Non-native Conifers Research Articles

Increased canopy seed-storage in post-fire pine invaders suggests rapid selection mediated by fire

Exotic species storing seeds in the canopy (serotinous species) can experience a clear advantage in fire-prone communities that lack native taxa with such fire-resistant traits. In addition, selection in the new environment can potentially increase the frequency of fire-adapted characteristics such as serotiny. We studied the potential role of fire favoring the serotinous, non-native conifer Pinus radiata in NW Patagonia. We characterized the degree of serotiny (percentage of serotinous cones) and the size of the canopy seed bank in the unburned plantation and in stands of trees recruited after a fire 30 years ago as a proxy for invasion potential. Fire had a positive effect, increasing serotiny in post-fire P. radiata stands. Post-fire recruited cohorts showed higher serotiny levels and a larger canopy seed bank compared with plantations. Our study suggests that fire-linked traits like serotiny may be subjected to a rapid, fire-driven selection process in fire-adapted species such as P. radiata invading fire-prone ecosystems. Thus, increased serotiny can lead to higher postfire invasion densities, which in turn create a positive feedback loop in invaded areas under recurrent fires.

Annual gaseous carbon budgets of forest-to-bog restoration sites are strongly determined by vegetation composition

Large areas of naturally open peatland in western Europe were drained and planted with non-native conifers in the twentieth century. Efforts are currently underway to restore many of these sites. Ultimately, forest-to-bog restoration aims to bring back functional peatlands that can sequester carbon but there is a lack of empirical evidence for whether current approaches are effective. Using a chronosequence design, we compared the annual gaseous carbon balance of two forest-to-bog restoration areas with an open area not subject to afforestation. A closed chamber method was used to determine gas fluxes (Net Ecosystem Respiration, Gross Primary Productivity, Net Ecosystem Exchange (NEE) and methane (CH4)) over a twelve-month period for locations spanning the range of peatland microtopography and vegetation communities. Relationships between gas fluxes, vegetation/cover and environmental factors were analysed and regression models used to estimate annual CO2 and CH4 budgets. During the study period, NEE estimates (total gaseous C expressed as CO2-eq) showed a net sink for the unafforested (−102 g C m−2 yr−1) and oldest (−131 g C m−2 yr−1) restoration area (17 years post-restoration ‘RES 17 YRS’), whilst the youngest restoration area (6 years post-restoration ‘RES 6YRS’), was a net source (35 g C m−2 yr−1). We observed significantly higher CH4 emissions from restoration areas dominated by Eriophorum angustifolium compared with other peatland vegetation types. Sampling points with higher cover of Sphagnum were found to be most effective for C sequestration. Overall, vegetation composition/cover was observed to be an important factor determining C emissions from forest-to-bog restoration areas. These results suggest that restoration is effective in returning the carbon sink function of peatlands damaged by commercial forestry and - depending on restoration techniques - timescales of >10 years may be required.

Increase in nonnative understorey vegetation cover after nonnative conifer removal and passive restoration

AbstractNonnative conifers are widespread in the southern hemisphere, where their use as plantation species has led to adverse ecosystem impacts sometimes intensified by invasion. Mechanical removal is a common strategy used to reduce or eliminate the negative impacts of nonnative conifers, and encourage native regeneration. However, a variety of factors may preclude active ecological restoration following removal. As a result, passive restoration – unassisted natural vegetation regeneration – is common following conifer removal. We asked, ‘what is the response of understorey cover to removal of nonnative conifer stands followed by passive restoration?' We sampled understorey cover in three site types: two‐ to ten‐year‐old clearcuts, native forest and current plantations. We then grouped understorey species by origin (native/nonnative) and growth form, and compared proportion and per cent cover of these groups as well as of bare ground and litter between the three site types. For clearcuts, we also analysed the effect of time since clearcut on the studied variables. We found that clearcuts had a significantly higher average proportion of nonnative understorey vegetation cover than native forest sites, where nonnative vegetation was nearly absent. The understorey of clearcut sites also averaged more overall vegetation cover and more nonnative vegetation cover (in particular nonnative shrubs and herbaceous species) than either plantation or native forest sites. Notably, 99% of nonnative shrub cover in clearcuts was the invasive nonnative species Scotch broom (Cytisus scoparius). After ten years of passive recovery since clearcutting, the proportion of understorey vegetation cover that is native has not increased and remains far below the proportion observed in native forest sites. Reduced natural regeneration capacity of the native ecosystem, presence of invasive species in the surrounding landscape and land‐use legacies from plantation forestry may inhibit native vegetation recovery and benefit opportunistic invasives, limiting the effectiveness of passive restoration in this context.Abstract in Spanish is available with online material.

