Understory Woody Species Research Articles

Species-specific and generalized allometric equations for improving aboveground biomass estimations of 33 understory woody species in northeastern China forest ecosystems

Understory small trees and shrubs play a crucial role in the biogeochemical cycles in forest ecosystems. However, their biomass in northeastern China is still uncertain and has been heavily disregarded due to the limited number of available allometric equations. For this study, 782 plants from 33 species obtained by the destructive method were used to develop species-specific and generalized aboveground allometric biomass equations based on collar diameter ( D) and height ( H) using weighted nonlinear seemingly unrelated regression. Each biomass component was shown to be well predicted by D alone, with R2 adj. values mostly greater than 0.80. The majority of species performed better in the models for wood and aboveground biomass when H was included as D2 H. Furthermore, generalized equations for the two components showed a comparatively large coefficient of variation but comparable Bias to species-specific equations, especially for small trees. It is recommended to estimate biomass using generalized equations for mixed species only when species-specific equations are unavailable at a given site. However, in the event when precision is not the primary concern, generalized equations are also suggested. The developed equations will help to improve the accuracy of biomass assessment of understory woody plants in northeastern China forest ecosystems.

Responses of plant carbon and nitrogen assimilations to nitrogen addition in a subtropical forest: Canopy addition vs. understory addition

The global atmospheric nitrogen (N) deposition has intensified in recent years, resulting in a complex impact on forest ecosystems. This study investigated the effects of canopy (CAN) and understory additions of N (UAN) on leaf carbon (C) and N assimilations, as well as growth parameters of representative woody plant species in an evergreen broad-leaved forest, i.e. Castanea henryi, Schefflera heptaphylla, Blastus cochinchinensis, and Lasianthus chinensis. The results showed that leaf N assimilation key enzyme nitrate reductase (NR) activities of B. cochinchinensis and S. heptaphylla were significantly decreased by UAN, and were significantly decreased by CAN for C. henryi. CAN significantly decreased the nitrite reductase activity of C. henryi, while significantly increased that of L. chinensis. However, the Amax values of each woody species were not significantly different among control (CK), CAN, and UAN. Community surveys demonstrated that CAN and UAN inhibited the growth (diameter at breast height, height, or crown width) of the representative large tree, C. henryi, while promoting the growths of understory woody species (B. cochinchinensis and L. chinensis). Overall, N addition was found to change the physiological processes of N and C metabolisms of the dominant woody species in an evergreen broad-leaved forest. The community of subtropical evergreen broad-leaved forests may further decline and its C fixation capacity may be detrimentally changed under N deposition in the future.

Leaf traits of understory woody species in the Congo Basin forests changed over a 60-year period

Background and aims – While tropical forests play an important role in carbon sequestration, they are assumed to be sensitive to rising temperatures and prolonged drought. Plant functional traits are useful for understanding and predicting the effects of such changes in plant communities. Here, we analyse the variation of leaf traits of understory woody species of the Congo Basin rainforests over a 60-year period using herbaria as tools and we verify if this variation is potentially related to recent climate change. Material and methods – Leaves of five shrub species were collected in 2019–2022 in Congolese old-growth forests (Yangambi Biosphere Reserve, DR Congo) from different positions on the shrub. These leaves were compared with herbarium specimens collected in the same area before 1960. For both periods, we assessed leaf size, specific leaf area, stomatal size, and stomatal density for all species. Key results – The variability of the functional traits of the understory woody species are independent of the position of the leaves in the crown. This allows for the use of historic herbarium collections for trait analyses on tropical understory shrubs. The traits of the recently collected leaves were notably different from the traits of herbarium leaves collected in pre-1960: recent leaves were significantly larger, had a higher Specific Leaf Area, a smaller stomata pore length, and, apart from Coffea canephora, showed a lower stomatal density. Conclusion – The difference in traits over time is probably related to the increase in temperature and to atmospheric CO2 concentration, as the average temperature at Yangambi over the past 60 years has shown an upward trend consistent with global increasing CO2 levels, while the average annual rainfall has remained unchanged. Our results provide a first insight into the response of forest species to climate change in the Congo Basin forests, and on how the understory species and the ecosystem will react in the long term, when the temperature further increases.

