Invasive Pine Research Articles

Reproductive biology and early establishment of Pinus elliottii var. elliottii in Brazilian sandy coastal plain vegetation: implications for biological invasion

Pinus is the most invasive woody taxon, exceeded only by herbaceous plants. This study reports the reproductive biology and early establishment of Pinus elliottii Engelm. var. elliottii, describing its invasive properties in a protected natural area of the Brazilian coastal sandy plains. We evaluated the seed germination and rain, longevity of seed viability and the initial dynamics of the seedlings of Pinus elliottii var elliottii through field and laboratory experiments. We recorded a continuous seed rain of about 204.0 viable seeds m- 2 per year, with a 90 % germination rate. The seeds exhibited a low longevity of viability in the soil and a dense, permanent seedling bank that may explain the high levels of pine invasion. The environmental impact caused by the pine's biological invasion suggests the recommendation for its immediate eradication, together with a restoration plan to restitute the native biodiversity gradually.

Can exotic pine trees assist in restoration?

AbstractInvasive exotic woody species, including conifers escaped from plantations, are usually regarded as serious threats to native biodiversity. Becerra & Montenegro (Applied Vegetation Science, 16, 2013, 2) present an interesting example of invasive Pinus radiata facilitating native woody regeneration in semi‐arid central Chile. However, the positive value of invasive conifers for restoration and conservation will not necessarily apply in all situations.

Comparison of water-use by alien invasive pine trees growing in riparian and non-riparian zones in the Western Cape Province, South Africa

Self-established stands of alien invasive pine trees are common in many parts of South Africa and elsewhere. They mainly invade non-riparian settings but sometimes invade riparian habitats. There are clear visual differences in the physical attributes of trees that occupy riparian and non-riparian zones. We have little information whether the differences between trees growing in these contrasting habitats reflect their water-use. The goal of this study was to establish the water-use of alien invasive pines growing adjacent to and away from a perennial stream, and to determine the driving factors behind the variations. The study was conducted in a self-established 20-year-old mixed pine forest occupied by roughly equal proportions of Pinus pinaster and Pinus halepensis. Individual tree transpiration rates were measured using the heat pulse velocity (HPV) sap flow method. Evapotranspiration (ET) from entire stands was determined from the surface energy balance equation using sensible heat flux data collected using a boundary layer scintillometer and measurements of the available energy (net radiation – soil heat flux). A simple two-layer model in which the stand ET was calculated as the algebraic sum of the outputs from transpiration (E) and soil evaporation sub-models was evaluated at the two contrasting sites. Annual transpiration and ET rates were higher in the riparian zone at 980 and 1417mm compared to 753 and 1190mm, respectively in the non-riparian area. The model predicted stand transpiration fairly accurately for both sites (average R2>0.75), but was less accurate for evapotranspiration (average R2<0.70) due to the difficulties in simulating soil evaporation. No significant differences in sap velocities were found between trees at the two sites so the greater water-use of trees in the riparian zone was due to the larger basal area per stem. Based on the measured transpiration data we conclude that self-sown pine stands growing in riparian zones use at least 36% more water than those occurring in non-riparian settings justifying the high priority given to clearing invasive trees in riparian zones.

Number of source populations as a potential driver of pine invasions in Brazil

To understand current patterns of Pinus invasion in an Araucaria forest in southern Brazil, we quantified invasion at the local scale and compared it with habitat characteristics, propagule size, and number of source populations, using generalized linear models. We also compared observed and expected invasive species status based on a previously developed model (Z scores) using Chi square and correlation tests to evaluate the predictability of species status based on their traits. Of the 16 Pinus species currently present in the site, three are invasive (P. elliottii, P. glabra, and P. taeda), three are naturalized (P. clausa, P. oocarpa, and P. pseudostrobus), and ten are present only as the originally planted individuals. While P. taeda spread the farthest, P. glabra had greater overall density, but none of the invasive species has spread more than 250 m in 45 years. Invasive Pinus plants were found where forest tree density was below 805 trees ha−1, and invasive Pinus density decreased log-linearly with an increase in native tree density. Number of individuals introduced and number of source populations were strong predictors of naturalization, thus both propagule size and propagule diversity can potentially be driving invasion success. Z scores based on species traits did not predict which species would invade in Rio Negro. Our findings suggest that Araucaria forests might not resist invasion by Pinus as recently suggested and support the hypothesis that propagule pressure is a fundamental driver of invasions with propagule diversity being a possible component of this mechanism.

