Canopy Disturbance Research Articles

Effect on Top-Coal Mass Failure under Load–Unload Induced by Shield Support

Prior to being released, the top coal of a fully mechanized caving face typically experiences the effects of mining-induced pressure and disturbances from the hydraulic support’s canopy. To investigate the promoting effect of canopy disturbances in the support-controlled area on the damage of top coal during fully mechanized top-coal caving mining, block instability theory and discrete block numerical simulation methods were employed. The instability modes during the initial support and cyclic disturbance stages were analyzed. The fracture evolution and displacement distribution curves were studied for coal bodies with strengths of 3.1 MPa, 15.0 MPa, and 29.5 MPa under the mining influence and support strengths of 0.5 MPa, 1.0 MPa, and 1.5 MPa during the initial support and cyclic disturbance processes. The results showed that during the initial support stage, the instability of the block-structured coal body exhibited four modes as follows: inter-block breakage, inter-block delamination, intra-block fracture expansion, and inter-block sliding. During the cyclic disturbance stage, the coal body instability showed three modes as follows: inter-block sliding, intra-block fracture expansion, and inter-block rotation. The disturbance from the support had little impact on the fragmentation of both low-strength and high-strength coal bodies. However, small support forces combined with multiple cycles of disturbance had an enhancing effect on the instability of medium-strength block-structured top coal.

Functional traits and ecological niches as correlates of the interspecific growth–mortality trade‐off among seedlings of 14 tropical tree species

Abstract The interspecific trade‐off between growth versus mortality rates of tree species is thought to be driven by functional biology and to contribute to species ecological niche differentiation. Yet, functional trait variation is often not strongly correlated with growth and mortality, and few studies have investigated the relationships of both traits and niches, specifically encompassing above and belowground resources, to the trade‐off itself. These relationships are particularly relevant for seedlings, which must often survive resource limitation to reach larger size classes. We investigated the functional basis of the interspecific growth–mortality trade‐off and its relationship with ecological niches for seedlings of 14 tree species in a tropical forest in southwest China. We found evidence for an interspecific growth–mortality trade‐off at the seedling stage using 15 functional traits and 15 ecological niche variables. None of the organ‐level traits correlated with growth, mortality, nor the trade‐off, whereas specific stem length (SSL), a biomass allocation trait, was the only trait to have a significant correlation (positive). Moreover, light‐defined niches were not correlated with growth, mortality or the trade‐off, but soil‐defined niches did. Species at the faster growth/higher mortality end of the trade‐off were associated with higher fertility defined by lower soil bulk density and slope, and higher soil organic matter concentration and soil total nitrogen. Our findings indicate the importance of stem elongation and soil fertility for growth, mortality and their trade‐off at the seedling stage in this Asian tropical forest. Our findings contrast with analogous studies in neotropical forests showing the importance of photosynthesis‐related leaf traits related to insolation. Therefore, the functional drivers of demographic rates and trade‐offs, as well as their consequences for ecological niches, can vary among forests, likely owing to differences in biogeography, canopy disturbance rates, topography and soil properties. Moreover, the effects of functional trait variation on demographic rates and trade‐offs may be better revealed when biomass allocation is accounted for in a whole‐plant context. Read the free Plain Language Summary for this article on the Journal blog.

Disturbance‐mediated community characteristics and anthropogenic pressure intensify understorey plant invasions in natural forests

Abstract Although disturbance is considered a major driver of plant invasions across many systems, our understanding of the mechanisms by which disturbance mediates understorey invasions in natural forests is limited. We used a national natural forest inventory dataset spanning New Zealand's wide climatic and soil fertility gradients to disentangle disturbance‐mediated community characteristics driving abundance, species richness and functional composition of understorey plant invasions. Disturbance‐mediated declines in canopy tree abundance and increases in soil fertility and pH increased non‐native plant richness and cover relative to co‐occurring native plant assemblages. Cover of non‐native species also increased with proximity to anthropogenic land cover. Non‐native plant assemblages had higher community‐weighted mean (CWM) values for specific leaf area (SLA) but lower CWM values for height and woodiness relative to native assemblages irrespective of disturbance. However, greater nearby anthropogenic land cover drove increased woodiness in non‐native assemblages but decreased woodiness in co‐occurring native assemblages. Synthesis: Our study provides the first national‐scale evidence that canopy disturbance effects on soil properties increase both richness and abundance of non‐native plants in natural forest understories. We also revealed functional trait differences between native and non‐native assemblages (SLA in particular), which could alter fundamental forest ecosystem processes like litter decomposition and nutrient cycling. Finally, landscape‐scale anthropogenic impacts may exacerbate forest invasions by increasing non‐native abundance and favouring woody invaders, which may achieve dominance in future forest communities over the longer‐term.