Scenario-led modelling of broadleaf forest expansion in Wales.

Significant changes in the composition and extent of the UK forest cover are likely to take place in the coming decades. Current policy targets an increase in forest area, for example, the Welsh Government aims for forest expansion by 2030, and a purposeful shift from non-native conifers to broadleaved tree species, as identified by the UK Forestry Standard Guidelines on Biodiversity. Using the example of Wales, we aim to generate an evidence-based projection of the impact of contrasting policy scenarios on the state of forests in the near future, with the view of stimulating debate and aiding decisions concerning plausible outcomes of different policies. We quantified changes in different land use and land cover (LULC) classes in Wales between 2007 and 2015 and used a multi-layer perceptron–Markov chain ensemble modelling approach to project the state of Welsh forests in 2030 under the current and an alternative policy scenario. The current level of expansion and restoration of broadleaf forest in Wales is sufficient to deliver on existing policy goals. We also show effects of a more ambitious afforestation policy on the Welsh landscape. In a key finding, the highest intensity of broadleaf expansion is likely to shift from southeastern to more central areas of Wales. The study identifies the key predictors of LULC change in Wales. High-resolution future land cover simulation maps using these predictors offer an evidence-based tool for forest managers and government officials to test the effects of existing and alternative policy scenarios.

How important are different non-native conifers in Britain to Common Crossbills Loxia curvirostra curvirostra?

ABSTRACTCapsulePines physically defend their seeds against seed-eating birds and mammals more than spruces or larches. Cone characteristics reflect the rate at which Common Crossbills Loxia curvirostra curvirostra extract seeds from different non-native conifers in Britain.AimsTo assess the profitability of different non-native conifers in Britain for Common Crossbills in winter.MethodsWe measured cone and seed parameters of conifers (Norway Spruce Picea abies, Sitka Spruce Picea sitchensis, Lodgepole Pine Pinus contorta and Japanese Larch Larix kaempferi) introduced into Britain and compared these with the native Scots Pine Pinus sylvestris. Feeding trials with captive Common Crossbills assessed intake rates.ResultsThe pines had thick and long scales, Japanese Larch had thin, short scales but thick seed coats and Sitka Spruce had thin, papery and short scales, and the thinnest seed coat. The two spruce species had more seeds per cone and the kernels had a higher energy content than the pines and larch. Feeding trials, simulating cones in winter, found that Common Crossbills failed to access seeds in closed Scots Pine cones. They also had difficulty in prising the scales of closed Lodgepole Pine cones but were able to forage on partially open cones. They took longer to extract seeds from large, open Lodgepole Pine cones than small ones, reflecting the effect of increasing scale thickness in larger pine cones. They also took longer to extract Lodgepole Pine seeds than Sitka Spruce and larch seeds. Although Common Crossbills could extract seeds quickly from open Sitka Spruce cones, the small seed size made the energy intake rate similar to Japanese Larch, if all seeds contained a kernel. However, after accounting for the proportion of seeds with a kernel, Sitka Spruce was the more profitable.ConclusionThe conifer food resource for crossbills in Britain has changed through the planting of non-native conifers. The physical properties of the cones and seeding phenology influence the rate at which Common Crossbills can extract seeds.

