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Thomas G. Whitham - One of the best experts on this subject based on the ideXlab platform.
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Relative Importance of Environmental Stress and Herbivory in Reducing Litter Fall in a Semiarid
2015Co-Authors: Thomas D. Schuster, Neil S. Cobb, Thomas G. Whitham, Stephen C HartAbstract:We examined the impact of soil stress (low water and nutrient availabilities) and two keystone insect herbivores on Pinyon Pine (Pinus edulis) needle litterfall. We compared trees growing on two dis-tinct soil types: volcanic cinders, which exhibit pronounced water and nutrient limitation, and sandy-loam soils, which have higher water-storage capacity and nutrient availability. Using two long-term herbivore removal experiments (15 and 18 years, respectively), we also examined the effects of the Pinyon needle scale (Matsucoccus acalyptus, which attacks juvenile trees) and the stem-boring moth (Dioryctria albovittella, which attacks mature trees) on Pinyon litterfall. These herbivores reac
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convergence in mycorrhizal fungal communities due to drought plant competition parasitism and susceptibility to herbivory consequences for fungi and host plants
Frontiers in Microbiology, 2014Co-Authors: Catherine A. Gehring, Rebecca C Mueller, Kristin E Haskins, Tine K Rubow, Thomas G. WhithamAbstract:Plants and mycorrhizal fungi influence each other’s abundance, diversity and distribution. How other biotic interactions affect the mycorrhizal symbiosis is less well understood. Likewise, we know little about the effects of climate change on the fungal component of the symbiosis or its function. We synthesized our long-term studies on the influence of mistletoe parasites, insect herbivores, competing trees, and drought on the ectomycorrhizal fungal communities associated with a foundation tree species of the southwestern United States, Pinyon Pine (Pinus edulis), and described how these changes feed back to affect host plant performance. We found that drought and all three of the biotic interactions studied resulted in similar shifts in ectomycorrhizal fungal community composition, demonstrating a convergence of the community towards dominance by a few closely related fungal taxa. Ectomycorrhizal fungi responded similarly to each of these stressors resulting in a predictable trajectory of community disassembly, consistent with ecological theory. Although we predicted that the fungal communities associated with trees stressed by drought, herbivory, competition, and parasitism would be poor mutualists, we found the opposite pattern in field studies. Our results suggest that climate change and the increased importance of herbivores, competitors and parasites that can be associated with it, may ultimately lead to reductions in ectomycorrhizal fungal diversity, but that the remaining fungal community may be beneficial to host trees under the current climate and the warmer, drier climate predicted for the future.
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drought negatively affects communities on a foundation tree growth rings predict diversity
Oecologia, 2010Co-Authors: Adrian C Stone, Catherine A. Gehring, Thomas G. WhithamAbstract:Understanding how communities respond to extreme climatic events is important for predicting the impact of climate change on biodiversity. The plant vigor and stress hypotheses provide a theoretical framework for understanding how arthropods respond to stress, but are rarely tested at the community level. Following a record drought, we compared the communities of arthropods on Pinyon Pine (Pinus edulis) that exhibited a gradient in physical traits related to environmental stress (e.g., growth rate, branch dieback, and needle retention). Six patterns emerged that show how one of the predicted outcomes of climate change in the southwestern USA (i.e., increased drought severity) alters the communities of a foundation tree species. In accordance with the plant vigor hypothesis, increasing tree stress was correlated with an eight to tenfold decline in arthropod species richness and abundance. Trees that were more similar in their level of stress had more similar arthropod communities. Both foliage quantity and quality contributed to arthropod community structure. Individual species and feeding groups differed in their responses to plant stress, but most were negatively affected. Arthropod richness (r 2 = 0.48) and abundance (r 2 = 0.48) on individual trees were positively correlated with the tree’s radial growth during drought. This relationship suggests that tree ring analysis may be used as a predictor of arthropod diversity, which is similar to findings with ectomycorrhizal fungi. A contrast of our findings on arthropod abundance with published data on colonization by mutualistic fungi on the same trees demonstrates that at low stress these two communities respond differently, but at high stress both are negatively affected. These results suggest that the effect of extreme climatic events such as drought on foundation tree species are likely to decrease multi-trophic diversity and shift arthropod community composition, which in turn could cascade to affect other associated taxa.