Species composition and influencing factors of understory woody species in Robinia pseudoacacia plantations on the Loess Plateau

Species composition and influencing factors of understory woody species in Robinia pseudoacacia plantations on the Loess Plateau

Restoration of woody plant communities in red pine plantations on abandoned agricultural land in northern Minnesota, USA

While tree plantations have increased globally, there is growing interest in restoring plantations to native ecosystems. Restoration efforts may be more successful when tree species native to original ecosystems are planted instead of species that are either introduced or native, but off site.Plantations of red pine (Pinus resinosa Ait.) in the western Great Lakes region of North America were commonly established on red pine sites, often after agricultural clearing and abandonment. Red pine is the only species in the overstory, compared to 10 or more species in red pine woodlands. Plantations differ in structure from woodlands, being single-aged versus multi-cohort and having high and spatially-uniform density versus lower and variable density.Our aim was to contrast changes in structure and woody species composition over a 15-year period in response to two silvicultural approaches in red pine plantations on the Red Lake Wildlife Management Area in northwestern Minnesota, USA. Approaches included: 1) uniform thinning followed by a dispersed retention harvest (dispersed treatment) and 2) variable density thinning followed by an expanding gap irregular shelterwood (aggregated treatment).Basal area of trees ≥10 cm dbh was reduced similarly in both treatments to 9 m2 ha−1, but there was high within-stand variability (coefficient of variation) in the aggregated treatment (>100%) compared to the dispersed treatment (20%). Understory woody species abundance increased with both treatments, including sapling, large regeneration, and small regeneration layers, with >100% increases. In the sapling layer, the increased abundance (to 2.5 m2 ha−1) reflected new recruitment into this layer, as there were few saplings before treatment (<0.5 m2 ha−1). By year 15, the majority of saplings were canopy capable species, while lower regeneration layers contained mixtures of canopy trees, sub-canopy trees, and shrubs.Composition in the understory changed, but was similar between treatments, including increases in woody shrubs in the small regeneration layer and increases in red pine regeneration in the large regeneration layer. The sapling layer had increased importance of several native tree species, especially in the aggregated treatment.Both of our silvicultural approaches moved structurally simple, species poor red pine plantations toward a condition reflecting native red pine woodlands, which had mixed-species composition and variably open canopy cover. The treatment using variable density thinning and an irregular shelterwood resulted in greater spatial variability of structure and greater importance of shade-intolerant tree species, both characteristics of native woodlands. While focused on red pine, this approach should have application to conifer restoration more generally.

Community biomass accumulation benefits from flexible plant nutrient homeostasis after wildfire

As an essential part of the stoichiometric flexibility in regulating the relative tissue contents of nitrogen (N) or phosphorus (P), their homeostasis (H) could determine plant growth under changing nutrient environments, such as under wildfire disturbance. Although a growing number of studies have suggested a coupling relationship between plant nutrient H and species abundance, how leaf N and P homeostasis (HN and HP) regulates the accumulation of community biomass remains unclear, especially in nutrient-fluctuating burned areas. We measured leaf N and P concentrations and total biomass of all species and the soil N and P concentrations across 15 forest plots at four recovery periods (2, 10, 20, and 30 years after burning) with low-severity wildfire and one unburned treatment (>80 years after burning) in boreal forests and associated the community-level HN and HP with biomass accumulation in the Great Xing’an Mountains, China. We found that the HN of trees peaked 20 years after burning under N limitation conditions, and the HP of shrubs approached a maximum at 10 years after burning due to P limitation. It is worth noting that the biomass of trees and herbs was significantly positively correlated with HN, while negative relationships between HP and biomass were shown in trees. Nutrient H explained 59% of the total variation in community biomass, and the joint effect of HN and HP mainly contributed to the biomass accumulation of trees (43.5%) and shrubs (68.5%), respectively. These findings greatly improve the theoretical foundation for understanding how plant nutrient H is associated with biomass accumulation after wildfire. Flexible nutrient H in woody plants highlights the vital role of understory woody species in postfire forest management. Therefore, clearly understanding the relationship between nutrient H and carbon sequestration is essential for a thorough assessment of postfire forest sustainability.

