Stem demography of Prosopis glandulosa var. torreyana in vegetation arcs and associated bare areas

Abstract. Vegetation arcs are characterized by dense patches of up to 100% plant cover surrounded by bare areas where cover is around 5%. These patterns of contracted vegetation have been reported mainly in alluvial fans of arid and semiarid zones (known as bajadas) with gentle slopes (0.2–2%), summer rainfall, and heavy showers that run as sheet-flow. It has been suggested that run-off water from bare areas is stopped at the front of the arcs and, depending on the amount of rainfall, may advance superficially into the arc, reaching the downslope limit less often. These dynamics would imply a gradient of water availability inside the arc, higher at the upper edge. We termed this the moisture gradient hypothesis. We indirectly tested this hypothesis by following the demography of a cohort of stems of the honey mesquite Prosopis glandulosa var. torreyana in three vegetation arcs and their corresponding bare areas during a yearly cycle at the Bolsón de Mapimí in the Chihuahuan Desert. We selectively removed different plant life-forms (trees, shrubs and herbs) in three positions within the arcs; no manipulation was possible in the bare areas due to low plant cover. The results suggested that removal of other plant life-forms, mainly grasses, affected growth and survival of mesquite low (but not high) stems, making them accessible to medium-sized mammal browsers (jackrabbits and packrats). These results suggest that, at least for the honey mesquite, water availability in vegetation arcs was not the most strongly limiting factor for shoot performance and demography, and that browsing damage, a biotic constraint, took precedence over resource limitation.     

Effects of sheep grazing and temporal variability on population dynamics of the clonal herb <Emphasis Type="Italic">Geranium sylvaticum</Emphasis> in an alpine habitat

An improved understanding of population-level consequences of grazing on plants can be facilitated by an assessment of grazing effects on all stages in the life-cycle. In this study, 6 years of demographic data for three populations of the perennial herb Geranium sylvaticum were analysed. We examined the effects of sheep grazing (high sheep density, low sheep density and no sheep) and interannual climatic variability on vital rates and population growth rates (λ). Grazing did not affect survival or flowering rates, but reduced rates of growth and increased rates of clonal reproduction. At the population level, high contributions from retrogression and clonal reproduction buffered reduced rates of growth and stasis, and no consistent differences in λ between populations exposed to different sheep densities were found. Instead, large between-year variability in λ, independent of sheep density, was detected, related to variation in the local summer climate. The results indicated, however, that grazing effects on λ were more severe in unfavourable than in normal years. Our study highlights that increased clonal reproduction rates functioned as a tolerance mechanism towards grazing in this herb, which forms a mechanism to explain how moderate population responses to grazing in some herbs can arise.     

Conservation of a rare plant requires different methods in different habitats: demographic lessons from Actaea elata

Understanding species decline and conserving endangered species requires demographic information, and variation in the environment may affect demography. Actaea elata is a globally rare, perennial herb found in a range of Pacific Northwest forest stand types that differ in canopy openness. Canopy openness increases reproductive output in this species and so was expected to have demographic impact. We performed a demographic analysis of A. elata in contrasting forest stands (broadleaved vs. coniferous) over two annual intervals, and predicted that population growth rate would be higher in the open-canopy broadleaved stand. Population growth was determined using stage-based matrix models, and the most influential transitions were identified using elasticity analyses. The finite rate of population increase (λ) was lower for the two transition periods at the broadleaved stand than at the coniferous stand (λ = 0.86 and 0.87 vs. 0.94 and 0.98), even though the former population was more fecund. The decline in the broadleaved stand reflects greater mortality and retrogression to previous stages, partly as a consequence of herbivory. In contrast, lower recruitment occurred in the coniferous stand, but there was also less mortality and retrogression. Our results suggest that management decisions for conservation of A. elata should be tailored to differing habitats, with a focus on preventing mortality in some populations and increasing recruitment in others.     