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shifts from competition to facilitation between a foundation tree and a pioneer shrub across spatial and temporal scales in a semiarid woodland
New Phytologist, 2007Co-Authors: Christopher M Sthultz, Catherine A. Gehring, Thomas G. WhithamAbstract:Summary • Theoretical and empirical research has supported the hypothesis that plant–plant interactions change from competition to facilitation with increasing abiotic stress. However, the consistency of such changes has been questioned in arid and semiarid ecosystems. • During a drought in the semiarid south-western USA, we used observations and a field experiment to examine the interactions between juveniles of a foundation tree (Pinyon Pine, Pinus edulis) and a common shrub (Apache plume, Fallugia paradoxa) in replicated areas of high and low stress. • The presence of F. paradoxa reduced P. edulis performance at low-stress sites, but had the opposite effect at high-stress sites. However, the intensity of the interactions depended on temporal variation in climate and age of P. edulis. Both above- and below-ground factors contributed to competition, while only above-ground factors contributed to facilitation. • These results support the hypothesis that interactions can change from competition to facilitation as abiotic stress increases in semiarid environments. A shift from competition to facilitation may be important for the recovery of P. edulis and other foundation species that have experienced large-scale mortality during recent droughts.
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differential tree mortality in response to severe drought evidence for long term vegetation shifts
Journal of Ecology, 2005Co-Authors: Rebecca C Mueller, Catherine A. Gehring, Crescent Scudder, Marianne E Porter, Talbot R Trotter, Thomas G. WhithamAbstract:Summary 1 Although drought frequency and severity are predicted to increase across numerous continental interiors, the consequences of these changes for dominant plants are largely unknown. Over the last decade, the south-western US has experienced six drought years, including the extreme droughts of 1996 and 2002, which led to widespread tree mortality across northern Arizona. 2 We examined the impact of these droughts on the co-dominant tree species of the Pinyon‐juniper woodland ( Pinus edulis and Juniperus monosperma ), a major vegetation type in the US. 3 Pinyon mortality following both droughts was 6.5-fold higher than juniper mortality. In addition, large Pinyons suffered 2‐6-fold greater mortality than small Pinyons, a pattern associated with higher mortality of reproductively mature trees and survival of smaller Pinyons resulting from facilitation by established vegetation. Differential mortality of large Pinyons resulted in a vegetation shift such that the Pinyon‐juniper woodlands are becoming dominated by juniper, a species that is typical of lower elevations and more arid conditions. 4 Sites that experienced high Pinyon mortality during the first drought suffered additional mortality during the second drought, so that reductions in tree densities and the resulting release from below-ground competition did not buffer surviving Pinyons against additional mortality during the second drought. Such repeated mortality events also suggest that these stands may suffer chronic stress. 5 Reductions in biotic associations (e.g. avian seed dispersers, ectomycorrhizas and nurse plants) that will probably result from extreme mortality of large Pinyons ensure that the observed vegetation shifts will be persistent. Because approximately 1000 species are associated with Pinyon Pine, the shift in the structure of these woodlands has large-scale community consequences.
Catherine A. Gehring - One of the best experts on this subject based on the ideXlab platform.
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ectomycorrhizal and dark septate fungal associations of Pinyon Pine are differentially affected by experimental drought and warming
Frontiers in Plant Science, 2020Co-Authors: Catherine A. Gehring, Sanna Sevanto, Adair Patterson, Danielle E M Ulrich, Cheryl R KuskeAbstract:Changing climates can cause shifts in temperature and precipitation, resulting in warming and drought in some regions. Although each of these factors has been shown to detrimentally affect forest ecosystems worldwide, information on the impacts of the combined effects of warming and drought is lacking. Forest trees rely on mutualistic root-associated fungi that contribute significantly to plant health and protection against climate stresses. We used a six-year, ecosystem-scale temperature and precipitation manipulation experiment targeted to simulate the climate in 2100 in the Southwestern United States to quantify the effects of drought, warming and combined drought and warming on the root colonization (abundance), species composition and diversity of ectomycorrhizal fungi (EMF), and dark septate fungal endophytes in a widespread woodland tree, Pinyon Pine (Pinus edulis E.). Our results show that Pinyon shoot growth after 6 years of these treatments was reduced more by drought than warming. The combined drought and warming treatment reduced the abundance and diversity of EMF more than either treatment alone. Individual ectomycorrhizal fungal taxa, including the drought tolerant Cenococcum geophilum, were present in all treatments but the combined drought and warming treatment. The combined drought and warming treatment also reduced the abundance of dark septate endophytes (DSE), but did not affect their diversity or species composition. The current year shoot growth of the trees correlated positively with ectomycorrhizal fungal diversity, highlighting the importance of diversity in mutualistic relationships to plant growth. Our results suggest that EMF may be more important than DSE to aboveground growth in P. edulis, but also more susceptible to the negative effects of combined climate stressors.
