Understorey Plant Communities Research Articles

Ecological indicator values of understorey plants perform poorly to infer forest microclimate temperature

AbstractQuestionEcological indicator values (EIVs) reflect species‘ optimal conditions on an environmental gradient, such as temperature. Averaged over a community, they are used to quantify thermophilization stemming from climate change, i.e. the reshuffling of communities toward more warm‐adapted species. In forests, understorey plant communities do not keep up with global warming and accumulate a climatic debt. Although the causes are still debated, this thermal lag may be partly explained by forest microclimate buffering. For the first time, we test whether community means of EIVs are able to capture microclimate (here, under forest canopies) temperature across, or also within forests.Location157 forest plots across three French deciduous forests covering a large macroclimatic gradient.MethodsTo assess whether EIVs can be used to infer the mean and range of microclimate temperature in forests, we measured understorey air temperature for ca. 1 year (10 months) with sensors located 1 m above the ground. We surveyed bryophytes and vascular plants within 400‐m2 plots, and computed floristic temperature from ordinal‐scale EIVs (Ellenberg, Julve) and degree‐scale EIVs (ClimPlant, Bryophytes of Europe Traits) for both temperature and continentality, i.e. temperature annual range. Finally, we fitted linear models to assess whether EIVs could explain the mean and range of microclimate temperature in forests.ResultsVascular plant and bryophyte communities successfully reflected differences in mean annual temperatures across forests but largely failed to do so for microclimate variation within forests. Bryophytes did not perform better than vascular plants to infer microclimate conditions. The annual range of microclimate temperatures was poorly associated with ordinal‐scale EIVs for continentality but was positively correlated with degree‐scale EIVs for annual range within lowland forests, especially for vascular plant communities.ConclusionOverall, the capabilities of EIVs to infer microclimate was inconsistent. Refined EIVs for temperature are needed to capture forest microclimates experienced by understorey species.

Nutrient‐demanding and thermophilous plants dominate urban forest‐edge vegetation across temperate Europe

AbstractQuestionsForests are highly fragmented across the globe. For urban forests in particular, fragmentation increases the exposure to local warming caused by the urban heat island (UHI) effect. We here aim to quantify edge effects on herbaceous understorey vegetation in urban forests, and test whether these effects interact with forest structural complexity.LocationWe set up a pan‐European study at the continental scale including six urban forests in Zurich, Paris, Katowice, Brussels, Bremen, and Stockholm.MethodsWe recorded understorey plant communities from the edge towards the interior of urban forests. Within each urban forest, we studied edge‐to‐interior gradients in paired stands with differing forest structural complexity. Community composition was analysed based on species specialism, life form, light, nutrient, acidity and disturbance indicator values and species' thermal niches.ResultsWe found that herbaceous communities at urban forest edges supported more generalists and forbs but fewer ferns than in forests' interiors. A buffered summer microclimate proved crucial for the presence of fern species. The edge communities contained more thermophilous, disturbance‐tolerant, nutrient‐demanding and basiphilous plant species, a pattern strongly confirmed by corresponding edge‐to‐interior gradients in microclimate, soil and light conditions in the understorey. Additionally, plots with a lower canopy cover and higher light availability supported higher numbers of both generalists and forest specialists. Even though no significant interactions were found between the edge distance and forest structural complexity, opposing additive effects indicated that a dense canopy can be used to buffer negative edge effects.ConclusionThe urban environment poses a multifaceted filter on understorey plant communities which contributes to significant differences in community composition between urban forest edges and interiors. For urban biodiversity conservation and the buffering of edge effects, it will be key to maintain dense canopies near urban forest edges.

