Introduced deer reduce native plant cover and facilitate invasion of non-native tree species: evidence for invasional meltdown

Invasive species are a major threat to native communities and ecosystems worldwide. One factor frequently invoked to explain the invasiveness of exotic species is their release in the new habitat from control by natural enemies (enemy-release hypothesis). More recently, interactions between exotic species have been proposed as a potential mechanism to facilitate invasions (invasional meltdown hypothesis). We studied the effects of introduced deer on native plant communities and exotic plant species on an island in Patagonia, Argentina using five 400 m² exclosures paired with control areas in an Austrocedrus chilensis native forest stand. We hypothesized that introduced deer modify native understory composition and abundance and facilitate invasion of introduced tree species that have been widely planted in the region. After 4 years of deer exclusion, native Austrocedrus and exotic Pseudotsuga menziesii tree sapling abundances are not different inside and outside exclosures. However, deer browsing has strongly inhibited growth of native tree saplings (relative height growth is 77% lower with deer present), while exotic tree sapling growth is less affected (relative height growth is 3.3% lower). Deer significantly change abundance and composition of native understory plants. Cover of native plants in exclosures increased while cover in controls remained constant. Understory composition in exclosures after only 4 years differs greatly from that in controls, mainly owing to the abundance of highly-browsed native species. This study shows that introduced deer can aid the invasion of non-native tree species through negatively affecting native plant species.     

Deciduous woodland conservation under heavy deer browsing on Devils Tower National Monument, USA

Native deciduous woodlands comprise <5% of the vegetation in the Black Hills and northern plains of the USA and appear to be in decline. Because deciduous woodlands comprise 18% of the land cover on Devils Tower National Monument (DTNM), this natural area is potentially important for the conservation of these plant communities. Lack of tree regeneration and accumulation of dead wood suggest that the native deciduous woodlands on DTNM may die out under persistent, heavy deer (Odocoileus virginianus, O. hemionus) browsing. We monitored vegetation changes for seven years (1989–1996) inside and outside 6 deer exclosures on DTNM to determine the resilience of native woodland communities when protected from browsing, and to parameterize models that evaluate specific fencing scenarios for managing woodland communities. The herbaceous woodland understory was not directly, but indirectly affected by deer browsing because graminoid biomass declined as tall shrubs recovered. Tall shrubs, dominated by Prunus spp. showed the most rapid and consistent recovery across sites. Models of shrub growth indicated that rotational fencing for short durations (10 years) produced the highest long-term increase in tall-shrub densities and the greatest evenness among size classes. Because deciduous woodlands currently are resilient and occur in small patches, fencing may provide a practical management alternative to deer culling programs.     

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.     

Shrubs as ecosystem engineers across an environmental gradient: effects on species richness and exotic plant invasion

Ecosystem-engineering plants modify the physical environment and can increase species diversity and exotic species invasion. At the individual level, the effects of ecosystem engineers on other plants often become more positive in stressful environments. In this study, we investigated whether the community-level effects of ecosystem engineers also become stronger in more stressful environments. Using comparative and experimental approaches, we assessed the ability of a native shrub (Ericameria ericoides) to act as an ecosystem engineer across a stress gradient in a coastal dune in northern California, USA. We found increased coarse organic matter and lower wind speeds within shrub patches. Growth of a dominant invasive grass (Bromus diandrus) was facilitated both by aboveground shrub biomass and by growing in soil taken from shrub patches. Experimental removal of shrubs negatively affected species most associated with shrubs and positively affected species most often found outside of shrubs. Counter to the stress-gradient hypothesis, the effects of shrubs on the physical environment and individual plant growth did not increase across the established stress gradient at this site. At the community level, shrub patches increased beta diversity, and contained greater rarified richness and exotic plant cover than shrub-free patches. Shrub effects on rarified richness increased with environmental stress, but effects on exotic cover and beta diversity did not. Our study provides evidence for the community-level effects of shrubs as ecosystem engineers in this system, but shows that these effects do not necessarily become stronger in more stressful environments.     

