Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as ‘dryland mechanisms’. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.

The distribution and stability of biomes are critical for understanding, modeling, and managing the land biosphere. While studies have emphasized abiotic factors such as climate, geology, or fire regimes, we here identify a biological mechanism—plant–plant competition for belowground resources—as critical for maintaining the boundary between the Fynbos and Afrotemperate Forest biomes in South Africa. We demonstrate an apparent general mechanism in which local competition triggers a biome-scale feedback between plant traits and soil resources, which, in turn, stabilizes the biome boundary by allowing plants to maintain their own preferred soil conditions. Our findings are of general importance for understanding the organization of biodiversity across landscapes, for managing alien plant invasions, and for modeling the future of biome boundaries. Recent findings point to plant root traits as potentially important for shaping the boundaries of biomes and for maintaining the plant communities within. We examined two hypotheses: 1) Thin-rooted plant strategies might be favored in biomes with low soil resources; and 2) these strategies may act, along with fire, to maintain the sharp boundary between the Fynbos and Afrotemperate Forest biomes in South Africa. These biomes differ in biodiversity, plant traits, and physiognomy, yet exist as alternative stable states on the same geological substrate and in the same climate conditions. We conducted a 4-y field experiment to examine the ability of Forest species to invade the Fynbos as a function of growth-limiting nutrients and belowground plant–plant competition. Our results support both hypotheses: First, we found marked biome differences in root traits, with Fynbos species exhibiting the thinnest roots reported from any biome worldwide. Second, our field manipulation demonstrated that intense belowground competition inhibits the ability of Forest species to invade Fynbos. Nitrogen was unexpectedly the resource that determined competitive outcome, despite the long-standing expectation that Fynbos is severely phosphorus constrained. These findings identify a trait-by-resource feedback mechanism, in which most species possess adaptive traits that modify soil resources in favor of their own survival while deterring invading species. Our findings challenge the long-held notion that biome boundaries depend primarily on external abiotic constraints and, instead, identify an internal biotic mechanism—a selective feedback among traits, plant–plant competition, and ecosystem conditions—that, along with contrasting fire regime, can act to maintain biome boundaries.

Mineral nutrition is essential for optimal plant growth. Phosphorus (P) is a relatively small component of leaf dry weight, with a concentration in plant foliage of less than 1%. Despite its low concentration, P is an essential element in plants, mainly used for energy transfer. Mapping P concentration using traditional methods is expensive and usually limited to a small area; it is time-consuming and covers only a few plant individuals or species. In this study, we demonstrate the use of remote-sensing (RS) data acquired from the feld and airborne hyperspectral sensors to predict and map the P concentration in leaves of different woody Mediterranean plant species. Comprehensive field work included leaf sampling, laboratory analyses, and spectral measurements using a visible, near-infrared and shortwave-infrared (VIS-NIR-SWIR) field spectrometer. Using different spectral configurations, we built accurate models to predict P concentration in leaf samples. The models were built using a NIR data analysis technique with the data mining software PARACUDA II. This software allowed us to identify the correlative wavelengths for P-bearing molecules in selected woody Mediterranean plant species. The hyperspectral-based model for leaf P concentration was extracted from the reflectance data acquired using a manned aircraft carrying a hyperspectral sensor (Specim AisaFenix 1 K). The model gave a reliable correlation between points extracted from the hyperspectral image and samples measured in the field. We believe that the methodology used in this study will help forest ecologists better understand the concentration of P in the foliage of woody Mediterranean plant species.

Aims In plant eco-physiology, less negative (enriched) carbon 13 (C-13) in the leaves indicates conditions of reducing leaf gas exchange through stomata, e.g. under drought. In addition, C-13 is expected to be less negative in non-photosynthetic tissues as compared with leaves. However, these relationships in delta C-13 from leaves (photosynthetic organs) to branches, stems and roots (non-photosynthetic organs) are rarely tested across multiple closely related tree species, multiple compartments, or in trees growing under extreme heat and drought. Methods We measured leaf-to-root C-13 in three closely related desert acacia species (Acacia tortilis, A. raddiana and A. pachyceras). We measured delta C-13 in leaf tissues from mature trees in southern Israel. In parallel, a 7-year irrigation experiment with 0.5, 1.0 or 4.0 L day(-1) was conducted in an experimental orchard. At the end of the experiment, growth parameters and delta C-13 were measured in leaves, branches, stems and roots. Important Findings The delta C-13 in leaf tissues sampled from mature trees was ca. -27 parts per thousand, far more depleted than expected from a desert tree growing in one of the Earth's driest and hottest environments. Across acacia species and compartments, delta C-13 was not enriched at all irrigation levels (-28 parts per thousand to ca. -27 parts per thousand), confirming our measurements in the mature trees. Among compartments, leaf delta C-13 was unexpectedly similar to branch and root delta C-13, and surprisingly, even less negative than stem delta C-13. The highly depleted leaf delta C-13 suggests that these trees have high stomatal gas exchange, despite growing in extremely dry habitats. The lack of delta C-13 enrichment in non-photosynthetic tissues might be related to the seasonal coupling of growth of leaves and heterotrophic tissues.

