Pinus Muricata Research Articles

Irene

Irene Jane Gillette (bio) Irene was nothing if not sensitive to her location, sensitive, too, to the names of the towns and cities through which she had conducted her personal hegira, and so it seemed significant that when Buck escaped from an upstairs window, the tree where he landed was located in Fort Bragg, some ten miles north of Mendocino, California. Buck made his escape while Mrs. Sanchez was shaking out the bathroom rug, and although she hadn’t noticed him perched on the towel rack, she immediately screamed for help and Irene got to the bathroom in time to see him flutter onto the low-lying branch of a neighbor’s tree. She immediately ran next door to coax him down: “I can’t explain, Mrs. Chin, but Buck’s in your tree.” And Mrs. Chin, accustomed to mysteries more opaque than this, welcomed her in and calmly watched her run through the house and out the back door to direct a plea at the Bishop pine that filled most of the back yard: “What do you think you’re doing? Come down here right now!” After a few minutes Irene calmed down and began to speak soothingly: “Buck, Buck, listen to me, please, please,” and Buck cocked his head and looked down, but refused to fly to her offered wrist. It wasn’t surprising. They’d never really practiced the come-on-command trick—or any other trick, for that matter. She’d let him out of the cage when she got home from work, and he’d wander about and eventually land on her bed and start pecking at her earrings, just to drive her crazy, and eventually she’d gently seize him and put him back in his cage. Irene was surprised that Buck could still manage to fly out a window, although of course he’d done it before, or something like it, at [End Page 407] least fifteen years ago now, when he’d flown into her ex-husband’s classroom right in the middle of a lecture on Frederick Law Olmsted, and even though they’d advertised, they’d not discovered whose window he’d flown out of. Who’d have thought he still had it in him? Maybe it was the continuing influence of his name, for they had named him Buckminster Freedbird, presumably after one of her favorite toddlers from the year she worked at the daycare center shortly after they got married. Buckminster Freedberg, Jr., son of the famous art historian by a third or fourth or maybe even a fifth wife. A slight gray-and-white speck against the dark green needles, Buck perched just out of reach, looking down at Irene, then fluttering almost to the top of the tree as Mrs. Chin struggled out the back door with a ladder. She was in her eighties and had dealt with all sorts of bad situations. A bird in a tree seemed a problem with an easy solution. “No, no,” Irene cautioned her quietly. “Don’t make any noise or he’ll fly away,” and Mrs. Chin clanked and clattered back inside. Buck stayed put on his high branch, and yet Irene knew he was paying attention, looking down, although probably not directly at her, just at the location of her voice. She wondered how keen-sighted an old cockatiel could be, old she assumed for she didn’t know how long they lived or, in this particular instance, Buck’s age—although it seemed as if he’d been her bird for a long, long time. Maybe he was all but blind, and so she muttered loud words of encouragement. “Nice bird, good bird, you’ve got a great view up there. You can probably see all the way to the Pacific Ocean, but come on down now, come down, and let’s go inside to your nice, warm … home.” And after only a few minutes of this palaver, Buck did flutter awkwardly down, more like falling than flying, but nevertheless landing safely on the next-to-lowest branch. Maybe he’d reassessed his options after a careful look at Fort Bragg: blocks and blocks of rundown houses...

The Microbiome of Pinus muricata Ectomycorrhizae: Community Assemblages, Fungal Species Effects, and Burkholderia as Important Bacteria in Multipartnered Symbioses.

Bacteria have been observed to grow with fungi, and those that associate with ectomycorrhizal fungi have often been thought of as symbionts that may either increase or decrease ectomycorrhizal formation rate or provide other unaccounted benefits. To explore this symbiosis from a community ecology perspective, we sampled ectomycorrhizal root tips over a 3-year period and used 454 pyrosequencing to identify the bacteria that live inside the ectomycorrhizal root tips. The results showed that fungal community composition within the same soil core and fungal taxonomic identity had a stronger effect on bacterial community composition than sample year or site. Members of the Burkholderiales and Rhizobiales were most highly represented, reflecting many previous reports of these bacteria in association with fungi. The repeated occurrences of these two bacterial orders suggest that they may be symbiotic with their fungal hosts, although the nature of such mechanisms, be it symbiotic diazotrophy or otherwise, remains to be thoroughly tested.

