Aims
We tested whether plant species niche breadths decrease with increasing species richness due to competition on temperature, precipitation and disturbance gradients. We assumed that niche optima, ...niche breadth and niche volume are related to plant functional traits, indicating competitive ability and adaptation to environmental stress. Finally, we wanted to identify habitats dominated by species with small niche breadths most prone to environmental change.
Location
Southern slopes of Mount Kilimanjaro, Tanzania.
Methods
We calculated species distribution models for 1,492 plant species based on the presence/absence data on 969 plots using generalized linear models. We derived niche breadths, optima and volumes and investigated their relationship with plant functional traits with principal component regression to account for high trait correlations.
Results
Niche breadths of individual species increased with elevation, precipitation and disturbance. Averaged community elevation niche breadth and niche volume decreased with increasing species richness. Plant functional traits explained about 40% and 50% of the variation in niche optima and breadths, respectively. Size and growth traits were significant predictors of niche breadths on all gradients, whereas traits indicating reproductive strategy were not significant on the precipitation gradient.
Main conclusions
Our results support the notion of decreasing niche breadth with increasing temperature, possibly a result of competition due to increased diversification rates and hence species richness. However, the complexity of the niche breadth–species richness patterns on the other gradients shows that additional covariables shape species niche breadths apart from competition. Plant species with narrow niche breadths dominated natural savannas around Mount Kilimanjaro, indicating strong sensitivity to the ongoing conversion of savanna to fields and grasslands.
Compared to other plant life‐forms, epiphytes remain understudied. Understanding the responses of epiphytes to changing environmental conditions is necessary to predict changes in ecosystem ...functioning especially in subtropical and tropical regions.
We investigated the functional traits of epiphytes along a large elevation gradient on Mount Kilimanjaro, Tanzania. We measured traits of co‐occurring trees and terrestrial non‐tree life‐forms and compared changes in community‐weighted means (CWM) of traits and trait spread, the range of observed trait values. We chose traits linked to growth and persistence: leaf area, specific leaf area, leaf dry matter content, stem specific density, plant height, leaf carbon, leaf nitrogen and leaf phosphorus.
For most traits, differences in CWM between life‐forms exceeded differences within life‐forms along the elevation gradient. Many CWM showed linear changes with elevation, but no response and unimodal patterns were also frequent. This was best explained by temperature, or a combination of temperature with precipitation or humidity, indicating effects of these factors on the distribution of epiphytic and non‐epiphytic species. Trait spread did not change with elevation in nearly half of the traits, but hump‐shaped patterns were also common, probably a result of weaker environmental filtering in the gradient centre. The magnitude of trait spread, that is, the variability between species of the same life‐form within communities, was highest for terrestrial non‐trees (TNT). Excluding ferns from the analyses lead to marked differences in trait patterns for epiphytes, as ferns made up 59% of the epiphytic species, while playing a minor role in the other groups.
The observed differences can be explained by a dichotomy in epiphytic life strategies, with tough‐leaved xerotolerant species on one side and succulent soft‐leaved species on the other. However, the influence of phylogeny was lower than expected from the taxonomic composition of the three life‐form groups.
Our results emphasize that environmental constraints act upon functional traits of epiphytes, trees and TNT. The differences in trait expressions, arguably adaptations of the different life‐forms, need to be taken into account in conservation contexts as well as when modelling the effects of global change on ecosystems.
A plain language summary is available for this article.
Plain Language Summary
Variations in the stable isotopic composition of carbon (δ13C)
and nitrogen (δ15N) of fresh leaves, litter, and topsoils were
used to characterize soil organic matter dynamics of 12 tropical
...ecosystems in the Mount Kilimanjaro region, Tanzania. We studied a total of
60 sites distributed along five individual elevational transects
(860–4550 m a.s.l.), which define a strong climatic and land-use gradient
encompassing semi-natural and managed ecosystems. The combined effects of
contrasting environmental conditions, vegetation, soil, and management
practices had a strong impact on the δ13C and
δ15N values observed in the different ecosystems. The relative
abundance of C3 and C4 plants greatly determined the
δ13C of a given ecosystem. In contrast, δ15N
values were largely controlled by land-use intensification and climatic
conditions. The large δ13C enrichment factors
(δ13Clitter − δ13Csoil) and low soil C∕N ratios observed
in managed and disturbed systems agree well with the notion of altered SOM
dynamics. Besides the systematic removal of the plant biomass characteristic of
agricultural systems, annual litterfall patterns may also explain the
comparatively lower contents of C and N observed in the topsoils of these
intensively managed sites. Both δ15N values and calculated
δ15N-based enrichment factors
(δ15Nlitter − δ15Nsoil) suggest the tightest nitrogen cycling at
high-elevation (> 3000 m a.s.l.) ecosystems and more open
nitrogen cycling both in grass-dominated and intensively managed cropping
systems. However, claims about the nature of the N cycle (i.e. open or closed)
should not be made solely on the basis of soil δ15N as other
processes that barely discriminate against 15N (i.e. soil nitrate
leaching) have been shown to be quite significant in Mount Kilimanjaro's forest
ecosystems. The negative correlation of δ15N values with soil
nitrogen content and the positive correlation with mean annual temperature
suggest reduced mineralization rates and thus limited nitrogen availability,
at least in high-elevation ecosystems. By contrast, intensively managed
systems are characterized by lower soil nitrogen contents and warmer
conditions, leading together with nitrogen fertilizer inputs to lower
nitrogen retention and thus significantly higher soil δ15N
values. A simple function driven by soil nitrogen content and mean annual
temperature explained 68 % of the variability in soil δ15N
values across all sites. Based on our results, we suggest that in addition to
land-use intensification, increasing temperatures in a changing climate may
promote soil carbon and nitrogen losses, thus altering the otherwise stable
soil organic matter dynamics of Mount Kilimanjaro's forest ecosystems.
