Vesicular mycorrhizal colonization of seedlings of Pinaceae and Betulaceae after spore inoculation with Glomus intraradices

 Although Pinaceae and Betulaceae have been reported to contain Glomus–type root endophytes, its ecological importance and the conditions influencing this symbiosis are poorly understood. Seedlings of Abies lasiocarpa, Alnus rubra, Pinus contorta, Pinus ponderosa, Pseudotsuga menziesii, and Tsuga heterophylla were inoculated with Glomus intraradices to determine the vesicular-arbuscular mycorrhizae (VAM) development and responsiveness of these hosts. The role of companion VAM host plants on mycorrhizal colonization and nutrient uptake by Pseudotsuga menziesii was also examined by growing seedlings of Pseudotsuga menziesii in dual culture with VAM hosts Thuja plicata or Calamagrostis rubescens. After 8 weeks, no seedlings were colonized. At 16 weeks, 8 of 17 Thuja plicata seedlings grown with Pseudotsuga menziesii and all 18 inoculated Thuja plicata seedlings grown alone were colonized with vesicles and hyphae. Two of 17 inoculated Pseudotsuga menziesii seedlings grown in dual culture with Thuja plicata were colonized with abundant vesicles and hyphae. No ectomycorrhizal seedlings grown in monoculture were colonized. At 9 months, all 10 Calamagrostis rubescens and all 10 inoculated Pseudotsuga menziesii seedlings grown in dual culture were colonized by vesicles and hyphae. Two of 10 inoculated Pseudotsuga menziesiiand 1 of 10 inoculated Pinus ponderosa seedlings grown in monoculture were similarly colonized. The mean phosphorus content in the needles of colonized Pseudotsuga menziesii seedlings grown with Calamagrostis rubescens was about twice as high as in noncolonized Pseudotsuga menziesiiseedlings grown with Calamagrostis rubescens. Tissue nitrogen did not differ between these treatments. The results show that Glomus intraradices colonization of Pinaceae is most successful when a VAM host is present, although some vesicular colonization of Pinaceae occurred in the absence of a VAM host. Accepted: 24 September 1997     

Mapped DNA probes from loblolly pine can be used for restriction fragment length polymorphism mapping in other conifers

A high-density genetic map based on restriction fragment length polymorphisms (RFLPs) is being constructed for loblolly pine (Pinus taeda L.). Consequently, a large number of DNA probes from loblolly pine are potentially available for use in other species. We have used some of these DNA probes to detect RFLPs in 12 conifers and an angiosperm. Thirty complementary DNA and two genomic DNA probes from loblolly pine were hybridized to Southern blots containing DNA from five species of Pinus (P. elliottii, P. lambertiana, P. radiata, P. sylvestris, and P. taeda), one species from each of four other genera of Pinaceae (Abies concolor, Larix laricina, Picea abies, and Pseudotsuga menziesii), one species from each of three other families of Coniferales [Sequoia sempervirens (Taxodiaceae), Torreya californica (Taxaceae) and Calocedrus decurrens (Cupressaceae)], and to one angiosperm species (Populus nigra). Results showed that mapped DNA probes from lobolly pine will cross-hybridize to genomic DNA of other species of Pinus and some other genera of the Pinaceae. Only a small proportion of the probes hybridized to genomic DNA from three other families of the Coniferales and the one angiosperm examined. This study demonstrates that mapped DNA probes from loblolly pine can be used to construct RFLP maps for related species, thus enabling the opportunity for comparative genome mapping in conifers.     

Anatomy and relationships of petrified seed cones of the Cupressaceae,Taxodiaceae, and Sciadopityaceae

Morphological features and the affinities of petrified seed cones of the Taxodiaceae, Cupressaceae, and Sciadopityaceae are reevaluated. TheCunninghamia-like plants are the earliest record of the families.Parataiwania explains the divergence ofTaiwania from aCunninghamia-like ancestral form by loss of ovuliferous scale and sclerenchyma.Sequoia-like allies probably diverged from a Cretaceous transitional plant likeYezosequoia and are most diversified in the Late Cretaceous.Yubaristrobus andArchicupressus suggest monophyly ofTaxodium and its allied genera of the Taxodiaceae and the Cupressaceae. The Cupressaceae may be derived from a taxodiaceous plant with orthotropous seeds, acquiring a reniform arrangement of the vascular bundles of the bract-scale complex, decussate or whorled phyllotaxis, and spherical cones consisting of a small number of bract-scale complexes. The Cretaceous fossils of the Sciadopityaceae suggest the ancient origin of the family.     

