Fire and the demography of camelthorn (Acacia erioloba Meyer) in the southern Kalahari – evidence for a bonfire effect?

Fires in arid environments are rare, so are not deemed as important as in mesic savannas. We investigated mortality and resprouting amongst camelthorn (Acacia erioloba) after two fires (at Vaalbos National Park and Susanna farm) in semi‐arid savanna near Kimberley, South Africa. Resprouting response 18 months after a fire was the greatest amongst <6.5 m high trees; extent of foliage damage by fire and bark thickness were the next best predictors of resprouting vigour amongst that size class. The largest size class (8–12 m height) of A. erioloba suffered the greatest mortality rates (40% and 83% at Vaalbos and Susanna respectively), with damage either severe or minor. We hypothesize that large tree mortality rates are partly attributable to well‐developed assemblages of flammable subcanopy plants producing a bonfire beneath trees. These mortality rates indicate that fire reduces both tree abundance and relative representation of large trees, and although able to resprout, A. erioloba is fire‐sensitive, which may explain its restriction to Kalahari sands where rainfall is less than 900 mm year^?1. Therefore, although relatively infrequent, fires shape Kalahari woodland structure, particularly as A. erioloba is long lived and slow growing. Large trees have been shown to be important to biodiversity in the southern Kalahari, so frequent fires could impact biodiversity.     

Reduced mycorrhizae on Juniperus monosperma with mistletoe: the influence of environmental stress and tree gender on a plant parasite and a plant-fungal mutualism

Summary We examined how an important plant mutualist (fungal mycorrhizae) interacted with a common tree parasite, a xylem-tapping mistletoe (Phoradendron juniperium Engelm.) growing on one-seeded juniper (Juniperus monosperma Engelm.). We also examined how host tree gender and environmental stress might be involved in this interaction. Four major patterns were observed. First, the mycorrhizal levels of trees of both sexes were negatively correlated with mistletoe density. In comparisons of heavily and lightly infested trees at the stressful site, high mistletoe levels were associated with 27% less mycorrhizae on male trees and 38% less mycorrhizae on the roots of female trees. Second, the reduction of mycorrhizae on trees with high mistletoe levels was slightly but significantly greater for female trees than male trees. These results are consistent with the hypothesis that severe mistletoe infestation suppresses mycoresis and that this suppression is more severe in female trees because of their greater energetic investment in reproduction. Third, female junipers growing in the stressful ash and cinder fields averaged three-fold higher levels of mistletoe infestation than male trees. Fourth, no differences in mistletoe infestation were observed between male and female trees growing in the more favorable soils. Comparisons with other systems suggest that both mistletoes and herbivores have similar interactions with mycorrhizae.     

The Relative Contribution of Specialists and Generalists to Mistletoe Dispersal: Insights from a Neotropical Rain Forest

Mistletoes rely on birds for seed dispersal, but the presumed importance of mistletoe‐specialist frugivores has not been critically examined nor compared with generalist frugivores and opportunistic foragers. The contribution of these three groups was compared directly by quantifying bird visitation to fruiting mistletoe plants ( Oryctanthus occidentalis: Loranthaceae) at Barro Colorado Island, Panama, and by comparing these results with proportions calculated from other empirical studies of mistletoe visitation conducted elsewhere. After more than 100 h of timed watches, 23 bird species were recorded visiting eight heavily infected host trees ( Luehea seemannii: Tiliaceae). Eight of these species visited mistletoe, of which five (all tyrannids) consumed mistletoe fruit. Although two mistletoe specialist frugivores ( Tyrannulus elatus and Zimmerius vilissimus) removed most fruit (73%), more than a quarter was consumed by one generalist frugivore ( Mionectes oleagineus) and two opportunists ( Myiozetetes cayanensis and Myiozetetes similis). Post consumption behaviour varied: the specialists flew from mistletoe to mistletoe, the generalist rested in the subcanopy and understory, and the opportunists spent most time hawking insects and resting high in the canopy. Integrating these data with previous work, the dietary specialization, short gut passage rate and strict habitat preferences of mistletoe specialists suggests that their services relate primarily to intensification and contagious dispersal, while species with broader diets are more likely to visit uninfected trees and establish new infections. The presumed importance of mistletoe‐specialist frugivores was not supported and mistletoes are considered to be comparable to many other bird‐dispersed plants, relying on both specialist and generalist frugivores, while opportunists may be disproportionately important in long‐distance dispersal.     

