Pieter Poot

The worldwide leaf economics spectrum

Bringing together leaf trait data spanning 2,548 species and 175 sites we describe, for the first time at global scale, a universal spectrum of leaf economics consisting of key chemical, structural and physiological properties. The spectrum runs from quick to slow return on investments of nutrients and dry mass in leaves, and operates largely independently of growth form, plant functional type or biome. Categories along the spectrum would, in general, describe leaf economic variation at the global scale better than plant functional types, because functional types overlap substantially in their leaf traits. Overall, modulation of leaf traits and trait relationships by climate is surprisingly modest, although some striking and significant patterns can be seen. Reliable quantification of the leaf economics spectrum and its interaction with climate will prove valuable for modelling nutrient fluxes and vegetation boundaries under changing land-use and climate.

Biomass allocation to leaves, stems and roots: meta analyses of interspecific variation and environmental control

We quantified the biomass allocation patterns to leaves, stems and roots in vegetative plants, and how this is influenced by the growth environment, plant size, evolutionary history and competition. Dose-response curves of allocation were constructed by means of a meta-analysis from a wide array of experimental data. They show that the fraction of whole-plant mass represented by leaves (LMF) increases most strongly with nutrients and decreases most strongly with light. Correction for size-induced allocation patterns diminishes the LMF-response to light, but makes the effect of temperature on LMF more apparent. There is a clear phylogenetic effect on allocation, as eudicots invest relatively more than monocots in leaves, as do gymnosperms compared with woody angiosperms. Plants grown at high densities show a clear increase in the stem fraction. However, in most comparisons across species groups or environmental factors, the variation in LMF is smaller than the variation in one of the other components of the growth analysis equation: the leaf area : leaf mass ratio (SLA). In competitive situations, the stem mass fraction increases to a smaller extent than the specific stem length (stem length : stem mass). Thus, we conclude that plants generally are less able to adjust allocation than to alter organ morphology.

Plant phenotypic plasticity in a changing climate

Climate change is altering the availability of resources and the conditions that are crucial to plant performance. One way plants will respond to these changes is through environmentally induced shifts in phenotype (phenotypic plasticity). Understanding plastic responses is crucial for predicting and managing the effects of climate change on native species as well as crop plants. Here, we provide a toolbox with definitions of key theoretical elements and a synthesis of the current understanding of the molecular and genetic mechanisms underlying plasticity relevant to climate change. By bringing ecological, evolutionary, physiological and molecular perspectives together, we hope to provide clear directives for future research and stimulate cross-disciplinary dialogue on the relevance of phenotypic plasticity under climate change.

Causes of increased nutrient concentrations in post-fire regrowth in an East African savanna

The aim of the present study was to investigate the causes of increased macronutrient concentrations in above-ground post-fire regrowth in an East African savanna (Northern Tanzania). Experiments were set up to discriminate between the following possible causes: (1) increased soil nutrient supply after fire, (2) relocation of nutrients from the roots to the new shoots, (3) rejuvenation and related changes in plant tissue composition and (4) changes in nutrient uptake in relation to above-ground carbon gains. N, P, K, Ca and Mg concentrations in post-burn graminoid vegetation were compared with clipped and with unburned, control vegetation during the post-burn growth season. One month after burning and clipping, nutrient concentrations in live grass shoots in the burned and clipped treatments were significantly higher than in the control. This effect, however, declined in the course of the season and, except for Ca, disappeared three months after onset of the treatments. There were no significant differences in live grass shoot nutrient concentrations between burned and clipped treatments which suggests that the increased nutrient concentration in post-fire regrowth is not due to increased soil nutrient supply via ash deposition. The relatively low input of nutrients through ash deposition, compared to the amount of nutrients released through mineralisation during the first month after burning and to the total nutrient pools, supports this suggestion. There was no difference between burned and unburned vegetation in total root biomass and root nutrient concentrations. Relocation of nutrients from the roots to the new shoots did not, therefore, appear to be a cause of higher post-fire shoot nutrient concentrations. The present study shows that in this relatively nutrient-rich savanna, the increased nutrient concentration in above-ground post-fire regrowth is primarily due to increased leaf:stem ratios, rejuvenation of plant material and the distribution of a similar amount of nutrients over less above-ground biomass.

Shallow-soil endemics: adaptive advantages and constraints of a specialized root-system morphology

Worldwide, many rare plant species occur in shallow-soil, drought-prone environments. For most of these species, the adaptations required to be successful in their own habitats, as well as their possible consequences for establishment and persistence in others, are unknown. Here, two rare Hakea (Proteaceae) species confined to shallow-soil communities in mediterranean-climate south-western Australia were compared with four congeners commonly occurring on deeper soils. Seedlings were grown for 7 months in a glasshouse in individual 1.8 x 0.2-m containers, to allow for unconstrained root development. In addition, a reciprocal transplant experiment was carried out. The rare Hakea species differed consistently from their common congeners in their spatial root placement. They invested more in deep roots and explored the bottom of the containers much more quickly. In the reciprocal transplant experiment they showed increased survival in their own habitat, but not in others. This research suggests that shallow-soil endemics have a specialized root system that allows them to explore a large rock surface area, thereby presumably increasing their chance to locate cracks in the underlying rock. However, this root-system morphology may be maladaptive on deeper soils, providing a possible explanation for the restricted distribution of many shallow-soil endemics.