杉科、柏科的系统发生关系研究进展

杉科、柏科是松柏类裸子植物中的重要类群,其系统分类研究一直是裸子植物的研究热点之一.但是杉科与柏科之间及其内部各属之间的系统发生关系却一直存在争议.一般认为杉科、柏科单独成科.近年来的分子系统学及分支系统学研究结果证实:除了金松属以外,杉科和柏科为一单系群,应合并为一个科Cupres-saceae sensu　lato（广义柏科）,其主要分支类群的系统发生关系也已经基本确立,而金松属则单独成立金松科.     

Structure and affinities of the petrified plants from the cretaceous of Northern Japan and Saghalien XIIIYubaristrobus gen. nov., A new Taxodiaceous cone from the upper cretaceous of Hokkaido

Yubaristrobus is a new genus of the Taxodiaceae based on a permineralized seed cone from the Upper Cretaceous of Hokkaido. The type species,Y. nakajimae sp. nov., is characterized by peltate bract-scale complexes consisting of a completely-fused bract and scale. The bract-scale complexes are spirally arranged as in most taxodiaceous genera. Their vascular arrangement is specialized and unique in the Taxodiaceae and suggests a relationship with the Cupressaceae. Consecutive number from the previous paper (Ohsawa, M. Nishida and H. Nishida, 1992b).     

Persistence of<Emphasis Type="Italic">Pseudotsuga menziesii</Emphasis> (Douglas-fir) in temperate coniferous forests of the Pacific Northwest Coast,USA

Old-growthPseudotsuga-Tsuga forests of the Pacific Northwest Coast of North America are characterized by the presence of large, old trees ofPseudotsuga menziesii var.menziesii (Douglas-fir). Colonizing soon after a stand-replacing disturbance,P. menziesii persists in these forests, coexisting for centuries with the late-successional species.P. menziesii survives by maintaining emergent status in the uppermost part of the forest canopy, above the crowns of competing late-successional species. After reaching maximum tree height and crown size,P. menziesii maintains shoots and foliage of the established crown by epicormic shoot production. In this review, we propose that attaining emergent status in the upper canopy combined with the process of crown maintenance contributes to the persistence ofP. menziesii into later stages of succession, making this species a long-lived pioneer that between infrequent disturbances can coexist with late-successional species for centuries.     

Occurrence and dominance of six Pacific Northwest conifer species

Questions: Can probability of occurrence and dominance be accurately estimated for six important conifer species with varying range sizes? Does range size impact the accuracy of species probability of occurrence models? Is species predicted probability of occurrence significantly related to observed dominance? Location: Pacific Northwest region, North America (60°–40°N, 140°–110°W). Methods: This study develops near range‐wide predictive distribution maps for six important conifer species (Pseudotsuga menziesii, Tsuga heterophylla, Pinus contorta, Thuja plicata, Larix occidentalis, and Picea glauca) using forest inventory data collected across the United States and Canada. Species model accuracies are compared with range size using a rank scoring system. A suite of climate and topographic predictor variables are used to investigate environmental constraints that limit species range and quantify relationships between species predicted probability of occurrence and dominance at both plot and landscape scales. Results: Evaluation statistics revealed significant and accurate probability of occurrence models were developed for all six species. Based on ranked evaluation statistics, Tsuga heterophylla had highest overall model accuracy (statistic rank score=5) and Pinus contorta the lowest (statistic rank score=17). Across species, ranked evaluation statistics also revealed a pattern of decreasing model accuracy with increasing range size. At plot level, correlations between dominance and probability of occurrence were weakly positive for all species with only half of the species having statistically significant correlations. Pseudotsuga menziesii had the highest correlation (r=0.36, P<0.001) and Thuja plicata lowest (r=0.038, P=0.799). At the 50‐km scale, correlations between dominance and probability of occurrence improved for all species except Pinus contorta. Pseudotsuga menziesii displayed the highest correlation (r=0.68, P<0.001) and Thuja plicata the lowest (r=0.07, P>0.709). Conclusions: Species probability of occurrence model accuracy decreased with increasing range size. The strength and significance of correlations between probability of occurrence and dominance varied considerably by species and across spatial scales. Apart from Pseudotsuga menziesii and L. occidentalis, the results suggest that probability of occurrence is not a consistently reliable surrogate for species dominance in Pacific Northwest forests. We demonstrate how the degree of correlation between species occurrence and dominance can be used as an indicator of how well predictions of occurrence characterize the optimal niche of a species.     

