Individual Plant Size Research Articles

Does forest gap size affects population size, plant size, reproductive success and pollinator visitation in Lantana camara, a tropical invasive shrub?

Invasive species are regarded as one of the largest global threats to biodiversity, but little is still known about the invasion of exotic plants into tropical forests. In this paper, we examine how the size and canopy openness of human-created gaps inside the Mabira forest-reserve in Uganda, affect the invasion of one of the world's most noxious weeds, Lantana camara L. Sub-population size inside gaps was positively related to gap size, and components of individual plant size increased with increasing gap size and canopy openness. Moreover, the reproductive effort of L. camara increased with gap size and canopy openness, whereas the fruit production of individual plants was unrelated to gap size and canopy openness. Finally, there was no relationship between gap size and pollinator visitation to inflorescence, but a marginally significant relationship occurred between canopy openness and pollinator visitation. Overall, our results suggest that the size of human-created gaps inside Mabira forest-reserve positively affect aspects of the population dynamics of L. camara. We propose strategies for a campaign aimed at removing L. camara from this forest-reserve, with minimal disturbance to intact forest habitats.

Biomass Yield and Stand Characteristics of Switchgrass in South Central U.S. Environments

Optimizing feedstock production from switchgrass (Panicum virgatum L.) requires careful matching of genotype to environment, especially for southern U.S. regions. Nine genotypes from four combinations of ecotype and morphological type were harvested once yearly in autumn for 3 or 4 yr at five locations across Texas, Arkansas, and Louisiana that varied in latitude and precipitation. Genotypes were evaluated for dry matter yield (DMY), plant density, tiller density, lodging, and rust (caused by Puccinia spp.) infection. Genotype × environment (G×E) interactions were identified for most traits. Biomass yield of all genotypes tended to increase with latitude, but lowland morphological types may have been more sensitive than upland morphological types to differences in moisture availability. Yield (5.82 vs. 14.97 Mg ha−1, respectively) and persistence (final stand density, 3.99 vs. 5.96 plants m−2) were lower for upland than for lowland genotypes, particularly at higher rainfall and more southern sites. Lowland genotypes were often able to compensate for stand thinning by increasing individual plant size, but upland genotypes were not. Lodging and rust scores were higher for upland than for lowland genotypes. Yield (13.65 vs. 9.75 Mg ha−1) and final plant density (5.58 vs. 4.95 plants m−2) were higher for southern than northern ecotypes. The southern‐lowland combination exhibited the best yield and persistence over the study region, and genotypes within this group exhibited variability in yield among sites. Therefore, development of switchgrass cultivars for biomass production in the southern USA should focus on the southern‐lowland genotypes.

Ancymidol and liquid media improve micropropagation of Hemerocallis hybrid cv. ‘Todd Monroe’ on a thin-film “rocker” bioreactor

SummaryShoots of the Hemerocallis hybrid cv. ‘Todd Monroe’ were micropropagated on semi-solid agar and in a thin-film “rocker” bioreactor for three repeated cycles of sub-culture, in the presence or absence of the gibberellin-antagonist, ancymidol. Ancymidol increased the numbers of plants multiplied, decreased residual sugar levels in the media, and reduced the size of individual plants. Liquid culture increased the availability of sugar and the biomass of plant tissue. There was an indirect linear relationship between residual sugar levels in the media following culture (°Brix) and plant biomass (g fresh weight). It was likely that 5% (w/v) sugar was inadequate for optimal growth in ancymidol-containing liquid media. Smaller plants, using ancymidol, and larger numbers of plants from larger bioreactor (e.g., “rocker”) vessels were more rapidly cut and transferred in the hood. Larger plants from ancymidol-free media, agar or liquid, were well-conditioned for greenhouse culture. However, the smaller plants from ancymidol-containing cultures on agar were not as fit for greenhouse culture. Liquid media increased the efficiency of micropropagation, but this required a greater quantity of available sugar than with conventional, agar-gelled media.