Age and phenology control photosynthesis and leaf traits in the understory woody species, Rhamnus cathartica and Prunus serotina.

Understory plants are often inadequately represented or neglected within analyses of forest ecosystem productivity. Further, the potential impacts of the biological factors of age class and growth form on carbon cycling physiology, and how it may vary across the growing season and amongst species of different native/non-native status, have not been thoroughly considered. Our study examines photosynthesis and associated physical leaf traits in two understory woody species, Rhamnus cathartica, introduced and invasive in North America, and Prunus serotina, a common subcanopy species native to North America. We estimated leaf-level photosynthesis as measured through light and carbon dioxide response curves, dark-adapted chlorophyll fluorescence and leaf traits (leaf mass per area and stomatal density) for each combination of species and age class at plots in the understory of a temperate deciduous research forest in the US Upper Midwest at two time points during the growing season, late spring (late May) and mid-summer (mid-July). Carbon assimilation rates from light response curves (A sat, A 400) and fluorescence capacity estimate F v/F m all increased between the two measurement points in both species and age class. Estimates of carbon reaction capacity (V cmax and J max) exhibited a different directional response to seasonal development, declining in seedlings of both species and P. serotina trees (~8-37 % reduction in V cmax, ~9-34 % reduction in J max), though increased in trees of R. cathartica (+24 % in V cmax, +9 % in J max). Divergent responses in photosynthetic parameters amongst these factors may be explained by species differences in leaf mass per area and stomatal density, which together are likely influenced by both growth form, canopy position and ontogeny. Overall, we believe our findings suggest complex, varied influences on photosynthesis that indicate environmental and biological plasticity which may contribute to the historic and continued expansion of R. cathartica in the US Upper Midwest region.

Hypotheses and lessons from a native moth outbreak in a low‐diversity, tropical rainforest

AbstractOutbreaks of defoliating insects in low‐diversity tropical forests occur infrequently but provide valuable insights about outbreak ecology in temperate environments and in general. We investigated an extensive outbreak of the endemic koa moth (Scotorythra paludicola), which defoliated endemic koa trees (Acacia koa) over a third of their range on Hawai‘i Island during 2013 and 2014. At Hakalau Forest National Wildlife Refuge, we observed the dynamics of the outbreak and its effects on host trees, nutrient cycling, and insectivorous consumers in reforestation stands of densely planted koa and in natural forest stands of mixed koa and ‘ōhi‘a (Metrosideros polymorpha). Contrary to predictions of the resource concentration hypothesis, caterpillar biomass and defoliation severity were greater in the natural forest sites, where koa density was relatively low. Caterpillars preferentially consumed the most palatable koa foliage type (phyllodes), and koa initially refoliated with the least palatable foliage type (true leaves). Lightly defoliated small trees refoliated more quickly than did heavily defoliated ones but the opposite was true for large trees, which also produced a greater proportion of phyllodes. Mortality was greatest for heavily defoliated small koa. Caterpillar frass caused larger increases in soil nitrogen (N) than phosphorus (P) availability, with the greatest N increases in fine‐textured soils. Foliar N increased in alien grasses under koa canopies compared to grasses away from koa and to native woody understory species. Bird activity was influenced by ‘ōhi‘a flower abundance and the severity of koa defoliation; birds switched to outbreaking caterpillar prey, and they gained weight during the outbreak. Bat foraging times decreased during the outbreak, apparently because they became satiated quickly each night. Parasitoid wasps increased with caterpillar abundance but had little influence on outbreak dynamics. Reducing alien grass cover and increasing tree diversity would likely reduce the impacts of insect outbreaks and similar perturbations to native forests.

