Suppression of Regeneration in New Zealand Mountain Beech Forests is Dependent on Species of Introduced Deer

We compared the impacts on forest regeneration of introduced sika (Cervus nippon) and red (Cervus elaphus) deer in New Zealand. Plot data were used to compare mountain beech (Nothofagus solandri var. cliffortioides) regeneration between a region with sika deer, and four regions without sika deer. All regions surveyed had red deer present. In the region where sika deer had been present for more than a decade, there was evidence of poor mountain beech seedling regeneration. In the four regions without sika deer, there was evidence of a strong regenerative response at stands with low occupancy by trees. When compared to larger deer species, sika deer have a digestive morphology allowing greater dietary versatility, which may result in them impeding forest regeneration where red deer do not. In contrast to mountain beech, some small-leaved shrub species may have been competitively advantaged by intensive browsing from sika deer. This is contra to a current view that small-leaved shrub species with interlacing branches were able to tolerate browsing from extinct ratite birds, but not introduced deer. Sika deer have been introduced into countries where other deer species are indigenous, such as Canada, Denmark, Great Britain, Czech Republic, Ireland and the United States of America. Because of their dietary advantage, sika deer may have a greater potential to impede forest regeneration and competitively exclude larger deer species, particularly at low basal area sites where impacts on tree regeneration are likely to be greatest.     

Positive interactions between herbivores and plant diversity shape forest regeneration

The effects of herbivores and diversity on plant communities have been studied separately but rarely in combination. We conducted two concurrent experiments over 3 years to examine how tree seedling diversity, density and herbivory affected forest regeneration. One experiment factorially manipulated plant diversity (one versus 15 species) and the presence/absence of deer (Odocoileus virginianus). We found that mixtures outperformed monocultures only in the presence of deer. Selective browsing on competitive dominants and associational protection from less palatable species appear responsible for this herbivore-driven diversity effect. The other experiment manipulated monospecific plant density and found little evidence for negative density dependence. Combined, these experiments suggest that the higher performance in mixture was owing to the acquisition of positive interspecific interactions rather than the loss of negative intraspecific interactions. Overall, we emphasize that realistic predictions about the consequences of changing biodiversity will require a deeper understanding of the interaction between plant diversity and higher trophic levels. If we had manipulated only plant diversity, we would have missed an important positive interaction across trophic levels: diverse seedling communities better resist herbivores, and herbivores help to maintain seedling diversity.     

Herbivory and competition slow down invasion of a tall grass along a productivity gradient

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced aboveground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.Plant nomenclature follows Van der Meijden et al. (1990)     

Competitive effects of the invasive shrub,Lonicera maackii (Rupr.) Herder (Caprifoliaceae), on the growth and survival of native tree seedlings

Invasive plants are often associated with reduced cover of native plants, but rarely has competition between invasives and natives been assessed experimentally. The shrub Lonicera maackii, native to northeastern Asia, has invaded forests and old fields in numerous parts of eastern North America, and is associated with reduced tree seedling density in Ohio forests. A field experiment was conducted to test the effects of established L. maackii on the survival and growth of transplanted native tree species. The experiment examined above-ground competition (by removing L. maackii shoots) and below-ground competition (by trenching around transplanted seedlings). The effects of above-ground competition with L. maackii were generally more important than below-ground competition, though both were detected. Shoot treatment was the key determinant for the survival of all species except P. serotina, whereas trenching only enhanced survival for A. saccharum caged and P. serotina, and only in the shoot removal treatment. For the surviving seedlings, L. maackii shoot removal increased growth of A. saccharum seedlings protected with cages, but actually reduced the growth of unprotected Q. rubra and A. saccharum seedlings, indicating that L. maackii shoots confer some protection from deer browsing. Significant interactions between root and shoot treatment on Q. rubra growth parameters, specifically greatest growth in the shoot present & trenched treatment, is attributed to protection from deer browsing combined with release from below-ground competition. Despite this protective function of L. maackii shoots, the overall effect of this invasive shrub is increased mortality of native tree seedlings, suggesting it impacts the natural regeneration of secondary forests.     