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PLANT ANIMAL INTERACTIONS
2014Co-Authors: Rebecca C Mueller, Catherine A. Gehring, Æ Brian, D. Wade, Æ Thomas, G. WhithamAbstract:Chronic herbivory negatively impacts cone and seed production, seed quality and seedling growth of susceptible Pinyon Pine
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convergence in mycorrhizal fungal communities due to drought plant competition parasitism and susceptibility to herbivory consequences for fungi and host plants
Frontiers in Microbiology, 2014Co-Authors: Catherine A. Gehring, Rebecca C Mueller, Kristin E Haskins, Tine K Rubow, Thomas G. WhithamAbstract:Plants and mycorrhizal fungi influence each other’s abundance, diversity and distribution. How other biotic interactions affect the mycorrhizal symbiosis is less well understood. Likewise, we know little about the effects of climate change on the fungal component of the symbiosis or its function. We synthesized our long-term studies on the influence of mistletoe parasites, insect herbivores, competing trees, and drought on the ectomycorrhizal fungal communities associated with a foundation tree species of the southwestern United States, Pinyon Pine (Pinus edulis), and described how these changes feed back to affect host plant performance. We found that drought and all three of the biotic interactions studied resulted in similar shifts in ectomycorrhizal fungal community composition, demonstrating a convergence of the community towards dominance by a few closely related fungal taxa. Ectomycorrhizal fungi responded similarly to each of these stressors resulting in a predictable trajectory of community disassembly, consistent with ecological theory. Although we predicted that the fungal communities associated with trees stressed by drought, herbivory, competition, and parasitism would be poor mutualists, we found the opposite pattern in field studies. Our results suggest that climate change and the increased importance of herbivores, competitors and parasites that can be associated with it, may ultimately lead to reductions in ectomycorrhizal fungal diversity, but that the remaining fungal community may be beneficial to host trees under the current climate and the warmer, drier climate predicted for the future.
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An elusive ectomycorrhizal fungus reveals itself: a new species of Geopora (Pyronemataceae) associated with Pinus edulis.
Mycologia, 2014Co-Authors: Lluvia Flores-rentería, Matthew K. Lau, Louis J. Lamit, Catherine A. GehringAbstract:Species of the genus Geopora are important ectomycorrhizal associates that can dominate the communities of some plant taxa, such as Pinyon Pine (Pinus edulis), a widespread tree of the western United States. Several members of the genus Geopora are known only from ectomycorrhizal root tips and thus have not been described formally. The sporocarps of some Geopora species occur infrequently because they depend on wet years for sporulation. In addition, Geopora sporocarps can be small and may be hypogeous at some developmental stage, limiting the opportunities for describing their morphology. Using molecular and morphological data, we have described a new species of fungus, Geopora Pinyonensis, which produced ascocarps after unusually high precipitation at a northern Arizona site in summer 2012. Based on analysis of the ITS and nuLSU regions of the rDNA, G Pinyonensis is a new species of Geopora. It has small sporocarps and ascospores relative to other members of the genus; however, these morphological features overlap with other species. Using rDNA data from sporocarps and ectomycorrhizal root tips, we show that the sporocarps correspond to an abundant species of ectomycorrhizal fungus associated with Pinyon Pines that is increasing in abundance in drought-affected landscapes and may promote drought tolerance.