Phenological niche overlap between invasive buckthorn (Rhamnus cathartica) and native woody species

Common buckthorn (Rhamnus cathartica L.) is a prolific invader of forest understories throughout eastern North America. Its ability to invade is partially attributable to its relatively high shade tolerance and ability to capture light both early and late in the growing season because of its phenological differences from native species. Competitors that mitigate this phenological advantage by casting shade early and late in the growing season may therefore increase biotic resistance against invasion. However, controlled comparisons between buckthorn and other woody understory species are scarce and to what extent buckthorn’s phenology is truly exceptional is incompletely known. Here, we compare the spring and autumn phenologies of five shade tolerant, native woody species (Sambucus canadensis, Sambucus racemosa, Corylus americana, Cornus racemosa, and Acer saccharum) to those of buckthorn in two common garden experiments. Spring phenology of buckthorn was often comparable to the five native species considered. All native species broke bud no later than 7 days after buckthorn, with S. racemosa reaching spring phenophases consistently earlier than buckthorn. In contrast to spring, buckthorn fall phenology was distinct in comparison to some natives but not all. Native species started to senesce up to 20 days earlier than buckthorn, but both Sambucus species senesced slowly and held their leaves equally long as buckthorn. These findings illustrate that buckthorn does not possess unique phenology amongst understory species. Forest communities rich in deciduous shrubs or trees that are phenologically similar to buckthorn (particularly S. racemosa) likely limit buckthorn's critical spring and fall carbon gains and exert greater biotic resistance to invasion by buckthorn.

Are we missing the forest for the trees? Conspecific negative density dependence in a temperate deciduous forest.

One of the central goals of ecology is to determine the mechanisms that enable coexistence among species. Evidence is accruing that conspecific negative density dependence (CNDD), the process by which plant seedlings are unable to survive in the area surrounding adults of their same species, is a major contributor to tree species coexistence. However, for CNDD to maintain community-level diversity, three conditions must be met. First, CNDD must maintain diversity for the majority of the woody plant community (rather than merely specific groups). Second, the pattern of repelled recruitment must increase in with plant size. Third, CNDD should extend to the majority of plant life history strategies. These three conditions are rarely tested simultaneously. In this study, we simultaneously test all three conditions in a woody plant community in a North American temperate forest. We examined whether understory and canopy woody species across height categories and dispersal syndromes were overdispersed-a spatial pattern indicative of CNDD-using spatial point pattern analysis across life history stages and strategies. We found that there was a strong signal of overdispersal at the community level. Across the whole community, larger individuals were more overdispersed than smaller individuals. The overdispersion of large individuals, however, was driven by canopy trees. By contrast, understory woody species were not overdispersed as adults. This finding indicates that the focus on trees for the vast majority of CNDD studies may have biased the perception of the prevalence of CNDD as a dominant mechanism that maintains community-level diversity when, according to our data, CNDD may be restricted largely to trees.

Effects of prescribed grazing by goats on non‐native invasive shrubs and native plant species in a mixed‐hardwood forest

Invasion of non‐native shrubs comprises a serious economic and ecological problem in North American forests. Prescribed grazing by goats may offer an effective alternative to traditional methods of control, but has received little study in forest settings. In a 5‐year field experiment, we varied the stocking of goats and number of grazing periods to determine effects of prescribed grazing on both invasive shrubs and native plant species. Specifically, we examined how varied stocking levels and grazing periods affect: (1) the cover and height of invasive shrub and native woody species (2) the cover and diversity of herbaceous species, and (3) the density of native tree seedlings. Data were analyzed with linear mixed‐effects models. We found that prescribed grazing by goats significantly reduced the cover and height of invasive shrubs, regardless of goat stocking or duration. Generally, the greatest reductions in invasive shrub cover occurred in treatments with two grazing periods, regardless of goat stocking. While we observed mostly neutral or positive effects on the herbaceous layer, all grazing treatments reduced the cover and height of native understory woody species. While changes were not significant, we observed a general decline in the aggregate height of native tree species regeneration. Our results suggest that prescribed grazing by goats provides an effective and environmentally friendly treatment for heavy invasions of non‐native shrubs such as Rosa multiflora, but grazing needs to be followed by focused mechanical or chemical treatments to maintain control while allowing the regeneration of native tree species.