Influence of ungulates on the vegetation composition and diversity of mixed deciduous and coniferous mountain forest in Austria

The often highly elevated stocks of ungulates (red and roe deer and chamois) in the Alps shape the composition of the woody vegetation. The aim of this study was to investigate the influence of ungulates on the mixed deciduous and coniferous mountain forest in the district of Reutte, which boasts the highest density of ungulates in Tyrol (Austria), with a special focus on the effect of browsing by ungulates on plant diversity of the herb layer, different shrub layers. and the tree layer. Our results showed that within the fenced ungulate exclosures, (1) the composition of trees shifted towards fir (Abies alba) and various deciduous trees, whereas outside the fences, spruce became the dominant species; (2) the cover of dwarf shrubs and upper and lower shrub layers (1.3–5.0 and 0.5–1.3 m, respectively) increased significantly; (3) the cover of grasses decreased significantly and (4) the diversity decreased as an increase in the diversity of the tree and shrub layer was overcompensated by a significant decrease in the diversity of the undergrowth vegetation. Browsing by ungulates benefited grass species in the understory and altered the relative abundance of tree species in the lower layer which could, over time, result in compositional shifts in the canopy.     

An unfortunate alliance: Native shrubs increase the abundance,performance, and apparent impacts of Bromus tectorum across a regional aridity gradient

Interspecific facilitation contributes to the assembly of desert plant communities. However, we know little of how desert communities invaded by exotic species respond to facilitation along regional-scale aridity gradients. These measures are essential for predicting how desert plant communities might respond to concomitant plant invasion and environmental change. Here, we evaluated the potential for Bromus tectorum (a dominant invasive plant species) and the broader herbaceous plant community to form positive associations with native shrubs along a substantial aridity gradient across the Great Basin, Mojave, and San Joaquin Deserts in North America. Along this gradient, we sampled metrics of abundance and performance for B. tectorum, all native herbaceous species combined, all exotic herbaceous species combined, and the total herbaceous community using 180 pairs of shrub and open microsites. Across the gradient, B. tectorum formed strong positive associations with native shrubs, achieving 1.6–2.2 times greater abundance, biomass, and reproductive output under native shrubs than away from shrubs, regardless of relative aridity. In contrast, the broader herbaceous community was not positively associated with native shrubs. Interestingly, increasing B. tectorum abundance corresponded to decreasing native abundance, native species richness, exotic species richness, and total species richness under but not away from shrubs. Taken together, these findings suggest that native shrubs have considerable potential to directly (by increasing abundance and performance) and indirectly (by increasing competitive effects on neighbors) facilitate B. tectorum invasion across a large portion of the non-native range.     

POST-FIRE REGENERATION OF SOUTHERN CALIFORNIA CHAPARRAL

Succession of chaparral shrubs was studied for the first 4 years after fire on the four major slope faces at three elevations in southern California. Although total cover fluctuated from year to year, shrub cover increased annually through the third year. There was little or no increase in shrub cover between the third and fourth years. Four years after fire the shrubs covered 55% ground surface at the highest elevation site but only 28% at the lowest elevation site. Shrub cover was similar between slope faces with one exception; at the lowest elevation site shrub cover was twice as great on the north and east-facing slopes as on the south and west-facing slopes. Post-fire recovery of shrubs was by seedlings and/or resprouts. Yucca whipplei was an exception in that it did not resprout once the aboveground parts were killed nor did seedlings establish after fire, however the aboveground parts of many Y. whipplei survived the fire. Species which reproduced entirely by seed did so in the first post-fire year from soil-stored seed with the exception of Ceanothus greggii at the highest elevation site. This species was entirely absent the first year after fire but abundant in the second year. Species producing both resprouts and seedlings varied from site to site in the proportion of resprouts:seedlings. Between 83–100% of the post-fire populations of Cercocarpus betuloides, Arctostaphylos glandulosa, and Xylococcus bicolor were resprouts whereas 12–13% of Ceanothus tomentosus were resprouts. For Adenostoma fasciculatum, resprouts constituted 27–54% of the population at the lowest elevation and 65–94% at the highest elevation; whereas, the Quercus dumosa population was 100% resprouts at the lowest elevation and 31–67% resprouts at the highest elevation. Data are presented on the height of resprouts and seedlings for all species at the end of the first post-fire year.     