Old-field seedling responses to insecticide,seed addition,and competition

The controls of seedling emergence and survival determine the potential distribution of adult plants and, thereby, plant community structure. Seed availability, competition from established neighbors, and seedling predation may all limit seedling recruitment. In this field experiment, we followed the emergence and survival of seedlings of three perennial forbs, Achillea millefolium, Hypericum perforatum, and Monarda fistulosa, in old-fields in southeastern Michigan, USA. As adults, all three have aromatic foliage that may deter herbivory, but seedlings may be more susceptible than adults. To establish the relative importance of potential controls on seedling numbers, we manipulated seed availability through seed additions, the influence of competitors by neighbor-removals, and the influence of insect herbivores with insecticide in a fully factorial field experiment. Seed addition and insecticide never affected seedling emergence for any species. Competition from established neighbors controlled seedling emergence for all three species and decreased Achillea survival. Insecticide significantly increased Monarda seedling survival in competition plots, significantly increased Hypericum survival in open plots, and had no effect on Achillea. Notably, insecticide increased survival of the native Monarda fistulosa more than the two introduced species. While neighbors strongly reduced emergence and survival of all three species, herbivores acted on a species-specific basis. These results suggest the differential effects of insects may contribute more to the seedling species composition and abundance patterns than the less-selective influence of competition.     

Herbivory on planted oak seedlings across a habitat edge created by timber harvest

Edge habitats create environmental gradients that affect plant community composition and herbivore behavior. Silvicultural disturbance creates edge habitat with direct (via changes in light) and indirect (via changes in herbivore behavior) consequences for the growth and survival of tree seedlings, and thus, the composition of the future forest stands. Herbivores, particularly ungulates, can be a major limiting factor in oak regeneration, and silvicultural disturbance may alter the abundance or behavior of herbivores following harvest. We measured the severity of herbivory on experimentally planted white (Quercus alba) and black oak (Quercus velutina) seedlings by white-tailed deer (Odocoileus virginianus) and eastern cottontail rabbits (Sylvilagus floridanus), as well as foliar damage from insects, across gradients created by clearcuts in a deciduous forest in Indiana, USA. Overall browse pressure on oaks was low in our study. Nonetheless, spatial variation in herbivory depended on herbivore taxa; herbivory by rabbits was highest inside harvest openings, whereas foliar damage by insects peaked in the forest. Intensity of deer herbivory was constant across the edge. In addition, we observed indirect interactions among herbivore species mediated by a seedling’s browsing history. Herbivore damage by deer was positively related to past browsing by rabbits, and foliar damage from insects was positively related to past browsing by both deer and rabbits. Increasing woody plant competition reduced herbivory on seedlings by both deer and rabbits. Given the lack of spatial variability in deer herbivory and low overall herbivory by rabbits, we suspect that interactions between timber harvesting and herbivory did not have a strong impact on oak seedlings at our study sites.     

The regeneration of pioneer tree species under browsing pressure of Sika deer in an evergreen oak forest

The effects of Sika deer (Cervus nippon) browsing on the regeneration of pioneer species were studied in relation to canopy gaps in a warm temperate evergreen oak forest in Kasugayama, Nara City. Four study sites, three in canopy gaps and one under a closed canopy, were selected and each divided into fenced and unfenced plots. Under the closed canopy, seedlings of all the pioneer species died irrespective of browsing pressure. However, in the canopy gap sites, seedlings of the pioneer species could establish and grow well. The seedling survival ratio in the fenced plots in the canopy gaps was >60% 1 year after germination. However, in the unfenced plots, only <20% of the seedlings survived 1 year, with all dying within 3 years after germination. Thus, the regeneration of pioneer trees in this forest was strongly inhibited by deer browsing. Successful regeneration of a pioneer,Zanthoxylum ailanthoides, occurred for several years even after two major wind disturbances during the past 90 years. This may be due to less browsing pressure from the deer.     

Herbivory and seedling performance in a fragmented temperate forest of Chile

Forest fragmentation alters plant-animal interactions, including herbivory. Relying manipulative experiments, we test if the reduction in insect herbivory associated with forest fragmentation translates into increased seedling growth and survival of three tree common species (Aristotelia chilensis, Cryptocarya alba and Persea lingue) in forest fragments and continuous forests in coastal Maulino forest, central Chile. Furthermore, we test if after protecting seedlings from herbivorous insects, plant performance is increased regardless of forest fragmentation. Nursery grown seedlings were transplanted into four forest fragments and a continuous forest during 2002. Insects, important herbivores in this forest, were excluded from half the seedlings by repeated applications of insecticides. Compared to continuous forests, in forest fragments, herbivory was reduced in all three species, seedling growth was greater in A. chilensis and C. alba but not in P. lingue, and survivorship was unaffected by herbivory or fragmentation in all three species. Protecting seedlings from insects reduced herbivory in the continuous forest to similar levels attained in the forest fragments. No change in herbivory results from by protecting seedlings in forest fragments. These results confirm that insects are important herbivores in the Maulino forest and also support the hypothesis that fragmentation can have strong indirect effects on plant communities as mediated through trophic interactions.     

