AbstractThe co‐existence of evergreen and deciduous oaks in Mediterranean‐climate savannas has motivated comparative studies on species’ physiological adaptations to light and drought, establishment niche differences in acorn production, dispersal and seedling herbivory, and differential sapling tolerance of ungulate browsing. Understanding how species’ differences collectively affect co‐occurrence or segregation requires long‐term studies that consider multiple life stages. We compare survival, height growth, and modeled population growth rate of the evergreen sclerophyll, Quercus agrifolia, to those of the winter‐deciduous, broadleaved Q. lobata in southern California savanna sites where the species naturally co‐occur. We evaluate species’ performances after 8.7 yr for four cohorts sown as acorns in 1997, 1998, 2000, and 2001 under four different levels of protection from cattle, deer, and rodents. Survival curves for the two species were closely similar, with large differences between cohorts associated with precipitation in the year acorns were sown. Survival and growth rates of Q. agrifolia seedlings and saplings were slightly lower, and were more reduced by ungulates and rodents, than those of Q. lobata. No Q. agrifolia plants (0/1431) exposed to ungulates (cattle and/or deer) and rodents survived to the end of the experiment. Of 1421 Q. lobata acorns planted in these treatments, only 19 plants (1.3%) survived 8.7 yr and none of these plants progressed out of the ungulate browse layer. However, when protected from both ungulates and rodents, survival rates increased significantly for both species, and Q. agrifolia increased height faster than Q. lobata. Simulated population growth rates increased slightly for plants protected from ungulates, and 95% confidence intervals spanned the stable population growth rate of 1. With exclusion of ungulates and rodents, sapling and small tree recruitment rates for both species far exceeded those needed to offset the rate of recent adult mortality, with modeled population growth rates of 1.07–1.11. Our results underscore the essential role of occasional high rainfall years for initial establishment, plus the key role of consumers in limiting early survival and growth of both evergreen and deciduous oaks in mixed oak savannas.

In 1977 an experiment was initiated in the Mojave Desert to investigate the relationship between shrub interactions and structure in a community dominated by Ambrosia dumosa and Larrea tridentata. Here, as in much of the Mojave, Larrea were regularly distributed, Ambrosia occurred in aggregations, and the two were randomly distributed relative to each other. Pre-dawn xylem pressure potentials (PDXPPs) of single Ambrosia or Larrea in centres of 100m(2) circular plots were monitored to assess effects of intraspecific, interspecific and total removals of neighbouring shrubs. Contrary with theory, results over the next 2years indicated that interspecific interference was more intense than intraspecific interference in both species. These plots were maintained through 2014. Measurements of seedling recruitment from 1980 to 2014, and of PDXPP, above-ground biomass, and canopy senescence from 2003 to 2014 were conducted. Recruitment of both species was substantial immediately after the removals, but declined to very low levels after 1983. Ambrosia recruited into all Ambrosia and Total-removal plots, but Larrea recruited only into plots that contained mature Ambrosia. Pre-dawn xylem pressure potentials of monitored shrubs continued to be enhanced in removal plots for at least 27years, but this changed from most being due to interspecific removals in both species to intraspecific removals causing most enhancement in Ambrosia and inter- and intraspecific removals causing nearly equal enhancements in Larrea. Above-ground biomasses of monitored Ambrosia and Larrea were 2.1x and 2.8x larger in Total-removal plots, 1.6x and 1.7x larger in intraspecific removal plots, respectively, and 1.1x larger in interspecific removal plots for both species than those in Control plots, indicating the absence of intraspecific interference had the dominant long-term effect. Canopy senescence differed between Ambrosia and Larrea in extent, timing and effect of specific removal treatments; it was greatest for both species in Controls, averaging 75% and 34%, respectively.Synthesis. Shrub interactions and their relations to community structure are mechanistically and spatially complex. Differences between short-term and long-term responses to removals reveal multi-tiered, temporally dynamic feedback loops between shrub interactions and community structure driven by demographics, species-specific root growth, resource competition, communications and territoriality.