Effect of the rust fungus Puccinia chondrillina TU 788 on plant size and plant size variability in Chondrilla juncea

Summary Chondrilla juncea is one of the most serious introduced weed species in Australia. Successful biological control has been achieved by the release of the rust fungus Puccinia chondrillina. Although P. chondrillina is thought to reduce the competitive ability of C. juncea by reducing plant size, the precise mechanism by which the pathogen controls its host is poorly understood. In this experiment, we examined the effect of P. chondrillina TU 788 infection on individual plant size, and size variability, in two genotypes (one resistant, one susceptible) of C. juncea. Exposure to P. chondrillina significantly reduced the average size of susceptible C. juncea plants, a reduction apparent in both roots and shoots. Plant size variability of the susceptible C. juncea genotype was also reduced by P. chondrillina. Plant size, and plant size variability of the resistant genotype were unaffected by exposure to rust, or association with rust‐affected plants. The effect of P. chondrillina on plant size in rust‐susceptible C. juncea plants shows how effective bio‐control of this weed may occur in field populations.

Density‐dependent regulation in an invasive seaweed: responses at plant and modular levels

Summary The effects of density on the vital rates, growth form and population size structure of the invasive seaweed Sargassum muticum (Yendo) Fensholt were evaluated experimentally. The initial difference in plant number between the highest and the lowest density (200 and 6400 plants m−2) was considerably reduced by the end of the experiment. Surprisingly, this was mostly due to numbers increasing at lower densities, probably because microscopic forms were not removed during the experimental thinning. The allometric length–dry mass relationship fitted a simple linear model on log‐log scale for both the highest and the lowest densities, but had different slopes. At higher densities plants became taller and thinner as a consequence of variations in the production and growth of modules. Mean size (dry mass) and the development of size hierarchies of plants were affected by both the addition of further (microscopic) recruits and asymmetrical competition among plants. Plant length distributions were also influenced by changes in the growth form of plants. The length hierarchies of main branches also suggested the presence of asymmetrical competition at this modular level. Density influenced both mean size and morphology of the plants and thus induced changes in reproduction. The negative effect of density on individual plant size reduced the percentage of fertile plants and possibly their annual reproductive allocation, but these effects may be attenuated by morphological responses. The responses of S. muticum to crowding are closely linked to its ability to colonize bare space. The massive reproductive output and very limited dispersal range account for local and dense recruitment patterns. Our results suggest that the responses of S. muticum to crowding allow the establishment of dense populations with high persistence and resistance to colonization by other species.

The effects of elevated CO2 on seed production and seedling recruitment in a sheep-grazed pasture.

Seed production and seedling recruitment were measured over 2years under ambient (360ppm) and elevated (475ppm) atmospheric CO2 in a free air carbon dioxide enrichment (FACE) experiment, carried out in a sheep-grazed pasture on dry, sandy soil in New Zealand. In both years elevated CO2 led to more dispersed seeds of the grasses Anthoxanthum odoratum, Lolium perenne and Poa pratensis, the legumes Trifolium repens and T. subterraneum and the herbs Hypochaeris radicata and Leontodon saxatilis. The increased seed dispersal in A. odoratum, H. radicata, Leontodon saxatilis and T. repens reflected both more inflorescences per unit area and more seeds per inflorescence under elevated CO2. The increased seed dispersal in Lolium perenne, P. pratensis and T. subterraneum was due solely to more inflorescences per unit area. The number of seedlings that emerged and survived to at least 7months of age was increased by elevated CO2 for H. radicata, Leontodon saxatilis, T. repens and T. subterraneum in both years and for A. odoratum and Lolium perenne in the first year. For species where increased seedling recruitment was noted, there was a significant positive correlation between seed production in summer and seedling emergence in the following autumn and winter, and sowing 200 extra seeds per species m-2 resulted in more seedlings compared to unsown controls. Elevated CO2 did not affect seedling survival in any species. There was no measurable effect of elevated CO2 on canopy and soil surface conditions or soil moisture at the time of seedling emergence. The results suggest the dominant effect of elevated CO2 on seedling recruitment in this pasture was an indirect one, reflecting effects on the number of seeds produced. The biomass of H. radicata, Leontodon saxatilis, T. repens and T. subterraneum in the above-ground vegetation was greater under elevated than ambient CO2. However, the size of individual seedlings and mature plants of these four species was unaffected by elevated CO2. The results indicate an important way elevated CO2 influenced plant species composition in this pasture was through changes in the pattern of seedling recruitment.

