Understorey Light Research Articles

Light quality and spatial variability influences on seedling regeneration in Hawaiian lowland wet forests

Abstract Tropical forest understories tend to be light‐limited. The red‐to‐far‐red ratio (R:FR) is a useful and reliable index of light quality and its spatial variability can influence competition between native and non‐native seedlings. While per cent light transmittance has been quantified in some Hawaiian lowland wet forests (HLWF), no information exists on how the spatial distribution of understorey light varies in relation to species invasion, or if patterns of seedling regeneration and light are linked. We measured the R:FR of light in the understorey to assess light quality in three HLWF forest types: native‐dominated, partially invaded and Psidium cattleyanum‐ (strawberry guava) dominated to quantify light quality in the understorey (0–50 cm height). We also identified relationships between light quality and native and non‐native seedling presence, diversity and abundance. Together, these data can help to determine the restoration potential of HLWF. Linear mixed‐effect modelling showed that native‐dominated forests had significantly greater R:FR than P. cattleyanum‐dominated forests, demonstrating a transformation in the light environment with increased invasion. Heterogeneity in R:FR varied more across sites than among forest types. In both native‐dominated and partially invaded forests, there were more native seedlings in the low‐quality R:FR (0.0–0.40) category and fewer in the medium‐ (0.41–0.70), and high‐quality (≥0.71) light categories than would be expected by chance, and there were no native seedlings in the P. cattleyanum‐dominated forests. Native‐dominated forests had greater species richness and abundance of native seedlings than the partially invaded forests, likely due to propagule availability. However, the spatial clustering of seedlings, the mismatch of native seedlings in light environments less suitable, and a considerable proportion of open high‐quality microsites, highlights that conditions are not optimal for native species in HLWF in the long term. Synthesis and applications. The native‐dominated and partially invaded forests still hold conservation value, despite variation among sites. Seedling additions could be targeted to different R:FR environments and at different spatial scales, but the lack of a strong relationship between R:FR and seedling number suggests that other factors besides light quality should be considered in seedling enrichment or other management activities.

Canopy structure and understorey light availability in Nothofagus and podocarp-broadleaf stands in a New Zealand forest

ABSTRACT The ability of some ectomycorrhizal fungi to access organic sources of nitrogen is known to influence decomposition, nitrogen mineralisation and soil carbon storage. The consequences for aboveground ecosystem structure and processes are less well-documented. We used the LAI-2200C Plant Canopy Analyzer to compare leaf area index and understorey light availability of ectomycorrhizal Nothofagus stands and arbuscular mycorrhizal podocarp-broadleaf stands growing on similar soils at a dry forest site in the Wairarapa district of New Zealand’s North Island. N. solandri was the sole overstorey tree in the three Nothofagus stands, and the ericaceous shrub Leucopogon fasciculatus was prominent in the understorey. The three podocarp-broadleaf stands were more diverse and more varied, the major contributors to basal area being Elaeocarpus dentatus, Hedycarya arborea, Cyathea dealbata, Carpodetus serratus and Dacrydium cupressinum. Leaf area index of Nothofagus stands averaged one unit lower (4.04) than that of neighbouring podocarp-broadleaf stands (5.11). Average light availability under Nothofagus stands (3.6–4.9%) was twice that beneath podocarp-broadleaf stands (1.6–2.1%). The lower leaf area index, greater light transmittance, and monodominance of Nothofagus stands probably reflects suppression of arbuscular mycorrhizal species by ectomycorrhizal short-circuiting of the nitrogen cycle. This study expands the evidence of positive feedback mechanisms stabilising ectomycorrhizal dominance in some forests.

Understorey light quality affects leaf pigments and leaf phenology in different plant functional types.

Forest understorey plants receive most sunlight in springtime before canopy closure, and in autumn following leaf‐fall. We hypothesised that plant species must adjust their phenological and photoprotective strategies in response to large changes in the spectral composition of the sunlight they receive. Here, we identified how plant species growing in northern deciduous and evergreen forest understoreys differ in their response to blue light and ultraviolet (UV) radiation according to their functional strategy. We installed filters in a forest understorey in southern Finland, to create the following treatments attenuating: UV radiation below 350 nm, all UV radiation (< 400 nm), all blue light and UV radiation (< 500 nm), and a transparent control. In eight species, representing different functional strategies, we assessed leaf optical properties, phenology, and epidermal flavonoid contents over two years. Blue light accelerated leaf senescence in all species measured in the understorey, apart from Quercus robur seedlings, whereas UV radiation only accelerated leaf senescence in Acer platanoides seedlings. More light‐demanding species accumulated flavonols in response to seasonal changes in light quality compared to shade‐tolerant and wintergreen species and were particularly responsive to blue light. Reduction of blue and UV radiation under shade reveals an important role for microclimatic effects on autumn phenology and leaf photoprotection. An extension of canopy cover under climate change, and its associated suppression of understorey blue light and UV radiation, may delay leaf senescence for understorey species with an autumn niche.

