Late-successional Species Research Articles

Plant Traits Variably Respond to Plant–Soil Interactions during Secondary Succession on the Loess Plateau

Knowledge of plant photosynthesis, biomass, and stress resistance could contribute to exploring the growth and restoration of vegetation. However, the response of these plant traits to plant–soil interactions at different successional stages remains poorly understood, which limits the understanding of secondary succession. A greenhouse experiment was designed to test the effects of rhizosphere soils collected from early- (EarlySoil), mid- (MidSoil), and late-successional (LateSoil) plant communities on plant traits of early-, mid-, and late-successional species (EarlySp, MidSp, and LateSp, respectively). We found that plant traits reacted in a specific direction to plant–soil interactions at different successional stages. Specifically, compared with treatments of plants growing in their own soil, the net photosynthetic rate and single-photon avalanche diode significantly increased in LateSp–EarlySoil (treatment of plants growing in soil) (20%–31%) and LateSp–MidSoil (10%–18%); the maximum quantum efficiency of photosystem II increased in MidSp–EarlySoil (1%) and LateSp–MidSoil (4%); belowground soluble sugar concentrations decreased in LateSp–EarlySoil (33%) and LateSp–MidSoil (45%); leaf, stem, and root biomass increased in MidSp–EarlySoil (76%–123%), LateSp–EarlySoil (180%–342%), and LateSp–MidSoil (83%–137%), and in turn they decreased in EarlySp–MidSoil (40%–73%) and EarlySp–LateSoil (53%–67%). The results indicated that soil conditioned by pre-successional species (early- or mid-successional species) would be conducive to plant functional traits of subsequent successional species (mid- or late-successional species). Constrained redundancy analysis and path analysis suggested that water-soluble ammonium N, total N, and available N concentrations were key soil factors affecting early-, mid-, and late-successional species, respectively. Our findings confirm the directionality of succession and provide new information for plant population dynamics during secondary succession.

Contrasting warming responses of photosynthesis in early- and late-successional tropical trees.

The productivity and climate feedbacks of tropical forests depend on tree physiological responses to warmer and, over large areas, seasonally drier conditions. However, knowledge regarding such responses is limited due to data scarcity. We studied the impact of growth temperature on net photosynthesis (An), maximum rates of Rubisco carboxylation at 25°C (Vcmax25), stomatal conductance (gs) and the slope parameter of the stomatal conductance-photosynthesis model (g1), in 10 early successional (ES) and 8 late-successional (LS) tropical tree species grown at three sites along an elevation gradient in Rwanda, differing by 6.8°C in daytime ambient air temperature. The effect of seasonal drought on An was also investigated. We found that warm climate decreased wet-season An in LS species, but not in ES species. Values of Vcmax25 were lower at the warmest site across both successional groups, and An and Vcmax25 were higher in ES compared with LS species. Stomatal conductance exhibited no significant site differences and g1 was similar across both sites and successional groups. Drought strongly reduced An at warmer sites but not at the coolest montane site and this response was similar in both ES and LS species. Our results suggest that warming has negative effects on leaf-level photosynthesis in LS species, while both LS and ES species suffer photosynthesis declines in a warmer climate with more pronounced droughts. The contrasting responses of An between successional groups may lead to shifts in species' competitive balance in a warmer world, to the disadvantage of LS trees.

Beaver activity and red squirrel presence predict bird assemblages in boreal Canada

AbstractWetlands and predation in boreal ecosystems play essential roles throughout the breeding season for bird assemblages. We found a positive association of beaver activity and a negative influence of American red squirrels (Tamiasciurus hudsonicus) on bird assemblages. We used a multispecies hierarchical model to investigate whether bird communities differ between two major wetland habitats in boreal Canada: beaver ponds and peatland ponds. In addition to including variables such as forest cover and latitude, we adopted a structural equation model approach to estimate the occupancy of American red squirrels and its potential influence on bird communities. Using automated recording stations deployed at 50 ponds, we detected 96 bird species in 2018 and 2019. Bird species were grouped into four taxonomic guilds according to their habitat successional requirements: early successional species, late successional species, generalists, and wetland species. Beaver ponds harbored higher species richness, a pattern driven primarily by early successional species. The occupancy of almost a quarter of the species was lower in the presence of red squirrels. Late successional species responded positively to the cover of forest surrounding the pond. Our results highlight the value of considering acoustic data of red squirrels to quantify habitat quality in boreal forests. We conclude that beaver activity shapes bird assemblages through modification of their habitat, and that some bird guilds are associated negatively with the presence of American red squirrels.

