Physiological and ecological constraints play key roles in the evolution of plant growth patterns, especially in relation to defenses against herbivores. Phenotypic and life history theories are unified within the growth-differentiation balance (GDB) framework, forming an integrated system of theories explaining and predicting patterns of plant defense and competitive interactions in ecological and evolutionary time. Plant activity at the cellular level can be classified as growth (cell division and enlargement) of differentiation (chemical and morphological changes leading to cell maturation and specialization). The GDB hypothesis of plant defense is premised upon a physiological trade-off between growth and differentiation processes. The trade-off between growth and defense exists because secondary metabolism and structural reinforcement are physiologically constrained in dividing and enlarging cells, and because they divert resources from the production of new leaf area. Hence the dilemma of plants: Th...

https://www.jstor.org/stable/pdfplus/2830650.pdf

The dilemma of plants: To grow or defend.

Summary Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, pre- diction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric

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Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?

I. CONTEXT * The Ecosystem Concept * Earth's Climate System * Geology and Soils * II. MECHANISMS * Terrestrial Water and Energy Balance * Carbon Input to Terrestrial Ecosystems * Terrestrial Production Processes * Terrestrial Decomposition * Terrestrial Plant Nutrient Use * Terrestrial Nutrient Cycling * Aquatic Carbon and Nutrient Cycling * Trophic Dynamics * Community Effects on Ecosystem Processes * III. PATTERNS * Temporal Dynamics * Landscape Heterogeneity and Ecosystem Dynamics * IV. INTEGRATION * Global Biogeochemical Cycles * Managing and Sustaining Ecosystem * Abbreviations * Glossary * References

Principles of Terrestrial Ecosystem Ecology

An important aim of plant ecology is to identify leading dimensions of ecological variation among species and to understand the basis for them. Dimensions that can readily be measured would be especially useful, because they might offer a path towards improved worldwide synthesis across the thousands of field experiments and ecophysiological studies that use just a few species each. Four dimensions are reviewed here. The leaf mass per area-leaf lifespan (LMA-LL) dimension expresses slow turnover of plant parts (at high LMA and long LL), long nutrient residence times, and slow response to favorable growth conditions. The seed mass-seed output (SM-SO) dimension is an important predictor of dispersal to establishment opportunities (seed output) and of establishment success in the face of hazards (seed mass). The LMA-LL and SM-SO dimensions are each underpinned by a single, comprehensible tradeoff, and their consequences are fairly well understood. The leaf size-twig size (LS-TS) spectrum has obvious consequences for the texture of canopies, but the costs and benefits of large versus small leaf and twig size are poorly understood. The height dimension has universally been seen as ecologically important and included in ecological strategy schemes. Nevertheless, height includes several tradeoffs and adaptive elements, which ideally should be treated separately. Each of these four dimensions varies at the scales of climate zones and of site types within landscapes. This variation can be interpreted as adaptation to the physical environment. Each dimension also varies widely among coexisting species. Most likely this within-site variation arises because the ecological opportunities for each species depend strongly on which other species are present, in other words, because the set of species at a site is a stable mixture of strategies.

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Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species

Herbivorous insects use diverse feeding strategies to obtain nutrients from their host plants. Rather than acting as passive victims in these interactions, plants respond to herbivory with the production of toxins and defensive proteins that target physiological processes in the insect. Herbivore-challenged plants also emit volatiles that attract insect predators and bolster resistance to future threats. This highly dynamic form of immunity is initiated by the recognition of insect oral secretions and signals from injured plant cells. These initial cues are transmitted within the plant by signal transduction pathways that include calcium ion fluxes, phosphorylation cascades, and, in particular, the jasmonate pathway, which plays a central and conserved role in promoting resistance to a broad spectrum of insects. A detailed understanding of plant immunity to arthropod herbivores will provide new insights into basic mechanisms of chemical communication and plant-animal coevolution and may also facilitate new approaches to crop protection and improvement.

Plant Immunity to Insect Herbivores

Since its accidental introduction from Asia, emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), has killed millions of ash trees in North America. As it continues to spread, it could functionally extirpate ash with devastating economic and ecological impacts. Little was known about EAB when it was first discovered in North America in 2002, but substantial advances in understanding of EAB biology, ecology, and management have occurred since. Ash species indigenous to China are generally resistant to EAB and may eventually provide resistance genes for introgression into North American species. EAB is characterized by stratified dispersal resulting from natural and human-assisted spread, and substantial effort has been devoted to the development of survey methods. Early eradication efforts were abandoned largely because of the difficulty of detecting and delineating infestations. Current management is focused on biological control, insecticide protection of high-value trees, and integrated efforts to slow ash mortality.