Parental UV-B radiation regulates the habitat selection of clonal Duchesnea indica in heterogeneous light environments.

Habitat selection behaviour is an effective strategy adopted by clonal plants in heterogeneous understorey light environments, and it is likely regulated by the parental environment's ultraviolet-B radiation levels (UV-B) due to the photomorphogenesis of UV-B and maternal effects. Here, parental ramets of Duchesnea indica were treated with two UV-B radiation levels [high (UV5 group) and low (UV10 group)], newborn offspring were grown under a heterogeneous light environment (ambient light vs shade habitat), and the growth and DNA methylation variations of parents and offspring were analysed. The results showed that parental UV-B affected not only the growth of the parent but also the offspring. The offspring of different UV-B-radiated parents showed different performances. Although these offspring all displayed a tendency to escape from light environments, such as entering shade habitats earlier, and allocating more biomass under shade (33.06% of control, 42.28% of UV5 and 72.73% of UV10), these were particularly obvious in offspring of the high UV-B parent. Improvements in epigenetic diversity (4.77 of control vs 4.83 of UV10) and total DNA methylation levels (25.94% of control vs 27.15% of UV10) and the inhibition of shade avoidance syndrome (denser growth with shorter stolons and internodes) were only observed in offspring of high UV-B parents. This difference was related to the eustress and stress effects of low and high UV-B, respectively. Overall, the behaviour of D. indica under heterogeneous light conditions was regulated by the parental UV-B exposure. Moreover, certain performance improvements helped offspring pre-regulate growth to cope with future environments and were probably associated with the effects of maternal DNA methylation variations in UV-B-radiated parents.

Applying the stress‐gradient hypothesis to curb the spread of invasive bamboo

AbstractThe stress‐gradient hypothesis (SGH) provides a conceptual framework for explaining how environmental context determines the nature of biotic interactions. It may be also useful for predicting geographic variability in the effect of management interventions on biological invasions. We aimed to test hypotheses consistent with the SGH to explain context dependency in bamboo invasion of secondary forests in Japan, and establish a predictive understanding of forest management impacts on invasion.We use a priori physiological knowledge of invasive giant bamboo,Phyllostachys bambusoides, to generate hypotheses consistent with the SGH. We modelled variation in giant bamboo occupancy within 810 secondary forest plots across the broad environmental gradients of Japan using a national vegetation database.Consistent with the SGH, we find that the effect of tree canopy cover on bamboo occupancy depends on interactions between solar radiation and mean annual temperature. In cool regions with high solar radiation—stressful conditions for bamboo—shade cast by dense canopies facilitates invasion. However, in warmer regions that are more benign, dense canopies tend to inhibit spread via competition for light, space and other resources.Synthesis and applications. We provide evidence that the stress‐gradient hypothesis can inform practical recommendations for invasive species control. We characterised geographic variability in the effect of forest thinning, a widespread management intervention used to enhance forest biodiversity, on the risk of bamboo spread into secondary forests in Japan. Thinning forest canopies to increase understorey light radiation should limit bamboo spread in cooler regions, while tree planting to increase canopy shade should limit bamboo spread in warmer regions.

Forest structure drives changes in light heterogeneity during tropical secondary forest succession.