Bark Stripping by Deer Disturbs Regeneration in a Larix–Abies Subalpine Forest

Deer can slow the natural succession of forests from light-demanding to shade-tolerant species or even keep forests in an early successional stage. This study examined the effects of bark stripping by deer (Cervus nippon) on the stand dynamics in a Larix kaempferi–Abies veitchii subalpine forest in central Japan. Bark stripping by C. nippon was concentrated on smaller A. veitchii individuals, and the number of stems with stripped bark increased over the study period (2007–2019). Unlike other tree species in the study plot, the main mortality factor of A. veitchii was bark stripping. In addition, the mortality factors of A. veitchii clearly differed among tree sizes: smaller A. veitchii trees (diameter at breast height ≤ 15 cm) died owing mainly to bark stripping, whereas larger individuals died owing mainly to uprooting, snapping, and standing dead trees. At this forest site, bark-stripping deer preferred the late-successional species (A. veitchii) to the early-successional species (L. kaempferi), and consequently, the progress of succession was disturbed.

Similar vegetation‐geomorphic disturbance feedbacks shape unstable glacier forelands across mountain regions

AbstractGlacier forelands are among the most rapidly changing landscapes on Earth. Stable ground is rare as geomorphic processes move sediments across large areas of glacier forelands for decades to centuries following glacier retreat. Yet, most ecological studies sample exclusively on stable terrain to fulfill chronosequence criteria, thus missing potential feedbacks between geomorphic disturbances and vegetation colonization. By influencing vegetation and soil development, such vegetation‐geomorphic disturbance feedbacks could be crucial to understand glacier foreland ecosystem development in a changing climate. We surveyed vegetation and environmental properties, including geomorphic disturbance intensities, in 105 plots located on both stable and unstable moraine terrain in two geomorphologically active glacier forelands in New Zealand and Switzerland. Our plot data showed that geomorphic disturbance intensities permanently changed from high/moderate to low/stable when vegetation reached cover values of around 40%. Around this cover value, species with response and effect traits adapted to geomorphic disturbances dominated. This suggests that such species can act as “biogeomorphic” ecosystem engineers that stabilize ground through positive feedback loops. Across floristic regions, biogeomorphic ecosystem engineer traits creating ground stabilization, such as mat growth and association with mycorrhiza, are remarkably similar. Nonmetric multidimensional scaling revealed a linked sequence of decreasing geomorphic disturbance intensities and changing species composition from pioneer to late successional species. We interpret this linked geomorphic disturbance‐vegetation succession sequence as “biogeomorphic succession,” a common successional pathway in unstable river and coastal ecosystems across the world. Soil and vegetation development were related to this sequence and only advanced once biogeomorphic ecosystem engineer species covered 40%–45% of a plot, indicating a crucial role of biogeomorphic ecosystem engineer stabilization. Different topoclimatic conditions could explain variance in biogeomorphic succession timescales and ecosystem engineer root traits between the glacier forelands. As glacier foreland ground is widely unstable, we propose to consider glacier forelands as “biogeomorphic ecosystems” in which ecosystem structure and function are shaped by geomorphic disturbances and their feedbacks with adapted plant species, similar to rivers and coasts.

Pioneer tree species accumulate higher neighbourhood diversity than late-successional species in a subtropical forest