Evaluating spatial patterns of drought-induced tree mortality in a coastal California pine forest

In a coastal, fog-influenced forest on Santa Cruz Island in southern California, we observed mortality of Bishop pine (Pinus muricata D.Don) trees following a brief (2year), yet intense, drought. While anecdotal evidence indicates that drought-induced Bishop pine mortality has occurred in the past in the stand we studied, this is the first attempt to capture the spatial distribution of mortality, and begin to understand the environmental drivers underlying these events. We used high spatial resolution remote sensing data to quantify the spatial extent of tree mortality using a 1m true color aerial photograph and a 1m LiDAR digital elevation model. We found the highest density of dead trees in the drier, more inland margins of the forest stand. We used the Random Forest decision tree algorithm to test which environmental variables (e.g., summertime cloud frequency, solar insolation, and geomorphic attributes) would best separate live and dead tree populations. We also included tree height as a variable in our analysis, which we used as a proxy for overall tree size and potential rooting distribution. Based on the Random Forest analysis, we generated a map of the probability of survival. We found tree survivorship after drought was best explained by the frequency of summertime clouds, elevation, and tree height. Specifically, survivorship was greatest for larger trees (∼8–10m tall) in more foggy parts of the stand located at moderate elevation. We found that probability of survival was lowest at the inland extent of the stand where trees occur at the upper limit of their elevation range (∼400m). The coexistence of these main factors with other landscape variables help identify areas of suitable habitat for Bishop pines across the stand, and extend our understanding of this species’ distribution.

Ecology and Distribution of the Introduced MossCampylopus introflexus(Dicranaceae) In Western North America

Abstract Campylopus introflexus (Hedw.) Brid. is a moss native to the southern hemisphere and well documented as an invasive species throughout Europe. The species was first collected in North America in 1967 in Del Norte Co., California, and now occurs primarily in coastal areas from Santa Barbara Co. to southern British Columbia. Herbarium specimens were assembled and verified to document the distribution of the species in western North America. Collection dates of specimens indicate a rapid invasion along the coast north and south from the first record, with a slower establishment of inland populations. The species is most prevalent in coastal, foggy areas. Many of the populations are associated with anthropogenic environments, but several populations appear to be invading relatively undisturbed sites. These are mostly characterized by poor soils along the coast, with the largest populations associated with Bishop Pine forests and stabilized sand dunes. A brief summary of ecological literature document...

Ectomycorrhizal fungal traits reflect environmental conditions along a coastal California edaphic gradient

Multispecies mutualisms, such as the association between trees and ectomycorrhizal fungi, are often shaped by environmental context. Here, we explored the functional mechanisms underlying this environmental filtering. Using a single population of Pinus muricata (Bishop pine) growing along a strong edaphic gradient, we examined how environmental stress affected ectomycorrhizal fungi. The gradient spans c. 400000 years of soil age, and reduced nutrient availability and increased water stress dwarf trees on older sites. Fungal community composition shifted with nutrient and water availability and with the stature of the P. muricata host trees. Not only did pygmy trees host a taxonomically different fungal subset as compared to nonpygmy trees, but associated fungal communities also differed in life history strategies: trees in more stressful conditions hosted fungi with more carbon-intensive foraging strategies. Our results indicate a link between environmental controls of host nutritional status and turnover in the ectomycorrhizal fungal community. The transition to more energy-intensive strategies under nutrient stress may allow for close recycling of recalcitrant nutrient pools within the root zone and facilitate transport of nutrients and water over long distances. These results highlight the value of life history data to understanding the mechanistic underpinnings of species distributions.

Does the Initial Floristic Composition model of succession really work?

AbstractHarvey & Holzman, in this issue of the Journal of Vegetation Science, describe divergent successional pathways after fire in Pinus muricata forest. Fourteen years after fire, the shrub Ceanothus thyrsiflorus had occupied flat parts of the area. They predicted it would continue to prevent succession to P. muricata forest, as in the initial floristic composition concepts of Clements and Egler.

Fungi at a small scale: spatial zonation of fungal assemblages around single trees.

Biological communities are often structured by environmental factors even at small spatial scales. Fungi are no exception, though the patterns and mechanisms underlying their community structure are usually unknown. Previous work documented zonation in fungi under tree canopies primarily through their fruiting patterns. Here we investigate the existence of zonation patterns in fungal communities around isolated Pinus muricata trees of different ages in northern coastal California. Using a combination of ingrowth bags and pyrosequencing to target underground mycelium we found highly diverse soil fungal communities associated with single trees. Both ectomycorrhizal and non-ectomycorrhizal fungi were present in all samples, but the latter were more species rich, dominated the samples by sequence read abundance, and showed partitioning by canopy-defined zones and tree age. Soil chemistry was correlated with fungal zonation, but host root density was not. Our results indicate different guilds of fungi partition space differently and are driven by distinct environmental parameters.

