Seeing the forest for the deer: Do reductions in deer-disturbance lead to forest recovery?

High levels of deer browsing can prevent canopy tree regeneration, but little is known about changes to forest size-structure following long-term deer herd reductions. We monitored changes in forest stand structure and composition in southwestern Ontario, Canada, over 28-years using permanent plots. Our study site was the largest remaining tract of Carolinian (deciduous) forest in Canada (11 km²), a habitat type that contains up to a fifth of Canada’s species at risk and is under intense anthropogenic pressures. We recorded declines in all tree size classes between 1981 and 1996, during which densities of white-tailed deer (Odocoileus virginianus) reached a peak of 55 deer km⁻². Despite significant and sustained deer herd reductions between 1996 and 2009, which reduced deer densities to 7 deer km⁻², there was limited recruitment of small trees and declines in basal area of tree species that were sensitive to deer browsing. Our results suggest that recovery from herbivory is a protracted process during which canopy tree regeneration may continue to decline despite a reduction in browsing pressure due to deer culling. Large declines in canopy-tree densities in Carolinian forests may lead to forest size-structures and herbaceous plant communities that resemble rare oak savanna habitat, creating difficult decisions for conservation managers aiming to protect rare and endangered species within native ecosystems. We recommend that managers protect Carolinian forest stands and encourage canopy tree regeneration by increasing seed sources of native trees. While deer control is essential in reducing forest damage, our results highlight the need to explore other forms of active management to expedite otherwise slow increases in tree density.     

Epigeic earthworms increase soil arthropod populations during first steps of decomposition of organic matter

Earthworms and soil arthropods are major groups involved in soil decomposition processes. Although the interaction between these organisms can influence decomposition rates, little is known about their population dynamics during the decomposition of organic matter. In this study, we used the pig manure decomposition process to evaluate the effects of the presence of the epigeic earthworm Eisenia fetida on seven groups of soil arthropods: springtails, astigmatid, prostigmatid, mesostigmatid and oribatid mites, psocids and spiders. We carried out an experiment in which low and high doses (1.5 and 3 kg, respectively) of pig manure were applied in consecutive layers to small-scale mesocosms with and without earthworms. The presence of E. fetida increased the overall number of soil arthropods regardless of the dose of manure applied. This result was mainly due to the presence of large populations of springtails and mesostigmatid mites. Springtails were more abundant in the new layers of the mesocosms, which indicated a preference for substrates with fresh organic matter and higher microbial biomass. The other arthropod groups were consistently favored by the presence of earthworms, but remained at low densities throughout the decomposition process. Only the psocids were negatively affected by the presence of E. fetida. These results suggest that the development of large populations of soil arthropods, mainly springtails, in the mesocosms with earthworms is a characteristic feature of the initial stages of the earthworm-driven decomposition process.     

Biodiversity in soil ecosystems: the role of energy flow and community stability

In a series of community food webs from native and agricultural soils, we modeled energetics and stability, and evaluated the role of the various groups of organisms and their interactions in energy flow and community stability. Species were aggregated into functional groups based on their trophic position in the food webs. Energy flow rates among the groups were calculated by a model using observations on population sizes, death rates, specific feeding preferences and energy conversion efficiencies. From the energetic organization of the communities we derived the strengths of the mutual effects among the populations. These interaction strengths were found to be patterned in a way that is important to community stability. The patterning consisted of the simultaneous occurrence of strong top down effects at lower trophic levels and strong bottom up effects at higher trophic levels. These patterns resulted directly from the empirical data we used to parameterize the model, as we found no stabilizing patterns with random but plausible parameter values. Also, the impact of each individual interaction on community stability was established. This analysis showed that some interactions had a relatively strong impact on stability, whereas other interactions had only a small impact. These impacts on stability were neither correlated with energy flow nor with interaction strength. Comparison of the seven food webs showed that these impacts were sometimes connected to particular groups of organisms involved in the interaction, but sometimes they were not, which might be due to different trophic positions in the food webs. We argue that future research should be directed to answer the question which energetic properties of the organisms form the basis of the patterning of the interaction strengths, as this would improve our understanding of the interrelationships between energetics, community stability, and hence the maintenance of biological diversity.     