Assessing Sustainability of Nontimber Forest Product Extractions: How Fire Affects Sustainability

Sustainable use of nontimber forest products (NTFPs) can be affected by levels of extractions as well as by other anthropogenic influences such as fire and grazing. We examined the effects of fire on the demography of Phyllanthus emblica, an important NTFP in the forests of Biligiri Rangan Hills, India. We then assessed demographic responses to the combined effects of fire and current fruit harvesting patterns. Fruits of Phyllanthus are commercially harvested by an indigenous forest dwelling people. Using matrix population models, we compared demographic indices across a chronosequence of time since last fire. Population growth rates (λ) ranged from 0.7692 to 1.1443 across the five times since last fire. λ was the lowest at times since last fire of 2 and 3 yr. Frequent fires increased time to maturity by altering growth and survival rates, thereby causing a demographic shift from growth to regressions or negative growth. Elasticity analysis revealed that stasis of adults makes the biggest contribution to λ. Simulations of periodic and stochastic fire regimes suggest that higher λ and population persistence can be achieved at fire-return intervals of ≥7 and ≥9 yr, respectively. These fire-return intervals became longer when the simulations included harvesting and fire. Extinction probabilities under the current fire regimes (every 2–3 yr) suggest that populations will decline to lower densities. Our findings provide critical information for developing guidelines for sustainable use and management of NTFPs in Biligiri Rangan Hills, and demonstrate the need to incorporate various human-generated physical regimes in assessing sustainability of NTFPs.     

Demographic differences between two sympatric lilies (<Emphasis Type="Italic">Calochortus</Emphasis>) with contrasting distributions,as revealed by matrix analysis

Related species of similar morphology can differ greatly in distribution and abundance. Elucidating reasons for such differences can contribute to an understanding of intrinsic limiting factors and the causes of rarity. We studied sympatric populations of two terrestrial lilies with contrasting distributions: Calochortus lyallii, which is geographically restricted but locally abundant, and C. macrocarpus, which is widespread but locally sparse. Marked plants of each species were monitored for 5 years in British Columbia, Canada. Matrix projection models were used to estimate annual and stochastic population growth rates (λ and λ_s) and to compare demographic traits. Annual λ-values ranged from 0.89 to 1.04 in C. lyallii and from 0.89 to 1.01 in C. macrocarpus. Stochastic projections yielded a long-term growth rate near 1 for C. lyallii, but indicated a decline for C. macrocarpus. Elasticity analysis indicated that over the 5-year period of the study, survival of flowering plants made a larger proportional contribution to λ in C. lyallii than in C. macrocarpus. LTRE analysis showed that temporal variation in λ was driven primarily by the dynamics of flowering individuals in C. lyallii, and by the dynamics of vegetative individuals in C. macrocarpus. Similarly, higher flowering rates in C. lyallii and greater vegetative stasis in C. macrocarpus made the largest contribution to the difference in λ between species. Thus, local persistence in these two morphologically similar species appears to be achieved via different demographic pathways. Our analyses show that extrapolations about demographic processes and population dynamics based on taxonomic relatedness, morphological similarity or habitat overlap may often not be justified. Electronic Supplementary Material The online version of this article contains supplementary material, which is available to authorised users.     

Effect of reproductive modes and environmental heterogeneity in the population dynamics of a geographically widespread clonal desert cactus