Light is a key resource for tree performance and hence, tree species partition spatial and temporal gradients in light availability. Although light distribution drives tree performance and species replacement during secondary forest succession, we yet lack understanding how light distribution changes with tropical forest development.This study aims to evaluate how changes in forest structure lead to changes in vertical and horizontal light heterogeneity during tropical forest succession.We described successional patterns in light using a chronosequence approach in which we compared 14 Mexican secondary forest stands that differ in age (8–32 years) since agricultural abandonment. For each stand, we measured vertical light profiles in 16 grid cells, and structural parameters (diameter at breast height, height and crown dimensions) for each tree.During succession, we found a rapid increase in stand size (basal area, crown area and length) and stand differentiation (i.e. a gradual leaf distribution along the forest profile), which leads to fast changes in light conditions and more light heterogeneity. The inflection points of the vertical light gradient (i.e. the absolute height at which 50% relative light intensity is attained) rapidly moved towards higher heights in the first 20 years, indicating that larger amounts of light are intercepted by canopy trees. Light attenuation rate (i.e. the rate of light extinction) decreased during succession due to slower accumulation of the crown area with height. Understorey light intensity and heterogeneity slightly decreased during succession because of an increase in crown size and a decrease in lateral gap frequency. Understorey relative light intensity was 1.56% at 32 years after abandonment.Synthesis. During succession, light conditions changed linearly, which should lead to a continuous and constant replacement of species. Especially in later successional stages, stronger vertical light gradients can limit the regeneration of light‐demanding pioneer species and increase the proportion of shade‐tolerant late‐successional species under the canopy. These changes in light conditions were largely driven by the successional changes in forest structure, as basal area strongly determined the height where most light is absorbed, whereas crown area, and to a lesser extent crown length, determined light distribution.

Light can modify density‐dependent seedling mortality in a temperate forest

AbstractQuestionsThere is growing evidence on how density‐dependent drivers of plant performance are influenced by habitat heterogeneity such as variation in understorey light availability. How these processes operate in temperate forests still remains an open question, however. We examined the interactive effects of density dependence and light availability on woody seedling mortality across multiple species, two size stages and growth forms.LocationAlluvial old‐growth temperate forest, southeastern Czech Republic.MethodsWe mapped seedlings of woody species (≥10 cm tall) and recorded their mortality over a two‐year period on 21 square seedling plots (25 m2) within one large square plot with all trees mapped (≥1 cm diameter at breast height). Seedling mortality was studied using a combined approach of generalised linear mixed models and spatial point pattern analyses.ResultsDensity‐dependent effects of tree and seedling neighbours influenced seedling mortality at different spatial scales, depending on their size class, growth form and by taxon. Conspecific inhibition raised the mortality of seedlings other than Acer campestre at low light levels, indicating that host‐specific enemies and/or intraspecific competition may be important in unfavourable habitats. While small seedlings experienced conspecific inhibition at low light levels, tall seedlings were additively affected by low‐light conditions and competition with trees. Both conspecific and heterospecific neighbours tended to affect tree seedlings at low light levels, whereas shrub seedlings experienced light‐ and density‐independent mortality. Some negative interspecific interactions indicated ongoing expansion of species like Acer campestre in currently flood‐free habitats.ConclusionsHow density‐dependent effects translate into demographic outcomes for woody seedling populations may be largely determined by variation in understorey light availability. Given that seedlings differ in size, growth form and taxa, we also demonstrate that both seedling and tree neighbours may contribute to the strength and spatial variation in density‐dependent effects at seedling neighbourhood scales.

Transforming even-aged coniferous stands to multi-aged stands: an opportunity to increase tree species diversity?

Abstract Transforming even-aged coniferous stands into multi-aged ones is attracting growing interest in Europe. However, applying this silvicultural treatment, maintaining a continuous cover and relying on natural regeneration require a deep understanding of the factors driving interspecific competition in the understorey. In particular, knowledge of species-specific response to different light conditions is needed to plan silvicultural treatments and forecast long-term stand composition. In this context, we assessed regeneration (±10–400 cm in height) and light conditions (±1–40 per cent of transmittance) in nine coniferous stands with ranging stand age (±20–120 years) and species composition (Norway spruce, Douglas fir, larch, silver fir and western hemlock) in Belgium. We then modelled interspecific differences in regeneration height growth to forecast the outcome of interspecific competition in different light conditions. Controlling understorey light seems an efficient way to control the interspecific competition, but with some limits, and taking into account sapling size. Maintaining low light conditions (transmittance &amp;lt; 15 per cent) probably reduces interspecific competition as it allows small saplings (height &amp;lt; 100 cm) of most species to grow at a comparable rate. Maintaining higher light conditions might allow a few species to rapidly overgrow the others. Species ranking in height growth changed across the studied light range only between spruce and larch, suggesting that the competition between these two species can be driven through the control of understorey light. On the other hand, controlling canopy openness was found to be insufficient, for example, to promote an advanced regeneration (height ≥ 200 cm) of fir over advanced regeneration of spruce, to promote any species over western hemlock or to promote Douglas fir. Western hemlock, a very shade-tolerant species, was found to grow three times faster than the other species in all the observed conditions (PACL = 5–20 per cent). Douglas fir saplings showed weak growth and marked defoliation, which we hypothetically relate to the recent outbreak of Contarinia pseudotsugae in Western Europe.