The presence of native aromatic species in understorey plant communities predicts reduced infestation by olive fruit flies in olive orchards on Lesvos Island

AbstractThe olive fruit fly (Bactrocera oleae) (Rossi) is the most economically important insect pest in many olive cultivation areas worldwide. However, levels of infestation by this pest show remarkable variability, even among orchards in close proximity to one another. While the factors driving this variability remain uncertain, there is evidence that the presence of native plants species can influence pest populations in agro‐ecosystems via various mechanisms, suggesting that variation in the composition of understorey plant communities might plausibly play a role. In this study, we first used field trapping and plant community surveys to explore the effect of the native understorey plants Cistus creticus (Cistaceae), Lavandula stoechas (Lamiaceae), Origanum onites (Lamiaceae) and Sarcopoterium spinosum (Rosaceae) on the abundance of B. oleae across 40 olive orchards on Lesvos Island (Greece). These four plant species are widely spread in many olive orchards. Our results revealed a relationship between the plant community and the intensity of flies captured in McPhail traps, and identified the plant species C. creticus and S. spinosum whose presence was negatively correlated with fly abundance (estimate = −0.047, −0.091, respectively). The subsequent laboratory behavioural experiments revealed that exposure to olfactory cues from C. creticus and S. spinosum reduced fly attraction to odours from the olive host (Olea europaea) by 10%. These results thus reveal a relationship between the infestation levels by B. oleae and the composition of the plant community in olive orchards on Lesvos and suggest a plausible mechanism via which olfactory cues from native aromatic species might influence fly recruitment. More generally, these findings highlight the potentially important role of understorey communities in mediating resistance to B. oleae and other agricultural pests.

High landscape‐scale forest cover favours cold‐adapted plant communities in agriculture–forest mosaics

AbstractAimThe ongoing climate warming is expected to reshuffle understorey plant community composition by increasing the occurrence of warm‐adapted species at the expense of cold‐adapted species. This process has been evidenced before by a warming community temperature index (CTI) over time. However, data indicate that the local tree canopy can partly explain an observed lag between understorey plant CTI and climate warming rates, though landscape‐scale forest cover effects have not yet been investigated. Here, we test the hypothesis that the amount of forest cover in the landscape lowers local CTI.LocationTemperate forests in France.Time period2005–2019.Major taxa studiedForest vascular plants.MethodsWe compared 2,012 pairs of neighbouring French forest inventory plots with contrasting percentages of forest cover within a 1‐km radius area (landscape forest cover). We computed the difference in the CTI of the understorey communities for each pair and tested the contributions of the landscape‐scale forest cover, local canopy cover, and soil conditions to the differences in CTI.ResultsPlots located in highly forested areas (> 80% in the 1‐km area) had an average CTI 0.26 °C lower (0.81 °C SD) than plots in sparsely forested areas (< 30% in the 1‐km area). Fifty percent of this difference was explained by landscape‐scale forest cover. Bioindicated soil conditions such as pH and available nutrients, which correlated with cold‐adapted species preferences, explained the remaining 50%.Main conclusionsHighly forested landscapes allow cold‐adapted species to survive in given macroclimatic conditions. These landscapes meet cold‐adapted species’ soil requirements and may cool the regional climate. Further microclimatic studies are needed to confirm the cooling capacity of landscape‐scale forest cover.

Plantation understorey legume functional groups enhance soil organic carbon sequestration by promoting species richness

AbstractDespite evidence showing that plant diversity or tree mixtures increase carbon storage in experimental grasslands or mixed forests, the effects of understorey plant communities on changes in soil organic carbon (SOC) storage in planted forests remain unclear, especially for monoculture plantations. Therefore, based on field observations in monoculture plantations on the Loess Plateau in China, we investigated the direct effects of understorey species richness (USR) and plantation age on SOC storage, and identified the indirect effects of understorey legume species richness (ULSR) and soil moisture (SM) mediated by USR. Results indicated that plantation age and USR significantly increased SOC sequestration with almost equal effects. The changes in the relationship between USR and SOC storage varied along the water availability gradient, from significantly positive (in plots with SM ≤11.14%) to insignificant (in plots with SM >11.14%). We observed that ULSR could increase SOC sequestration by promoting USR, while SM had negative and positive effects on ULSR and USR, respectively. Our findings can provide insights into plantation management in semiarid regions: Thinning mature plantations and introducing understorey legumes can increase SOC sequestration and maintain water resource sustainability.