Species' functional traits set the blueprint for pair-wise interactions in ecological networks. Yet, it is unknown to what extent the functional diversity of plant and animal communities controls ...network assembly along environmental gradients in real-world ecosystems. Here we address this question with a unique dataset of mutualistic bird-fruit, bird-flower and insect-flower interaction networks and associated functional traits of 200 plant and 282 animal species sampled along broad climate and land-use gradients on Mt. Kilimanjaro. We show that plant functional diversity is mainly limited by precipitation, while animal functional diversity is primarily limited by temperature. Furthermore, shifts in plant and animal functional diversity along the elevational gradient control the niche breadth and partitioning of the respective other trophic level. These findings reveal that climatic constraints on the functional diversity of either plants or animals determine the relative importance of bottom-up and top-down control in plant-animal interaction networks.
Patterns of insect diversity along elevational gradients are well described in ecology. However, it remains little tested how variation in the quantity, quality, and diversity of food resources ...influence these patterns. Here we analyzed the direct and indirect effects of climate, food quantity (estimated by net primary productivity), quality (variation in the specific leaf area index, leaf nitrogen to phosphorus and leaf carbon to nitrogen ratio), and food diversity (diversity of leaf traits) on the species richness of phytophagous beetles along the broad elevation and land use gradients of Mt. Kilimanjaro, Tanzania. We sampled beetles at 65 study sites located in both natural and anthropogenic habitats, ranging from 866 to 4,550 m asl. We used path analysis to unravel the direct and indirect effects of predictor variables on species richness. In total, 3,154 phytophagous beetles representing 19 families and 304 morphospecies were collected. We found that the species richness of phytophagous beetles was bimodally distributed along the elevation gradient with peaks at the lowest (∼866 m asl) and upper mid-elevations (∼3,200 m asl) and sharply declined at higher elevations. Path analysis revealed temperature- and climate-driven changes in primary productivity and leaf trait diversity to be the best predictors of changes in the species richness of phytophagous beetles. Species richness increased with increases in mean annual temperature, primary productivity, and with increases in the diversity of leaf traits of local ecosystems. Our study demonstrates that, apart from temperature, the quantity and diversity of food resources play a major role in shaping diversity gradients of phytophagous insects. Drivers of global change, leading to a change of leaf traits and causing reductions in plant diversity and productivity, may consequently reduce the diversity of herbivore assemblages.
Temperature, primary productivity, plant functional traits, and herbivore abundances are considered key predictors of leaf herbivory but their direct and indirect contributions to community‐level ...herbivory are not well understood along broad climatic gradients.
Here, we determined elevational herbivory patterns and used a path analytical approach to disentangle the direct and indirect effects of climate, land use, net primary productivity (NPP), herbivore abundance, and plant functional traits on community‐level invertebrate herbivory along the extensive elevational and land use gradients at Mt. Kilimanjaro, Tanzania.
We recorded standing leaf herbivory caused by leaf chewers, leaf miners and leaf gallers on 55 study sites distributed in natural and anthropogenic habitats along a 3,060 m elevation gradient. We related the total community‐level herbivory to climate (temperature and precipitation), NPP, plant functional traits (specific leaf area, leaf carbon‐to‐nitrogen CN ratio and leaf nitrogen‐to‐phosphorus NP ratio) and herbivore abundances.
Leaf herbivory ranged from 5% to 11% along the elevation gradient. Total leaf herbivory showed unimodal pattern in natural habitats but a strongly contrasting bimodal pattern in anthropogenic habitats. We also detected some variation in the patterns of leaf herbivory along environmental gradients across feeding guilds with leaf chewers being responsible for a disproportionally large part of herbivory. Path analyses indicated that the variation in leaf herbivory was mainly driven by changes in leaf CN and NP ratios which were closely linked to changes in NPP in natural habitats. Similarly, patterns of leaf herbivory in anthropogenic habitats were best explained by variation in leaf CN ratios and a negative effect of land use.
Our study elucidates the strong role of leaf nutrient stoichiometry and its linkages to climate and NPP for explaining the variation in leaf herbivory along broad climatic gradients. Furthermore, the study suggests that climatic changes and nutrient inputs in the course of land use change may alter leaf herbivory and consequently energy and nutrient fluxes in terrestrial habitats.
The authors' findings advance the debate on the role of plant functional traits in predicting ecosystem functions by showing its relevance across broad climatic gradients and at the level of ecological communities.