The invasive tree, Ailanthus altissima, impacts understory nativity, not seedbank nativity

Ailanthus altissima, the invasive forest tree commonly known as the tree-of-heaven, has been associated with decreased levels of plant species richness and native species diversity. However, this relationship with resident plants has been inconsistently found and the tree’s influence on the seedbank has yet to be studied. To further understand the long-term impact of this tree, ten paired invaded-uninvaded sites were identified in Virginia, USA in a variety of different-aged stands. The herbaceous and woody understories for each plot were inventoried and soil samples were collected and grown out for 34 weeks in a greenhouse. All plants were identified to the most detailed taxonomic level possible. In total, 35 woody understory species, 62 herbaceous understory taxa, and 77 seedbank taxa were identified. The relationship between A. altissima presence and i) the proportion of individual plants that are native, ii) the proportion of species that are native, iii) the native diversity, and iv) the nonnative diversity were analyzed. In addition, models including the invasion age were also considered. We show that A. altissima invasions were associated with a decrease in the proportion of native plants and species in the understory, but not in the seedbank. Nonnative woody diversity also increased with A. altissima presence. Additionally, the impact on the nativity of the woody understory became more extreme over time. We end by discussing the benefits of both managing A. altissima invasions early to limit its overall impact and including the management of other nonnative plants in A. altissima restoration plans.

Plant–animal interactions of understory species in an area of tropical rainforest, north‐eastern Brazil

AbstractWe studied the reproductive attributes (reproductive phenology, pollination and seed dispersion) of understory woody species from a rainforest area in the Araripe‐Apodi National Forest (FLONA), NE Brazil. The FLONA has a representative number of fauna and flora species that are important for the local climatic, hydrological and ecological equilibrium of the region. The community was composed of 68 species distributed in 43 genera and 26 plant families. Melastomataceae was the family with the highest species richness. Most species (61.76%) shared floral traits that promoted bee pollination, for example white, dish type, medium‐sized flowers with pollen as the main resource. Zoochory was the most frequent dispersion system (76.47%), followed by autochory (16.17%) and anemochory (7.35%). Resource distribution (flowers and fruits) was uniform throughout the year, and pollination and dispersion networks were significantly modular, composed of five and four modules, respectively. In the pollination network, bees occurred in almost all modules, but flies, hummingbirds, butterflies and hawk moths were more restricted to certain modules, partially highlighting the idea of pollination syndromes. In the dispersion network, the modules were composed of birds from different families; only one module was comprised of all sampled mammals. Our results show that most species depend on the action of biotic vectors during their reproductive processes (pollination and dispersal). Those interactions are fundamental for maintaining biological diversity in different tropical ecosystems.

Topography and vegetation structure mediate drought impacts on the understory of the South American Atlantic Forest

Droughts have increased in frequency, duration, and severity across most of the tropics but their effect on forest communities remain not fully understood. Here we assessed the effects of a severe El Niño-induced drought event on dominant and low abundance understory plant species and the consequent impacts on ecosystem functions in the South American Atlantic Forest. We established 20 permanent plots with contrasting vegetation structure and topography. In each plot, we measured the stem diameter at breast height (DBH) of every understory woody plant (i.e. 1 to 10 cm stem diameter) before and after a severe 4-year drought event to calculate relative growth and mortality rates after drought. Litter biomass, litter nutrient content and soil nutrients, as well as tree canopy cover, were also quantified. High stem density reduced survival to drought for both dominant and low abundance understory woody species. The growth rate of dominant and low abundance species was lower on steeper slopes during the drought. Dominant species were the main contributor of litter biomass production whereas low abundance species were important drivers of litter quality. Overall, our findings suggest that habitats with low tree density and larger trees on flat areas, such as in valleys, can act as refuges for understory plant species during drought periods. These habitats are resource-rich, providing nutrients and water during unfavorable drought periods and might improve forest resilience to climate change in the long term.

Trade-offs between xylem water and carbohydrate storage among 24 coexisting subtropical understory shrub species spanning a spectrum of isohydry.