Effects of introduced ungulates on forest understory communities in northern Patagonia are modified by timing and severity of stand mortality

Natural disturbances such as fires, windstorms, floods, and herbivory often act on plant communities, affecting their structure and the abundance and composition of their species. Most research has focused on the effects of single disturbances on plant communities whereas the synergistic effects of several disturbances have received less attention. In this study, we evaluated how timing and severity of tree mortality modified plant use by introduced deer and early post-mortality successional trajectories in northern Patagonian conifer forests. We sampled understory composition and deer use in Austrocedrus chilensis (ciprés de la cordillera) forest stands undergoing varying timing and severity of forest mortality as reconstructed using dendroecological techniques. In addition, we evaluated the effect of fallen logs on plant composition and deer use of plants by monitoring areas of massive dieback where fallen logs had been removed for fire hazard reduction, and nearby control areas not subjected to such removal. Stepwise regression analyses showed that history and severity of tree mortality strongly influence plant composition and deer use of plants. For deer use (with pellet counts and browsing index as response variables), results showed a positive relationship with degree of stand mortality and a negative relationship with cover of fallen logs. Similarly, cover of unpalatable shrub species was explained by canopy mortality history, whereas cover of palatable shrub species was positively associated with severity of canopy mortality. In areas where fallen logs had been removed, pellet counts were six times higher than those in control areas. Though total shrub species cover was similar between log removal and control areas, proportion of unpalatable shrubs increased in areas where fallen logs had been removed. In conclusion, deer use of plants was strongly limited by tall fallen logs, allowing palatable species to establish and grow. Fallen log removal accelerated deer entrance and changed understory composition toward more browse-resistant and unpalatable species. These results underscore the importance of considering the dynamics (timing, severity, and extent) of fallen woody debris influencing understory herbivory and post-disturbance succession. In addition, experimental results underpin the importance of maintaining snags and large woody debris in disturbed landscapes where salvage logging is a routine procedure.     

Fallow deer impacts on Wakatipu beech forest

This study uses data from forty-nine 20 m × 20 m permanent plots measured in 1976, 1982, 1989 and 1997-2002 in Wakatipu Forest, western Otago. We relate changes in red (Nothofagus fusca), silver (Nothofagus menziesii) and mountain beech (Nothofagus solandri var. cliffortioides) forest vegetation to the presence of fallow deer (Dama dama). Vegetation composition is likely to have been altered prior to plot establishment, and results show that there was little change in vegetation composition during the study. There are some signs of fallow deer reducing silver beech seedling abundance at some plots, but most beech stands appear to be in the late phase of development and undergoing self thinning, so are probably not immediately vulnerable to suppression of canopy regeneration. In the future, widespread disturbance is likely to release plants in the understorey from competition for light and nutrients, and at that stage fallow deer browsing of beech species may be able to alter successional pathways more than has occurred over the past two decades. Seedling and sapling density of the palatable Griselinia littoralis (broadleaf) was probably insufficient to ensure successful regeneration. Comparisons of seedling densities with other mixed beech forests throughout New Zealand suggest that fallow deer density will need to be nearly zero in Wakatipu Forest before regeneration of all palatable subcanopy hardwood species is assured.     

Disturbance, Herbivory, and Propagule Dispersal Control Dominance of an Invasive Grass

Despite the dramatic changes invasive plants cause to ecosystems and communities, factors that control dominance of invasive species after establishment in a community are poorly understood. Most active management relies on catastrophic disturbances of invasive-dominated communities to increase richness and diversity of plant communities. This study examines the importance of propagule dispersal and deer herbivory on continued dominance of Phalaris arundinacea after a non-catastrophic, short-term disturbance to monotypic stands of this invasive grass. The disturbance caused no change in P. arundinacea cover among treatments during any year of the study and, thus, simulates disturbance intensity more likely to be encountered in unmanaged settings. Despite the small disturbance, the combinations of disturbance + seeding and disturbance + seeding + deer exclusion caused greater species richness than controls even three years after disturbance. Increased invasion of P. arundinacea stands caused few effects on the dominant, as P. arundinacea biomass was unaffected after the first year. Selective herbivory by deer of species other than P. arundinacea increased the effects of disturbance and seeding, and aided in continued dominance of the grass. The tolerance of P. arundinacea for direct anthropogenic effects, including poor water quality and hydroperiod fluctuations, and indirect effects, such as increased herbivory by historically high deer populations, indicates the complexity of determining persistence of invasive species.     