Effect of forest structure and management on the functional diversity and composition of understorey plant communities

AbstractQuestionsDo forest structural parameters related to stand heterogeneity enhance functional diversity (FD) of understorey plant communities? Do FD and functional composition of understorey plant communities vary between high‐forest (HF) and old coppices‐with‐standard (oldCWS) management types? Are HF stands characterized by a higher FD than oldCWS?LocationSubmediterranean beech forests of Montagne della Duchessa Reserve (central Italy).MethodsWe sampled 57 (20 m × 20 m) forest plots, 29 oldCWS and 28 HF stands, where we recorded plant species cover abundance. We used Leaf–Height–Seed and clonal traits, and run multiple separate linear models to quantify the effect of forest structural parameters and management types on FD and functional composition of understorey plant communities.ResultsWe found that increasing lying deadwood enhanced FD of specific leaf area (SLA) through micro‐environmental heterogeneity of soil fertility regardless of management type. We also found that increased horizontal stratification filtered the range of plant sizes, probably through light reduction. HF management enhanced FD of SLA and clonal traits through micro‐environmental heterogeneity, while reducing the FD of plant size and seed mass. HF tended to select plant communities characterized by high resource acquisition strategies but lower persistence between ramets, possibly as an effect of more mature forest conditions.ConclusionOur study suggests that understorey plant community diversity and composition change in response to forest structure and management. Combining Leaf–Height–Seed with clonal traits offers a promising framework for understanding and predicting plant response to management practices.

Stronger legacy effects of cropland than of meadows or pastures on soil conditions and plant communities in French mountain forests

AbstractQuestionDifferences in understorey vegetation between ancient and recent forests have been thoroughly explored; however, few studies have investigated the legacies of different former land uses in recent forests. Indeed, due to more intense agricultural practices (tillage and fertilisation), legacy effects are expected to be stronger in former cropland or meadows than in former pastures. Our objectives were to compare soil conditions, taxonomic composition and functional composition of understorey plant communities in recent forests located on former pastures, meadows or cropland, with ancient forests as a reference.LocationTarentaise Valley, Savoy, France.MethodsBased on land‐use maps surveyed between 1862 and 1864, we selected 82 forest sites with different former land uses in mountain forests in the French Alps and carried out soil sampling and botanical surveys. To account for potential confounding factors (altitude, canopy cover, tree species composition), we applied multiple linear regressions to analyse soil properties, canonical correspondence analysis to analyse plant taxonomic composition and multispecies generalised linear mixed‐effects models to analyse relationships between plant functional composition and former land uses.ResultsThe soils of former cropland were richer in nutrients and more alkaline compared to other past land uses, while soils on former pastures and meadows differed only slightly from ancient forests. Ancient forests were characterised by acidophilic, shade‐tolerant, low‐stature, forest‐dependent species, whereas former cropland was characterised by calcicolous non‐forest species. Former pasture and meadow communities displayed a distinct taxonomic composition compared to other past land uses, but a functional composition closer to ancient forest than to former cropland.ConclusionFormer cropland has a stronger legacy effect than former pastures or meadows. This could explain small differences between ancient and recent forests observed in previous studies conducted in mountain landscapes where former cropland was rare.