Soil‐plant relationships,micronutrient contents,and cardenolide production in natural populations of Digitalis obscura

The production of secondary metabolites by plants growing in natural populations is conditioned by environmental factors. In the present study, we have investigated the relationships among soil properties, micronutrients in soils and plants, and cardenolide production from wild Digitalis obscura (Scrophulariaceae) populations. Young and mature leaves and soil samples were collected in ten different populations, corresponding to three Mediterranean bioclimatic belts (Thermo‐, Meso‐, and Supramediterranean belts). Soil (total and EDTA‐extractable) and leaf micronutrients (Fe, Mn, Zn, and Cu), and leaf cardenolide accumulation have been determined. Significant negative correlations were observed between Fe, Mn or Zn concentration in leaves and soil pH, as well as between Fe or Mn in leaves and carbonate content of soils. Only EDTA‐extractable Mn was significantly correlated with Mn content in the plants. With regard to cardenolide content in leaves, this parameter was negatively correlated with Zn_leaf in young leaves and with Mn_leaf in old leaves. Positively correlated, however, were Fe and cardenolide content in young leaves. The influence of environmental conditions and leaf micronutrient contents on cardenolide accumulation is discussed.     

Compositional nutrient diagnosis and main nutrient interactions in yellow pepper grown on desert calcareous soils

Mineral‐nutrient stress is one of the main factors limiting crop production, especially in arid lands. The mineral requirement of a crop is difficult to determine, and the interpretation of foliar chemistry composition is not easy. This study was conducted to compute the minimum yield target for fresh fruit of yellow pepper (Capsicum annuum L.) and the corresponding Compositional Nutrient Diagnosis (CND) as well as to identify significant nutrient interactions of this crop in desert calcareous soils. Preliminary CND norms were developed using a cumulative variance‐ratio function and the chi‐square distribution function. From a small database, we computed means and standard deviations of row‐centered log ratios, V_X, of five nutrients (N, P, K, Ca, and Mg) and a filling value, R, which comprises all nutrients not chemically analyzed and quantified them in 54 foliar samples of the popular yellow pepper cv. ‘Santa Fé’. This cultivar is widely grown in northwest Mexico under arid conditions. These norms are associated to fresh fruit yields higher than 15.04 t ha^–1. Principal‐component analyses, performed using estimated CND nutrient indexes, allowed us to identify four interactions: negative P‐Ca, P‐Mg, and N‐K, and positive Ca‐Mg. Pepper plants growing on calcareous soils tend to take up more Ca and Mg than considered as optimum in other soil conditions.     

CO2 and inorganic N supply modify competition for N between co-occurring grass plants, tree seedlings and soil microorganisms

Plant-plant and plant-soil interactions play a key role in determining plant community structure and ecosystem function. However, the effects of global change on the interplay between co-occurring plants and soil microbes in successional communities are poorly understood. In this study, we investigated competition for nitrogen (N) between soil microorganisms, grass plants and establishing tree seedlings under factorial carbon dioxide (CO₂) and N treatments. Fraxinus excelsior seedlings were germinated in the presence or absence of grass competition (Dactylis glomerata) at low (380 μmol mol⁻¹) or high (645 μmol mol⁻¹) CO₂ and at two levels of N nutrition in a mesocosm experiment. Pulse ¹⁵N labelling was used to examine N partitioning among plant and soil compartments. Dactylis exerted a strong negative effect on Fraxinus biomass, N capture and ¹⁵N recovery irrespective of N and CO₂ treatment. In contrast, the presence of Dactylis had a positive effect on the microbial N pool. Plant and soil responses to N treatment were of a greater magnitude compared with responses to elevated CO₂, but the pattern of Fraxinus- and microbial-N pool response to N and CO₂ varied depending on grass competition treatment. Within the Dactylis competition treatment, decreases in Fraxinus biomass in response to N were not mirrored by decreases in tree seedling N content, suggesting a shift from below- to above-ground competition. In the Dactylis-sown pots, ¹⁵N recovery could be ranked Dactylis > microbial pool > Fraxinus in all N and CO₂ treatment combinations. Inequalities between Fraxinus and soil microorganisms in terms of ¹⁵N recovery were exacerbated by N addition. Contrary to expectations, elevated CO₂ did not increase plant-microbe competition. Nevertheless, microbial ¹⁵N recovery showed a small positive increase in the high CO₂ treatment. Overall, elevated CO₂ and N supply did not interact on plant/soil N partitioning. Our data suggest that the competitive balance between establishing tree seedlings and grass plants in an undisturbed sward is relatively insensitive to CO₂ or N-induced modifications in N competition between plant and soil compartments.     