A long-standing paradigm in ecology is that species diversity in a community accrues over succession. Central to this process are the mechanisms that regulate neighbourhood diversity around species of different functional groups such as early vs late-successional species. However, there is poor understanding of the accumulation of this neighbourhood diversity and its contribution to community diversity. We calculated neighbourhood species and phylogenetic diversities for adult trees of 10 species belonging to three groups: early-, mid-, and late-successional species in a 25-ha forest stem-mapping plot in subtropical China. We compared these diversities with homogeneous and heterogeneous Poisson null models to assess the roles of individual species and habitat heterogeneity in community assembly. We also calculated the neighbourhood species composition to quantify beta diversity turnover across different species groups. We observed that species richness, phylogenetic diversity and beta diversity all decreased from early- to mid- and late-successional species. Compared with homogeneous Poisson null model, early-successional trees had higher neighbourhood species but a lower phylogenetic diversity. Mid-successional trees showed no significant departure in richness and phylogenetic diversity from the null model. In contrast, late-successional trees had lower neighbourhood species richness and also a lower phylogenetic diversity than the null model. Both neighbourhood richness and phylogenetic diversity of all three species groups were well fitted by the heterogeneous Poisson model, indicating that environmental filtering predominated neighbourhood diversity. Our results underscore the importance of the early successional species in the accumulation of neighbourhood diversity and their contributions to diversity of plant communities. We suggest that secondary forests, which are mostly composed of early successional species, warrant a due attention in biodiversity conservation.

Differences in leaf photoprotection strategies of tree species at different successional stages in subtropical forests under seasonal climate change and their relationship to construction cost strategies

Differences in response patterns to spatio-temporal changes in environmental stressors mediated by seasonal climatic differences reflect the underlying adaptation strategies of plants, and can provide key information on how tree species respond to seasonal climatic changes at different successional stages. However, in forests dominated by subtropical broadleaf evergreen species, the relationship between leaf photoprotection strategies and their construction cost strategies in different seasons remains unclear. It limits the assessment of the impact of seasonal climate change on community growth and exploration of potential drivers of community succession. In this study, we measured photoprotective substances, chlorophyll fluorescence, photosynthetic pigments and leaf construction cost (CC) in leaves of three mid-successional and three late-successional tree species in a subtropical broadleaf evergreen forest in different seasons. Our data showed that the young leaves of mid-successional trees had higher photoprotection ability in summer compared to winter, and were therefore better adapted to high temperature and high light conditions in summer, but not to low temperature and high light conditions in winter. The young leaves of late-successional trees had strong and flexible photoprotection in both summer and winter, which ensured normal growth of the young leaves under high temperature and high light in summer, and under low temperature and high light in winter. In addition, the photoprotection of the leaves was positively correlated with CC, suggesting that an increase in photoprotective capacity of plants was accompanied by an increase in its investment in CC. This was a response of the plants to environmental stress and the sacrifices it needed to make to maintain its survival. The differential growth patterns of late-successional species, which could grow normally in summer and winter, and mid-successional species, which were adapted to grow in summer but not in winter, may be key clues to the gradual replacement of mid-successional species by late-successional species.

What is the role of disturbance in catalyzing spatial shifts in forest composition and tree species biomass under climate change?

Mounting evidence suggests that climate change will cause shifts of tree species range and abundance (biomass). Abundance changes under climate change are likely to occur prior to a detectable range shift. Disturbances are expected to directly affect tree species abundance and composition, and could profoundly influence tree species spatial distribution within a geographical region. However, how multiple disturbance regimes will interact with changing climate to alter the spatial distribution of species abundance remains unclear. We simulated such forest demographic processes using a forest landscape succession and disturbance model (LANDIS-II) parameterized with forest inventory data in the northeastern United States. Our study incorporated climate change under a high-emission future and disturbance regimes varying with gradients of intensities and spatial extents. The results suggest that disturbances catalyze changes in tree species abundance and composition under a changing climate, but the effects of disturbances differ by intensity and extent. Moderate disturbances and large extent disturbances have limited effects, while high-intensity disturbances accelerate changes by removing cohorts of mid- and late-successional species, creating opportunities for early-successional species. High-intensity disturbances result in the northern movement of early-successional species and the southern movement of late-successional species abundances. Our study is among the first to systematically investigate how disturbance extent and intensity interact to determine the spatial distribution of changes in species abundance and forest composition.