Drought-induced positive feedback in xylophagous insects: Easier invasion of Scots pine leading to greater investment in immunity of emerging individuals

We studied the infestation rate of Scots pine Pinus sylvestris by xylophagous insects in relation to distance from forest lakes in eastern Latvia, northern Europe. In summers of 2008 and 2009, we felled 72 pines of approximately 65years age. Sections of the logs were incubated in insect emergence traps. The trees located near lakes were significantly less infested by xylophagous insects than those sampled at greater distances from the lakes. We also tested the ability of Tomicus piniperda, the most abundant species of xylophagous insects in our samples, to resist the entomopathogenic fungus Beauveria bassiana. The results show that beetles captured near lakes were more susceptible to the fungal infection than individuals sampled away from forest lakes, indicating that the beetles far from lakes afforded to invest more in immunity during the larval phase. During the warmest days in summer the mean maximum ambient temperature was 2.32°C lower near lakes than away from lakes. Since increased temperatures not only trigger drought stress, but may potentially also cause temperature-sensitive mortality of conifers, a warmer microclimate may lower resistance of pine trees to attacks of xylophagous beetles. In weakened trees, insects can invade more easily resulting in increased investment in immunity in bark- and wood-boring insects. This is the first demonstration of drought-induced positive feedback in xylophagous insects infesting Scots pine. Since bark beetles are common pests of conifers, our results may be important in forest pest management.

Effects of pine invasion on anurans assemblage in southern Brazil coastal ponds

The destruction of wetlands due to afforestation areas is a common activity in temperate and subtropical regions in Southern America. The expansion of pine in the Coastal Plain of Southern Brazil is out of control and its impacts on aquatic biodiversity are little known. We tested the following hypotheses: the pine occurrence diminishes the anuran richness and abundance in ponds and it changes the anuran composition; the beta-diversity between pine and native grassland matrix ponds (natural ponds) is determined mainly by nestedness. Sampling was carried out from 2007 to 2009 in five ponds in pine invasion matrix and five ponds in native grassland matrix. The natural ponds showed a greater richness of tadpoles (10 species) than pine ponds (5 species). The mean richness of adults was higher in natural ponds than pine ponds throughout the entire study. The species composition was different between natural and pine ponds, for both adults and tadpoles. Comparing natural ponds with each other and the pine ponds with each other, our results showed that neither nestedness nor turnover was determinant for beta-diversity. However, when natural ponds were compared with pine ones, we found that the contribution of nestedness was higher for the anurans beta-diversity than turnover. Increases in the nestedness mechanism indicated that the pine occurrence results in species loss in Southern Brazil ponds. Since 90% of its wetland has been already affected, the removal ofPinusin the conservation areas in southern Brazil is important to minimize their impacts on aquatic biodiversity.

Negative effects of exotic pine invasion on macroinvertebrate communities in southern Brazil coastal ponds

Exotic pine invasion influences native wetland assemblages by changing environmental conditions such as hydrological regime and physicochemical characteristics. The expansion of cultivated pine has been a concern in southern Brazil and its impacts on aquatic biodiversity are poorly known. We tested the hypotheses that: (1) pine invasion decreases aquatic macroinvertebrate richness and abundance, modifying composition and macroinvertebrate functional feeding groups in ponds; and (2) β-diversity between natural and pine ponds is determined mainly by species nestedness. Five ponds in native grassland and five in a pine-planted matrix were sampled seven times from 2007 to 2009. The pine ponds had reduced macroinvertebrate richness and abundance, and different taxa and functional feeding groups. Comparing ponds in natural and cultivated pine areas, β-diversity as determined by nestedness did not differ from the value resulting from the turnover. Reduction of surface in ponds in pine areas may be one of the main causes for the lower macroinvertebrate richness and abundance because many taxa do not have adaptations to tolerate or escape the dry phase. Our results suggest that pine invasion has a negative impact on macroinvertebrate structure in southern Brazil coastal ponds.