Resource Abundance and Insect Herbivore Diversity on Woody Fabaceous Desert Plants

This study addresses four hypotheses that may account for differences in the number of insect herbivore species among plant species. These hypotheses are based on the assumption that insect diversity is a function of the number, quantity, and distribution of plant resources used by herbivores. The study investigated predictions that herbivore species richness will increase as a function of increasing the following: (1) host plant distribution over the landscape (host plant geographical distribution hypothesis), (2) host plant density within a habitat (resource concentration hypothesis), (3) size of individual plants (plant size hypothesis), or (4) abundance of resources (resource abundance hypothesis). We tested predictions from these hypotheses by examining the species richness of insect herbivores on five sympatric species of fabaceous plants that varied in their local dispersion of individual plants and plant architecture. Among these five species, plant geographical distribution varied threefold, density varied 38-fold, plant size and food resources available to insect herbivores varied ≈100-fold. Plant geographical distribution, plant size, and the resource concentration hypotheses were not corroborated in this study. Resource abundance, measured as plant dry weight, accounted for the differences in number and abundance of insect species between host plant species. Leaf biomass accounted for 44.15% of the variation in number of insect herbivore species and 51.76% of the variation in abundance of insect herbivores using leaves as resources. Flower biomass accounted for 32.86% of the variation in the number of insect herbivore species using flowers. Therefore, plants that offered a greater quantity of resources, especially leaves and flowers, had greater species richness and abundance in insect herbivores.

Demographic and genetic invasion history of a 9-year-old roadside population of Bunias orientalis L. (Brassicaceae).

The population history of a 9-year-old roadside population of the invasive plant Bunias orientalis was reconstructed by demographic analysis including size, position, age (determined by herbchronology) and RAPD-PCR patterns of individual plants. We evaluated emerging patterns of population growth and genetic structure during a full period of population development under typical site conditions (anthropogenic disturbance) and their possible consequences for the invasion potential of the species. The population has grown rapidly and continuously (though with slowing geometric population increase) during the 9 years since its foundation, filling the space available in the study area. Genetic variation (RAPD markers) was already high in the founder cohorts and remained at the same level throughout population development (variance fluctuations <15%). Both results may be related to the mowing management at the site which seems to promote population growth of B. orientalis relative to other co-occuring species and to prevent the genetic drift and the development of spatial genetic structure that would be expected under isolation-by-distance models. Large founder plants had comparatively low genetic variance and were more closely related to younger cohorts than were small founder plants, indicating that selection acted during population development. Overall, the current anthropogenic disturbance regimes may contribute to high genetic variability by artificially increasing gene flow and thereby promoting the adaptability of invasive species to the often unpredictable conditions at disturbed sites. Our approach using retrospective demographic investigation allows the detection of spatio-temporal microscale patterns in genetic and phenotypic variation. Thus it allows a thorough understanding of local invasions of perennial herbaceous plants.

Genome size variations within Dasypyrum villosum: correlations with chromosomal traits, environmental factors and plant phenotypic characteristics and behaviour in reproduction

Feulgen/DNA cytophotometric determinations carried out in the root meristem of seedlings showed that substantial quantitative alterations in the nuclear genome are present between and within 15 natural populations of Dasypyrum villosum in Italy. When the most variant values are considered, there is a 17.6% difference between the mean genome size of the populations, and a 66.2% difference between the genome size of individual plants within a population. A highly significant, positive correlation was found to exist between the genome size of D. villosum plants and the altitude of their stations, and differences in DNA contents between individual plants were greater in populations from mountain sites. Karyological analyses showed all chromosome pairs to differ largely in size between plants with differing DNA contents. A highly significant, positive correlation was found to exist between genome size and both the length of the chromosome complement at metaphase and the length and arm ratio of pair VII. Significant correlations were also found between DNA content and certain phenotypic characteristics of the plants. The mean genome size of the populations was negatively correlated with the mean leaf length and width. In contrast, the genome size of individual plants was positively correlated with the weight of the seed from which they originated and their flowering interval. A large range of genome sizes was found in the half-sib progeny of a plant having a relatively large genome. In contrast, in the half-sib progeny of a plant having a small genome, the genome sizes of the individual plants were less divergent and similar to that of the mother plant. All siblings from crosses between plants with differing genome sizes had similar DNA contents, which were intermediate between those of the parental plants, even if closer to the DNA content of the parent plant having the smaller genome size. Size polymorphism within pairs was never observed in plants obtained from these crosses or in half-sibs whose genome size differed from that of the mother plant. The intraspecific alterations observed in the nuclear genome and their effects on plant development and phenotype are briefly discussed as evolutionary factors which allow D. villosum populations to withstand different environmental conditions as well as the variability of conditions in a given environment.

