Understory Communities Research Articles

Long‐term and landscape impacts of buffel grass on arid plant communities: Ecosystem shifts and acceleration by fire

AbstractPlant invasions drive biodiversity loss, transform ecosystems, and promote positive‐feedback cycles between invasion and fire. However, the long‐term impacts of invasive grasses across landscapes with diverse plant communities and interactions with fire are poorly known. Our objectives were to examine whether buffel grass (Cenchrus ciliaris), a globally significant plant invader, altered the abundance of understory and overstory plants, homogenized plant composition, and shifted ecosystems from woodlands to grassland and to explore interrelationships between invasion and fire. We combined two methodological approaches to assess invasion spread and impacts of buffel grass in the Aṉangu Pitjantjatjara Yankunytjatjara (APY) Lands of arid central Australia: a before‐after‐control‐impact (BACI) experiment over 25 years at 15 sites and a paired‐plot (randomized‐block) experiment at 18 sites. Both experiments spanned two geographic regions and multiple vegetation communities situated on flat plains and rocky hills. We used generalized linear mixed models to analyze predictions about plant abundance and permutational multivariate ANOVA (PERMANOVA) and permutational multivariate analysis of dispersion (PERMDISP) to examine changes in community composition. Fire and invasion interactions were explored using fire history or the relative fire tolerance of plant species as covariates, predictors, or responses. Fire interacted with the invasion process in multiple ways. Invaded sites had burnt more frequently and recently than native sites in one region, and where propagules were present in 1995, buffel grass abundance increased most when fires ensued. Abundance of understory plant functional groups (native grasses, ferns, and vines) decreased with invasion, and understory shrubs decreased due to frequent fires in invaded sites. Overstory composition shifted from fire‐sensitive species toward fire‐tolerant species, but this was not directly attributable to invasion. Partial evidence for ecosystem regime shifts included homogenization of understory communities in invaded rocky hills, and an increase in woody shrub cover at native but not invaded sites over 25 years, resulting in a 5% cover difference by 2019. Impacts were detected across heterogeneous ecological communities at a scale not previously tested amongst high background community variability. Although invasion is not dependent on fire, the acceleration of invasion spread and impacts with fire is a critical consideration for future research and management of grass invaders.

Changes in understory plant communities following soil scarification and amendments in a Northern Hardwood Forest

The regeneration dynamics of sugar maple-dominated stands in northeastern North America are intricately influenced by global changes such as soil acidification, invasive species, forest management, and climate shifts. A notable challenge is the expansion of American beech and its bark disease, threatening mature trees and ecosystem services. Although sylvicultural practices aim to control beech regeneration and enhance high-value species, their impact on understory plant communities remains poorly documented. In Valcartier, eastern Canada, we conducted a field experiment to assess the short-term effects of soil scarification and diverse soil amendments (nitrogen fertilization, liming, wood ash) on understory plant communities in a beech-invaded sugar maple stand. Over four years, we inventoried vegetation and measured various biotic and abiotic variables. Results showed that soil scarification significantly reduced diversity metrics like Simpson dominance, functional diversity, and redundancy (a metric correlated to community stability and resilience to disturbance). Negative effects of scarification were exacerbated by nitrogen, lime, or wood ash additions. Lime amendment without scarification enhanced diversity, suggesting its potential to support understory community stability. While scarification and amendments can promote sugar maple, yellow birch, and pin cherry seedling establishment, they have collateral effects on the understory, impacting productivity, diversity, and resilience in the short term. Land managers should exercise caution when combining these treatments, as they may transiently alter the stability and resilience of the understory and other forest functions. A more sustainable approach involves applying scarification and amendments on small, well-distributed patches within the targeted stands for treatment, avoiding extensive application over large areas.