The Triggering Effect of Gaps on Seedling Germination of the Soil Seed Bank in Tropical Rain Forests, Hainan Island, South China

Light, soil temperature, and soil moisture likely change when a gap or clearing is formed in closed-canopy forests, triggering seed germination in the soil seed bank. However, which factors induce such seed germination remains elusive. In this study, we explored the triggering mechanism of gaps on seed germination in the soil seed bank without felling trees. Power sources were supplied in the forest, and three types of lamps were used to simulate the photo-thermal, light, and temperature environments of gaps (i.e., photo-thermal separation in the field), respectively. The photo-thermal separation experiment was carried out in the tropical rain forests of Bawangling in Hainan Island, South China. Three common pioneers and one late-successional species of the Bawangling area were selected for an indoor photo-thermal separation experiment. The field experiment results showed a significant difference in the average seedling number between groups exposed to light and the control group (13.2 ± 4.0 and 1.4 ± 1.7, respectively; p < 0.01), indicating that light in gaps can initiate seed germination of some species in the soil seed bank. Further indoor validation experiments supported this conclusion. No significant difference was observed in the average seedling number between the thermal group (2.1 ± 1.6) and the control group. The indoor validation experiment showed that changes in temperature alone could not trigger the seed germination of the three pioneer species in darkness. However, a higher average seedling number was observed in the photo-thermal group (15.7 ± 5.6) compared to the light group, indicating that the combined effect of light and temperature can initiate seed germination in the soil seed bank, which was also supported by the indoor verification experiment. We further showed that the ratio of species requiring only light for seed germination to those requiring both light and temperature was 2:3. More case studies are necessary to determine if such outcomes are common in forest soil seed banks.

Downregulation of PSI regulates photosynthesis in early successional tree species. Evidence from a field survey across European forests

Photosynthesis rates and electron transport are key physiological traits that distinguish the successional status of plants (early successional – ES, and late successional – LS) in a community. ES plants can respond quickly to sudden changes in sunlight radiation exposure and display a greater photosynthetic plasticity, with respect to the LS ones, thanks to their ability to regulate the photosynthesis. Photosynthesis regulation is connected to the efficiency of the two photosystems (PSI and PSII). PSI plays an important role in the photosynthetic machinery functioning to respond against sudden changes of metabolism in response to heterogeneous environmental conditions. It can be downregulated under high pressure of reductants, to maintain the balance between the reductant pressure and the metabolic demand. The efficiency of PSII and of PSI can be estimated, respectively, by the maximum quantum yield of photosystem II (FV/FM) and the contribution of the thermal phase in the chlorophyll fluorescence emission (expressed by the parameter ΔVIP, that quantify the I-P phase in the fluorescence emission transient). Both parameters are evaluated in dark-adapted samples and can be measured by prompt fluorescence technique. Here, we analyzed a large dataset from a European-wide survey on mature forests, considering four representative forest tree species growing at six sampling sites distributed along a latitudinal gradient. Two early successional (Pinus sylvestris and Betula pendula) and two late successional (Picea abies and Fagus sylvatica) species were considered. The correlations between FV/FM and I-P phase were mostly positive for different plant species at each sampling site for FV/FM values < 0.82 (assumed as optimal threshold for this parameter), but not for FV/FM values > 0.82. In this last case, the correlations were mostly negative for early successional species, and not significant for late successional species. Foliar nitrogen concentration plays a relevant role for the PSI regulation in early successional species: trees with high nitrogen content have higher I-P phase values and negative correlations between FV/FM and I-P phase for FV/FM > 0.82. An opposite pattern was found in tree species with low foliar nitrogen level. We conclude that the late successional tree species show a substantial regulation and stability of the photosynthetic machinery and photosystem stoichiometry, whereas early successional species have more dynamic behavior of PSI. Early successional species can modulate the photosynthetic efficiency and are able both to up-regulate and down-regulate the PSI concentration and activity in relation to quantum yield capacity and leaf nitrogen content.

Differential elemental stoichiometry of two Mediterranean evergreen woody plants over a geochemically heterogeneous area