Comparative water use and nitrogen relationships in a mistletoe and its host

Summary The impact of the xylem-tapping mistletoe Phoradendron juniperinum on the nitrogen and water relations of its host Juniperus osteosperma was investigated under natural field conditions. Leaf conductance, leaf water potential, and leaf Kjeldahl nitrogen contents were followed through the growing season on mistletoes, infected junipers (separating infected from uninfected stems) and uninfected junipers. Infected trees experienced lower leaf water potentials than uninfected trees and also had lower leaf conductances and lower leaf nitrogen contents. Infected juniper stems had higher conductances than uninfected stems. Mistletoes had higher leaf nitrogen contents than their hosts and much of this nitrogen appeared as arginine, a potential nitrogen storage compound. Photosynthetic rates (per unit leaf area) were significantly higher in junipers than in the mistletoe, and higher in the uninfected than infected junipers. Water use efficiencies as estimated by carbon isotope ratios were significantly lower in mistletoes than in their hosts. Increased mistletoe infestation appeared to increase absolute water use efficiency of both host and mistletoe.Dedicated to Professors D. Wiens (Salt Lake City) and H. Ziegler (München) for initiating our curiosity in mistletoes     

From protégé to nurse plant: establishment of thorny shrubs in grazed temperate woodlands

Question: Thorny shrubs play keystone roles in grazed ecosystems by defending non‐protected plants against herbivores, but their establishment in grazed ecosystems is poorly understood. Which factors control establishment of recruits of thorny nurse shrubs in grazed temperate woodlands? Location: Ancient grazed temperate woodlands (52°32′N, 6°36′E), The Netherlands. Methods: We surveyed biotic and abiotic factors for saplings of thorny nurse shrubs in plots with and without saplings. To disentangle these factors, we performed a transplantation experiment over two growing seasons with nurse shrub saplings (Prunus spinosa and Crataegus monogyna) planted in two dominant vegetation types – tall unpalatable swards and short grazed lawns – half of them protected from herbivory via exclosures. Results: Plots with shrub saplings had taller surrounding vegetation, higher soil pH and higher soil moisture than plots without saplings. These plots predominantly contained unpalatable sward species, while plots without saplings mainly contained palatable lawn species. After transplantation, sapling survival was higher in exclosures than in the open, and higher in sward exclosures than in lawn exclosures. Sapling growth was higher in swards than in lawns, higher inside than outside exclosures, and higher for Prunus than Crataegus, while browsing on saplings was higher in lawns. Conclusion: Unpalatable swards form essential establishment niches for thorny shrubs in grazed temperate woodlands: they protect against herbivores before thorns fully develop in saplings, and sapling growth is better due to improved micro‐environmental conditions. Once established and thorny, shrub saplings grow out of the protective range of the swards and in turn facilitate tree seedlings, which are essential for long‐term persistence of grazed temperate woodlands. This study shows that nurse plants may start as protégés before becoming facilitators for other plants in a later life stage. This may be common for nurse plants in various ecosystems. We argue that improved understanding of establishment of nurse plants and their constraining factors is crucial for effective conservation and restoration in various ecosystems.     

