Bark beetle infestation is a major driver of tree mortality that may be critical for forest persistence under climate change and the forecasted increase of extreme heat and drought episodes. Under ...this context, the environmental position of host tree populations within the species’ climatic niche (central vs. marginal populations) is expected to be a determinant in the dynamics of insect–host systems. Here, we analyzed the recent patterns of bark beetle disturbance and forest resistance across European coniferous forests during the 2010–2018 period. We obtained bark beetle attack and tree mortality data from successive continental‐scale forest condition surveys on 130 plots including five host trees and five bark beetle species, and characterized the climatic niche of each species. Then, we analyzed the overall forest resistance and species‐specific responses, in terms of bark beetle attack and induced tree mortality, in relation to the distance to the niche optimum of both host tree and beetle species, previous drought events, and plot characteristics. Regional patterns of recent disturbance revealed that forests in central, north, and east of Europe could be at risk under the attack of multivoltine bark beetle species. We found that overall forest resistance to beetle attack was determined by several driving factors, which varied among species responses. Particularly, the environmental position of the affected forest within the host and beetle species’ climatic niche and plot characteristics mediated the influence of drought on the resistance to beetle attack. In turn, forest resistance to induced tree mortality was determined exclusively by the maximum intensity and duration of drought events. Our findings highlight the importance of disturbance interactions and suggest that the joint influence of drought events and bark beetle disturbance will threaten the persistence of European coniferous forests, even in those tree populations close to their species' climatic optimum.
We analyzed the overall forest resistance and species responses, in terms of bark beetle attack and tree mortality, in relation to the distance to the species climatic niche optimum, previous drought events, and plot characteristics. A confluence of driving factors determined the overall forest resistance to beetle attack, which varied among species. Particularly, the environmental position of the forest within the host and beetle species' climatic niche and plot characteristics mediated the influence of drought on the resistance to beetle attack. In turn, forest resistance to induced tree mortality was determined exclusively by the intensity and duration of drought events.
The recent expansion of mountain pine beetle (MPB, Dendroctonus ponderosae) from its native range in western North America into northern boreal pine forests in Alberta, Canada has resulted in ...conditions for tree regeneration that are dramatically different from those after wildfire, the predominant natural disturbance in these forests. We assessed natural regeneration post-MPB for northern boreal lodgepole pine sites in Alberta, Canada via intensive surveys of small plots at 33 severely attacked pine stands and using data from 205 permanent sample plots representing various site types and levels of MPB mortality. We used model selection to identify factors explaining regeneration. Overall, pine regeneration was very poor 6–9 years post-MPB; only 42% of the 33 intensively surveyed plots and only 9% of the 205 permanent plots had pine seedlings. This poor regeneration is attributed to high levels of cone serotiny in these populations, unsuitable regeneration microsites due to undisturbed litter or feathermoss layers, and competition from the residual canopy and understory vegetation. Other species (aspen, birch, poplar, and black and white spruce) were found on most sites, either as post-attack regeneration or regeneration established in advance. Without intervention, many of these stands will likely transition away from pine, to broadleaf and other conifer species.
The high biodiversity of the Mexican montane forests is concentrated on the Trans-Mexican Volcanic Belt, where several Protected Natural Areas exist. Our study examines the projected changes in ...suitable climatic habitat for five conifer species that dominate these forests. The species are distributed sequentially in overlapping altitudinal bands: Pinus hartwegii at the upper timberline, followed by Abies religiosa, the overwintering host of the Monarch butterfly at the Monarch Butterfly Biosphere Reserve, P. pseudostrobus, the most important in economic terms, and P. devoniana and P. oocarpa, which are important for resin production and occupy low altitudes where montane conifers merge with tropical dry forests. We fit a bioclimatic model to presence–absence observations for each species using the Random Forests classification tree with ground plot data. The models are driven by normal climatic variables from 1961 to 1990, which represents the reference period for climate-induced vegetation changes. Climate data from an ensemble of 17 general circulation models were run through the classification tree to project current distributions under climates described by the RCP 6.0 watts/m² scenario for the decades centered on years 2030, 2060 and 2090. The results suggest that, by 2060, the climate niche of each species will occur at elevations that are between 300 to 500 m higher than at present. By 2060, habitat loss could amount to 46–77%, mostly affecting the lower limits of distribution. The two species at the highest elevation, P. hartwegii and A. religiosa, would suffer the greatest losses while, at the lower elevations, P. oocarpa would gain the most niche space. Our results suggest that conifers will require human assistance to migrate altitudinally upward in order to recouple populations with the climates to which they are adapted. Traditional in situ conservation measures are likely to be equivalent to inaction and will therefore be incapable of maintaining current forest compositions.