Silvicultural management and altitude prevail on soil properties and fungal community in shaping understorey plant communities in a Mediterranean pine forest

Understorey vegetation plays a key role in Mediterranean forest ecosystem functioning. However, we still lack a thorough understanding of the patterns and drivers of understorey composition and diversity. As a result, understoreys are often ignored during assessments of forest functioning under climate change. Here we studied the effect of silvicultural management, topography, soil fungal community composition and soil physical and chemical properties on understorey community composition and diversity. The plant cover and number of individuals of understorey perennial plants, shrubs and non-dominant trees was recorded on 24 plots (paired: control-thinned) in a Mediterranean pine-dominated mountainous area in Northeast Spain. The study area represented a broad thinning intensity gradient (from 0 to 70 % in removed stand basal area) along a 400-m altitudinal range (from 609 m to 1013 m). Our results showed that thinning intensity and topography explained the greatest proportion of the total variance in the understorey species composition, i.e., 18 % and 16 %, respectively. Interestingly, the effects of the silvicultural treatments were significant only when considering the altitudinal effect, so that, the main impacts of thinning on the understorey community composition occurred at low altitudes (between 609 m and 870 m). Moreover, we found a significant decrease in both richness and abundance of understorey species in both the control and thinned plots with increasing altitude, with thinned plots being significantly richer in species compared to the control plots. The difference in the understorey community sensitivity to forest thinning along the altitudinal gradient suggests changes in factors that limit plant growth. Low elevation plots were restrained by light availability while high altitudes plots limited by winter freezing temperature.

Does intraspecific trait variability affect understorey plant community assembly?

Increasing evidence points to the major impact of intraspecific variability in species characteristics on community assembly. Contrasting theories predict that high trait variability might either steepening dominance orders or promote diversity. Here, we try to test these predictions with understorey plants in a Polish mixed temperate forest. We focus on four important traits (leaf dry mass LDW, plant height SH, specific leaf area SLA, and number of flowers NF) of the small balsam Impatiens parviflora, the dominant species in this forest system. We relate trait variability to community species richness and abundances, as well as to soil characteristics. Trait variability was high irrespective of species richness. Contrary to theory, abundances of non-I. parviflora species significantly decreased with increasing variance in I. parviflora trait expression. Trait variability did not significantly influence species composition and was not significantly correlated with soil characteristics. Our results emphasis the need to study trait variability in terms of appropriate statistical standards as provided by the allometric variance - mean relationship. We argue that an individual based study of local trait variability might return important insights into the composition and assembly of local communities.

Significance of Soil Seed Bank in Forest Vegetation—A Review

Seed banks present in soils provide information on past/existing standing vegetation and the probable future vegetation of the area. We reviewed 187 articles related to seed banks that were published worldwide from 1859 to 2022 to understand the dynamics of soil seed banks. The heterogeneity and biodiversity of most ecosystems appear vulnerable due to the over-exploitation of soil over the years. The studies on seeds present in soil provide knowledge on species cover, which supports the restoration of degraded areas. An interesting feature observed in most references was that species identified in seed banks do not necessarily represent or reflect the standing vegetation—this is attributed to the varied dispersal mechanisms of different species. The most significant contributions of a seed bank are conservation and the provision of propagules to the ecosystem. These banks are critical for identifying the composition and spatial organisation of understorey plant communities in degraded forests following soil disturbances, such as wildfire, harvesting, and/or logging. Regeneration from soil seed banks enables biodiversity recovery in tropical forests. There has been little understanding of the function soil seed banks play in forest restoration, which is critical to the natural regeneration of forest reserves. The impact of invasive alien species and the associated soil seed banks on vegetation dynamics is poorly researched. With landscape restoration gaining significance in the present decade, it becomes exceedingly important to maintain existing vegetation. It is relevant to India, which has international commitments to restore 26 mha of degraded and deforested land by 2030. This review reveals the importance of soil seed banks and their role in the future maintenance of forest cover.