Molecular fingerprinting of Helicanthus elastica (Desr.) Danser growing on five different hosts by RAPD

Mistletoes are hemiparasitic plants growing on aerial parts of other host trees. Many of the mistletoes are reported to be medicinally important. The hemiparasitic nature of these plants makes their chemical composition dependent on the host on which it grows. They are shown to exhibit morphological dissimilarities also when growing on different hosts. Helicanthus elastica (Desr.) Danser (mango mistletoe) is one such less explored medicinal mistletoe found on almost every mango tree in India. Traditionally, the leaves of this plant are used for checking abortion and for removing stones in the kidney and urinary bladder while significant antioxidant and antimicrobial properties are also attributed to this species of mistletoe. The current study was undertaken to evaluate molecular differences in the genomic DNA of the plant while growing on five different host trees using four random markers employing random amplified polymorphic DNA (RAPD) followed by similarity matrix by Jaccard’s coefficient and distance matrix by hierarchal clustering analysis. Similarity and distance matrix data employing just 4 random markers, separately and the pooled data as well, revealed significant difference in the genomic DNA of H. elastica growing on five different hosts. Pooled data of similarity from all the 4 primers cumulatively showed similarity between 0.256 and 0.311. Distance matrix ranged from of 0.256 to 0.281 on pooling the data from all the four primers. The result employing a minimum number of primers could conclude that genomic DNA of H. elastica differs depending upon the host on which it grows, hence the host must be considered while studying or utilizing this mistletoe for medicinal purposes.     

Host physiological condition regulates parasitic plant performance: <Emphasis Type="Italic">Arceuthobium vaginatum</Emphasis> subsp. <Emphasis Type="Italic">cryptopodum</Emphasis> on <Emphasis Type="Italic">Pinus ponderosa</Emphasis>

Much research has focused on effects of plant parasites on host-plant physiology and growth, but little is known about effects of host physiological condition on parasite growth. Using the parasitic dwarf mistletoe Arceuthobium vaginatum subsp. cryptopodum (Viscaceae) and its host Pinus ponderosa, we investigated whether changes in host physiological condition influenced mistletoe shoot development in northern Arizona forests. We conducted two studies in two consecutive years and used forest thinning (i.e., competitive release) to manipulate host physiological condition. We removed dwarf mistletoe shoots in April, before the onset of the growing season, and measured the amount of regrowth in the first season after forest thinning (Study I: n=38 trees; Study II: n=35 trees). Thinning increased tree uptake of water and carbon in both studies, but had no effect on leaf N concentration or δ¹³C. Mistletoe shoot growth was greater on trees with high uptake of water and carbon in thinned stands than trees with low uptake in unthinned stands. These findings show that increased resource uptake by host trees increases resources to these heterotrophic dwarf mistletoes, and links mistletoe performance to changes in host physiological condition.     

Spatial patterns and dynamics of woody vegetation in an arid savanna

The spatial distribution of woody plants was studied in an arid savanna in Botswana. The study included stands of mixed species and sizes as well as monospecific even-sized stands of different size classes of the tree Acacia erioloba and the shrub Acacia mellifera. In the case of A. mellifera both dense stands on overgrazed land and more open stands were included. The analysis used all plant-to-plant distances, and individuals were represented with a realistic canopy extension. The mixed stands showed aggregated distribution of individuals, mainly caused by strong clumping of small shrubs. In A. erioloba saplings were aggregated, small trees were randomly or regularly distributed and large trees were randomly spaced. In open stands of A. mellifera aggregation increased with size of the shrubs, while in dense stands with overgrazing aggregation decreased with increasing size. The different patterns are discussed in relation to the relative importance of inter- and intraspecific competition for water and of disturbance by fire as regulatory mechanisms for total amount and spatial distribution of woody plants in this savanna.     

Effect of brousse tigrée on the dynamics of nurse–protégé interactions of a cactus in the Chihuahuan Desert

A nurse–protégé relationship is a frequent facilitation interaction in deserts that allows the recruitment of new individuals of many species. Our aim was to evaluate the relationship of Echinocereus enneacanthus during its life cycle and its nurse plants in the brousse tigrée (tiger bush) vegetation of the Chihuahuan Desert. The population structure of the cactus is skewed toward adult stages, which are commonly found in the bare areas of the brousse tigrée, whereas juveniles are located in the vegetated bands. The vegetation structure and the nurse–protégé relationship were determined using censuses of 19 permanent plots monitored between 2008 and 2019. The differential association of E. enneacanthus with potential nurse plants was evaluated using an interaction network, under the hypothesis of strict association of the seedlings and juveniles with nurse species. In addition, χ² tests and standardized residuals were fitted among likely nurse species, weighted by the cover of the nurse and of the areas without vegetation. The study species, as well as the nurse plants, had an aggregated distribution pattern. The interaction network showed that the frequency of the association with nurse plants decreased as the size of the protégé individuals increased. At the same time, there were differences in the establishment of E. enneacanthus under particular nurse plants; Prosopis glandulosa and Hilaria mutica were the most important. The nurse–protégé relationship for seeds and seedlings is integral to the vegetation bands of the brousse tigrée.     