Forest floor environment overrules global change treatment effects on understorey communities in a mesocosm experiment.

Light availability profoundly influences plant communities, especially below dense tree canopies in forests. Canopy disturbances, altering forest floor light conditions, together with other environmental changes such as climate change, nitrogen deposition and legacy effects from previous land-use will simultaneously impact forest understorey communities. Yet, knowledge on the individual effects of these drivers and their potential interactions remains scarce. Here we performed a forest mesocosm experiment to assess the influence of warming, illumination (simulating canopy opening), nitrogen deposition and soil land-use history (comparing ancient and post-agricultural forest soil) on understorey community composition trajectories over a 7-year period. Strikingly, understorey communities primarily evolved in response to the deeply shaded ambient forest conditions, with experimental treatments exerting only secondary influences. The overruling trajectory steered all mesocosms towards slow-colonizing forest specialist communities dominated by spring geophytes with lower nutrient-demand. The illumination treatment and, to a lesser extent, warming and agricultural land-use legacy slowed down this trend by advancing fast-growing resource-acquisitive generalist species. Warm ambient temperatures induced thermophilization of plant communities in all treatments, including control plots, towards higher dominance of warm-adapted species. Nitrogen addition accelerated this thermophilization process and increased the community light-demand signature. Land-use legacy effects were limited in our study. Our findings underscore the essential role of limited light availability in preserving forest specialists in understorey communities and highlight the importance of maintaining a dense canopy cover to attenuate global change impacts. It is crucial to integrate this knowledge in forest management adaptation to global change, particularly in the face of increasing demands for wood and wood products and intensified natural canopy disturbances.

Changes in planned and unplanned canopy openings are linked in Europe’s forests

Canopy openings are increasing in Europe’s forests, yet the contributions of anthropogenic and ecological agents of disturbance to this increase remain debated. Here we attribute the root cause of all stand-replacing canopy disturbances identified for Europe in the period 1986–2020 from Landsat data (417,000 km²), distinguishing between planned and unplanned canopy openings (i.e., disturbance by human land use versus by wind, bark beetles, and wildfire). We show that canopy openings by humans dominate the European forest disturbance regime, accounting for 82% of the area disturbed. Both planned and unplanned canopy openings increased in the early 21st century (+24% and +30% relative to the late 20th century). Their changes are linked, with simultaneous increases in planned and unplanned canopy openings on 68% of Europe’s forest area. We conclude that an important direction for tackling disturbance change in policy and management is to break the link between planned and unplanned canopy openings in Europe’s forests.

Fire responses of flora in a sclerophyll–rainforest vegetation complex in the Nightcap Range, North Coast, New South Wales

Context Species fire responses were investigated in a mixed sclerophyll–rainforest ecosystem in the Nightcap Range, North Coast, New South Wales. Aims To examine rates of seedling recruitment and resprouting in functional and phytogeographic components of wet sclerophyll forest (WSRf), and adjacent open forest (OF) and rock outcrop shrubland (RO). Methods Species resprouting and seedling recruitment traits (fire responses) were recorded in 45 stem plots and 225 seedling subplots in WSRf, OF and RO. Species fire responses were classified, community fire-response spectra compiled and rates of seedling recruitment and resprouting in WSRf examined in relation to primary fire response, growth-form, habitat and broad functional and phytogeographic species groupings. Species size-regenerative class distribution was used to analyse population structure, fire impact, regeneration and recruitment in resprouter species that comprised most of the mesic-Gondwanan element of the WSRf flora. Key results WSRf, OF and RO habitats had distinctively different fire-response spectra. In WSRf, there was a high proportion of mesophyll resprouter species of Gondwanan origin with nil or very low seedling recruitment, a distinct component of mesophyll seeders of Indo-Malayan origin, as well as sclerophyll seeders and resprouters that also comprised most of the OF and RO floras. Resprouters comprised 75% of the WSRf flora, 50% OF and 10% pavement shrubland. Continuous size-class distributions indicated recruitment between fire events in the majority of mesophyll resprouters in WSRf. Lower total seedling density appeared to reflect inherent species traits and less canopy disturbance by fire. Large sclerophyll species forming the unburnt canopy of WSRf had very low seedling recruitment. Conclusions Different habitats (WSRf, OF and RO) and functional and phytogeographic clades in WSRf display distinctive patterns of resprouting and seedling-recruitment fire response. Fire responses of species that maintain species population and community composition are governed by fire regime, habitat variables and inherent species traits. Implications The distinctive fire-response spectrum of WSRf appears to be a direct consequence of the overlap of ‘new’ and ‘old’ floras in this broad vegetation type.

