Lonnie W. Aarssen

Three keys to the radiation of angiosperms into freezing environments

Early flowering plants are thought to have been woody species restricted to warm habitats. This lineage has since radiated into almost every climate, with manifold growth forms. As angiosperms spread and climate changed, they evolved mechanisms to cope with episodic freezing. To explore the evolution of traits underpinning the ability to persist in freezing conditions, we assembled a large species-level database of growth habit (woody or herbaceous; 49,064 species), as well as leaf phenology (evergreen or deciduous), diameter of hydraulic conduits (that is, xylem vessels and tracheids) and climate occupancies (exposure to freezing). To model the evolution of species’ traits and climate occupancies, we combined these data with an unparalleled dated molecular phylogeny (32,223 species) for land plants. Here we show that woody clades successfully moved into freezing-prone environments by either possessing transport networks of small safe conduits and/or shutting down hydraulic function by dropping leaves during freezing. Herbaceous species largely avoided freezing periods by senescing cheaply constructed aboveground tissue. Growth habit has long been considered labile, but we find that growth habit was less labile than climate occupancy. Additionally, freezing environments were largely filled by lineages that had already become herbs or, when remaining woody, already had small conduits (that is, the trait evolved before the climate occupancy). By contrast, most deciduous woody lineages had an evolutionary shift to seasonally shedding their leaves only after exposure to freezing (that is, the climate occupancy evolved before the trait). For angiosperms to inhabit novel cold environments they had to gain new structural and functional trait solutions; our results suggest that many of these solutions were probably acquired before their foray into the cold.

On the relationship between r/K selection and environmental carrying capacity : a new habitat templet for plant life history strategies

We propose a revision of the habitat templet approach for modelling the relationship between r/K selection and selection related to the level of resource impoverishment within vegetation. The latter is represented as a continuum on one axis of the templet by the mean annual environmental carrying capacity of the habitat. This is perpendicular to the second axis representing the traditional r/K selection continuum and defined by the mean annual distance below environmental carrying capacity that the vegetation is maintained at, usually as a consequence of different levels of disturbance (...)

Resource manipulations in natural vegetation: a review

This paper reviews the effects that resource manipulations, especially nutrient additions, have on productivity, diversity, species interactions and succession in natural and semi-natural low-growth-form terrestrial restrial vegetation. In most studies, fertilization has resulted in an increase in productivity with a concomitant decrease in species diversity. Plant community productivity in nutrient-poor habitats (e.g. arctic tundra) however, has been relatively little affected by nutrient additions. Results from several fertilization experiments in nutrient-poor habitats indicate that competition may be an important component of community dynamics. Resource additions generally do not affect co-occurring species equally. In many cases, this results from an alteration of the relative competitive abilities of species. Several short-term fertilization studies have suggested that the rate of succession is increased by the addition of nutrients. Opposite trends however, have been reported in long-term studies. Contradictory results among some resource manipulation studies appear to stem largely from design limitations (e.g. only one resource was manipulated). Variability in habitat fertility, the type of resource added and the duration of enrichment also contribute to the difficulties involved in interpreting and comparing field experiments. A pluralistic approach combining glasshouse studies with both neighbour and resource manipulations in the field can be expected to lead to a greater depth of understanding of current controversies over the effects of habitat fertility on the intensity of competition and the relationship between competitive ability and tolerance of resource impoverishment.

A global meta‐analysis of the relative extent of intraspecific trait variation in plant communities

Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.

Neighbour manipulations in natural vegetation a review

. This paper reviews the literature on neighbour manipulation experiments on species intemctions in natural of semi-natural vegetation. Three major approaches have been used: 1) introductions into vegetation, 2) trenching, and 3) vegetation removals. Some studies have used a combination of approaches, especially 1) and 3). The removal approach has been used most commonly and is given the greatest emphasis in this paper accordingly. Details of over 50 such studies are summarized here, 48 of which have appeared within only the last two decades. We compare and contrast the experimental designs of these studies and examine their contributions to the understanding of species interactions in vegetation. Evidence for competition is virtually universal among these studies. Several studies have also detected evidence of beneficence between plant species. Numerous factors may confound interpretations in removal experiments including: life stage dependent species responses, species-dependent timing and speed of response, inadequacy of controls due to temporal and/or spatial variability in site quality, indirect effects of treatment on soil moisture content or nutrient levels, or on the activities of predators or decomposers, and several constraints inherent in particular experimental designs. Another level of complexity arises as a consequence of several variables related to plant attributes that directly determine the nature of interactions between neighbours (e.g. relative competitive abilities, the magnitude of beneficial interactions, the extent to which neighbours make demands on the same resource units). These may interact in a complex manner to affect the response of a ‘target’ plant to the removal of neighbours. Recommendations for future studies are considered and a neighbourhood experimental design is proposed which enables analysis of the extent to which the fates of naturally established individuals following vegetation removal can be accounted for (in multiple regressions) by several variables that reflect different properties and circumstances of interaction with immediate Thiessen neighbours in the field.