The relative importance of livestock grazing in limiting or enhancing oak recruitment remains unclear because results from previous studies have been contradictory. In Santa Barbara County, we have replicated large-scale planting experiments from 1997 to 2001 to determine the effects of cattle and other factors on seedling establishment of valley oak (Quercus lobata) and coast live oak (Q. agrifolia). We manipulated cattle grazing (ungrazed vs. winter-spring rotational grazing) and protection from small and large mammals. Rainfall and seed predation and herbivory by small mammals—most likely gophers and ground squirrels—significantly impacted rates of seedling recruitment. Exclusion of cattle alone has not significantly increased establishment of either valley or coast live oak. However, protected seedlings in pastures with cattle have higher survival and growth rates than protected seedlings in plots excluding cattle. Our results suggest that winter-spring livestock grazing can have indirect positive effects on oak establishment, by reducing herbaceous biomass and associated small mammals adjacent to protected seedlings. Contrary to expectations based on the relative abundance of natural oak seedling recruits, establishment and survival of coast live oak planted in our experiments has been significantly and consistently lower than that of valley oak.

The Santa Barbara County Oak Restoration Program was initiated in 1994 to determine the major factors limiting recruitment of valley oak (Quercus lobata) and coast live oak (Q. agrifolia). At Sedgwick Reserve in Santa Barbara County, California, we have replicated large-scale planting experiments in four different years to determine the effects of cattle and other ecological factors on oak seedling establishment in oak savannas and woodlands. In 33 large experimental plots (50 x 50 m) we planted acorns collected from Q. lobata and Q. agrifolia on the site. Fifteen of these large plots are controls, open to grazing, fifteen exclude cattle with the use of electric fence, and three are ungrazed in large ungrazed pastures. Within the plots, experimental treatments included: 1) protection from small mammals such as gophers and ground squirrels, 2) protection from large animals such as cattle, deer, and pigs, and 3) no protection from mammalian grazers. In winters 1997, 1998, 2000, and 2001, we planted approximately 1,000 acorns of each species. Results confirm that seed predation and herbivory by small mammals are a significant "bottleneck" to oak seedling recruitment on the landscape scale. Comparing results among years indicates that lack of late winter rainfall can significantly reduce oak emergence and establishment. Survivorship of protected acorns and seedlings is comparable in grazed and ungrazed areas.

Tree recruitment is rare and oak populations are declining in many Mediterranean-climate oak savannas. Factors affecting acorn production, seedling establishment and initial seedling growth have been much studied in short-term experiments. However, fecundity and early survival rates have not been placed in a demographic framework that also considers sapling survival and growth and adult tree mortality. We use matrix models and life table response experiments to analyze long-term experimental and observation data on California valley oak (Quercus lobata Nee) near its southern range limit in Santa Barbara County to answer three questions: (1) How sensitive is oak population growth rate to variation in acorn production and initial seedling establishment vs. sapling survivorship and growth? (2) How do mammalian consumers-specifically, cattle, deer and rodents-impact valley oak seedling establishment and sapling survival and growth? (3) Can vertebrate consumers account for the observed population decline of valley oak in savannas in its southern range? We find that population growth is far more sensitive to consumer-mediated variation in sapling survivorship and growth than to variation in fecundity or seedling establishment and that consumers exert strong influence on the demography of the species. Deterministic, finite population growth rate (lambda) is <1 for unprotected plants and for plants that are protected from cattle but still exposed to mule deer and rodents. Population growth rate increases to 1.03 with protection from both cattle and deer, mainly because plants are able to quickly reach the tree layer when they are protected from ungulate browsing. Population growth rate jumps to 1.15 for plants protected from both ungulates and rodents as a result of increased survivorship and height growth of established individuals and because of increased seedling establishment during the first year. Our experimental findings are consistent with observed patterns in natural populations in Santa Barbara County, where tree recruitment is rare in both grazed and non-grazed savannas but more common in areas such as roadsides that are refuges from cattle and deer.