Monitoring lianas from space: Using Sentinel-2 imagery to observe liana removal in logged tropical forests

Liana removal – the cutting of over-abundant woody climbing plants (lianas) – has the potential to substantially increase tree growth and biomass accumulation across millions of hectares of degraded tropical forest. Satellite imagery could provide data capable of observing the effect of liana removal on the forest canopy, enabling the large-scale monitoring and validation of liana removal, which remains a key hurdle to its widespread implementation. Using a 320-ha liana removal experiment in Sabah, Malaysian Borneo, we tested whether a time series of Sentinel-2 images could observe the canopy signature of liana removal. Calculating a range of metrics derived from the Normalized Burn Ratio – a vegetation index based on spectral reflectance that differentiates leaf from non-leaf – we quantified satellite-derived canopy disturbance and fragmentation across a range of liana removal intensities and examined how canopy disturbance changed in the 12-months following removal treatments. We find that liana removal significantly increases canopy disturbance and fragmentation metrics one month after removal, with partial removal having a smaller effect than complete removal. The impact of liana removal on the canopy metrics declined over time, with measures of canopy disturbance and fragmentation largely indistinguishable from control forest within 12-months of treatment. Our findings evidence that freely available satellite imagery can be used to efficiently monitor large-scale liana removal applied at a range of intensities and suggest that partial liana removal could significantly reduce canopy disturbance of this restoration method.

Combining multiple investigative approaches to unravel functional responses to global change in the understorey of temperate forests

AbstractPlant communities are being exposed to changing environmental conditions all around the globe, leading to alterations in plant diversity, community composition, and ecosystem functioning. For herbaceous understorey communities in temperate forests, responses to global change are postulated to be complex, due to the presence of a tree layer that modulates understorey responses to external pressures such as climate change and changes in atmospheric nitrogen deposition rates. Multiple investigative approaches have been put forward as tools to detect, quantify and predict understorey responses to these global‐change drivers, including, among others, distributed resurvey studies and manipulative experiments. These investigative approaches are generally designed and reported upon in isolation, while integration across investigative approaches is rarely considered. In this study, we integrate three investigative approaches (two complementary resurvey approaches and one experimental approach) to investigate how climate warming and changes in nitrogen deposition affect the functional composition of the understorey and how functional responses in the understorey are modulated by canopy disturbance, that is, changes in overstorey canopy openness over time. Our resurvey data reveal that most changes in understorey functional characteristics represent responses to changes in canopy openness with shifts in macroclimate temperature and aerial nitrogen deposition playing secondary roles. Contrary to expectations, we found little evidence that these drivers interact. In addition, experimental findings deviated from the observational findings, suggesting that the forces driving understorey change at the regional scale differ from those driving change at the forest floor (i.e., the experimental treatments). Our study demonstrates that different approaches need to be integrated to acquire a full picture of how understorey communities respond to global change.

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.

Forest understorey flowering phenology responses to experimental warming and illumination.

Species are altering their phenology to track warming temperatures. In forests, understorey plants experience tree canopy shading resulting in light and temperature conditions, which strongly deviate from open habitats. Yet, little is known about understorey phenology responses to forest microclimates. We recorded flowering onset, peak, end and duration of 10 temperate forest understorey plant species in two mesocosm experiments to understand how phenology is affected by sub-canopy warming and how this response is modulated by illumination, which is related to canopy change. Furthermore, we investigated whether phenological sensitivities can be explained by species' characteristics, such as thermal niche. We found a mean advance of flowering onset of 7.1 d per 1°C warming, more than previously reported in studies not accounting for microclimatic buffering. Warm-adapted species exhibited greater advances. Temperature sensitivity did not differ between early- and later-flowering species. Experimental illumination did not significantly affect species' phenological temperature sensitivities, but slightly delayed flowering phenology independent from warming. Our study suggests that integrating sub-canopy temperature and light availability will help us better understand future understorey phenology responses. Climate warming together with intensifying canopy disturbances will continue to drive phenological shifts and potentially disrupt understorey communities, thereby affecting forest biodiversity and functioning.