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molecular characterization of pezizalean ectomycorrhizas associated with Pinyon Pine during drought
Mycorrhiza, 2011Co-Authors: Galena J Gordon, Catherine A. GehringAbstract:Recent studies using molecular analysis of ectomycorrhizas have revealed that ascomycete fungi, especially members of the order Pezizales, can be important members of ectomycorrhizal (EM) fungal communities. However, little is known about the ecology and taxonomy of many of these fungi. We used data collected during a wet and a dry period to test the hypothesis that pezizalean EM fungi associated with Pinyon Pine (Pinus edulis) responded positively to drought stress. We also assessed the phylogenetic relationships among six, unknown pezizalean EM fungi, common to our study sites, using rDNA sequences from the internal transcribed spacer and large subunit (LSU) regions of the ribosomal DNA. Sequences of these fungi were then compared to sequences from known taxa to allow finer-scale identification. Three major findings emerged. First, at two sites, pezizalean EM were 44–95% more abundant during a dry year than a wetter year, supporting the hypothesis that pezizalean EM fungi respond positively to dry conditions. Second, four of the six unknown pezizalean EM fungi associated with P. edulis separated from one another consistently regardless of site or year of collection, suggesting that they represented distinct taxa. Third, comparison with LSU sequences of known members of the Pezizales indicated that these four taxa grouped within the genus Geopora of the family Pyronemataceae. Our results provide further evidence of the importance of pezizalean fungi in the ectomycorrhizal symbiosis and demonstrate high local abundance of members of the genus Geopora in drought-stressed Pinyon–juniper woodlands.
Neil S. Cobb - One of the best experts on this subject based on the ideXlab platform.
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Relative Importance of Environmental Stress and Herbivory in Reducing Litter Fall in a Semiarid
2015Co-Authors: Thomas D. Schuster, Neil S. Cobb, Thomas G. Whitham, Stephen C HartAbstract:We examined the impact of soil stress (low water and nutrient availabilities) and two keystone insect herbivores on Pinyon Pine (Pinus edulis) needle litterfall. We compared trees growing on two dis-tinct soil types: volcanic cinders, which exhibit pronounced water and nutrient limitation, and sandy-loam soils, which have higher water-storage capacity and nutrient availability. Using two long-term herbivore removal experiments (15 and 18 years, respectively), we also examined the effects of the Pinyon needle scale (Matsucoccus acalyptus, which attacks juvenile trees) and the stem-boring moth (Dioryctria albovittella, which attacks mature trees) on Pinyon litterfall. These herbivores reac
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Pinyon Pine mortality alters communities of ground dwelling arthropods
Western North American Naturalist, 2014Co-Authors: Robert Delph, Neil S. Cobb, Michael J Clifford, Paulette L Ford, Sandra L BrantleyAbstract:We documented the effect of drought-induced mortality of Pinyon Pine (Pinus edulis Engelm.) on communities of ground-dwelling arthropods. Tree mortality alters microhabitats utilized by ground-dwelling arthropods by increasing solar radiation, dead woody debris, and understory vegetation. Our major objectives were to determine (1) whether there were changes in community composition, species richness, and abundance of ground-dwelling arthropods associated with Pinyon mortality and (2) whether specific habitat characteristics and microhabitats accounted for these changes. We predicted shifts in community composition and increases in arthropod diversity and abundance due to the presumed increased complexity of microhabitats from both standing dead and fallen dead trees. We found significant differences in arthropod community composition between high and low Pinyon mortality environments, despite no differences in arthropod abundance or richness. Overall, 22% (51 taxa) of the arthropod community were identified as being indicators of either high or low mortality. Our study corroborates other research indicating that arthropods are responsive to even moderate disturbance events leading to changes in the environment. These arthropod responses can be explained in part due to the increase in woody debris and reduced canopy cover created by tree mortality.