We quantitatively compared phenology and water relations of a fully deciduous shrub, Styrax officinalis, and an evergreen shrub, Arctostaphylos glauca, in shared microsites in a sandstone outcrop in southern California during a multi-year drought. Pre-dawn xylem pressure potentials, Ψpd, were similar for the two species during most months of 2 years, but occasional differences and watering experiment results suggest S. officinalis may have phreatophytic roots that tap water in deep rock cavities and joint traces, while A. glauca may have primarily shallow roots. Neither species varied in maximum or minimum Ψpd between years of very different rainfall totals. Twig elongation and leaf production of S. officinalis began earlier during spring, and its leaves matured more quickly and more synchronously than A. glauca. Leaves lived a mean of 180 days for S. officinalis and 849 days for A. glauca. Leaf life spans varied among years in both species. S. officinalis leaf senescence occurred mostly in August and September and was not discernibly related to Ψpd or drought avoidance. A. glauca leaf senescence occurred throughout the year, but especially coincided with leaf production. In A. glauca most senescence occurred at the beginning of a leaf cohort's third growing season, but numbers of retained older leaves increased during 4 years of drought. Timing of twig elongation and leaf and flower production appeared to be related to current rainfall, but amount of twig growth and numbers of leaves and flowers produced appeared to be related to rainfall of the previous year or years for S. officinalis and, more complexly, A. glauca. Because of an interrupted pattern of flower production, number of flowers produced by A. glauca may be responsive to rainfall amounts during two environmental periods. Morphological differences, including much higher above ground allocation, many more leaves/twig, lower allocation to stem mass, and longer duration of leaves/year in A. glauca, are probably responsible for A. glauca having >6 times more above ground biomass per plant than S. officinalis. During the multi-year drought S. officinalis changed little, indicating either superb adaptation or growth pattern rigidity, while A. glauca underwent extensive phenological and morphological changes, indicating either stress or adaptive flexibility.

Natural recruitment of oaks appears to be declining throughout the northern hemisphere. Summer drought poses a potentially important barrier to oak recruitment in southern California. To evaluate this barrier, we grew evergreen Quercus agrifolia and deciduous Q. lobata from seeds near parental trees. We measured water relations, chlorophyll fluorescence, and gas exchange during these seedlings' fourth and fifth summers and compared them to neighboring adults. Most seedlings had substantially lower values for predawn xylem pressure potential (Ψ(pd)), minimum photosystem II (PSII) quantum efficiency (Φ(PSIIMIN)), maximum quantum efficiency for PSII under dark-adapted leaf conditions (Fv/Fm), and maximum photosynthetic assimilation (Amax), and higher values for maximum nonphotochemical quenching (NPQmax) than did conspecific adults. The high, unvarying Ψ(pd) values of the adults suggest they use perennially available groundwater. Quercus lobata seedlings commonly had lower values for Ψ(pd) than did Q. agrifolia, and values for Ψ(pd) and Φ(PSIIMIN) were significantly related to size in Q. lobata but not in Q. agrifolia. These data suggest important interspecific differences in root architecture. Lower values for Φ(PSIIMIN), Fv/Fm, and higher NPQmax in Q. agrifolia indicate that Q. agrifolia seedlings were usually under more stress than Q. lobata, which typically had higher Amax rates than did Q. agrifolia seedlings. Diurnal photosynthesis curves were quite flat for Q. agrifolia, but they peaked in the morning for Q. lobata. Established seedlings appeared to be under more stress than adults, but this stress did not appear severe enough to cause death. Access to perennially available groundwater may be crucial for the seedling to sapling transition.