Novel light regimes in European forests.

Tree canopies are one of the most recognizable features of forests, providing shelter from external influences to a myriad of species that live within and below the tree foliage. Canopy disturbances are now increasing across European forests, and climate-change-induced drought is a key driver, together with pests and pathogens, storms and fire. These disturbances are opening the canopy and exposing below-canopy biodiversity and functioning to novel light regimes-spatial and temporal characteristics of light distribution at forest floors not found previously. The majority of forest biodiversity occurs in the shade within and below tree canopies, and numerous ecosystem processes are regulated at the forest floor. Altered light regimes, in interaction with other global change drivers, can thus strongly impact forest biodiversity and functioning. As recent European droughts are unprecedented in the past two millennia, and this has initiated probably the largest pulse of forest disturbances in almost two centuries, we urgently need to quantify, understand and predict the impacts of novel light regimes on below-canopy forest biodiversity and functions. This will be a crucial element in delivering much-needed information for policymakers and managers to adapt European forests to future no-analogue conditions.

Mechanical Analysis of Rice Canopy Using Explicit Dynamics and Practical Applications of Canopy Opener

Open-canopy pesticide application technology is not affected by the canopy structure and can substantially increase droplet penetration and deposition in the middle and lower rice canopy. However, the optimal working position of the canopy opener is difficult to determine. The purpose of this study was to examine the characteristics of rice canopy disturbances under the action of a canopy opener, verify simulation results using high-speed photography tests, and identify the spraying effect of the optimal operating parameters of a canopy opener through spray tests. The mechanical characteristic parameters of the rice heading stage were determined through experiments, and the canopy-opening process was simulated based on the finite element method of explicit dynamics. The simulation results show that when the speed of the canopy opener is 1.2 m s−1 and the height is 0.6 m, the rice canopy is the most disturbed. The spray experiment results show that under this condition, the overall coverage rate of the rice canopy is relatively high, and the coverage rates of the upper, middle, and lower layers of rice are 92.18%, 66.43%, and 52.84%, respectively.

Effect of seaweed canopy disturbance on understory microbial communities on rocky shores

IntroductionThe collapse of macroalgal habitats is altering the structure of benthic communities on rocky shores globally. Nonetheless, how the loss of canopy-forming macroalgae influences the structure of epilithic microbial communities is yet to be explored. MethodsHere, we used experimental field manipulations and 16S-rRNA-gene amplicon sequencing to determine the effects of macroalgal loss on the understory bacterial communities and their relationship with epiphytic bacteria on macroalgae. Beds of the fucoid Hormosira banksii were exposed to different levels of disturbance resulting in five treatments: (i) 100% removal of Hormosira individuals, (ii) 50% removal, (iii) no removal, (iv) a procedural control that mimicked the removal process, but no Hormosira was removed and (v) adjacent bare rock. Canopy cover, bacterial communities (epilithic and epiphytic) and benthic macroorganisms were monitored for 16 months.ResultsResults showed that reductions in canopy cover rapidly altered understory bacterial diversity and composition. Hormosira canopies in 50% and 100% removal plots showed signs of recovery over time, but understory epilithic bacterial communities remained distinct throughout the experiment in plots that experienced full Hormosira removal. Changes in bacterial communities were not related to changes in other benthic macroorganisms.DiscussionThese results demonstrate that understory epilithic bacterial communities respond rapidly to environmental disturbances at small scales and these changes can be long-lasting. A deeper knowledge of the ecological role of understory epilithic microbial communities is needed to better understand potential cascading effects of disturbances on the functioning of macroalgal-dominated systems.