Ability of native ectomycorrhizal fungi from northern Spain to colonize Douglas-fir and other introduced conifers

 Thirty-six isolates from 27 species of native ectomycorrhizal fungi collected in northern Spain were tested for ectomycorrhiza formation with Pseudotsuga menziesii seedlings in pure culture syntheses. Thirteen of those species were also tested for ectomycorrhiza formation with six other species of conifers (two native and four introduced) to compare their colonization potential. Twenty-three fungal isolates from 18 species formed ectomycorrhizas with Pseudotsuga menziesii. The colonization level of the root system varied markedly among the different fungal species. Eight fungi colonized over 50% of the short roots. Nine fungi did not form ectomycorrhizas even though some of them were collected in pure stands of Pseudotsuga menziesii. Laccaria laccata, Lyophyllum decastes, Pisolithus tinctorius, and Scleroderma citrinum formed abundant ectomycorrhizas on all the conifers tested. Lactarius deliciosus, Rhizopogon spp., and Suillus luteus showed the greatest host specificity. The success in the introduction of some exotic conifers for reforestation in northern Spain is discussed in relation to their compatibility with native ectomycorrhizal fungi. Accepted: 28 August 1995     

Molecular phylogeny and evolution of host-plant use in conifer seed chalcids in the genus Megastigmus (Hymenoptera: Torymidae)

Abstract. Phylogenetic relationships amongst Megastigmus species (Chalcidoidea: Torymidae) associated with conifer seeds were inferred from DNA sequence data. Twenty‐nine species of seed chalcids were analysed using two different genes, cytochrome b (mitochondrial DNA) and the D2 domain of the 28S ribosomal DNA. Maximum‐parsimony and maximum‐likelihood analyses showed that taxa formed two monophyletic groups, one clade comprising all species associated with Cupressaceae and Taxodiaceae hosts with the exception of Chamaecyparis, and the other clade composed of species associated with Pinaceae. Species infesting Cupressaceae and Taxodiaceae seemed to be specialized to particular host genera or even to be species specific, which was consistent with a taxonomic radiation following initial host adaptation. By contrast, Megastigmus species associated with Pinaceae appeared capable of shifting onto different congeneric species or even onto a new host genus, with their evolution apparently less constrained by plant association. We hypothesized that the Megastigmus group associated with Pinaceae may have a much higher invasive potential than that related to Cupressaceae. The study also confirmed the presence of invasive Nearctic species in the Palaearctic, and demonstrated the existence of a cryptic species complex.     

Genetic maladaptation of coastal Douglas-fir seedlings to future climates

Climates are expected to warm considerably over the next century, resulting in expectations that plant populations will not be adapted to future climates. We estimated the risk of maladaptation of current populations of coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) to future climates as the proportion of nonoverlap between two normal distributions where the means and genetic variances of current and future populations are determined from genecological models derived from seedling common garden studies. The risk of maladaptation was large for most traits when compared with the risk associated with current transfers within seed zones, particularly for the more drastic climate change scenario. For example, the proportion of nonoverlap for a composite trait representing bud set, emergence, growth, and root : shoot ratio was as high as 0.90. We recommend augmenting within-population variation by mixing local populations with some proportion of populations from lower elevations and further south. Populations expected to be adapted to climates a century from now come from locations as far down in elevation as 450–1130 m and as far south in latitude as 1.8–4.9°.     

Mycorrhiza-like interaction by Morchella with species of the Pinaceae in pure culture synthesis

 Isolates from two species of Morchella were tested for ability to form mycorrhizae in pure culture synthesis with Arbutus menziesii, Larix occidentalis, Pinus contorta, Pinus ponderosa, andPseudotsuga menziesii. Ectomycorrhizal structures (mantle and Hartig net) formed with the four species of the Pinaceae but not with A. menziesii. Results are compared to previous studies on morel mycorrhizae and discussed in an ecological context. Accepted: 23 October 1999     