The dynamics of plant populations in arid environments are largely affected by the unpredictable environmental conditions and are fine-tuned by biotic factors, such as modes of recruitment. A single species must cope with both spatial and temporal heterogeneity that trigger pulses of sexual and clonal establishment throughout its distributional range. We studied two populations of the clonal, purple prickly pear cactus, Opuntia macrocentra, in order to contrast the factors responsible for the population dynamics of a common, widely distributed species. The study sites were located in protected areas that correspond to extreme latitudinal locations for this species within the Chihuahuan Desert. We studied both populations for four consecutive years and determined the demographic consequences of environmental variability and the mode of reproduction using matrix population models, life table response experiments (LTREs), and loop and perturbation analyses. Although both populations seemed fairly stable (population growth rate, λ∼1), different demographic parameters and different life cycle routes were responsible for this stability in each population. In the southernmost population (MBR) LTRE and loop and elasticity analyses showed that stasis is the demographic process with the highest contributions to λ, followed by sexual reproduction, and clonal propagation contributed the least. The northern population (CR) had both higher elasticities and larger contributions of stasis, followed by clonal propagation and sexual recruitment. Loop analysis also showed that individuals in CR have more paths to complete a life cycle than those in MBR. As a consequence, each population differed in life history traits (e.g., size class structure, size at sexual maturity, and reproductive value). Numerical perturbation analyses showed a small effect of the seed bank on the λ of both populations, while the transition from seeds to seedlings had an important effect mainly in the northern population. Clonal propagation (higher survival and higher contributions to vital rates) seems to be more important for maintaining populations over long time periods than sexual reproduction.     

Variation in life-cycle between three rare and endangered floodplain violets in two regions: implications for population viability and conservation

We studied the demography of Viola elatior, V. pumila, and V. stagnina, three rare and endangered Central European floodplain species, to (i) analyse variation in life-cycles among congeners and between regions (Dyje-Morava floodplains, Czech Republic; Upper Rhine, Germany), (ii) to define sensitive stages in the life-cycles, and (iii) to identify possible threats for population viability and species conservation. Matrix models were based on the fate of marked individuals from a total of 27 populations over two years. We analysed population growth rate (λ), stage distribution, net reproductive rate (R ₀), generation time, age at first reproduction, and elasticity and calculated a life table response experiment (LTRE). Most populations were declining and λ did not differ between species or regions during the observed interval. Despite higher probabilities for survival and flowering in the Dyje populations, R ₀ was higher in the Rhine populations. Also other demographic traits showed consistent differences between regions and/or species. Complex life-cycles and large variation in λ precluded unequivocal identification of sensitive stages or vital rates for conservation. Variation between regions may be a consequence of differences in habitat quality. Our results suggest that deterministic processes such as reduced management, succession, habitat destruction, and lack of disturbance through reduced or eliminated flooding present the strongest threat for the viability and persistence of populations of the three floodplain violets as compared with stochastic processes. However, the persistent seed bank of the species may buffer populations against environmental variation and represents a reservoir for recovery after resumption of suitable land-use management.     

Reed die-back related to increased sulfide concentration in a coastal mire in eastern Hokkaido,Japan

A drastic decline of Phragmites australis was observed along the middle reaches of Ichibangawa River in Kiritappu Mire, eastern Hokkaido, Japan, during the last 50 years. In an area of ~30 ha, reed-sedge vegetation and alder forest have been replaced by bare soil and patches of salt marsh vegetation. A gradual increase in frequency of flooding by brackish water probably was the ultimate cause of the vegetation change. We measured redox potentials and oxygen and sulfide concentrations in soil profiles using needle electrodes. Measurements were carried out in areas where reed has disappeared and in sites where reed stands were still healthy. The concentration of selected ions in the surface water was also measured at various sites. Surface water in low-lying areas was clearly influenced by seawater. Very high sulfide concentrations were measured in bare peat sites (more than 600 mol l^–1), which exceeded P. australis tolerance 2 – 3 times. In a healthy reed zone adjacent to an area with poor fen vegetation, sulfide concentration in the rooting zone of Phragmites was also high (300–400 mol l^–1), particularly during the night. The fact that Phragmites in this zone was still healthy indicates that sulfide did not reach toxic levels in the direct vicinity of the roots. Sulfide that is produced in this area is probably fixed by iron, which is supplied through a continuous discharge of iron-rich groundwater. An increase in frequency of flooding by brackish water could be related to ongoing subsidence of this part of the Pacific coast which is located at the Kuril subduction zone. Sea level rise could also contribute to a stronger inflow of seawater into the mire system.     