Summary
Non‐structural carbohydrates (NSCs), as the labile fraction and dominant carbon currency, are essential mediators of plant adaptation to environments. However, whether and how NSC coordinates ...with plant economic strategy frameworks, particularly the well‐recognized leaf economics spectrums (LES) and root economics space (RES), remains unclear.
We examined the relationships between NSC and key plant economics traits in leaves and fine roots across 90 alpine coniferous populations on the Tibetan Plateau, China.
We observed contrasting coordination of NSC with economics traits in leaves and roots. Leaf total NSC and soluble sugar aligned with the leaf economic spectrum, conveying a trade‐off between growth and storage in leaves. However, NSC in roots was independent of the root economic spectrum, but highly coordinated with root foraging, with more starch and less sugar in forage‐efficient, thinner roots. Further, NSC‐trait coordination in leaves and roots was, respectively, driven by local temperature and precipitation.
These findings highlight distinct roles of NSC in shaping the above‐ and belowground multidimensional economics trait space, and NSC‐based carbon economics provides a mechanistic understanding of how plants adapt to heterogeneous habitats and respond to environmental changes.
Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is ...challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil‐vegetation‐atmosphere‐transport (SVAT) model LWF‐Brook90 taking advantage of regionalized depth‐resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant‐available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT‐derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback‐driven quantification of ecosystem water fluxes in the soil–plant‐atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes.
Swiss early‐wilting occurrence in August 2018 (adapted after Brun et al., 2020) was best explained by mean soil matric potential in the rooting zone (ψs) and actual to potential transpiration Ta/Tp. Mean soil matric potential in the rooting zone and actual to potential transpiration in pixels with and without early wilting are significantly different and indicate early wilting thresholds.
In recent decades, European temperate forests have repeatedly suffered from severe droughts. Drought‐weakened forests have often become more susceptible to pest outbreaks such as bark beetle ...infestations. Tree species diversity is expected to increase resistance to drought and pests, but evidence for a positive tree diversity effect on insect pest reduction is largely circumstantial.
Here we tested the effects of tree diversity and biogeographic origin of tree species on bark beetle infestation in a large, young tree diversity experiment, with six broadleaved and six conifer species from Europe and North America. Lower infestation risk was expected for the exotic tree species in each congeneric pair (spruce, larch and pine) and for mixtures with higher species richness and higher broadleaf proportion. Following a severe drought in summer 2018, the conifer trees were attacked by the six‐toothed spruce bark beetle Pityogenes chalcographus. Bark beetle boreholes were recorded in winter 2018/2019 on all conifer species.
Norway spruce Picea abies and European larch Larix decidua were the most infested species and thus considered main hosts of the bark beetle. For these two species, probability of infestation decreased with increasing tree diversity (although this was only significant for Larix). In contrast, Pinus, which were less infested overall, were more likely to be infested in plots with high tree diversity. Exotic trees tended to be less infested, with clearest support for enemy release found at the level of infestation intensity when considering pure conifer stands. Overall, the effects of tree diversity and tree species origin were not as strong as the effect of position within the experimental site, where higher rates of infestation were observed at the edge than in the centre.