Characteristics and Driving Mechanisms of Understory Vegetation Diversity Patterns in Central and Southern China

Large-scale forest restoration projects significantly reduce the net rates of forest loss. However, as a key component of forest restoration, planted forests have failed to restore biodiversity. China has implemented a large-scale afforestation program, which includes pure planted forests in particular, leading to various changes in ecosystem processes. Despite this, a comprehensive analysis of understory vegetation diversity patterns in these pure planted forests is still lacking. This study aimed to analyze the data on understory vegetation diversity from three typical pure and natural forest ecosystems of Hunan ecological forests to reveal their diversity patterns. The results revealed no significant difference in the understory diversity index between natural and pure forest types, although natural forests had a bigger species pool. The Zipf–Mandelbrot model was a better fit for species abundance distribution. The fitted results suggested that both environmental filtering and neutral processes affected the species abundance distribution and pure understory communities during restoration succession. Natural forests had the most stable understory diversity structure, whereas pure Phyllostachys heterocycla (Carr.) Mitford forests had the least stable structure. Multivariate regression tree analysis identified indicator species for each community. The gradient boosting model indicated that isothermality and slope direction were the most important factors affecting diversity. The β-diversity analysis showed that community establishment in the four forest types was affected via different mechanisms. The findings of this study have significant implications for understanding the impact of afforestation on the mechanisms for maintaining diversity.

Frequent and strong cold-air pooling drives temperate forest composition.

Cold-air pooling is an important topoclimatic process that creates temperature inversions with the coldest air at the lowest elevations. Incomplete understanding of sub-canopy spatiotemporal cold-air pooling dynamics and associated ecological impacts hinders predictions and conservation actions related to climate change and cold-dependent species and functions. To determine if and how cold-air pooling influences forest composition, we characterized the frequency, strength, and temporal dynamics of cold-air pooling in the sub-canopy at local to regional scales in New England, USA. We established a network of 48 plots along elevational transects and continuously measured sub-canopy air temperatures for 6-10 months (depending on site). We then estimated overstory and understory community temperature preferences by surveying tree composition in each plot and combining these data with known species temperature preferences. We found that cold-air pooling was frequent (19-43% seasonal occurrences) and that sites with the most frequent inversions displayed inverted forest composition patterns across slopes with more cold-adapted species, namely conifers, at low instead of high elevations. We also observed both local and regional variability in cold-air pooling dynamics, revealing that while cold-air pooling is common, it is also spatially complex. Our study, which uniquely focused on broad spatial and temporal scales, has revealed some rarely reported cold-air pooling dynamics. For instance, we discovered frequent and strong temperature inversions that occurred across seasons and in some locations were most frequent during the daytime, likely affecting forest composition. Together, our results show that cold-air pooling is a fundamental ecological process that requires integration into modeling efforts predicting future forest vegetation patterns under climate change, as well as greater consideration for conservation strategies identifying potential climate refugia for cold-adapted species.

Bayesian joint species distribution model selection for community‐level prediction

AbstractAimJoint species distribution models (JSDMs) are an important tool for predicting ecosystem diversity and function under global change. The growing complexity of modern JSDMs necessitates careful model selection tailored to the challenges of community prediction under novel conditions (i.e., transferable models). Common approaches to evaluate the performance of JSDMs for community‐level prediction are based on individual species predictions that do not account for the species correlation structures inherent in JSDMs. Here, we formalize a Bayesian model selection approach that accounts for species correlation structures and apply it to compare the community‐level predictive performance of alternative JSDMs across broad environmental gradients emulating transferable applications.InnovationWe connect the evaluation of JSDM predictions to Bayesian model selection theory under which the log score is the preferred performance measure for probabilistic prediction. We define the joint log score for community‐level prediction and distinguish it from more commonly applied JSDM evaluation metrics. We then apply the joint community log score to evaluate predictions of 1918 out‐of‐sample boreal forest understory communities spanning 39 species generated using a novel multinomial JSDM framework that supports alternative species correlation structures: independent, compositional dependence and residual dependence.Main conclusionsThe best performing JSDM included all observed environmental variables and compositional dependence modelled using a multinomial likelihood. The addition of flexible residual species correlations improved model predictions only within JSDMs applying a reduced set of environmental variables highlighting potential confounding between unobserved environmental conditions and residual species dependence. The best performing JSDM was consistent across successional and bioclimatic gradients regardless of whether interest was in species‐ or community‐level prediction. Our study demonstrates the utility of the joint community log score to compare the predictive performance of JSDMs and highlights the importance of accounting for species dependence when interest is in community composition under novel conditions.

