The effects of species mixture on the growth and yield of mid-rotation mixed stands of Scots pine and silver birch

The effect of tree species mixture on stand volume yield and on tree-species-specific diameter and height growth rates were analysed in managed mixed stands of Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Ehrn.).Data were obtained from 14 repeatedly measured stands located in Southern Finland on mineral soil sites with varying admixture of Scots pine and silver birch. Statistical analysis was carried out for studying the effect of species mixture on the development of stand characteristics. For the analysis, the plots were categorised into three groups (plot types) according to the species dominance. In order to analyse species-specific growth rates, individual-tree mixed linear growth models for tree diameter and height growth were developed for both tree species.The results clearly show that the yield of the managed mid-rotation, mixed stands was greater for stands dominated by Scots pine than for stands dominated by birch, and the stand volume increment decreased with an increasing proportion of silver birch. Analysis of diameter and height growth by tree species revealed that the main reason for this pattern is the negative impact of birch competition on the growth of pine trees. The increase in diameter of pine was clearly hampered if the proportion of birch was high. An abundance of birch also slightly decreased the growth in height of Scots pine, although the effect was less than on diameter growth. Species mixture did not affect the diameter growth of birch but did have a significant effect on height development. Height growth of birch was considerably greater in pine-dominated stands than in birch-dominated stands. In pine-dominated mixed stands, the height growth of birch was quite close to that of dominant pine trees, and birches can endure in competition with pines for light.The results apply for even-aged and single-storey managed stands, where stocking density and structure are controlled with pre-commercial and commercial thinnings. The results are not applicable to unmanaged mixed stands undergoing self-thinning. This study provides new information on mixed stands from a silvicultural perspective, which can be applied in decisions involving the management of mixed stands.     

Temperate forest development during secondary succession: effects of soil,dominant species and management

With the increase in abandoned agricultural lands in Western Europe, knowledge on the successional pathways of newly developing forests becomes urgent. We evaluated the effect of time, soil type and dominant species type (shade tolerant or intolerant) on the development during succession of three stand attributes: above-ground biomass, stand height (HT) and stem density (SD). Additionally, we compared above-ground biomass (AGB) in natural and planted forests, using ten chronosequences (8 from the literature and 2 from this study). Both AGB and HT increased over time, whereas SD decreased. HT, SD and AGB differed among species types. For example, birch had greater HT than alder, willow and ash at a similar age and had higher SD than pine and oak at a similar age. However, birch showed lower AGB than pine. HT and AGB differed among soil types. They were higher in rich soil than in poor soils. Comparative analysis between chronosequences showed an effect of the regeneration method (natural regeneration vs plantation) on above-ground biomass. Planted sites had higher AGB than natural regeneration. Time, soil type, species and regeneration method influenced the mechanism of stand responses during secondary succession. These characteristics could be used to clarify the heterogeneity and potential productivity of such spontaneously growing temperate forest ecosystems.     

SECONDARY SUCCESSION OF NATURAL KOREAN PINE FORESTS

ThereisatractofsecondaryforestsinnatUfKoreanghn(Pinuskorniensis)forestndon-FenlinNatUrReserveswithanareaofl8l65.4hIn,locatedinthesouthslopeofXiaOking'anlingMountain.ThisresearchwascAnedoutonthesuccessionofhosstandNATURAL0unS0FwiSTANDSInl93l,therewasawildfireinthevir-ginKoreanPineforests,whichstretchedfromShanggedingalongTangwangffivertonorth-eastuntilstandNo.32OfFenglinNatureRe-serves.Thefiredareawasabout2,44Ohm2.TOdate,thestandsisbroadleavedmixedfOr-estsdOndnatedbywhitebirch(…     

Species Interactions in the Dynamics of Even- and Uneven-Aged Boreal Forests

Many boreal tree stands are neither clearly even-aged nor clearly uneven-aged. The stands may undergo a series of stages, during which an even-aged stand is transformed into two-storied mixed stand, and finally to multistoried or uneven-aged stand structure. The species composition often changes during the succession of stand stages. This study developed models for stand dynamics that can be used in different stand structures and species compositions. The model set consists of species-specific individual-tree diameter increment and survival models, and models for ingrowth. Separate models were developed for Scots pine, Norway spruce, and hardwood species. The models were used in a growth simulator, to give illustrative examples on species influences and stand dynamics. Methods to simulate residual variation around diameter increment and ingrowth models are also presented. The results suggest that mixed stands are more productive than one-species stands. Spruce in particular benefits from an admixture of other species. Mixed species improve diameter increment, decrease mortality, and increase ingrowth. Pine is a more beneficial admixture than birch. Simulations showed that uneven-aged management of spruce forests is sustainable and productive, and even-aged conifer stands growing on medium sites can be converted into uneven-aged mixed stands by a series of strong high thinnings.     