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precipitation thresholds and drought induced tree die off insights from patterns of pinus edulis mortality along an environmental stress gradient
New Phytologist, 2013Co-Authors: Michael J Clifford, David D. Breshears, Neil S. Cobb, Patrick D Royer, Paulette L FordAbstract:Summary � Recent regional tree die-off events appear to have been triggered by a combination of drought and heat – referred to as ‘global-change-type drought’. To complement experiments focused on resolving mechanisms of drought-induced tree mortality, an evaluation of how patterns of tree die-off relate to highly spatially variable precipitation is needed. � Here, we explore precipitation relationships with a die-off event of Pinyon Pine (Pinus edulis Engelm.) in southwestern North America during the 2002–2003 global-change-type drought. Pinyon die-off and its relationship with precipitation was quantified spatially along a precipitation gradient in north-central New Mexico with standard field plot measurements of die-off combined with canopy cover derived from normalized burn ratio (NBR) from Landsat imagery. � Pinyon die-off patterns revealed threshold responses to precipitation (cumulative 2002– 2003) and vapor pressure deficit (VPD), with little to no mortality (< 10%) above 600 mm and below warm season VPD of c. 1.7 kPa. [Correction added after online publication 17 June 2013; in the preceding sentence, the word ’below’ has been inserted.] � Our results refine how precipitation patterns within a region influence Pinyon die-off, revealing a precipitation and VPD threshold for tree mortality and its uncertainty band where other factors probably come into play – a response type that influences stand demography and landscape heterogeneity and is of general interest, yet has not been documented.
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Long-term sexual allocation in herbivore resistant and susceptible Pinyon Pine ( Pinus edulis )
Oecologia, 2002Co-Authors: Neil S. Cobb, Robert T. Trotter, Thomas G. WhithamAbstract:Chronic herbivory by the stem-boring moth (Dioryctria albovittella) alters the sexual expression of a monoecious tree, Pinyon Pine (Pinus edulis) by reducing female function and increasing male function. Observations and long-term moth removal experiments show that 55% of susceptible trees can lose all female function. Moth herbivory has little effect on male function in young trees, but has an important effect on older trees, where moth-susceptible trees produced 1.5 times more pollen than moth-resistant trees. Susceptible trees were 6.5 times more likely to exhibit male-only function than either resistant trees or susceptible trees that have had their moths experimentally removed. This herbivore-induced sex change is caused primarily by differential moth attack and the resulting mortality of the shoots that bear female reproductive structures. Moth attack rates were positively correlated with individual stem biomass (female stems >non-reproductive stems >male stems). Moth attack also increased conelet abortion on unattacked shoots, indicating that moths indirectly reduce female function. Moth-induced altering of sexual function is also expressed at the population level. Male function is relatively greater in stands with high moth densities on stressful soils than in stands with few moths. Under certain conditions, sexual selection theory suggests that the negative effects on female function could be overcome with greater investment in male function. Because susceptible trees produce large amounts of pollen and are more abundant than moth-resistant trees, frequency-dependent selection may counteract selection against susceptible genotypes. Plant-herbivore interaction studies typically examine negative impacts of herbivory on female function, but not positive effects on male function. Here we demonstrate that herbivory may have important effects on the evolutionary ecology of Pinyon by both promoting male function and depressing female function.
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increased moth herbivory associated with environmental stress of Pinyon Pine at local and regional levels
Oecologia, 1997Co-Authors: Neil S. Cobb, Catherine A. Gehring, Susan Mopper, Kerry M Christensen, Matt Caouette, Thomas G. WhithamAbstract:Using 6 years of observational and experimental data, we examined the hypothesis that water and nutrient stress increase the susceptibility of Pinyon Pine (Pinus edulis) to the stem- and cone-boring moth (Dioryctria albovittella). At two geographic levels, a local scale of 550 km2 and a regional scale of 10,000 km2, moth herbivory was strongly correlated with an edaphic stress gradient. At a local scale, from the cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten soil macro- and micronutrients, and soil water content were lowest in cinder-dominated soils. Herbivore damage was six times greater on trees growing in the most water and nutrient deficient site at Sunset Crater compared to sites with well-developed soils. Percentage silt-clay content of soil, which was highly positively correlated with soil nutrient and soil moisture at a local scale, accounted for 56% of the variation in herbivory at a regional scale among 22 sites. Within and across sites, increased stem resin flow was positively associated with reduced moth attack. On the basis of moth distribution across a stress gradient, we predicted that Pinyons growing in highly stressful environments would show increased resistance to herbivores if supplemented with water and/or nutrients. We conducted a 6-year experiment at a high-stress site where individual trees received water only, fertilizer only, and water + fertilizer. Relative to control trees, stem growth and resin flow increased in all three treatments, but only significantly in the water + fertilizer treatment. Although there was no significant difference in herbivore damage among these three treatments, there was an overall reduction in herbivore damage on all treatment trees combined, compared to control trees. This experiment suggests that release from stress leads to increased resistance to insect attack and is consistent with our observational data. While other studies have predicted that short-term stress will result in herbivore outbreaks, our studies extend this prediction to chronically stressed host populations. Finally, while flush-feeders are not predicted to respond positively to stressed host plants, we found a positive association between herbivore attack and stressed Pinyon populations.