Community-identified key research questions for the future of UK afforested peatlands

Large areas of UK peatland were planted with non-native conifers in the twentieth century, changing many aspects of the ecosystem. As these plantations reach harvesting age there are important questions about what should be done with them next, with key options including restocking for continued forestry and restoration. Making decisions on the future of these sites is difficult and the underlying evidence base is often incomplete. In order to prioritise future evidence needs we conducted a two-phase consultation exercise to identify what a large body of stakeholders in science, policy and practice consider to be the most important outstanding research questions. The five most popular questions identified were: How does the greenhouse gas balance of peatland forestry differ between deep and shallow peat and compare to forestry on mineral soils?; How does the greenhouse gas budget of a peatland change with initial afforestation, restocking or restoration?; Is it possible to have trees on peat without loss of biodiversity and carbon storage?; What are the limits to the achievability of forest-to-bog restoration in terms of factors such as peat condition, depth and site extent?; and What is the financial value of natural capital in natural and afforested peatlands and how does this change with restoration? Notable subsidiary themes included flooding, biodiversity and compensatory planting. These questions form potential foci for future research and particularly emphasise the importance of understanding carbon cycling in afforested peatlands.

Ecology and consequences of invasion by non-native (wilding) conifers in New Zealand

Invasion by non-native woody species into largely treeless vegetation such as grasslands and shrublands is widespread, and has prompted both research and management in response. Here I review the current situation of invasions by non-native Pinaceae, better known as ‘wilding conifers’ in New Zealand, and how both research and management are working to better understand and manage these invaders. The success of wildings is explained by a combination of history (e.g., deforestation of previously woody vegetation), biological traits of the species (rapid growth and early reproduction), and propagule pressure (introduction effort). Wildings represent a major land use change affecting about 2 million ha, including many grasslands, rare ecosystems and subalpine habitats. Wilding invasions into grasslands have profound impacts on biological diversity, but also have important ecosystem impacts including legacy effects belowground by altering nutrient cycling and soil biota. Recent expanded efforts are underway to control and co-ordinate management to avoid or mitigate the negative impacts of wilding conifers. The long-term value of managing invasions, and whether additional management interventions are needed to restore grasslands or woody vegetation is in progress, but is urgently needed given recent moves to widely establish new woody vegetation at large scales in New Zealand.

Seed predation of non-native species along a precipitation gradient

Generalist seed predators are an important factor shaping non-native species invasion. Their effect is highly influenced by abiotic conditions, yet how the importance of this biotic filter changes in a gradient of abiotic conditions is still poorly understood. In this study, we assessed seed predation of non-native conifer species along a precipitation gradient in north-western Argentinean Patagonia. We performed a seed removal experiment over a 15-day period during the fall in sites where annual precipitation ranges from 600 to 1600 mm and vegetation ranges from steppe to forest. We used the three most common forestry species in the study area, including two species known to invade areas adjacent to plantations and one native species. Seed predation was higher in sites with higher precipitation, but no significant differences in seed predation were found among species. Small mammals were the dominant group predating seeds on this gradient. These findings highlight the importance of abiotic conditions mediating plant-granivore interactions, and the influence that abiotic conditions may have on biotic resistance to invasion.

Insects emerging from novel species of host trees attacked by mountain pine beetle, Dendroctonus ponderosae Hopkins, 1902 (Coleoptera: Curculionidae: Scolytinae), in the University of Idaho Arboretum

A naturally-occurring infestation of mountain pine beetle, Dendroctonus ponderosae Hopkins, 1902 (Coleoptera: Curculionidae: Scolytinae), in the Shattuck Arboretum on the campus of the University of Idaho, Moscow, was found to be attacking non-native conifers. The arboretum had experienced a prior infestation in similar tree species in the late 1960s (Furniss & Schenk 1969). Infested trees were harvested, and an approximately 0.3-m section of each tree was cut from a height of 10 m on the bole and maintained under laboratory conditions. All insects that emerged from the bolts were collected and identified. In the current infestation, trees successfully attacked by mountain pine beetle included jack pine (Pinus banksiana Lamb.), Austrian pine (P. nigra L.), red pine (P. resinosa Sol. Ex Alton), pitch pine (P. rigida Mill.), eastern white pine (P. strobus L.), Scots pine (P. sylvestris L.), Norway spruce (Picea abies H. Karst.) and white spruce (P. glauca (Moench) Voss). A total of 777 mountain pine beetle were captured as they emerged from the bolts, and emergence densities of the beetle were similar across tree species. Insects from three orders, 15 families and 27 species emerged from the bolts. Species richness of the emerged insect assemblages were similar across tree species. Shortleaf pine had the largest number of emerged species of insects captured, followed by white spruce and eastern white pine.

