Woody Species as Landscape Modulators and Their Effect on Biodiversity Patterns

Natural vegetation biomass and the dimension of forest quality in tropical agricultural landscapes

Forest cover has been a pivotal indicator of biological conservation and carrying capacity for wildlife in forest ecoregions. Such a relationship underpins policies focused on the extension of protected lands. Here, we estimate aboveground biomass (AGB) as a proxy for habitat quality in seminatural rural patches and provide a comparison with approaches that only consider forest cover. We hypothesize that recommendations for biological conservation in agricultural landscapes are substantially improved if habitat quality is also taken into account, and thus consider the possibility of forest quality being modulated by land-use amount, type, and age. We assessed AGB in a densely farmed Brazilian region using a straightforward approach designed to be affordable at large scales, focusing on two expanding and contrasting land uses: sugarcane, and eucalyptus plantations. At a detailed scale, we confirmed through field surveys and AGB estimation using 3D-multispectral imagery (i.e., AGB = 0.842 × vegetation heightNDVI+1) that AGB variation could be predicted with forest degradation classes that are visually distinguishable with high-resolution images: 9.33 t ha-1 (90% predictive intervals [PI] = [3.23, 26.97]) in regenerating fields (RF), 31.12 t ha-1 (90% PI = [10.77, 89.90]) in pioneer woods (PW), and 149.04 t ha-1 (90% PI = [51.59, 430.58]) in dense forests (DF). Applying these values to land units sampled across the study region, we found an average land use of 88.5%, together with 11.5% of land set aside for conservation, which reduced AGB to less than 4.2% of its potential (averages of 5.85 t ha-1 in sugarcane-dominated areas and 6.56 t ha-1 in eucalyptus-dominated areas, with secondary forests averaging 149.04 t ha-1). This imbalance between forest cover and AGB resulted from forest quality decay, which was similarly severe among land-use types, ages, and extensions. Therefore, the shortage of trophic resources is likely more critical to wildlife than spatial limitations in vastly deforested tropical ecoregions, where AGB and carbon sinks can be more than doubled just by restoring forests in lands currently spared by agriculture.

Geodiversity impacts plant community structure in a semi-arid region

Geodiversity refers to the variety of geological and physical elements as well as to geomorphological processes of the earth surface. Heterogeneity of the physical environment has an impact on plant diversity. In recent years, the relations between geodiversity and biodiversity has gained attention in conservation biology, especially in the context of climate change. In this study, we assessed the spatial and temporal change in plant's community structure in a semi-arid region, Sayeret Shaked Long Term Ecosystem Research (LTER) station, Israel. Vegetation surveys were conducted on different hillslopes, either with or without rock covers in order to study the spatial trends of hillslope geodiversity. The surveys were conducted for two consecutive years (2016 and 2017), of which the second year was drier and hotter and therefore permitted to investigate the temporal change of plant's community structure. The results of the spatial trends show that (1) geodiversity increases vegetation biodiversity and promotes perennial plants and those of the temporal change show that (2) the positive effect of geodiversity on plants' community structure and species richness is greater in the drier year than that in a wetter year. The main insight is that in these drylands, hillslopes with higher geodiversity appear to buffer the effect of drier years, and supported a more diverse plant community than lower geodiversity hillslopes.

Ant Guild Identity Determines Seed Fate at the Post-Removal Seed Dispersal Stages of a Desert Perennial

Simple Summary

Ants play a dual role in their interaction with plant seeds. Many ant species, mainly harvester ants, consume plant seeds (granivory), whereas other ants, mainly scavengers, provide a beneficial seed dispersal service. Granivory by ants is frequently documented in deserts, but beneficial seed dispersal is rarely studied in such ecosystems. We followed the handling of seeds of Sternbergia clusiana, an ant-dispersed plant, by two guilds of desert ants. We focused on the treatment of seeds within the nest and on the redispersal of seeds after ejection from the nest. Scavenging ants rarely consumed the seed itself, they deposited most seeds away from the nest in apparently suitable microhabitats. In addition, we found that most of the seeds that were relocated by scavenging ants arrived at sites under the canopy of shrubs. Such sites might be beneficial for the establishment and success of plants in the arid environment. Indeed, we found that the subject plant was more likely to be found under shrubs than elsewhere, which suggests that it benefits from being placed there. Such documentation of seed dispersal activity by scavenging ants in arid ecosystems suggests they may be efficient seed dispersers in deserts, as they are in other ecosystems.