GROUNDNUT GROWTH AND DEVELOPMENT IN CONTRASTING ENVIRONMENTS. 1. GROWTH AND PLANT DENSITY RESPONSES

Data from studies of growth and development, and response to plant density in common groundnut (Arachis hypogaea) cultivars were examined from published studies. Data were available from the humid tropics of Indonesia, the semi-arid tropics of north-west Australia and the humid coastal and inland elevated areas of north-east Australia. Temperature and irradiance played a major role in determining crop duration, individual plant size and partitioning of dry matter to pods across environments, and these plant characteristics provided the major determinants of pod yield and response to plant density. Crop duration was shortest in humid tropical and subtropical environments, with both high and low temperatures apparently delaying crop maturity. A relatively small individual plant size in humid tropical environments was due to a combination of low incident irradiance and short duration, with very high plant densities needed to maximize dry matter production. The progressive decline in harvest indices in more tropical environments was due to a decline in pod numbers per plant. Although increased plant density resulted in greater numbers of pod initials in the humid tropics, a high proportion of these pods did not contain developed seeds and pod yield at high densities remained relatively low at ≤2.5 t ha−1.

Relationships between flowering phenology, plant size and reproductive success in Lotus corniculatus (Fabaceae)

Over three years the flowering phenology of individuals of Lotus corniculatus has been studied in relation to fruit set and seed predation to determine the relationships between four components of flowering time, plant size and reproductive success. Timings of first and peak flowering, and duration and synchrony of flowering differed between individuals in the same years. Between years, timing of first flowering was highly correlated for the same individuals, and was closely correlated with plant size and duration of flowering–larger plants flowered earlier and for a longer period. Peak flowering and synchrony were not correlated between-years for individuals. Fruit production and seed predation were correlated with some of the components of flowering phenology in some years, but not in others. The inconstancy of these relationships suggests that directional or stabilising selection is not acting consistently on the aspects of reproductive success studied in this work. The inconstancy of selection may result in the rather asynchronous flowering phenologies of individuals of L. corniculatus observed. We emphasize the importance of studying different components of flowering phenology in relation to individual plant size over several seasons. This work has shown that plant size not only has a direct effect on individual plant fecundity but also can influence flowering time and hence indirectly affect reproductive output.

The gall maker Neopelma baccharidis Burck. (Homoptera:Psyllidae) on Baccharis dracunculifolia DC. (Asteraceae): individual, local, and regional patterns

Baccharis dracunculifolia, the host plant of the gall maker Neopelma baccharidis Burck., is locally distributed in delimited patches. The effects of the host plant individual (plant height, crown diameter, and crown volume), local (host plant patch), and regional (locality) characteristics on the number of galls were analyzed, in Viçosa (three patches) and in Rio Doce State Park, Marliéria (two patches), Minas Gerais State (20°45'S, 42°50'W; and 19°50'S, 42°30'W, respectively). The number of galls/individual plant increased homogenousliy with plant dimensions on local and regional scale. Gall load was also homogenous, but the proportion of galled plants differed on a local scale. The differences were related to local plant size differences, and not to host plant patch size. Plants without galls presented different frequency distributions of plant dimensions than galled plants, with greater number of little plants, which indicated a minimum plant size to be gall maker prone. Local factors prevailed over regional factors in determining the number of N. baccharidis galls on 5. dracunculifolia, affecting the size of individual plants and suitability to the gall maker, without modifying the relationship between gall load and plant dimensions.