Reproductive allocation and resource compensation in male-sterile and hermaphroditic plants of Plantago lanceolata (Plantaginaceae).

Gynodioecy is a breeding system in which hermaphrodites coexist with male steriles. Theoretical models predict that without any compensation in female fitness male steriles will disappear from a population due to their reproductive disadvantage. In the present study I investigated whether male-sterile (MS), partially male-sterile (IN), and hermaphroditic (H) plants of Plantago lanceolata differed in reproductive growth and allocation. Offspring of three interpopulation crosses segregating all three sex morphs were grown under nitrogen-limited conditions in a growth chamber. Independent of the genetic background MS plants attained a higher vegetative and reproductive dry mass and a higher reproductive output than H plants, whereas IN plants had intermediate values. When corrected for the mass of the pollen, the dry mass differences between the sex morphs were much reduced but still present. However, when whole-plant allocation was expressed on the basis of nitrogen, the differences between the sex morphs disappeared. Thus the sex morphs took up similar amounts of nitrogen but distributed them differently. The MS and IN plants used the nitrogen saved by not producing pollen for additional vegetative as well as reproductive growth. The data presented in this study suggest that resource compensation is one of the main mechanisms responsible for the maintenance of MS and IN plants in gynodioecious P. lanceolata.

Structure and functioning of cluster roots and plant responses to phosphate deficiency

Introduction H. Lambers, et al. The biology of cluster roots and the acquisition of P from the rhizosphere. Structure, ecology and physiology of root clusters a review B.B. Lamont. The evolution of physiology and development in the cluster root: teaching an old dog new tricks? K.R. Skene. Organic acid behaviour in soils misconceptions and knowledge gaps D.L. Jones, et al. Origins of root-mediated pH changes in the rhizosphere and their responses to environmental constraints: A review P. Hinsinger, et al. Strategies to isolate transporters that facilitate organic anion efflux from plant roots P.R. Ryan, et al. Phosphate transport in plants F.W. Smith, et al. Development of the adhesive pad on climbing fig (Ficus punila) stems from clusters of adventitious roots E.P. Groot, et al. Pathways of carbon metabolism related to enhanced exudation of organic acids and acid phosphatase. Acclimation of white lupin to phosphorus deficiency involves enhanced expression of genes related to organic acid metabolism C. Uhde-Stone, et al. Phosphorus deficiency-induced modifications in citrate catabolism and in cystolic pH as related to citrate exudation in cluster roots of white lupin A. Kania. Secreted acid phosphatase is expressed in cluster roots of lupin in response to phosphorus deficiency J. Wasaki, et al. Effects of altered citrate synthase and isocitrate dehydrogenase expression on internal citrate concentrations and citrate efflux from tobacco (Nicotiana tabacum L.) roots E. Delhaize, et al. Characterization of NAPD-isocitrate dehydrogenase expression in a carrot mutant cell line with enhanced citrate excretion T. Kihara, et al. The influence of organic acid exudation and cluster roots on P and N acquisition. Do cluster roots of Hakea actities (Proteaceae) acquire complex organic nitrogen? S. Schmidt, et al. Lupinus luteus cv. Wodjil takes up more phosphorus and cadmium than Lupinus angustifolius cv. Kalya R.F. Brennan, M.D.A. Bolland. Chick pea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake E.J. Veneklaas, et al. Role of phosphorus nutrition in development of cluster roots and release of carboxylates in soil-grown Lupinus albus J. Shen, et al. The influence of plant nutrition on plant growth and development of cluster roots. Effects of external phosphorus supply on internal phosphorus concentration and the initiation, growth and exudation of cluster roots in Hahea prostrata R. Br. M.W. Shane, et al. Differences in cluster root formation and carboxylate exudation in Lupinus albus L. under different nutrient deficiencies R. Liang, C. Li. Is Fe deficiency rather than P deficiency the cause of cluster root formation in Casuarina species? E.H. Zaid, et al. The formation, morphology and anatomy of cluster root of Lupinus albus L. as dependent on soil type and phosphorus supply C.S. Peek, et al. Localized supply of phosphorus induces root morphological and architectural changes of rice in split and stratified soil cultures Y. He, et al. Interaction of nitrogen and phosphorus nutrition in determining growth C.C.de Groot, et al. Implications of root architecture, root-soil interactions and mycorrhiza on plant P nutrition. Phosphorus acquisition from soil by white lupin (Lupinus albus L.) and soybean (Glycine max L.) species with contrasting root development M. Watt, J.R. Evans. Plantago lanceolata L. and Rumex acetosella L. differ in their utilisation of soil phosphorus fractions A.-M. Fransson, et al. Chickpea facilitates phosphorus uptake by intercropped wheat from an organic phosphorus source L. Li, et al. Using competitive and facilitative interactions in intercropping systems enhances crop productivity and nutrient-use efficiency F. Zhang, L. Li. Phosphorus uptake by a community of arbuscular mycorrhizal fungi in jarrah forest M.Z. Solaiman, L.K. Abbott. Relationships between cluster root-bearing taxa and laterite across landscapes in southwest Western Australia: an approach using airborne radiometric and digital elevation models W.H. Verboom, J.S. Pate. Volume contents. Author index.