Emerald Ash Borer Invasion of North America: History, Biology, Ecology, Impacts, and Management

Alien invasive insects such as gypsy moth, hemlock woolly adelgid, and emerald ash borer continue to disturb the mixed deciduous and hemlock forests of eastern North America by causing wide-scale defoliation, decline and/or mortality of their hosts. Some of the most devastating species are spreading in “defense free space”, causing extensive mortality of hosts that are inherently susceptible, perhaps due to their lack of coevolutionary history with the invader. These disturbances have altered the dynamics of canopy gaps, coarse woody debris, biogeochemical cycling, and ecological interactions among organisms in terrestrial and aquatic systems, with consequent effects on forest composition, structure, and function. Populations of indigenous species specialized to particular habitats and/or host trees are most likely to decrease, while some generalist and opportunistic species may increase in invaded forests, including exotic plants as their facilitation by alien insects sparks an “invasional meltdown”. Although poorly documented, alien insects may induce positive feedback effects on ecological processes and interactions. For example, effects of herbivory on foliar chemistry may indirectly alter tri-trophic interactions of indigenous herbivores on their shared hosts, slow rates of terrestrial nutrient cycling, and decrease productivity of aquatic habitats based on allochthonous inputs. Tactics used to eradicate or suppress alien insects in forests such as insecticide applications, biological control, and silvicultural prescriptions can also have ecological impacts. As alien insects continue to establish and spread in forests of eastern North America, their already pervasive effects on ecological interactions and ecosystem processes will continue to magnify.

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Direct and indirect effects of alien insect herbivores on ecological processes and interactions in forests of eastern North America

Urbanization affects communities of herbivorous arthropods and provides opportunities for dramatic changes in their abundance and richness. Underlying these changes are creation of impervious surfaces; variation in the density, diversity, and complexity of vegetation; and maintenance practices including pulsed inputs of fertilizers, water, and pesticides. A rich body of knowledge provides theoretical underpinnings for predicting and understanding impacts of urbanization on arthropods. However, relatively few studies have elucidated mechanisms that explain patterns of insect and mite abundance and diversity across urbanization gradients. Published accounts suggest that responses to urbanization are often taxon specific, highly variable, and linked to properties of urbanization that weaken top-down and/or bottom-up processes, thereby destabilizing populations of herbivores and their natural enemies. In addition to revealing patterns in diversity and abundance of herbivores across urbanization gradients, a primary objective of this review is to examine mechanisms underlying these patterns and to identify potential hypotheses for future testing.

Ecology of Herbivorous Arthropods in Urban Landscapes

We conducted a 3-yr study to compare the susceptibility of selected North American ash and an Asian ash species to emerald ash borer, Agrilus planipennis Fairmaire, an invasive wood-boring beetle introduced to North America from Asia. Because of a coevolutionary relationship between Asian ashes and emerald ash borer, we hypothesized an Asian ash species, Manchurian ash, is more resistant to the beetle than its North American congeners. Consistent with our hypothesis, Manchurian ash experienced far less mortality and yielded far fewer adult beetles than several cultivars of North American green and white ash. Surprisingly, a black ash (North American) × Manchurian ash hybrid was highly susceptible to emerald ash borer, indicating this cultivar did not inherit emerald ash borer resistance from its Asian parent. A corollary study investigated the efficacy of soil-applied imidacloprid, a systemic, neonicotinoid insecticide, for controlling emerald ash borer in each of the five cultivars. Imidacloprid had no effect on emerald ash borer colonization of Manchurian ash, which was low in untreated and treated trees. In contrast, imidacloprid did enhance survival of the North American and hybrid cultivars and significantly reduced the number of emerald ash borer adults emerging from green and white ash cultivars. We identify a possible mechanism of resistance of Manchurian ash to emerald ash borer, which may prove useful for screening, selecting, and breeding emerald ash borer-resistant ash trees.

https://bioone.org/journals/Environmental-Entomology/volume-37/issue-1/0046-225X(2008)37[242:IVIRTE]2.0.CO;2/Interspecific-Variation-in-Resistance-to-Emerald-Ash-Borer-Coleoptera/10.1603/0046-225X(2008)37[242:IVIRTE]2.0.CO;2.pdf

Daniel A. Herms

Papers

The dilemma of plants: To grow or defend.

Emerald Ash Borer Invasion of North America: History, Biology, Ecology, Impacts, and Management

Direct and indirect effects of alien insect herbivores on ecological processes and interactions in forests of eastern North America

Ecology of Herbivorous Arthropods in Urban Landscapes

Interspecific Variation in Resistance to Emerald Ash Borer (Coleoptera: Buprestidae) Among North American and Asian Ash (Fraxinus spp.)