Abstract

Ants play a dual role in their interaction with plant seeds. In deserts, the consumption of seeds by granivorous ants is common, whereas mutualistic seed dispersal, often associated with scavenging ants, is rarely documented. We evaluated the contribution of both ant guilds to efficient seed dispersal of an ant-dispersed plant, Sternbergia clusiana, in a desert ecosystem. We presented seed to colonies of three species of desert ants from the Cataglyphis (scavengers) and Messor (granivorous) genera. We recorded seed consumption, ejection from the nest, and seed transportation to potentially beneficial microhabitats. We evaluated microhabitat quality by testing the association between habitat types and the plant at various life stages. As expected, granivores mainly consumed the seeds, whereas scavengers consumed the elaiosome (seed appendage serving as a reward), but left the seeds intact. Moreover, scavenging ants relocated the seeds much further than granivores, mainly to shrub patches. The disproportional distribution of the plant under shrubs at several life stages suggests that this microhabitat is beneficial for the plant. Overall, while granivores seem to mainly harm seed dispersal, we provide the first evidence for the beneficial contribution of scavenging ants in deserts, showing they exhibit the same suite of characteristics that render them efficient seed dispersers in other ecosystems.

Positive ecological effects of wind farms on vegetation in China's Gobi desert

With the rapid development of wind power, there are increasing concerns about the negative ecological effects of its construction and operation. However, previous studies have mainly focused on the effects of wind farms on flying fauna (i.e., birds and bats) or climate change separately from communities or ecosystems, and little attention has been paid to vegetation during wind farm operation. Furthermore, few studies have referred to vulnerable ecosystems with low biomass and biodiversity. In this research, a field study was conducted to investigate the effects of wind farms on the individual traits, community structures and ecosystem functions of Gobi Desert ecosystems. The effects were measured by comparing interfering areas (IAs, located between 40 m and 90 m in the downstream direction of the wind turbine) with non-interfering areas (NIAs, located over 200 m from the wind turbine matrixes). The results showed that (1) plant individuals in IAs were less stressed and in better physiological states than those in NIAs; (2) for community structures, IA plants tended to be shorter and denser and had a higher coverage condition than that of NIA plants; and (3) ecosystem functions in IAs were significantly improved due to the existence of shrubs and higher biomass. Meanwhile, significant correlations were identified between the wind wake caused by the large spinning blades and the community structures. Constructing wind turbines in the Gobi Desert is a win-win strategy that both contributes to the growth of desert vegetation with a favourable microclimate and sufficiently utilizes wind power to produce clean energy.

Symbiotic dinitrogen fixation is seasonal and strongly regulated in water-limited environments

Plants, especially perennials, growing in drylands and seasonally dry ecosystems are uniquely adapted to dry conditions. Legume shrubs and trees, capable of symbiotic dinitrogen (N₂ ) fixation, often dominate in drylands. However, the strategies that allow symbiotic fixation in these ecosystems, and their influence on the nitrogen cycle, are largely unresolved. We evaluated the climatic, biogeochemical and ontogenetic factors influencing nitrogen fixation in an abundant Mediterranean legume shrub, Calicotome villosa. We measured nodulation, fixation rate, nitrogen allocation and soil biogeochemistry in three field sites over a full year. A controlled experiment evaluated differences in plant regulation of fixation as a function of soil nutrient availability and seedling and adult developmental stages. We found a strong seasonal pattern, shifting between high fixation rates during the rainy season at flowering and seed-set times to almost none in the rainless season. Under controlled conditions, plants downregulated fixation in response to soil nitrogen availability, but this response was stronger in seedlings than in adult shrubs. Finally, we did not find elevated soil nitrogen under N₂ -fixing shrubs. We conclude that seasonal nitrogen fixation, regulation of fixation, and nitrogen conservation are key adaptations influencing the dominance of dryland legumes in the community, with broader consequences on the ecosystem nitrogen cycle.

Vegetation demographics in Earth System Models: A review of progress and priorities

Numerous current efforts seek to improve the representation of ecosystem ecology and vegetation demographic processes within Earth System Models (ESMs). These developments are widely viewed as an important step in developing greater realism in predictions of future ecosystem states and fluxes. Increased realism, however, leads to increased model complexity, with new features raising a suite of ecological questions that require empirical constraints. Here, we review the developments that permit the representation of plant demographics in ESMs, and identify issues raised by these developments that highlight important gaps in ecological understanding. These issues inevitably translate into uncertainty in model projections but also allow models to be applied to new processes and questions concerning the dynamics of real-world ecosystems. We argue that stronger and more innovative connections to data, across the range of scales considered, are required to address these gaps in understanding. The development of first-generation land surface models as a unifying framework for ecophysiological understanding stimulated much research into plant physiological traits and gas exchange. Constraining predictions at ecologically relevant spatial and temporal scales will require a similar investment of effort and intensified inter-disciplinary communication.