Seasonal Patterns in Spectral Irradiance and Leaf UV-A Absorbance Under Forest Canopies.

Plants commonly respond to UV radiation through the accumulation of flavonoids and related phenolic compounds which potentially ameliorate UV-damage to crucial internal structures. However, the seasonal dynamics of leaf flavonoids corresponding to epidermal UV absorbance is highly variable in nature, and it remains uncertain how environmental factors combine to govern flavonoid accumulation and degradation. We studied leaf UV-A absorbance of species composing the understorey plant community throughout two growing seasons under five adjacent tree canopies in southern Finland. We compared the relationship between leaf flavonol index (Iflav—repeatedly measured with an optical leaf clip Dualex) and measured spectral irradiance, understorey and canopy phenology, air temperature and snowpack variables, whole leaf flavonoid extracts, and leaf age. Strong seasonal patterns and stand-related differences were apparent in Iflav of both understorey plant communities and individual species, including divergent trends in Iflav during spring and autumn. Comparing the heterogeneity of the understorey light environment and its spectral composition in looking for potential drivers of seasonal changes in Iflav, we found that unweighted UV-A irradiance, or the effective UV dose calculated according to the biological spectral weighting function (BSWF) for plant growth (PG action spectrum), in understorey shade had a strong relationship with Iflav. Furthermore, understorey species seemed to adjust Iflav to low background diffuse irradiance rather than infrequent high direct-beam irradiance in sunflecks during summer, since leaves produced during or after canopy closure had low Iflav. In conclusion, we found the level of epidermal flavonoids in forest understorey species to be plastic, adjusting to climatic conditions, and differing according to species' leaf retention strategy and new leaf production, all of which contribute to the seasonal trends in leaf flavonoids found within forest stands.

Determinants of leaf area index and understorey light availability in New Zealand old‐growth forests

AbstractAimGlobal studies show strong climatic control of leaf area index (LAI) and related ecosystem properties. However, alternative states stabilized by positive feedbacks have the potential to produce appreciable differences in LAI in a given environment. On certain sites in New Zealand, ectomycorrhizal (EM) southern beech forest (Nothofagaceae) appear to form a stable alternative to more species‐rich conifer‐broadleaved forest dominated by arbuscular mycorrhizal (AM) trees. We asked which climate variables best predict LAI of New Zealand's remaining old‐growth forests, and if stands dominated by Nothofagaceae have lower LAI and better‐lit understories than conifer‐broadleaved forests, when climate and other site factors are held constant.LocationNew Zealand.TaxonSeventy‐two tree species belonging to 37 angiosperm, conifer and pteridophyte families, the most frequent occurrences being Cunoniaceae, Cyatheaceae, Dicksoniaceae, Lauraceae, Nothofagaceae, Podocarpaceae and Violaceae.MethodsWe quantified overstorey composition of 45 old‐growth stands, and used the LAI‐2200C Plant Canopy Analyzer to measure LAI and understorey light. We used generalized linear mixed models to determine how these variables were influenced by environment and by dominance of Nothofagaceae.ResultsLAI ranged from 3.5 to 6.9, and increased linearly with mean annual temperature (MAT); percentage light transmission to the understorey spanned an order of magnitude and decreased exponentially with increasing MAT. However, at a given MAT, LAI of stands dominated by Nothofagaceae averaged 0.8–0.9 units lower than conifer‐broadleaf stands, resulting in twofold higher understorey light levels in the former. Soil water deficits also significantly depressed LAI, and increased understorey light; atmospheric water deficits had the opposite effect.Main conclusionsTemperature is the best climatic predictor of LAI and understorey light in New Zealand's remaining old‐growth forests. However, the lower LAI and canopy light interception in southern beech stands presumably reflects the light‐demanding nature of Nothofagaceae, and their apparent ability to inhibit invasion by more shade‐tolerant AM trees with heavier crowns. This inhibitory effect may reflect reduced N supply to AM plants because of suppression of N mineralization by EM fungi, as observed in some EM systems elsewhere.