Bioclimatic velocity: the pace of species exposure to climate change

AbstractAimTo investigate the velocity of species‐specific exposure to climate change for mid‐ and late 21st century and develop metrics that quantify exposure to climate change over space and time.LocationCalifornia Floristic Province, south‐western USA.MethodsOccurrences from presence/absence inventories of eight Californian endemic tree species (Pinus balfouriana [Grev.&Balf.], Pinus coulteri [D.Don], Pinus muricata [D.Don.], Pinus sabiniana [D.Don], Quercus douglasii [Hook.&Arn.], Quercus engelmannii [Greene], Quercus lobata [Nee] and Quercus wislizeni [A.DC.]) were used to develop eight species distribution models (SDMs) for each species with the BIOMOD platform, and this ensemble was used to construct current suitability maps and future projections based on two global circulation models in two time periods [mid‐century: 2041–2070 and late century (LC): 2071–2100]. From the resulting current and future suitability maps, we calculated a bioclimatic velocity as the ratio of temporal gradient to spatial gradient. We developed and compared eight metrics of temporal exposure to climate change for mid‐ and LC for each species.ResultsThe velocity of species exposure to climate change varies across species and time periods, even for similarly distributed species. We find weak support among the species analysed for higher velocities in exposure to climate change towards the end of the 21st century, coinciding with harsher conditions. The variation in the pace of exposure was greater among species than for climate projections considered.Main conclusionsThe pace of climate change exposure varies depending on period of analysis, species and the spatial extent of conservation decisions (potential ranges versus current distributions). Translating physical climatic space into a biotic climatic space helps informing conservation decisions in a given time frame. However, the influence of spatial and temporal resolution on modelled species distributions needs further consideration in order to better characterize the dynamics of exposure and species‐specific velocities.

Divergent successional pathways of stand development following fire in aCalifornia closed‐cone pine forest

AbstractQuestionsWhat are the major trends in vegetation community structure and forest stand structure over a 14‐yr post‐fire period in aCalifornia closed‐cone pine forest? Which biotic and abiotic factors best explain variation in stand structure at different stages of post‐fire succession, and does the relative importance of these factors remain constant? Is there evidence of multiple successional pathways of forest stand development?LocationPost‐firePinus muricata(bishop pine) forests atPointReyesNationalSeashore,CA,USA.MethodsWe quantified post‐fire vegetation change from field data collected 1, 2, 6 and 14 yr following stand‐replacing wildfire that occurred in 1995. General linear models were used to assess trends in composition (plant functional groups and species diversity) and generalized linear models were used to assess trends in stand structure (post‐fireP. muricatadensity) and determine the relative importance of abiotic and biotic factors on stand structure in different early‐successional post‐fire years.ResultsSpecies richness and diversity peaked in the first 2 yr following fire, and then declined through year 14. Initial post‐fireP. muricatatree regeneration was high (mean 249 750 stems·ha−1in year 1) and remained well above pre‐fire stand density levels by year 14 (mean 15 179 stems·ha−1). Post‐fireP. muricataseedling density was associated with topographic factors in years 1 and 2, negatively associated with cover of a non‐native herb in year 2, and negatively associated with cover of an early/mid‐successional shrub and positively associated with slope in years 6 and 14. Two alternative pathways of post‐fire stand development have emerged by year 14. A high‐density, closed‐canopy pathway (mean 40 875 stems·ha−1) with early intra‐specific thinning resulted on steep slopes and ridges with low shrub cover. In contrast, a low‐density, open‐canopy pathway (mean 1250 stems·ha−1) resulted on gentle slopes and where shrub cover was high.ConclusionsThis study provides evidence of divergent successional pathways and illustrates the importance of early‐successional species interactions and topography on longer‐term stand development trajectories in serotinous conifer forests. Early heterogeneity in vegetation establishment set the course for variability in stand structure in mid‐seral stages and may persist into later stages.