Soil zoology I: arthropod communities in open landscapes of former brown coal mining areas

In different habitat types of the former coal mining area of Lower Lusatia, distribution and abundance of species of various arthropod groups was studied as to the colonization dynamics and the formation of community patterns. Heteroptera, Auchenorrhyncha, different groups of Coleoptera, Araneida, and Orthoptera were included in the study. In total, about 850 species were captured by pitfall trapping and sweepnet sampling. A detailed analysis of species–environment-relations was performed by means of gradient and eigenvector analysis (DCA, CCA). It is shown that colonization of bare sand habitats, pioneer vegetation with ruderal herbs, short grass prairie with Corynephorus and xerophytic herbs, tall grass prairie with Calamagrostis, and shrubs takes place rather quickly. In all the analysed habitats an adequate degree of the colonization was attained by the studied groups. Both the formation of patterns of species assemblages and population dynamics in upper layers of vegetation mainly depend on the patterns of plant communities and vegetation architecture. In lower layers micro-climatic conditions as well as abiotic soil parameters were shown to be of special importance. Differences of community patterns between predators and mainly phytophagous arthropod groups were discussed.     

Impact of different‐sized herbivores on recruitment opportunities for subordinate herbs in grasslands

Abstract. Potential effects of herbivores on plant species diversity depend on herbivore size, species and density. In this study we examine the effect of different‐sized herbivores (cattle and rabbits) on recruitment of subordinate herbs in grasslands. We show that in a grazed floodplain, grassland plant species richness is mainly determined by the presence of many species of subordinate herbs. These herbs experience high colonization and extinction rates. We conclude that the creation of colonization opportunities for subordinate herbs plays a crucial role in maintaining plant species richness in productive grasslands. We found that cattle disperse large amounts of seeds via their dung, over ten times more than rabbits. Rabbits create more and on average larger bare soil patches than cattle. In a field experiment artificial disturbances improved germination success tremendously for four tested herb species. We found that bare soil is the best regeneration site, while cattle dung gave a too strong nutrient stimulus, resulting in tall vegetation and therefore light limitation. These results can be confirmed with results from field monitoring plots where plant species richness was positively related to the occurrence of bare soil patches. Therefore both large and small herbivores have a major impact on dispersal and colonization, but for different reasons. Cattle are identified as most important for seed dispersal whereas rabbits have a main effect as creators of disturbances. These results emphasize the importance of distinguishing between herbivore species in assessing their (potential) effects.     

Evaluation of the Degraded Riparian Ecosystems in the Geum River Watershed in Korea

Riparian vegetation, an important mediator of land–water interactions, provides habitat for animals and other organisms; however, riparian vegetation zones have been altered by agricultural and urban development in Korea. This riparian vegetation survey was conducted to obtain information vital for the ecological restoration and management of the Korean Geum River ecosystem. At 100 study sites, along the Geum River, we recorded the vegetation of the Geum riparian zone. We then surveyed the riparian vegetation associations in the area and overlaid those areas corresponding to trees, shrubs, perennial herbs, annual herbs, exotic plants, cultivated lands, and damaged lands on a geographical map. We also reconstructed the cross-sectional landscape. The mean values of vegetation diversity, exotic plant area (%), annual plant area (%), and species richness were 6.47 ± 0.26, 5.44 ± 1.01, 11.98 ± 1.20, and 22.69 ± 0.93, respectively. The landscape elements of the herbaceous plants were more spread out, compared with those of the woody plants, and 23 sites were composed strictly of herbs. Our results indicate significant differences in vegetation structure among the study sites. For example, at some sites, exotic plants, cultivated lands, and damaged lands dominated the landscape comprising 25.7, 62, and 68.9%, respectively, of the area. The riparian landscape reference model suggested by these results may be applied to studies of other well-conserved riparian zones. We propose that the material pathways and transport of organisms from land to water at Geum River depend on the patchy distribution of these diverse landscape elements.     