Early response of late-successional species to nurse shrub manipulations in degraded high country, New Zealand

In post-disturbance grasslands in New Zealand shrubs can serve as nurse plants. However, while many canopy trees may establish under a shrub canopy, shade may also restrict seedling establishment. We assessed the potential of shrubs and canopy manipulations in restoring mature forest species in a stressful environment. Mountain beech (Fuscospora cliffortioides, Nothofagaceae) seedlings were planted in October 2015 and December 2016 in post-disturbance grasslands. Growth and survival were monitored over three growing seasons in five canopy treatments: open grassland, mānuka (Leptospermum scoparium, Myrtaceae) shrubland with intact canopy, mixed-species shrubland with intact canopy, artificial gaps in mānuka shrubland; and the grassland edge of the mixed-species shrubland. Understorey light transmission and canopy openness were quantified during the first growing season using hemispherical photos. Canopy manipulations caused significant differences in the light availability and canopy openness among treatments. We observed that seedlings performed better in the mānuka shrub with gap and mixed-species shrub canopies (intact and edge treatments), perhaps due to moderate canopy openness in these treatments compared to open grasslands and intact mānuka canopy. Growth and survival were positively associated with moderate log(light) levels, which correspond to light conditions in manuka with gaps and intact mixed-species shrubs and are likely to represent the net effects of these treatments. The results demonstrated that early-successional shrubs could be managed as nurse plants to control the effect of over-shading and facilitate the growth and survival of canopy species in a stressful environment.

Drivers of above‐ground understorey biomass and nutrient stocks in temperate deciduous forests

AbstractThe understorey in temperate forests can play an important functional role, depending on its biomass and functional characteristics. While it is known that local soil and stand characteristics largely determine the biomass of the understorey, less is known about the role of global change. Global change can directly affect understorey biomass, but also indirectly by modifying the overstorey, local resource availability and growing conditions at the forest floor.In this observational study across Europe, we aim at disentangling the impact of global‐change drivers on understorey biomass and nutrient stocks, from the impact of overstorey characteristics and local site conditions. Using piecewise structural equation modelling, we determine the main drivers of understorey biomass and nutrient stocks in these forests and examine potential direct and indirect effects of global‐change drivers.Tree cover, tree litter quality and differences in former land use were the main drivers of understorey biomass and nutrient stocks, via their influence on understorey light and nitrogen availability and soil acidity. Other global‐change drivers, including climate and nitrogen deposition, had similar indirect effects, but these were either weak or only affecting nutrient concentrations, not stocks.Synthesis.We found that direct effects of global‐change drivers on understorey biomass and nutrient stocks were absent. The indirect effects of global change, through influencing resource availability and growing conditions at the forest floor, were found to be less important than the effects of overstorey cover and composition. These results suggest that understorey biomass and nutrient stocks might respond less to global change in the presence of a dense overstorey, highlighting the buffering role of the overstorey in temperate forests.

Cascading effects of canopy mortality drive long‐term changes in understorey diversity in temperate old‐growth forests of Europe

AbstractQuestionsWe investigated the influence of protracted mortality of a dominant canopy tree (Abies alba) on long‐term understorey dynamics. We ask (a) how tree regeneration and understorey species diversity and composition changed over 32 years; and (b) whether the observed changes were mainly driven by mortality of A. alba.LocationThree old‐growth forest reserves dominated by A. alba and Fagus sylvatica in the Dinaric Mountains of Slovenia.MethodTree layer and understorey regeneration and herbs were surveyed in 147 plots across the three forest reserves in 1983 and 2015. Soils were also sampled in 2015. The study period coincides with a protracted period of increased A. alba mortality in the canopy layer associated with anthropogenic emissions.ResultsBetween 1983 and 2015, the decline in canopy layer A. alba caused a recruitment pulse of F. sylvatica regeneration to the subcanopy tree layer across the three reserves. These changes were accompanied by a significant decline in plot level herb species richness. A model‐based analysis of beta‐diversity revealed significant community convergence during the study period, mainly caused by the loss of rare species. Ellenberg values indicate that these changes were mainly driven by loss of understorey light, while an increase in soil pH may have played a role also.ConclusionsThis observational study suggests that the long‐term decline of A. alba resulted in a cascade of processes — widespread F. sylvatica recruitment that impeded penetration of light to the forest floor, and possibly a change in soil conditions due to the decline of coniferous litter. These changes caused a significant loss of herb diversity and homogenization of the understorey community across the three sites. This study sheds light on the potential cascading consequences triggered by episodes of increased tree mortality resulting from global‐change‐type drivers.