Hydraulic capacitance and carbohydrate storage are two drought adaptation strategies of woody angiosperms. However, we currently lack information on their associations and how they are associated with species' degree of isohydry. We measured total stem xylem nonstructural carbohydrate (NSC) concentration in the dry and wet seasons, xylem hydraulic capacitance, native leaf water potentials, pressure-volume curve parameters and photosynthetic performance in 24 woody understory species differing in their degree of isohydry. We found a trade-off between xylem water and carbohydrate storage both in storage capacitance and along a spectrum of isohydry. Species with higher hydraulic capacitance had lower native NSC storage. The less isohydric species tended to show greater NSC depletion in the dry season and have more drought-tolerant leaves. In contrast, the more isohydric species had higher hydraulic capacitance, which may enhance their drought avoidance capacity. In these species, leaf flushing in the wet season and higher photosynthetic rates in the dry season resulted in accumulation rather than depletion of NSC in the dry season. Our results provide new insights into the mechanisms through which xylem storage functions determine co-occurring species' drought adaptation strategies and improve our capacity to predict community assembly processes under drought.

Leaf nonstructural carbohydrate concentrations of understory woody species regulated by soil phosphorus availability in a tropical forest.

Leaf soluble sugars and starch are important components of nonstructural carbohydrates (NSCs), which are crucial for plant growth, development, and reproduction. Although there is a large body of research focusing on the regulation of plant NSC (soluble sugars and starch) concentrations, the response of foliar NSC concentrations to continuous nitrogen (N) and phosphorus (P) addition is still unclear, especially in tropical forests. Here, we used a long‐term manipulative field experiment to investigate the response of leaf NSC concentrations to continuous N and P addition (3‐, 5‐, and 8‐year fertilization) in a tropical forest in southern China. We found significant species‐specific variation in leaf NSC concentrations in this tropical forest. Phosphorus addition dramatically decreased both leaf soluble sugar and starch concentrations, while N addition had no significant effects on leaf soluble sugar and starch concentrations. These results suggest that, in plants growing in P‐limiting tropical soil, leaf NSC concentrations are regulated by soil P availability rather than N availability. Moreover, the negative relationships between NSC concentrations and leaf mass per area (LMA) revealed that NSCs could supply excess carbon (C) for leaf expansion under P addition. This was further supported by the increased structural P fraction after P fertilization in our previous study at the same site. We conclude that soil P availability strongly regulates leaf starch and soluble sugar concentrations in the tropical tree species included in this study. The response of leaf NSC concentrations to long‐term N and P addition can reflect the close relationships between plant C dynamics and soil nutrient availability in tropical forests. Maintaining relatively higher leaf NSC concentrations in tropical plants can be a potential mechanism for adapting to P‐deficient conditions.

Lack of vulnerability segmentation among woody species in a diverse dry sclerophyll woodland community

Abstract Recent findings suggest that tree mortality and post‐drought recovery of gas exchange can be predicted from loss of function within the water transport system. Understanding the susceptibility of plants to hydraulic damage requires knowledge about the vulnerability of different plant organs to stress‐induced hydraulic dysfunction. This is particularly important in the context of vulnerability segmentation between plant tissues which is believed to protect more energetically ‘costly’ tissues, such as woody stems, by sacrificing ‘cheaper’ leaves early under drought conditions. Differences in vulnerability segmentation between co‐occurring plant species could explain divergent behaviours during drought, yet there are few studies considering how this characteristic may vary within a plant community. Here we investigated community‐wide vulnerability segmentation by comparing leaf/shoot and stem vulnerability in all coexistent dominant canopy and understory woody species in a diverse dry sclerophyll woodland community, including multiple angiosperms and one gymnosperm. Previously published terminal leaf/shoot vulnerability to loss of water transport capacity was compared with stem xylem vulnerability to embolism measured on the same species at the same site. We calculated hydraulic safety margins for stems to determine variation in the risk of hydraulic failure during drought among species. The xylem of all species was found to be highly resistant to hydraulic dysfunction, with only two of the eight species exhibiting significantly different vulnerability to the overall mean. No evidence of vulnerability segmentation between shoots/leaves and stems was found in seven of the eight species. Phylogenetically diverse canopy and understory species in this evergreen sclerophyll woodland appear to have evolved similar strategies of drought resistance, including low xylem vulnerability to embolism and general lack of vulnerability segmentation. This convergence in hydraulic safety indicates a lack of hydraulic niche partitioning in this woodland community. A free plain language summary can be found within the Supporting Information of this article.

