Plant physiological ecology: An essential link for integrating across disciplines and scales in plant ecology

A sizeable number of scientists and funding organisations are of the opinion that the relevance of plant physiological ecology as an important discipline has declined to the point that it is no longer considered as one of the important topics of ecological research. Plant physiological ecology is typically associated with the autecological plant research conducted during the latter portion of the 20th century or, even worse, simply with gas exchange measurements. However, taking a closer look, it becomes obvious that, by focusing on the intermediate integration levels (individuals, populations), this discipline represents an essential link between the high integration levels (communities, ecosystems, biosphere) and the disciplines at the bottom of the complexity hierarchy (physiology, molecular biology). In this paper we show that the principal question of all ongoing community and ecosystem level research – What is the mechanistic background of vegetation composition, biodiversity structure and dynamics and how is this linked to fluxes of matter at the community and higher levels of organisation? – can only be answered if the mechanism of interactions between the relevant organisms are understood. In consequence, the classical discipline of plant physiological ecology will continuously develop into a truly interdisciplinary experimental ecology of interactions and its importance will rather increase than diminish. Promising activities of this kind are already underway. Scientists needed for this new direction should have a rather broad scientific perspective, including knowledge and experience in fields outside of typical ecological research, instead of being specialists for single ecophysiological aspects.     

The physiological ecology of strandline plants

Factors which affect the vegetation of strandlines on sandy shores were investigated with particular attention to the growth of four annual species Atriplex glabriuscula, Atriplex laciniata, Cakile maritima and Salsola kali which are widely distributed around the coasts of North-West Europe. The response of these species to increases in salinity and sand accretion are reported from laboratory experiments. Field observations were also made of photosynthesis and the availability of, and the response to, nitrogen. The results are used to discuss the adaptation of these plants to growth on strandlines in an attempt to explain their restriction to this habitat.     

Small plants, large plants: the importance of plant size for the physiological ecology of vascular epiphytes

Recently, a number of publications have reported that many physiological properties of vascular epiphytes are a function of plant size. This short review will summarize what is known to date about this phenomenon, describe the possible mechanism and will discuss the consequences for the present understanding of epiphyte biology. Size-related changes are also known from other plant groups and it is argued that close attention should be paid to the size of the organisms under study in order to understand the performance and survival of a species in the field. In the light of these findings, the results of many earlier studies on epiphyte ecophysiology are now difficult to interpret because essential information on the size of the specimens used is missing.     

The importance of physiological ecology in conservation biology

Many of the threats to the persistence of populations of sensitivespecies have physiological or pathological mechanisms, and thosemechanisms are best understood through the inherently integrativediscipline of physiological ecology. The desert tortoise waslisted under the Endangered Species Act largely due to a newlyrecognized upper respiratory disease thought to cause mortalityin individuals and severe declines in populations. Numeroushypotheses about the threats to the persistence of desert tortoisepopulations involve acquisition of nutrients, and its connectionto stress and disease. The nutritional wisdom hypothesis positsthat animals should forage not for particular food items, butinstead, for particular nutrients such as calcium and phosphorusused in building bones. The optimal foraging hypothesis suggeststhat, in circumstances of resource abundance, tortoises shouldforage as dietary specialists as a means of maximizing intakeof resources. The optimal digestion hypothesis suggests thattortoises should process ingesta in ways that regulate assimilationrate. Finally, the cost-of-switching hypothesis suggests thatherbivores, like the desert tortoise, should avoid switchingfood types to avoid negatively affecting the microbe communityresponsible for fermenting plants into energy and nutrients.Combining hypotheses into a resource acquisition theory leadsto novel predictions that are generally supported by data presentedhere. Testing hypotheses, and synthesizing test results intoa theory, provides a robust scientific alternative to the popularuse of untested hypotheses and unanalyzed data to assert theneeds of species. The scientific approach should focus on hypothesesconcerning anthropogenic modifications of the environment thatimpact physiological processes ultimately important to populationphenomena. We show how measurements of such impacts as nutrientstarvation, can cause physiological stress, and that the endocrinemechanisms involved with stress can result in disease. Finally,our new syntheses evince a new hypothesis. Free molecules ofthe stress hormone corticosterone can inhibit immunity, andthe abundance of "free corticosterone" in the blood (thoughtto be the active form of the hormone) is regulated when thecorticosterone molecules combine with binding globulins. Thesex hormone, testosterone, combines with the same binding globulin.High levels of testosterone, naturally occurring in the breedingseason, may be further enhanced in populations at high densities,and the resulting excess testosterone may compete with bindingglobulins, thereby releasing corticosterone and reducing immunityto disease. This sequence could result in physiological andpathological phenomena leading to population cycles with a periodthat would be essentially impossible to observe in desert tortoise.Such cycles could obscure population fluctuations of anthropogenicorigin.     

