Sapling Richness Research Articles

High beta diversity of gaps contributes to plot-level tree diversity in a tropical forest.

Canopy gaps are widely recognized as being crucial for maintaining the diversity of forest tree communities. But empirical studies have found mixed results because the differences in diversity between individual gaps and non-gaps are often small and statistically undetectable. One overlooked factor, however, is how small individual gap versus non-gap differences may accumulate across sites and potentially have a large effect on forest diversity at the plot scale. Our study investigated sapling richness, density, and composition in 124 treefall gaps, and 200 non-gap sites in the 50-ha tropical forest plot at Barro Colorado Island (BCI), Panama. Additionally, we analyzed species accumulation curves to understand how species richness increases with increasing stem numbers. We observed that sapling richness and density were only slightly higher in gaps 7 years after formation and statistically indistinguishable from non-gaps after 12 years. However, species accumulation curves across multiple gaps were substantially higher than those across non-gaps. Species composition showed small differences between individual gaps and non-gaps but differed significantly between collections of gaps and non-gaps. Specifically, 55 species specialized in 7-year-old gaps compared with 24 in non-gaps; of these, 23 gap-specialized species and zero non-gap species were pioneers. Our results indicate that tree species richness is higher in gaps because of both higher stem density and the presence of gap-specialized species. Our study has finally provided compelling evidence to support the idea that gaps enhance the overall diversity of tropical forest tree communities.

Long‐term stability of sapling dynamics is regulated by soil phosphorus availability in subtropical forest

Abstract The abiotic and biotic factors that regulate stability in species‐rich forests are poorly resolved, and this limits the ability to predict how climate change and other perturbations impact forest dynamics. Phosphorus limitation and nitrogen deposition are important factors affecting the dynamics and functioning of tropical and subtropical forests, but how long‐term temporal stability of tree communities is affected by availability and heterogeneity of soil phosphorus and nitrogen remains unclear. We collected annual dynamic data of 20,768 regenerating saplings, which were censused from 2008 to 2019 in a subtropical forest, to investigate how soil nutrients affect the temporal stability of productivity at both population and community levels. We found that concentrations of soil inorganic phosphorus were significantly and positively correlated with sapling richness and phylogenetic diversity, leading to significantly higher species asynchrony and community temporal stability. By contrast, higher concentrations of organic phosphorus weakened community stability via a negative effect on species richness. Structural equation models provide further strong evidence that increasing concentrations of soil inorganic phosphorus strongly promoted community temporal stability via increased species diversity, species asynchrony and population stability, while organic phosphorus displayed opposite effects. Meanwhile, soil concentrations of available and unavailable forms of nitrogen showed much weaker and negative associations with community stability. Synthesis. Using a 12‐year data set of sapling demography from a subtropical forest of south China, our study demonstrates that soil phosphorus is an important determinant of the long‐term stability of sapling dynamics and consequently the whole‐community structure, and the composition of soil phosphorus pools predicts the temporal stability and diversity of tree communities in phosphorus‐limited forests.

Understory plant communities show resistance to drought, hurricanes, and experimental warming in a wet tropical forest

Global climate change has led to rising temperatures and to more frequent and intense climatic events, such as storms and droughts. Changes in climate and disturbance regimes can have non-additive effects on plant communities and result in complicated legacies we have yet to understand. This is especially true for tropical forests, which play a significant role in regulating global climate. We used understory vegetation data from the Tropical Responses to Altered Climate Experiment (TRACE) in Puerto Rico to evaluate how plant communities responded to climate warming and disturbance. The TRACE understory vegetation was exposed to a severe drought (2015), 2 years of experimental warming (4°C above ambient in half of the plots, 2016–2017 and 2018–2019), and two major hurricanes (Irma and María, September 2017). Woody seedlings and saplings were censused yearly from 2015 to 2019, with an additional census in 2015 after the drought ended. We evaluated disturbance-driven changes in species richness, diversity, and composition across ontogeny. We then used Bayesian predictive trait modeling to assess how species responded to disturbance and how this might influence the functional structure of the plant community. Our results show decreased seedling richness after hurricane disturbance, as well as increased sapling richness and diversity after warming. We found a shift in species composition through time for both seedlings and saplings, yet the individual effects of each disturbance were not significant. At both ontogenetic stages, we observed about twice as many species responding to experimental warming as those responding to drought and hurricanes. Predicted changes in functional structure point to disturbance-driven functional shifts toward a mixture of fast-growing and drought-tolerant species. Our findings demonstrate that the tropical forest understory community is more resistant to climatic stressors than expected, especially at the sapling stage. However, early signs of changes in species composition suggest that, in a warming climate with frequent droughts and hurricanes, plant communities might shift over time toward fast-growing or drought-tolerant species.