Longleaf pine restoration on hydric sites: Understorey plant community responses to site preparation through 15 years

AbstractQuestionLongleaf pine (Pinus palustris) restoration is an important management objective throughout the southeastern United States. Site preparation prior to planting longleaf pine seedlings is often required to overcome challenges to seedling establishment on hydric sites. We know site preparation improves growth and survival of planted longleaf pine on hydric sites, but what are the impacts of site preparation on the understorey plant community?LocationMarine Corps Base Camp Lejeune in Onslow County, NC, USA.MethodsThis study tested eight site preparation treatments, including an untreated control, six combinations of two vegetation control treatments (chopping or herbicide) with three soil manipulation treatments (mounding, bedding, or flat‐planting [no treatment]), and a chopping–herbicide–bedding treatment. We collected data on understorey plant abundance, diversity, and composition through the first three and at 15 years after plantation establishment. We used ANOVA procedures and multivariate ordination to test for differences in understorey responses among the study treatments and through time.ResultsSite preparation had lasting impacts on the understorey plant community. The chop‐only treatment was the only treatment that resulted in dominance of herbaceous vegetation and was among the treatments with the highest diversity and greatest abundance of Aristida stricta (wiregrass). Herbicide produced lasting reductions in Aristida stricta abundance and apparent shifts in community composition. Reduced graminoid abundance was particularly pronounced when herbicide was combined with soil manipulation treatments.ConclusionsUnderstorey responses to site preparation through 3 years differed from responses observed in year 15, highlighting the importance of long‐term monitoring. For example, herbicides controlled shrubs early, but by year 15, herbicide treatments were dominated by shrubs. Restoration scenarios often must balance ecological objectives (e.g., maintenance of a diverse understorey community) with forestry objectives (e.g., timely establishment of overstorey trees), and our findings can be used as a guide to balance these trade‐offs.

Changes in forest structure drive temperature preferences of boreal understorey plant communities

Abstract The local climate in forest understories can deviate substantially from ambient conditions. Moreover, forest microclimates are often characterized by cyclic changes driven by management activities such as clear‐cutting and subsequent planting. To understand how and why understorey plant communities change, both ambient climate change and temporal variation in forest structure have to be considered. We used inventories from 11,436 productive forest sites in Sweden repeated every 10th year 1993–2017 to examine how variation in forest structure influences changes in the average value of minimum and maximum temperature preferences of all species in a community, that is, community temperature indices (CTIs). We then evaluated to what extent these changes were driven by local extinctions and colonizations, respectively, and to what extent the difference in CTI value between two inventories was related to changes in forest density and in macroclimate. Lastly, we tested whether effects on CTI change by these two drivers were modified by topography, soil moisture and tree species composition. CTI values of the understorey plant communities increased after clear‐cutting, and decreased during periods when the forest grew denser. During the period immediately after clear‐cutting, changes were predominately driven by colonizations of species with a preference for higher temperatures. During the forest regeneration phase, both colonizations by species preferring lower temperatures and local extinctions of species preferring higher temperatures increased. The change in understorey CTI over 10‐year periods was explained more by changes in forest density, than by changes in macroclimate. Soil moisture, topography and forest tree species composition modified to some extent the effects of changes in forest density and in macroclimate on understorey CTI values. Synthesis. Via stand manipulation, forest management impacts the effects of regional climate on understorey plant communities. This implies that forest management by creating denser stands locally even can counterbalance the effects of regional changes in climate. Consequently, interpretations of changes in the mean temperature preference of species in forest understorey communities should take forest management regimes into account.

Forest understorey communities respond strongly to light in interaction with forest structure, but not to microclimate warming.

Forests harbour large spatiotemporal heterogeneity in canopy structure. This variation drives the microclimate and light availability at the forest floor. So far, we do not know how light availability and sub-canopy temperature interactively mediate the impact of macroclimate warming on understorey communities. We therefore assessed the functional response of understorey plant communities to warming and light addition in a full factorial experiment installed in temperate deciduous forests across Europe along natural microclimate, light and macroclimate gradients. Furthermore, we related these functional responses to the species' life-history syndromes and thermal niches. We found no significant community responses to the warming treatment. The light treatment, however, had a stronger impact on communities, mainly due to responses by fast-colonizing generalists and not by slow-colonizing forest specialists. The forest structure strongly mediated the response to light addition and also had a clear impact on functional traits and total plant cover. The effects of short-term experimental warming were small and suggest a time-lag in the response of understorey species to climate change. Canopy disturbance, for instance due to drought, pests or logging, has a strong and immediate impact and particularly favours generalists in the understorey in structurally complex forests.