Nurse plants,tree saplings and grazing pressure: changes in facilitation along a biotic environmental gradient

Current conceptual models predict that an increase in stress shifts interactions between plants from competitive to facilitative; hence, facilitation is expected to gain in ecological importance with increasing stress. Little is known about how facilitative interactions between plants change with increasing biotic stress, such as that incurred by consumer pressure or herbivory (i.e. disturbance sensu Grime). In grazed ecosystems, the presence of unpalatable plants is reported to protect tree saplings against cattle grazing and enhance tree establishment. In accordance with current conceptual facilitation-stress models, we hypothesised a positive relationship between facilitation and grazing pressure. We tested this hypothesis in a field experiment in which tree saplings of four different species (deciduous Fagus sylvatica, Acer pseudoplatanus and coniferous Abies alba, Picea abies) were planted either inside or outside of the canopy of the spiny nurse shrub Rosa rubiginosa in enclosures differing in grazing pressure (low and high) and in exclosures. During one grazing season we followed the survival of the different tree saplings and the level of browsing on these; we also estimated browsing damage to the nurse shrubs. Shrub damage was highest at the higher grazing pressure. Correspondingly, browsing increased and survival decreased in saplings located inside the canopy of the shrubs at the high grazing pressure compared to the low grazing pressure. Saplings of both deciduous species showed a higher survival than the evergreens, while sapling browsing did not differ between species. The relative facilitation of sapling browsing and sapling survival – i.e. the difference between saplings inside and outside the shrub canopy – decreased at high grazing pressure as the facilitative species became less protective. Interestingly, these findings do not agree with current conceptual facilitation-stress models predicting increasing facilitation with abiotic stress. We used our results to design a conceptual model of facilitation along a biotic environmental gradient. Empirical studies are needed to test the applicability of this model. In conclusion, we suggest that current conceptual facilitation models should at least consider the possibility of decreasing facilitation at high levels of stress.     

Bottom-up trophic cascades caused by moose browsing on a natural enemy of a galling insect on balsam fir

To elucidate the impact of browsing by moose on the indigenous insect community, we investigated the abundance of a specialist insect herbivore (galler; Paradiplosis tumifex) and its natural enemy (inquiline; Dasineura balsamicola) on balsam fir saplings in the field using exclosures in Newfoundland, Canada. The number of current-year shoots on each sapling, but not the density of balsam fir saplings, was significantly higher inside exclosures, where moose had been excluded for 9–12 years, than outside exclosures. Current-year shoots were longer and contained more, but smaller, needles inside than outside exclosures. Moose browsing had a negative impact on the abundance of both the galler and its natural enemy. Gall density was 85.6% and 60.4% lower in 2006 and 2007, respectively, outside than inside exclosures. Similarly, the attack rate of D. balsamicola was 76.5% lower outside than inside exclosures in 2007. The apparent survival rate of P. tumifex larvae was similar outside and inside exclosures due to higher rates of unexplained mortality in galls outside exclosures. Survival of P. tumifex larvae was positively related to shoot length and inversely related to gall density. Only gall density was negatively related to D. balsamicola survival. Our results demonstrate that a large mammal can cause strong bottom-up trophic cascades.     