Drivers of species richness and community integrity of small forest patches in an agricultural landscape

AbstractQuestionsLong‐term fragmentation and land use in Europe have created a landscape pattern where small forest patches are embedded among agricultural landscapes. These small forest patches can be one of the few habitats left to maintain the species richness and ecosystem functions within intensively managed agroecosystems. We ask, which factors determine vascular plant species richness, community composition and forest community integrity in small forest patches in an agricultural landscape?LocationNE Estonia.MethodsWe combine island biogeographic theory (patch area and isolation) with the properties of the surrounding landscape and local environmental conditions within a patch to study the drivers of species richness and community integrity.ResultsPatch area together with local environmental factors (understorey light conditions and soil reaction) determined both species richness and community integrity. Total species richness and forest generalists were related to patch area alone, whereas forest specialists were additionally dependent on patch light conditions. Species richness of grassland specialists in the forest patches increased with the amount of natural habitat in the surrounding landscape, while the presence of synanthropic species was positively related to soil reaction. Forest community integrity was higher in larger, more shaded patches with low soil reaction, which together defined suitable conditions for forest communities and hindered the intrusion of species from other habitats.ConclusionsUnder a suitable set of conditions, encompassing both favourable landscape and local environmental conditions, even small forest patches can provide habitat for both forest and grassland communities in agricultural landscapes. Comprehensive approaches, considering species composition, environment and landscape conditions simultaneously, are needed for making reliable predictions of biodiversity patterns.

Assessing scale-wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance.

Forest canopies create dynamic light environments in their understorey, where spectral composition changes among patterns of shade and sunflecks, and through the seasons with canopy phenology and sun angle. Plants use spectral composition as a cue to adjust their growth strategy for optimal resource use. Quantifying the ever‐changing nature of the understorey light environment is technically challenging with respect to data collection. Thus, to capture the simultaneous variation occurring in multiple regions of the solar spectrum, we recorded spectral irradiance from forest understoreys over the wavelength range 300–800 nm using an array spectroradiometer. It is also methodologically challenging to analyze solar spectra because of their multi‐scale nature and multivariate lay‐out. To compare spectra, we therefore used a novel method termed thick pen transform (TPT), which is simple and visually interpretable. This enabled us to show that sunlight position in the forest understorey (i.e., shade, semi‐shade, or sunfleck) was the most important factor in determining shape similarity of spectral irradiance. Likewise, the contributions of stand identity and time of year could be distinguished. Spectra from sunflecks were consistently the most similar, irrespective of differences in global irradiance. On average, the degree of cross‐dependence increased with increasing scale, sometimes shifting from negative (dissimilar) to positive (similar) values. We conclude that the interplay of sunlight position, stand identity, and date cannot be ignored when quantifying and comparing spectral composition in forest understoreys. Technological advances mean that array spectroradiometers, which can record spectra contiguously over very short time intervals, are being widely adopted, not only to measure irradiance under pollution, clouds, atmospheric changes, and in biological systems, but also spectral changes at small scales in the photonics industry. We consider that TPT is an applicable method for spectral analysis in any field and can be a useful tool to analyze large datasets in general.

Spectral-temporal modelling of bamboo-dominated forest succession in the Atlantic Forest of Southern Brazil