A synthesis on the impact of non-native conifer plantations on ant and beetle diversity in north-western Patagonia

Softwood forestry with non-native tree species is increasing worldwide and especially in many developing countries of the Southern Hemisphere. Tree plantations are beneficial in environmental and socioeconomic aspects, but at the same time there are recognised costs associated with afforestation. Our aim was to revise the existing information on the impact of exotic conifer plantations in north-western Patagonia on insect biodiversity. A total of five studies were selected and, in these, not every insect group responded in a similar manner to the habitat replacement. There was a tendency towards a reduction in abundance and species richness of several insects inside pine plantations. This change in abundance and richness was especially evident for ant assemblages and when pine plantations were dense. Beetle assemblages, in turn, showed diverse responses to the replacement of native vegetation with forests depending on the native habitat matrix. Our findings confirm that practices that reduce tree density (via thinning or during plantation) should be recommended to minimise their impact on insect biodiversity in north-western Patagonia. The consistent behaviour of ant assemblages, coupled with their abundance, ease to sample and unambiguous taxonomy make them reliable candidates for long-term monitoring of the impact conifer forestation in north-western Patagonia, as well as probably in other regions of the world in which non-native pines replace natural environments.

Projecting tree-growth responses into future climate: A study case from a Danish-wide common garden

Assessing growth responses to climate variations from common garden experiments is vital to identify a species portfolio matching future climate. In the present study we make use of a 50 years old common garden experiment spanning six sites with different soil types across Denmark to (i) analyse climate-growth responses and resilience to drought and (ii) model future growth predictions for six non-native conifers and two native broadleaved tree species. Species-specific response-functions and Superposed Epoch Analysis of drought events are used to assess differences in sensitivity to drought.The results show that the growth of all species, except for Quercus robur L., are significantly (P<0.05) and negatively correlated with summer drought from June-August in at least one of the sites, whereby Larix kaempferi (Lamb.) Carr, Abies grandis (Dougl.) Lindl., Picea sitchensis (Bong.) Carr., and Picea abies (L.) Karst. are the less resilient. Negative effects of previous warm autumn or late summer were found for P. abies, A. grandis, Abies alba Mill. and Pseudotsuga menziesii (Mirb.) Franco. Moreover, independently of the site conditions, our model projections of growth responses under future climate prediction (RCP4.5 emission scenario) forecast that growth of L. kaempferi, A. grandis, P. abies and Fagus sylvatica L. will be reduced by up to 10–16% by 2100. Minor changes in growth responses are expected for P. sitchensis, A. alba and P. menziesii, while Q. robur will increase by 12%.This study demonstrates how such projections based on old common garden experiments could be used as inputs to today’s forest management decisions.

Testate amoebae as functionally significant bioindicators in forest-to-bog restoration

In north-west Europe, large areas of open peatland have been drained and planted with spatially homogenous stands of non-native conifers. The detrimental impact of afforestation on peatland carbon and biodiversity have led to large-scale attempts to restore these landscapes back to their open, tree-less form. The responses of dominant microbial consumers − testate amoebae − to peatland forest-to-bog restoration are largely unexplored. We studied changes in testate amoebae with forest-to-bog restoration in the largest expanse of blanket bog in the UK and compared testate amoeba communities in relatively undisturbed open bog with forested and forest-to-bog restoration sites. Forested areas contained testate amoeba communities which were functionally different from open bog, characterised by a lack of mixotrophic taxa known to contribute to primary production. Seventeen years after restoration management, the microbial communities in the forest-to-bog sites remained more similar to forested areas than to the open bog community. Our results suggest that afforestation has reduced the trophic level of testate amoeba communities, which are only beginning to recover post-restoration in the wettest areas where Sphagnum has re-colonized. This study also highlights the need to consider a wide-range of reference sites to encompass the natural variability within ombrotrophic blanket bog. We conclude that testate amoebae have the potential to act as functionally-significant bio-indicators in peatlands undergoing forest-to-bog restoration.