Moms are better nurses than dads: gender biased self‐facilitation in a dioecious Juniperus tree

Questions: Can gender of nurse plants affect regeneration patterns and spatial population structure? Is there a seed‐seedling conflict in the regeneration process? What factors are responsible for the clumped spatial population structure observed for adult trees? Location: Mediterranean cold semi‐arid high mountains in Spain. Methods: The spatial pattern of adult Juniperus thurifera trees was studied by means of Ripley's K‐analysis. χ² analyses were used to test for natural seedling frequency in each of three main microhabitats: (1) under female and (2) male tree canopies and (3) in open interspaces. The observed pattern was explained experimentally by studying seed and seedling survival for two years. Survival probabilities were calculated across life stages for each of three main microhabitats. Results: Adult J. thurifera trees were aggregated in space. Most seedlings were found underneath female J. thurifera trees. Experimental studies demonstrated that from seed dispersal to seedling survival all life stages showed the same positive or negative trend within a given microhabitat, indicating stage coupling and no seed‐seedling conflicts. Attraction of frugivo‐rous birds by reproductive female junipers and improvement of environmental conditions beneath tree canopies were the main factors responsible for the variation in seedling density among microhabitats; highest underneath female trees and lowest in open interspaces. Conclusions: In dioecious species, the gender of nurse plants can significantly determine the spatial population structure. In J. thurifera forests, facilitation beneath female trees occurs among all life stages without any sign of seed‐seedling conflict. The most critical factors shaping the spatial population structure were directed seed dispersal and environmental amelioration beneath female conspecific trees.     

AGE STRUCTURE OF PHORADENDRON JUNIPERINUM (VISCACEAE), A XYLEM-TAPPING MISTLETOE: INFERENCES FROM A NON-DESTRUCTIVE MORPHOLOGICAL INDEX OF AGE

We investigated the age-structure of the xylem-tapping mistletoe Phoradendron juniperinum in relation to characteristics of its host tree, Juniperus osteosperma. We first correlated branch structure in the mistletoe with age of the mistletoe infection as determined anatomically; this correlation provided a nondestructive, field method of obtaining age structure information. We then surveyed the mistletoe plants, applying our aging index, within a population of their host trees in southwestern Utah; the majority of mistletoe plants were 2–12 years old. This peak in abundance of mistletoe infections showed no correlation to total annual precipitation within or 1 year previous to the peaks, minimum winter temperature, or to warmer than average winter temperatures. However, there was a positive correlation (r = 0.51, P < 0.06) with the amount of summer precipitation. A log-linear analysis indicated that a greater than expected number of mistletoe infections occurred at 5–7 years of age and at approximately 3 m in height among all host trees. We suggest that this pattern resulted because this canopy position had greater leaf and branch areas and was visited most frequently by seed-dispersing birds. The log-linear analysis also revealed that fewer than expected mistletoe infections occurred at ages older than 10 years, yet our data indicate that plants can reach 20 years of age. The lack of infections > 10 years of age was correlated to a period of below average precipitation, especially during the growing season, but not with cold winter temperatures, which in other studies had been suggested as a factor influencing mortality. We feel that drought may play an important role in influencing mistletoe mortality through its direct affect on host tree water status, but in addition we offer two alternative hypotheses to explain mistletoe longevity; the first is concerned with the relationship between carbon and nitrogen costs and maintaining large leaf areas in older plants, and the second addresses how increased hydraulic resistance in older and larger plants may be too costly for the plant, and stems are abscised.     

