A modular concept of plant foraging behaviour: the interplay between local responses and systemic control

In this paper we examined the notion that plant foraging for resources in heterogeneous environments must involve: (1) plasticity at the level of individual modules in reaction to localized environmental signals; and (2) the potential for modification of these responses either by the signals received from connected modules that may be exposed to different conditions, or by the signals reflecting the overall resource status of the plant. A conceptual model is presented to illustrate how plant foraging behaviour is achieved through these processes acting in concert, from the signal reception through signal transduction to morphological or physiological response. Evidence to support the concept is reviewed, using selective root placement under nutritionally heterogeneous conditions and elongation responses of stems and petioles to shade as examples. We discussed how the adoption of this model can promote understanding of the ecological significance of foraging behaviour. We also identified a need to widen the experimental repertoires of both molecular physiology and ecology in order to increase our insight into both the regulation and functioning of foraging responses, and their relationship with the patterns of environmental heterogeneity under which plants have evolved.

Differences in flooding tolerance between species from two wetland habitats with contrasting hydrology: implications for vegetation development in future floodwater retention areas

BACKGROUND AND AIMS

Plants need different survival strategies in habitats differing in hydrological regimes. This probably has consequences for vegetation development when former floodplain areas that are currently confronted with soil flooding only, will be reconnected to the highly dynamical river bed. Such changes in river management are increasingly important, especially at locations where increased water retention can prevent flooding events in developed areas. It is therefore crucial to determine the responses of plant species from relatively low-dynamic wetlands to complete submergence, and to compare these with those of species from river forelands, in order to find out what the effects of such landscape-scale changes on vegetation would be.

METHODS

To compare the species' tolerance to complete submergence and their acclimation patterns, a greenhouse experiment was designed with a selection of 19 species from two contrasting sites: permanently wet meadows in a former river foreland, and frequently submerged grasslands in a current river foreland. The plants were treated with short (3 weeks) and long (6 weeks) periods of complete submergence, to evaluate if survival, morphological responses, and changes in biomass differed between species of the two habitats.

KEY RESULTS

All tested species inhabiting river forelands were classified as tolerant to complete submergence, whereas species from wet meadows showed either relatively intolerant, intermediate or tolerant responses. Species from floodplains showed in all treatments stronger shoot elongation, as well as higher production of biomass of leaves, stems, fine roots and taproots, compared with meadow species.

CONCLUSIONS

There is a strong need for the creation of temporary water retention basins during high levels of river discharge. However, based on the data presented, it is concluded that such reconnection of former wetlands (currently serving as meadows) to the main river bed will strongly influence plant species composition and abundance.

Improving the scale and precision of hypotheses to explain root foraging ability

BACKGROUND

Numerous hypotheses have been proposed to explain the wide variation in the ability of plants to forage for resources by proliferating roots in soil nutrient patches. Comparative analyses have found little evidence to support many of these hypotheses, raising the question of what role resource-foraging ability plays in determining plant fitness and community structure.

SCOPE

In the present viewpoint, we respond to Grime's (2007; Annals of Botany 99: 1017-1021) suggestion that we misinterpreted the scope of the scale-precision trade-off hypothesis, which states that there is a trade-off between the spatial scale over which plant species forage and the precision with which they are able to proliferate roots in resource patches. We use a meta-analysis of published foraging scale-precision correlations to demonstrate that there is no empirical support for the scale-precision trade-off hypothesis. Based on correlations between foraging precision and various plant morphological and ecophysiological traits, we found that foraging precision forms part of the 'fast' suite of plant traits related to rapid growth rates and resource uptake rates.

CONCLUSIONS

We suggest there is a need not only to examine correlations between foraging precision and other plant traits, but to expand our notion of what traits might be important in determining the resource-foraging ability of plants. By placing foraging ability in the broader context of plant traits and resource economy strategies, it will be possible to develop a new and empirically supported framework to understand how plasticity in resource uptake and allocation affect plant fitness and community structure.