Ungulates can control tree invasions: experimental evidence from nonnative conifers and sheep herbivory

Invasive conifer species are increasingly recognized as a serious problem in many parts of the world, where they are having large ecological and economic impacts. Understanding factors that trigger and can control invasion is key to management. Grazing and browsing by large herbivores have been suggested as a mechanism that may halt conifer invasions, although conflicting results have been reported (i.e. positive, negative or no effect of grazing on invasion). We believe that some of these opposing responses arise due to the absence of well-planned and replicated experiments, since current evidence is mostly observational, and for example, differences in animal densities can produce different results. Thus, in this study, we tested whether large herbivores can control invasion by nonnative conifers and whether the severity of the invasion process would be lessened by increased herbivory intensity. We evaluate experimentally herbivore damage on Pinus contorta, a highly invasive species in many countries of the Southern Hemisphere, under different sheep stocking rates in Patagonia, Argentina. We used four stocking rates, corresponding to 1, 2, 4 and 8 times the recommended sheep herbivory intensity for the study area. The response was not linear but rather presented a threshold. The greater stocking rate, the greater the browsing, higher reduction in seedling height, and decrease of survival of P. contorta. Also, the highest stocking rate damaged and killed 99% of them. This study provides evidence that large domestic herbivores can play a key role in the invasion process and, if managed properly, would provide a tool to help limit conifer invasion.

First record of the steppic relict Astragalus exscapus L. subsp. exscapus in the Apennines (Abruzzo, Italy), and biogeographic implications

Astragalus exscapus L. subsp. exscapus (Fabaceae) is a rare and declining perennial herb with a very fragmented distribution range. It is interpreted as a relict of the glacial steppe flora. In Italy, it was known until now only from the Alpine “dry valleys”, namely from Val d’Aosta and Val Venosta/Vinschgau. We report on the discovery of a population on the slopes of the Fucino basin in the Central Apennines (Abruzzo, Italy). The finding significantly expands the known species range (the site is >600 km away from the nearest known populations), suggesting that during Pleistocene steppic phases it had a wide distribution across Europe. A. exscapus adds to a number of other steppic relicts found in the Fucino basin, highlighting that some inner valleys of the Central Apennines, featuring a relatively continental climate, perform as “interglacial refugia” for the steppe flora. Further research is needed to ascertain the species’ local distribution and abundance, and to clarify its vicariance history; however, the finding underlines the need to re-evaluate priorities in landscape management, as in the Apennines dry grasslands are often given low values compared to forests and are frequently destroyed by afforestation practices with non-native conifers.

Bat use of commercial coniferous plantations at multiple spatial scales: Management and conservation implications

Commercial plantations are primarily managed for timber production, and are frequently considered poor for biodiversity, particularly for mammalian species. Bats, which constitute one fifth of mammal species worldwide, have undergone large declines throughout Europe, most likely due to widespread habitat loss and degradation. Bat use of modified landscapes such as urban or agricultural environments has been relatively well studied, however, intensively managed plantations have received less attention, particularly in Europe. We assessed three of the largest, most intensively managed plantations in the UK for the occurrence of bats, activity levels and relative abundance in response to environmental characteristics at multiple spatial scales, using an information theoretic approach. We recorded or captured nine species; Pipistrellus pipistrellus and P. pygmaeus were the most commonly recorded species on acoustic detectors and female P. pygmaeus were the most commonly captured. The influence of environmental characteristics on bat activity varied by species or genus, although all bat species avoided dense stands. Occurrence and activity of clutter and edge adapted species were associated with lower stand densities and more heterogeneous landscapes whereas open adapted bats were more likely to be recorded at felled stands and less likely in areas that were predominantly mature conifer woodland. In addition, despite morphological similarities, P. pipstrellus and P. pygmaeus were found foraging in different parts of the plantation. This study demonstrates that with sympathetic management, non-native conifer plantations may have an important role in maintaining and supporting bat populations, particularly for Pipistrellus spp.