Holes and the sums of parts in Ghanaian forest: regeneration, scale and sustainable use

SynopsisThe current state of Ghana's forest is summarised. Considerable changes have occurred in the last decade, since Hall &amp; Swaine's account and classification, due mainly to fire and logging. The requirements and potential for sustainable forest use are explored through a summary of patterns of regeneration, and of local and national distribution of individual species.Incisive indices of forest quality and condition are vital to good forest management. Various forest quality indices, summarising different properties of the plant community, are examined. These indices gloss over the statistically noisy behaviour of single species in small forest areas. The indices are: Forest Type – Hall &amp; Swaine's forest ordination and classification; a Pioneer Index (PI) revealing the balance of ‘regeneration guilds’; a Genetic Heat Index (GHI), based mainly on the rarity value (Star rating) of all forest species, highlighting ‘hotspots’; and an Economic Index (EI) based on the concentration of common species (‘reddish Stars’) threatened by exploitation. Guild and Star are defined for all species and encapsulate trends of local and of global distribution and ecology. The national and local patterns and response to disturbance of the indices derived from the representation of these various guilds and stars are discussed.Scale is crucial to all discussions. A strictly hierarchical model of forest ecology/biogeography is less suitable than a continuum-of-significant-scale, and non-hierarchical model. For instance, refugia are usually perceived as discrete biogeographical units. However, major biological ‘hotspots’, which are often described as refugia and attributed to Pleistocene climatic variation, differ only in position along a continuum of scale from mini-refugia as small as individual plants. The biogeographic Dahomey gap has much in common with a canopy gap, with scale as the main distinction.There are conspicuous trends across Ghana's forests in the abundance of pioneer, rare or economic species. These differ in detail, but ‘hysteresis’ – the forest memory – and other factors related to the concept of refugia apply to all these aspects of forest quality. Major hotspot refugia are crucial to the national framework of biodiversity, but local refugia, between the size of individual plants and single forest blocks, are crucial to local regeneration and sustainable use, as they shape the probability cloud which defines the anatomy of and processes within each species' range. Short-term sustainable use depends on local refugia; longer-term sustainability requires maintenance of refugia on a wider range of scale.The implications of these phenomena to forest management are discussed in conclusion. Forest health is a multi-scale, but particularly a broad-scale, phenomenon. Local processes like the regeneration of forest under canopy gaps, are subordinate to larger-scale patterns and not determined simply by a match between species physiology and gap dynamics or patterns in the physical environment. Success of a species in a certain landscape does not automatically imply the species can be successful in similar conditions in a different landscape elsewhere: the context of the landscape in terms of the broader mosaic is also important. Managers, whether of plantations or natural forest, need to monitor, plan, and protect indigenous species on all scales. Forest managers need also to be aware of and work with the ‘forest memory’ factor. Protective measures for rare or economically threatened species should be based on current refugia and, like them, be arranged on all scales from single trees to large forest blocks.Researchers need to pay more attention to processes between the ecological and biogeographical, if they are to provide information for managers which has a useful synergy with existing types of data. Exploration is needed of the anatomy of the ‘probability clouds’ defining the statistics of dispersal and regeneration of rare or threatened species with respect to parent populations. What are the chances of a mahogany establishing at a point 500 metres from a mother tree? How is this statistic influenced by soil type? There is much to be learnt on scales between the canopy and the Dahomey Gap.

Effects of Tillage and Mulch on Weed Seed Production and Seed Banks in Sweet Corn

Effects of tillage (till, no-till), mulch (none, dead Secale cereale), and crop competition (none, sweet corn) on weed seed production and seed banks are reported. Data are presented for Amaranthus retroflexus, Chenopodium album, Portulaca oleracea and Digitaria sanguinalis. These constituted the majority of the weeds present during the cropping season. Seed bank data for a suite of winter annuals active during the fallow season are also presented. Weed seed production was estimated by dimension analysis. Most seeds were produced by the first cohort of plants to emerge. Amaranthus and Digitaria showed significantly greater seed production in no-till than in till treatments. Seed production by Amaranthus, Chenopodium and Portulaca was significantly greater in the absence of a corn crop. The only effect of Secale cereale mulch was to decrease seed production by Portulaca in 1 year. Differences among treatments in the number of seeds produced per unit area were primarily due to effects on individual plant size and seed production. Effects on seedling density and rate of survival to maturity were of secondary importance. Seed banks of Digitaria were greater in no-till than in till treatments, whereas the seed banks of winter annual forbs were greater in till treatments. Presence of a corn crop did not affect seed banks until the final year of the study, at which time seed banks of Amaranthus, Chenopodium and Portulaca were greater in the absence of a crop. An uncropped treatment which did not receive herbicides had exceptionally large seed production and seed banks of Amaranthus and Chenopodium, but not Portulaca and Digitaria, which apparently suffered from competition with the two larger species.