Long‐term mortality patterns of the deep‐rooted Acacia erioloba: The middle class shall die!

Question: Is there a relationship between size and death in the long‐lived, deep‐rooted tree, Acacia erioloba, in a semi‐arid savanna? What is the size‐class distribution of A. erioloba mortality? Does the mortality distribution differ from total tree size distribution? Does A. erioloba mortality distribution match the mortality distributions recorded thus far in other environments? Location: Dronfield Ranch, near Kimberley, Kalahari, South Africa. Methods: A combination of aerial photographs and a satellite image covering 61 year was used to provide long‐term spatial data on mortality. We used aerial photographs of the study area from 1940, 1964, 1984, 1993 and a satellite image from 2001 to follow three plots covering 510 ha. We were able to identify and individually follow ca. 3000 individual trees from 1940 till 2001. Results: The total number of trees increased over time. No relationship between total number of trees and mean tree size was detected. There were no trends over time in total number of deaths per plot or in size distributions of dead trees. Kolmogorov‐Smirnov tests showed no differences in size class distributions for living trees through time. The size distribution of dead trees was significantly different from the size distribution of all trees present on the plots. Overall, the number of dead trees was low in small size classes, reached a peak value when canopy area was 20 ‐ 30 m², and declined in larger size‐classes. Mortality as a ratio of dead vs. total trees peaked at intermediate canopy sizes too. Conclusion: A. erioloba mortality was size‐dependent, peaking at intermediate sizes. The mortality distribution differs from all other tree mortality distributions recorded thus far. We suggest that a possible mechanism for this unusual mortality distribution is intraspecific competition for water in this semi‐arid environment.     

Thorn‐dwelling ants provide antiherbivore defence for camelthorn trees,Vachellia erioloba,in Namibia

Ants are widely employed by plants as an antiherbivore defence. A single host plant can associate with multiple, symbiotic ant species, although usually only a single ant species at a time. Different plant‐ant species may vary in the degree to which they defend their host plant. In Kenya, ant–acacia interactions are well studied, but less is known about systems elsewhere in Africa. A southern African species, Vachellia erioloba, is occupied by thorn‐dwelling ants from three different genera. Unusually, multiple colonies of all these ants simultaneously and stably inhabit trees. We investigated if the ants on V. erioloba (i) deter insect herbivores; (ii) differ in their effectiveness depending on the identity of the herbivore; and (iii) protect the tree against an important herbivore, the larvae of the lepidopteran Gonometa postica. We show that experimental exclusion of ants leads to greater levels of herbivory on trees. The ants inhabiting V. erioloba are an effective deterrent against hemipteran and coleopteran, but not lepidopteran herbivores. Defensive services do not vary among ant species, but only Crematogaster ants exhibit aggression towards G. postica. This highlights the potential of the V. erioloba–ant mutualism for studying ant–plant interactions that involve multiple, simultaneously resident thorn‐dwelling ant species.     

Parasites boost productivity: effects of mistletoe on litterfall dynamics in a temperate Australian forest

The importance of litter in regulating ecosystem processes has long been recognised, with a growing appreciation of the differential contribution of various functional plant groups. Despite the ubiquity of mistletoes in terrestrial ecosystems and their prominence in ecological studies, they are one group that have been overlooked in litter research. This study evaluated the litter contribution from a hemiparasitic mistletoe, Amyema miquelii (Lehm. ex Miq.) Tiegh., in an open eucalypt forest (Eucalyptus blakelyi, E. dwyeri and E. dealbata), at three scales; the forest stand, single trees and individual mistletoes. Litter from mistletoes significantly increased overall litterfall by up to 189%, the amount of mistletoe litter being proportional to the mistletoe biomass in the canopy. The high litter input was due to a much higher rate of mistletoe leaf turnover than that of host trees; the host litterfall and rate of leaf turnover was not significantly affected by mistletoe presence. The additional litter from mistletoes also affected the spatial and temporal distribution of litterfall due to the patchy distribution of mistletoes and their prolonged period of high litterfall. Associated with these changes in litterfall was an increase in ground litter mass and plant productivity, which reflects similar findings with root-parasitic plants. These findings represent novel mechanisms underlying the role of mistletoes as keystone resources and provide further evidence of the importance of parasites in affecting trophic dynamics.     