Cloud shading and fog drip influence the metabolism of a coastal pine ecosystem

Assessing the ecological importance of clouds has substantial implications for our basic understanding of ecosystems and for predicting how they will respond to a changing climate. This study was conducted in a coastal Bishop pine forest ecosystem that experiences regular cycles of stratus cloud cover and inundation in summer. Our objective was to understand how these clouds impact ecosystem metabolism by contrasting two sites along a gradient of summer stratus cover. The site that was under cloud cover ~15% more of the summer daytime hours had lower air temperatures and evaporation rates, higher soil moisture content, and received more frequent fog drip inputs than the site with less cloud cover. These cloud-driven differences in environmental conditions translated into large differences in plant and microbial activity. Pine trees at the site with greater cloud cover exhibited less water stress in summer, larger basal area growth, and greater rates of sap velocity. The difference in basal area growth between the two sites was largely due to summer growth. Microbial metabolism was highly responsive to fog drip, illustrated by an observed ~3-fold increase in microbial biomass C with increasing summer fog drip. In addition, the site with more cloud cover had greater total soil respiration and a larger fractional contribution from heterotrophic sources. We conclude that clouds are important to the ecological functioning of these coastal forests, providing summer shading and cooling that relieve pine and microbial drought stress as well as regular moisture inputs that elevate plant and microbial metabolism. These findings are important for understanding how these and other seasonally dry coastal ecosystems will respond to predicted changes in stratus cover, rainfall, and temperature.

Stayin' alive: survival of mycorrhizal fungal propagules from 6-yr-old forest soil

Spores and sclerotia are the main propagules that allow fungi to persist through unfavorable conditions and disperse into new environments. Despite their importance, very little is known about their longevity and dormancy, especially in ectomycorrhizal fungi. To assess the viability of ectomycorrhizal fungal spores in forest soil, we collected and buried non-sterile forest soil, in pots, in the field distant from an inoculum source. After 6 yr, a subset of this soil was assayed for viable spores by baiting the fungi with Bishop pine (Pinus muricata) seedlings. Our results show that the three most frequent colonizers in year 1 continued to colonize significant percentages of seedlings in year 6: Wilcoxina mikolae (77 %), Rhizopogon vulgaris (13 %) and Suillus brevipes (9 %). While three species that colonized low percentages of seedlings in year 1, Suillus pungens (1 %), Rhizopogon salebrosus (2 %), and Thelephora terrestris (5 %) were not recovered in year 6. Laccaria proxima, a species not seen in year 1, was recovered on a single seedling in year 6. This is the first report of long-term survival of S. brevipes and W. mikolae. Our results reveal a more complete picture of ectomycorrhizal fungal spore longevity in soil spore banks.

First Report of Grass Species (Poaceae) as Naturally Occurring Hosts of the Pine Pathogen Gibberella circinata.

Gibberella circinata (anamorph Fusarium circinatum) causes pitch canker in pines and is not known to have any hosts outside the Pinaceae. However, G. circinata is closely related to and interfertile with G. subglutinans, which is associated with grasses both as a pathogen and a commensal endophyte. Furthermore, studies under controlled conditions have shown that G. circinata can colonize corn (Zea mays) without inducing symptoms (4). To determine if G. circinata can also infect grasses under natural conditions, plants were collected in proximity to trees with symptoms of pitch canker in native stands of Pinus radiata (Monterey pine) on the Monterey Peninsula and P. muricata (bishop pine) at Pt. Reyes National Seashore on the California coast during July and August of 2011. Leaves and stems were rinsed in 0.1% Tween 20, immersed in 70% ethanol for 30 s followed by 1 min in 1% NaOCl, and placed on a Fusarium selective medium (FSM) (1). Single-spore subcultures of colonies growing from cultured plant material were transferred to 0.6% KCl agar and identified as G. circinata based on morphological criteria as described by Gordon et al. (2). G. circinata isolates were recovered from Holcus lanatus and Festuca arundinacea on the Monterey Peninsula and H. lanatus at Pt. Reyes National Seashore. Three isolates from each of these sources (nine total) and one known G. circinata isolate from pines (GL 17) were tested for virulence by inoculating 1-year-old, greenhouse-grown Monterey pine trees; three trees were inoculated, once for each isolate. Trees were inoculated by depositing 250 spores in a wound on the main stem by the method described by Gordon et al. (3). Two weeks later, all grass isolates had induced resinous branch cankers with lesions comparable in length (17 to 24 mm) and appearance to those caused by GL 17. Similar results were obtained when inoculations were repeated. One isolate from F. arundinacea and one from H. lanatus (collected at Pt. Reyes National Seashore) were tested and shown to be somatically compatible with tester strains for vegetative compatibility groups C6 and C1, respectively, both of which are associated with isolates previously recovered from diseased pines (2). GL 17 and one isolate each from F. arundinaceae and H. lanatus were tested for their ability to infect F. arundinaceae cv. Fawn. For each isolate, 20 14-day-old seedlings (10 pots with two plants per pot) were sprayed to runoff with an aqueous suspension of 106 spores per ml. All inoculations were repeated once. Two weeks after inoculation, leaves and stems were rinsed briefly in 0.1% Tween 20, immersed for 10 s in 70% ethanol, followed by 30 s in 1% NaOCl, and cultured on FSM. All tested isolates were recovered from at least some of the inoculated plants (range 20 to 100%), from living stems and leaves, as well as from senescing tissue. These results show that grass species can be symptomless hosts for G. circinata, constituting the first documentation of any host for this pathogen outside the Pinaceae. Studies are underway to further characterize the host range of G. circinata and assess the epidemiological implications of grasses as alternate hosts for the pitch canker pathogen.

