Bertrand Boeken

Plant species richness and ecosystem multifunctionality in global drylands

Global Ecosystem Analysis The relationship between species richness and the functional properties of their ecosystems has often been studied at small scales in experimental plots. Maestre et al. (p. 214; see the Perspective by Midgley) performed field measurements at 224 dryland sites from six continents and assessed 14 ecosystem functions related to carbon, nitrogen, and phosphorus cycling. Positive relationships were observed between perennial plant species richness and ecosystem functionality. The relative importance of biodiversity was found to be as large as, or larger than, many key abiotic variables. Thus, preservation of plant biodiversity is important to buffer negative effects of climate change and desertification in drylands, which collectively cover 41% of Earths land surface and support over 38% of the human population. Plant species richness is positively related to ecosystem multifunctionality in drylands at a global scale. Experiments suggest that biodiversity enhances the ability of ecosystems to maintain multiple functions, such as carbon storage, productivity, and the buildup of nutrient pools (multifunctionality). However, the relationship between biodiversity and multifunctionality has never been assessed globally in natural ecosystems. We report here on a global empirical study relating plant species richness and abiotic factors to multifunctionality in drylands, which collectively cover 41% of Earth’s land surface and support over 38% of the human population. Multifunctionality was positively and significantly related to species richness. The best-fitting models accounted for over 55% of the variation in multifunctionality and always included species richness as a predictor variable. Our results suggest that the preservation of plant biodiversity is crucial to buffer negative effects of climate change and desertification in drylands.

The germination of mucilaginous seeds of t Plantago coronopus, Reboudia pinnata, and t Carrichtera annua on cyanobacterial soil crust from the Negev Desert

We studied the effect of intact, crushed or sterile cyanobacterial soil crust from the Negev Desert highlands of Israel as substrates for the germination of seeds of three annual plant species from local populations that produce mucilaginous seeds t Plantago coronopus, Reboudia pinnata and t Carrichtera annua. Mucilaginous seeds of these species were wetted on local intact cyanobacterial soil crust which inhibited their germination in comparison with their germination on filter paper. However, the percentage of germination of each species differed on sterile and crushed soil crust after 72 h of wetting. Germination of t Plantago coronopus seeds was inhibited mechanically and biologically on intact soil crust, since it was significantly higher on both crushed and sterile soil crust. Germination of t Reboudia pinnata seeds was slightly mechanically and biologically inhibited on intact soil crust, but germination on live crushed crust was higher than on sterile crushed crust. By contrast, germination of t Carrichtera annua seeds was not mechanically inhibited by live soil crust but there was significant biological acceleration of germination on intact soil crust relative to crushed soil crust. Each of the three species exhibited different requirements for germination. Germination of t P. coronopus is promoted by disturbances and removing the crust components (including its biological and structural effects). t R. pinnata and t C. annua germination is less affected by crust disturbance but is generally inhibited after removing the live components of the cyanobacterial crust.These species differences suggest different functions of the mucilage of the seed coat for the three species.

Woody Species as Landscape Modulators and Their Effect on Biodiversity Patterns

ABSTRACT Ecological research on organism-environment interactions has developed asymmetrically. Modulation of organisms by the environment has received much attention, while theoretical studies on the environmental impact of organisms have until recently been limited. We propose a theoretical framework for studying the environmental impacts of woody plants in order to understand their effects on biodiversity. We adopt pattern formation theory to discuss how woody plants organize ecological systems on the patch and landscape levels through patch formation, and how organism patchiness creates resource patchiness that affects biodiversity. We suggest an integrative model that links organisms as landscape modulators through resource distribution and species filtering from larger to smaller spatial scales. Our “biodiversity cycling hypothesis” states that in organism-modulated landscapes, disturbance enables the coexistence of different developmental stages of vegetation patches, thereby increasing biodiversity. This hypothesis emphasizes that species and landscape diversity vary with the development, renewal, maturation, and decay of biotically induced patches.