Long-Term Regional Shifts in Plant Community Composition Are Largely Explained by Local Deer Impact Experiments

The fact that herbivores and predators exert top-down effects to alter community composition and dynamics at lower trophic levels is no longer controversial, yet we still lack evidence of the full nature, extent, and longer-term effects of these impacts. Here, we use results from a set of replicated experiments on the local impacts of white-tailed deer to evaluate the extent to which such impacts could account for half-century shifts in forest plant communities across the upper Midwest, USA. We measured species'' responses to deer at four sites using 10–20 year-old deer exclosures. Among common species, eight were more abundant outside the exclosures, seven were commoner inside, and 16 had similar abundances in- and outside. Deer herbivory greatly increased the abundance of ferns and graminoids and doubled the abundance of exotic plants. In contrast, deer greatly reduced tree regeneration, shrub cover (100–200 fold in two species), plant height, plant reproduction, and the abundance of forbs. None of 36 focal species increased in reproduction or grew taller in the presence of deer, contrary to expectations. We compared these results to data on 50-year regional shifts in species abundances across 62 sites. The effects of herbivory by white-tailed deer accurately account for many of the long-term regional shifts observed in species'' abundances (R² = 0.41). These results support the conjecture that deer impacts have driven many of the regional shifts in forest understory cover and composition observed in recent decades. Our ability to link results from shorter-term, local experiments to regional long-term studies of ecological change strengthens the inferences we can draw from both approaches.     

Cumulative effects of wild ungulate and livestock herbivory on riparian willows

We examined the effects of wild ungulates (deer and elk) and domestic sheep browsing on the growth, structure, and reproductive effort of two common willow species, Salix boothii and S. geyeriana, in a montane northeast Oregon riparian zone. With the use of exclosures, large herbivore effects on willows were studied in an area browsed by native mammals only and an adjacent area in which domestic sheep also lightly grazed during summer months. Growth variables were repeatedly measured on individual plants over a 5-year period to understand physiognomic and flowering responses of native willows to different levels of browsing pressure. At the beginning of the study, all willows were intensely browsed but were significantly taller in the area browsed only by native mammals than in the area also grazed by sheep (69 versus 51 cm, respectively). Willows inside exclosures responded with pronounced increases in height, crown area, and basal stem diameters while the stature of browsed plants outside exclosures stayed constant or declined. In the area browsed by both sheep and wild herbivores, the size of browsed plants remained at pre-treatment levels (<60 cm in height) for the duration of the study. There was no significant difference in growth rates of enclosed willows, indicating that current herbivory was the primary cause of growth retardation in the study area. Foliar area was strongly correlated with basal stem numbers for enclosed plants but much less so for browsed plants. Willows inside exclosures had more than twice as much foliar area per stem. Stem diameters were a positive function of crown area: stem-number ratios, suggesting lower photosynthetic potential was correlated with diminished radial growth among browsed plants. No flowering was observed until 2 years after exclusion when plants inside all exclosures and browsed willows in the wild ungulate area responded with a large pulse in flowering. Browsed plants in the sheep + wild ungulate area did not flower. The number of catkins produced per plant was significantly associated with willow height and plants <70 cm in height did not flower, thus suggesting a size threshold for reproduction in these species. Our results suggest that even relatively light levels of domestic livestock grazing, when coupled with intense wild ungulate browsing, can strongly affect plant structure and limit reproduction of riparian willows.     

Mortality of resprouting chaparral shrubs after a fire and during a record drought: physiological mechanisms and demographic consequences