Atmospheric correction (ATC) of radiance image data is a preliminary and necessary procedure to reach a coherent unsupervised classification. Though ATC results in removal of noise artefacts related to path radiance, loss of some data is inherent by the process. The unsupervised principal component analysis-based classification (PCABC) was harnessed in this paper using radiance data that bypass the ATC protocol. Being primarily based on the variability of the input hyperspectral remote sensing (HRS) image regardless of its physical attributes, it was assumed that PCABC can be applied to radiance HRS image just as already shown on reflectance domain. To test this assumption, PCABC was tested on a radiance HRS image of Specim's AisaFENIX taken over the Mediterranean forest of Mount Horshan, Israel. With no application of ATC or noise reduction, while tested unsupervised classification methods were insufficient, PCABC was able to classify four different plant species with an overall accuracy of 68%.

In Mediterranean grasslands, the composition of vegetation and its nutritional quality for animals are strongly affected by the climatic conditions prevailing during winter and spring. Therefore, these seasonal ecosystems provide an opportunity to examine how variability in climatic conditions affects the regeneration and quality of pasture vegetation. The intensity of grazing in this seasonal system can moderate, or alternatively exacerbate, climatic effects on the nutritional quality of the vegetation. Herein, we analyzed the interactive effects of climate variables, grazing intensity, and grazing exclusion on herbage quality parameters using long-term vegetation and climate data collected during 2005-2018 from an extensive experiment in Galilee, Israel. We evaluated the contribution of different climate variables to the prediction of herbage quality parameters. Our results showed that climate variables have a dramatic effect on herbage quality and that this effect interacts with grazing intensity. Herbage quality improved in temperate rainy years compared to warm and dry years. High grazing intensity improved herbage quality under temperate climate conditions, but this effect was moderated or completely disappeared as winter conditions become warmer and drier. The results of the study foresee negative effects of warming and drying on the carrying capacity of natural pastures.

Previous observational analyses have shown a declining rainfall trend over Israel, mostly statistically insignificant. The current study, for the period 1975-2020, undermines these findings, and the alarming future projections, and elaborates other ingredients of the rain regime. No trend is found for the annual rainfall, reflecting a balance between a negative trend in the number of rainy days and a positive trend in the daily rainfall intensity, both on the order of 2.0%/decade. In the mid-winter, the rainfall and the daily intensity increased, while both declined in the autumn and spring, implying a contraction of the rainy season. The time span between accumulation of 10% and 90% of the annual rainfall, being 112 days on the average, shortened by 7 days during the study period. This is also expressed by an increase of the Seasonality Index, indicating that the regional climate is shifting from ``markedly seasonal with a long dry season'' to ``most rain in <= 3 months.'' The intra-seasonal course of the rainfall trend corresponds to that of the occurrence and intensity of the Cyprus Lows and the Mediterranean Oscillation. The contraction of the rainy season and the increase in the daily intensity have far-reaching environmental impacts in this vulnerable region.