With access to collections of continuous satellite imagery over a 40-year period, spectral-temporal patterns extracted from multi-temporal imagery offer a potential new tool to model mechanisms of forest succession and monitor changes in forested landscapes. Specifically, spectral-temporal trajectories associated with successional forest change occurring over prolonged periods of time may enhance periodic ‘snapshot’ monitoring methods, especially for species that exhibit complex and non-linear dynamics. In this paper, Landsat time-series are used to examine the spectral-temporal signatures of bamboo-dominated forest succession occurring within the critically threatened Araucaria Forest, a pine-dominated subtype of the Atlantic Forest in southern Brazil. Alteration of canopy structure through ongoing anthropogenic disturbance has increased understorey light climate and given opportunity for native invasive bamboos to flourish, resulting in drastic reduction of tree regeneration and loss of biodiversity. We aimed to evaluate how spectral-temporal signatures could be used to (1) characterize stages of bamboo-dominated forest succession, (2) identify synchrony of bamboo lifecycle dynamics and (3) classify regions of bamboo-dominated forest. Changepoint analysis was performed using an extracted sample spectral-temporal signature and trajectories were fit to the resulting segments using linear regression. Based on slope values of the fitted segments, a novel description incorporating temporal information of bamboo-dominated forest succession was developed which identified four broad phases: pioneer predominance, mature bamboo, dieback and pioneer regeneration. To determine the spatial and temporal synchrony of bamboo-dominated forest succession, a hybrid model was developed by combining the modelled segments and compared to a 32-year Landsat time-series of vegetation indices by calculating root-mean square error between each pixel in the study area. The hybrid model proficiently classified regions of bamboo-dominance, achieving between 77% and 90% accuracy, which also indicated lifecycle synchrony of bamboo populations within the study area. To further assess the performance of the hybrid model, a time-weighted dynamic time warping model approach was used to determine synchrony and classify regions of bamboo. The time-weighted dynamic time warping classifier had lower overall accuracy (68%–82%), but is still considered a useful tool for automated classification purposes that take advantage of multi-temporal imagery. To compare classification performance between ‘snapshot’ and multi-temporal imagery classifiers, a maximum-likelihood classification was performed, which attained lower overall accuracies than the hybrid model (75%–84%). Overall, the use of spectral-temporal signatures offers a novel and effective approach to both describing and modelling bamboo-dominated forest succession (and forest successional processes more generally) on a landscape-scale.

Acclimation of leaf traits in seasonal light environments: Are non‐native species more plastic?

Abstract In temperate deciduous forests, understorey light environments vary dramatically throughout spring, summer and autumn due to tree canopy leaf display. This variability in light level is a physiological challenge for understorey species that produce sun‐adapted leaves in the spring before being shaded by the tree canopy. Similarly, some understorey species display leaves late into autumn after the tree canopy senesces. Many species in North American deciduous forests with extended leaf display are not native to North America. Since many non‐native species have been shown to have greater plasticity than natives, we hypothesized that leaves of non‐native species may be more plastic with respect to seasonal light changes than natives, and that this plasticity may allow them to assimilate more carbon in the same environment. We measured leaf traits and photosynthetic capacity of 17 native and 13 non‐native understorey shrub species in a common garden in Syracuse, New York, during spring, summer and autumn. We tested for the contribution of seasonal mean and variance (plasticity) of leaf traits to a species’ average photosynthetic rate and total leaf production. We also analysed the extent to which leaf adjustments depended on whether plants continued to produce new leaves over the growing season. Leaf traits of both native and non‐native species varied seasonally, but plasticity varied in extent and contribution to overall carbon gain. Non‐native species had the highest seasonal plasticity, but, contrary to our hypothesis, such plasticity did not contribute to their overall carbon gain. However, leaf trait plasticity was adaptive for native species that continued to produce leaves throughout the year, primarily due to increases in quantum efficiency and electron transport rate in leaves produced mid‐year compared to leaves produced early in the year. Synthesis. Despite large adjustments in leaf traits across seasonal light environments for both native species and non‐native species, we found little evidence that leaf‐level plasticity drives non‐native invasion or contributes strongly to annual carbon gain or productivity in understorey species. Instead, differences in mean leaf traits across seasons are sufficient to explain carbon gain advantages of non‐native woody species in deciduous forests.

A comparison of ground-based methods for estimating canopy closure for use in phenology research