Temperate forests respond in a non-linear way to a population gradient of wild deer

Abstract Wild deer exert strong top–down control on forest composition by browsing on palatable trees, and these effects are exacerbated as red, fallow, and roe deer populations increase in northern temperate forests. However, the relationship between deer abundance and plant recruitment remains poorly documented. Here, we combined camera trap and vegetation plot data to quantify the shape of the relationship between habitat utilization by deer (red, fallow and roe deer) with different components of the forest understory in ten sites distributed across a temperate mixed forest in the Veluwe, the Netherlands. The list of forest attributes included the density, richness and diversity of saplings, the proportion of conifers to broadleaves, the number of browsed broadleaves and conifers, the forest basal area, the understory cover of shrubs, moss and bare soil and the depth of the litter layer. When applying General Linear Mixed Models to those camera trap data, six of the eleven forest variables were related to the utilization level (UL) by deer. With increasing UL by fallow deer, there was a decrease in sapling species richness (β = −0.26). With increasing UL by red deer, there was a decrease in litter depth (β = −0.14) and an increase in browsed broadleaf stems (β = 0.40). With increasing UL by roe deer, there was a decrease in species richness of sapling plants (β = −0.26), Shannon diversity (β = −0.11) and shrub cover (β = −0.36), whereas there was an increase in stem density (β = 0.06). When combining all deer species into one guild, a negative relation was found between UL by deer and sapling richness (β = −0.21), diversity (β = −0.09) and litter depth (β = −0.14) and a positive relation with sampling density (β = 0.04). The relationship between UL by deer and different forest attributes followed a linear log10 shape. When back transforming, the response was curvilinear with large changes at a low UL and small changes when the UL was high. Yet, the exact shape of the curve varied according to (a) biotic factors specific to each study location. The conservation of temperate forests can benefit from limiting the size of deer populations and the time deer spend browsing in forest patches with high biodiversity value.

Interaction of land management and araucaria trees in the maintenance of landscape diversity in the highlands of southern Brazil.

In the southern Brazilian highlands, pre-Columbian societies created domesticated landscapes through the use and management of forests, including nurse Araucaria angustifolia trees, a common conifer in these regions. Nowadays, local smallholders still use traditional practices, such as burning, to promote vegetation for cattle grazing in highland grasslands. Even though burning is normally of small extent and low frequency, such management can slow down natural forest expansion and contribute to the maintenance of grasslands, by opposing the facilitative effect of nurse araucaria trees. To comprehend the interplay between human cultural management, species interactions and the environment, it is important to better understand how these relations affect diversity and composition. Our goal was to investigate how land management, biotic interactions and abiotic factors affect saplings species richness, abundance and composition. We hypothesized that (1) land management would decrease sapling richness and abundance and change sapling composition, (2) nurse araucaria trees would increase species richness and abundance and change sapling composition, and (3) the interactive effect between land management and nurse araucaria trees would shape sapling richness, abundance and composition. Data were collected in unmanaged and managed conditions, both beneath araucaria crowns and in nearby treeless areas. Our results indicate that abundance and species composition are affected by land management and araucaria crown influence. The highest values of sapling abundance were found beneath crowns in unmanaged areas. Species composition changed between all assessed combinations of land management and crown influence. Our study demonstrates the major roles of land management and facilitation in structuring communities, despite the effects of rock and grass cover. Moreover, our results clarify patterns and processes that may emerge in natural highland grasslands, such as the conversion of grasslands into forests and the loss of cultural landscapes when the main local management actions are excluded.