Rapid leaf litter decomposition of deciduous understory shrubs and lianas mediated by mesofauna

Leaf litter decomposition rates (LDRs) of understory woody species vary substantially across species in temperate forest ecosystems. Using litter traits and LDR data for 78 shrub and liana species from a previous study, plus an additional litterbag experiment with varying mesh bag sizes for a subset of 17 species with rapid LDRs, we report that litter traits have nonlinear effects on LDR. In addition, we show that mesofauna, including nonnative earthworms, in general increase LDR and that the effects are greater for species of high LDR. Our results suggest that the acceleration of LDR by the co-invasion of plant species with high LDR and soil biota can promote nutrient cycling, potentially disrupting the stability of native forest ecosystems.

The role of fire frequency and severity on the regeneration of Mediterranean serotinous pines under different environmental conditions

Fire frequency and burn severity may increase in pine forests in the Mediterranean Basin under the warmer and drier climate projected for this region. Our study aims to evaluate the role of fire frequency and burn severity in the post-fire recruitment and development of Mediterranean serotinous pines under different environmental conditions. Two pine forests representing contrasting climatic conditions and soil types that support serotinous pines in the Iberian Peninsula and affected by large wildfires in summer 2012, were selected. In these two study sites, we determined the number of wildfires between 1978 and 2012 and the burn severity of the last fire (2012 at both sites) through the dNBR spectral index. Three and four years after the wildfires, we sampled the density, cover and height of pine seedlings and the cover of woody understory species in 1296 1 m2 plots. The results indicated that the density and cover of pine seedlings was low after two fires combined with high severities, as well as after three fires, regardless of burn severity. Seedling recruitment after three fires was particularly threatened in the most arid study site (0.01 seedlings m−2), resulting in low seedling cover (0.01%). Seedling height decreased with fire frequency in both study sites, and with burn severity owing to fire-induced shifts in soil fertility and microclimatic conditions. There was a significant negative effect of the cover of woody understory species on the recruitment and cover of pine seedlings. Our results suggest that the effects of increasing fire frequency and severity on pine regeneration may be aggravated under arid conditions. Additionally, this study encourages forest managers to avoid the occurrence of frequent crown fires in order to prevent the loss of serotinous pine forest, and provides useful information to predict the scenarios in which post-fire restoration actions would be helpful.

Invasive shrub removal benefits native plants in an eastern deciduous forest of North America

AbstractIn eastern deciduous forests of North America, invasive shrubs are increasing in richness and abundance at the expense of native species across taxa. Invasive shrubs create an understory that is more dense than both recent and historical preinvasion conditions. Interest in invasive shrub removal to restore native habitat is growing, but our understanding of natural regeneration following treatment of a diverse invasive shrub community is lagging. Using an invasive shrub removal experiment, we provide insight into the effect of repeated removal of a suite of 18 invasive shrub species dominated by border privet (Ligustrum obtusifolium Siebold &amp; Zucc.). In 2009, invasive shrubs were removed from five 20-m-diameter treatment plots, each with a paired control plot. Seven years later, we find an increase in plant diversity, native understory species abundance, and overstory tree species regeneration for individuals under a meter in height. For plants 1 to 4 m in height, the removal treatment has a positive effect on understory woody species, but there has been no change in regenerating overstory trees. A lack of overstory tree regeneration to greater heights is not surprising, given the time frame and the closed-canopy conditions. However, other factors, such as white-tailed deer (Odocoileus virginianus Zimmermann) browse, could be serving as an impediment to taller tree regeneration in the forest understory. An ambient sampling approach in unmanaged, invaded, and uninvaded forest has been used in other studies to estimate the potential impacts of invasive shrub species to native plant communities. However, in this study the ambient sampling approach underestimated the impacts of invasive shrubs compared to their experimental removal. Overall, invasive shrub removal increased plant diversity and allowed passive natural regeneration of native plants that exceeded native cover in the unmanaged, ambient forest under minimal invasive shrub abundance.