We examined postfire regeneration of chaparral shrubs during an intense drought. This study focused on the demography and physiology of shrub species that resprout from a basal lignotuber following fire. We found significant levels of resprout mortality when intense drought occurred in the year following fire during the period of shrub recovery. Three of the seven sampled resprouting species had the greatest or near greatest levels of mortality ever recorded when compared to previous studies. Most shrub mortality occurred during the drought after individuals had resprouted (i.e. individuals survived fire, resprouted and then subsequently died). Physiological measurements of species with high mortality suggested that resprout stems were highly embolized and xylem hydraulic conductivities were close to zero during the peak of the drought. In addition, lignotubers of two of the three species experiencing high mortality were depleted of starch. Population densities of most shrub species declined after the drought compared with their prefire levels, with the exception of one drought tolerant obligate seeding species. Resprouting shrub species may deplete their carbohydrate reserves during the resprouting process, making them particularly vulnerable to drought because of the need to transpire water to acquire the CO₂ that is used to supply energy to a large respiring root system. Drought appears to interact with fire by altering postfire shrub recovery and altering species abundances and composition of chaparral communities.     

Species‐specific influences of shrubs on the non‐dormant soil seed bank of native and exotic plant species in central‐northern Monte Desert

Deserts shrubs are well known to facilitate vegetation aggregation, mostly through seed trapping, and stress amelioration during and after plant establishment. Because vegetation aggregation effects are a by‐product of shrub presence, beneficiary species may not only be native, but also exotic. However, despite the high risk that exotic invasive species pose to ecosystem services, little is known of the role of desert shrubs on plant invasions. We assessed the influence of two shrub species on the non‐dormant soil seed bank (i.e. the number of seeds that readily germinate with sufficient water availability) of an invasive annual grass (Schismus barbatus) and of coexisting native species in a central‐northern Monte Desert (Argentina). Soil samples were collected beneath the canopies of two dominant shrub species (Bulnesia retama and Larrea divaricata) and in open spaces (i.e. intercanopies) in May 2001. Overall, the density of germinated seedlings of Schismus and that of the native species were negatively associated across microsite types. Schismus density was similar to that of all native species pooled together (mostly annuals), and was highest in Larrea samples (with no significant differences between Bulnesia and intercanopies). On the contrary, the density of all native species pooled together was highest in Bulnesia samples. Our results suggest that shrubs may contribute to plant invasions in our study system but, most importantly, they further illustrate that this influence can be species specific. Further research is needed to assess the relative importance of in situ seed production (and survival) and seed redistribution on soil seed bank spatial patterns.     

Vegetation dynamics and exotic plant invasion following high severity crown fire in a southern California conifer forest

Early post-fire vegetation dynamics following large, severe forest fires are largely unknown for the southern California mountains owing to historic fire suppression. Vegetation in 38 forest stands was surveyed (2004, 2005, and 2007) following the 2003 Cedar Fire in the Cuyamaca Mountains, Peninsular Ranges, San Diego County, California, USA. Each stand was sampled using four 10-m radius plots for the tree stratum, and 20 1-m² quadrats for shrub and herb strata. Changes in canopy cover by species, origin (native and exotic) and life form were analyzed. 2007 data were subjected to clustering to examine the divergence in species composition of the stands with time. Shrub cover increased from 3 to 31%, and exotic herbaceous cover increased from 3 to 40%. Cover of native annuals had increased from 2004 (17%) to 2005 (33%), but then dropped to 15% in 2007. Forty percent of the stands were dominated by the shrub species Ceanothus palmeri, and associated with higher pre-fire conifer cover and fire severity. More than 50% of the stands were dominated by exotic annuals and associated with lower fire severity and less steep slopes. The remaining stands (<10%) were dominated by chaparral shrubs and occurred on lower elevation, steep west-facing slopes. Species traits predict their dynamics following disturbance, as environmental conditions change. Establishment and increasing abundance of species dependent on dispersal to reach a site, including exotic and native herbaceous species, occurred in years 2–4. Differences among stands in species composition 4 years post-fire were associated with topographic and fire severity gradients.     