Natural Canopy Disturbance Patterns and Ecological Silviculture in the Alabama Fall Line Hills

Abstract As a natural disturbance-based approach to silviculture is increasingly embraced by forest managers, quantitative reference conditions of natural disturbance patterns are paramount. We used LiDAR data to quantify resultant structural patterns from an EF3 tornado on the Oakmulgee Ranger District of Talladega National Forest in the Alabama Fall Line Hills, USA. We found the zone of catastrophic disturbance totaled 123 ha, had a mean width of 360 m, and residual trees were randomly distributed. This zone was buffered by an edge zone of intermediate-severity disturbance where trees were removed individually and in groups. The edge zone averaged 220 m in width. In total, the study area included seventy-nine stands and the percentage of stand area damaged ranged from < 1% to 94%. We suggest that clearcut, seed tree, and shelterwood with reserves regeneration methods may be appropriate analogs to natural canopy disturbance in the Fall Line Hills region. These catastrophic disturbance entries should be buffered from undisturbed neighborhoods by a system that retains mature forest structures and would constitute a zone of intermediate-severity disturbance. Our approach may be used as a template to expand our understanding of natural canopy disturbance patterns in other regions and forest types. Study Implications: In regions where tornadoes are a natural component of the disturbance regime, including much of the southeastern United States, we suggest that clearcut, seed tree, or shelterwood with reserves regeneration methods may be used to approximate the zone of catastrophic disturbance that corresponds to the tornado track. We observed that reserve tree spatial patterns were random, but residual trees could be selected based on desired species or protection of important biophysical features such as seeps. Surrounding the catastrophic disturbance zone was an edge zone of intermediate-severity disturbance where trees were removed individually and in variably sized groups. We suggest that the structural patterns documented in this edge zone may be created through variable retention harvesting.

Monitoring selective logging intensities in central Africa with sentinel-1: A canopy disturbance experiment

Forest degradation is a major threat to tropical forests, and effective monitoring using remotely sensed data is subject to significant challenges. In particular, consistent methods for detecting subtle changes in the forest canopy structure caused by selective logging are lacking. Here, using a unique dataset collected in southeastern Cameroon, covering over 22,000 ha of monthly harvesting areas, >6000 locations of harvested trees, and an independent canopy gap dataset developed from an uninhabited aerial vehicle flight (UAV; RGB camera) of approximately 1500 ha, a new method was designed to monitor canopy disturbance and logging intensity in Central Africa. Using Sentinel-1 synthetic aperture radar (SAR) data, the method was conceptualised using a two-step, two-scale approach, which better matched logging practices. First, (non-)harvesting activity areas were identified using textural indices at a spatial resolution of 300 m (step 1), and within these harvesting activity areas, canopy gaps were detected at a resolution of 10 m (step 2). Both steps were based on monthly differences in the Sentinel-1 SAR time series computed using the average of the 12 months preceding and the average of the three months following the month of interest. This method identified harvesting activity areas (step 1 at 300 m resolution) of over 12,004 km2 with high accuracy (omission and commission errors for both classes ≤0.05) and, within them, detected canopy gaps (step 2 at 10 m resolution) with a global accuracy of 0.89. Although some canopy gaps were subject to omission and commission errors (0.39 and 0.05, respectively), this method yielded better results than other available approaches. Compared to the UAV canopy gaps, this method detected most of the small gaps (≤ 500 m2), which represent 80% of all disturbed areas, whereas other available approaches missed at least 70% of these and consequently missed most of the disturbance events occurring in a selectively logged forest. Furthermore, the predictions were correlated with logging intensity, i.e., the number of trees and volume cut per hectare, which are two important criteria for assessing the sustainability of logging activities. This two-step two-scale method for short-term, monthly monitoring of logging disturbances and intensity has strong practical implications for forest administration and certification bodies in Central Africa.

Effects of Canopy Damage and Litterfall Input on CO2-Fixing Bacterial Communities

Extreme weather events often cause canopy disturbance and litter deposition. To study the CO2-fixing bacterial response to forest damage, we simulated the canopy damage caused by extreme weather with four different treatments: control (CN), canopy trimming + removal of branches and leaves debris (TR), canopy trimming + retaining of branches and leaves debris (TD), and undamaged + transplantation of branches and leaves debris (UD). We used the cbbL gene, which encodes ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO), for Miseq sequencing to analyze the dynamics of community composition of soil CO2-fixing bacteria for five consecutive years after canopy damage. Double treatments of canopy damage and litterfall inputs (TD) facilitate forest restoration better than single treatments (TR or UD). Most soil CO2-fixing bacteria are facultative autotrophic bacteria, and Nitrosospira, Streptomyces, and Saccharomonospora are the main carbon-fixing microorganisms, which have significant differences during the restoration of damaged forest canopy. The forest ecosystem restoration after canopy damage lasted 4–5 years. Rainfall and pH showed a significant negative correlation with most soil CO2-fixing bacteria communities. This study provides a theoretical basis for improving the carbon sequestration capacity of forest soil CO2-fixing bacteria after extreme weather and also provides guidance for forest ecosystem management.