Deer exclusion unveils abiotic filtering in forest understorey plant assemblages

Background and AimsThe role of deer (family Cervidae) in ecosystem functioning has traditionally been neglected by forest ecologists due to the animal’s scarcity in most parts of the northern hemisphere. However, the dramatic rebound in deer populations throughout the 20th century has brought deer browsing to the forefront of forest ecological questioning. Today there is ample evidence that deer affect tree regeneration, understorey plant and animal diversity, and even litter decomposition. However, the mechanisms underlying the effects of deer on forest ecosystems remain unclear. Among others, the relative role of abiotic factors versus biotic interactions (e.g. herbivory) in shaping plant assemblages remains largely unknown.MethodsWe used a large-scale experiment with exclosures distributed along abiotic gradients to understand the role of black-tailed deer (Odocoileus hemionus sitchensis) on the forest understorey on the Haida Gwaii archipelago (western Canada), a unique context where most of the key ecological effects of deer presence have already been intensively studied.Key ResultsOur results demonstrate that 20 years of deer exclusion resulted in a clear increase in vascular plant richness, diversity and cover, and caused a decline in bryophyte cover. Exclusion also unveiled abiotic (i.e. soil water availability and fertility) filtering of plant assemblages that would otherwise have been masked by the impact of abundant deer populations. However, deer exclusion did not lead to an increase in beta diversity, probably because some remnant species had a competitive advantage to regrow after decades of over browsing.ConclusionsWe demonstrated that long-term herbivory by deer can be a dominant factor structuring understorey plant communities that overwhelms abiotic factors. However, while exclosures prove useful to assess the overall effects of large herbivores, the results from our studies at broader scales on the Haida Gwaii archipelago suggest that exclosure experiments should be used cautiously when inferring the mechanisms at work.     

Fire intensity and herbivory effects on postfire resprouting of Adenostoma fasciculatum in southern California chaparral

Summary Resprouting is the main regeneration mechanism after fire in Mediterranean-type ecosystems. Herbivores play an important role in controlling postfire seedling establishment, but their influence on regeneration by resprouting is less well known. To study the effects of fire intensity on resprouting of Adenostoma fasciculatum in southern California chaparral, and its interaction with herbivory, we conducted an experimental burn at three levels of fire intensity. We found that increasing fire intensity increased plant mortality, reduced the number of resprouts per plant, and delayed the time of resprouting. Herbivory increased with fire intensity, and was related to the time of resprouting. Plants resprouting later in the season and out of synchrony with the main flush were attacked more readily by herbivores. Post-resprouting mortality also increased with fire intensity and was significantly associated with herbivory in the higher fire intensity plots. Fire intensity effects on chaparral regeneration by resprouting may be farreaching through effects on the population structure, resprout production, and growth of Adenostoma fasciculatum.     

Diversity in plants and other Collembola ameliorate impacts of Sminthurus viridis on plant community structure

Five experiments investigated the importance of herbivory by Sminthurus viridis in structuring botanical composition in developing grasslands, and how these effects may be modified by diversity in collembolan and plant species. Differential susceptibility to S. viridis feeding was demonstrated in 23 dicotyledonous and three monocotyledonous plants assayed as seedlings at the first true leaf stage. The composition of seedling communities developing from natural and artificially constructed soil seed banks varied with the level of S. viridis infestation, with plant species least susceptible to herbivory making the greatest contribution to plant biomass. The combined effect of herbivory by S. viridis and Bourletiella hortensis on Trifolium repens biomass was shown to be less than the effect of S. viridis alone, indicating competitive interference. The adverse effects of herbivory by S. viridis on T. repens biomass was reduced by increased diversity of plants growing in association with the legume, and the presence of four non-herbivorous arthropleonan Collembola. S. viridis was shown to reduce seedling numbers, species diversity and biomass in communities developing from the soil seed bank, but the presence of non-herbivorous arthropleonan species reduced the effect of S. viridis. The experiments demonstrate the potential for herbivory by S. viridis to significantly alter species composition in developing grassland communities. However, interactions with collembolan and plant species profoundly modified S. viridis herbivory impacts, either by reducing feeding intensity or enhancing plant growth. These results highlight the fact that data from simple, synthetic systems may be poor predictors of herbivory impacts under field conditions where more complex species interactions occur.     

