Decline In Plant Diversity Research Articles

Gains in native species promote biotic homogenization over four decades in a human‐dominated landscape

Summary A long‐term perspective is needed to understand how disturbance is affecting plant communities in human‐dominated landscapes. Increased human disturbance often results in declining local native species richness, gains in exotic species and a decline in beta diversity. However, it is far from certain whether a general decline in plant diversity is occurring across all disturbed landscapes, and knowledge gaps remain concerning how the spread of exotic species influences beta diversity over long time‐scales. We resurveyed 184 vegetation plots in three broad vegetation types on southern Vancouver Island, Canada, originally surveyed in the late 1960s. This landscape has experienced a high degree of human disturbance over the past 40 years due to urbanization. We examined changes in total diversity, local diversity and beta diversity over time. We also compiled information on the traits of each species and tested for correlations between traits and plant species success over four decades. We found striking increases in local and total plant species richness driven by both native and exotic species. The most successful species tended to be exotic, disturbance tolerant, shade tolerant and shrubs. Biotic homogenization occurred, but not as a result of exotic species colonization, instead being significantly correlated with gains in native species. The loss in beta diversity has resulted in a shrinking of the gradient of vegetation types, blurring the distinction between them. Synthesis. Our study shows that human‐mediated disturbance is the dominant driver of plant community changes, but the net result has actually been an increase in richness, for each plot and for all plots pooled, and for both natives and exotics, despite a decline in variability among plant communities on the landscape. Contrary to conventional definitions of biotic homogenization, this decline in beta diversity was not correlated with the spread of exotic species, but with the colonization of common, disturbance‐tolerant natives.

Simulated N deposition negatively impacts sugar maple regeneration in a northern hardwood ecosystem

Summary1. During the next century, atmospheric nitrogen (N) deposition is projected to more than double, potentially leading to a decline in plant diversity as well as a change in plant community composition and structure.2. In a decade‐long field experiment, simulated atmospheric N deposition has slowed litter decay, resulting in an accumulation of forest floor (i.e. Oi & Oe horizons). We reasoned that a greater forest floor mass under simulated N deposition would impose a physical barrier to sugar maple Acer saccharum seedling establishment, thereby reducing seedling populations of an ecologically and economically important tree species.3. To test this idea, we first quantified sugar maple seedling abundance in replicate northern hardwood forest stands receiving ambient atmospheric N (7–12 kg N ha−1 year−1) and experimental atmospheric N deposition, simulating future amounts in eastern North America (ambient plus 30 kg NO3− N ha−1 year−1). Then, we experimentally manipulated forest floor mass under ambient and simulated N deposition treatments. Finally, we transplanted first‐year established seedlings into areas receiving ambient and simulated N deposition and quantified their mortality after 1 year.4. First‐year seedling abundance did not differ under ambient and simulated N deposition; however, there were greater abundances of second‐ and third‐to‐fifth‐year seedlings under ambient N deposition (P < 0·001). In all cases, experimental manipulation to increase forest floor mass, equivalent to that under simulated N deposition, resulted in significantly (P = 0·001) fewer established individuals, regardless of whether the greater forest floor mass occurred under ambient or simulated N deposition. Finally, fewer 1‐year‐old transplanted seedlings survived when grown under simulated N, albeit that result was not statistically significant.5. Synthesis and applications. The slowing of decay and the accumulation of forest floor under anthropogenic N deposition can negatively impact seedling survival and potentially alter stand development and structural diversity. As atmospheric N deposition increases globally, it becomes necessary to understand the mechanisms that lead to population changes for ecologically important tree species. The responses we document should be considered in simulations of future of forest’dynamics, as atmospheric N deposition continues to increase, specifically when sugar maple life‐history traits are included to simulate regeneration, structural diversity and stand development.

Plant community impacts on the structure of earthworm communities depend on season and change with time

Declining plant diversity potentially threatens essential ecosystem functions driven by the decomposer community, such as litter decomposition and nutrient cycling. Currently, there is no consensus on the interrelationships between plant diversity and decomposer performance and previous studies highlighted the urgent need for long-term experiments. In the Jena Experiment we investigated the long-term impacts of plant community characteristics on the structure of earthworm communities representing key decomposers in temperate grassland. We repeatedly sampled plots varying in plant species richness (1–16 species), plant functional group richness (1–4 groups), and presence of certain plant functional groups (grasses and legumes) three, four, and six years after establishment of the experiment in spring and autumn. The results show that earthworm performance is essentially driven by the presence of certain plant functional groups via a variety of mechanisms. Plant productivity (root biomass) explained most of the detrimental grass impacts (decrease in earthworm performance), while beneficial legume effects likely were linked to high quality inputs of plant residues (increase in earthworm performance). These impacts depended on the functional group of earthworms with the strongest effects on surface feeding anecic earthworms and minor effects on soil feeding endogeic species. Remarkably, effects of plant community characteristics on the composition and age structure of earthworm communities varied between seasons. Moreover, plant diversity effects reported by a former study decreased and detrimental effects of grasses increased with time. The results indicate that plant community characteristics, such as declining diversity, indeed affect the structure of earthworm communities; however, loss of key plant functional groups is likely to be more important than plant species number per se. However, in frequently disturbed ecosystems plant species richness might be important for the recovery and resilience of belowground functions. Moreover, the results accentuate the importance of long-term repeated measurements to fully appreciate the impacts of plant community composition and diversity on ecosystem properties. Single point observations may be misleading and potentially mask the complexity of above-belowground interrelationships.