Leaf nutrient composition and stoichiometry reflect complex interactions of the plant with its environment and are useful traits to explore ecological processes and relationships. In the present study, the foliar elemental compositions of two common Mediterranean woody species, the evergreen broad-leaved Quercus ilex and the coniferous Pinus pinaster growing in an area of Central Italy known for geochemical and geothermal anomalies, were investigated. To assess the site-specific and age-dependent pattern of foliar composition and stoichiometry, macronutrients (C, N, P, K, Mg, S) and trace elements (Al, As, Ba, Cd, Cr, Cu, Fe, Hg, Ni, Pb, Sb, V, Zn) were determined in leaves and needles of three different ages (6-, 12- and 24-month-old) collected from metalliferous (geothermal, mining) and rural areas. Leaves of Q. ilex showed comparatively high concentrations of micronutrients (i.e., Cu, Fe and Zn), while needles of P. pinaster accumulated significantly high concentrations of potentially toxic elements (i.e., As, Pb and S). No significant trend was found in elemental concentrations in relation to the age of leaves and needles. Multi-element stoichiometry of P. pinaster was driven by the geochemical heterogeneity of the sites, suggesting plastic adaptation at the sites with the most selective edaphoclimatic conditions (i.e., patches with nutrient poor and metalliferous soils). On the other hand, the content of both nutrients and potentially toxic elements in Q. ilex leaves varied little across the study area, reflecting stoichiometric stability; this is consistent with the ecophysiological features of Q. ilex as a late-successional species with a dominant role in the ecosystems of the Mediterranean area. Our findings demonstrate the value of foliar stoichiometric traits for understanding plant adaptation in a heterogeneous environment and also the consequences of biotic interactions during succession.

Understorey Seedling Bank in Forest Areas with a Differing Period of Recovery in Mabira Central Forest Reserve, South Central Uganda.

Seedling banks are very important in forest regeneration following forest disturbances such as crop cultivation. In 2011 and 2013 the Uganda National Forestry Authority (NFA) evicted encroachers from parts of Mabira Central Forest Reserve that had been under crop cultivation for over 40 years. This gave an opportunity for the vegetation to recover. In this study, we assessed the recovery process based on seedling bank diversity, richness and density in three blocks differing in recovery time. Two disturbed blocks, the Western Block (WB) (abandoned by encroachers 1-3 years), and the Eastern Block (EB) (abandoned 4-5 years) before this study and a nearby undisturbed area (intact) were included in the study. We recorded 48 seedlings species; 37 in WB, 30 in EB and 27 in intact. Differences in species richness were not statistically significant among blocks (F2, 88 = 1.2420, p = 0.294). All seedling species found in the intact were found in the EB and WB. There were statistically significant differences in species diversity (Shannon-Wiener: F2, 88 = 5.354, p = 0.006), density (P < 0.001) and composition (ANOSIM; R = 0.55, p = 0.001) among blocks. Apart from Broussonetia papyrifera, other species contributing to the dissimilarity (Acalypha neptunica, Antiaris toxicaria, Blighia unijugata, Funtumia elastica were late succession species. Animal dispersed species dominated intact. Seed-regenerating species were found in both WB and EB, but re-sprouts were more common in EB. These results show that proximity to intact forest aids forest recovery, even for areas with long-term cultivation history.

Soil detachment caused by flowing water erosion in six typical herbaceous plant root systems on the Loess Plateau, China

The effects of herbaceous plant root systems on soil erosion differ due to differences among plant species and root types. Six typical herbaceous plants that commonly appear during vegetation succession on the Loess Plateau were selected to detect the effects of plant root traits on the soil detachment at a species level. In total, 180 undisturbed soil samples were collected, and their soil detachment capacities were tested by overland flow by six specific combinations of slope (3) and flow discharge (4). Soil detachment capacity was estimated effectively using shear stress, bulk density, and root surface area density [ R 2 = 0.78; Nash–Sutcliffe efficiency ( NSE ) = 0.63]. The results showed that mean values of soil detachment capacity were ranged from 0.09 to 1.17 kg m −2 s −1 . Pioneer species of Artemisia capillaris and Leymus secalinus had a high soil detachment capacity and were 6.32–12.57 times greater than that of late succession species. The mean soil detachment capacity of plant with tap root system ( Artemisia argyi , A. capillaris , and A. vestita ) was 1.23 times greater than that of other plants with fibrous root systems ( Bothriochloa ischcemum , L. secalinus , and Stipa bungeana ). Soil detachment capacity decreased with soil bulk density as a power function ( p < 0.01) and decreased with root traits of root length density, root surface area density, root area ratio, and root mass density as exponential functions ( p < 0.01). Thus the effects of plant roots on soil erosion differed among herbaceous species were well reflected by the root surface area density. • Effects of plant roots on soil detachment detected at species level. • Exponential functions were found between root traits and soil detachment capacity. • Effects of the plant root system on soil detachment reflected by root surface area density.

