impacts of fire than in mesic soils. Soil is a good insulator, so soil microorganisms will survive a few centimeters deep even in hot surface fires. Immediately postfire there is often an increase in mineral nitrogen (N) and a decrease in soil carbon (C) and organic N, but these changes are often minimal in desert soils, except under fertile shrub islands that have higher fuel loads and fire temperature. Both hot and cold deserts have experienced slow recovery of native shrubs and increased growth of invasive grasses following fire. Invasive species may either increase or decrease soil N and C depending on fire temperature and site and species characteristics. Mineralization and fixation of N are often high enough after fire that subsequent productivity balances N losses. The elimination of islands of fertility coupled with postfire erosion may be a major impact after fire in grass-invaded shrub lands. In the long term, the interaction of fire and invasive species may result in more frequent fires that eliminate fertile islands and reduce the productivity of deserts. Managers may use fire as a tool to control desert invasives without the concern that N will be irrevocably lost, but this must be done judiciously to avoid eliminating shrubs and further increasing invasive species.

Productivity in desert ecosystems is primarily limited by water followed by nitrogen availability. In the deserts of southern California, nitrogen additions have increased invasive annual plant abundance. Similar findings from other ecosystems have led to a general acceptance that invasive plants, especially annual grasses, are nitrophilous. Consequently, reductions of soil nitrogen via carbon amendments have been conducted by many researchers in a variety of ecosystems in order to disproportionately lower invasive species abundance, but with mixed success. Recent studies suggest that resource-use traits may predict the efficacy of such resource manipulations; however, this theory remains largely untested. We report findings from a carbon amendment experiment that utilized two levels of sucrose additions that were aimed at achieving soil carbon to nitrogen ratios of 50:1 and 100:1 in labile sources. Carbon amendments were applied once each year, for three years, corresponding with the first large precipitation event of each wet season. Plant functional traits measured on the three invasive and 11 native herbaceous species that were most common at the study site showed that exotic and native species did not differ in traits associated with nitrogen use. In fact, plant abundance measures such as density, cover, and biomass showed that carbon amendments were capable of decreasing both native and invasive species. We found that early-germinating species were the most impacted by decreased soil nitrogen resulting from amendments. Because invasive annuals typically germinate earlier and exhibit a rapid phenology compared to most natives, these species are expected to be more competitive than native annuals yet more susceptible to early-season carbon amendments. However, desert annual communities can exhibit high interannual variability in species composition and abundance. Therefore, the relative abundance of native and invasive species at the time of application is critical to the success of carbon amendments at our study site. For land management purposes, carbon amendments remain relatively impractical and may only be useful at small scales or in conjunction with other invasive species removal techniques.

Native annual plant species constitute a large proportion of the plant diversity found in arid vegetation types within the southwestern United States; yet, little is known about controls on diversity patterns along natural and anthropogenic gradients. In this study we evaluated native species richness and exotic species cover across overlapping gradients of precipitation, wind, and N deposition in the Colorado Desert of southern California. Factors allowing native diversity to persist under high N deposition and high wind were also evaluated in a second, focused study at one end of the gradient. We found that gradients in precipitation, nitrogen deposition, and wind were the most important factors to native richness and exotic species cover across the landscape, while local heterogeneity in bare ground influenced richness and cover at the high deposition/windy, or high-disturbance, end of the gradient. Patterns of native diversity were evaluated across the gradients using non-metric multidimensional scaling, which showed diversity was split into two axes: one strongly correlated to precipitation and the other strongly correlated with disturbance factors. The disturbance factors were also positively associated with exotic grass and forb cover. In total, these results indicate that large-scale patterns in disturbance and exotic species cover negatively affect native annual plant species diversity but native species can also persist due to local heterogeneity.

Creosote bush scrub vegetation typically contains high diversity of native annual plants relative to shrubs, cacti, perennial herbaceous species, or other plant life forms. This vegetation type is also very susceptible to exotic, invasive annual plants, which promote fire by changing fuel properties. The impact of fire on most perennial species is severe but the impact on native annual plants is not well understood. We measured annual species composition in five sites that each contained paired burned and unburned stands in the western Colorado Desert, California. The burned stands at each site ranged in time since fire from 3 to 29 years ago. Annual plant cover, species richness, and soil chemical and physical properties were compared in the paired burned and unburned reference stands. Differences between paired stands at the time of each fire are assumed negligible since shrub cover across fuel breaks did not differ prior to each fire based on aerial photographs. Fires elevated soil pH but otherwise had little effect on other soil properties. In recently burned stands, invasive annual grass abundance increased while native annual plant cover and species richness decreased. However, in older burned stands, annual plant composition did not always differ between paired stands because invasive annual plant abundance was very high in both stands. Thus, while fires can have long-lasting negative impacts to perennial components of creosote bush scrub, invasive species can displace native annual plants regardless of whether or not a site burns, although fire disturbance appears to accelerate invasive plant dominance. ____________________________________ In Monaco, T.A. et al. comps. 2011. Proceedings – Threats to Shrubland Ecosystem Integrity; 2010 May 18-20; Logan, UT. Natural Resources and Environmental Issues, Volume XVII. S.J. and Jessie E. Quinney Natural Resources Research Library, Logan Utah, USA.