T. G. Whitham - One of the best experts on this subject based on the ideXlab platform.
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Predictors of Ips confusus Outbreaks During a Record Drought in Southwestern USA: Implications for Monitoring and Management
2013Co-Authors: M. J. Santos, T. G. WhithamAbstract:Abstract In many ecosystems the effects of disturbance can be cryptic and disturbance may vary in subtle spatiotemporal ways. For instance, we know that bark beetle outbreaks are more frequent in temperate forests during droughts; however, we have little idea about why they occur in some locations and not others. Understanding biotic and abiotic factors promoting bark beetle outbreaks can be critical to predicting and responding to pest outbreaks. Here we address the environmental factors which are associated with Ips confusus outbreaks during the 2002 widespread drought within the distribution range of Pinyon Pine woodlands in Arizona. We used univariate statistics to test if whether tree characteristics, other herbivores, stand properties, soil type, wind, and topography were associated with I. confusus outbreak, and logistic regression to create a predictive model for the outbreaks. We found that I. confusus attacks occur in low elevation stands on steeper slopes, where favorable winds for I. confusus dispersion occur. I. confusus select larger trees, in high density stands with understory shrubs that exhibit phenotypic traits characteristic of resistance to stem-boring moths. The mode
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Predictors of Ips confusus Outbreaks During a Record Drought in Southwestern USA: Implications for Monitoring and Management
Environmental Management, 2010Co-Authors: M. J. Santos, T. G. WhithamAbstract:In many ecosystems the effects of disturbance can be cryptic and disturbance may vary in subtle spatiotemporal ways. For instance, we know that bark beetle outbreaks are more frequent in temperate forests during droughts; however, we have little idea about why they occur in some locations and not others. Understanding biotic and abiotic factors promoting bark beetle outbreaks can be critical to predicting and responding to pest outbreaks. Here we address the environmental factors which are associated with Ips confusus outbreaks during the 2002 widespread drought within the distribution range of Pinyon Pine woodlands in Arizona. We used univariate statistics to test if whether tree characteristics, other herbivores, stand properties, soil type, wind, and topography were associated with I. confusus outbreak, and logistic regression to create a predictive model for the outbreaks. We found that I. confusus attacks occur in low elevation stands on steeper slopes, where favorable winds for I. confusus dispersion occur. I. confusus select larger trees, in high density stands with understory shrubs that exhibit phenotypic traits characteristic of resistance to stem-boring moths. The model was highly accurate, and explained 95% of the variability in occurrence (98% of the absences and 95% of the presences). Accurate prediction of the impacts of disturbance allow us to anticipate, minimize or mitigate for and eventually counteract its effects, especially those affecting diversity and ecosystem function. Identification of outbreak risk areas can guide regional and national management towards the reduction of infestation risk and enhancing conservation of Pinyon-juniper woodlands.
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A genetic linkage map of Pinyon Pine (Pinus edulis) based on amplified fragment length polymorphisms
Theoretical and Applied Genetics, 1998Co-Authors: Steven E. Travis, T. G. Whitham, Kermit Ritland, Paul KeimAbstract:Amplified fragment length polymorphisms (AFLP) were used to rapidly generate a dense linkage map for Pinyon Pine (Pinus edulis). The map population consisted of 40 megagametophytes derived from one tree at Sunset Crater, Arizona. A total of 78 primer combinations, each with three to five selective nucleotides, amplified 542 polymorphic markers. Of these, 33 markers showed significant deviation from the expected Mendelian genotypic segregation ratio of 1 : 1, and 164 showed complete linkage with another marker. This resulted in 338 unique markers mapping to 25 linkage groups, each of which ranged from 2 to 22 markers, averaging 80 centiMorgans (cM) in size and covering 2,012 cM (2,200 cM with the inclusion of 25 cM for each of 7 unlinked markers). Pairwise linkage values gave a genome size estimate of 2,390 cM, suggesting comprehensive coverage of the genome. A search for subsets of primer combinations giving the best map coverage found 10 primer combinations which together marked 72% of the linkage map to within 10 cM; an additional 10 primer combinations increased this percentage to 85%. Our map represents an initial step towards the identification of quantitative trait loci associated with pest resistance and water stress in Pinyons and will further allow us to examine introgression rates between P. edulis and P. californiarum.