Plant Demography and Habitat: A Comparative Approach

Abstract Progress in plant demography will depend upon being able to synthesize a large body of data and this requires a means of comparing populations between sites and species. We have employed a comparative technique using elasticity analysis of stage-projection matrices to partition the contributions of fecundity (F), survival (L) and growth (G) to the finite rate of increase λ. Ordination of populations of 77 perennial herbs and trees in G-L-F space has shown that species segregate in this triangular space according to their life history and habitat. In the present paper we use the correspondence between demographic parameters and habitat revealed by this method to predict how succession and a variety of environmental factors such as grazing and fire are likely to alter the demography of populations and ultimately to change the composition of communities.     

Source-sink dynamics within a plant population: the impact of substrate and herbivory on palm demography

Site factors have frequently been shown to affect survival, growth, and reproduction in plant populations. The source-sink concept proposed by Pulliam is one way of integrating this spatial demographic variation into population models. Source-sink models describe a population where propagules from “source” habitats sustain less productive “sink” areas. We adapted this concept to model the population dynamics of the understory palm Chamaedorea radicalis on two substrates, rock outcrops and forest floor. In our model, sources and sinks correspond to fine-scale demographic structure within the population, rather than spatially discrete subpopulations as described in the Pulliam model. We constructed a stage-structured population matrix model that integrates the site-specific demography of individuals across two habitats types that are linked by migration. We then parameterized this model with field data from C. radicalis. To address whether observed differences in palm demography between rock outcrops and the forest floor were due to natural variation between microsites or due to differences in browsing intensity from free range livestock, we parameterized separate models based on the substrate-specific demography of protected, non-browsed palms and of palms exposed to burro browse. Results showed that herbivory reduced survival and fecundity on the forest floor, which in the absence of seed migration resulted in a projected decline of forest floor palms (sinks). However with seed dispersal, palms persisted and total population growth (both substrates) was projected to be positive, indicating that seed dispersal from non-browsed palms on rock outcrops (sources) was sufficient to sustain C. radicalis on the forest floor.     

Artificial bare patches increase habitat for the endangered Ohlone tiger beetle (Cicindela ohlone)

The endangered Ohlone tiger beetle (Cicindela ohlone) depends on bare ground areas in California coastal grasslands to encounter mates, oviposit, and find prey. We tested habitat creation as a potential management strategy to increase the availability of oviposition sites for C. ohlone. We compared three different bare ground treatments by scraping off surface vegetation, ripping, and tamping the plots. We also tested whether bare ground creation expands C. ohlone range within a habitat patch by scraping plots at increasing distances from the core habitat and monitoring C. ohlone colonization. C. ohlone oviposited significantly more in artificial bare ground plots compared to controls both one and 2 years after the scrapes were created. Distance from the core habitat did not affect colonization nor did decompaction of scraped plots. Percent bare ground significantly predicted incidence of colonization. For the conservation of the endangered Ohlone tiger beetle, we recommend continued creation of scraped plots every 2 years in order to maintain bare ground and to ensure maximum usage by female C. ohlone as oviposition sites.     

The impact of extreme run-off events from the Sakasagawa river on the Senjogahara ecosystem, Nikko National Park. IV. Changes in tree and understory vegetation distribution patterns from 1982 to 1992

In this study, a survey was made of a sample area adjacent to Senjogahara moor, Nikko National Park, in central Japan, to discover the change of vegetation over the 10 years from 1982 to 1992. The sample area, which is dominated byMalus toringo, has often suffered alluvial deposition from flooding of the river. The accumulation depth of alluvial materials deposited for 1984–1987 were measured at 266 points in the area, and for 1987–1992 at 347 points. Each tree species showed its own changing pattern of distribution area in relation to the accumulation depth of alluvial deposits. For example,M. toringo showed an increase in the number of trunks, and extended its distribution area in spite of heavy deposition, whereasBetula platyphylla var.japonica andLarix kaempferi shifted their distribution to the area that suffered only light deposition. The distribution area of shrubby and herbaceous species of understory vegetation also showed different patterns of change. In addition, the recovery of bare ground by understory vegetation was by different species in heavily and in lightly deposited areas. These facts show that there is a difference in the tolerance of different species to burial by alluvial deposits, and this difference is an important factor in successional change and mosaic formation of vegetation in a repeatedly disturbed area.     