Deer exclusion unveils abiotic filtering in forest understorey plant assemblages.

The role of deer (family Cervidae) in ecosystem functioning has traditionally been neglected by forest ecologists due to the animal's scarcity in most parts of the northern hemisphere. However, the dramatic rebound in deer populations throughout the 20th century has brought deer browsing to the forefront of forest ecological questioning. Today there is ample evidence that deer affect tree regeneration, understorey plant and animal diversity, and even litter decomposition. However, the mechanisms underlying the effects of deer on forest ecosystems remain unclear. Among others, the relative role of abiotic factors versus biotic interactions (e.g. herbivory) in shaping plant assemblages remains largely unknown. We used a large-scale experiment with exclosures distributed along abiotic gradients to understand the role of black-tailed deer (Odocoileus hemionus sitchensis) on the forest understorey on the Haida Gwaii archipelago (western Canada), a unique context where most of the key ecological effects of deer presence have already been intensively studied. Our results demonstrate that 20 years of deer exclusion resulted in a clear increase in vascular plant richness, diversity and cover, and caused a decline in bryophyte cover. Exclusion also unveiled abiotic (i.e. soil water availability and fertility) filtering of plant assemblages that would otherwise have been masked by the impact of abundant deer populations. However, deer exclusion did not lead to an increase in beta diversity, probably because some remnant species had a competitive advantage to regrow after decades of over browsing. We demonstrated that long-term herbivory by deer can be a dominant factor structuring understorey plant communities that overwhelms abiotic factors. However, while exclosures prove useful to assess the overall effects of large herbivores, the results from our studies at broader scales on the Haida Gwaii archipelago suggest that exclosure experiments should be used cautiously when inferring the mechanisms at work.

Temperature effects on forest understorey plants in hedgerows: a combined warming and transplant experiment.

Hedgerows have been shown to improve forest connectivity, leading to an increased probability of species tracking the shifting bioclimatic envelopes. However, it is still unknown how species in hedgerows respond to temperature changes, and whether effects differ compared with those in nearby forests. We aimed to elucidate how ongoing changes in the climate system will affect the efficiency of hedgerows in supporting forest plant persistence and migration in agricultural landscapes. Here we report results from the first warming experiment in hedgerows. We combined reciprocal transplantation of plants along an 860-km latitudinal transect with experimental warming to assess the effects of temperature on vegetative growth and reproduction of two common forest herbs (Anemone nemorosa and Geum urbanum) in hedgerows versus forests. Both species grew taller and produced more biomass in forests than in hedgerows, most likely due to higher competition with ruderals and graminoids in hedgerows. Adult plant performance of both species generally benefitted from experimental warming, despite lower survival of A. nemorosa in heated plots. Transplantation affected the species differently: A. nemorosa plants grew taller, produced more biomass and showed higher survival when transplanted at their home site, indicating local adaptation, while individuals of G. urbanum showed greater height, biomass, reproductive output and survival when transplanted northwards, likely owing to the higher light availability associated with increasing photoperiod during the growing season. These findings demonstrate that some forest herbs can show phenotypic plasticity to warming temperatures, potentially increasing their ability to benefit from hedgerows as ecological corridors. Our study thus provides novel insights into the impacts of climate change on understorey plant community dynamics in hedgerows, and how rising temperature can influence the efficiency of these corridors to assist forest species' persistence and colonization within and beyond their current distribution range.