How robust is the Australian Weed Risk Assessment protocol? A test using pine invasions in the Northern and Southern hemispheres

The Australian Weed Risk Assessment protocol (WRA) is often considered the standard approach for pre-border screening of new plant introductions. Here we assess its robustness against three key criteria: ability to discriminate success or failure of species at three stages of the invasion process (introduction, naturalisation and spread); sensitivity to taxonomic range and target region; and dependence on knowledge of invasive behaviour elsewhere. We address these issues by retrospectively testing the WRA using pine (Pinus) introductions to New Zealand and Great Britain. For both regions we calculated WRA scores for 115 species, and classified all species according to whether they had been introduced, which of these had naturalised, and the extent of their naturalised distribution (spread). Using regression models, we assessed whether WRA scores could predict success at each stage. We repeated this procedure using WRA scores calculated without information on species naturalisation behaviour elsewhere. In both regions, the WRA could discriminate among species in the same genus at the introduction and naturalisation stages, but not at the spread stage. The outcome at the naturalisation stage depended on prior knowledge of naturalisation behaviour elsewhere. Without this information the WRA may be unable to distinguish among closely related species, and should be used cautiously where data on invasive behaviour elsewhere is lacking. Human selection played a strong role in the invasion process both through introducing pine species likely to naturalise in New Zealand and Great Britain in the first instance, and subsequent use of many of these species for forestry in the target regions.

Pioneer effects on exotic and native tree colonizers: Insights for Araucaria forest restoration

We integrate analyses of spatial association with empirical measures of juvenile performance to develop a strategy for Araucaria forest restoration in Brazil. Because pioneer species can differentially facilitate or inhibit target species, we measure native Araucaria angustifolia and exotic Pinus taeda responses to two common pioneer species, Baccharis uncinella and Vernonia discolor in a clear-cut pine plantation. P. taeda establishment and growth was greater than that of A. angustifolia in all treatments, showing its potential for invasiveness. B. uncinella showed significant positive spatial associations with P. taeda and negative spatial associations with A. angustifolia. V. discolor showed significant negative spatial associations with P. taeda but neutral spatial associations with A. angustifolia, it also supported a higher diversity of native species under its canopy than B. uncinella. Experimental results show that these patterns are due to differential establishment restrictions imposed by both pioneers on target species. We conclude that B. uncinella promotes pine invasion while V. discolor restrains it. We suggest strategies for Araucaria forest restoration and highlight the broad applicability of our approach to restoration programs.

Does pine occurrence influence the macrophyte assemblage in Southern Brazil ponds?

The destruction of wetlands due to afforestation areas is a common activity in temperate and subtropical regions in Southern America. The expansion of pine (Pinus elliottii Engelm.) in the Coastal Plain of Southern Brazil is critical and its impacts on aquatic biodiversity are little known. We tested the hypotheses that: (1) pine occurrence diminishes the macrophyte richness and changes the macrophyte composition in ponds; (2) beta-diversity between natural and pine ponds is determined mainly by species nestedness (species loss). Sampling was carried out from 2007 to 2009 in five ponds in pine invasion matrix and five ponds in native grassland matrix. In natural ponds, the total richness was 87 species, followed by 51 species in pine ponds. From the total richness, 42 species were shared between natural and pine ponds. The natural ponds were richer than the pine ponds along the entire study, and the composition of macrophyte species was different between natural and pine ponds. Comparing natural ponds with each other and pine ponds with each other, the species turnover (species replacement) was determinant for beta-diversity, however, when we compared natural and pine ponds, we found that nestedness and species turnover were equivalent for beta-diversity. The increase in the nestedness mechanism indicates that the pine occurrence implies in species loss in Southern Brazil ponds. The change of hydroperiod may be one of the causes for the macrophyte species loss. The removal of pine from areas destined to conservation in Southern Brazil is urgent as well as a proper management of pine plantations in order to minimise its expansions and impacts in the aquatic biodiversity, since 90% of its wetlands have been already lost.