The evolutionary origins of Lévy walk foraging

We study through a reaction-diffusion algorithm the influence of landscape diversity on the efficiency of search dynamics. Remarkably, the identical optimal search strategy arises in a wide variety of environments, provided the target density is sparse and the searcher’s information is restricted to its close vicinity. Our results strongly impact the current debate on the emergentist vs. evolutionary origins of animal foraging. The inherent character of the optimal solution (i.e., independent on the landscape for the broad scenarios assumed here) suggests an interpretation favoring the evolutionary view, as originally implied by the Lévy flight foraging hypothesis. The latter states that, under conditions of scarcity of information and sparse resources, some organisms must have evolved to exploit optimal strategies characterized by heavy-tailed truncated power-law distributions of move lengths. These results strongly suggest that Lévy strategies—and hence the selection pressure for the relevant adaptations—are robust with respect to large changes in habitat. In contrast, the usual emergentist explanation seems not able to explain how very similar Lévy walks can emerge from all the distinct non-Lévy foraging strategies that are needed for the observed large variety of specific environments. We also report that deviations from Lévy can take place in plentiful ecosystems, where locomotion truncation is very frequent due to high encounter rates. So, in this case normal diffusion strategies—performing as effectively as the optimal one—can naturally emerge from Lévy. Our results constitute the strongest theoretical evidence to date supporting the evolutionary origins of experimentally observed Lévy walks.

How organisms improve the search for food, mates, etc., is a key factor to their survival. Mathematically, the best strategy to look for randomly distributed re-visitable resources—under scarce information and sparse conditions—results from Lévy distributions of move lengths (the probability of taking a step ℓ is proportional to 1/ℓ²). Today it is well established that many animal species in different habitats do perform Lévy foraging. This fact has raised a heated debate, viz., the emergent versus evolutionary hypotheses. For the former, a Lévy foraging is an emergent property, a consequence of searcher-environment interactions: certain landscapes induce Lévy patterns, but others not. In this view, the optimal strategy depends on the particular habitat. The evolutionary explanation, in contrast, is that Lévy foraging strategies are adaptations that evolved via natural selection. In this article, through simulations we exhaustively analyze the influence of distinct environments on the foraging efficiency. We find that the optimal procedure is the same in all situations, provided density is low and landscape information is scarce. So, the best search strategy is remarkably independent of details. These results constitute the strongest theoretical evidence to date supporting the evolutionary origins of experimentally observed Lévy walks.

The influence of species, density, and diversity of macroalgal aggregations on microphytobenthic settlement

Intertidal macroalgae can modulate their biophysical environment by ameliorating physical conditions and creating habitats. Exploring how seaweed aggregations made up of different species at different densities modify the local environment may help explain how associated organisms respond to the attenuation of extreme physical conditions. Using Silvetia compressa, Chondracanthus canaliculatus, and Pyropia perforata, we constructed monocultures representing the leathery, corticated and foliose functional forms as well as a mixed tri-culture assemblage including the former three, at four densities. Treatment quadrats were installed in the intertidal where we measured irradiance, temperature, particle retention, and water motion underneath the canopies. Additionally, we examined the abundance and richness of the understory microphytobenthos with settlement slides. We found that the density and species composition of the assemblages modulated the amelioration of extreme physical conditions, with macroalgal aggregations of greater structural complexity due to their form and density showing greater physical factor attenuation. However, increasing the number of species within a patch did not directly result in increased complexity and therefore, did not necessarily cause greater amelioration of the environment. Microphytobenthic composition was also affected by species composition and density, with higher abundances under S. compressa and C. canaliculatus canopies at high and mid densities. These results support the idea that the environmental modifications driven by these macroalgae have a significant effect on the dynamics of the intertidal environment by promoting distinct temporal and spatial patchiness in the microphytobenthos, with potentially significant effects on the overall productivity of these ecosystems.

Size and Reproductive Traits Rather than Leaf Economic Traits Explain Plant-Community Composition in Species-Rich Annual Vegetation along a Gradient of Land Use Intensity