Can mixed stands of native and non-native tree species enhance diversity of epigaeic arthropods in plantation forests?

In regions with low cover of natural forests and high cover of plantations predominately comprised of non-native species, inclusion of a native tree species with a more productive non-native species has the potential to enhance biodiversity and meet production goals. In this context, we tested the alternative hypotheses that: (i) equitable mixes of a non-native and a native tree species support greater diversity of ground-dwelling arthropods than single species stands; or, (ii) native ash stands support greater diversity of ground-dwelling arthropods than mixed or single species stands that include a non-native conifer species. Active epigaeic spiders (Araneae) and beetles (Coleoptera: Carabidae, Staphylinidae) were sampled using pitfall traps in three forest types in Ireland: single species stands of non-native Norway spruce (Picea abies) or native ash (Fraxinus excelsior), and mixed stands of these species.Stands of Norway spruce did not negatively influence spider and staphylinid diversity, suggesting that they maintain a similar range of biodiversity to mixed plantations or stands of native ash. However, carabid beetle richness (but not abundance) was negatively affected by the presence of spruce suggesting caution when drawing conclusions about biodiversity impacts from single taxon studies. We found that equitable mixes of spruce and ash supported many species associated with native ash stands. Thus, we recommend that mixes with an equitable species ratio (e.g. 50:50) and containing a native species will enhance epigaeic arthropod diversity and heterogeneity in plantations. Furthermore, our finding that ash stands supported greater beta diversity than spruce stands supports current guidelines that recommend a range of stand types, including native species, to enhance diversity within and between stands.

Beyond cool: adapting upland streams for climate change using riparian woodlands.

Managed adaptation could reduce the risks of climate change to the world's ecosystems, but there have been surprisingly few practical evaluations of the options available. For example, riparian woodland is advocated widely as shade to reduce warming in temperate streams, but few studies have considered collateral effects on species composition or ecosystem functions. Here, we use cross-sectional analyses at two scales (region and within streams) to investigate whether four types of riparian management, including those proposed to reduce potential climate change impacts, might also affect the composition, functional character, dynamics and energetic resourcing of macroinvertebrates in upland Welsh streams (UK). Riparian land use across the region had only small effects on invertebrate taxonomic composition, while stable isotope data showed how energetic resources assimilated by macroinvertebrates in all functional guilds were split roughly 50:50 between terrestrial and aquatic origins irrespective of riparian management. Nevertheless, streams draining the most extensive deciduous woodland had the greatest stocks of coarse particulate matter (CPOM) and greater numbers of 'shredding' detritivores. Stream-scale investigations showed that macroinvertebrate biomass in deciduous woodland streams was around twice that in moorland streams, and lowest of all in streams draining non-native conifers. The unexpected absence of contrasting terrestrial signals in the isotopic data implies that factors other than local land use affect the relative incorporation of allochthonous subsidies into riverine food webs. Nevertheless, our results reveal how planting deciduous riparian trees along temperate headwaters as an adaptation to climate change can modify macroinvertebrate function, increase biomass and potentially enhance resilience by increasing basal resources where cover is extensive (>60m riparian width). We advocate greater urgency in efforts to understand the ecosystem consequences of climate change adaptation to guide future actions.

Do non-native conifer plantations provide benefits for a native forest specialist, the wood ant Formica lugubris?