Synthesis. Increasing tree diversity may reduce the risk of bark beetle infestation for genera prone to high infestation rates (Picea and Larix), but risk for less preferred genera (Pinus, and to some extent the exotic tree species) may increase with tree diversity due to spillover from preferred hosts. In mixed forests, the risk of infestation, even by relatively specialized insect pests, may be redistributed among tree species rather than reduced for all.
Zusammenfassung
In den letzten Jahrzehnten haben die Wälder der gemäßigten Zone Europas wiederholt unter schweren Dürreperioden gelitten. Durch Trockenheit geschwächte Wälder sind oft anfälliger für Befall mit Schädlingen wie z.B. Borkenkäfer geworden. Es wird erwartet, dass Baumartenvielfalt die Resistenz gegen Trockenheit und Schädlinge erhöht, aber bisher gibt es nur wenige Hinweise dafür, dass Baumdiversität den Befall mit Insektenschädlingen reduziert.
In dieser Studie haben wir die Auswirkungen der Baumdiversität und der biogeographischen Herkunft der Baumarten auf den Borkenkäferbefall in einem großen, jungen Baumdiversitätsexperiment mit sechs Laub‐ und sechs Nadelbaumarten aus Europa und Nordamerika getestet. Nach einer starken Trockenheit im Sommer 2018 wurden die Nadelbäume vom Kupferstecher Pityogenes chalcographus befallen. Bohrlöcher durch den Borkenkäfer wurden im Winter 2018/2019 an allen Nadelbaumarten festgestellt. Ein geringeres Befallsrisiko wurde für die exotischen Baumarten in jedem Gattungspaar (Fichte, Lärche und Kiefer) und für Mischungen mit höherem Artenreichtum und höherem Laubbaumanteil erwartet.
Die Gemeine Fichte Picea abies und Europäische Lärche Larix decidua waren die am stärksten befallenen Arten und galten somit als Hauptwirte des Borkenkäfers. Für diese beiden Arten nahm die Befallswahrscheinlichkeit mit zunehmender Baumdiversität ab (obwohl dies nur für Larix signifikant war). Im Gegensatz dazu waren Pinus, die insgesamt weniger befallen waren, in Parzellen mit hoher Baumdiversität häufiger befallen. Exotische Bäume waren tendenziell weniger befallen, wobei der deutlichste Beleg dafür auf der Ebene der Befallsintensität bei reinen Nadelbaumbeständen gefunden wurde. Insgesamt waren die Effekte der Baumvielfalt und der Baumartenherkunft nicht so stark wie der Effekt der Position innerhalb der Versuchsfläche, wo am Rand höhere Befallsraten als in der Mitte beobachtet wurden.
Synthese. Eine zunehmende Baumvielfalt kann das Risiko eines Borkenkäferbefalls für Gattungen, die für hohe Befallsraten anfällig sind (Picea und Larix), verringern, aber das Risiko für weniger bevorzugte Gattungen (Pinus und bis zu einem gewissen Grad die exotischen Baumarten) kann mit der Baumvielfalt aufgrund von spill‐over von bevorzugten Wirten zunehmen. In Mischwäldern kann das Befallsrisiko, selbst durch relativ spezialisierte Schadinsekten, eher auf die Baumarten umverteilt als für alle reduziert werden.
Increasing tree diversity may reduce the risk of bark beetle infestation for genera prone to high infestation rates (Picea and Larix), but risk for less preferred genera (Pinus, and to some extent the exotic tree species) may increase with tree diversity due to spillover from preferred hosts. In mixed forests, the risk of infestation, even by relatively specialized insect pests, may be redistributed among tree species rather than reduced for all. Aerial view forest photograph: Kyle R. Kovach.
• Many ecologically important forest trees from dry areas have been insufficiently investigated for their ability to adapt to the challenges posed by climate change, which hampers the implementation ...of mitigation policies. We analyzed 14 common-garden experiments across the Mediterranean which studied the widespread thermophilic conifer Pinus halepensis and involved 157 populations categorized into five ecotypes.
• Ecotype-specific tree height responses to climate were applied to projected climate change (2071–2100 AD), to project potential growth patterns both locally and across the species’ range.