Agricultural land use imposes a major disturbance on ecosystems worldwide, thus greatly modifying the taxonomic and functional composition of plant communities. However, mechanisms of community assembly, as assessed by plant functional traits, are not well known for dryland ecosystems under agricultural disturbance. Here we investigated trait responses to disturbance intensity and availability of resources to identify the main drivers of changes in composition of semiarid communities under diverging land use intensities. The eastern Mediterranean study region is characterized by an extended rainless season and by very diverse, mostly annual communities. At 24 truly replicated sites, we recorded the frequency of 241 species and the functional traits of the 53 most common species, together with soil resources and disturbance intensity across a land use gradient ranging from ungrazed shrubland to intensively managed cropland (six land use types). Multivariate RLQ analysis (linking functional traits, sites and environmental factors in a three-way ordination) and fourth corner analysis (revealing significant relations between traits and environmental factors) were used in a complementary way to get insights into trait-environment relations. Results revealed that traits related to plant size (reflecting light absorption and competitive ability) increased with resource availability, such as soil phosphorus and water holding capacity. Leaf economic traits, such as specific leaf area (SLA), leaf nitrogen content (LNC), and leaf dry matter content showed low variation across the disturbance gradient and were not related to environmental variables. In these herbaceous annual communities where plants grow and persist for just 3-5 months, SLA and LNC were unrelated, which together with relatively high SLA values might point to strategies of drought escape and grazing avoidance. Seed mass was high both at higher and lower resource availability, whereas seed number increased with the degree of disturbance. The strong response of size and reproduction traits, and the missing response of leaf economic traits reveal light interception and resource competition rather than resource acquisition and litter decomposition as drivers of plant community composition. Deviations from trait relationships observed in commonly studied temperate ecosystems confirm that climatic conditions play a fundamental role by filtering species with particular life forms and ecological strategies.

Vegetation patches improve the establishment of Salvia mexicana seedlings by modifying microclimatic conditions

Human disturbance has disrupted the dynamics of plant communities. To restore these dynamics, we could take advantage of the microclimatic conditions generated by remaining patches of vegetation and plastic mulch. These microclimatic conditions might have great importance in restoring disturbed lava fields located south of Mexico City, where the rock is exposed and the soil is shallow. We evaluated the effects of both the shade projected by vegetation patches and plastic mulch on the mean monthly soil surface temperature (Tss) and photosynthetic photon flux density (PPFD) and on the survival and growth of Salvia mexicana throughout the year. This species was used as a phytometer of microsite quality. Shade reduced the T ss to a greater extent than mulch did. Both survival and growth were enhanced by shade and mulch, and the PPFD was related with seedling growth. During the dry season, plant biomass was lost, and there was a negative effect of PPFD on plant growth. At micro-meteorological scales, the use of shade projected by patches of vegetation and mulch significantly reduced the mortality of S. mexicana and enhanced its growth. Survival and growth of this plant depended on the environmental quality of microsites on a small scale, which was determined by the environmental heterogeneity of the patches and the landscape. For plant restoration, microsite quality must be evaluated on small scales, but on a large scale it may be enough to take advantage of landscape shade dynamics and the use of mulch to increase plant survival and growth.

Periodic versus scale-free patterns in dryland vegetation

Two major forms of vegetation patterns have been observed in drylands: nearly periodic patterns with characteristic length scales, and amorphous, scale-free patterns with wide patch-size distributions. The emergence of scale-free patterns has been attributed to global competition over a limiting resource, but the physical and ecological origin of this phenomenon is not understood. Using a spatially explicit mathematical model for vegetation dynamics in water-limited systems, we unravel a general mechanism for global competition: fast spatial distribution of the water resource relative to processes that exploit or absorb it. We study two possible realizations of this mechanism and identify physical and ecological conditions for scale-free patterns. We conclude by discussing the implications of this study for interpreting signals of imminent desertification.

Poplar plantation has the potential to alter the water balance in semiarid Inner Mongolia

Poplar plantation is the most dominant broadleaf forest type in northern China. Since the mid-1990s plantation was intensified to combat desertification along China's northwestern border, i.e., within Inner Mongolia (IM). This evoked much concern regarding the ecological and environmental effects on areas that naturally grow grass or shrub vegetation. To highlight potential consequences of large-scale poplar plantations on the water budget within semiarid IM, we compared the growing season water balance (evapotranspiration (ET) and precipitation (PPT)) of a 3-yr old poplar plantation (Kp(3)) and a natural shrubland (Ks) in the Kubuqi Desert in western IM, and a 6-yr old poplar plantation (Bp(6)) growing under sub-humid climate near Beijing. The results showed that, despite 33% lower PPT at Kp(3), ET was 2% higher at Kp(3) (228 mm) as compared with Ks (223 mm) in May-September 2006. The difference derived mainly from higher ET at the plantation during drier periods of the growing season, which also indicated that the poplars must have partly transpired groundwater. Estimated growing season ET at Bp(6) was about 550 mm and more than 100% higher than at Kp(3). It is estimated that increases in leaf area index and net radiation at Kp(3) provide future potential for the poplars in Kubuqi to exceed the present ET and ET of the natural shrubland by 100-200%. These increases in ET are only possible through the permanent use of groundwater either directly by the trees or through increased irrigation. This may significantly change the water balance in the area (e.g., high ET at the cost of a reduction in the water table), which renders large-scale plantations a questionable tool in sustainable arid-land management.