Fire severity drives understory community dynamics and the recovery of culturally significant plants

AbstractAnthropogenic influences are altering fire regimes worldwide, resulting in an increase in the size and severity of wildfires. Simultaneously, throughout western North America, there is increasing recognition of the important role of Indigenous fire stewardship in shaping historical fire regimes and fire‐adapted ecosystems. However, there is limited understanding of how ecosystems are affected by or recover from contemporary “megafires,” particularly in terms of understory plant communities that are critical to both biodiversity and Indigenous cultures. To address this gap, our collaborative study, in partnership with Secwépemc First Nations, examined understory community recovery following a large, mixed‐severity wildfire that burned in the dry and mesic conifer forests of British Columbia, Canada, with a focus on plants of high cultural significance to Secwépemc communities. To measure the effect of a continuous gradient of fire severity across forest types, we conducted field assessments of fire severity and sampled understory plants 4 years postfire. We found that native species richness and richness of species of high cultural significance were lowest in areas that burned at high severity, with distinct compositional differences between unburned areas and those that burned at high severity. These findings were consistent across forest types characterized by distinct historical fire regimes. In contrast, richness of exotic species increased with increasing fire severity in the dominant montane interior Douglas‐fir forests, with exotic species closely associated with areas that burned at high severity. Our study indicates that recent megafires may be pushing ecosystems outside their historical range of variability, with negative implications for ecosystem recovery and cultural use across these fire‐affected landscapes. We also found consistently higher plant diversity, and both native and cultural species richness, in subalpine forests. Collectively, our results provide strong evidence of the ecological and cultural significance of low‐ to moderate‐severity fire and subalpine forests, and the longstanding and ongoing role of Indigenous peoples in shaping these landscapes. As wildfires continue to impact ecosystems and human communities, this study offers novel insights into the recovery of important ecological and cultural values, while highlighting the need to support ethical research collaborations with Indigenous communities and Indigenous‐led revitalization of fire and plant stewardship.

Understory vegetation diversity patterns of Platycladus orientalis and Pinus elliottii communities in Central and Southern China.

As a vital component of arbor forests, understory vegetation serves as an essential buffer zone for storing carbon due to its strong capacity for community regeneration. This study aimed to identify the diversity pattern and construction mechanism of Platycladus orientalis and Pinus elliottii understory vegetation based on large-scale sample surveys. The Bayesian Information Criterion value of species abundance distribution (SAD) indicated that the Zipf and Zipf-Mandelbrot models were the best-fitting models. The SAD and gambin fitting results suggested that the Pi. elliottii community had a more balanced structure, with most species being relatively abundant. The multiple regression tree model detected four and six indicator species in P. orientalis and Pi. elliottii communities, respectively. The α-diversity index increased with a rise in altitude and showed a wavy curve with latitude. Linear regression between the β diversity and environmental and geographic distance indicated that the P. orientalis and Pi. elliottii understory communities tended to be dominated by different ecological processes. The partition of β diversity indicated that both communities were dominated by turnover processes, which were caused by environmental classification or spatial constraints. This study helped to understand the diversity maintenance in the P. orientalis and Pi. elliottii understory vegetation communities, and will benefit for diversity restoration and conservation of pure conifer forests.