Taxonomic, phylogenetic and functional diversity of understorey plants respond differently to environmental conditions in European forest edges

Abstract Forest biodiversity world‐wide is affected by climate change, habitat loss and fragmentation, and today 20% of the forest area is located within 100 m of a forest edge. Still, forest edges harbour a substantial amount of terrestrial biodiversity, especially in the understorey. The functional and phylogenetic diversity of forest edges have never been studied simultaneously at a continental scale, in spite of their importance for the forests' functioning and for communities' resilience to future change. We assessed nine metrics of taxonomic, phylogenetic and functional diversity of understorey plant communities in 225 plots spread along edge‐to‐interior gradients in deciduous forests across Europe. We then derived the relative effects and importance of edaphic, stand and landscape conditions on the diversity metrics. Here, we show that taxonomic, phylogenetic and functional diversity metrics respond differently to environmental conditions. We report an increase in functional diversity in plots with stronger microclimatic buffering, in spite of their lower taxonomic species richness. Additionally, we found increased taxonomic species richness at the forest edge, but in forests with intermediate and high openness, these communities had decreased phylogenetic diversity. Functional and phylogenetic diversity revealed complementary and important insights in community assembly mechanisms. Several environmental filters were identified as potential drivers of the patterns, such as a colder macroclimate and less buffered microclimate for functional diversity. For phylogenetic diversity, edaphic conditions were more important. Interestingly, plots with lower soil pH had decreased taxonomic species richness, but led to increased phylogenetic diversity, challenging the phylogenetic niche conservatism concept. Synthesis. Taxonomic, phylogenetic and functional diversity of understorey communities in forest edges respond differently to environmental conditions, providing insight into different community assembly mechanisms and their interactions. Therefore, it is important to look beyond species richness with phylogenetic and functional diversity approaches when focusing on forest understorey biodiversity.

The climatic debt is growing in the understorey of temperate forests: Stand characteristics matter

AbstractAimClimate warming reshuffles biological assemblages towards less cold‐adapted but more warm‐adapted species, a process coined thermophilization. However, the velocity at which this process is happening generally lags behind the velocity of climate change, generating a climatic debt the temporal dynamics of which remain misunderstood. Relying on high‐resolution time series of vegetation data from a long‐term monitoring network of permanent forest plots, we aim at quantifying the temporal dynamics – up to a yearly resolution – of the climatic debt in the understorey of temperate forests before identifying the key determinants that modulate it.LocationFrance.Time period1995–2017.Taxa studiedVascular plants.MethodsWe used the community temperature index (CTI) to produce a time series of understorey plant community thermophilization, which we subsequently compared to a time series of mean annual temperature changes over the same period and for the same sites. The direction and magnitude of the difference (i.e., the climatic debt) was finally analysed using linear mixed‐effect models to assess the relative contributions of abiotic and biotic determinants, including forest stand characteristics.ResultsWe found a significant increase in CTI values over time (0.08–0.09 °C/decade), whereas the velocity of mean annual temperature changes was three times higher over the same period (0.22–0.28 °C/decade). Hence, the climatic debt increased over time and was greater in forest stands with higher basal area or older trees as well as under warmer macroclimate. By contrast, a greater frequency of anthropogenic disturbances decreased the climatic debt, while natural disturbances and herbivory had no impact.ConclusionsAlthough often overlooked in understanding the climatic debt of forest biodiversity, changes in forest stand characteristics may modulate the climatic debt by locally modifying microclimatic conditions. Notably, the buffering effect of the upper canopy layer implies microclimate dynamics that may provide more time for understorey plant communities to locally adapt.

Woody species have stronger facilitative effects on soil biota than on plants along an aridity gradient