Spatial variability in stand structure and density-dependent mortality in newly established post-fire stands of a California closed-cone pine forest

Fire is an important process in California closed-cone pine forests; however spatial variability in post-fire stand dynamics of these forests is poorly understood. The 1995 Vision Fire in Point Reyes National Seashore burned over 5000 ha, initiating vigorous Pinus muricata (bishop pine) regeneration in areas that were forested prior to the fire but also serving as a catalyst for forest expansion into other locales. We examined the post-fire stand structure of P. muricata forest 14 years after fire in newly established stands where the forest has expanded across the burn landscape to determine the important factors driving variability in density, basal area, tree size, and mortality. Additionally, we estimated the self-thinning line at this point in stand development and compared the size–density relationship in this forest to the theorized (−1.605) log–log slope of Reineke’s Rule, which relates maximum stand density to average tree size. Following the fire, post-fire P. muricata density in the expanded forest ranged from 500 to 8900 live stems ha −1 (median density = 1800 ha −1). Post-fire tree density and basal area declined with increasing distance to individual pre-fire trees, but showed little variation with other environmental covariates. Self-thinning (density-dependent mortality) was observed in nearly all stands with post-fire density >1800 stems ha −1, and post-fire P. muricata stands conformed to the size–density relationship predicted by Reineke’s Rule. This study demonstrates broad spatial variability in forest development following stand-replacing fires in California closed-cone pine forests, and highlights the importance of isolated pre-fire trees as drivers of stand establishment and development in serotinous conifers.

Colonization-competition tradeoffs as a mechanism driving successional dynamics in ectomycorrhizal fungal communities.

Colonization-competition tradeoffs have been shown to be important determinants of succession in plant and animal communities, but their role in ectomycorrhizal (ECM) fungal communities is not well understood. To experimentally examine whether strong spore-based competitors remain dominant on plant root tips as competition shifts to mycelial-based interactions, we investigated the mycelial competitive interactions among three naturally co-occurring ECM species (Rhizopogon occidentalis, R. salebrosus, and Suillus pungens). Each species was grown alone and in all pair-wise combinations on P. muricata seedlings in experimental microcosms and culture assays. Competitive outcomes were assessed from ECM root tip colonization, soil mycelial abundance, and mycelial growth in culture. In the microcosm experiment, we observed a clear competitive hierarchy of R. salebrosus>R. occidentalis>S. pungens. Competitive effects were also apparent in the culture assays, however, no similar hierarchy was present. These results contrast with our previous findings from spore-based competition, suggesting that ECM competitive outcomes can be life-stage dependent. The differing competitive abilities observed here also showed general correspondence with patterns of ECM succession in Pinus muricata forests, indicating that competitive interactions may significantly influence temporal patterns of ECM community structure.