A Decade of Understory Community Dynamics and Stability in a Mature Second-Growth Forest in Western Washington

A Decade of Understory Community Dynamics and Stability in a Mature Second-Growth Forest in Western Washington

Higher soil moisture increases microclimate temperature buffering in temperate broadleaf forests

Forest canopies can buffer the understory against temperature extremes, often creating cooler microclimates during warm summer days compared to temperatures outside the forest. The buffering of maximum temperatures in the understory results from a combination of canopy shading and air cooling through soil water evaporation and plant transpiration. Therefore, buffering capacity of forests depends on canopy cover and soil moisture content, which are increasingly affected by more frequent and severe canopy disturbances and soil droughts. The extent to which this buffering will be maintained in future conditions is unclear due to the lack of understanding about the relationship between soil moisture and air temperature buffering in interaction with canopy cover and topographic settings. We explored how soil moisture variability affects temperature offsets between outside and inside the forest on a daily basis, using temperature and soil moisture data from 54 sites in temperate broadleaf forests in Central Europe over four climatically different summer seasons. Daily maximum temperatures in forest understories were on average 2 °C cooler than outside temperatures. The buffering of understory temperatures was more effective when soil moisture was higher, and the offsets were more sensitive to soil moisture on sites with drier soils and on sun-exposed slopes with high topographic heat load. Based on these results, the soil–water limitation to forest temperature buffering will become more prevalent under future warmer conditions and will likely lead to changes in understory communities. Thus, our results highlight the urgent need to include soil moisture in models and predictions of forest microclimate, understory biodiversity and tree regeneration, to provide a more precise estimate of the effects of climate change.

A Decade of Understory Community Dynamics and Stability in a Mature Second-growth Forest in Western Washington

A Decade of Understory Community Dynamics and Stability in a Mature Second-growth Forest in Western Washington

Invasion by Callery pear (Pyrus calleryana) does not affect understory abundance or diversity in early-successional meadows

AbstractTrait differences between invasive plants and the plants in their recipient communities moderate the impact of invaders on community composition. Callery pear (Pyrus calleryana Decne.) is a fast-growing, stress-tolerant tree native to China that has been widely planted for its ornamental value. In recent decades, P. calleryana has naturalized throughout the eastern United States, where it spreads rapidly and achieves high abundance in early-successional environments. Here we compare the impacts of low-density, establishment-phase P. calleryana to those of functionally similar native trees on the understory community diversity and total cover of three early-successional meadows in Indiana’s Eastern Corn Belt Plains. In contrast to our prediction that P. calleryana would have greater negative effects on the total abundance and diversity of the understory plant community compared with native tuliptree (Liriodendron tulipifera L.), American sycamore (Platanus occidentalis L.), or non-tree control plots, we found that these low-density populations of P. calleryana had no significant impact on total cover, species richness, or diversity indices for the understory community compared with the native trees and non-tree control plots. Likewise, the studied populations of P. calleryana had no significant impact on the native, introduced, woody, or native tree subsets of the understory community. These results indicate that in young, low-density populations situated in early-successional meadows, the trait differences between P. calleryana and functionally similar native trees are not of a great enough magnitude to produce changes in community composition. Going forward, complementary research on the impacts of P. calleryana on community composition and ecosystem processes in areas with long-established, dense invasions or invasions in more sensitive ecosystems would allow us to more fully understand how this widespread invader disrupts its host ecosystems.

Soil organic carbon accumulation and microbial carbon use efficiency in subalpine coniferous forest as influenced by forest floor vegetative communities

The importance of forest floor plants (herbs and mosses) and understory communities on soil C dynamics has been grossly understudied in forest ecosystems; however, there is currently very little knowledge on the impact of forest floor vegetation composition on soil organic C (SOC) accumulation and the microbial metabolic processes. To bridge this gap of knowledge, a forest floor vegetation-removal experiment involving nonvascular mosses (Pleurozium schreberi (PS); Rhizomnium tuomikoskii (RT); and Hylocomiastrum pyrenaicum (HP)) and vascular sedges (Carex sp., CS) was conducted in a subalpine coniferous forest on the eastern edge of Tibetan Plateau, to investigate the associations of different forest floor vegetation communities with mineral soil C accumulation and microbial physiology (C use efficiency (CUE) and microbial biomass turnover). Soils beneath the forest floor vegetative communities differed in soil C and nitrogen (N) concentrations and had distinctively different microbial community structure and physiology. Compared to bare soils, sedge soils had significantly greater SOC and dissolved organic C (DOC) accumulation, greater microbial DNA, biomass C and phospholipid fatty acids (PLFAs) concentrations, and higher microbial CUE and shorter microbial biomass turnover time. While effects of mosses differed among species, P. schreberi had similar effects as sedges, but the effects of H. pyrenaicum and R. tuomikoskii were minimal. Relative to bare soil, P. schreberi and Carex sp. soils were 61.5% and 51.6% higher in microbial CUE and had an obviously shorter microbial biomass turnover time. Variations in the level of DOC and PLFAs (rather than their portion relative to SOC) were the most important regulators of microbial CUE and biomass turnover rate in soils with different forest floor vegetation covers. These results highlight how differences in soil organic matter quality that are directly related to the forest floor vegetation community influence the microbial CUE and biomass turnover and the long-term soil C dynamics.