Climate change is influencing tree phenology, causing earlier and more prolonged canopy closure in temperate forests. Canopy closure is closely associated with understorey light, so shifts in its timing have wide-reaching consequences for ecological processes in the understorey. Widespread monitoring of forest canopies through time is needed to understand changes in light availability during spring in particular. Canopy openness, derived from hemispherical photography, has frequently been used as a proxy for understorey light. However, hemispherical photography is relatively resource intensive, so we tested a range of inexpensive alternatives for monitoring variability in canopy closure (visual estimation, canopy scope, smartphone photography, smartphone photography with fisheye attachment; and image analysis with specialist hemispherical photography software or with simpler, open access image analysis software). Smartphone photography with an inexpensive fisheye lens attachment proved the most reliable estimator of canopy closure. We found no significant difference in canopy estimations from three widely-owned smartphone models with differing resolutions and fields of view, and no significant effect of camera operator on the results. ImageJ, a free image analysis software, detected canopy variability in a similar way to HemiView specialist hemispherical photography software. We recommend a combination of smartphone photography with fisheye attachment and analysis with ImageJ for identifying changes in the timing of canopy closure (but not for estimating absolute canopy closure). We discuss how large-scale citizen science using this approach could generate meaningful and comparative data on the timings of canopy closure in different forests, year-to-year.

Long‐term responses of canopy–understorey interactions to disturbance severity in primary Picea abies forests

AbstractQuestionsHow do canopy–understorey interactions respond to variation in disturbance severity over extended periods of time? For forests with different disturbance histories, do light availability and understorey cohort densities converge towards a common old‐growth structure, or do historical legacies influence populations indefinitely?LocationsRemnants of primary spruce (Picea abies (L.) Karst.) forests throughout Germany, Slovakia, Ukraine and Romania.MethodsA disturbance history of &gt;200 yr was reconstructed from 11 278 tree cores collected from forest plots (n = 520). Understorey tree densities of two size classes and hemispherical photo‐based light availabilities were inventoried and modelled as functions of the severity of the main disturbance and time since the event.ResultsVariation in understorey tree densities had a hump‐shaped distribution through time. Stem densities were approximately static in the least disturbed sites, and declined in relation to disturbance severity over approx. 100 yr. Similar to patterns of stem densities, initially high understorey light availability also reached a minimum at 100 yr, which indicated crown closure. Following this, light availability and stem densities both increased as stands transitioned from stem exclusion to understorey re‐initiation. The effect of disturbance severity on understorey densities and patchiness in light availability persisted for &gt;200 yr.ConclusionsLong‐term trends in canopy–understorey interactions validate current conceptual models of forest development. Furthermore, we empirically validate that these conceptual models generalized over gradients in disturbance severity. Higher disturbance sites exhibited a more even‐aged character with more pronounced periods of stem exclusion, canopy closure and understorey re‐initiation; forests with low‐severity disturbance histories yielded a more stationary uneven‐aged structure. The model identified the extent of variation in disturbance severity within which these P. abies forests are able to regenerate and retain their monospecific character, which is increasingly relevant as disturbance regimes continue to shift under global climate change.

Lianas reduce community‐level canopy tree reproduction in a Panamanian forest

AbstractLianas are a key component of tropical forests, where they compete intensely with trees, reducing tree recruitment, growth and survival. One of the most important potential outcomes of liana competition is the reduction of tree reproduction; however, no previous study has experimentally determined the effects of lianas on tree reproduction beyond a single tree species.We used a large‐scale liana removal experiment to quantify the effect of lianas on community‐level canopy and understorey tree and palm reproduction. In 2011, we removed lianas from eight 6,400‐m2plots (eight plots served as controls) and surveyed understorey tree reproduction in 2012, canopy tree and palm reproduction in 2013, and a second census of all plants in 2016.We found that lianas significantly reduced canopy tree community flowering and fruiting after liana removal. Two years after liana removal, the number of canopy trees with fruits was 173% higher, fruiting individuals had 50% more of their canopy covered by fruits and the number of tree species with fruits was 169% higher than in control plots where lianas were present. Five years after liana removal, the number of canopy trees with fruits was 150% higher, fruiting individuals had 31% more of their canopy covered by fruits and the number of tree species with fruits was 109% higher than in unmanipulated control plots.Liana removal had only a slight positive effect on palms and on understorey tree flower and fruit production, even though understorey light levels had increased 20% following liana cutting.Synthesis. Our findings provide the first experimental demonstration that competition from lianas significantly reduces community‐level canopy tree reproduction. Reduced reproduction increases canopy tree seed and dispersal limitations, and may interfere with deterministic mechanisms thought to maintain tropical canopy tree species diversity, as well as reduce food availability to many animal species. Because lianas are increasing in abundance in many neotropical forests, the effects of lianas on tree reproduction will likely increase, and if the effects of lianas on tree reproduction vary with tree species identity, lianas ultimately could have a destabilizing effect on both tree and animal population dynamics.