Effect of continuous and alternate water regimes on methane efflux from rice under greenhouse conditions

In a greenhouse study, the effect of moisture regimes (continuously flooded, continuously nonflooded, alternately flooded) on methane efflux from an alluvial soil planted to rice was studied using the closed chamber method. Methane efflux was almost 10 times more pronounced under continuously flooded conditions than under continuously nonflooded conditions. Intermittently flooded regimes (alternately flooded and nonflooded cycles of 40 or 20 days each) emitted distinctly less methane than the continuously flooded system. A significant negative correlation was found between methane emission under different water regimes and rhizosphere redox potential. Extractable Fe²⁺, readily mineralizable carbon (RMC) and root biomass presented a significant positive correlation with cumulative methane emission. The correlation of methane emission with other plant parameters and microbial biomass was not significant. Our results further suggest the possibility of reduced methane emissions through appropriate water management in a rainfed rice ecosystem. Received: 4 June 1996     

Oribatid mite community structure and tree species diversity: A link?

Differences in tree species may lead to contrasting soil environments via differences in litter chemical quality and physical environmental factors, such as soil type and soil moisture. However, separating the effects of litter quality and physical environment is difficult under field conditions. Both litter quality and soil environment affect the species composition of the soil animal community. A diversity gradient of canopy tree species (11–25 species) located on homogeneous soil substrate at Tomakomai Experimental Forest of Hokkaido University was used to analyse the relationship between tree species diversity and oribatid mite community structure. Soil samples were collected from three levels of tree species richness (high, intermediate and low) with three replicates each, in July 2000. Leaf area index (LAI) was positively correlated with tree species diversity suggesting higher litter input into the soils with increasing tree diversity. However, the tree species diversity gradient affected neither accumulation of litter on the forest floor nor abundance and species richness of oribatid mites. Canopy and understory plant species richness, LAI, total soil carbon and biomass of epigeic and endogeic earthworms did not significantly affect mite community structure as indicated by redundancy analysis (RDA). The results suggest that oribatid mite community structure is minimally affected by tree species diversity and associated changes in litter diversity.     

Early decomposer assemblages of soil organisms in litterbags with vetch and rye roots

The assemblages of microbial (bacteria and fungi), microfaunal (protozoa and nematodes) and mesofaunal (microarthropods) populations were studied in decomposing root residues from hairy vetch (Vicia villosa Roth) and rye (Secale cereale L.) in a litterbag field experiment. Litterbags containing vetch or rye root residues were buried in soil at the same day as either vetch or rye winter catch crops were incorporated into the field soil from which the materials were gathered. The litterbags were sampled after 6 weeks in the field. In vetch, bacterial and fungal biomasses were similar whereas fungi dominated microbial biomass in rye. The biomass of the bacterial consuming fauna dominated by nematodes and microarthropods was similar to the biomass of bacteria in vetch as opposed to in rye where bacterivore biomass was lower than bacterial biomass. This suggests a much higher bacterial production in vetch compared to rye. Furthermore, in vetch dauer larvae of bacteria feeding nematodes prevailed, which is also a sign of high bacterial production followed by food shortage for the bacterivores. Bacterivorous and predatory nematodes with capability of consuming protozoa showed an inverse relationship to flagellated protozoa. This suggests that these nematodes controlled the protozoan biomass constituting a lower fraction of the bacterivore biomass in vetch compared to in rye. Such intraguild predator-prey relationship is therefore indicated for microbivorous organisms among bacterivorous and predatory nematodes (the intraguild predator) protozoa (the intraguild prey) and bacteria (the common prey). The much higher fungal biomass in rye than in vetch litterbags was not reflected in the biomass of the fungal feeders. Due to the generally lower intrinsic rate of increase of the fungivores, as well as of the omnivores and predators, in comparison with the bacterial feeders, they were not able to generate dense populations at this early stage of decomposition.