Spatial pattern of trees influences species productivity in a mature oak–pine mixed forest

Spatial pattern has a key role in the interactions between species in plant communities. These interactions influence ecological processes involved in the species dynamics: growth, regeneration and mortality. In this study, we investigated the effect of spatial pattern on productivity in mature mixed forests of sessile oak and Scots pine. We simulated tree locations with point process models and tree growth with spatially explicit individual growth models. The point process models and growth models were fitted with field data from the same stands. We compared species productivity obtained in two types of mixture: a patchy mixture and an intimate mixture. Our results show that the productivity of both species is higher in an intimate mixture than in a patchy mixture. Productivity difference between the two types of mixture was 11.3 % for pine and 14.7 % for oak. Both species were favored in the intimate mixture because, for both, intraspecific competition was more severe than interspecific competition. Our results clearly support favoring intimate mixtures in mature oak–pine stands to optimize tree species productivity; oak is the species that benefits the most from this type of management. Our work also shows that models and simulations can provide interesting results for complex forests with mixtures, results that would be difficult to obtain through experimentation.     

Using a retrospective dynamic competition index to reconstruct forest succession

Understanding forest dynamics and stand structures is crucial for predicting forest succession. However, many forests have been altered due to century-long land-use practices, which complicates the reconstruction of past and current successional trajectories. For a better understanding of successional processes, we suggest studying the intra- and interspecific competition among single trees across time. We introduce a tree-ring based competition index to reconstruct the competitive dynamics of individual trees over time. This new retrospective dynamic competition index combines a temporal and a spatial component by calculating the yearly ratio between the basal area increments (bai) of the neighbouring trees and the subject tree. The new index is applied to mixed Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) stands in the inner-Alpine dry-valley Valais, for which a change in species composition is hypothesised. The aim is to analyse current stand structures in terms of recent changes in the competitive interactions at the single tree level and to relate these competitive dynamics to land-use change and increasing drought due to climate change. On five plots, the positions of 456 trees were recorded and increment cores were taken to derive bai data. The individual dynamic competition index curves were aggregated in clusters, which define typical patterns of competitive dynamics in both tree species. A large percentage of the trees (87% in oak, 70% in pine) were clustered into a group of trees with constant competition at a relatively low level. However, a smaller group of pines (20%) had recently faced increasing competition. In addition, stand structure analyses indicated a change towards a higher proportion of oak. This change in the competitive ability between oak and pine was found to be related to drought, in that oak had a competitive advantage in dry years. Furthermore, the high proportion of dead branches in pines with decreasing competitive abilities indicated increasing competition for light as a consequence of natural development towards a later successional stage that favours the more shade-tolerant oak. The new retrospective dynamic competition index proved to be promising in studying forest succession. The tree-ring based method allows us to identify changes in the competitive ability of single trees with a high temporal resolution and without repeated assessments.     

Effects of the lack of forest management on spatiotemporal dynamics of a subalpine Pinus cembra forest

Knowledge about the stand structure and dynamics of subalpine forests is crucial to preserve their multifunctionality. In the present study, we reconstructed the spatiotemporal dynamics of a subalpine Pinus cembra forest in the eastern Italian Alps in response to natural disturbances and forest management. We adopted a concurrent point pattern, dendroecological and growth dominance (GD) analysis. We mapped and measured all trees of Pinus cembra and Larix decidua in a 1?ha plot. We analyzed intra- and interspecific spatial patterns and spatial autocorrelation of tree size and age. We explored establishment dynamics and shifts in competition by analyzing growth suppression/release patterns and GD trends. Results showed a clumped, uneven-aged, multilayered structure where pine was dominant. The synergic action of ecological and human-induced factors is discussed to explain the prevalence of pine over time. Spatial pattern and autocorrelation analyses suggest a different colonization strategy of the two species, in which pine established after small-scale perturbations and experienced a stronger inter- and intra-specific competition. The interruption of tree establishment and shift in GD toward large trees resulting from the lack of forest management are the most important findings of this research. This highlights the importance of an active management to avoid the homogenization of the forest structure that is generally associated with a reduction in biodiversity and protective ability of forests.     