Annual plant community responses to density of small-scale soil disturbances in the Negev Desert of Israel

Abstract We investigated whether plant diversity and productivity in small-scale soil disturbances, which is known to be higher than in undisturbed soil, decreases as the density of the disturbances increases. We studied this in an experiment with soil diggings (15 cm diameter and 15 cm depth) dug at a range of densities, on a north- and a south-facing slope of a watershed in the central Negev Desert of Israel. The diggings were similar to the commonly occurring pits made by porcupines (Hystrix indica) as they forage for below-ground plant parts. We used four levels of digging density, within the naturally occurring range in the region, represented by a rectangular plot with rows of diggings dug at four distances between diggings. The plots were laid out in a blocked design with three replications on both slopes, with each block containing all four levels of digging density. In the spring of 1992, 1994 and 1995 we measured plant density, species richness and plant productivity in the diggings, and in adjacent equal-sized undisturbed control areas (“soil matrix”) and on the mounds made by the removed excess soil. Plant density, species richness and productivity of annual plants were higher in the diggings than in the undisturbed matrix, while these responses were very low on the mounds. Plant density, species richness and productivity in the diggings, but not in the matrix or mounds, decreased as digging density increased. This effect varied slightly with location within a watershed and with annual rainfall. The density of seeds captured in the diggings from outside the digging during the 1995 dispersal season decreased with increasing digging density, but only on one of the slopes. At the highest digging density, plant density and species number in the diggings did not decrease down the slope, as expected if interference between diggings in runoff water capture were the cause of the digging density effect. There was a weak decrease in biomass production in 1994–1995 down the slope. We used a simple mathematical model to estimate whether the distribution of rainfall intensities that occurred during the winter of 1994–1995 could result in differences between digging densities in the amount of water captured by the diggings, and whether this could explain the observed effect of digging density. The model showed that there were four events during which less water was captured by the diggings at high digging densities, except in the topmost row of diggings. Soil moisture measurements, however, showed very little difference between diggings at different digging densities. We explain our findings as the result of the interaction between the properties of the disturbance patch with its surroundings, as the diggings capture resources in the form of runoff water, and seeds moved primarily by wind. The additional resources and seeds captured in diggings increase plant density, species richness and productivity relative to the undisturbed matrix. However, the contrast in plant responses between the disturbed patches and undisturbed soil diminishes at higher digging densities. We explain this as interference among diggings at close proximity. As we did not detect a decrease in plant responses down the slopes, we conclude that interference is due to interception of the wind-driven, non-directional flow of seeds. Interception of the down-slope flow of runoff water by upslope diggings is insufficient to affect plant density, determined at the beginning of the season. Later in the season, runoff interception may become important for biomass production.

Environmental factors affecting dispersal, germination and distribution of Stipa capensis in the Negev Desert, Israel

The effects of postmaturation ambient temperatures, light, as well as chemical and structural properties of the substrate on germination and patch distribution of Stipa capensis in a shrubland landscape were studied. This species is a dominant annual grass in exposed intershrub areas covered with biological loess soil crusts in the northern Negev Desert.Freshly matured caryopses do not germinate. After 7–8 months of dry storage at high temperatures, there was a significant reduction in primary dormancy and an increase in the rate and percentage of germination of caryopses stored at high temperatures. The speed and percentages of germination were lower on various sterile substrates, such as organic matter and loose loess soil, in comparison with similar live substrates or the control on filter paper.In 3 of 4 years of field observations in semi-arid shrubland, the density of Stipa capensis plants on soil crust in the exposed intershrub areas was significantly higher than below shrubs. The positive effects of higher temperatures on the dry soil during summer, before the season with rains, and of light during germination, can favor germination in exposed patches.Although germination responses to storage temperature and light regime differentiate between patch types before and during germination, other processes may be critical for the pattern of distribution of Stipa capensis. These include dispersal of caryopses, arrival, soil penetration and density of local seed banks, as well as substrate properties affecting germination and plant density, in different landscape patches.

Linking Community and Ecosystem Processes: The Role of Minor Species

The link between species and ecosystem functioning is a central issue in ecology. In natural plant communities, the dominant species determine most of the productivity-related processes but what is the function of minor species? A recent hypothesis suggests that after disturbance, minor species facilitate the recruitment and abundance of dominants during re-colonization, thus indirectly determining ecosystem function. We tested this hypothesis using a long-term dataset of annual plant communities in a semiarid shrubland by comparing plant density and biomass from plots in which all vegetation had been removed; plots from which only the dominant (the annual grass Stipa capensis) had been removed, and control plots. In the absence of vegetation, the dominant failed to re-establish during the following growing season. After being removed the dominant re-established similarly to the controls. An ant exclosure experiment excluded the possibility that this was due to seed predation. In an experiment with individual dispersal units of S. capensis, we demonstrated the mechanism by which minor species can control the dominant’s abundance. Minor species indirectly govern ecosystem processes by providing structures facilitating seed soil penetration and thus recruitment of the dominant.