The incidence of mistletoe (<Emphasis Type="Italic">Viscum album</Emphasis> ssp. <Emphasis Type="Italic">abietis</Emphasis>) on silver fir (<Emphasis Type="Italic">Abies alba</Emphasis>) in Croatia

The research on incidence of mistletoe (Viscum album ssp. abietis) on silver fir (Abies alba) was carried out in natural fir stands in Croatia. In the area of Gorski Kotar 32.8 % of the examined dominant fir trees were infected with mistletoe. The mistletoe incidence was presented according to the damage degrees of silver fir from 0 (healthy trees) to 4 (dead trees), rated by the crown defoliation and needle discoloration. With the increase of incidence, mistletoe spreads more onto more vital, i.e. less damaged trees. In the Dinaric Alps 28.6% of the examined fir trees were infected, this percentage amounting to 27.1% for the mountainous regions between the Sava and Drava rivers. The site and stand parameters (exposure, elevation, site quality, forest community and crown closure) were analysed in order to establish whether there was a correlation between these parameters and mistletoe incidence. There was a negative correlation between the elevation and mistletoe incidence. Among the compartments with a closed stands there were considerably less compartments with higher mistletoe incidence than among compartments with a sparsely closed and understocked stands. The other analysed site and stand parameters individually had no significant influence on mistletoe incidence. The correlation of silver fir mortality in 2004 and mistletoe incidence in 2002/03 was analysed, and their strong correlation was established. Mistletoe could be considered as a bioindicator of silver fir decline, and probably a significant contributor to that decline. In the areas where mistletoe incidence is great it can be presumed that silver fir is significantly damaged.     

Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants

Question: How can we disentangle facilitation and seed dispersal from environmental heterogeneity as mechanisms causing spatial associations of plant species? Location: Semi‐arid savanna in the Kimberley Thorn Bushveld, South Africa. Methods: We developed a two‐step protocol for the statistical differentiation of association‐promoting mechanisms in plants based on the Acacia erioloba–Grewia flava association. Individuals of the savanna shrub G. flava and the tree A. erioloba were mapped on four study plots. Disentangling the mechanism causing the association of G. flava and A. erioloba involved tests of three spatial and one non‐spatial null model. The spatial null models include homogeneous and heterogeneous Poisson processes for spatial randomness based on the bivariate spatial point patterns of the four plots. With the non‐spatial analysis, we determined the relationship between the canopy diameter of A. erioloba trees and presence or absence of G. flava shrubs in the tree understorey to find whether shrub presence requires a minimum tree canopy diameter. Results: We first showed a significant positive spatial association of the two species. Thereafter, the non‐spatial analysis supported an exclusion of environmental heterogeneity as the sole cause of this positive association. We found a minimum tree size under which no G. flava shrubs occurred. Conclusions: Our two‐step analysis showed that it is unlikely that heterogeneous environmental conditions caused the spatial association of A. erioloba and G. flava. Instead, this association may have been caused by seed dispersal and/or facilitation (e.g. caused by hydraulic lift and/or nitrogen fixation by the host tree).     

Effect of mistletoes (Amyema preissii) on host (Acacia victoriae) survival

Abstract The effect of mistletoes Amyema preissii on the survival of fast growing trees of Acacia victoriae was investigated in arid central Australia. Trees with different levels of experimentally induced mistletoe infection were monitored for 4 years. Analysis of covariance failed to reveal a treatment effect of infection on host survival. Loss of trees early in the experiment reduced the statistical power of the analysis, and variable infection success and the establishment of bird-dispersed mistletoes on experimental trees blurred the distinction between treatments. However regression analysis showed a significant relationship between the volume of mistletoe and tree mortality. The implications for the design of future experiments are discussed.     