Shrub invasion into subalpine vegetation: implications for restoration of the native ecosystem

The ability of plant communities to recover after non-native species invasion will depend upon the nature of their soil seed bank and seed rain characteristics. This study assessed changes in the soil seed bank and seed rain associated with the invasion of the non-native shrub Cytisus scoparius in subalpine vegetation. Soil seed bank and seed rain composition, density and richness were investigated at three areas of different stages of invasion: (i) recent (8–10 years), (ii) mature (15–16 years) and (iii) long-term (25 years). There were few changes in seed bank composition or richness regardless of invasion stage. By contrast, the seed rain composition, richness and density was substantially different within long-invaded areas. Very few seeds were able to colonise the dense barrier characteristic of larger, more mature C. scoparius stands. Some prominent herbs from the native vegetation were under-represented or absent from the seed bank, both in invaded and uninvaded areas. Laboratory germination experiments demonstrated that most native species germinate easily, which may imply a transient seed bank, rather than a persistent one. The majority of herbaceous and shrub species were capable of resprouting vegetatively. Therefore, regeneration appeared more reliant on the bud and tuber bank than a persistent soil seed bank. The dominance of graminoid species and C. scoparius rather than other herbaceous, shrub or tree species suggests that the regenerating vegetation will be dominated by grass species and/or C. scoparius. Hence, in areas where long-invaded C.␣scoparius stands are present the recovery of native subalpine vegetation maybe difficult. Recovery may only be possible through wind dispersal from the surrounding intact vegetation or through actively reseeding the area. This study highlights the importance of early intervention in invasive species management.     

Demographic modeling and monitoring cycle in a long-lived endangered shrub

Demographic monitoring is a well established tool for conservation managers. But, monitoring programs typically do not offer methods to explore the relationship between projected population trends and the probability of trend detection. Moreover, conservation biologists need to evaluate and incorporate the effect of variability and habitat perturbation on the efficiency of monitoring programs.We studied population demography of an endangered shrub endemic to East Central Spain, Vella pseudocytisus subsp. paui in order to understand its present demographic performance, how it is affected by increasing variability and perturbation and how these two factors are related to monitoring and monitoring thresholds.Using Lefkovich demographic matrices on six years of data, we produced 19 different population projections, and we compared these modeled projections against observed projection. We designated three conceptually important detection thresholds: an early warning; an unequivocal signal; and, a quasi-extinction threshold. Based on calculated detection times of all models for every threshold, we produced an averaged monitoring cycle (minimum visit frequency) to provide managers with a tool to design a consistent monitoring program for this plant.Our results indicate a relatively stable current population along with large detection times for critical threshold changes of modeled populations when considering small management changes (low population variability and low perturbation intensity). In contrast, model combinations incorporating high population variance and high perturbation produce disproportionally short times to detection of a significant population trend. In terms of designing studies to maximise results while minimising monitoring effort, designing a monitoring program capable of providing an early warning system of potential population failure requires more effort than detecting an unequivocal signal of decline.Finally, we propose two alternatives when calculating monitoring cycle (MC) values. One of them implies demographic MC if population perturbation and variation are high. For that, an optimal monitoring cycle is estimated based on a range of possible scenarios for population trends. Alternatively, we propose environmental MC, when low values of perturbation and variation are expected.     