Remnant trees location and abundance play different roles in forest landscape recovery

Remnant trees have great ecological importance in post-disturbance forest landscape recovery which is strongly affected by their spatial configuration. Location and abundance, the two key attributes of spatial configuration, often act jointly, but their relative roles have rarely been investigated. Here, we spatially reconstructed a 300-year time series (1710–2010) of the post-volcanic-eruption forest landscape in Changbai Mountain. We designed a factorial experiment for location (expected vs random) and abundance (expected vs 50–100 % increase) of remnant trees to quantify their relative effects on forest recovery at landscape level and by species. Results showed that random location and increased abundance had significantly higher total basal area than that with the expected location and expected abundance in most simulation periods (P < 0.01). Location of remnant trees had greater effects on total basal area (more than twofold) and landscape pattern (measured by aggregation index) than abundance. Meanwhile, abundance of remnant trees displayed larger effects (∼or > 50 %) on the importance of coniferous species and large-seeded hardwood species. Abundance also played a greater role on the coverage area of the late-successional species. Our study could provide important management implications that planting individual patches in a dispersed pattern and increasing the abundance of late-successional species can best facilitate the recovery in a disturbed forest landscape.

Succession of Ambrosia Beetles Colonizing the Logs of Fallen Alder and Birch Trees.

Simple SummaryTo determine the succession of ambrosia beetles after tree felling, two Betulaceae tree species, an alder (Alnus hirsuta), and a white birch (Betula platyphylla var. japonica) were felled as bait logs in central Hokkaido, Japan, in 2016. From 2016 to 2018, the bait logs were dissected late in each flying season to collect ambrosia beetles. During the period of monitoring, most beetles were collected during the first 2 years. The ambrosia beetle faunas colonizing the two plant species were found to be similar, and it was established that each beetle species appeared in the same sequence in each of the two plant species, albeit with differing temporal patterns. These observations thus indicate a similar sequence of beetle succession in the two tree species, and that the relative niches of beetle species in the successional sequence after tree felling can be determined using this methodology.Ambrosia beetles bore into the xylem of woody plants, reduce timber quality, and can sometimes cause devastating damage to forest ecosystems. The colonization by different beetle species is dependent on host status, from healthy trees to the early stages of wood decay, although the precise factors influencing their host selection are not well known. Classic studies on plant ecology have determined the niches of different plant species in vegetation succession, based on comparisons of successions in different locations using ordination analyses, although the factors influencing the colonization of each species are largely undetermined. In this study, to characterize the succession of ambrosia beetles after tree felling, two Betulaceae tree species, an alder (Alnus hirsuta), and a white birch (Betula platyphylla var. japonica) were felled as bait logs in central Hokkaido, Japan, in 2016. From 2016 to 2018, the bait logs were dissected late in each flying season, and ambrosia beetles were collected from the logs. During the period of monitoring, the beetle colonization in both tree species was mostly concentrated in the first 2 years. We observed similarities in the beetle faunas colonizing the two plant species, and that individual species appeared in the same sequence in the logs of the two plant species, although the temporal patterns of colonization differed. Consequently, significant differences in beetle community compositions in the two host species were detected in each of the first 2 years of the study, whereas the difference in the overall composition of beetle assemblages (=pooled over 3 years) between the two plant species was smaller than that in either 2016 or 2017. We speculated that the differences in the temporal pattern of colonization could be attributable to differences in the rates at which the wood of the two tree species deteriorated. Treptoplatypus severini and Xylosandrus crassiusculus were considered to be early-successional species that commenced log colonization soon after felling, although T. severini has a wide niche and was collected during all 3 years of the study. Conversely, Xyleborinus attenuatus and Heteroborips seriatus were identified as probable late-successional species that showed a preference for older logs.