Pine invasions: climate predicts invasion success; something else predicts failure

AbstractAim Explaining why some invasions fail while others succeed is a prevailing question in invasion biology. Different factors have been proposed to explain the success or failure of exotics. Evidence suggests that climate similarities may be crucial. We tested this using 12 species of the genusPinusthat have been widely planted and shown to be highly invasive.Pinusis among the best‐studied group of exotic species and one that has been widely introduced world‐wide, so we were able to obtain data on invasive and non‐invasive introductions (i.e. unsuccessful invasions; areas where after many decades of self‐sowing seeds there is no invasion).Location World‐wide.Methods We developed species distribution models for native ranges using a maximum entropy algorithm and projected them across the globe. We tested whether climate‐based models were able to predict both invasive and non‐invasive introductions.Results Appropriate climatic conditions seem to be required for these long‐lived species to invade because climates accurately predicted invasions. However, climate matching is necessary, but not sufficient to predict the fate of an introduction because most non‐invasive introductions were predicted to have triggered an invasion.Main conclusions Other factors, possibly including biotic components, may be the key to explaining why some introductions do not become invasions, because many areas wherePinusis not invading were predicted to be suitable for invasion based solely on climate.

INVASÃO E MANEJO DE Pinus taeda EM CAMPOS DE ALTITUDE DO PARQUE ESTADUAL DO PICO PARANÁ, PARANÁ, BRASIL

Espécies exóticas invasoras são espécies introduzidas e estabelecidas que produzem descendentes viáveis e que se propagam a distâncias significativas da planta-mãe, podendo causar danos ao meio ambiente. O presente estudo teve como objetivo realizar as primeiras avaliações sobre esse processo de invasão e uma ação de manejo da planta exótica invasora Pinus taeda em três morros no Parque Estadual do Pico Paraná, Paraná, Brasil. O manejo foi realizado mecanicamente, com corte das plantas, e as primeiras respostas sobre a cobertura vegetal e a riqueza de espécies foram mensuradas dez meses após a ação de corte. Os resultados indicam que a população da espécie invasora no local do estudo tem aproximadamente 36 anos, que as plantas crescem num ritmo inferior à média de outros ambientes, que sob a influência de pínus a vegetação nativa apresentou menor porcentagem de cobertura e, finalmente, que após o corte e derrubada de pínus a vegetação apresentou menor riqueza. O manejo teve custo total de R$ 199,00 por hectare. Concluiu-se que os campos de altitude são habitats suscetíveis a invasão biológica por pínus e que o período de 10 meses foi insuficiente para se obterem conclusões consistentes sobre o controle mecânico por corte raso.Palavras-chave:Espécies exóticas invasoras; invasão biológica; Pinus taeda; Campos de Altitude; Parque Estadual do Pico Paraná. AbstractInvasion and managment of Pinus taeda in mountain top grasslands of Pico Paraná State Park, Paraná, Brazil. Invasive alien species are introduced from other ecosystems and establish, producing viable descendency that spread for significant distances from the mother plants, potentially cause damage to the environment. The aim of the current study is to evaluate the first responses of the invasion process and the results of Pinus taeda control on three mountain areas in the Pico Paraná State Park, Southern Brazil. The control was conducted mechanically by felling adult trees and pulling out seedlings. The first responses were measured ten months after felling, measuring plant coverage and species richness. Results indicate that the population of invasive pines in these areas is around 36 years old, growing at a slower rate when compared to the average growth in other habitats, that the pine trees impact the native vegetation by producing lower coverage and finally, that after felling the pine trees the native vegetation showed lower richness. The control method had a cost of R$ 199,00 (about US$ 100) per hectare. Conclusions are that high mountain grasslands are susceptible habitats for biological invasion by pine trees and that recurrent work needs to be done for more consistent elucidations.Keywords:Invasive alien species; biological invasion; Pinus taeda; grasslands; Atlantic Forest; invasive conifers.