广义柏科主要分类群起源时间的推测

通过构建分子钟对广义柏科主要分类群的起源时间进行探讨。采用相对速率检验法分析广义柏科mat K、rbc L进化速率的稳定性,结果显示rbc L的非同义替代速率只在Pinaceae与Taxodiaceae的分类群及柏科北半球的支系(Cupressoideae)之间通过相对速率检验,而Pinaceae与柏科南半球分支(Callitrodeae)之间没有通过相对速率检验。mar K基因的非同义替代速率在Pinaceae与广义柏科的所有分类群之间通过相对速率检验。根据通过相对速率检验的分类群之间的遗传距离和基因进化速率,计算它们发生分歧的时间。据此推测,杉科的主要分类群Taiwanioideae、Athrotaxidoideae、Sequoioideae与其他分支发生分歧的时间均在侏罗纪,支持现存杉科在侏罗纪就已经建立起来的观点;Cupressaceae(s．s．)的两个支系(亚科)发生分歧的时间在124Ma之前,相当于早白垩世早期,可能由于南北古大陆的完全分离,其祖先居群被分隔成两个亚群,随后各自演化为不同的支系。Callitirodeae、Cupressoideae各属发生分歧的时间也均在白垩纪,表明Cupressaceae(s．s．)在白垩纪就已经建立起来。     

Inoculation and field testing of Sitka spruce and Douglas fir with ectomycorrhizal fungi in the United Kingdom

Picea sitchensis and Pseudotsuga menziesii seedlings were grown in containers, inoculated with ectomycorrhizal fungi, and planted in British forestry sites. Root samples taken during the year after planting were assessed for mycorrhiza formation. Survival and shoot height were assessed at the end of each year. Observations were made each autumn on the occurrence of sporophores of ectomycorrhizal fungi. Pot experiments were used to assess the colonization potential of soils from the experimental locations. Assessment of mycorrhiza formation by the inoculant fungi both before planting and the following year showed much variation among the fungi used. Similar variation was found among field sites. Inoculation with Laccaria isolates was most successful. Height measurements are reported for the first 2 years after planting, at which time there were few significant effects on growth of Picea sitchensis or Pseudotsuga menziesii seedlings. Experimental assessment of colonization potential was of little value in this work for predicting events in the forest.     

杉科、柏科的系统发生分析——来自28S rDNA序列分析的证据

对杉科（Taxodiaceae）与柏科（Cupressaceae s.s.）的28S rRNA基因的部分序列（约630 bp）进行PCR扩增、序列测定和系统发生关系分析,用简约法和邻接法构建的系统发生树基本一致。结果表明,杉科与柏科构成一个单系群,支持将杉科、柏科（Sciadopitys除外）合并为一个科——广义柏科（Cupressaceae sensu lato）的观点。在广义柏科中,Taiwania、Athrotaxis分别形成一支系;Metasequoia、Sequoia、Sequoiadendron关系较近,聚成一支系; Taxodium、Glyptostrobus、Cryptomeria聚成一支系;柏科聚成一支系。这一分析结果与叶绿体基因序列的分析结果相吻合,但是由于28S rRNA基因的进化速率较慢,尚不能分辨上述各个支系之间的系统演化关系。 Abstract:DNA sequences from 28S rDNA were used to assess relationships between and within traditional Taxodiaceae and Cupressaceae s.s.The MP tree and NJ tree generally are similar to one another.The results show that Taxodiaceae and Cupressaceae s.s.form a monophyletic conifer lineage excluding Sciadopitys.In the Taxodiaceae-Cupressaceae s.s.monophyletic group,the Taxodiaceae is paraphyletic.Taxodium,Glyptostrobus and Cryptomeria forming a clade(Taxodioideae),in which Glyptostrobus and Taxodium are closely related and sister to Cryptomeria;Sequoia,Sequoiadendron and Metasequoia are closely related to each other,forming another clade (Sequoioideae),in which Sequoia and Sequoiadendron are closely related and sister to Metasequoia;the seven genera of Cupressaceae s.s.are found to be closely related to form a monophyletic lineage (Cupressoideae).These results are basically similar to analyses from chloroplast gene data.But the relationships among Taiwania,Sequoioideae,Taxodioideae,and Cupressoideae remain unclear because of the slow evolution rate of 28S rDNA,which might best be answered by sequencing more rapidly evolving nuclear genes.     

