Effects of species evenness can be derived from species richness – ecosystem functioning relationships

Although species richness effects on ecosystem functioning have been studied thoroughly in countless experiments, the effects of the other side of diversity – species evenness – remain less identified, especially at high species richness. Due to the large number of different model ecosystems that need to be created, the explanatory power of the experimental approach for evenness is indeed limited. We show here that experimental studies on the influence of species richness on ecosystem functions contain hidden information on the influence of species evenness. Both the effects of maximum and minimum evenness, and of a key set of intermediate evenness levels, can be derived from species richness – ecosystem function curves, and that for every richness level, by using communities with low species richness as the equivalent of highly uneven communities with higher richness. We show that evenness effects on ecosystem functioning have the same direction as richness effects, however with increasing effect sizes at higher richness levels. We validated our technique for a wide range of ecosystem functions and applied it to the species richness – community biomass data from an existing biodiversity experiment. Our approach could provide a fast and easy alternative to resource‐intensive experiments in which evenness is experimentally varied, as we can build on the elaborate existing literature on species richness to assess its effects.     

How important are species richness,species evenness and interspecific differences to productivity? A mathematical model

We present a theoretical model to quantify the influence of diversity on productivity and nutrient acquisition in plant communities during exponential growth. The model fractionates diversity into three components, namely species richness ( S ), species evenness ( E ) and the degree of difference between species ( D ). The influence of each of these components is assessed individually: S is varied by changing the number of species, E by changing their population size, and D by changing the range of species traits critical to productivity (specific nutrient uptake rate, Σ _r , or nutrient use efficiency, NUE ). D was quantified as the coefficient of variation of Σ _r or NUE . All three components of diversity enhance the biomass and nutrient stocks in the community, but the response patterns are different. Species richness has a saturating influence, whereas effects of E and D are linear and exponential, respectively. In all cases the non-linear dependence of productivity and nutrient acquisition on Σ _r and NUE during exponential growth was the single mechanism underlying these effects. This causes the presence of plants with extreme traits to promote productivity, and S , E and D all affect the abundance and/or intensity of these extremes. The model offers a framework to explain differences between experimental observations, and suggests a concept of diversity where S and E are structural components and D a qualitative or functional component, which modulates the influence of the two others. We propose to explicitly recognise D as an integral constituent of plant diversity in future studies.     

Effects of genotype and temperature on pollen tube growth in perennial ryegrass (Lolium perenne L.)

Summary Low yield in seed crops of perennial ryegrass is related to low fertilization efficiency and low temperature during anthesis. To study the effect of genotype and temperature on pollen performance, we conducted greenhouse experiments at controlled temperatures. Individual florets of four genotypes that are known to differ in seed production were hand pollinated at four temperatures (14°, 18°, 22°, 26° C) both in vivo and via a semiin-vitro method involving excised florets on agar. Pollen germination and tube growth were determined with UV-fluorescence microscopy and scored in six classes at 2 h after pollination in vitro and after 0.5, 2 and 5 h in vivo. In vitro, both genotype and temperature had a significant effect on the performance of self-pollen. Pollen tube growth increased with temperature. In cross-pollinations, the pistil parent had a significant effect on pollen tube growth, and there was also a significant pistil-by-temperature interaction. In vivo, genotype and temperature significantly affected pollen performance. The genotype-by-temperature interaction was only significant 5 h after pollination. One genotype with low seed yield was pseudoself-compatible and was a relatively poor mother after cross-pollination. The effects of genotype and temperature on the growth of self-pollen might be exploited in a breeding programme.A.G. Stephenson was on a sabbattical leave at SVP in 1987     

Shielded environments reduce stress in alien Asteraceae species during hot and dry summers along urban‐to‐rural gradients