Life histories of desert geophytes―the demographic consequences of reproductive biomass partitioning patterns

SummaryDuring five consecutive growing seasons (winters) ca. 110 plants of the desert geophyte Bellevalia desertorum were marked individually in a 25 m2 plot on a south-facing slope in the central Negev Desert of Israel. The number of rosette leaves of each plant was recorded, as well as whether it flowered and produced seeds. Multiple regression analysis of the data on the B. desertorum individuals showed that the reproductive state of a plant was determined by 1) its previous size and 2) the current conditions (rainfall), but not by previous conditions, nor by previous reproductive activity. Plant surveys supported these findings. These demographic results were consistent with the current understanding of the reproductive resource allocation pattern of B. desertorum. Flowering was most affected by rainfall until January of the same season, the number of leaves by rainfall until March and seed set was by the total annual amount. Surveys in populations ofB. eigii, a species of more mesic habitats in the Negev Desert suggested that in this species there is a negative effect of previous reproduction in combination with the previous and current conditions, which is also expected from its biomass partitioning pattern.The significance of demographic studies of individual plants in natural populations for the interpretation of experimentally determined resource partitioning patterns was discussed.

Interactions of Biological Soil Crusts with Vascular Plants

Biocrusts and vascular plants interact on many levels. The nature and consequences of these interactions vary with biocrust and plant characteristics and environmental conditions and throughout the plants’ life cycle. Biocrust structure and surface texture—shaped by its species composition and the environment—interacting with seed shape and size, determine whether the crust facilitates or deters seed capture and thus seedling establishment. In general, biocrusts tend to enhance plant growth through improved availability of nutrients, but root architecture plays a role in determining the effect of crusts on nutrient uptake. Furthermore, exchange of nutrients between biocrusts and vascular plants can occur through different pathways, including fungal linkages. Vascular plant communities also affect biocrust development, composition, and function through canopy shading, litterfall, and root activity and their effects on microclimate. The vascular plant canopy tends to favor certain biocrust species groups over others and usually enhances biocrust formation; however, a dense canopy can deprive crusts of adequate light for photosynthesis. Likewise, light litterfall may protect or favor biocrusts by improving the microclimatic conditions, while heavy litterfall can bury, damage, or destroy the crusts.

Flowering Affected by Daylength and Temperature in the Leafless Flowering Desert Geophyte Colchicum tunicatum, Its Annual Life Cycle and Vegetative Propagation

Colchicum tunicatum has an underground corm with three fleshy stalk internodes. The corm is replaced annually by a bud in the axil of the first foliage leaf. Sometimes the second or third axillary bud also develops into an independent shoot. Flowering takes place during autumn, before the onset of rain, when the leaves are still very small and positioned deep in the soil. At the time of leaf appearance during winter, the new replacement buds are visible on the newly developing corm. Corms of C. tunicatum were planted 10 cm below the soil surface outdoors at Sede Boqer in the area where C. tunicatum occurs naturally. The plants received 9, 12, 15, or 18 h light/day from sunlight, augmented with artificial low-level light after 4 P.M. Photoperiod affected flowering. The shorter the daylength, the earlier flowering started, as long as the daily minimum soil temperature was above 29 C.

Life histories of two desert species of the bulbous genus Bellevalia

SummaryAnnual biomass increment and biomass partitioning to leaves, roots and reproduction, and biomass storage in the below-ground bulb was measured in plants of two species of the geophytic genus Bellevalia grown outdoors at three levels of soil moisture. The differences between the species were in accordance with the hypothesis that plants of more arid environments should rely more on internal reserves than plants of more productive environments. In Bellevalia desertorum, a shallow rooted species of the most arid habitats in the Central Negev, leaf and root development during outgrowth at the beginning of winter was rather variable, and followed soil moisture availability to a certain degree. A small portion of its biomass budget was committed to seed production, which varied little among the irrigation regimes. The rest of the biomass was stored in the bulb. The amount of biomass devoted annually to reproduction was mainly determined by the amount of reserves already present in the bulb. In contrast, in B. eigii, which grows in the more productive wadis with its bulb at a depth of 15 to 30 cm, leaf and root growth was not only determined by water availability, but also by initial bulb mass. This resulted in a greater potential relative growth rate than in B. desertorum, but also in a greater risk of accumulating less biomass than it spent in root and leaf construction under poor soil moisture conditions. In this species, reproductive biomass and seed yield were proportional to current biomass gain and, in contrast to B. desertorum, independent of initial bulb mass, provided that the bulb was large enough to initiate flowering.