Influence of herbivores on a perennial plant: variation with life history stage and herbivore species

Herbivores have diverse impacts on their host plants, potentially altering survival, growth, fecundity, and other aspects of plant performance. Especially for longer-lived plant species, the effects of a single herbivore species can vary markedly throughout the life of the host plant. In addition, the effects of herbivory during any given life history stage of a host plant may also vary considerably with different types of herbivores. To investigate the effects of herbivory by black-tailed deer (Odocoileus hemionus columbianus) and snails (Helminthoglypta arrosa and Helix aspersa) on a nitrogen-fixing shrub, Lupinus chamissonis, we established three exclosure experiments in a sand dune system on the coast of northern California. These experiments documented that deer browsing significantly reduced the volume and growth rate of lupines in the seedling and juvenile life stages. Since plant volume was strongly correlated with aboveground dry biomass for lupines, such herbivore-induced reductions in volume should translate into losses of aboveground biomass. Deer browsing also significantly altered the likelihood of attack by and density of a leaf-galling cecidomyid fly (Dasineura lupinorum), suggesting that a vertebrate herbivore indirectly affected an invertebrate herbivore in this system. Although deer did not significantly affect the survival of lupine seedlings and juveniles, individuals protected from deer had consistently greater survival in the two separate experiments. Our results revealed that snails did not have a significant effect on the survival or growth of juvenile plants, despite being common on and around lupines. An exclosure experiment revealed that herbivory by deer significantly reduced the shoot lengths of mature shrubs, but led only to a minimal reduction in growth rates. In addition, we found that browsed shrubs had significantly greater inflorescence production, but also produced individual seeds with significantly reduced mass. Collectively, these data indicate that deer and snails have widely differing effects on their shared host plant; browsing by deer indirectly affects insect herbivores, and the impacts of deer change markedly with the life history stage of their host plant.     

Effects of white-tailed deer on the population dynamics of acorns,seedlings and small saplings of Quercus buckleyi

To measure the effects of white-tailed deer (Odocoileus virginianus) herbivory on seeds, seedlings, and young saplings of Quercus buckleyi on the eastern Edwards Plateau of central Texas, USA, experimental fenced deer exclosures were constructed. Acorns or small Q. buckleyi transplants were placed in each exclosure and in each unfenced control plot. Deer did not significantly affect acorn survival and germination, but did significantly reduce transplant survival and growth rates. The results support the hypothesis that deer are responsible for the failure of recruitment into adult size classes in Q. buckleyi populations in this region. Without adult recruitment of Q. buckleyi, the species composition and possibly even the physiognomy of woodlands on the eastern Edwards Plateau will change markedly. The results of this experiment also indicate that, although juniper (Juniperus ashei) and Q. buckleyi presumably compete for water, light, and nutrients, in the presence of deer junipers can have a positive effect on seedlings and saplings of Q. buckleyi, a case of facilitation. In the presence of deer the transplants increased in height significantly more slowly away from juniper saplings than they did beneath juniper saplings, probably due to the physical protection from browsing that junipers provided to the transplants.     

Tolerance to deer herbivory and resistance to insect herbivores in the common evening primrose (Oenothera biennis)

The evolution of plant defence in response to herbivory will depend on the fitness effects of damage, availability of genetic variation and potential ecological and genetic constraints on defence. Here, we examine the potential for evolution of tolerance to deer herbivory in Oenothera biennis while simultaneously considering resistance to natural insect herbivores. We examined (i) the effects of deer damage on fitness, (ii) the presence of genetic variation in tolerance and resistance, (iii) selection on tolerance, (iv) genetic correlations with resistance that could constrain evolution of tolerance and (v) plant traits that might predict defence. In a field experiment, we simulated deer damage occurring early and late in the season, recorded arthropod abundances, flowering phenology and measured growth rate and lifetime reproduction. Our study showed that deer herbivory has a negative effect on fitness, with effects being more pronounced for late‐season damage. Selection acted to increase tolerance to deer damage, yet there was low and nonsignificant genetic variation in this trait. In contrast, there was substantial genetic variation in resistance to insect herbivores. Resistance was genetically uncorrelated with tolerance, whereas positive genetic correlations in resistance to insect herbivores suggest there exists diffuse selection on resistance traits. In addition, growth rate and flowering time did not predict variation in tolerance, but flowering phenology was genetically correlated with resistance. Our results suggest that deer damage has the potential to exert selection because browsing reduces plant fitness, but limited standing genetic variation in tolerance is expected to constrain adaptive evolution in O. biennis.     