Questions: How do community-weighted means of traits (CWM) and functional dispersion (FDis), a measure of trait variability, change in response to gradients of temperature, precipitation, soil ...nutrients and disturbance? Is the decrease in trait similarity between plots continuous or discontinuous? Is species turnover between plots linked to trait turnover? Location: Mount Kilimanjaro, Tanzania, Africa. Methods: Sixty plots were established in 12 major vegetation types on Mount Kilimanjaro, covering large gradients of temperature, precipitation, soil nutrients and anthropogenic disturbance representing the dominant ecosystems in East Africa. Environmental data, plant abundances and plant traits were recorded for each plot. Trait CWM and FDis were related to environmental factors with partial least squares regressions. Trait similarity between pairs of plots was assessed with a null model approach. Results: Both CWM and FDis of most traits responded strongly to environmental factors, particularly to precipitation and disturbance. FDis of traits associated with growth and reproduction mostly increased with temperature and precipitation, and decreased with disturbance. Pair-wise plot comparisons revealed an inverse relationship of trait similarity with differences in temperature, precipitation and anthropogenic disturbance. However, changes in similarity were often discontinuous rather than continuous. Several vegetation types differed strongly in species composition but not in traits. Conclusions: Trait dispersion indicating functional niches increased with productivity and temperature. Conversely, low-productivity conditions were characterized by trait convergence. Discontinuous changes in trait similarity between plots suggested tipping points at which trait expressions change strongly to adjust to environmental conditions. Large sections of the temperature gradient were characterized by species turnover with only minor changes in traits, indicating that the functional composition may be resilient to gradual environmental changes until a tipping point is reached.
Tropical forests represent the largest store of terrestrial biomass carbon (C) on earth and contribute over-proportionally to global terrestrial net primary productivity (NPP). How climate change is ...affecting NPP and C allocation to tree components in forests is not well understood. This is true for tropical forests, but particularly for African tropical forests. Studying forest ecosystems along elevation and related temperature and moisture gradients is one possible approach to address this question. However, the inclusion of belowground productivity data in such studies is scarce. On Mt. Kilimanjaro (Tanzania), we studied aboveground (wood increment, litter fall) and belowground (fine and coarse root) NPP along three elevation transects (c. 1800–3900 m a.s.l.) across four tropical montane forest types to derive C allocation to the major tree components. Total NPP declined continuously with elevation from 8.5 to 2.8 Mg C ha
−1
year
−1
due to significant decline in aboveground NPP, while fine root productivity (sequential coring approach) remained unvaried with around 2 Mg C ha
−1
year
−1
, indicating a marked shift in C allocation to belowground components with elevation. The C and N fluxes to the soil via root litter were far more important than leaf litter inputs in the subalpine
Erica
forest. Thus, the shift of C allocation to belowground organs with elevation at Mt. Kilimanjaro and other tropical forests suggests increasing nitrogen limitation of aboveground tree growth at higher elevations. Our results show that studying fine root productivity is crucial to understand climate effects on the carbon cycle in tropical forests.
Tropical forests are carbon-dense and highly productive ecosystems. Consequently, they play an important role in the global carbon cycle. In the present study we used an individual-based forest model ...(FORMIND) to analyze the carbon balances of a tropical forest. The main processes of this model are tree growth, mortality, regeneration, and competition. Model parameters were calibrated using forest inventory data from a tropical forest at Mt. Kilimanjaro. The simulation results showed that the model successfully reproduces important characteristics of tropical forests (aboveground biomass, stem size distribution and leaf area index). The estimated aboveground biomass (385 t/ha) is comparable to biomass values in the Amazon and other tropical forests in Africa. The simulated forest reveals a gross primary production of 24 tcha(-1) yr(-1). Modeling above- and belowground carbon stocks, we analyzed the carbon balance of the investigated tropical forest. The simulated carbon balance of this old-growth forest is zero on average. This study provides an example of how forest models can be used in combination with forest inventory data to investigate forest structure and local carbon balances.
The effect-response framework states that plant functional traits link the abiotic environment to ecosystem functioning. One ecosystem property is the body size of the animals living in the system, ...which is assumed to depend on temperature or resource availability, among others. For primary consumers, resource availability may directly be related to plant traits, while for secondary consumers the relationship is indirect. We used plant traits to describe resource availability along an elevational gradient on Mount Kilimanjaro, Tanzania. Using structural equation models, we determined the response of plant traits to changes in precipitation, temperature and disturbance with and assessed whether abiotic conditions or community-weighted means of plant traits are stronger predictors of the mean size of bees, moths, frugivorous birds, and insectivorous birds. Traits indicating tissue density and nutrient content strongly responded to variations in precipitation, temperature and disturbance. They had direct effects on pollination and fruit traits. However, the average body sizes of the animal groups considered could only be explained by temperature and habitat structure, not by plant traits. Our results demonstrate a strong link between traits and the abiotic environment, but suggest that temperature is the most relevant predictor of mean animal body size. Community-weighted means of plant traits and body sizes appear unsuitable to capture the complexity of plant-animal interactions.
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