Spatial Variation in Nitrogen Availability in Three Successional Plant Communities

1 Spatial variability in soil nitrogen and moisture levels was determined using a geostatistical analysis in a newly abandoned field, a mid-late successional field and a second-growth forest in south-western Michigan. 2 A greater proportion of the total variation for all variates associated with nitrogen availability was spatially dependent in the mid-successional field; the newly abandoned field and forest had similar patterns of spatial dependence in these variates. 3 The distance (range) over which there was spatial dependence was greater in the mid-successional field than the other two communities, indicating a more coarsegrained pattern of spatial heterogeneity in soil nitrogen, particularly in the surface soils (0-5 cm), than in the other two sites. 4 Examination of patterns of spatial dependence using different lag intervals generally gave similar results; though a nested pattern of N03-N availability in the midsuccessional field was detected at a finer scale of analysis, indicating spatial variation at multiple scales. 5 The results suggest that patterns of spatial variation in soil nitrogen change over time in successional plant communities, perhaps reflecting changes in the species composition or size of individual plants in these communities.

REGULATION OF GENDER AND FLOWERING BEHAVIOR IN A SEXUALLY DIMORPHIC DESERT SHRUB (OCHRADENUS BACCATUS DELILE [RESEDACEAE

The major goal of this study was to examine patterns of gender variation in Ochradenus baccatus, a shrub found in the Judean Desert and Arava Valley of Israel, whose breeding system was previously considered dioecious (separate male and female individuals). We conducted detailed measurements on 150 marked plants over two years to (1) quantitatively describe the variation in sex expression and elucidate the factors responsible for gender variation in males, (2) determine the role of plant size in regulating the flowering behavior through the year, and (3) document pollinator visitation to males and females. The variability in sex expression differed between males and females. Females only reproduced by seed. In contrast, gender in males was extremely variable: 35% of males reproduced only by pollen donation (pure males) and 65% produced pollen and varying amounts of fruit and seeds (inconstant males). The ability to produce fruit was highly correlated with individual plant size. Inconstant males were significantly larger than pure males. Individual plant size also determined the flowering pattern through the year. Unlike most other desert plant species that typically flower after the winter rains, large O. baccatus plants flowered all year. Small plants, regardless of sex, flowered only during the winter months. The flowers of O. baccatus were visited by a diverse suite of insects including wasps, bees, ants, beetles, flies, and butterflies. Perhaps owing to the greater attractiveness of the floral display of males (due to the presence of yellow pollen), males were visited by many more insects than females. Over four observation days, 526 insects were counted on male plants compared to only 39 on females.

Adaptability and virulence specificity in Australian strains of blackleg [Leptosphaeria maculans (Desm.) Ces. et De Not.] on different host genotypes of rapeseed (Brassica napus L.)

Five clone-lines of rapeseed (Brassica napus L.) were artificially inoculated with 24 isolates of blackleg (Leptosphaeria maculans (Desm.) Ces. et De Not.) from 12 sites around Australia to determine the proportion of virulence genes in L. maculans which interacted with genes for specific resistance in B. napus. Resistance/susceptibility was evaluated both as external lesion size (%G) and internal lesion area (%II) of individual plants of the clone-lines. Using two different regression techniques (stability analysis) involving levels of infection caused by the isolates on the clone-lines, isolates were ranked according to levels of specific virulence. There were differences in stability between isolates in their capacity to exhibit consistent virulence over the range of genotypes used. Using deviations from regression as an indicator of the proportion of genes for specific- and non-specific virulence, it was observed that a third of all isolates used possessed significant levels of specific virulence. The significance of this finding in relation to glasshouse screening programs for blackleg resistance is discussed.

