Bill Shipley

The global spectrum of plant form and function

Earth is home to a remarkable diversity of plant forms and life histories, yet comparatively few essential trait combinations have proved evolutionarily viable in today’s terrestrial biosphere. By analysing worldwide variation in six major traits critical to growth, survival and reproduction within the largest sample of vascular plant species ever compiled, we found that occupancy of six-dimensional trait space is strongly concentrated, indicating coordination and trade-offs. Three-quarters of trait variation is captured in a two-dimensional global spectrum of plant form and function. One major dimension within this plane reflects the size of whole plants and their parts; the other represents the leaf economics spectrum, which balances leaf construction costs against growth potential. The global plant trait spectrum provides a backdrop for elucidating constraints on evolution, for functionally qualifying species and ecosystems, and for improving models that predict future vegetation based on continuous variation in plant form and function.

Confirmatory path analysis in a generalized multilevel context

This paper describes how to test, and potentially falsify, a multivariate causal hypothesis involving only observed variables (i.e., a path analysis) when the data have a hierarchical or multilevel structure, when different variables are potentially defined at different levels of such a hierarchy, and when different variables have different sampling distributions. The test is a generalization of Shipleys d-sep test and can be conducted using standard statistical programs capable of fitting generalized mixed models.

Competitive hierarchies in herbaceous plant communities

Using data from eight published competition experiments with diallel designs, and a new method of analyzing such data, we present evidence that competition among herbaceous plants has two general properties: pairwise interactions are predominantly asymmetric, and competitive hierarchies exist, i.e. competitive networks are primarily transitive. This empirical evidence is compared with what is known about the mechanisms of plant competition. Finally we present an hypothesis to explain some of the variability in the degree to which competitive networks display asymmetry and transitivity, and explore some of the consequences of this in terms of developing a predictive theory of plant competition.

Abiotic drivers and plant traits explain landscape-scale patterns in soil microbial communities

The controls on aboveground community composition and diversity have been extensively studied, but our understanding of the drivers of belowground microbial communities is relatively lacking, despite their importance for ecosystem functioning. In this study, we fitted statistical models to explain landscape-scale variation in soil microbial community composition using data from 180 sites covering a broad range of grassland types, soil and climatic conditions in England. We found that variation in soil microbial communities was explained by abiotic factors like climate, pH and soil properties. Biotic factors, namely community-weighted means (CWM) of plant functional traits, also explained variation in soil microbial communities. In particular, more bacterial-dominated microbial communities were associated with exploitative plant traits versus fungal-dominated communities with resource-conservative traits, showing that plant functional traits and soil microbial communities are closely related at the landscape scale.

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.

THE ALLOMETRY OF SEED PRODUCTION IN HERBACEOUS ANGIOSPERMS

The number of seeds produced per ramet of 285 ramets of 57 species of herbaceous angiosperms in 1 yr was regressed on aboveground vegetative weight and average individual seed weight, as well as on categorical variables indicating minimum time to reproduction, growth form, occurrence in disturbed habitats, and phylogeny. Seed number per ramet (Ns) was weakly correlated to average seed weight (Wa, r2 = 0.36) and to vegetative ramet weight (Wv, r2 = 0.38), but a combination of these two variables accounted for 81% of the variance in seed number. The allometric equation was Ns = 1.4 Wv 0.93Wa -0.78. The main source of variation was interspecific. The nonphylogenetic categorical variables accounted for little of the residual variance, with occurrence in disturbed habitats being the most important. This indicates that most of the variation in seed output among plants of different habitats or successional status is due to size-related effects.