Landscape-Scale Vegetation Change Following Fire in Point Reyes, California, USA

Fire is an important factor in determining plant community composition and distribution. This study quantifies landscape-scale vegetation change following a large fire at Point Reyes National Seashore, California, USA. Vegetation in the Point Reyes region is characterized by a complex mosaic of grassland, shrub, and forest plant communities, and by high levels of plant diversity. Although large fires are relatively rare on the coast of California north of San Francisco Bay, they are important in determining the distributions of plant communities at the landscape scale. We mapped vegetation communities throughout the study area using aerial imagery and analyzed how vegetation shifted following fire. We found substantial areas had transitioned from coastal scrub to ceanothus scrub (Ceanothus thyrsiflorus Eschsch.) or Bishop pine (Pinus muricata D. Don) forest following fire. Transitions from shrub to tree vegetation following fire have rarely been documented in this region. Logistic regression analysis was used to examine the factors influencing the post-fire distribution of Bishop pine and ceanothus scrub. Proximity to pre-fire Bishop pine stands and pre-fire vegetation type were the most important predictors of post-fire Bishop pine regeneration. Pre-fire vegetation type, burn severity, and topography were the most important predictors of post-fire ceanothus scrub distribution. This study demonstrates the capacity of these ecosystems for substantial change over short time periods in response to fire, and identifies some of the factors driving this change.

Suillus quiescens, a new species commonly found in the spore bank in California and Oregon

Suillus quiescens sp. nov. is common under Pinus muricata on Santa Cruz and Santa Rosa Islands in the northern Channel Islands of California, and we subsequently found it fruiting at Point Reyes National Seashore on the central coast of California. Sequences from the internal transcribed spacer region show that it is distinct from all 44 species of Suillus tested, and features of its morphology separate it from all other unsequenced species. Suillus quiescens has a broader distribution than coastal California because it also was encountered as ectomycorrhizae on roots of pine seedlings from the eastern Sierra Nevada, coastal Oregon and the southern Cascade Mountains. The reason it had not been identified from these areas might be due to its resemblance to S. brevipes at maturity or it might be a rare fruiter that persists in the spore bank.

Testing the ecological stability of ectomycorrhizal symbiosis: effects of heat, ash and mycorrhizal colonization on Pinus muricata seedling performance

Understanding how abiotic conditions mediate the outcome of biotic interactions is a key question in community ecology. This is particularly interesting in the case of mutualisms because changing environmental conditions may be a source of evolutionary or ecological instability in the relationship between symbiotic partners. For the mycorrhizal symbiosis, elevated nutrient levels may make the carbon cost to plants of supporting mycorrhizal fungi outweigh the benefits of mycorrhizal nutrient acquisition. In this study, we simulated the effects of post-fire nutrient enrichment on the ectomycorrhizal symbiosis between Rhizopogon occidentalis and Pinus muricata by using factorial combinations of soil heating, ash addition and mycorrhizal colonization. In general, the presence of R. occidentalis and the heat and ash treatments increased growth of P. muricata seedlings. However, significant interactions between the treatments indicated that there were context-dependent effects on seedling growth. Soil heating increased soil mineral nitrogen content, but did not decrease the benefit of mycorrhizal colonization relative to unheated controls. This appeared to be driven by a decrease in the proportion of the root system occupied by R. occidentalis, indicating that host-plants can optimize colonization in a way that maximizes growth. Because seedlings in heat-treated soils were larger than unheated controls, reduced percent colonization did not cause any decrease in fungal biomass in heat-treated soils. Since there was no decrease in the performance of either partner, our results show that, in this system, ectomycorrhizal mutualisms are likely robust within the natural degree of environmental variability caused by moderate intensity wildfires.

Root tip competition among ectomycorrhizal fungi: Are priority effects a rule or an exception?

Competition for root colonization among ectomycorrhizal fungi is well documented, but the mechanisms determining competitive outcomes are not clearly understood. In a previous study, we observed that timing of colonization (i.e., a priority effect) had a significant effect on the outcome of competition between two ectomycorrhizal (EM) fungi in the genus Rhizopogon. In this study, we explicitly tested the role of priority effects in competition among EM fungi by experimentally manipulating the timing of colonization of four Rhizopogon species on Pinus muricata seedlings. In a first experiment, we set up 12 two-species combinations, in which seedlings were first inoculated from spores with one species, grown for three months, and then inoculated with an equal density of spores of a second species and grown for an additional three months. Root tip occupation in the two-species treatments was determined by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis of internal transcribed spacer region (ITS) of rDNA. In a second experiment, we further examined competitive interactions between two Rhizopogon species using split-root P. muricata seedlings. One side of the root system was pre-colonized by one species, and spores of the second species were added to the other side of the root system in all same and different species pair-wise combinations. We found that for three of the four species (R. occidentalis, R. salebrosus, R. vulgaris), the outcome of competition in the first experiment depended strongly on the timing of colonization, with the first colonizing species always being the competitive dominant. For R. evadens, however, initial colonization did not prevent significant subsequent colonization by R. occidentalis and R. vulgaris. This appeared to be caused by the lower colonization of R. evadens compared to the three other species. In the second experiment, we observed that the portion of the split root system that was initially uncolonized remained receptive to colonization when spores were added. The amount of colonization of R. occidentalis and R. salebrosus on the side of the root system to which they were added was not significantly influenced by species identity on the other side of the seedling. In combination, these results confirm that priority effects do play a major role in dynamics of EM root tip colonization, at least in the early colonization of seedlings, and that the proportion of the root system occupied by a species appears to be a key factor determining competitive success.