Ericaceous dwarf shrubs affect ectomycorrhizal fungal community of the invasive Pinus strobus and native Pinus sylvestris in a pot experiment

This study aimed to elucidate the relationship between ericaceous understorey shrubs and the diversity and abundance of ectomycorrhizal fungi (EcMF) associated with the invasive Pinus strobus and native Pinus sylvestris. Seedlings of both pines were grown in mesocosms and subjected to three treatments simulating different forest microhabitats: (a) grown in isolation and grown with (b) Vaccinium myrtillus or (c) Vaccinium vitis-idaea. Ericaceous plants did not act as a species pool of pine mycobionts and inhibited the ability of the potentially shared species Meliniomyces bicolor to form ectomycorrhizae. Similarly, Ericaceae significantly reduced the formation of Thelephora terrestris ectomycorrhizae in P. sylvestris. EcMF species composition in the mesocosms was strongly affected by both the host species and the presence of an ericaceous neighbour. When grown in isolation, P. strobus root tips were predominantly colonised by Wilcoxina mikolae, whereas those of P. sylvestris were more commonly colonised by Suillus and Rhizopogon spp. Interestingly, these differences were less evident (Suillus + Rhizopogon spp.) or absent (W. mikolae) when the pines were grown with Ericaceae. P. strobus exclusively associated with Rhizopogon salebrosus s.l., suggesting the presence of host specificity at the intrageneric level. Ericaceous plants had a positive effect on colonisation of P. strobus root tips by R. salebrosus s.l. This study demonstrates that the interaction of selective factors such as host species and presence of ericaceous plants may affect the realised niche of the ectomycorrhizal fungi.

Recovery of Native Grasslands after Removing Invasive Pines

The advance of exotic tree and shrub species is one of the main threats to conservation of the last relicts of natural grassland in South America; however, control actions in the region are still scarce and there are almost no evaluations of the recovery of natural ecosystems after removing invasive plants. Monitoring of the vegetation during the years after removal of invasive trees is critical in order to decide whether an active restoration strategy is necessary. The recovery of montane grassland four years after the control of a dense invasion of Aleppo pine (Pinus halepensis) is described in this study. Experimental clearing areas were followed during four years and compared to grassland controls. Variation was seen in the levels of recovery in function of the proximity of sectors of grassland that are free of invasive species and/or the density of invasive trees before control. Native species slowly replaced many exotic herbs that had appeared as pioneers, there was low recruitment of pine seedlings in spite of the quantity of seeds from trees that surrounded the clearings, and species richness and diversity were restored, including cover of the typical grasses in the controls. Recovery of grassland after felling was shown to be successful and does not seem to be seed limited if tree removal occurs early in the invasion process.

Density‐dependent impacts of exotic conifer invasion on grassland invertebrate assemblages