A Comparison Between European Continental and British Provenances of Some British Native Trees: Growth, Survival and Stem Form

Data describing the growth, survival and stem form of: a) Europeancontinental, and b) British provenances of trees native to Britainwere collated from Forestry Commission records. The growth ratesof European continental provenances of Scots pine (Pinus sylvestris),silver birch (Betulo pendula), sessile oak (Quercus petraea)and common alder (Alnus glutinosa) were inferior to those ofBritish provenances in 90 per cent of cases. European provenancesof these species also showed inferior survival. The growth ratesof continental provenances of beech (Fagus sylvatica) were superiorto British provenances in about 50 per cent of cases, provenancesfrom northern France, Belgium and Holland being the fastestgrowing. Relationships between the relative height growth offoreign provenances and latitude were demonstrated for Scotspine, silver birch and beech (relative height growth = the heightgrowth of a foreign provenance expressed relative to that ofBritish provenances at the same site). The continental provenancesthat grew best originated at 0–4° south of the latitudesof the planting sites. Relative height growth decreased bothnorth and south of these latitudes. In the case of oak, relativeheight growth did not vary widely between origins. The survivalof continental provenances of Scots pine was particularly pooron exposed upland sites. The stem form of British provenancesof Scots pine was intermediate between Fennoscandian provenancesand provenances from the rest of Europe. In oak and beech therewas generally little difference in stem form between Britishand continental provenances, but the best continental provenances(Belgian beech; French oak) were most reliable. The growth andsurvival of native trees is interpreted in the light of thepost-glacial history of these species. The implications regardingthe continued use of seed of native hardwoods imported fromcontinental Europe are discussed.     

Dynamics of some mineral nutrients and heavy metals in decomposing forest litter

The dynamics of three major mineral nutrients (K, Ca, Mg) and six heavy metals (Fe, Mn, Zn, Cu, Pb and Cd) were studied in decomposing natural, unpolluted litter in two forest types: Scots pine needle litter in a pure Scots pine forest and oak‐hornbeam leaf litter in a mixed oak‐hornbeam stand. Of the mineral nutrients, only K in the oak‐hornbeam system showed a significant decrease in concentration during the course of litter decomposition. For Ca and Mg in both systems and for K in the Scots pine system no clear relation between concentration and accumulated litter‐mass loss was observed. On the other hand, for Fe, Zn, Pb and Cd at both sites, as well as for Mn in the oak‐hornbeam stand and Cu in the Scots pine stand highly significant positive relations were found between heavy metal concentrations and accumulated mass loss. A decrease in concentration was noted only in the case of Mn in the Scots pine system. No clear pattern was found for Cu in the oak‐hornbeam stand. Concentrations of heavy metals in the decomposing litter reached levels at or above those reported to retard decomposition in other studies. At the oak‐hornbeam stand increased also the absolute amounts of Fe, Zn, Pb and Cd.     

Multipurpose conversion management of Scots pine towards mixed oak–birch stands—A long-term simulation approach

The forest growth model 4C was used to investigate how conversion management of a Scots pine (Pinus sylvestris L.) stand towards a mixed oak–birch stand would affect stand structural development – and hence biodiversity and productivity – in the long term. For this purpose the 4C model was parameterised for natural regeneration of light demanding species and extended for management of multi-layered stands. A series of structural indicators was selected to describe key factors of forest biodiversity at the stand scale. Two consecutive aspects of Scots pine conversion were tested: (1) the choice of conversion strategy between thinning and gap creation and (2) the choice of conversion regime in terms of cutting cycle, thinning type and pine tree retention. Three simulated conversion strategies aim at the gradual removal of the pine canopy but differ in the spatial organisation of pine cuttings and hence result in different light conditions for regeneration. Only the directed gap creation strategy was able to maintain and increase birch admixture to the stand and to approach natural stand structural development. Simulation of 12 conversion regimes for the directed gap creation strategy indicated that thinning type (from above or from below), pine tree retention at final felling (50% of the standing volume or none) and cutting cycle (6, 9 or 12 years) all significantly influence stand structural development. These effects were clearest for oak development. Birch occurred in a few mixed clusters, but tended to disappear when longer cutting cycles were used. Based on a multi-criteria analysis we conclude that the optimal conversion regime – in which both stand productivity and biodiversity objectives can be combined – implies thinning from above, pine tree retention, and cutting cycles of 6 years. The conceptual validity of the model as well as the applicability of the results are discussed.     