Moderate pine cover maximizes 10-year survival and growth in late-successional species of contrasting functional strategies

Monospecific pine forests are widespread due to extensive afforestation efforts and natural colonization of abandoned croplands in the Mediterranean Basin. It was originally thought that pines would facilitate the natural colonization of native late-successional resprouter species (e.g., hardwoods), but these species can be compromised if competition with pines outweighs their facilitative effect on these hardwood species. Managing the density or canopy cover of these widespread pine forests can potentially provide some “optimum” balance between facilitation and competition to maximize success in the introduction of late successional species while maintaining a tree stratum. We tested the response (survival and growth across 10 years) of six resprouter species covering a wide range of plant functional strategies, from drought-tolerant sclerophyllous shrubs and trees to malacophyllous drought-sensitive trees, across an experimental gradient of Aleppo pine canopy cover. Seedling performance varied according to the functional strategy, pine cover and time. High pine cover generally enhanced seedling survival, whereas moderate pine cover generally enhanced seedling growth, although this response was modulated by the functional strategy of the seedling species. Interactions between pines and seedlings were only detectable 2–3 years after plantation, increasing in intensity with time. The latter highlights the need of medium to long-term studies to evaluate plant-plant interactions in these water-limited environments with slow successional trajectories. Our results could be attributed to the shade tolerance of most of the introduced trees, combined with their low tolerance to the combination of high sunlight radiation and drought. We found an optimal pine cover of ca. 50% (equivalent to 300–400 trees/ha) in which both survival and growth of late successional species can be maximized, which help to select best locations for more efficient reforestation programs and set a threshold value to decide whether or not to perform tree thinning to enhance ecosystem diversity and, subsequently, resilience.

Genetic variation in leaf traits and gas exchange responses to vapour pressure deficit in contrasting conifer species

Abstract Mechanistically predicting the evolutionary response of tree species to climate change requires an understanding of genetic variation in relevant traits. Here we compared the phenotypic and genetic variation in the Leaf Economics Spectrum (LES) traits and the response of gas exchange to vapour pressure deficit (VPD) in lodgepole pine (Pico) and white spruce (Pigl), an early and a late successional species dominating the boreal forests of western Canada. We measured gas exchange, foliar nitrogen and lamina mass to area ratio in 697 c. 30‐year‐old trees in two field progeny trials. We analysed the response of gas exchange rates to VPD using a novel quantitative genetic model, the function‐valued trait approach. Pico showed greater phenotypic variation in the LES traits and greater genetic variation in photosynthetic rate than Pigl, but the species showed no significant difference in their phenotypic correlations between the LES traits. Pico showed a less sensitive stomatal response to VPD than Pigl and no significant genetic variation in stomatal sensitivity. In contrast, Pigl showed a positive correlation between the genetic values of stomatal sensitivity to VPD and stomatal conductance under low VPD. Our study region is projected to see an increase in VPD with climate change; the less sensitive and genetically diverse stomatal response to VPD in Pico could make this species more vulnerable to climate‐change‐induced droughts. A free Plain Language Summary can be found within the Supporting Information of this article.

Salix shrub encroachment along a 1000 m elevation gradient triggers a major ecosystem change in the European Alps

Shrub encroachment, a globally recognized response to climate warming, usually involves late successional species in mountain environments, without alteration to climax communities. We show that a major ecosystem change is occurring in the European Alps across a 1000 m elevation gradient, with pioneer hygrophilous Salix shrubs, previously typical of riparian forests, wetlands and avalanche ravines, encroaching into the climax communities of subalpine and alpine belts shrublands and grasslands, as well as snowbeds, pioneer vegetation and barren grounds in the nival belt.We analyzed Salix recruitment through dendrochronological methods, and assessed its relationships with climate and atmospheric CO2 concentration. The dendrochronological data indicated that Salix encroachment commenced in the 1950s (based on the age of the oldest Salix individuals, recruited in 1957), and that it was correlated with increasing atmospheric CO2 concentration, spring warming and snow cover decrease. Hygrophilous Salix shrubs are expanding their distribution both through range filling and upwards migration, likely achieving competitive replacement of species of subalpine and alpine climax communities. They benefit from climate warming and CO2 fertilization and are not sensitive to spring frost damage and soil limitations, being observed across a gradient of soil conditions from loose glacial sediments in recently deglaciated areas (where soils had not had sufficient time to develop) to mature soils such as podzols (when colonizing late successional subalpine shrublands).Salix encroachment may trigger ecosystem and landscape transformations, promoting the development of forests that replace pre‐existing subalpine shrublands, and of open woodlands invading alpine grasslands and snowbeds, making the alpine environment similar to sub‐Arctic and Arctic areas. This results in a new threat to the conservation of the plant species, communities and landscapes typical of the alpine biota, as mountain ranges such as the Alps provide limited opportunities for upward migration and range‐shift.