As has been reported in other oak habitats throughout the Northern Hemisphere, natural recruitment rates of young oaks in California are very low for some species and in some regions. The majority of experimental studies that contribute to our understanding of this oak recruitment pattern in California have been relatively short-term, conducted on a small-scale. Thus, while we have valuable information about the array of factors that are able to limit seedling establishment, we know much less about their relative importance, and how they vary across years, sites, or age classes. To investigate the impacts of factors limiting seedling and sapling establishment of valley oak (Quercus lobata) and coast live oak (Quercus agrifolia) across a landscape and over time we replicated experiments in four different years in Santa Barbara County, California. Experimental factors manipulated included cattle grazing, and access by deer and small mammals. Plots were distributed across three sites with a cumulative area of 200-ha, and seedling survival was recorded for a minimum of 5 years. We used classification and regression tree analysis (CART) to examine the relative importance of factors influencing survival of planted oaks at different life stages.The relative importance of limiting factors varied among age classes for both species. For initial seedling emergence and survival to 6 months planting year was the most important factor and rodent access was the second most important factor for both Q. lobata and Q. agrifolia. For survival of seedlings through their 1st year rodents, planting year, and site were major limiting factors, though their relative importance varied between the two species. For survival from 1.5 to 5 years, Q. lobata was strongly affected by rodents and site, while Q. agrifolia was mainly affected by site, deer browsing (which reduced survival), and indirect effects of winter-spring cattle grazing (which improved survival). Contrary to our expectations, based on observed patterns of natural recruitment, Q. agrifolia had equivalent or lower survivorship than Q. lobata in all seedling age classes. Overall, in addition to other factors, there were strong year and site effects controlling oak seedling emergence and survival and the relative importance of limiting factors depended on seedling age. Our results suggest that large-scale, long-term experiments encompassing multiple sites will improve our understanding of controls on recruitment in oak woodlands in California and elsewhere. (c) 2008 Elsevier B.V. All rights reserved.

Experiments in which related and unrelated plants were grown together reveal the ability of roots to recognize their kin. The ecological and evolutionary implications are tantalizing topics for future studies.

Root hydrotropism could be a means by which plants forage for limited and patchy distributions of soil water. While root hydrotropism has been induced in distinctly artificial conditions, it is unclear if it operates in natural settings. Here, we tested for this possibility in seedlings of two species of dune shrubs.Growth of individual roots in sand-filled observation chambers was monitored in response to moisture-rich patches and resultant soil water gradients. Chambers were designed so that roots could intercept the moisture gradients but not the moisture-rich patches simply through gravitropism.While up to 12% of the Eriogonum parvifolium roots grew into the moisture-rich patches, comparable root growth was observed in the control. None of the Artemisia californica roots grew into the patches.Thus, in a reasonable simulation of field conditions, we found no compelling evidence for hydrotropic root behavior in seedlings of these two dune shrubs. Our results leave the ecological significance of root hydrotropism in question.

Efforts to arrest the spread of invasive weeds with herbivory may be hindered by weak effects of the herbivores or strong compensatory responses of the invaders. We conducted a greenhouse experiment to study the effects of defoliation and soil fungi on competition between the invasive weed Centaurea solstitialis and C. solstitialis and Avena barbata, a naturalized Eurasian annual grass, and Nassella pulchra, a native California bunchgrass. Surprisingly, considering the explosive invasion of grasslands by C. solstitialis, Avena and Nassella were strong competitors and reduced the invader’s biomass by 80.2% and 80.1% over all defoliation and soil fungicide treatments, respectively. However, our experiments were conducted in artificial environments where competition was probably accentuated. When fungicide was applied to the soil, the biomass of C. solstitialis was reduced in all treatment combinations, but reduction in the biomass of the invader had no corollary impact on the grasses. There was no overall effect of defoliation on the final biomass of C. solstitialis as the invader compensated fully for severe clipping. In fact, the directional trend of the clipping effect was +6.4% over all treatments after eight weeks. A significant neighbor × soil fungicide × clipping effect suggested that the compensatory response was the strongest without soil fungicide and when C. solstitialis was alone (+ 19%). Our key finding was that the compensatory response of C. solstitialis in all treatments was associated with an increase in the weed’s negative effects on Nassella and Avena – there was a significant decrease in the total biomass of both grasses and the reproductive biomass of Avena in pots with clipped C. solstitialis. Our results were obtained in controlled conditions that may have been conducive to compensatory growth, but they suggest the existence of mechanisms that may allow C. solstitialis, like other Centaurea species, to resist herbivory.