Facilitation by nurse shrubs of resprouting behavior in a post-fire shrubland in northern Patagonia,Argentina

Abstract. The nurse-plant syndrome is a widely recognized example of positive (facilitative) influences of plant species on the establishment and growth of other species. Most studies of the nurse-plant syndrome have been on species that reproduce mainly from seed rather than vegetatively. In this study, we experimentally compared the influences of two species of nurse shrubs, Schinus patagonicus and Berberis buxifolia, on the survival and growth of vegetatively reproducing herbaceous and woody plants in a post-fire shrubland in northern Patagonia, Argentina. The vegetation beneath shrubs was removed by clipping and, in a paired-sample design, one half of the canopy of each shrub was removed. We determined species richness, counted number of resprouts, and measured photon flux density and soil moisture beneath cut and uncut halves of each shrubs. Abundances of resprouts were several times greater beneath the uncut vs. the cut shrubs, as was the mean number of species. Thus, shrubs have a strong facilitating influence as measured by resprout densities and the number of species. Numbers of resprouts and of species were twice as high beneath Schinus as beneath Berberis, implying important differences in the facilitative effects of the two shrubs species. Microsites beneath Schinus were characterized by lower and more heterogeneous light levels but by greater soil moisture. Even though the reproductive mode in this post-fire shrubland is overwhelmingly vegetative, facilitation by nurse shrubs is important and differentially effective for different species of nurse shrubs.     

Facilitative interactions between an exotic mammal and native and exotic plants: hog deer (<Emphasis Type="Italic">Axis porcinus</Emphasis>) as seed dispersers in south-eastern Australia

Endozoochory by exotic mammalian herbivores could modify vegetation composition by facilitating the dispersal and establishment of exotic and native plant species. We examined the potential for endozoochoric dispersal of native and exotic plants by exotic hog deer (Axis porcinus) in south-eastern Australia. We quantified the germinable seed content of hog deer faecal pellets collected in five vegetation types within a 10,500-ha study area that was representative of their Australian range. Twenty exotic and 22 native species germinated from hog deer faecal pellets and significantly more native species germinated compared to exotic species. Seedlings of the encroaching native shrub Acacia longifolia var. sophorae emerged, but no native trees emerged and the percentage of grasses that germinated was low (11%). The species composition of germinants was similar among the five vegetation types. We estimated that the hog deer population in our study area could potentially disperse >130,000 viable seeds daily. Our study shows how an exotic mammal can disperse seeds from both native and invasive plants and highlights the need for endozoochory to be considered more widely in studies assessing the impacts of exotic mammals on plant communities.     

Restoration of Dry Afromontane Forest Using Pioneer Shrubs as Nurse-Plants for Olea europaea ssp. cuspidata

Shrubs are often considered competitive barriers for seedlings planted in reforestation programs, although they can facilitate tree recruitment, especially in ecosystems under high abiotic stress. An alternative reforestation technique using pioneer shrubs as nurse‐plants for Olea europaea ssp. cuspidata was tested in exclosures in northern Ethiopia. Seedlings were planted in three different microhabitats, and their survival was monitored. The microhabitats were bare soil patches between shrubs, patches under the dominant shrub Acacia etbaica, and patches under Euclea racemosa, an evergreen shrub, which supports the majority of naturally established Olea recruits. The ability of shrubs to offer protection against browsing goats was tested experimentally. Controlled shading was used to determine whether solar irradiation causes seedling mortality in environments without water stress. Data were analyzed using Kaplan–Meier survival analysis, Kruskal–Wallis analysis of variance (ANOVA), and one‐way ANOVA. Olea survival was significantly higher and shoot damage by goats was lower when planted under shrub cover compared to bare soil patches, particularly under Euclea canopies, although high shade levels reduced seedling performance. Reduction of solar radiation by shrub canopies and thus control of soil–water evaporation and seedling transpiration most likely controlled the observed facilitation. Planting under shrubs may increase seedling survival and assist regeneration of dry Afromontane vegetation. Preserving pioneers also reduces soil erosion and conserves biodiversity. Excluding livestock is essential for Olea woodland restoration and allows persistent but morphologically modified Olea shrubs to develop vigorous regrowth. Facilitative processes are guiding principles for assisted forest restoration, but above‐average rains may be critical to restore higher biomass levels in semiarid areas.     

