John Markham

Herbivore-like damage induces increased strength and toughness in a seaweed

The seaweed Ascophyllum nodosum responds to simulated herbivore damage with an induced increase in tensile strength and toughness (force and work required for breakage), demonstrating a capability in plants for an induced response that reduces the probability of breakage. These properties are especially valuable because Ascophyllum is prone to losing much more tissue indirectly owing to herbivore-caused breakage than directly as a result of consumption by herbivores. Furthermore, the amount of past damage to Ascophyllum fronds is an indication of the probability of future damage, favouring an induced response in particular. In contrast, this damage induces a decrease in potential chemical feeding deterrents and an increase in edibility. Plant responses to herbivory appear to be complex and may reflect trade-offs between the costs of different kinds of responses.

Herbivory and harvesting: Effects on sexual recruitment and vegetative modules of Ascophyllum nodosum

Abstract Populations of Ascophyllum nodosum (Phaeophyta) have the potential to grow from either sexual recruits or by vegetative propagation of modules (i.e., fronds). In stands that are harvested in southwestern Nova Scotia there are two major potential regulators of these sources of natality, herbivores and commercial harvesting activities. It was shown here that herbivory has a major effect on sexual recruitment. Nearly all of the zygotes (99.9 %) which settled naturally in 1989 were consumed within one year. Slow growth of the survivors and continued herbivore pressure indicated that remaining germlings would not survive in subsequent years. Our results also suggested that harvesting may enhance the production of zygotes but this effect may not result in increased recruitment as most germlings do not survive grazing. Grazing did not affect module survivorship or breakage to any major extent. Hence our study suggested that increased size represents an escape from herbivory. Because sexual reproduction ...

Reciprocal interactions between plants and soil in an upland grassland

Through the production of litter, plants with different life history strategies are predicted to both affect and be affected by the properties of soil. Competitive species are expected to increase the fertility of, and have a positive growth feedback with, soil, whereas stress-tolerant species should decrease fertility but show no growth feedback. We maintained monocultures of competitive (Lolium perenne and Agrostis capillaris) and stress-tolerant (Festuca ovina and Nardus stricta) grasses on an unproductive grassland for six years. The Nardus soil developed significantly greater inorganic nitrogen than the Agrostis and Festuca soil, and significantly greater soil moisture content than the Festuca soil. However, there were no differences in organic matter content, phosphate or bulk density between the soil types. In a greenhouse assay, each species was grown in soil cores from the different monocultures as well as natural turf. There were significant differences in growth between plant species and soil types. As expected, L. perenne produced the greatest amount of biomass. However, plants grown on Nardus soil were twice as large and had a 21% lower root allocation than plants grown on any of the other soil types. Lolium perenne, A. capillaris and F. ovina had significant negative growth feedbacks with their own soil (−0.460, −0.821 and −0.792, respectively) and N. stricta had a significant positive feedback (0.560). This study highlights the difficulties of predicting how plant traits will affect soil properties.

Alnus rubra (Bong.) nodule spore type distribution in southwestern British Columbia

In red alder (Alnus rubra Bong.) the nitrogen fixing actinomycete Frankia can be divided into strains which form spores (sp+) within nodules and those that do not. Red alder nodules were collect from 42 sites in southwestern British Columbia and their spore type was determined. Sp+ nodules were found in 18 of the sites, where their proportion ranged from 4 to 93%. The distribution of sites with sp+ nodules was clumped, with significant autocorrelation at the 0–10 km interval. No sp+ nodules were found in any of the eight stands in the submaritime region, indicating a possible continentality effect on spore type distribution. There was a significant positive association between the presence of Rubus spectabilis and the presence sp+ nodules. Stand and soil chemical variables were generally poor predictors of the proportion of sp+ nodules, although total soil nitrogen and exchangeable phosphorus accounted for 36.4 and 28.5% respectively, of the variation in the proportion of sp+ nodules on sites where they were present. There was also a small but significant negative relation between soil pH and the proportion of sp+ nodules and young stands generally did not have any sp+ nodules.

Alnus rubra nodulation capacity of soil under five species from harvested forest sites in coastal British Columbia

Nodulation of Alnus rubra seedlings after inoculation with soil from under A. rubra, Betula papyrifera. Rubus lacianutus, R. spectabilis, and R.ursinus on 2 recently harvested sites was compared. Nodulation capacity was low compared to other published reports, ranging from 0 to 18.9 infective units cm-3 of soil and was significantly affected by the site and plant species. Nodulation capacity of soil under alder was significantly higher than under all other species except R. spectabilis, regardless of site. The lowest nodulation capacity was found in soil under B. papyrifera.