Invasive annual grasses have become increasingly important components of desert vegetation in North America. They are especially problematic because they increase the extent, severity, and frequency of fire in desert shrublands that normally experience fire very rarely, or not at all. After fire, invasive grasses and forbs are often dominant and restoration methods are required to promote native plant recovery. Three treatments to control invasive annual grasses and forbs were implemented in the first 3 years following a fire in creosote bush scrub vegetation. Treatments included early season mechanical removal (raking) of all annuals, grass‐specific herbicide (Fusilade II), and Fusilade II plus hand pulling of exotic forbs. In the first year, all treatments reduced invasive annual grass abundance by about half but had little effect on native annuals. Treatment effectiveness was minimal in the first year due to low and irregular distribution of rainfall. In the second year, insufficient rainfall prevented the germination of any annual plants and no treatments were applied. In the third year, precipitation onset occurred later in the season and was above average. Although the raking treatment performed poorly, treatments utilizing Fusilade II nearly eliminated invasive grasses and forbs, achieved native annual dominance, and increased native perennial abundance. These results indicate that in the absence of invasive grasses and forbs, the native annual community can be resilient to fire disturbance and native perennials can recover. The results also suggest that burned creosote bush shrublands can be managed after fire to decrease the chance of invasive plant–fire feedback.

Type-conversion of coastal scrub to exotic annual grassland has been extensive in certain parts of southern California, especially in drier inland locales. Field observations suggest that rock outcrops harbor native perennials associated with coastal scrub vegetation after conversion to exotic annual grassland has occurred. Surveys were conducted to test this observation. In addition, measures of fossorial mammal disturbance, soil depth, exotic annual plant abundance, and soil moisture were collected and used to infer potential mechanisms that may influence patterns of native and exotic vegetation. Results showed that larger outcrops are associated with more native perennial species than smaller outcrops, consistent with predictions based on species-area relationships. In addition, the edges of rock outcrops were shown to have greater native perennial species richness than the surrounding exotic annual grassland matrix. Higher soil moisture, lower soil depth, and less disturbance by Botta's pocket gopher (Thomomys bottae) were also found at the edge of rock outcrops compared to the surrounding grassland matrix. Furthermore, soil depth was positively correlated with pocket gopher disturbance and with exotic annual grass cover. Rock outcrops serve as refuge habitat for native coastal scrub perennials in areas where exotic annual grasses have become dominant. Rock outcrops may become increasingly important for conservation efforts if remaining coastal scrub vegetation continues to deteriorate.

Plant communities in the southern Coast Range of California form a mosaic with discrete to gradual transitions between multiple vegetation types. To accurately portray this pattern and to quantify the areal coverage of ecotonal space, a new method of mapping vegetation was developed. Vegetation stands were classified and mapped in separate GIS layers to the full extent of their respective suite of indicator species. Since all stands were mapped in this way, the overlap of different communities in the GIS represents ecotonal space. Vegetation mapping was entirely ground-based using a GPS receiver. Vegetation classification followed the Holland and Keil scheme. Eleven plant communities were identified within the 92.6 ha study area. This mapping method revealed that 32% of the total area was ecotonal and that the majority of plant communities exhibited a greater portion of their total area as ecotone than as discrete space. This finding suggests that typical vegetation maps depicting discrete boundaries between all vegetation types may misrepresent a nontrivial proportion of the area mapped. In addition, because ecotones are ecologically significant and important to conservation, the portrayal of transitional space between communities is worth consideration in the future creation of vegetation maps within California.

Plant communities in the southern Coast Range of California form a mosaic with discrete to gradual transitions between multiple vegetation types. To accurately portray this pattern and to quantify the areal coverage of ecotonal space, a new method of mapping vegetation was developed. Vegetation stands were classified and mapped in separate GIS layers to the full extent of their respective suite of indicator species. Since all stands were mapped in this way, the overlap of different communities in the GIS represents ecotonal space. Vegetation mapping was entirely ground-based using a GPS receiver. Vegetation classification followed the Holland and Keil scheme. Eleven plant communities were identified within the 92.6 ha study area. This mapping method revealed that 32% of the total area was ecotonal and that the majority of plant communities exhibited a greater portion of their total area as ecotone than as discrete space. This finding suggests that typical vegetation maps depicting discrete boundaries between all vegetation types may misrepresent a nontrivial proportion of the area mapped. In addition, because ecotones are ecologically significant and important to conservation, the portrayal of transitional space between communities is worth consideration in the future creation of vegetation maps within California.

The western Mojave Desert is downwind of nitrogen emissions from coastal and inland urban sources, especially automobiles. The objectives of this research were to measure reactive nitrogen (N) in the atmosphere and soils along a N-deposition gradient at Joshua Tree National Park and to examine its effects on invasive and native plant species. Atmospheric nitric acid (HNO 3 ) and ozone (O 3 ) were elevated in western Joshua Tree National Park, and there were some high levels of atmospheric ammonia (NH 3 ) in the east that may be related to local sources. The central areas of Joshua Tree National Park were lowest in reactive atmospheric N; HNO 3 was higher in summer, while NH 3 was higher in winter. Extractable soil N was generally higher in sites that had higher atmospheric reactive N. Invasive grasses and forbs, such as Mediterranean split grass (Schismus barbatus), red brome (Bromus madritensis), and stork's bill (Erodium cicutarium) have become more productive and widespread in the last two decades. To test the hypothesis that elevated N may be related to the success of invasive species, N fertilizer experiments were done at four sites in Joshua Tree National Park at levels of 5 and 30 kg N/ha for each of two years. Sites with higher