Kerry M Christensen - One of the best experts on this subject based on the ideXlab platform.
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increased moth herbivory associated with environmental stress of Pinyon Pine at local and regional levels
Oecologia, 1997Co-Authors: Neil S. Cobb, Catherine A. Gehring, Susan Mopper, Kerry M Christensen, Matt Caouette, Thomas G. WhithamAbstract:Using 6 years of observational and experimental data, we examined the hypothesis that water and nutrient stress increase the susceptibility of Pinyon Pine (Pinus edulis) to the stem- and cone-boring moth (Dioryctria albovittella). At two geographic levels, a local scale of 550 km2 and a regional scale of 10,000 km2, moth herbivory was strongly correlated with an edaphic stress gradient. At a local scale, from the cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten soil macro- and micronutrients, and soil water content were lowest in cinder-dominated soils. Herbivore damage was six times greater on trees growing in the most water and nutrient deficient site at Sunset Crater compared to sites with well-developed soils. Percentage silt-clay content of soil, which was highly positively correlated with soil nutrient and soil moisture at a local scale, accounted for 56% of the variation in herbivory at a regional scale among 22 sites. Within and across sites, increased stem resin flow was positively associated with reduced moth attack. On the basis of moth distribution across a stress gradient, we predicted that Pinyons growing in highly stressful environments would show increased resistance to herbivores if supplemented with water and/or nutrients. We conducted a 6-year experiment at a high-stress site where individual trees received water only, fertilizer only, and water + fertilizer. Relative to control trees, stem growth and resin flow increased in all three treatments, but only significantly in the water + fertilizer treatment. Although there was no significant difference in herbivore damage among these three treatments, there was an overall reduction in herbivore damage on all treatment trees combined, compared to control trees. This experiment suggests that release from stress leads to increased resistance to insect attack and is consistent with our observational data. While other studies have predicted that short-term stress will result in herbivore outbreaks, our studies extend this prediction to chronically stressed host populations. Finally, while flush-feeders are not predicted to respond positively to stressed host plants, we found a positive association between herbivore attack and stressed Pinyon populations.
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Herbivory and tree mortality across a Pinyon Pine hybrid zone
Oecologia, 1995Co-Authors: Kerry M Christensen, Thomas G. Whitham, Paul KeimAbstract:We examined the abundances of three common insect herbivores on pure and hybrid Pinyon Pines along a 250-km transect in west-central Arizona, United States. Using six morphological traits, we developed a hybrid index to classify trees as pure Pinus californiarum , hybrid, or pure Pinus edulis . The insects (the stem-boring moth, Dioryctria albovittella , the scale insect, Matsucoccus acalyptus , and several species of pitch moths that produce wounds on the trunk and branches) exhibited different distributional patterns across tree types. Stem-boring moths were significantly more abundant on trees at “hybrid” sites compared to trees at “pure” sites. In addition, within hybrid sites, hybrids supported significantly more moth larvae than pure trees of either species. These two patterns support the hybrid susceptibility hypothesis in which hybrid breakdown results in increased susceptibility to herbivory. In contrast to stem-borers, there were significantly more pitch moth wounds on trees at pure P. californiarum sites than at hybrid and pure P. edulis sites. Within the hybrid zone, pitch moth abundance was equal on pure P. californiarum and hybrids, and both were significantly greater than on pure P. edulis . These within-site comparisons support the dominance hypothesis where hybrid resistance differs from one tree species, but not the other. Scale insects exhibited the most restricted distribution; over the 250 km transect they were found only in the hybrid zone. This supports the hybrid susceptibility and/or the stress hypothesis (i.e., species at the edge of their range suffer greater stress and are more susceptible to herbivory). We summed the mean numbers of these three common herbivores across sites and found that hybrid sites supported 2.1 and 3.9 times more herbivores than pure P. californiarum and P. edulis sites, respectively. Furthermore, tree mortality was on average, 35 times greater within the hybrid zone compared to pure zones of each species and was associated with the cumulative abundance of herbivores ( r ^2=0.646). Regardless of whether this mortality is due to insect infestation, stress or a combination of both, these results suggest that hybrid zones are important arenas of natural selection.