Evidence that ethylene signalling is not involved in selective root placement by tobacco plants in response to nutrient-rich soil patches

Ethylene is a strong controller of root development, and it has been suggested that it is involved in the increase of lateral root development in nutrient-rich soil patches (selective root placement). Here, this contention was tested by comparing selective root placement of an ethylene-insensitive transgenic tobacco (Nicotiana tabacum) genotype (Tetr) with that of wild-type (Wt) plants. Wt and Tetr plants were grown in pots with locally increased nitrate or phosphate concentrations, after which the root growth patterns were compared with those of plants grown in pots with homogeneous nutrient distribution. Tetr plants responded to nutrient patches in a similar way to Wt plants, by placing their roots preferentially at locations with higher nutrient content, and other aspects of plant morphology were also not greatly influenced by ethylene insensitivity. The response of both Wt and Tetr plants to high-nitrate patches was considerably stronger than to locally high phosphate, suggesting that the two responses are not linked in any functional or regulatory way. As the response to nutrient patches was similar in ethylene-sensing and ethylene-insensitive plants, it is concluded that selective root placement in response to nitrate or phosphate is, at least in tobacco, not mediated or modified by ethylene action.

Testing sustainability by prospective and retrospective demographic analyses: evaluation for palm leaf harvest

Harvesting nontimber forest products (NTFPs) is a major economic activity in tropical forests. As many NTFPs are overexploited, sustainability analyses are required to set harvest guidelines. Here we introduce and apply a new approach to evaluating sustainability, which combines prospective (elasticity) and retrospective (Life Table Response Experiments [LTRE]) demographic analyses of matrix population models. We relate the elasticity of vital rates (representing their importance for population growth rate, lamda) to their contribution to harvest-induced change in lamda ("LTRE contribution"). When high-elasticity vital rates have a low LTRE contribution, exploitation is potentially sustainable as negative effects for population growth are buffered. If the reverse is found, there is little scope for sustainability because crucial vital rates are affected. Our approach is less sensitive to chance fluctuations than the commonly used sustainability criterion of lamda = 1.0, as it does not depend on the absolute value of lamda. We applied this analysis to Geonoma deversa, a clustered forest understory palm. We studied three experimentally defoliated and control populations in a Bolivian rainforest during two years. Cutting all leaves of large ramets did not change mortality but strongly affected growth and reproduction. In spite of severe changes in some vital rates, population growth rate was not significantly reduced after defoliation. A literature review revealed that six other understory palms species responded very similarly to defoliation. The combination of LTRE contributions and elasticity analyses showed that low-elasticity vital rates were mainly responsible for the defoliation-induced change in lamda for Geonoma deversa. For two other understory palms (Astrocaryum mexicanum and Chamaedorea radicalis) new demographic analyses yielded very similar results. For Geonoma, the LTRE contribution-elasticity relation strongly changed when we mimicked harvest damage. Adding 5% mortality to defoliated palms caused stronger change in lamda, mainly due to changes in a high-elasticity vital rate (survival). Therefore, harvest practices that involve stem killing are clearly unsustainable. Our results show that commercial leaf cutting in Geonoma deversa is potentially sustainable, and that this is likely the case for understory palms in general. Our approach to evaluating harvest sustainability can be applied to other NTFPs.

Supported living in residential homes for the elderly: impact on patients and elder care workers

To enable older people with severe and persistent mental illness to live in the community, the Dutch mental health sector has developed a program for supported living in residential homes for the elderly. It provides for the permanent stationing of mental health workers (MHWs) in elder care facilities to support both the resident patients and the elder care staff. The authors examined associations between the number of MHW staff and the degree to which (1) patients were integrated into the community and (2) elder care workers had developed effective working alliances with their patients. Participants included 110 patients participating in 18 supported living programs in the Netherlands. Community integration was assessed in face-to-face interviews with the patients about their perceived influence over daily life, involvement in social activities, and social network size. The quality of the worker-patient relationship was assessed using the Dutch Working Alliance Questionnaire for Community Care, completed by the elder care worker primarily responsible for each patient. After differentiation of the MHW staff into medically trained and nurse-trained professionals, associations with outcome measures were found only for the nurse-trained staff. The more hours of nurse-trained staff capacity per patient, the more influence perceived by the patients, and the more directiveness shown by the elder care workers in their contacts with patients. The impact of supported living programs in residential homes for the elderly appears to be determined in part by the caseloads of the on-site MHWs.