A matter of scale: Local biotic differentiation and potential regional homogenization of understory plant communities in a highly fragmented tropical landscape

Plant communities in highly fragmented tropical landscapes can undergo biotic homogenization and differentiation after land-use changes, such as forest fragmentation. Here we evaluated the beta diversity of understory plant communities in fragmented forest remnants and assessed species geographic distribution to infer potential local and regional scale dependency of homogenization and differentiation pathways in the Atlantic Forest in southern Brazil. More specifically we assessed: (i) community composition; (ii) the frequency of locally rare species, generalists, interior and edge specialists; (iii) the relative contribution of beta diversity components; and (iv) the influence of widely, intermediate and narrowly distributed species to the observed patterns. Understory communities were sampled in a nested design with plots allocated to the interior and edge of forest fragments, and geographic distributions were estimated based on occurrence records from herbaria collections. Our results revealed differences in species composition among fragments and habitat types (edge and interior). Most of the sampled species were classified as locally rare. The turnover component of beta diversity explained a higher proportion of variation in species composition among fragments and habitats. Species richness and abundance were higher in the interior of fragments and most of the recorded species in the study had a wide geographic distribution. Our results support a local community differentiation (high beta diversity) among fragments and habitat types and suggest a potential regional homogenization, given the predominance of widely distributed generalist species in a domain historically characterized by high levels of endemism. We recommend urgent protection and restoration actions on the few remaining fragments of the Atlantic Forest given that each remnant can support different plant assemblages that have been increasingly threatened. Future studies should explicitly consider multiple spatial and temporal scales across fragmented tropical landscapes to better understand links with biotic homogenization and differentiation of ecological communities.

Microhabitat variation affects the understory community assembly of a neotropical rainforest

Deterministic and stochastic processes combine with biotic interactions in community assembly. Stochasticity and biotic interactions are thought to operate on fine scales, and determinism on coarse scales. Would determinism be detected on a fine scale in an understory community of a tropical rainforest? We measured the altitude, index of convexity (IC), slope, and canopy openness, counted the individuals and species and calculated Simpson's dominance index of plants with height ≥ 30 cm and DBH < 5 cm for each of 50 randomly drawn 3 × 3 m plots from a one-hectare grid. We used redundancy analysis and spatial modeling with principal coordinates of neighbor matrices to assess how much of the community variation each variable explained. The same was done for each set of plots with 20%, 60%, and 20% respectively of the lower, intermediate, and higher values of the microrelief variables. Most of the community variation remained unexplained, but the effect of the microrelief variables was significant. IC had the highest contribution and was four times higher in the plots with 20% higher values, stressing the increasing importance of determinism towards the higher extremes of the variable's local range. Species richness decreased and dominance increased in convex plots (high IC), and the most abundant species differed between concave and convex plots. Microrelief variation can act as a local filter with a growing influence towards the extremes of the local range of microenvironmental variables in the community space. Niche-based processes rely on the strength of the environmental filter.