Enemy release or invasional meltdown? Deer preference for exotic and native trees on Isla Victoria,Argentina

Abstract How interactions between exotic species affect invasion impact is a fundamental issue on both theoretical and applied grounds. Exotics can facilitate establishment and invasion of other exotics (invasional meltdown) or they can restrict them by re‐establishing natural population control (as predicted by the enemy‐release hypothesis). We studied forest invasion on an Argentinean island where 43 species of Pinaceae, including 60% of the world's recorded invasive Pinaceae, were introduced c. 1920 but where few species are colonizing pristine areas. In this area two species of Palearctic deer, natural enemies of most Pinaceae, were introduced 80 years ago. Expecting deer to help to control the exotics, we conducted a cafeteria experiment to assess deer preferences among the two dominant native species (a conifer, Austrocedrus chilensis, and a broadleaf, Nothofagus dombeyi) and two widely introduced exotic tree species (Pseudotsuga menziesii and Pinus ponderosa). Deer browsed much more intensively on native species than on exotic conifers, in terms of number of individuals attacked and degree of browsing. Deer preference for natives could potentially facilitate invasion by exotic pines. However, we hypothesize that the low rates of invasion currently observed can result at least partly from high densities of exotic deer, which, despite their preference for natives, can prevent establishment of both native and exotic trees. Other factors, not mutually exclusive, could produce the observed pattern. Our results underscore the difficulty of predicting how one introduced species will effect impact of another one.     

Cattle change plant reproductive phenology,promoting community changes in a post-fire Nothofagus forest in northern Patagonia,Argentina

Aims Variations in rates and length of flowering and fruiting not only affect the reproduction of a given plant species but also the behavior and reproduction of associated taxa. Flowering and fruiting variations may be influenced by herbivory, especially by large mammals. The aim of this study was to determine the effects of cattle browsing on the reproductive phenology of understory species in a subalpine post-fire Nothofagus forest in Patagonia.Methods The effects of herbivory on plant reproductive phenology were studied in a set of experimental exclosures (fenced plots) installed since 2001 in a post-fire N. pumilio forest, located in Nahuel Huapi National Park (NHNP), Argentina. We monitored the beginning and duration of each reproductive phenological stage: floral bud, open flower, immature fruit and mature fruit. We also counted the number of flowers, fruits, seeds and viable seeds of the dominant plants to assess whether browsing modifies temporal patterns of the flowering and fruiting periods.Important findings Cattle reduced the total number of species flowering and fruiting and changed the reproductive phenology of some species. We found that palatable species seem to be negatively affected by browsing in terms of reduced fitness due to changes in flowering and fruiting periods. In contrast, cattle benefitted the reproduction of non-palatable species and could promote the invasion of shade-intolerant exotic forbs such as Cirsium vulgare. The effects of livestock reported in this study are important to understanding how browsing could alter native species establishment and possibly alter successional trajectories during recolonization after fire.     

Apparent competition: an impact of exotic shrub invasion on tree regeneration

Invasion of habitats by exotic shrubs is often associated with a decrease in the abundance of native species, particularly trees. This is typically interpreted as evidence for direct resource competition between the invader and native species. However, this may also reflect indirect impacts of the exotic shrubs through harboring high densities of seed predators––known as apparent competition. Here I present data from separate seed predation experiments conducted with two shrub species exotic to North America; Rosa multiflora, an invader of abandoned agricultural land, and Lonicera maackii, an invader of disturbed or secondary forest habitats. Both experiments showed significantly greater risks of seed predation for tree seeds located under shrub canopies when compared to open microhabitats within the same site. These results indicate the potential importance of indirect impacts of exotic species invasions on native biota in addition to the direct impacts that are typically the focus of research.