Summary 1. Globally, temperate grasslands have been heavily modified by agricultural intensification. The ecological integrity of many remaining semi‐natural grasslands is further threatened by the encroachment of invasive alien woody plant species, such as conifers. 2. Although the ecological impact of conifers is often inferred to be directly related to increasing conifer density at invaded sites, interpretation of these density‐dependent effects on native biodiversity is almost always confounded by strong correlations between density and time since invasion. Here, we isolate the direct effects of exotic conifer density on native invertebrate assemblages from the confounding effects due to time since invasion, by establishing replicated experimental plantings of the invasive Pinus nigra (densities of 400, 800, 1600, 2500 and 3500 trees per ha, established in 1993) in New Zealand. 3. From the 29 549 invertebrates captured, the relative abundance of major classes and orders was largely unaffected by conifer invasion at densities below 800 trees per ha at age 14 years, but differed substantially in higher‐density conifer stands (canopy cover &gt;50%). At the species level, beetle species composition was highly sensitive to conifer invasion at densities as low as 400 trees per ha at age 14 years. Changes in beetle species composition were correlated with reduced soil moisture, increases in canopy cover, and trap distance from the nearest semi‐natural grassland. 4. Synthesis and applications. The effects of exotic conifer invasion on grassland invertebrate assemblages were strongly dependent on conifer density. In our study, the negative effects of conifer invasion on biodiversity were comparatively low at densities below ≤800 trees per ha at 14 years after invasion. To conserve invertebrate diversity in semi‐natural grasslands we recommend a dual focus of conservation actions targeting: (i) immediate control, or thinning, of high density conifer infestations (&gt;800 trees per ha) to restore habitat to a state suitable for grassland species, and (ii) longer‐term removal of sparse, pre‐coning infestations in order to prevent further spread. We propose the use of canopy cover as a proxy for the confounded relationship between conifer density and time since invasion.

Relationships between leaf-litter traits and the emergence and early growth of invasive Pinus radiata seedlings

Baker AC & Murray BR (2010). Relationships between leaf-litter traits and the emergence and early growth of invasive Pinus radiata seedlings. Weed Research 50, 586–596. Summary Pinus radiata is an invasive weed in Australia that spreads from plantations and establishes in surrounding native eucalypt woodlands. To determine factors linked to the invasiveness of P. radiata, we compared emergence and growth traits of P. radiata seedlings at five different depths of leaf litter under pine needles, native eucalypt leaves and an equal mix of the two. Seedling emergence, height, survival and establishment were significantly reduced as leaf-litter depth increased. Seedlings had lower root:shoot ratios and higher specific leaf area (SLA) under deeper litter treatments, shifts linked to the provision of more surface area for light capture and greater light access by seedlings. Total seedling dry weight was highest in treatments with 1 cm of litter cover due to greater moisture retention provided by a small amount of leaf litter outweighing the costs of seedling penetration through leaf litter. Importantly, we found that at any given depth of leaf litter, there were no significant differences in emergence and growth traits between pine, eucalypt or mixed leaf-litter treatments. The ability of P. radiata seedlings to succeed equally well under a range of different leaf-litter types is undoubtedly an important trait linked to its invasiveness. Given ethical concerns of introducing highly invasive species, such as P. radiata into remnant native woodland in field-based studies, glasshouse research is highly desirable and invaluable in elucidating important factors underpinning the invasiveness of weed species such as P. radiata.

Co‐invasion by Pinus and its mycorrhizal fungi

*The absence of co-evolved mutualists of plants invading a novel habitat is the logical corollary of the more widely recognized 'enemy escape'. To avoid or overcome the loss of mutualists, plants may co-invade with nonnative mutualists, form novel associations with native mutualists or form associations with native cosmopolitan mutualists, which are native but not novel to the invading plant. *We tested these hypotheses by contrasting the ectomycorrhizal fungal communities associated with invasive Pinus contorta in New Zealand with co-occurring endemic Nothofagus solandri var. cliffortioides. *Fungal communities on Pinus were species poor (14 ectomycorrhizal species) and dominated by nonnative (93%) and cosmopolitan fungi (7%). Nothofagus had a species-rich (98 species) fungal community dominated by native Cortinarius and two cosmopolitan fungi. *These results support co-invasion by mutualists rather than novel associations as an important mechanism by which plants avoid or overcome the loss of mutualists, consistent with invasional meltdown.