Arctic foxes as ecosystem engineers: increased soil nutrients lead to increased plant productivity on fox dens

Top predators can provide fundamental ecosystem services such as nutrient cycling, and their impact can be even greater in environments with low nutrients and productivity, such as Arctic tundra. We estimated the effects of Arctic fox (Vulpes lagopus) denning on soil nutrient dynamics and vegetation production near Churchill, Manitoba in June and August 2014. Soils from fox dens contained higher nutrient levels in June (71% more inorganic nitrogen, 1195% more extractable phosphorous) and in August (242% more inorganic nitrogen, 191% more extractable phosphorous) than adjacent control sites. Inorganic nitrogen levels decreased from June to August on both dens and controls, whereas extractable phosphorous increased. Pup production the previous year, which should enhance nutrient deposition (from urine, feces, and decomposing prey), did not affect soil nutrient concentrations, suggesting the impact of Arctic foxes persists >1 year. Dens supported 2.8 times greater vegetation biomass in August, but δ15N values in sea lyme grass (Leymus mollis) were unaffected by denning. By concentrating nutrients on dens Arctic foxes enhance nutrient cycling as an ecosystem service and thus engineer Arctic ecosystems on local scales. The enhanced productivity in patches on the landscape could subsequently affect plant diversity and the dispersion of herbivores on the tundra.

Effect of neighbour presence and soil volume on the growth of Andropogon gerardii Vitman

Background: Theory predicts that plants can reduce their fitness in the presence of neighbours by allocating resources to root growth, in order to pre-empt resource capture. A number of studies that have tested this idea have done so by using experiments where neighbour presence is confounded with soil volume. Aims : To avoid confounding effects of neighbour presence and soil volume we adjusted these variables independently from one another. Methods: We grew Andropogon gerardii with and without neighbours, holding soil volume available to each plant constant, and compared plant performance with a treatment where both neighbour presence and soil volume were varied. We also grew plants with a quarter of the soil volume but four times the nutrient concentration to determine if changes in plant growth in response to soil volume are caused by access different levels of soil resources. Results: We found no evidence that plants adjust root growth to the presence of neighbour roots alone. We did, however, find a significant reduction in plant growth when soil volume was reduced. The reduction was overcome by increasing nutrient concentrations in the growth media. Conclusions: Our results suggest the effects of soil volume on plant growth are mainly due to changes in nutrient availability.

Rare species occupy uncommon niches

The fact that temperate grasslands often contain upwards of 30 vascular plant species per m2 yet these species seem to have relatively similar life histories and resource requirements has made explaining species coexistence in these communities a major focus of research. While the reduction of competition by disturbance has been a popular explanation for species coexistence, in tallgrass prairies any level of disturbance either has no effect, or decreases diversity, since it favors the dominant plants. Although there has long been speculation that grassland species could coexist by niche partitioning the concept received renewed interest when it was shown that soil hydrology could explain species coexistence. One aspect of community structure that has not been explained by niche partitioning is the rareness and commonness of species within communities. There are three classes of explanations for rareness: narrow habitat requirements, low competitive ability combined with frequency dependent fitness and, dispersal ability. However, evidence for these explanations tend to be anecdotal, focusing on particular species. Here I show that in tallgrass prairies common and rare species consistently occupy different parts of niche space, with rare species being restricted by the cover of common species and occupying the rare available niches.

Stand- and plot-level changes in a boreal forest understory community following wildfire

Background: Boreal forest understory plant communities are known to be resilient to fire – the species composition of stands after a fire is quite similar to the pre-fire composition. However, we know little about recovery of individual plants within particular locations in forest stands (i.e. plot-level changes) since we usually do not have pre-fire data for plots. Aims: We wanted to determine whether species recruited into the same or different locations in a Pinus banksiana stand that experienced a severe wildfire. Methods: We used pre-existing permanent plots to evaluate the cover of understory after an unplanned wildfire. Results: Across the entire stand nine of 47 species showed a significant change in cover. The largest change in a plant functional group was in the mosses, with all species present before fire being eliminated. There was no change in species diversity or total cover. At the plot level, species composition showed a much greater change. An average of 47% of the species present in a plot before the fire were absent in the same plot after the fire, and the total species turnover in plots was 88% of the species present before the fire. The plots showed a similar shift in species composition. Conclusions: These results confirm that boreal forest communities show a high degree of resilience to fire, but within a forest stand species will be found in different locations following fire, potentially exposing them to a different set of biotic and abiotic conditions in these new locations.

Biotic and Abiotic Effects of Remnant and Restoration Soils on the Performance of Tallgrass Prairie Species

Interactions between plants and soil microbes are increasingly recognized as an important component in the functioning of ecosystems. Because these interactions affect and are affected by soil abiotic conditions, restoration efforts must consider the interactions between the plant community, the soil community, and the soil abiotic conditions. We sampled soil from 20 independently restored tallgrass prairies and 8 natural prairie remnants in southern Manitoba. Soils from the restored sites had 4.5 times higher phosphate levels than soils from the remnants. In whole soil assays, big bluestem (Andropogon gerardii ) and Culver’s root (Veronicastrum virginicum) had significantly greater growth in soil from the remnant sites. A second growth assay using sterile and inoculated soil from a subset of these sites showed that while big bluestem benefited from soil biota on both remnants and restored sites, the effect was twice as strong on the remnant sites. Our results suggest that plants on restored prairies are less reliant on soil microbes due to the higher fertility found within their soils. Our data suggests that like other ecosystems, residual high fertility in tallgrass prairies may facilitate invasion by non-native plants.