Changes in rainfall pH after passing through the forest canopy: Increase in throughfall but decrease in stemflow

Acid rainfall has the potential to harm the structure and function of forest ecosystems. Forest canopy, which is the first surface of contact for rainfall entering the forest ecosystem, may play a role in filtering acid and increasing pH, acting as a natural protective layer for the understory communities in the forest. However, there is currently a lack of global quantitative assessments on the impacts of forest canopy on rainfall pH. To better understand and predict the relationship between forest canopy and rainfall pH that reaches the forest floor (i.e., throughfall and stemflow), we systematically combined data from 244 published articles, including 2563 paired observations for throughfall and 1431 for stemflow, to investigate whether forest canopy has a buffering effect on rainfall pH. We found that (1) throughfall pH significantly increased by an average of 0.14 after passing through forest canopy, while stemflow pH significantly decreased by 0.74; (2) understory rainfall (throughfall and stemflow) ΔpH was higher in broadleaved compared to needleleaf forests, while stemflow ΔpH was lower in evergreen forests and plantations than in deciduous, primary or secondary forests; (3) throughfall ΔpH was mainly influenced by tree height, rainfall K+ concentration and leaf type, while stemflow ΔpH was mainly affected by stemflow ΔMg2+, ΔNO3- and leaf phenology. Our results showed that forest canopy has a significant impact on the pH of rainfall entering forests, which can be influenced not only by rainfall partitioning, but also by vegetation type and rainfall characteristics. Our findings have important implications for predicting understory rainfall pH in the context of global climate change and will contribute to a better understanding of nutrient cycling associated with hydrological processes in forests.

Factors Driving Changes in Plant Understory Communities Between Coniferous and Broadleaf Deciduous Boreal Forests

Factors Driving Changes in Plant Understory Communities Between Coniferous and Broadleaf Deciduous Boreal Forests

Soil microbial community characteristics and the influencing factors at different elevations on the eastern slope of Helan Mountain, Northwest China.

As an important bridge connecting aboveground communities and belowground biological processes, soil microorganisms play an important role in regulating belowground ecological processes. The altitudinal changes and driving factors of soil microbial community in mountain ecosystem in arid region are still unclear. We measured soil physicochemical properties at seven altitudes in the range of 1300-2800 m in Helan Mountains, and investigated the understory community composition, soil physicochemical properties, and soil microbial community. The driving factor for soil microbial community was explored by variance partitioning analysis and redundancy analysis. The results showed that the total amount of soil microorganisms and bacterial biomass first increased and then decreased with the increases of altitude, fungi, actinomyces, arbuscular mycorrhizal fungi, Gram-positive bacteria, and Gram-negative bacteria groups showed a gradual increase. The variation of fungal-to-bacterial ratio (F/B) along the altitude showed that the cumulative ability of soil bacteria was stronger than that of fungi at low altitudes, while the pattern is opposite at high altitudes. The ratio of Gram-positive bacteria to Gram-negative bacteria (GP/GN) showed an overall decreasing trend with the increases of altitude, indicating that soil bacteria and organic carbon availability changed from "oligotrophic" to "eutrophication" and from "low" to "high" transition as the altitude increased. Vegetation properties, soil physical and chemical properties jointly accounted for 95.7% of the variation in soil microbial community. Soil organic carbon (SOC), soil water content (SWC), and total nitrogen (TN) were significantly correlated with soil microbial community composition. Our results revealed the distribution pattern and driving factors of soil microbial communities at different elevations on the eastern slope of Helan Mountain, which would provide theoretical basis and data support for further understanding the interaction between plant-soil-microorganisms in arid areas.