Fire exclusion vs. a fire-free interval following repeated prescribed fire: Consequences for forest stand structure and species composition in an upland oak forest

In the Central Appalachian forest region, oaks (Quercus L.) and other disturbance-dependent tree species are experiencing widespread regeneration failure after decades of fire suppression. Managers use prescribed fire to decrease basal area with the aim of facilitating oak regeneration and shifting species composition toward pyrophytic species that dominate the overstory. Although fire may set the stage for improved oak regeneration, a fire-free period is necessary for oak seedlings to develop into fire-resistant stems. The effects of prescribed fire have been amply investigated across the region, but few studies have examined changes to stand structure and oak regeneration after cessation of burning. We followed the effects of three treatments, fire-excluded, 3x-burned, and 4x-burned, for 20 years. After a fire-free interval of approximately ten years, burned treatments had significantly lower total basal area and greater relative density of oaks in the subcanopy (10–20 cm DBH) than the fire-excluded treatment. The 4x-burned, which had the greatest reduction in basal area, also had greater relative density of midstory oaks (2–10 cm DBH), and higher stem density of oak regeneration (<2 cm DBH) than the fire-excluded. Despite these trends towards increased regeneration of oaks and other pyrophytes, red maple (Acer rubrum L.) stem density also increased in the midstory of burned treatments, suggesting a legacy effect of decades of fire suppression and the concomitant mesophication that has occurred. We also examined the effects of continued fire exclusion on the fire-excluded reference treatment. Additional evidence of mesophication on this treatment was demonstrated by increased overstory (≥20 cm DBH) and subcanopy red maple stem density as well as midstory (2–10 cm DBH) eastern white pine (Pinus strobus L.) stem density, a prominent mesophyte on these sites. This study supports the idea that repeated prescribed burning followed by a 10-year fire-free interval can alter stand structure and lead to improved oak regeneration by increasing canopy openness. However, the results were variable across burned treatments (3x vs. 4x) and the ingrowth of red maple and other mesophytes after only 10 years of fire exclusion will require additional control measures. Successful maintenance of oak-dominated forests on upland sites in the Central Appalachian hardwoods region may be contingent on a subsequent canopy disturbance that further increases light availability followed by a fire-free interval. Importantly, in the absence of burning or other disturbance, red maple and other mesophytic species will continue to increase in relative stem density.

Depth-dependent effects of ericoid mycorrhizal shrubs on soil carbon and nitrogen pools are accentuated under arbuscular mycorrhizal trees.

Plant mycorrhizal associations influence the accumulation and persistence of soil organic matter and could therefore shape ecosystem biogeochemical responses to global changes that are altering forest composition. For instance, arbuscular mycorrhizal (AM) tree dominance is increasing in temperate forests, and ericoid mycorrhizal (ErM) shrubs can respond positively to canopy disturbances. Yet how shifts in the co-occurrence of trees and shrubs with different mycorrhizal associations will affect soil organic matter pools remains largely unknown. We examine the effects of ErM shrubs on soil carbon and nitrogen stocks and indicators of microbial activity at different depths across gradients of AM versus ectomycorrhizal (EcM) tree dominance in three temperate forest sites. We find that ErM shrubs strongly modulate tree mycorrhizal dominance effects. In surface soils, ErM shrubs increase particulate organic matter accumulation and weaken the positive relationship between soil organic matter stocks and indicators of microbial activity. These effects are strongest under AM trees that lack fungal symbionts that can degrade organic matter. In subsurface soil organic matter pools, by contrast, tree mycorrhizal dominance effects are stronger than those of ErM shrubs. Ectomycorrhizal tree dominance has a negative influence on particulate and mineral-associated soil organic matter pools, and these effects are stronger for nitrogen than for carbon stocks. Our findings suggest that increasing co-occurrence of ErM shrubs and AM trees will enhance particulate organic matter accumulation in surface soils by suppressing microbial activity while having little influence on mineral-associated organic matter in subsurface soils. Our study highlights the importance of considering interactions between co-occurring plant mycorrhizal types, as well as their depth-dependent effects, for projecting changes in soil carbon and nitrogen stocks in response to compositional shifts in temperate forests driven by disturbances and global change.