Long-term changes in an oak forest's woody understory and herb layer with repeated burning1

Although fire exclusion is thought to be linked with declining plant diversity in oak forests, few studies have examined long-term changes in their shrub and ground layers resulting from repeated burning. In this study, we compare the composition and structure of woody understory and ground layer vegetation in burned and unburned oak forest after 17 years of annual dormant season low-intensity burns. Over time, burned forest had 97% reduction of shrubs and small saplings, but only 38% loss of stems in the > 5–10 cm size class. Canopy openness was similar in burned and unburned forest plots prior to the onset of burning, but it was significantly greater in burned forest after 17 years of fire. Ground layer vegetation structure also changed significantly, with responses differing by guilds. Spring herbs were the dominant guild before burning and did not change over time. However, cover and abundance of summer herbs increased over time in burned forest, probably in response to greater light assimilation under the more open canopy. This resulted in greater overall species richness in burned plots without loss of the spring herbs. Burning eliminated most alien shrubs, although common buckthorn persisted in small numbers. The alien herb garlic mustard also persisted and had greater abundance in burned plots, apparently by re-colonizing from unburned micro-habitats and adjacent forest. These results indicate that long-term burning can eliminate shrub and small sapling canopy cover, thereby increasing canopy openness and promoting greater richness and cover of summer forbs. Fire also probably had a positive effect on seedling establishment through removal of litter. Resulting tradeoffs to this gain in diversity include loss of native vines, shrubs, understory trees and forest interior bird habitat, as well as persistence of alien plants.

Underlying effects of spatial aggregation (clumping) in relationships between plant diversity and resource uptake

In the debate on the influence of declining plant diversity on ecosystem functioning, spatial aggregation has received little attention, in spite of the fact that the local distribution of species determines interaction. Using a simple conceptual model, we visualize the effect that clumping has on below ground resource uptake (RU). Two general conclusions can be drawn from the model: (1) clumping results in a loss of RU, and does so to a larger extent when species differ more in rooting depth and when plant density increases; (2) the current view on complementarity may be flawed, as we find that neighborhood species richness only promotes spatial complementarity directly at the transition from monocultures to higher species richness levels, but not at transitions between higher richness levels. The number of species in a community does, however, affect the probability of the occurrence of clumping. In randomly assembled communities, clumps are statistically larger when the number of species is lower, leading to reduced overall below ground complementarity. Complementarity, and thus also RU, will therefore increase indirectly with increasing species richness. As a result of our findings, we propose that clumping could have a number of ‘hidden’ consequences, for example on invasibility of a community and on competition between species. Clumping could also prove to be a new and important explanatory factor in diversity studies, as it extends the definition of complementarity effects to include the effect of spatial heterogeneity of plant positions within the community.

Rapid effects of plant species diversity and identity on soil microbial communities in experimental grassland ecosystems

Changes in plant community structure, including the loss of plant diversity may affect soil microbial communities. To test this hypothesis, plant diversity and composition were experimentally varied in grassland plots cultivated with monocultures or mixtures of 2, 3 or 4 species. We tested the effects of monocultures versus mixtures and of plant species composition on culturable soil bacterial activity, number of substrates used and catabolic diversity, microbial biomass N, microbial respiration, and root biomass. These properties were all measured 10 months after seeding the experiment. Soil bacterial activity, number of substrates used and catabolic diversity were measured in the different plant communities using BIOLOG GN and GP microplates, which are redox-based tests measuring capacity of soil culturable bacteria to use a variety of organic substrates. Microbial biomass N, microbial respiration, and root biomass were insensitive to plant diversity. Culturable soil microbial activity, substrates used and diversity declined with declining plant diversity. Their activity, number of substrates used and diversity were significantly higher in plots with 3 and 4 plant species than in monocultures and in plots with 2 species. There was also an effect of plant species composition. Culturable soil microbial activity and diversity was higher in the four-species plant community than in any of the plant monocultures suggesting that the effect of plant diversity could not be explained by the presence of a particular plant species. Our results showed that changes in plant diversity and composition in grassland ecosystems lead to a rapid response of bacterial activity and diversity.