Recent increases in plantation forestry are starting to reverse the global decline in forest cover, in some areas of the world. Britain has practiced afforestation, primarily with non-native conifers, for over a century. It is unclear whether these new plantations have the potential to support native forest species.We quantify afforestation across the North York Moors National Park, UK, deriving a chronology of afforestation from historic maps at six time points from 1854 to 2013. We map the location of current wood ant (Formica lugubris) nests and set their distribution in the context of historic forest cover. We use these nest locations and the features of the habitat in which they occur to model the suitability of recently established conifer plantations for wood ants using MaxEnt. We determine whether non-native conifers offer suitable habitat for a forest specialist species, and assess the lag between establishment of conifer plantations and colonisation by wood ants from historic woodland fragments.Forest cover increased by 229% over 160years and is now dominated by non-native conifer plantations. Our survey data show that current wood ant populations extend hundreds of metres from where forest was in the past, demonstrating geographical population expansions into newly formed forest, comprised of non-native conifer plantations. Both our data and model reveal that the recently planted non-native conifer plantations are a suitable habitat for this forest specialist species. Our model reveals that Formica lugubris has not yet spread through all available suitable habitat due to very poor dispersal ability, displaying a severe lag behind the availability of habitat.Managers should not assume that unoccupied habitat is unsuitable nor should they expect to see immediate colonisation of plantations. Future forest creation should be targeted close to existing forests to facilitate colonisation of forest specialists.

Impact of future climate change on a genetic plantation of hybrid pine

European black pine (Pinus nigra Arnold) and Japanese red pine (Pinus densiflora Siebold &amp; Zucc.) are nonnative conifers that have been introduced into the United States and have the ability to hybridize with each other and can potentially become an invasive threat. Interspecific hybridization is an important component of applied genetic tree improvement programs. This study retrospectively examined the influence of climate on interannual growth patterns of 12 full-sib families of hybrid pine that were derived from the hybridization of European black pine and Japanese red pine. The hybrids (Pinus nigra × Pinus densiflora) were initially planted in 1982 in southern mid-Michigan at Michigan State University (MSU) Sandhill Research Area. Tree-ring analysis methods (dendrochronology) were used to quantify the influence of climate (i.e., mean temperature and moisture index) on interannual basal area growth rates of the hybrid pine over an 18 year period (1991–2008). The productivity of the 12 full-sib families of hybrid pine were also projected under future climate change in the 21st century based on the IPCC A1B emission scenario. Climatic sensitivities that were identified in the hybrid pine families included high temperature stress in the summer and fall, moisture stress in the summer, winter harshness, and the timing of the start of the growing season. According to projections, by the end of the 21st century, 8 out of the 12 pine families will show significant decreases in growth outside of the historical norm under the temperature-based dendrochronology models, while 4 families will show significant declines under the moisture index models. The results indicate that the hybrid pine families appear to be generally resilient to future changes in moisture, but will likely be extremely vulnerable to future climatic warming, and thus do not appear to be a future invasive threat. This study represents the first dendrochronological examination of hybrid pine in North America.

Réponse à la sécheresse de conifères indigènes et exotiques: une étude dendroécologique

Drought response of native and non-native conifers: a dendroecological study The sensitivity to drought of five conifers was studied along a climatic gradient covering the northern flank of the Swiss Alps, the Swiss Plateau, the Jura foothills and the central Alps. Three native species, i.e., Norway spruce, Scots pine and European larch, and two non-native species, i.e., Douglas fir and black pine, were analyzed. A total of 770 trees on 14 sites were sampled. To assess the sensitivity of conifers to drought, dendroecological analyses were performed for the period 1941–2006. On dry sites in the central Alps, spring water deficits sharply reduce growth, while on mesic sites located on the Swiss Plateau and the Jura foothills summer droughts significantly restrict growth. Spruce, larch and Scots pine are, depending on the site conditions, the most vulnerable species to extreme droughts, which implies that their long-term performance and survival may be compromised if the climate becomes warmer and drier. Conversely, black pine and Douglas fir were less drought-sensitive and grew faster than native species along the gradient. Selecting species with different but complementary strategies regarding drought could secure the productivity and efficiency of water use of forest stands and increase their resistance to climate change.