Seedlings of shade-tolerant tree species are more vulnerable to chilling rain under a forest gap

The manner in which tree species differ in their responses to chilling rain in warm and species-rich (sub-)tropical forests is not well understood. Understanding this variation between species is essential for linking the responses of individual plants to chilling rain with ecological consequences at the forest community and ecosystem levels. We hypothesized that chilling rain can induce detrimental effects on leaf photochemical processes, and the negative impacts are more evident for shade-tolerant species than light-demanding species. This trade-off between species' tolerance to shade and chilling rain may depend on the light environment to which plants are exposed. To test these hypotheses, we conducted two sequential experiments with five subtropical tree species, measuring their leaf photochemical processes during the phases of a chilling rain event and assessing their resistance and recovery. We determined species shade tolerance by integrating their functional traits associated with resource acquisition in a shade environment. Our results showed that Fv/Fm, a measure of maximum quantum yield of photosystem II, was co-determined by the plant's exposure to light and chilling rain. Seedlings exposed to a gap before the chilling rain generally had lower Fv/Fm than those exposed to shade before the rain treatment. Chilling rain, relative to ambient rain, significantly reduced Fv/Fm during the cold and sunny phase. However, the effects of chilling rain were only evident for shade-tolerant species with gap-exposure history. The reduction in Fv/Fm induced by chilling rain led to higher resistance of Fv/Fm to chilling rain for light-demanding species than shade-tolerant species, leading to a trade-off between species' tolerances to shade and chilling rain for plants with gap-exposure history, thus supporting the hypotheses. This trade-off was primarily due to variation in functional traits related to light-use strategy, such as specific leaf area, leaf area ratio, leaf thickness, relative volume growth rate, and root-shoot ratio. As light availability and species' tolerance to shade are critical abiotic and biotic factors determining forest community succession, respectively, these findings may improve the scalability of the physiological responses of individual plants to chilling rain to the dynamics of forest communities. Our results suggest that frequent chilling rain events and canopy disturbances may favor light-demanding species and alter the species composition of subtropical forests.

Seedling drought responses governed by root traits, site-soil moisture regimes and overstory competition-facilitation

Periodic drought may interact with spatially patterned soil moisture regimes, canopy disturbance and species seedling traits to influence tree regeneration processes in northern temperate forests. In northern Michigan, USA, we assessed the effects of regional upland soil moisture regimes (six sites classified from dry to mesic) and tree canopy shading (% light) on planted-seedling dry mass, probability of mortality and functional traits for eight tree species varying in shade tolerance and natural distribution over soil moisture gradients (i.e., drought tolerance): Quercus velutina, Quercus alba, Quercus rubra, Prunus serotina, Acer rubrum, Acer saccharum, Fraxinus americana, and Betula alleghaniensis. The assessment period included droughts during the seedlings’ 2nd and 3rd growing seasons. High mortality (especially on drier-classified sites and in more open, high-light environments) and negative associations of soil water minima with mortality collectively suggest drought-driven mortality across sites. In contrast, canopy shade driven mortality occurred only on mesic sites at <10 % light. Overstory canopies facilitated seedling survival relative to more open environments, with survival maxima ranging from 3 to 20 % light among sites. Among species, probability of survival was 1) highest for Quercus species (∼0.70) and lowest for B. alleghaniensis (0.10), 2) positively and generally associated with drought tolerance, but not associated with shade tolerance, and 3) positively and strongly associated with species seed-size related differences in total dry mass, root dry mass and rooting depth. In turn, rooting depth (range 3–68 cm) was positively related to seedling water status, and water status to survival during drought. In summary, site soil moisture regimes, facilitation vs. competition via variation in canopy density, and size-related species traits drove patterns of young seedling mortality during an interval that included droughts. Results suggests current drought regimes likely have strong influence on canopy composition across the region’s spatially patterned dry to mesic soil gradients, and may have larger impacts if drought intensifies with climate change. Our results can aid development of planting strategies for assisted migration and other emerging needs, including planting 1) species more drought adapted than those dominating current overstories, 2) large stock with well-developed root systems, and 3) in microsites that maximize canopy facilitation.