Molecular phytogeny of conifers using RFLP analysis of PCR-amplified specific chloroplast genes

We investigated the molecular phylogeny of conifers using restriction endonuclease fragment length polymorphism of six polymerase chain reaction-amplified chloroplast genes — frxC, rbcL, psbA, psbD, trnK, and 16S. We detected 227 total site changes among species, representing 23, 26, 38, 48, 67, and 25 site changes in frxC, psbA, psbD, rbcL, trnK and 16S, respectively. The mean nucleotide substitution was 10.75% (SD 0.573) among species in five families. Forty maximally parsimonious trees were obtained using the Wagner parsimony method, and a 50% majority-rule consensus tree was obtained from them. Data analysis produced similar basic patterns when both the Wagner parsimony and the neighbor-joining methods were applied, and the main lineages were clearly separated. Taxaceae and Cephalotaxaceae species were used as the out-groups when applying Wagner parsimony methods. With the Wagner method, the consistency index was 0.510, the retention index was 0.879, and tree length was 435 steps. Our results indicated that Cupressaceae and Taxodiaceae are closely related families and that Sciadopitys verticillata is the basal lineage of Cupressaceae and Taxodiaceae. The neighbor-joining tree is similar to the 50% majority-rule consensus of the 40 Wagner parsimony trees except for the position of Keteleeria daversifolia, the Picea and Cedrus group, and the divergence within Cupressaceae.     

Phylogeny of Taxaceae and Cephalotaxaceae Genera Inferred from Chloroplast matK Gene and Nuclear rDNA ITS Region

Phylogeny of the Taxaceae genera and the monotypic family Cephalotaxaceae has been extraordinarily controversial. In this paper chloroplast matK genes and nuclear ITS sequences were determined for all six genera of the two families and representatives of other conifer families. Analysis using either the nonsynonymous sites or the deduced amino acid sequences of matK genes strongly indicates that taxad genera and Cephalotaxaceae are monophyletic, with the Taxodiaceae/Cupressaceae clade as their sister group. Cephalotaxus is basal to the taxad genera, among which two clades, Torreya/Amentotaxus and Taxus/Pseudotaxus/Austrotaxus, are resolved. They correspond to Janchen's two tribes, Torreyeae and Taxeae. In Taxeae, Austrotaxus is the first to branch off. Analyses of the nuclear ITS sequence data corroborated the topology of the matK gene tree. These results refute the views that Cephalotaxaceae has no alliance with Taxaceae and that Austrotaxus and Amentotaxus should be excluded from the Taxaceae. We estimated the divergence time between the Taxodiaceae/Cupressaceae and the Cephalotaxaceae/Taxaceae clades to be 192–230 Myr ago and the divergence time between taxads and Cephalotaxus to be 149–179 Myr ago. Soon after the latter divergence event, within 6–8 Myr, the two taxad tribes originated. In conclusion, our data do not support Florin's claim that taxads could be traced to Devonian psilophytes (359–395 Myr ago).     

Water flux and canopy conductance of natural versus planted forests in Patagonia, South America

In NW Patagonia, South America, natural shrublands and mixed forests of short Nothofagus antarctica (G. Forst.) Oerst. trees are currently being replaced by plantations with Pseudotsuga menziesii (Mirb) Franco. This land use change is controversial because the region is prone to drought, and replacement of native vegetation by planted forests may increase vegetation water use. The goal of this study was to examine the physiological differences, especially the response of water flux and canopy conductance to microclimate, that lead to greater water use by exotic trees compared to native trees. Meteorological variables and sapflow density of P. menziesii and four native woody species were measured in the growing season 2005–2006. Canopy conductance (gc) was estimated for both the exotic (monoculture) and native (multi-species) systems, including the individual contributions of each species of the native forest. Sapflow density, stand-level transpiration and gc were related to leaf-to-air vapor pressure difference (VPD). All native species had different magnitudes and diurnal patterns of sapflow density compared to P. menziesii, which could be explained by the different gc responses to VPD. Stomatal sensitivity to VPD suggested that all native species have a stronger stomatal control of leaf water potential and transpiration due to hydraulic limitations compared to P. menziesii. In conclusion, differences in water use between a P. menziesii plantation and a contiguous native mixed forest of similar basal area could be explained by different gc responses to VPD between species (higher sensitivity in the native species), in addition to particular characteristics of the native forest structure.     

What happens to the mycorrhizal communities of native and exotic seedlings when Pseudotsuga menziesii invades Nothofagaceae forests in Patagonia,Argentina?