Structured interspecific determinants of specific leaf area in 34 species of herbaceous angiosperms

1. Specific leaf area is a variable implicated in a number of functional aspects of plant ecology, including gas exchange, relative growth rate and palatability. 2. Six leaf attributes (water content, lamina thickness, protruding vein thickness, dry weight, lamina area and specific leaf area) were determined on a collection of 572 leaves. The data set was derived from 194 individual plants, 34 species, 32 genera, 18 families and both classes of angiosperms. All species except one (a shrub) were herbaceous. All species were from sunny habitats. 3. Structured relationships between these six variables were obtained. These relationships consisted of path analyses, multiple regressions and major axis (allometric) regressions. 4. There was an interspecific allometric trend for leaf dry weight to increase more rapidly than leaf surface area, resulting in larger leaves having a lower specific leaf area. This trend did not exist at an intraspecific level. 5. Path analysis showed that the most important direct effect on specific leaf area was the water content of the leaf. Increasing water content resulted in a larger specific leaf area. The direct effect of lamina thickness was to decrease specific leaf area but lamina thickness itself was affected by leaf water content. 6. These effects on specific leaf area were mediated through variation in leaf dry weight. Leaf surface area appeared to be independent of the other five variables. 7. These results help to explain two unresolved general patterns in plant comparative ecology: (1) why net photosynthetic rate is not positively correlated with maximum relative growth rates and (2) why there is a negative correlation between maximum relative growth rate and seed size

Regeneration and Establishment Strategies of Emergent Macrophytes

The questions are; Are the traits possessed by juveniles (seed/seedlings) of an assemblage of twenty-five emergent macrophyte species independent of the traits possessed by adults of these species? What are the patterns of covariation among twenty morphological traits of these species of widely different taxonomy? Is there any predictable relationships between these traits and the environments in which the species occur?

The relationship between relative growth rate and sensitivity to nutrient stress in twenty-eight species of emergent macrophytes

(1) Stress sensitivity is defined as the decrease in the relative growth rate of a species relative to a decline in resource supply. This definition is used to test the hypothesis of a trade-off between maximum relative growth rates and sensitivity to stress. (2) The data used to test this hypothesis were collected from twenty-eight species of emergent macrophytes, occurring in north-eastern North America. Seedlings of these species were grown from seed in washed sand in a glasshouse in two nutrient conditions: full strength Hoaglands solution and one-tenth strength Hoaglands solution. Relative growth rates were determined from seedlings aged ten to thirty days old. (3) There was no correlation between relative growth rate in the full strength Hoaglands solution and relative growth rate in the one-tenth Hoaglands solution (r = 029, P> 0 1). (4) This resulted in a strong positive correlation between relative growth rate of these species in full strength Hoaglands solution and their sensitivity to decreasing nutrient supply (stress) (r=080, P<0 001), indicating that those species whose relative growth rate was highest in the more fertile conditions had their relative growth rate most depressed in the less fertile conditions. (5) This relationship held even on a proportional basis, such that the proportional decrease in relative growth rate was correlated with sensitivity to nutrient stress (r = 0 55, P<0 1).

Germination Responses of 64 Wetland Species in Relation to Seed Size, Minimum Time to Reproduction and Seedling Relative Growth Rate

Three germination attributes (lag time, maximum germination rate, and final germination proportion) were measured for 64 species of herbaceous wetland plants. The environmental conditions approximated the drawdown environment known to stimulate germination in wetland plants: a period of cold stratification followed by position of the seed on the surface of wet, but not inundated, substrate in the presence of light and with a 20/300C daily temperature cycle. Correlations were sought between the three germination attributes and average individual seed weight, seedling relative growth rate and a categorical variable indicating miminum time to reproduction (annuals, facultative annuals and obligate perennials). Average seed weight was not correlated with any of the three germination attributes. Seedling relative growth rate was negatively correlated with time to initiation of germination. Species capable of setting seed their first year (annuals and facultative annuals) initiated germination sooner, a larger proportion germinated per day once germination began, and a larger proportion of seeds had germinated by the end of the experiment in comparison with species which require more than 1 year to set seed (obligate perennials). A discriminant analysis showed that the time to initiation of germination could accurately classify 89% of the perennial species as being either facultative annuals or obligate perennials. Key-words: Annuals, drawdown, facultative annuals, germination, perennials, plant strategies, regeneration, seed size, wetlands