The biology and ecology of narrow endemic and widespread plants: a comparative study of trait variation in 20 congeneric pairs

The objective of this study is to examine whether habitat, herbivory and traits related to resource acquisition, resource conservation, reproduction and dispersal differ between narrow endemic plant species and their widespread congeners. We undertook pairwise contrasts of 25 ecological characteristics and biological traits in 20 congeneric pairs of narrow endemic and widespread plant species in the French Mediterranean region. Within each pair, the two species had the same life-form, pollination mode and dispersal mode. Endemic species differed significantly from widespread congeners for a number of attributes. Endemic species occur in habitats on steeper slopes, with higher rock cover and in lower and more open vegetation than their widespread congeners. Endemic species are significantly smaller than widespread species, but show no differences in traits related to resource acquisition (specific leaf area, leaf nitrogen content, maximum photosynthetic rate) or resource conservation (leaf dry matter content). After accounting for their smaller stature, endemic species produce fewer and smaller flowers with less stigma-anther separation and lower pollen/ovule ratios and produce fewer seeds per plant than their widespread congeners. No consistent variation in seed mass and propagule structure was found between congeneric species. Herbivory levels did not differ between congeneric species. Ecological characteristics, notably the occupation of rocky habitats with low aboveground competition, may thus have played an important role in the differentiation of narrow endemic species in the Western Mediterranean. Morphological and ecophysiological traits of narrow endemic species indicate that they are not more stress-tolerant than their widespread congeners. Lower investment in pollen transfer and seed production suggest that local persistence is a key feature of the population ecology of narrow endemic species.     

Some like it hot: the physiological ecology of C4 plant evolution

Oecologia - The evolution of C4 photosynthesis requires an intermediate phase where photorespiratory glycine produced in the mesophyll cells must flow to the vascular sheath cells for metabolism by...     

Interactions between plants and herbivores: A review of plant defense

Ecologists have long ignored or underestimated the importance of plant–herbivore interactions owing to the diversities of herbivores, plant defensive strategies and ecological systems. In this review, we briefly discussed the categories of herbivores. Then we reviewed the major types of plant defenses against herbivores. Selective forces of herbivore pressures have led to the evolution of various defensive mechanisms in plants, which can be classified into (i) resistance traits that reduce the amount of damage received, including physical, chemical, and biotic traits; (ii) tolerance mechanisms that decrease the impact of herbivore damage, and (iii) escape strategies that reduce the probability of plants to be found by herbivores. These strategies have been studied at different levels from molecular genetics and genomics, to chemistry and physiology, to community and ecosystem ecology. We summarized the development of the methodology for studying plant defenses against herbivores. Particularly, 24 of those hypotheses and models, which are influential in the international community concerning the relationship between plants and herbivores, including the defensive mimicry hypothesis, the compensatory continuum hypothesis, the slow-growth-high-mortality hypothesis, etc, were introduced and grouped into four categories according to plant defense strategies in the present review. Finally, we also reviewed the research progress of plant–herbivore interactions in China, and discussed the perspectives of studies on plant–herbivore interactions.     

Putting the plants back into plant ecology: six pragmatic models for understanding and conserving plant diversity

BACKGROUND: There is a compelling need to protect natural plant communities and restore them in degraded landscapes. Activities must be guided by sound scientific principles, practical conservation tools, and clear priorities. With perhaps one-third of the world's flora facing extinction, scientists and conservation managers will need to work rapidly and collaboratively, recognizing each other's strengths and limitations. As a guide to assist managers in maintaining plant diversity, six pragmatic models are introduced that are already available. Although theoretical models continue to receive far more space and headlines in scientific journals, more managers need to understand that pragmatic, rather than theoretical, models have the most promise for yielding results that can be applied immediately to plant communities. SIX PRAGMATIC MODELS: For each model, key citations and an array of examples are provided, with particular emphasis on wetlands, since "wet and wild" was my assigned theme for the Botanical Society of America in 2003. My own work may seem rather prominent, but the application and refinement of these models has been a theme for me and my many students over decades. The following models are reviewed: (1) species-area: larger areas usually contain more species; (2) species-biomass: plant diversity is maximized at intermediate levels of biomass; (3) centrifugal organization: multiple intersecting environmental gradients maintain regional landscape biodiversity; (4) species-frequency: a few species are frequent while most are infrequent; (5) competitive hierarchies: in the absence of constraints, large canopy-forming species dominate patches of landscape, reducing biological diversity; and (6) intermediate disturbance: perturbations such as water level fluctuations, fire and grazing are essential for maintaining plant diversity. CONCLUSIONS: The good news is that managers faced with protecting or restoring landscapes already have this arsenal of tools at their disposal. The bad news is that far too few of these models are appreciated.     

7. Bioactivity in plants: the link between phytochemistry and medicine

The development of medicinal plant research over the last 30 years is reviewed with reference to the search for new active principles. Difficulties inherent to activity guided isolation and the specific requirements of bioassays are discussed. An overview is given on currently used systems for various bioactivities, with emphasis on simple bioassays for phytochemical laboratories. The progress in medicinal plant research is illustrated by selected examples of plant derived compounds of importance as drugs or pharmacological tools.