Contrasting patterns of local richness of seedlings, saplings, and trees may have implications for regeneration in rainforest remnants

AbstractRemnants of lowland rain forest remain following deforestation, but the longer‐term effects of fragmentation remain poorly understood, partly due to the long generation times of trees. We study rain forest trees in three size classes: seedlings (<1 cm dbh), saplings (1–5 cm dbh), and trees (>5 cm) that broadly reflect pre‐ and post‐fragmentation communities, and we examine the impacts of fragmentation on forest regeneration in Sabah, Malaysian Borneo. We found that seedling richness (measured as the number of genera per plot) in fragments was about 30 percent lower than in plots in undisturbed forest, and about 20 percent lower than in an extensive tract of selectively logged forest, providing evidence of recruitment declines in fragments. Seedling richness was lowest in small, isolated, and disturbed fragments, potentially signaling an extinction debt given that these fragmentation impacts were not observed in trees. Unlike seedlings, saplings showed no declines in richness in fragments, suggesting that density dependent mortality (where rare individuals have a higher survival rate) and/or year‐to‐year variation in which species are recruiting could potentially compensate for the reductions in seedling richness we observed. Longer‐term studies are required to determine whether sporadic or failed recruitment in small fragments will eventually translate into reduced richness of mature trees, or whether the processes that currently retain high sapling richness will continue in fragments.

Do composition and richness of woody plants vary between gaps and closed canopy patches in subtropical forests?

AbstractQuestionsDo composition and richness of woody plants differ between gaps and closed canopy in subtropical forests, and does this difference vary across life stages of tree species? Is tree species richness in gaps a function of regeneration density?LocationSubtropical Shorea robusta Gaertn (Sal) forest, central Nepal.MethodsWe collected vegetation data from two old‐growth S. robusta forest stands. We sampled 128 plots of 100 m2 equally spread between the two habitats: gap and closed canopy. In each plot, we recorded the total number of woody species, number of individuals of seedlings and saplings of tree species and measured the DBH of all saplings. We compared species richness and composition of total woody species, seedlings and saplings between the two habitats. We used ordination to analyse species composition, and an individual‐based species accumulation curves to illustrate the effect of density on species richness.ResultsThe species composition of total woody species and seedlings was similar in both habitats, but species composition of saplings differed between habitats. Total woody and seedling richness were similar between habitats at one site, but were richer under closed canopy at the other site. Sapling richness was higher in gaps at both sites and was a function of stem density at one site, but not at the other site.ConclusionsGaps are not always areas of higher woody species richness and therefore may be less important than expected for the overall species richness of woody plants. Instead, they are potentially important for enhancing local tree richness by increasing sapling richness. Gap disturbance is the primary driver of structural heterogeneity in forests where topographic and edaphic gradients are negligible.

Regeneration potential in South African forest fragments: extinction debt paid off or hampered by contemporary matrix modification?

Deforestation generates forest fragments within modified matrices with frequently detrimental outcomes for species persistence. In historically fragmented forests, however, species have evolved under fragmented conditions and might be more resistant to matrix modification. We examined the effect of contemporary matrix modification on forest regeneration potential in historically fragmented scarp-forest fragments in KwaZulu-Natal, South Africa. We compared tree, seedling and sapling richness, abundance and composition in four forest-fragment types varying in size and matrix modification: large and small forest fragments surrounded by indigenous grassland and small forest fragments enclosed by eucalypt and sugarcane plantations. Additionally, seedlings and saplings were classified according to their origin as local/immigrant to assess matrix permeability for seed dispersers. Trees and saplings were slightly, and seedlings were strongly affected by matrix modification: seedling richness and abundance were significantly lower in fragments with sugarcane compared to fragments with grassland matrix. Furthermore, all stage classes revealed compositional differences with more light-tolerant species occurring in fragments with modified matrices. We suggest that human activities associated with matrix modification have altered abiotic factors favouring light-tolerant species. Changes in biotic interactions seemed to be less important as immigrant recruitment occurred in all fragments implying matrix permeability. Our findings demonstrate that contemporary matrix modification might threaten forest regeneration potential in historical scarp-forest fragments. Reduced seedling recruitment might indicate a time lag in local species extinction, as trees regenerate slowly. We recommend using both recruitment stages as early warning indicators. Further, including small scarp-forest fragments into protected area networks will minimise human impact associated with matrix modification.