Drivers of contrasting boreal understory vegetation in coniferous and broadleaf deciduous alternative states

AbstractAlternative states defined by tree‐canopy dominance result in different ecosystem functioning and shape habitat conditions for the understory vegetation. One example in the boreal forest is the alternation between broadleaf deciduous and coniferous forests. Disturbances related to natural fires and human land uses have produced changes in tree‐canopy dominance in the boreal region where coniferous forests change to broadleaved forests, affecting understory community dynamics and their related ecosystem processes and functions. To analyze the factors driving changes in understory vegetation and the resistance of its vegetation to shifts between alternative states, we compared the effects of changes in the system between two contrasting boreal forest types (black spruce vs. trembling aspen) in adjacent stands with similar topoedaphic conditions. We performed a 5‐year in situ experiment using alternative states as a theoretical framework including two approaches: (1) the ecosystem approach, manipulating environmental conditions of light, litter, and nutrients in each forest type to determine the main mechanisms associated with tree‐canopy dominance that affect the diversity and composition of understory communities; and (2) the community approach, physically exchanging understory communities between alternative states, to determine their resistance under a new tree‐canopy dominance through time, as well as the resilience of the forest understory after a small‐scale disturbance. Results indicate that the understory vegetation of trembling aspen forests were resistant through time both after changes in local conditions in the ecosystem approach and in the new black spruce‐dominated alternative state in the community approach. In contrast, mosses and ericaceous plants that typically dominate the forest floor of black spruce forests were negatively affected by the physical effect of broadleaf litter addition in our ecosystem approach and they were not resistant when transplanted to trembling aspen forests in the community approach, as they decreased in abundance and were invaded by aspen understory community species over time. The understory vegetation is a key forest ecosystem driver that can contribute to maintain the resilience of the boreal system and help to preserve their ecosystem services, which is a key aspect to consider in forest management faced with the effects of climate change.

Long-term community shifts driven by local extinction of an iconic foundation species following an extreme marine heatwave.

Gradual ocean warming combined with stronger marine heatwaves (MHWs) can reduce abundances of foundation species that control community structures, biodiversity, and ecosystem functioning. However, few studies have documented long-term succession trajectories following the more extreme events that cause localized extinctions of foundation species. Here, we documented long-term successional changes to marine benthic communities in Pile Bay, New Zealand, following the Tasman 2017/18 MHW, which caused localized extinctions of dominant southern bull kelp (Durvillaea sp.). Six years on, multiscale annual and seasonal surveys show no sign of Durvillaea recolonization. Instead, the invasive annual kelp (Undaria pinnatifida), rapidly colonized areas previously dominated by Durvillaea, followed by large changes to the understory community, as Durvillaea holdfasts and encrusting coralline algae were replaced by coralline turf. Between 3 and 6 years after the total loss of Durvillaea, smaller native fucoids colonized in high densities. Although Undaria initially colonized plots throughout Durvillaea's tidal range, later in the succession Undaria only retained dominance in the lower intertidal zone and only in spring. Ultimately, the tidal zone was slowly replaced by alternative foundation species, composed of different canopy-forming brown seaweeds that dominated different intertidal elevations, resulting in a net increase in canopy and understory diversity. This study is a rare example of long-term effects following an extreme MHW that caused extinctions of a locally dominant canopy-former, but these events and their associated dramatic changes to community structures and biodiversity are expected to become increasingly common as MHWs continue to increase in strength, frequency, and duration.

Peeking under the canopy: anomalously short fire-return intervals alter subalpine forest understory plant communities.

Climate change is driving changes in disturbance regimes world-wide. In forests adapted to infrequent, high-severity fires, recent anomalously short fire-return intervals (FRIs) have resulted in greatly reduced postfire tree regeneration. However, effects on understory plant communities remain unexplored. Understory plant communities were sampled in 31 plot pairs across Greater Yellowstone (Wyoming, USA). Each pair included one plot burned at high severity twice in < 30 yr and one plot burned in the same most recent fire but not burned previously for > 125 yr. Understory communities following short-interval fires were also compared with those following the previous long-interval fire. Species capable of growing in drier conditions and in lower vegetation zones became more abundant and regional differences in plant communities declined following short-interval fire. Dissimilarity between plot pairs increased in mesic settings and decreased with time since fire and postfire winter snowfall. Reduced postfire tree density following short-interval fire rather than FRI per se affected the occurrence of most plant species. Anomalously short FRIs altered understory plant communities in space and time, with some indications of community thermophilization and regional homogenization. These and other shifts in understory plant communities may continue with ongoing changes in climate and fire across temperate forests.