An analysis of tree mortality in southern California using high spatial resolution remotely sensed spectral radiances: a climatic change scenario

The value of remotely sensed data for ecological monitoring and modeling studies that address landscape changes due to climatic change is widely recognized. However, these data can be collected at a variety of spatial resolutions which was significant implications in terms of the information that can be derived from the data. Most readily available remotely sensed data tend to have ground resolutions substantially greater than the size of individual plants so pixels may contain a mixture of vegetation types, background cover, illumination intensity and shadow. These mixtures make it difficult to evaluate fine-scale vegetation dynamics. This paper examines the potential utility of high spatial resolution remotely sensed data for assessing vegetation condition within a climatic change context. The drought in southern California that started in 1987 and the associated increase in tree mortality due to bark beetle infestations, provided the climatic change scenario for the study. A possible consequence of anthropogenically induced climatic change is the occurrence of more intense and prolonged droughts in some regions leading to greater mortality rates for susceptible vegetation species. Studies of present-day patterns and processes of vegetation mortality associated with droughts may help to identify future consequences of hypothesized climatic changes. High spatial resolution (0.5 m) reflected spectral radiances were collected over the Cuyamaca State Park in southern California and related to levels of bark beetle infestations that increased during the drought years. The results from this demonstration project indicated that high spatial resolution remotely sensed data allow investigators to isolate individual plants from the scene background and are likely to provide valuable information for assessing vegetation condition. The characteristics of the reflected spectral radiances and their geostatistical properties may be potential indicators of differences in vegetation condition. These data are also likely to be valuable for determining biophysical quantities required in models of ecosystem function and for monitoring key species that may be particularly sensitive to climatic change (indicator species).

Resprouting vigour of two mediterranean shrub species after experimental fire treatments

Experimental fire treatments were carried out by applying a propane torch flame to individual stumps ofArbutus unedo andErica arborea, two dominant ericaceous shrubs living in relatively moist maquis of the western Mediterranean Basin. No mortality was observed in either species. Individual plant size, measured as the individual stump area, was the most important factor determining both number and biomass of resprouts for all fire treatments. The number of resprouts at 3 months was less at the higher temperature, but no difference could not be detected at 18 months. Duration of flame application had no effect on resprouting success. This effect was not statistically significant 18 months after treatments were applied. Biomass of resprouts was not clearly affected by different fire treatments either 3 months or 18 months after the start of the resprouting process.

Population distributions of plant size and light environment of giant ragweed (Ambrosia trifida L.) at three densities.

Plots in a naturally occurring population of giant ragweed (Ambrosia trifida L.) near Ames, Iowa, USA were left unthinned (high density,=693 plants/m2) or were thinned in early June 1989 to create low and medium densities of 10 and 50 plants/m2. Size and light environment of individual plants were measured at monthly intervals from June to September. By September, low density plants had 15 times greater biomass/plant and 30 times greater leaf area/plant than high density plants, although biomass and leaf area per unit land area decreased with decreasing density. Plants at high density allocated more biomass to stem growth, but plants at medium and low density had successively higher leaf area ratios, higher potential photosynthetic rates, higher allocation to leaves, and higher growth rates. Average light on leaves decreased with increasing density and also decreased over the growing season in the low and medium densities. The distribution of light environments of individual plants was non-normal and skewed to the left in most months, in contrast to the rightwards skew of distributions of plant size parameters. Inequality in the distributions, as measured by coefficient of variation and Gini coefficients, increased over most of the growing season. There was little effect of density on inequality of stem diameter, height, or estimated dry weight, but inequality in reproductive output greatly increased with density. There was greater inequality in number of staminate flowers produced than in number of pistillate flowers and seeds produced. Path analysis indicated that early plant size was the most important predictor of final plant size and reproductive output; photosynthesis, conductance, and light environment were also significantly correlated with size and reproduction but usually were of minor importance. Variation in growth rate apparently increased inequality in plant size at low density, whereas belowground competition and death of smaller plants may have limited increases in inequality at high density.