Urban environments often host a greater abundance and diversity of alien plant species than rural areas. This is frequently linked to higher disturbance and propagule pressure, but could also be related to the additional establishment of species from warmer native ranges in cities, facilitated by the latter''s higher air temperatures and drier soils. A hitherto unresolved question is how stressful the urban environments become during climate extremes such as heatwaves and droughts. Do such episodes still favor alien plant species, or set them back? We used in situ measured phenotypic leaf and development trait responses of the six most widespread alien Asteraceae species from various native climates along Belgian urban‐to‐rural gradients, measured during two unusually warm and dry summers. Urbanization was characterized by three factors: the percentage of artificially sealed surfaces (urbanity, measured at three spatial scales from in situ to satellite‐based), the vegetation cover and the sky view factor (SVF, fraction of the hemisphere not blocked by buildings or vegetation). Across species, either from colder or warmer native climates, we found a predominant protective effect of shielded environments that block solar radiation (low SVF) along the entire urban‐to‐rural gradient. Such environments induced lower leaf anthocyanins and flavonols indices, indicating heat stress mitigation. Shielded environments also increased specific leaf area (SLA), a typical shade response. We found that vegetated areas had a secondary importance, increasing the chlorophyll content and decreasing the flavonols index, but these effects were not consistent across species. Finally, urbanity at the organism spatial scale decreased plant height, while broader‐scale urbanity had no significant influence. Our results suggest that sealed surfaces constrain alien Asteraceae during unusually warm and dry summers, while shielded environments protect them, possibly canceling out the lack of light. These findings shed new light on alien plant species success along urban‐to‐rural gradients in a changing climate.     

Mountain roads shift native and non‐native plant species' ranges

Roads are known to act as corridors for dispersal of plant species. With their variable microclimate, role as corridors for species movement and reoccurring disturbance events, they show several characteristics that might influence range dynamics of both native and non‐native species. Previous research on plant species ranges in mountains however seldom included the effects of roads. To study how ranges of native and non‐native species differ between roads and adjacent vegetation, we used a global dataset of plant species composition along mountain roads. We compared average elevation and range width of species, and used generalized linear mixed models (GLMMs) to compile their range optimum and amplitude. We then explored differences between roadside and adjacent plots based on a species’ origin (native vs non‐native) and nitrogen and temperature affinity. Most non‐native species had on average higher elevational ranges and broader amplitudes in roadsides. Higher optima for non‐native species were associated with high nitrogen and temperature affinity. While lowland native species showed patterns comparable to those in non‐native species, highland native species had significantly lower elevational ranges in roadsides compared to the adjacent vegetation. We conclude that roadsides indeed change the elevational ranges of a variety of species. These changes are not limited to the expansion of non‐native species along mountain roads, but also include both upward and downward changes in ranges of native species. Roadsides may thus facilitate upward range shifts, for instance related to climate change, and they could serve as corridors to facilitate migration of alpine species between adjacent high‐elevation areas. We recommend including the effects of mountain roads in species distribution models to fine‐tune the predictions of range changes in a warming climate.     

Are heat and cold resistance of arctic species affected by successive extreme temperature events?

Extreme temperature events are projected to increase in frequency in a future climate. As successive extremes could occur more frequently, patches of vulnerable tundra vegetation were exposed to two consecutive heat waves (HWs) of 10 d each, with a 5-d recovery period in between. Surface temperatures during the HWs were increased approximately 6 degrees C using infrared irradiation sources. In three of the four target species (Pyrola grandiflora, Polygonum viviparum and Carex bigelowii), plant conditions improved upon the first exposure. Depending on species, leaf relative growth, leaf chlorophyll content or maximal photochemical efficiency was increased. In P. grandiflora the positive effects of the heat on the photosynthetic apparatus led to augmented net photosynthesis. By contrast, Salix arctica responded mainly negatively, indicating species-specific responses. During the second HW, leaf mortality suddenly increased, indicating that the heat stress induced by the extreme events lasted too long and negatively influenced the species resistance to high temperature. After the HWs, when plants were exposed to (low) ambient temperatures again, plant performance deteriorated further, indicating possible loss of cold resistance.     

Effects of resource availability on the trade-off between seed and vegetative reproduction

Aims To explore whether the trade-off between seed and vegetative reproductive modes is flexible in environments with different amounts of available resources to maintain optimal behaviors.Methods A transition matrix model was established to determine the optimal trade-off between seed and vegetative reproduction in resources–variable habitats.Important findings The model predicts that plants allocate more resources to seed reproduction when available resources are scarce. With increasing resources, more vegetative propagules are produced. However, if resources keep increasing to a harmful level, plants would switch to seeds again.