We studied the effects of soil fungi on interactions between Centaurea melitensis, an exotic invasive weed in central California, and two co-occurring grasses, Nassella pulchra and Avena barbata, The fungicide benomyl reduced the abundance of arbuscular mycorrhizal (AM) fungi in plant roots but did not affect non-AM fungi. Centaurea plants grown alone were >50% smaller with the resident microbial community intact than when benomyl was applied. When grown with Nassella, the effect of benomyl was reversed. Centaurea grew almost five times larger with the resident microbial community intact. Fungicide had no effect on the biomass of Centaurea grown with Avena, but biomass of Centaurea was significantly lower when grown with Avena than when grown with Nassella or alone. Photosynthetically fixed carbon may have been transferred from Nassella via soil fungi to Centaurea, constituting a form of soil fungi-mediated parasitism, but such a transfer did not occur from Avena to Centaurea. Second. Nassella may have been more inhibited by soil pathogens in the presence of Centaurea than when alone. and the inhibition of Nassella may have released Centaurea from competition. A third possibility is that Nassella has strong positive effects on the growth of Soil fungi. but the positive feedback of beneficial soil fungi to Nassella is less than the positive feedback to Centaurea. Regardless of the mechanism, the difference in soil fungicide treatment effects on competition between Centaurea and Nassella vs. Centaurea and Avena has important implications for the invasion of California grasslands.

We used 16s rDNA sequences to identify novel species of. cyanobacteria beneath translucent quartz pebbles in the desert pavement on an alluvial piedmont of the Coxcomb Mountains in the southern Mojave Desert, California, USA. Transmission of light, as measured with an integrating sphere, was about 0.08% beneath the thickest pieces of quartz (25 mm) harboring these hypolithic autotrophs. The photosynthetic rate ranged from 0.1 to 1.0 mumol(.)m(-2.)s(-1) in the linear range of its response to light (PAR of 0-50 mumol(.)m(-2.)s(-1)), over which the apparent quantum-use efficiency was 0.019. Light-saturated rates of 1.7-2.7 mumol(.)m(-2.)s(-1) were recorded at light intensities of 200-400 mumol(.)m(-2.)s(-1). The hypolithic community had an upper thermal tolerance of >90degreesC in laboratory conditions. The quartz pebbles confer a modest greenhouse effect that may be important for photosynthetic activity during cool, wet, wintertime periods that prevail in the Mojave Desert.

Summary 1 Spatial pattern analyses were used to generate hypotheses about the processes that shape the structure of a plant community in the Mojave Desert of North America, with a focus on the semi‐shrub Ambrosia dumosa . We analysed spatial distributions and sizes of this species relative to other semi‐shrubs, shrubs and annuals, and the relationships between spatial patterns and abiotic and biotic habitat characteristics. 2 The analyses were based on maps of sample plots placed along a transect spanning two adjacent geological substrates: aeolian sand and gravelly, sandy to loamy alluvium. Of these two substrates, sand supported higher total biomasses of Ambrosia and of all woody perennials, while alluvium supported on average higher biomasses of winter annuals. 3 Annuals and seedlings of Ambrosia were much more strongly aggregated with Ambrosia canopies on sand than on alluvium, suggesting that these small plants were more strongly facilitated by Ambrosia on sand than on alluvium. 4 Ambrosia semi‐shrubs were spatially segregated on sand but aggregated on alluvium, and the degree of segregation on sand increased with the total above‐ground biomass of Ambrosia per unit area, indicating that negative interactions between Ambrosia plants were stronger in more productive habitats. Canopy sizes of Ambrosia in all mapped plots increased with distance to the nearest conspecific neighbour, which suggests that neighbour interactions negatively affected plant sizes. 5 Ambrosia plants on sand were spatially aggregated with Acamptopappus sphaerocephalus semi‐shrubs, suggesting that at least one of these species may benefit from the association. Ambrosia plants were spatially segregated from Larrea tridentata shrubs on both substrates, possibly due to negative effects of Larrea roots on Ambrosia roots reported in previous studies. 6 Subtle differences in substrate characteristics were correlated with strong differences in the spatial distribution of Ambrosia plants relative to their neighbours, which suggests that edaphic conditions may affect the spatial structure of the community by modifying complex positive and negative interactions between neighbouring plants.