Aboveground biomass and nutrient accumulation dynamics in young black alder, silver birch and Scots pine plantations on reclaimed oil shale mining areas in Estonia

The growth, aboveground biomass production and nutrient accumulation in black alder (Alnus glutinosa (L.) Gaertn.), silver birch (Betula pendula Roth.) and Scots pine (Pinus sylvestris L.) plantations during 7 years after planting were investigated on reclaimed oil shale mining areas in Northeast Estonia with the aim to assess the suitability of the studied species for the reclamation of post-mining areas. The present study revealed changes in soil properties with increasing stand age. Soil pH and P concentration decreased and soil N concentration increased with stand age. The largest height and diameter of trees, aboveground biomass and current annual production occurred in the black alder stands. In the 7-year-old stands the aboveground biomass of black alder (2100 trees ha⁻¹) was 2563 kg ha⁻¹, in silver birch (1017 trees ha⁻¹) and Scots pine (3042 trees ha⁻¹) stands respective figures were 161 and 1899 kg ha⁻¹. The largest amounts of N, P, K accumulated in the aboveground part were in black alder stands. In the 7th year, the amount of N accumulated in the aboveground biomass of black alder stand was 36.1 kg ha⁻¹, the amounts of P and K were 3.0 and 8.8 kg ha⁻¹, respectively. The larger amounts of nutrients in black alder plantations are related to the larger biomass of stands. The studied species used N and P with different efficiency for the production of a unit of biomass. Black alder and silver birch needed more N and P for biomass production, and Scots pine used nutrients most efficiently. The present study showed that during 7 years after planting, the survival and productivity of black alder were high. Therefore black alder is a promising tree species for the reclamation of oil shale post-mining areas.     

Fine root morphological adaptations in Scots pine, Norway spruce and silver birch along a latitudinal gradient in boreal forests

Variability in short root morphology of the three main tree species of Europe's boreal forest (Norway spruce (Picea abies L. Karst.), Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth)) was investigated in four stands along a latitudinal gradient from northern Finland to southern Estonia. Silver birch and Scots pine were present in three stands and Norway spruce was present in all stands. For three fertile Norway spruce stands, fine root biomass and number of root tips per stand area or unit basal area were assessed from north to south. Principal component analysis indicated that short root morphology was significantly affected by tree species and site, which together explained 34.7% of the total variability. The range of variation in mean specific root area (SRA) was 51-74, 60-70 and 84-124 m(2) kg(-1) for Norway spruce, Scots pine and silver birch, respectively, and the corresponding ranges for specific root length were 37-47, 40-48 and 87-97 m g(-1). The range of variation in root tissue density of Norway spruce, Scots pine and silver birch was 113-182, 127-158 and 81-156 kg m(-3), respectively. Sensitivity of short root morphology to site conditions decreased in the order: Norway spruce > silver birch > Scots pine. Short root SRA increased with site fertility in all species. In Norway spruce, fine root biomass and number of root tips per m(2) decreased from north to south. The differences in morphological parameters among sites were significant but smaller than the site differences in fine root biomass and number of root tips.     

Does tree species composition control soil organic carbon pools in Mediterranean mountain forests?

We compared soil organic carbon (SOC) stocks and stability under two widely distributed tree species in the Mediterranean region: Scots pine (Pinus sylvestris L.) and Pyrenean oak (Quercus pyrenaica Willd.) at their ecotone. We hypothesised that soils under Scots pine store more SOC and that tree species composition controls the amount and biochemical composition of organic matter inputs, but does not influence physico-chemical stabilization of SOC. At three locations in Central Spain, we assessed SOC stocks in the forest floor and down to 50 cm in the mineral in pure and mixed stands of Pyrenean oak and Scots pine, as well as litterfall inputs over approximately 3 years at two sites. The relative SOC stability in the topsoil (0-10 cm) was determined through size-fractionation (53 μm) into mineral-associated and particulate organic matter and through KMnO₄-reactive C and soil C:N ratio.Scots pine soils stored 95-140 Mg ha⁻¹ of C (forest floor plus 50 cm mineral soil), roughly the double than Pyrenean oak soils (40-80 Mg ha⁻¹ of C), with stocks closely correlated to litterfall rates. Differences were most pronounced in the forest floor and uppermost 10 cm of the mineral soil, but remained evident in the deeper layers. Biochemical indicators of soil organic matter suggested that biochemical recalcitrance of soil organic matter was higher under pine than under oak, contributing as well to a greater SOC storage under pine. Differences in SOC stocks between tree species were mainly due to the particulate organic matter (not associated to mineral particles). Forest conversion from Pyrenean oak to Scots pine may contribute to enhance soil C sequestration, but only in form of mineral-unprotected soil organic matter.     