• We found contrasting ecotypic sensitivities to annual precipitation but comparatively uniform responses to mean temperature, while evidence of local adaptation for tree height was limited to mesic ecotypes. We projected intriguing patterns of response range-wide, implying either height inhibition or stimulation of up to 75%, and deduced that the ecotype currently experiencing more favorable (wetter) conditions will show the largest inhibition. Extensive height reductions can be expected for coastal areas of France, Greece, Spain and northern Africa.
• Our findings underline the fact that intraspecific variations in sensitivity to precipitation must be considered when projecting tree height responses of dry forests to future climate. The ecotype-specific projected performances call for management activities to ensure forest resilience in the Mediterranean through, for example, tailored deployment strategies.
We report a new version and an empirical evaluation of a forest reflectance model based on photon recollision probability (p). For the first time, a p-based approach to modeling forest reflectance ...was tested in a wide range of differently structured forests from different biomes. To parameterize the model, we measured forest canopy structure and spectral characteristics for 50 forest plots in four study sites spanning from boreal to temperate biomes in Europe (48°–62°N). We compared modeled forest reflectance spectra against airborne hyperspectral data at wavelengths of 450–2200 nm. Large overestimation occurred, especially in the near-infrared region, when the model was parameterized considering only leaves or needles as plant elements and assuming a Lambertian canopy. The model root mean square error (RMSE) was on average 80%, 80%, 54% for coniferous, broadleaved, and mixed forests, respectively. We suggest a new parameterization that takes into account the nadir to hemispherical reflectance ratio of the canopy and contribution of woody elements to the forest reflectance. We evaluated the new parameterization based on inversion of the model, which resulted in average RMSE of 20%, 15%, and 11% for coniferous, broadleaved, and mixed forests. The model requires only few structural parameters and the spectra of foliage, woody elements, and forest floor as input. It can be used in interpretation of multi- and hyperspectral remote sensing data, as well as in land surface and climate modeling. In general, our results also indicate that even though the foliage spectra are not dramatically different between coniferous and broadleaved forests, they can still explain a large part of reflectance differences between these forest types in the near-infrared, where sensitivity of the reflectance of dense forests to changes in the scattering properties of the foliage is high.
•First extensive empirical evaluation of a forest reflectance model using p-theory.•New parameterization taking into account woody elements and directional scattering.•Uncertainties of the modeled forest reflectance reduced in the near-infrared region.•Forest spectra and their relationships with plant area index simulated correctly.•Based on field and airborne data in 50 forest plots from boreal to temperate biomes.
Know Your Neighbours Vitali, Valentina; Forrester, David I.; Bauhus, Jürgen
Ecosystems (New York),
09/2018, Letnik:
21, Številka:
6
Journal Article
Recenzirano
Norway spruce is a widely cultivated species in Central Europe; however, it is highly susceptible to droughts, which are predicted to become more frequent in the future. A solution to adapt spruce ...forests to droughts could be the conversion to mixed-species stands containing species which are less sensitive to drought and do not increase the drought stress in spruce. Here we assessed the drought response of spruce and the presumably more drought-tolerant silver fir and Douglas fir in mixed-conifer stands. We measured tree ring widths of 270 target trees, which grew in mixed and mono-specific neighbourhoods in 18 managed stands in the Black Forest, to quantify the complementarity effects caused by species interactions on growth during the extreme drought event of 2003 and for a number of years with “normal” growth and climatic conditions. Mixed-species neighbourhoods did not significantly affect tree ring growth in normal years. However, during the drought, silver fir benefitted from mixing, while Douglas fir was more drought-stressed in the mixture. The drought response of spruce was dependent on the density and species composition of the neighbourhood, showing both positive and negative mixing effects. Mixed stands containing these tree species could improve adaptation to drought because the risks of extreme events are spread across species, and the performance of individual species is improved. Our knowledge about specific species interactions needs to be improved to manage tree mixtures more effectively with regard to the participating species and stand density.
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