Gnawing damage by rodents to the seedlings ofFagus crenata andQuercus mongolica var.grosseserrata in a temperateSasa grassland-deciduous forest series in southwestern Japan

The effects of dwarf bamboo,Sasa, cover on the initial morrality of hardwood seedlings were investigated by transplanting 1-year-old beech (Fagus crenata) and current-year oak (Quercus mongolica var.grosseserrata) seedling to three different stands; old-growth beech and secondary oak forests withSasa undergrowth, and aSasa grassland in a grassland-forest series near the top of Mt Jippo, southwestern Japan. The most frequent cause of seedling morrality was gnawing of the stems by rodents. In the beech forest, the gnawing was more likely to occur underSasa cover, suggesting that it provides a good habitat for rodents on the beech forest floor. TheSasa under growth may thus play an imporrant role in regeneration of beech forest. In the oak floor, mortality of both species was low and only a little gnawing occurred during a year. However, no natural oak seedling were found in the forest even after a mast year. This may be because most of the acorns disappeated before establishment. The early-stage demography of hardwood seedling as oak may thus play an imporrant role in regeneration of oak forest. In theSasa grassland where the seed supply is small, almost all of the seedlings died fromo gnawing regardless of the presence ofSasa cover. These factors prevent the recruitment of a sizable seedling bank. Rodents may thus play an imporrant role in maintenance of theSasa grassland.     

Limitations to seedling establishment in a mesic Hawaiian forest

While invasive species may be visible indicators of plant community degradation, they may not constitute the only, or even the primary, limitation to stand regeneration. We used seed-augmentation and grass-removal experiments under different canopy conditions to assess the relative importance of dispersal limitation, resource availability, and competition on seedling establishment in the understory shrubs Sophora chrysophilla, Dodonea viscosa, and Pipturus albidus in a montane mesic forest in Hawaii. The study location was an Acacia koa-Metrosideros polymorpha forest at 1000–1500 m elevation on the leeward side of Hawaii Island; it is a closed-canopy forest historically subject to logging and grazing by cattle and sheep and currently dominated by the exotic grass, Ehrharta stipoides, in the herb layer. Seedling establishment after 1 and 2 years was strongly dispersal limited in Sophora and Dodonea, but not in Acacia, a non-augmented species in which abundant seedlings established, nor in Pipterus, in which only one seedling established in 2 years. Grass cover reduced seedling establishment in Acacia, Sophora, and Dodonea and, for the latter two species, seedling establishment was substantially greater in the warmer, more moist forest at the lowest elevation. Light, moisture, and resin-captured N and P were more strongly affected by elevation and canopy composition than by grass cover, but in most cases seedling establishment was not positively correlated with resource availability. Limitations to the establishment of woody seedlings in this forest-grassland mixture vary among species; however, both dispersal limitation and competition from a shade-tolerant grass are important deterrents to regeneration in these forests.     

Positive and negative effects of herbivory on the population dynamics of Senecio jacobaea L. and Cynoglossum officinale L.

Summary Herbivore effects were studied on populations of the biennial plant species Senecio jacobaea and Cynoglossum officinale. During a three year period (1985–1988) population characteristics (herbivory, number of seedlings, rosettes and flowering plants) were compared in-and outside exclosures, as well as parameters reflecting vegetation cover. In S. jacobaea, a strong negative effect of Tyria jacobaeae was found on seedling establishment, rosette growth and flowering. On the other hand, vertebrate herbivores (mainly rabbits) had an indirect positive effect by limiting the development of the surrounding vegetation (esp. grasses). The increasing vegetation cover in protected populations caused a reduction in germination, seedling- and rosette-growth. Herbivory on C. officinale was low (<10%), no direct effects of herbivores on plant populations were shown. Indirect effects of herbivory through an increasing vegetation were even more pronounced as in S. jacobaea. Therefore, although both plant species may first benefit from herbivore-exclusion, their populations are dependent on rabbits eating other plants (esp. grasses) and reducing competition.Publication of the Meijendel comité, New Series no. 108