No consistent effects of plant diversity on root biomass, soil biota and soil abiotic conditions in temperate grassland communities

We examined the effects of decreasing plant diversity and functional group identity on root biomass, soil bulk density, soil nitrate and ammonium concentrations, microbial basal respiration, density of predaceous and non-predaceous nematodes, earthworm biomass and density and Shannon–Wiener indices of earthworm diversity in a temperate grassland. Plant species and functional group diversity did not have significant effects on any of these measured variables. However, functional group identity of the plants did significantly affect root biomass and soil abiotic factors. In addition, root biomass, Shannon–Wiener indices of earthworm diversity and density of epigeic earthworms were significantly higher in the presence of legumes, while we found no correlation between functional group identity and other groups of soil biota. We also found several significant relationships among root biomass density, soil microbial basal respiration, nematodes and earthworms. Thus, how can the lack of correlation between soil variables and plant diversity manipulation be explained? Firstly, plant litter input and historical land use have long-term effects on soil properties, which suggests effects of plant diversity manipulation might only be manifested over the long-term. However, we found significant relationships between root biomass density and soil biota suggesting that changes in root biomass density with declining plant diversity could lead to changes in soil biota that might be detectable despite large temporal lags in the soil system. Secondly, we did not find significant effects of plant diversity on root biomass but we found that it was significantly related to the identity of functional groups. Thirdly, we observed complex interactions among trophic levels of soil organisms, which may counteract and buffer effects of changes in plant diversity on soil biota. Fourth, our results suggest that belowground properties might be much more influenced by the identity (and litter quality) of plant functional groups than plant species diversity per se.

Declining plant diversity: empty niches or functional shifts?

Declining plant diversity: empty niches or functional shifts?

Declining plant diversity: empty niches or functional shifts?

Declining plant diversity: empty niches or functional shifts?

Understanding the Role of Allelopathy in Weed Interference and Declining Plant Diversity1

Several weed species have been reported to have allelopathic activities. However, most of these studies indicate the probable involvement of allelochemicals but are not conducted in field settings. In addition to their adverse effects on growth and yield of many crop species, many troublesome weeds such as mugwort and lantana influence biodiversity. More studies on the ecological, physiological, and molecular aspects of weed allelopathy should be conducted in order to better understand community structure and declining biodiversity. Nomenclature: Lantana, Lantana camara L. #3 LANCA; mugwort, Artemisia vulgaris L. # ARTVU. Additional index words: Allelochemicals, plant diversity, allelopathy, phytotoxicity.

Plant carbon balance, evolutionary innovation and extinction in land plants

SummaryThe plant fossil record was reviewed to highlight how consideration of plant carbon balance strengthens our understanding of various evolutionary innovation and extinction events. Following a brief physiological primer to carbon acquisition and allocation in C3‐plants, specific evolutionary events are discussed in connection with postulated carbon‐based mechanisms. Primary topics include: (i) the evolution of plants with the C4‐photosynthetic pathway; (ii) the surprising lack of plant extinctions during the Pleistocene (1.6 million years ago, Ma); (iii) the trend toward declining plant diversity and increasing rates of herbivory across the Palaeocene/Eocene transition (57–52 Ma); and (iv) megaherbivore extinctions at the end of the Pleistocene (10 thousand years ago, Ka). A framework is presented for testing hypotheses on the cause–effect relationships between global carbon cycling, plant carbon dynamics and the evolution of terrestrial ecosystems.

Diversity of the understory vascular vegetation in 40 year‐old and old‐growth forest stands on Vancouver Island, British Columbia

Abstract. We studied plant diversity of the understory vascular vegetation in 40 yr‐old plantations (immature stands) and old‐growth forest stands on southwestern Vancouver Island, British Columbia, Canada. Site‐specific comparisons using several indices of species diversity were made between: (1) immature stands segregated according to the canopy cover and dominant canopy tree species; and (2) immature and old‐growth stands. There were no significant differences (P < 0.05) among immature stands in species richness (S) and the Shannon‐Wiener index (H′), in relation to the canopy cover or in S, H′ and evenness (E) in relation to the dominant canopy tree species. Using the same indices, the plant diversity varied with edaphic conditions (represented by five site associations) and time (represented by two developmental stages). At both stand‐ and site levels, plant diversity increased with increasing soil moisture, from slightly dry to moist sites, and with increasing plant‐available soil nitrogen in both immature and old‐growth stands; and the plant diversity of immature stands across the sites studied was considerably lower than in old‐growth stands, regardless of site association. The indices of plant diversity, floristic similarity indices, and species turnover rates indicated that the immature stands had their plant diversity at a minimum, but a drastic loss of diversity expected in the stem exclusion stage had not materialized. We attributed decline in plant diversity to the absence of old‐growth structural features in immature stands. Several measures to foster the stand‐level diversity were proposed.