Influence of litter on emergence and early growth of Quercusrugosa: a laboratory study

Species-rich old-growth forests dominated by Quercus wereextensive in the highlands of Chiapas until a few decades ago. Current land-use is resulting in replacement of Quercus by Pinusspp. in the canopy of the remaining forest fragments, which areless diverse, drier, and more exposed to freezing temperatures.Forest floor and soil are also modified and may limit theregeneration of many woody species. We studied the influence oflitter type (pine needles vs. oak leaves), litter depth (0, 3, 6and 14 cm), and litter cover (3 cm vs. 0 cm of loose litter ontop of sowed acorns) on the emergence and growth of seedlings ofQuercus rugosa, a dominant tree in pine-oak forests. Seedlingemergence and establishment were affected by the interaction ofexperimental factors. Uncovered acorns on pine litter were moreexposed to desiccation; this effect was more evident with deeperlitter. Acorns sowed on oak litter were not affected by levels oflitter cover and litter depth. The results can be of use indefining further field studies, and practices of direct seedingfor restoration of pine-dominated stands.     

Photosynthetic utilization efficiency of absorbed photosynthetically active radiation by Scots pine and birch forest stands in the southern Taiga

Absorption and utilization of photosynthetically active radiation (PAR) were investigated in Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth.) stands that were 41 years old at the end of the experimental period. Canopy depth of the Scots pine stand was about half that of the birch stand (6.5 versus 11.0 m), but absorption of PAR was similar in the two stands. The Scots pine forest canopy, with a leaf area index of 8.9, absorbed 90% of the incoming PAR (APAR), whereas the birch forest canopy, with a leaf area index of 5.9, absorbed 92% of APAR. During maximum foliage development, the upper Scots pine canopy absorbed more PAR than the upper birch canopy (75 versus 66%). The upper, middle and lower layers of the Scots pine canopy contained 37, 48 and 15% of the total needle surface area, respectively. The corresponding distribution of foliage surface area in the three layers of the birch canopy was 50, 30 and 20%, respectively. Measurements of photosynthetic rate were combined with estimates of leaf area index and stand phytomass to determine rates of primary production on a sunny day, a cloudy day, and on an annual basis. The energy equivalents of short- and long-term carbon gain were used with determinations of APAR to calculate photosynthetic utilization efficiency. Throughout the growing season, photosynthetic utilization efficiency of APAR in the upper canopy layer of the Scots pine forest was almost twice that in the lower canopy layer. In the birch forest, photosynthetic utilization efficiency was greater in the lower canopy layer than in the upper canopy layer. In all cases, utilization efficiency was higher in the birch stand than in the Scots pine stand (52 versus 29 J kJ(-1)). Taking account of respiration of the non-photosynthetic parts of each stand (night respiration of needles or leaves; respiration of branches, trunk and roots), estimated utilization efficiency of APAR for net primary production was 11 J kJ(-1) for Scots pine and 19 J kJ(-1) for birch. Solar conversion ratios, expressed as whole-plant net primary productivity per unit of APAR for the growing season, were 0.81 g MJ(-1) for Scots pine and 0.93 g MJ(-1) for birch.     

Quantitative estimates of tree species selectivity by moose (Alces alces) in a forest landscape

Abstract

An extensive literature is available on browsing preference for certain tree species. However, useful predictive tools for estimating the impact of deer on forests production and biodiversity can still be improved. A step in that direction is not only to rank preference among tree species but also to quantify the relative risk of being browsed. The foraging selectivity of moose was evaluated using three different statistical methods developed to study habitat utilization. The general pattern for the three methods was consistent. From the results, groups of forage species were clustered and a quantitative index of selectivity was calculated for the groups. The selectivity index showed that rowan (Sorbus aucuparia), willow (Salix ssp.) and aspen (Populus tremula) had a 14 times higher probability of being browsed than a group consisting of Scots pine (Pinus sylvestris) and downy birch (Betula pubescens), while juniper (Juniperus communis) and silver birch (Betula pendula) had a 3.5 times higher probability than Scots pine and downy birch. Since the most preferred species were the least abundant, one should be cautious about the generality of the index between areas, as it may indicate that preference depends on plant species composition. The method used can easily be applied in forest management. Information on quantitative selectivity indices may improve the possibility of managing moose in accordance with acceptable browsing damage.     