Spatial and temporal variation in dispersal pattern of an invasive pine

Understanding dispersal ability of an invasive species is crucial for describing its potential spread. Despite this, we still know little about the dispersal potential of many invasive species. We explored dispersal spectra in Pinus strobus, an invasive tree in sandstone areas in Central Europe. We studied dispersal of the species using distribution of self-sown trees in the field. We compared these observed data with theoretical dispersal curves derived using information on wind speed, seed terminal velocity and tree height. Finally, we fitted various empirical dispersal curves to the observed data. All the analyses based on the observed field data were done for the whole dataset, and for the dataset divided by habitat types and age categories of the self-sown trees. P. strobus seeds can disperse up to 757.5 m from the source. The observed data fall within the confidence intervals of the predictions based on a negative exponential model. When comparing different dispersal functions fitted to the data, it was not easy to decide which of the dispersal curves provides the overall best fit. This was because different functions were the best predictors of different parts of the dispersal curve. We suggest that future studies should provide not only empirical fitted dispersal curves but also observed data and provide estimates of confidence intervals. Such information will provide insights into the reliability of the dispersal estimates in general and help to evaluate the predictive power of the different models.

A study of the carbon dynamics of Japanese grassland and forest using 14C and 13C

Abstract We quantified the carbon contents of grassland and forest soil using conventional methods and studied the changes in their dynamics by measuring δ13C and Δ14C. Soil samples were taken from a neighboring Miscanthus sinensis grassland and Pinus densiflora forest in central Japan. Both had been maintained as grassland until the 1960s, when the latter was abandoned and became a pine forest by natural succession. The soil carbon content of the forest was much lower than that of the grassland, implying that the soil carbon decreased as the grassland became forest. The δ13C values were very similar in the grassland and forest, at approximately −20‰, suggesting that M. sinensis (a C4 plant) contributed to carbon storage, whereas there was little carbon accumulation from P. densiflora (a C3 plant) in forest soil. The Δ14C values and calculated soil carbon mean residence time (MRT) showed that the soil carbon in the upper A horizon was older, and that in the lower A horizon was younger in forest than in grassland. From these results, we conclude that young, fast-MRT soil carbon is decomposed in the upper A horizon, and old, stable soil carbon was decomposed in the lower A horizon after the pine invasion.

Disentangling the effects of land use, shrub cover and climate on the invasion speed of native and introduced pines in grasslands

AbstractAims To determine how changes in land use, climate and shrub cover affect the invasion dynamics of native (Pinus sylvestris L.) and introduced (Pinus nigra Arn. subsp. nigra) pines in grasslands. To analyse how these factors interact and affect seedling recruitment, a bottleneck in the lifecycle of many trees. Such information is required to manage the dynamics of these species.Location Grands Causses, calcareous plateaus (Southern France).Methods We used both published and unpublished demographic and dispersal data to assess population growth and invasion speed of invading pines. A demographic and spatially explicit model, which included density dependence and stochasticity in dispersal, demography and environment, was run for different scenarios of sheep grazing pressure (nil, extensive or intensive), shrub cover (0, 10 or 20%) and drought frequency (past‐to‐present or future). For each scenario, population growth rate, invasion speed and elasticity of invasion speed to each demographic and dispersal parameter were computed.Results Grazing was the main factor for limiting invasion speed. Shrub cover reduced tree spread under nil or extensive grazing pressure, but increased it under intensive grazing pressure. Although dry years led to nil seedling establishment rates, an increase in their frequency had surprisingly few effects on pine invasion speed. This last result remained unchanged when very dry years, inducing seedling, but also sapling mortality were introduced. In most environmental conditions, population growth rate and invasion speed were higher for the introduced than for the native pine. Elasticity analysis highlighted the importance of demographic parameters on invasion speed, notably adult and sapling survival.Main conclusion Tree invasion speed may rely at least as much on human activities, like sheep grazing, tree cutting and non‐native trees introduction, as on changes in climate factors. Therefore, human activities need to be explicitly taken into account in the prediction and management of tree dynamics.