AbstractQuestionsWoody‐shrub encroachment affects community structure and composition. However, most studies focus on their effects on understorey plant communities, and the relative importance of shrubs in affecting plants vs soil biota communities is poorly known.LocationInner Mongolian Steppe, China.MethodsWe examined the effect of shrubs on multiple community attributes, including plants, soil biota (bacteria, fungi and nematodes), and soil fertility, and quantified how these effects changed from semi‐arid to hyperarid conditions (from 281 mm to 110 mm of mean annual precipitation). In addition, we assessed whether the effects of shrubs on plant communities were directly mediated by biotic filtering in seed germination and establishment, or indirectly mediated by plant biomass or soil fertility in the case of soil organisms.ResultsThe effect of shrubs on soil biota was generally more positive than on plants, and it increased with aridity. We found that a larger proportion of below‐ground taxa depended on shrub presence (36%) than plants (20%). Soil nematodes and soil bacteria were directly influenced by shrub presence whereas soil fungi were indirectly influenced by enhanced soil fertility. Shrubs also increased plant biomass under all conditions but only increased plant species richness in the most arid conditions. Despite the generally positive effect of shrubs, and the fact that they weakened the filtering effects of aridity on seed germination, aridity was a stronger predictor of changes in species composition than shrub presence was, particularly for plants.ConclusionsOur results illustrate the variety of positive effects of shrubs and show that they are particularly important in supporting biodiversity in the most arid conditions. These strong and positive effects could partially buffer the impacts of increasing aridity on dryland soil biodiversity, but our study suggests that facilitative interactions may not be able to completely mitigate the impacts of increasing aridity on drylands.

Rapid thermophilization of understorey plant communities in a 9 year‐long temperate forest experiment

Abstract The vast majority of plant biodiversity associated with temperate forests is harboured by the understorey layer. This layer also plays crucial roles in ecosystem functions such as tree regeneration, nutrient cycling and carbon dynamics. Research using space‐for‐time substitutions and resurveys of vegetation plots has shown that climate warming, changes in forest management and resource availability are key determinants of forest understorey biodiversity change and functioning. However, long‐term experiments are needed to better unravel their complex interactive effects. Here we study the influence of nearly a decade of experimental warming, light addition using fluorescent tubes (as a proxy for management‐driven changes in forest‐floor light levels) and nitrogen input on understorey plant communities of temperate broadleaved forest. Plant communities shifted towards a higher dominance of warm‐adapted species, a process referred to as thermophilization. We detected a marked community shift in all treatments including the control plots, reflecting ongoing ambient environmental changes. This reordering over time was greater than the shift induced by the treatments. Thermophilization was, however, greatest when temperature and/or light availability were enhanced. Communities were also taller in response to warming and increased light availability. Synthesis. Our experiment provides important insights into 9 years of vegetation changes in a temperate forest and how canopy density and forest management can be adapted to limit thermophilization of forest understorey biodiversity under climate change. [Correction added on 27 April 2021, after first online publication: The Synthesis section in the abstract has been updated to reflect the original text supplied.]

Dominant plant facilitation can generate indirect competition in a South‐American desert plant community

AbstractAimsDesert dominant plants commonly facilitate plant communities within their canopies. Although substantial research has examined the direct consequences of this effect, a mechanistic understanding of indirect effects mediated via beneficiary plants is still relatively limited. We tested the hypothesis that the net positive outcome of dominant plants on beneficiaries extends to a series of interactions including indirect competition or facilitation. To test this hypothesis, we aggregated two years of field surveys with a manipulative experiment.LocationAtiquipa, Southern Peru.MethodsWe surveyed the understorey plant community of the dominant tree Caesalpinia spinosa and compared it to that of open microsites. Field manipulations included removal of plant neighbourhoods of two target annual species in the understorey of the dominant plant Caesalpinia spinosa and adjacent open microsites. In the surveys, we measured plant density, and in the removal experiment, plant height, fruit set, and biomass of the targets.ResultsIn the surveys, density of Fuertisimalva peruviana, a target species, was negatively dependent on understorey neighbours’ density. Neighbourhood removal did not affect fruit set or biomass of Fuertisimalva peruviana in tree understories or open microsites. Neighbourhood removal around Plantago limensis increased biomass, plant height and fruit set, and changed the effect of the dominant tree from positive to negative for biomass.ConclusionsUnderstorey plant neighbours can mediate the direct effects of dominant plants in deserts, but responses to these indirect effects can be species‐specific. The facilitative effects of dominant plants represent key coexistence mechanisms because they reduce stress and generate extended series of interactions not necessarily present in the open.