Long- and short-term impacts of a defoliating moth plus mistletoe on tree growth,wood anatomy and water-use efficiency

Climate warming and biotic stressors are expected to reduce tree radial growth and performance at short and long time scales. However, the impacts of different biotic stressors on performance throughout a tree’s life are largely understudied. Here we assessed the effects of a past nun moth (Lymantria dispar) outbreak and related defoliation on Scots pine (Pinus sylvestris) trees, which were later severely infested by the hemiparasite mistletoe (Viscum album subsp. austriacum). We compared the responses of trees severely infested or not infested by mistletoe in a wet vs. a dry site to quantify the relative importance of biotic stressors under different climate conditions. We used dendrochronology to quantify: long- and short-term changes in radial growth (resilience), differences in wood anatomy during the outbreak, and recent changes in intrinsic water-use efficiency (WUEi). The outbreak caused a sharp growth reduction in 1953 (50% decrease in basal area increment –BAI) and the formation of tracheids of small transversal lumen diameter (33% decrease in diameter). Recent mistletoe infestation caused a difference in growth between infested and non-infested trees lasting 34 and 21 years in the wet and dry sites, respectively. Growth (BAI) decreased more steeply in severely infested than in non-infested trees, the post-drought resilience decreased in severely infested trees, and the WUEi increased, particularly in the dry site. The BAI of severely infested trees was more negatively impacted by warm and dry conditions during the growing season than in non-infested trees, particularly in the dry site. Tree rings recorded historical effects of biotic stressors (L. monacha outbreak), which may constrain responses to recent stressors (mistletoe).     

Leaf gas exchange characteristics,daily carbon and water balances of the host/mistletoe pair Eucalyptus behriana F. Muell. and Amyema miquelii (Lehm. ex Miq.) Tiegh. at permanently low plant water status in the field

Summary Leaf gas exchange and xylem pressures of the xylem-tapping mistletoe, Amyema miquelii (Lehm. ex Miq.) Tiegh., and its host, Eucalyptus behriana F. Muell., both growing under permanently low plant water status, are studied under controlled and naturally fluctuating field conditions. Stomata of both plants regulate transpiration with respect to light and air humidity, but leaf conductances and total daily transpiration are up to 7.3 times higher in the host. Despite this, water-use efficiency in the mistletoe is lower than in the host. In the light of the mistletoe's low transpiration rates observed in spring, a commonly accepted pattern of higher water losses from parasites is likely to be inconsistent over a season. The role of partial parasitic carbon heterotrophy is discussed.     

The influence of mistletoes on nutrient cycling in a semi-arid savanna, southwest Zimbabwe

Parasitic plants are increasingly becoming the focus of research in many ecosystems. They have been shown to alter litterfall properties and decomposition rates in environments where they occur. Despite this recognition, the role of mistletoes in nutrient cycling in semi-arid savanna remains poorly understood. We investigated the litter input, element returns, and associated below-canopy soil nutrient concentrations of three mistletoe species (Erianthemum ngamicum, Plicosepalus kalachariensis, and Viscum verrucosum) that parasitize Acacia karroo trees in a semi-arid savanna, southwest Zimbabwe. Element concentrations in mistletoe leaf litter were enriched relative to the host. Litterfall from mistletoes significantly increased overall litterfall by up to 173 %, with E. ngamicum and P. kalachariensis having greater litterfall than their host trees. Associated with these changes in litterfall was an increase in element returns and the below-canopy soil nutrient concentrations. The increase in nutrient returns was due to both the effect of enriched mistletoe litter and increased volumes of litterfall beneath host trees. Litterfall, element returns, and the below-canopy soil nutrient concentrations were significantly influenced by mistletoe density, with higher values at high mistletoe density. Overall, E. ngamicum and P. kalachariensis had greater influence on litterfall, element returns, and soil nutrient concentrations than V. verrucosum. These findings are consistent with current understanding of enhanced nutrient cycling in the presence of parasitic plants particularly in nutrient-poor ecosystems. We conclude that the introduction of nutrients and associated increase in resource heterogeneity play an important role in determining ecosystem structure and function in semi-arid savannas.