. Abiotic factors are often thought to be the predominant forces shaping desert plant communities. But both positive and negative interactions between plants are frequently observed in deserts, and it is an open question whether they can strongly affect the spatial structure of a desert community. The goal of this study was to answer this question for a plant community in the North American Mojave Desert. Two semi-shrub species, Ambrosia dumosa and Acamptopappus sphaerocephalus , were the focus of this study. At the study site, seedlings emerged predominantly on the northern side of shrubs, indicating positive effects of canopy shading on emergence, but survival of Ambrosia seedlings was much higher in open areas than at the edge of conspecific shrubs. Negative intraspecific interactions also affected Ambrosia shrubs, which did not increase in size over a 4-year period unless the nearest conspecific neighbor had been removed. These negative intraspecific interactions among different life stages of Ambrosia appear to contribute to spatial segregation observed among shrubs of this species. In contrast, Acamptopappus shrubs and their seedlings were aggregated with Ambrosia shrubs, and occurred more often on the northern side of Ambrosia than expected by chance. Removal of Ambrosia neighbors positively affected growth of Acamptopappus , but only when the neighbor was removed on the northern side. For Acamptopappus , an Ambrosia neighbor on the southern side may have some positive effects, which appear to neutralize the negative effects found for northern neighbors. These positive effects were likely at least partly due to shading. Removal of Ambrosia neighbors negatively affected predawn xylem pressure potentials of Acamptopappus , but this effect was only found during one growing season and was briefly reversed during the next. In summary, negative intraspecific interactions appear to cause spatial segregation of Ambrosia shrubs, while a combination of positive and negative interactions apparently contribute to the directional association between Ambrosia and Acamptopappus . Thus plant interactions in this desert appear to shape community structure in at least two dimensions by influencing the distances and in which directions to their neighbors plants can grow and survive.

Compensatory responses to herbivory by invasive weeds may foil attempts to arrest their spread with biological controls. We conducted an experiment to study the effects of defoliation and soil fungi on interactions between Centaurea melitensis, an invasive annual from Eurasia, and Nassella pulchra, a native Californian bunchgrass. Defoliation of C. melitensis reduced its final biomass in all species–fungicide treatments, except when C. melitensis was grown with both Nassella and non‐treated soil fungi at the same time. In this treatment, the biomass of clipped C. melitensis plants was equal to that of unclipped plants, indicating that soil fungi and Nassella promoted a compensatory response in the weed. Overall, the biomass of C. melitensis was 44% lower when soil fungi were reduced. However, in soil not treated with fungicide, the total biomass of C. melitensis increased in the presence of Nassella, but decreased when it was grown alone. When stressed by defoliation, C. melitensis may benefit from a form of mycorrhizae‐mediated parasitism through a common mycorrhizal network, or Nassella may alter the fungal community in a way that enhances the positive direct effects of soil fungi on Centaurea.

The Santa Barbara County Oak Restoration Program was funded as alternative mitigation for the loss of more than 2000 oaks during installation of the All American Pipeline (AAPL). As described in the original request for proposals, this program was intended to promote the regeneration of oak habitats within Santa Barbara County through fencing and cattle grazing management. Initiated in 1995 by investigators at the University of California at Santa Barbara, the Oak Restoration Program was designed as a program of research and restoration that would give practical guidance to resource managers and land owners concerned with management and restoration of local oak woodlands. This report provides a summary of the research findings and work completed within the initial 10-year contract period.