Study of landscape change under forest harvesting and climate warming-induced fire disturbance

We examined tree species responses under forest harvesting and an increased fire disturbance scenario due to climate warming in northern Wisconsin where northern hardwood and boreal forests are currently predominant. Individual species response at the ecosystem scale was simulated with a gap model, which integrates soil, climate and species data, stratified by ecoregions. Such responses were quantified as species establishment coefficients. These coefficients were used to parameterize a spatially explicit landscape model, LANDIS. Species response to climate warming at the landscape scale was simulated with LANDIS, which integrates ecosystem dynamics with spatial processes including seed dispersal, fire disturbance, and forest harvesting. Under a 5 °C annual temperature increase predicted by global climate models (GCM), our simulation results suggest that significant change in species composition and abundance could occur in the two ecoregions in the study area. In the glacial lake plain (lakeshore) ecoregion under warming conditions, boreal and northern hardwood species such as red oak, sugar maple, white pine, balsam fir, paper birch, yellow birch, and aspen decline gradually during and after climate warming. Southern species such as white ash, hickory, bur oak, black oak, and white oak, which are present in minor amounts before the warming, increase in abundance on the landscape. The transition of the northern hardwood and boreal forest to one dominated by southern species occurs around year 200. In the sand barrens ecoregion under warming conditions, red pine initially benefits from the decline of other northern hardwood species, and its abundance quickly increases. However, red pine and jack pine as well as new southern species are unable to reproduce, and the ecoregion could transform into a region with only grass and shrub species around 250 years under warming climate. Increased fire frequency can accelerate the decline of shade-tolerant species such as balsam fir and sugar maple and accelerate the northward migration of southern species. Forest harvesting accelerated the decline of northern hardwood and boreal tree species. This is especially obvious on the barrens ecoregion, where the intensive cutting regime contributed to the decline of red pine and jack pine already under stressed environments. Forest managers may instead consider a conservative cutting plan or protective management scenarios with limited forest harvesting. This could prolong the transformation of the barrens into prairie from one-half to one tree life cycle.     

Why and where do adult trees die in a young secondary temperate forest? The role of neighbourhood

? The density and identity of tree neighbourhood is a key factor to explain tree mortality in forests, especially during the stem exclusion phase.

? To understand this process, we built a logistic model for mortality in a spatially explicit context, including tree and neighbourhood predictors. Additionally, we used this model to build mortality risk frequency distributions. Finally, we tested this model against a random mortality model to predict the spatial pattern of the forest.

? Annual mortality rate was high for pedunculate oak (Quercus robur, 6.99%), moderate for birch (Betula celtiberica, 2.19%) and Pyrenean oak (Q. pyrenaica, 1.58%) and low for beech (Fagus sylvatica, 0.26%). Mortality risk models for pedunculate oak and birch included stem diameter, tree height, canopy position and neighbourhood. Mortality was affected by the specific nature of the neighbourhood showing a clear competitive hierarchy: beech > pedunculate oak > birch. Models based on random mortality and logistic regression model were able to predict the spatial pattern of survivors although logistic regression predictions were more accurate.

? Our study highlights how simple models such as the random mortality one may obscure much more complex spatial interactions.

     

Differences in Agrobacterium infections in silver birch and Scots pine

The aim of this study was to investigate the differences in infections caused by Agrobacterium tumefaciens in a conifer, Scots pine (Pinus sylvestris), and in a non-host deciduous species, silver birch (Betula pendula). All the Agrobacterium tumefaciens strains tested caused crown-gall formation in both tree species, but the infection rates varied remarkably. In Scots pine, the development of galls was rare, and slower than in silver birch. Inoculation into the base of the stem were the most successful in gall induction. Silver-birch galls were large, often surrounding the whole stem, in contrast to Scots pine galls, which were characterized by their small size and neck-like connection with the host plant. In silver birch, no other morphological changes could be seen. In Scots pine, abnormal phenotypes with proliferating short shoots above the galls were observed during the second and third growing season. The results indicate that, of the two non-host tree species, the deciduous one, silver birch, is more susceptible to an A. tumefaciens infection than the conifer, Scots pine. The matrix for A. tumefaciens infection in silver birch differs from that in Scots pine, since the terpene compounds of Scots pine seem either to kill the agrobacteria or to suppress their growth. The differences between the species could be partly caused by their difference in sensitivity to phytohormones. These features reflect evolutionary incompatibility between A. tumefaciens and a gymnosperm.