Discriminating between woody plant species using a single image is not straightforward due to similarity in their spectral signatures, and limitations in the spatial resolution of many sensors. Seasonal changes in vegetation indices can potentially improve vegetation mapping; however, for mapping at the individual species level, very high spatial resolution is needed. In this study we examined the ability of the Israel/French satellite of VEN mu S and other sensors with higher spatial resolutions, for identifying woody Mediterranean species, based on the seasonal patterns of vegetation indices (VIs). For the study area, we chose a site with natural and highly heterogeneous vegetation in the Judean Mountains (Israel), which well represents the Mediterranean maquis vegetation of the region. We used three sensors from which the indices were derived: a consumer-grade ground-based camera (weekly images at VIS-NIR; six VIs; 547 individual plants), UAV imagery (11 images, five bands, seven VIs) resampled to 14, 30, 125, and 500 cm to simulate the spatial resolutions available from some satellites, and VEN mu S Level 1 product (with a nominal spatial resolution of 5.3 m at nadir; seven VIs; 1551 individual plants). The various sensors described seasonal changes in the species' VIs at different levels of success. Strong correlations between the near-surface sensors for a given VI and species mostly persisted for all spatial resolutions <= 125 cm. The UAV ExG index presented high correlations with the ground camera data in most species (pixel size <= 125 cm; 9 of 12 species with R >= 0.85; p < 0.001), and high classification accuracies (pixel size <= 30 cm; 8 species with >70%), demonstrating the possibility for detailed species mapping from space. The seasonal dynamics of the species obtained from VEN mu S demonstrated the dominant role of ephemeral herbaceous vegetation on the signal recorded by the sensor. The low variance between the species as observed from VEN mu S may be explained by its coarse spatial resolution (effective ground spatial resolution of 7.5) and its non-nadir viewing angle (29.7 degrees) over the study area. However, considering the challenging characteristics of the research site, it may be that using a VEN mu S type sensor (with a spatial resolution of similar to 1 m) from a nadir point of view and in more homogeneous and dense areas would allow for detailed mapping of Mediterranean species based on their seasonality.

The potential impacts of species colonization on the structure and functioning of ecosystems are poorly understood. We propose a novel approach for understanding the consequences of habitat colonization, highlighting the influence of colonists on the availability of limiting resources to resident species. We studied how colonization of dry oak woodlands by pines (Pinus halepensis) is affecting water stress of resident oaks (Quercus calliprinos). We monitored predawn leaf water potential (PLWP) of oaks monthly for 2 years. Using maximum likelihood and multi-model inference, we evaluated how the PLWP of oaks was affected by pine colonists. The influence of colonizing pines on PLWP of resident oaks varied in time and space from negative to positive depending on season, oak size, pine size, and proximity to pines presence. The water stress of oaks increased along the dry season (- 1.5 to - 4.5 MPa), with small oaks becoming more severely stressed than large ones (up to 60% difference). During the dry season, neighboring pine colonists increased the water stress of oaks (up to - 0.4 MPa difference), but during the wet season, they reduced the water stress mainly for large oaks. Our findings indicate that pine colonization differentially affects water limitation for resident oaks with implications for future development and regeneration. The influence of pine colonists shifted from positive to negative along an increasing water stress gradient, contrary to predictions by the stress gradient hypothesis. Our work demonstrates how colonization by non-resident species can influence key ecosystem processes through the redistribution of limiting resources. Identifying these processes is fundamental for understanding the consequences of colonization, mitigating these influences, and predicting future change in the structure and function of ecosystems.

Leaf nitrogen concentration often is higher in leguminous plants, which associate with dinitrogen-fixing bacteria, compared with nonlegume plants. However, the range of nitrogen concentrations in legumes is wide, likely related to the range of nitrogen fixation strategies. We evaluated how carbon and nitrogen allocation to roots, stems and leaves is influenced by the type of strategy of nitrogen fixation regulation. We grew herbaceous annual legumes (Medicago truncatula, Hymenocarpos circinnatus and Vicia palaestina) under two nitrogen availability treatments (none/sufficient), with and without bacterial inoculation. We found facultative downregulation of the rate of nitrogen fixation when nitrogen was available in H. circinnatus, and an obligate similar fixation rate in both nitrogen treatments in M. truncatula and V. palaestina. Uninoculated plants invested more biomass in roots and contained lower nitrogen concentrations. However, nitrogen concentration in the entire plant and in the leaves was lower and more plastic in the species with a facultative fixation strategy, whereas species with an obligate fixation strategy also maintained high nitrogen concentrations. Our results suggest a suite of functional traits associated with the strategies of allocation and symbiotic nitrogen fixation. This suite of traits probably shapes successional and functional niches of different leguminous species in specious plant communities.