Relationships between the density of two potential restoration tree species and plant species abundance and richness in a degradedAfromontane forest ofKenya

AbstractIn recent years, there have been considerable efforts to restore degraded tropical montane forests through active restoration using indigenous tree species. However, little is known about how these species used for restoration influence other species. In this study, two potential restoration species,Albizia gummiferaandNeoboutonia macrocalyx, are investigated with regard to the relationship between their density and the abundance and richness of other plant species. The study was conducted in a degraded forest consisting of disturbed transition zones and secondary forest. Our results show positive relationships between the density ofA. gummiferaand the abundance of tree seedling and sapling richness in the transition zones and in the secondary forest. Shrub richness was negatively related to the density ofA. gummifera. Abundance and richness of tree saplings and shrubs were positively related toN. macrocalyxdensity both in the transition zones and in the secondary forest. Herb species richness declined withN. macrocalyxdensity in the transition zones but increased withN. macrocalyxdensity in the secondary forest. The positive relationships between the density of the two tree species and species richness of other woody species suggest that bothA. gummiferaandN. macrocalyxcan be suitable for active restoration of degraded mountain forests within their natural range.

Sapling richness and composition in canopy gaps of a southern Appalachian mixed Quercus forest1

Canopy gaps create distinct microenvironments within the broader forest environment and provide a mechanism for regeneration and recruitment. In this study, we investigated patterns of sapling richness and composition in 40 canopy gaps in a secondary, mixed Quercus forest on the Cumberland Plateau in Tennessee. We found that sapling richness was high, with 34 species represented in canopy gaps. A species-gap area curve revealed that sapling richness increased steadily with gap sizes up to ca. 200 m2, but then tended to level off. While minor components in the main forest canopy, Acer saccharum, Fagus grandifolia, and Acer rubrum were the most widespread and abundant species in gaps. Our analyses of floristic composition using ordination (NMS) and randomization techniques (MRPP) clarified the mechanisms responsible for structuring patterns of gap composition and species-environment relationships, including the effects of individual factors (gap formation mechanism, gap size, aspect, and soils) and their collective impacts. Snag-formed gaps were smaller in size and most common on southern exposures with stony loam soils. Gaps formed by uprooted trees were larger and most common on north-facing slopes with shallow, gravelly loam soils. Gap composition varied along these gradients. Saplings of current canopy dominant genera (Quercus and Carya) were largely restricted to small gaps on xeric sites, suggesting that gap-phase succession will result in a transition from a Quercus–Carya system to one with a much stronger Acer–Fagus component.

Recovery of plant species richness and composition after slash-and-burn agriculture in a tropical rainforest in Madagascar

Slash-and-burn agriculture is an important driver of deforestation and ecosystem degradation, with large effects on biodiversity and carbon sequestration. This study was conducted in a forest in Madagascar, which consists of fragments of slash-and-burn patches, within a matrix of secondary and primary forest. By recording species richness, abundance, and composition of trees, shrubs, and herbs in fallows of various age and slash-and-burn history, and in the secondary and primary forest, we show how slash-and-burn intensity (number of cycles, duration of abandonment), years since last abandonment, and environmental factors (distance to primary forest and topography) affect the natural succession and recovery of the forest ecosystem. We used ordination analyses to examine how the species composition varied between the different successions stages, and to examine tree recruitment. Our results show shrub dominance the first years after abandonment. Thereafter, a subsequent increase in species richness and abundance of tree seedlings and saplings suggests a succession towards the diversity and composition of the secondary and primary forest, although a big gap between the oldest fallows and the secondary forest shows that this will take much more than 30 years. A high number and frequency of slash-and-burn cycles decreased tree seedling and sapling richness and abundance, suggesting that reducing slash-and-burn intensity will increase the speed of tree recruitment and fallow recovery. Trees can be planted into fallows to speed up vegetation and soil recovery, such that fallows can be usable within needed time and thus the extension of cultivated areas reduced. We recommend further testing of six potential species for restoration based on their early colonization of the fallows and their survival through vegetation succession.