Significance of summer fog and overcast for drought stress and ecological functioning of coastal California endemic plant species

AbstractAim Fog drip is a crucial water source for plants in many ecosystems, including a number of global biodiversity hotspots. In California, dozens of rare, drought‐sensitive plant species are endemic to coastal areas where the dominant summer moisture source is fog. Low clouds that provide water to these semi‐arid ecosystems through fog drip can also sharply reduce evaporative water losses by providing shade. We quantified the relative hydrological importance of cloud shading vs. fog drip. We then examined how both factors influence the range dynamics of an apparently fog‐dependent plant species spanning a small‐scale cloud gradient.Location The study area is on Santa Cruz Island off the coast of southern California. It is near the southern range limit of bishop pine (Pinus muricata D. Don), a tree endemic to the coasts of California and Baja, Mexico.Methods We measured climate across a pine stand along a 7 km, coastal–inland elevation transect. Short‐term (1–5 years) monitoring and remote sensing data revealed strong climatic gradients driven primarily by cloud cover. Long‐term (102 years) effects of these gradients were estimated using a water balance model.Results We found that shade from persistent low clouds near the coast reduced annual drought stress by 22–40% compared with clearer conditions further inland. Fog drip at higher elevations provided sufficient extra water to reduce annual drought stress by 20–36%. Sites located at both high elevation and nearer the coast were subject to both effects. Together, these effects reduced average annual drought stress by 56% and dramatically reduced the frequency of severe drought over the last century. At lower elevation (without appreciable fog drip) and also near the inland edge of the stand (with less cloud shading) severe droughts episodically kill most pine recruits, thereby limiting the local range of this species.Main conclusions Persistent cloud shading can influence hydrology as much as fog drip in cloud‐affected ecosystems. Understanding the patterns of both cloud shading and fog drip and their respective impacts on ecosystem water budgets is necessary to fully understand past species range shifts and to anticipate future climate change‐induced range shifts in fog‐dependent ecosystems.

Within-population genetic variability in mycorrhizal interactions

The geographic mosaic theory of coevolution hypothesizes that natural selection on species interactions varies among ecosystems, partly because the genes involved in species interactions differ in their fitness effects among environments. This selection mosaic may be expressed, at the extreme, as ecological outcomes ranging from mutualism to parasitism among environments. In a recent laboratory experiment on the interaction between a plant, bishop pine (Pinus muricata), and a root-symbiotic ectomycorrhizal fungus, Rhizopogon occidentalis, we demonstrated the potential for selection mosaics in that interaction, and the existence of substantial within-population genetic variation for symbiotic compatibility in the interaction. Here, we present the results from a second experiment on the interaction between the same ectomycorrhizal fungus and a different plant, shore pine (Pinus contorta var. contorta), designed to test for the presence of genetic variation for symbiotic compatibility in another similar system, and also to test whether such variation might be generated in part by adaptation of fungal lineages to individual trees. In this experiment, we found no genetic variation among plant lineages for compatibility with the fungal symbiont, and no evidence for adaptation of fungal lineages to individual plants, but the two fungal genotypes differed greatly in their compatibility with the plant hosts. Specifically, one of the two fungal genotypes not only colonized host plants less intensively than the other, but also had a negative effect on plant growth. Altogether, these results suggest the potential for ongoing natural selection on the ectomycorrhizal fungus, R. occidentalis, for different levels of symbiotic compatibility with particular pine hosts, but the mechanisms generating and maintaining genetic variation for symbiotic compatibility remain unclear. Such results will aid in efforts to develop realistic models of how plants and their symbionts coevolve over broad geographic ranges in which they co-occur.