Remote-sensing tools and satellite data are often used to map and monitor changes in vegetation cover in forests and other perennial woody vegetation. Large-scale vegetation mapping from remote sensing is usually based on the classification of its spectral properties by means of spectral Vegetation Indices (VIs) and a set of rules that define the connection between them and vegetation cover. However, observations show that, across a gradient of precipitation, similar values of VI can be found for different levels of vegetation cover as a result of concurrent changes in the leaf density (Leaf Area Index-LAI) of plant canopies. Here we examine the three-way link between precipitation, vegetation cover, and LAI, with a focus on the dry range of precipitation in semi-arid to dry sub-humid zones, and propose a new and simple approach to delineate woody vegetation in these regions. By showing that the range of values of Normalized Difference Vegetation Index (NDVI) that represent woody vegetation changes along a gradient of precipitation, we propose a data-based dynamic lower threshold of NDVI that can be used to delineate woody vegetation from non-vegetated areas. This lower threshold changes with mean annual precipitation, ranging from less than 0.1 in semi-arid areas, to over 0.25 in mesic Mediterranean area. Validation results show that this precipitation-sensitive dynamic threshold provides a more accurate delineation of forests and other woody vegetation across the precipitation gradient, compared to the traditional constant threshold approach.

In recent years, hyperspectral remote sensing (HRS) has become common practice for remote analyses of the physiognomy and composition of forests. Supervised classification is often used for this purpose, but demands intensive sampling and analyses, whereas unsupervised classification often requires information retrieval out of the large HRS datasets, thereby not realizing the full potential of the technology. An improved principal component analysis-based classification (PCABC) scheme is presented and intended to provide accurate and sequential image-based unsupervised classification of Mediterranean forest species. In this study, unsupervised classification and reduction of data size are performed simultaneously by applying binary sequential thresholding to principal components, each time on a spatially reduced subscene that includes the entire spectral range. The methodology was tested on HRS data acquired by the airborne AisaFENIX HRS sensor over a Mediterranean forest in Mount Horshan, Israel. A comprehensive field-validation survey was performed, sampling 257 randomly selected individual plants. The PCABC provided highly improved results compared to the traditional unsupervised classification methodologies, reaching an overall accuracy of 91%. The presented approach may contribute to improved monitoring, management, and conservation of Mediterranean and similar forests. © 2019 by the authors.

Plants, especially perennials, growing in drylands and seasonally dry ecosystems are uniquely adapted to dry conditions. Legume shrubs and trees, capable of symbiotic dinitrogen (N 2 ) 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 N 2 -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. © 2018 The Authors. New Phytologist © 2018 New Phytologist Trust

Abstract The importance of symbiotic dinitrogen (N2) fixation in shaping the coupled nitrogen–carbon cycle is now known for most humid terrestrial ecosystems. However, whether N2 fixation can play a key role in the nitrogen and carbon budget of water-limited and seasonally dry ecosystems remains a mystery. The maintenance of metabolically and physiologically costly symbiotic fixation in water-limited environments is highly complex. These costs are particularly high during the first developmental season, when allocation to deep rooting and drought resistance mechanisms is essential for seedling survival of prolonged seasonal drought. We, therefore, evaluated how drought-adapted legume species change their allocation to symbiotic nitrogen fixation as a function of soil nitrogen availability. We tested this on seedlings of a suite of four common Mediterranean legume shrubs with a strong seasonal behaviour, which we grew under controlled nitrogen and phosphorus availabilities. We asked: (1) Do species differ in their investment and regulation of nitrogen fixation? (2) Is fixation regulated via plant allocation to nodules, fixation rate or both? and (3) Does phosphorus availability limit symbiotic nitrogen fixation? All Mediterranean perennial legumes in the experiment established and grew, nodulated, and fixed nitrogen, even under severe nitrogen limitation. The four species reacted similarly to nitrogen supply, by strongly downregulating fixation through both decreased nodulation and lower rate of fixation. However, we found a significant interspecific difference in fixation (both nodulation and rate), biomass production and growth rate. Our experimental species presented a range of fixation investment strategies corresponding to life-history and resource partitioning patterns. Phosphorus limitation had a minor influence on both fixation and plant growth. Synthesis. The high physiological cost of symbiotic fixation imposes the need to tightly regulate fixation in perennial legumes coping with severe water stress. Control of fixation allows legume species to colonize recently disturbed nitrogen-deficient habitats, cope with grazing, survive long seasonal droughts and recover nitrogen fixation later in the wet season, and survive over time by reducing nitrogen inputs to the ecosystem.

Empirically validated mathematical models show that a combination of intraspecific competition between subterranean social-insect colonies and scale-dependent feedbacks between plants can explain the spatially periodic vegetation patterns observed in many landscapes, such as the Namib Desert ‘fairy circles’.