Pseudotsuga menziesii is one of the most widely planted conifers in the Patagonian Andes of Argentina, having invading characteristics that are broadly reported. We studied the mycorrhizal status of seedlings along six Nothofagaceae + P. menziesii invasion matrices to investigate their role in the invasive process, according to these hypothesis: a) The abundance and richness of EM will be higher in seedlings grown in their own soil; b) In the presence of native EM inoculum, the invasive plant will be associated with generalist mycorrhizae (EM and/or AM), c) AM associations will be more abundant in P. menziesii seedlings grown in Interface or native forest soils, d) Mycorrhizal community differences between treatments will alter host fitness (growth and nutritional parameters). Seedlings from Nothofagus dombeyi, N. antarctica, Lophozonia alpina, L. obliqua and Pseudotsuga menziesii were set up in a soil-bioassay that included soils from non-invaded Nothofagaceae forests, pure P. menziesii plantations, and the interface between both. Pseudotsuga menziesii seedlings showed a decreasing, although never null, ectomycorrhizal (EM) colonization pattern from plantations to non-invaded forests, mainly with exotic EM species. Hebeloma mesophaeum and Wilcoxina sp. 1, two EM species with cosmopolitan distribution, were found to be shared by both tree species. Hebeloma hiemale and Wilcoxina sp. 1, common mycorrhizal partners of P. menziesii in Patagonia although not registered from Nothofagaceae forest, were found to be associated with N. antarctica, being the first report for both fungal species. Pseudotsuga menziesii seedlings showed the ability to form different arbuscular mycorrhiza (AM) colonization types (Paris-, Arum-, Both- and Intermediate-types) depending on the treatments, with significantly higher presence of Intermediate-type in the Interface treatment, where colonization was low. The shared EM species and the presence of different AM colonization types imply enhanced possibilities for invasive P. menziesii seedlings establishment and development. Seedling features and EM colonization rates evidenced that P. menziesii invasion could produce maladaptation (defined as a relative decline in host fitness due to altered mycorrhizal communities from native settings) of mycorrhizal communities, seriously injuring native ecosystem.     

Facilitation and Inhibition of Seedlings of an Invasive Tree (Acer platanoides) by Different Tree Species in a Mountain Ecosystem

Facilitation is known to be an important process structuring natural plant communities. However, much less is known about its role in facilitating the invasion of ecosystems by non-native plant species. In this study we evaluated the effects of invasive (Acer platanoides) and native (Pseudotsuga menziesii) forest types on the performance of A. platanoides seedlings, and related these effects to structural and functional properties associated with the two forest types, in a native P. menziesii forest that is being invaded by A. platanoides. Acer platanoidesseedlings had higher densities, recruitment, and survival, and experienced less photoinhibition and water stress when beneath conspecific canopies than in the adjacent P. menziesii forest. Soil moisture and canopy cover were greater in the invaded patch than the native forest. There was no difference in soil fertility or understory light levels between locations. These demographic (i.e. seedling survival), physiological, and environmental differences appeared to be due to the effects of A. platanoides and P. menziesii trees. Thus, Acer trees appear to produce a more mesic environment by modifying the structure and phenology of the forest canopy and by altering the timing of transpirational water loss relative to P. menziesii. Environmental modification by invaders that lead to positive effects on conspecifics may help us to understand the dramatic success and lag periods of some invasive species     

Ectomycorrhizal Networks of Pseudotsuga menziesii var. glauca Trees Facilitate Establishment of Conspecific Seedlings Under Drought

Ectomycorrhizal (EM) networks are hypothesized to facilitate regeneration under abiotic stress. We tested the role of networks in interactions between P. menziesii var. glauca trees and conspecific seedlings along a climatic moisture gradient to: (1) determine the effects of climatic factors on network facilitation of Pseudotsuga menziesii (Mirb.) Franco var. glauca (Mayr) seedling establishment, (2) infer the changing importance of P. menziesii var. glauca parent trees in conspecific regeneration with climate, and (3) parse the competitive from facilitative effects of P. menziesii var. glauca trees on seedlings. When drought conditions were greatest, seedling growth increased when seedlings could form a network with trees in the absence of root competition, but was reduced when unable to form a network. Survival was maximized when seedlings were able to form a network in the absence of root competition. Seedling stem natural abundance δ¹³C increased with drought due to increasing water use efficiency, but was unaffected by distance from tree or network potential. We conclude that P. menziesii seedlings may benefit from the presence of established P. menziesii trees when growing under climatic drought, but that this benefit is contingent upon the establishment of an EM network prior to the onset of summer drought. These results suggest that networks are an important mechanism for EM plants establishing in a pattern consistent with the stress-gradient hypothesis, and therefore the importance of EM networks to facilitation in regeneration of EM trees is expected to increase with drought.