Estimates of biomass and primary productivity in a high-altitude maple forest of the west central Himalayas

The paper describes the biomass and productivity of maple (Acer cappadocicum) forest occurring at an altitude of 2,750 m in the west central Himalayas. Total vegetation biomass was 308.3 t ha⁻¹, of which the tree layer contributed the most, followed by herbs and shrubs. The seasonal forest-floor litter mass varied between 5.4 t ha⁻¹ (in rainy season) and 6.6 t ha⁻¹ (in winter season). The annual litter fall was 6.2 t ha⁻¹, of which leaf litter contributed the largest part (59% of the total litter fall). Net primary productivity of total vegetation was 19.5 t ha⁻¹ year⁻¹. The production efficiency of leaves (net primary productivity/leaf mass) was markedly higher (2.9 g g⁻¹ foliage mass year⁻¹) than those of the low-altitude forests of the region.     

The demographic costs of nectar production in the desert perennial Prosopis glandulosa (Mimosoideae): a modular approach

Nectar production in angiosperms is considered to represent a reproductive cost, and has been associated with a decrease in fruit set or an overall decrease in the energetic budget of the plant. Populations of Prosopis glandulosa var. torreyana (honey mesquite) are a suitable system to evaluate the demographic costs of nectar production, as populations are composed of a 1:1 proportion of nectarful to nectarless individuals. The study was carried out in a population of 404 individuals of Prosopis g1andulosa var. torreyana found in an area with differing water availability in the Southern Chihuahuan Desert. The possible costs of nectar production were assessed on 1212 shoots of the honey mesquite that were tagged in 1994 and followed until 1998. We used two methods of analysis to describe the effect of nectar production on modular population dynamics: matrix analysis and log-xlinear models. Water availability and the varying environmental conditions affected plant growth, but nectar production did not have an effect on the demographic parameters we measured. The values of λ did not differ between nectar morphs and the only important effects we detected were the year to year variation in precipitation and microclimate differences at each site. Furthermore, the elasticity of each demographic process (growth, fecundity, retrogression and stasis) between nectar morphs did not differ. The log-linear models suggested a similar pattern but could discriminate the importance of each factor (nectar morph, year and site) on module fate. We were not able to detect a demographic cost of nectar production in the honey mesquite. The absence of a demographic response could be due to the negligible cost of producing nectar for this species or that the resources allocated for growth are different from those allocated for reproduction. Our results suggest that the modular fates of mesquites are mainly determined by environmental factors. This revised version was published online in August 2006 with corrections to the Cover Date.     

Alternative spatial sampling in studies of plant demography: consequences for estimates of population growth rate

Ecologists commonly use matrix models to study the population dynamics of plants. Most studies of plant demography use plot-based methods to collect data, in part, because mapped individuals are easier to relocate in subsequent surveys and survey methods can be standardized among sites. However, there is tremendous variation among studies, both in terms of plot arrangement and the total area sampled. In addition, there has been little discussion of how alternative sampling arrangements influence estimates of population growth rates (λ) calculated with matrix models. We surveyed the literature to determine what sampling designs are most used in studies of plant demography using matrix models. We then used simulations of three common sampling techniques—using a single randomly placed plot, multiple randomly placed plots, and systematically distributed plots—to evaluate how these alternative strategies influenced the precision of estimates of λ. These simulations were based on long-term demographic data collected on 13 populations of the Amazonian understory herb Heliconia acuminate (Heliconiaceae). We found that the method used to collect data did not affect the bias or precision of estimates in our system—a surprising result, since the advantage in efficiency that is gained from systematic sampling is a well-known result from sampling theory. Because the statistical advantage of systematic sampling is most evident when there is spatial structure in demographic vital rates, we attribute this result to the lack of spatially structured vital rates in our focal populations. Given the likelihood of spatial autocorrelation in most ecological systems, we advocate sampling with a systematic grid of plots in each study site, as well as that researchers ensure that enough area is sampled—both within and across sites—to encompass the range of spatial variation in plant survival, growth, and reproduction.