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impact of insect herbivores on competition between birds and mammals for Pinyon Pine seeds
Ecology, 1993Co-Authors: Kerry M Christensen, Thomas G. WhithamAbstract:Few studies have considered impacts of insect cone herbivores on bird and mammal seed consumers. Harvests of Pinyon Pine (Pinus edulis) cones by insects, birds, and mammals were negatively correlated both within and across sites. Birds and mammals significantly increased their cone harvest when the other group was experimentally pre- vented access to trees, and mammals harvested more cones when insects were removed from trees with an insecticide. These results suggest that stem- and cone-boring insects (primarily Dioryctria albovittella), birds (Clark's Nutcrackers, Pinyon Jays, and Scrub Jays), and mammals (cliff chipmunks and rock squirrels) compete for Pinyon Pine seeds. Addi- tionally, insects affected the interaction between birds and mammals because the ratio of mammal to bird harvest increased up to 350-fold as insect cone consumption increased. Because insect-infested and uninfested Pinyon Pines are genetically different, these obser- vations and experiments suggest that genetic interactions between plants and herbivores can affect the structure of a seed-consumer community.
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genetic differentiation and heterozygosity in Pinyon Pine associated with resistance to herbivory and environmental stress
Evolution, 1991Co-Authors: Susan Mopper, Neil S. Cobb, Thomas G. Whitham, Jeffry B. Mitton, Kerry M ChristensenAbstract:Arizona's Sunset Crater began erupting in 1064 AD and for the next 200 years buried over 2,000 km2 in ash, cinders, and lava. Soil analyses indicate that Pinyon Pines (Pinus edulis) currently colonizing the cinder fields are faced with a highly stressful environment. Many of these Pinyons suffer chronic, intense insect herbivory that reduces plant growth and eliminates female cone production. In contrast, herbivory among Pinyons growing in neighboring sandy-loam soils is minimal. Furthermore, numerous trees within the heavily infested cinder field population suffer relatively low herbivory and maintain normal growth and reproduction. We used four polymorphic enzymes to examine the relationship between herbivore attack, environmental stress and genotypes of the adjacent cinder field, and sandy-loam soil Pinyon populations. Our results demonstrate that 1) resistant trees display significant genetic differences and are more heterozygous for two enzymes associated with herbivory than susceptible trees; and 2) the cinder-soil Pinyons exhibit significant genetic differences and are more heterozygous for an enzyme associated with environmental stress than the neighboring sandy-loam soil Pinyons. We conclude that heterozygosity of specific or closely linked loci may facilitate Pinyon resistance to herbivory and environmental stress, and that strong selection across narrow geographic boundaries resulted in rapid genetic differentiation of Pinyon populations.
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indirect herbivore mediation of avian seed dispersal in Pinyon Pine
Ecology, 1991Co-Authors: Kerry M Christensen, Thomas G. WhithamAbstract:During a 4-yr study, we examined how an insect herbivore indirectly influ- enced Pinyon Pine by affecting its avian seed dispersal agents. Colorado Pinyon Pine, Pinus edulis, suffered reduced cone initiation and increased cone mortality primarily from in- festation by the stem- and cone-boring moth Dioryctria albovitella. Because avian dispersal agents selectively foraged where cone crops were highest, individual trees and stands of trees with greater insect abundances were avoided. Even herbivore-resistant trees with substantial cone crops did not receive dispersal services if they were surrounded by sus- ceptible trees because the birds often ignored entire stands of trees. A cone removal ex- periment showed that avian seed dispersers also ignored trees with artificially reduced crops even though many cones remained. We argue that masting, the production of large cone crops at irregular intervals, may have evolved to insure dispersal success. The decline in attractiveness of entire stands of trees to dispersal agents due to infestation may functionally alter the sex expression of uninfested individuals.