THE SPECIALIZATION HYPOTHESIS FOR PHENOTYPIC PLASTICITY IN PLANTS

Adaptive plasticity in plants is commonly interpreted for fitness estimates like size and fecundity. The specialization hypothesis, however, predicts that plasticity in such characters is not a product of selection but, rather, a product of specialized (i.e., ecotypic) adaptation to particular environmental conditions. In response to a recent test of this hypothesis (Emery et al. 1994), we refine its predictions to recognize that the evolution of specialized ecotypes may be accompanied by an increase, decrease, or no change in the plasticity of size or fecundity. These predictions depend on whether specialization is associated with the less favorable or more favorable end of an environmental gradient and on whether specialization to one end of the gradient comes at a cost of reduced performance at the other end. We argue that, for size or fecundity characters, a plastic response to environmental deterioration is adaptive only if the alternative is dormancy or death and is generally less adaptive than phenotypic stability. Based on analysis of reaction norms for reciprocal transplants, we illustrate how it is possible to reject the specialization hypothesis and how to recognize results that are consistent with this hypothesis.

Competitive ability and species coexistence: a 'plant's-eye' view

This paper develops a «plants-eye» view of avoiding competitive exclusion which interprets coexistence at the species level as a consequence of ongoing selection resulting from genetically-based differences in competitive abilities within local neighborhoods. That competitive ability may change as a consequence of selection is supported by results from a multi-generation competition experiment involving Senecio vulgaris and Phleum pratense

Between-species patterns of covariation in plant size, seed size and fecundity in monocarpic herbs

Abstract Covariation in plant mass, seed mass and fecundity was investigated for 15 species of monocarpic herbs (annuals and biennials) harvested at final developmental stage from recently disturbed habitats. Above-ground vegetative mass and fecundity varied by over three orders of magnitude and seed mass varied by over two orders of magnitude across species. Eighty-eight percent of the variation in fecundity across species was explained by covariation in the other two characters as predictor variables in a least squares regression model; fecundity increased significantly and proportionately with increasing above-ground vegetative mass but decreased significantly and proportionately with increasing individual seed mass. Individual seed mass and above-ground vegetative mass were positively correlated across species when fecundity was held constant under partial correlation. The results indicate that none of the relationships between these characters are size-dependent, i.e., the traditional trade-off, predicted by life history theory, between seed size and fecundity is isometric across species, as is the increase in both fecundity and total seed mass with increasing vegetative mass. Allometric (size-dependent) relationships for these characters have been reported in previous studies where developmental stage was not controlled across species; the detection of isometric relationships in the present study may therefore be a consequence of measuring whole-genet, lifetime fecundity and above-ground vegetative mass for all study species. A general hypothesis is presented for the interpretation of covariation in plant size, seed size and fecundity across species in which the principal selection mechanism involves ‘time limitation’. The amount of time available for growth (before density-independent mortality, e.g., from disturbance) selects for the level of precocity necessary to reproduce before death. This affects the level of constraint on maximum attainable plant size, i.e., smaller when shorter-lived. This, in turn, affects reproductive output, which can be greater when longer-lived, expressed as greater fecundity and/or larger seed size. However, both cannot be maximized because of the inherent trade-off between seed size and fecundity for plants of a given size.

Death without sex—the ‘problem of the small’ and selection for reproductive economy in flowering plants

Most of the resident plants within vegetation fail to leave descendants because of death without sex—i.e. sexual reproduction fails (zero fecundity), primarily because of relatively small plant size. I propose that this ‘problem of the small’ represents one of the principal driving forces of evolution by natural selection, and that the main product of this selection is ‘reproductive economy’, manifested by several plant traits that are widely distributed among angiosperms: sexual maturity at a relatively young age and small size, relatively small seed size, selfing (including through mixed mating), and of particular interest here, clonality. In non-clonal species, an offspring develops from a zygote into a single ‘rooted unit’, i.e. a distinct vascular transition point between live shoot and root tissue. Clonal species can produce an indeterminate number of these rooted unit offspring asexually, all as products of a single zygote. Clonality is a common strategy in angiosperms because it confers a potential two-fold fitness benefit—especially in relatively small species—by promoting longevity of the zygote product, while at the same time providing a fecundity supplement (through asexual multiplication of rooted units), thereby allowing offspring production economically, i.e. without requiring large adult size, and without even requiring the fertilization of ovules. The primary fitness benefit from clonality, therefore, is that the somatic product of a zygote can effectively avoid an intrinsic limitation predicted for all non-clonal plants: the trade-off between longevity and the potential rate of offspring/descendant production. These major fitness benefits of clonality are explored in considering why clonality is less common in larger species, why the largest species (trees) generally do not have the longest-lived zygote product, and in re-assessing traditional and recent views concerning the loss of sex in clonal plants, the predicted trade-off between the size and number of clonal offspring, and the predicted trade-off between sexual and asexual reproduction.

Allometry and development in herbaceous plants: functional responses of meristem allocation to light and nutrient availability.

We examined the relationship between meristem allocation and plant size for four annual plant species: Arabidopsis thaliana, Arenaria serphyllifolia, Brassica rapa, and Chaenorrhinum minus. Gradients of light and nutrient availability were used to obtain a range of plant sizes for each of these species. Relative allocation to reproductive, inactive, and growth meristems were used to measure reproductive effort, apical dominance, and branching intensity, respectively. We measured allocation to each of these three meristem fates at weekly intervals throughout development and at final developmental stage. At all developmental stages reproductive effort and branching intensity tended to increase with increasing plant size (i.e., due to increasing resource availability) and apical dominance tended to decrease with increasing plant size. We interpret these responses as a strategy for plants to maximize fitness across a range of environments. In addition, significant differences in meristem response among species may be important in defining the range of habitats in which a species can exist and may help explain patterns of species competition and coexistence in habitats with variable resource availability.