Light Competition Contributes to the Death of Masson Pines of Coniferous-Broadleaf Mixed Forests in Subtropical China

In the process of subtropical forest succession, it has long been recognized that population decline of Masson pines in coniferous-broadleaf mixed forest is caused by shading from broadleaf trees. However, little is known about the mechanism underlying the interaction between them. Here, we first chose two sets of Masson pine plots approximately aged 60 years in subtropical mountainous areas in eastern China (i.e., pure coniferous forest vs. coniferous-broadleaf mixed forest). Then, we measured and compared tree height, diameter at breast height, first branch height (FBH), live crown ratio (LCR) of Masson pines between the two sets of plots, and also determined the difference in growth performance of Masson pines relative to their neighboring broadleaf trees in the mixed forest stand. Compared with plots in pine forests, Masson pines in mixed plots had lower tree height and crown breadth, higher FBH, lower LCR, and leaf area. Furthermore, the difference of mean FBH between reference trees (Masson pines) and their neighboring trees (i.e., broadleaf trees) in mixed forest plots was greater than that in pine forest plots, and the ratio of LCR between Masson pines and their neighbors (0.46) in mixed forest was significantly smaller than in pine forest (1.05), indicating that those broadleaf trees around Masson pines probably affected their growth. The mean distance between Masson pines and neighboring trees (1.59 m) in mixed forest plots was significantly shorter than in pine forest plots (2.77 m) (p &lt; 0.01), suggesting that strong competition may occur between reference trees and their neighbors. There was a significant difference in the ratio of crown volume between reference tree Masson pine and its neighboring trees in mixed forests (p &lt; 0.01), indicating that the ratio of biomass synthesis to consumption of pines was much lower than their nearby broadleaf trees in mixed forest. Our results have demonstrated for the first time that Masson pines’ population decline is affected by shade-tolerant broadleaf late-successional species, which can be primarily attributed to the distinctive light transmittance of dominant species nearby (pure pine vs. mixed forest). This study provides a new perspective for future studies on the mechanism of forest succession.

Invertebrate herbivory rather than competition with tussocks will increasingly delay highland forest regeneration in degraded areas under active restoration

Determining barriers to tree regeneration along elevational gradients is important to predict shifts in regeneration patterns under climate change scenarios. The stress-gradient hypothesis predicts that facilitation predominates at high elevations and competition at low elevations. Invertebrate herbivory may also play an important, yet hardly recognized role, in low elevation and degraded areas. Our objective was to understand the relative changes in facilitation, competition and invertebrate herbivory along an elevational gradient degraded by long-term livestock rearing and repeated wildfires. Our study area was a seasonally dry ecosystem in central Argentina subjected to forest restoration activities. We planted 3000 saplings of the dominant early and late successional tree species in a full factorial design that included elevation (3 levels: low, 1300; intermediate, 1800; and high, 2300 m a.s.l.), microsite treatment, where we manipulated facilitation and competition (4 levels: tussock grasses mowed to the ground, tussock grasses mowed to 15 cm above the ground, unmowed tussock grasses with an average height of 70 cm, and tussock grasses mowed to the ground near rock outcrops) and 5 blocks per elevation. We monitored sapling survival, change in height, and damage by invertebrate herbivores during one year. For both species, sapling survival significantly increased with elevation, while sapling change in height decreased. Survival and change in height for microsite treatments suggest weak competition for the early successional species and facilitation for the late successional species. Notably, we did not find the elevation and microsite treatment interaction predicted by the stress-gradient hypothesis. The main invertebrate herbivores were leaf-cutting ants, which damaged 42, 33 and 0%, and 25, 27 and 0% of the saplings according to species, for the low, intermediate and high elevations, respectively. Damage did not differ significantly between microsite treatments for either species. Survival for saplings with evidence of damage by leaf-cutting ants was 2.5 and 3.4 times lower as compared saplings with no evidences, according to species. Our findings imply that under a climate warming scenario, future facilitation and competition effects will be similar to current effects, whereas leaf-cutting ants rather than competition may increasingly limit tree regeneration.