Climate and landscape drivers of tree decline in a Mediterranean ecoregion

Climate change and anthropogenic land use are increasingly affecting the resilience of natural ecosystems. In Mediterranean ecoregions, forests and woodlands have shown progressive declines in health. This study focuses on the decline of an endemic woodland tree species, Eucalyptus wandoo (wandoo), occurring in the biodiversity hotspot of southwest Western Australia. We determined the change in health of wandoo stands between 2002 and 2008 across its geographic and climatic range, and associated this change in health with non-biotic variables focusing on: (1) fragment metrics; (2) topography; (3) soil characteristics; and (4) climate. Only fragment metrics and climate variables were found to be significantly related to the observed change in health. Stands that were small with high perimeter/area ratios were found to be most sensitive to health declines. Recent increases in autumn temperatures and decreases in annual rainfall were negatively affecting health of wandoo most prominently in the low rainfall zone of its climatic range. Together, these results suggest the onset of range contraction for this ecologically important species, which is likely to be exacerbated by projected future changes in climate. Our results emphasize the importance of establishing monitoring programs to identify changes in health and decline trends early to inform management strategies, particularly in the sensitive Mediterranean ecoregions.

Seasonal patterns in water use and leaf turnover of different plant functional types in a species-rich woodland, south-western Australia

Woodlands in south-western Australia are evergreen and transpire throughout the year despite the long, hot and dry summers of the Mediterranean climate. Results from a case study in a species-rich Banksia woodland are used to discuss the ecological and physiological properties that appear to be essential features of this and similar communities. Tree, shrub and perennial herbaceous species with long-lived leaves dominate the community, whereas winter-green herbaceous species with short-lived leaves constitute a minor group. The total leaf area index is therefore reasonably constant in all seasons. Leaf area index is low and canopies are open, causing good coupling between the vegetation and the atmosphere, and making stomatal control an effective regulator of transpiration. Mean maximum (winter) stomatal conductances were high at approximately 300 mmol m−2 s−1. Deep-rootedness allows the dominant species to access soil moisture throughout the unsaturated zone, and down to the capillary fringe of the saturated zone. Shrubs and herbs with shallow roots experience greater drought stress during summer. Rates of community evapotranspiration are limited by leaf area index in the wet season, and further reduced by stomatal closure in the dry season. Deep-rooted plants appear to decrease their stomatal conductance before the development of severe drought stress. Such conservative behaviour, possibly related to plant hydraulic constraints, is a contributing factor to the limited seasonality in community water use.

Exploring rock fissures: does a specialized root morphology explain endemism on granite outcrops?

BACKGROUND AND AIMS Worldwide, many plant species are confined to open, shallow-soil, rocky habitats. Although several hypotheses have been proposed to explain this habitat specificity, none has been convincing. We suggest that the high level of endemism on shallow soils is related to the edaphic specialization needed to survive in these often extremely drought-prone habitats. Previous research has shown that species endemic to ironstone communities in SW Australia have a specialized root morphology that enhances their chance to access fissures in the underlying rock. Here we test the generality of these findings for species that are confined to a shallow-soil habitat that is of much greater global significance: granite outcrops. METHODS We compared temporal and spatial root growth and allocation of three endemic woody perennials of SW Australian granite outcrop communities with those of congeners occurring on nearby deeper soils. Seedlings of all species were grown in 1·2 m long custom-made containers with a transparent bottom that allowed monitoring of root growth over time. KEY RESULTS The granite outcrop endemics mostly differed in a predictable way from their congeners from deeper soils. They generally invested a larger portion of their biomass in roots, distributed their roots faster and more evenly over the container and had a lower specific root length. In different species pairs the outcrop endemics achieved their apparent advantage by a different combination of the aforementioned traits. CONCLUSIONS Our results are consistent with earlier work, indicating that species restricted to different types of drought-prone shallow-soil communities have undergone similar selection pressures. Although adaptive in their own habitat in terms of obtaining access to fissures in the underlying rock, these root system traits are likely to be maladaptive in deeper soil habitats. Therefore, our results may provide an explanation for the narrow endemism of many shallow-soil endemics.