Increased Soil Water Availability Research Articles

Expression of the limited‐transpiration trait under high vapour pressure deficit in peanut populations: Runner and virginia types

AbstractDrought can be a critical limitation on peanut yield. A physiological trait that may help to ameliorate drought is limited transpiration (TRlim), defined as a limitation on further increases in transpiration rate (TR) under high vapour pressure deficit (VPD) conditions. The advantage of the TRlim trait is that it allows plant water conservation to increase soil water availability for use during late‐season drought. While this trait has been identified in peanut, there is no information of how readily the trait may be transfer to progeny lines. The objective of this study was to obtain preliminary information on the expression of the TRlim trait in two peanut progeny populations. One population was runner type of 88 RILs derived from the mating of Tifrunner × NC 3033. The second set was selected for the virginia‐type phenotype of large pods obtained from mating of PI 585005 (ICGV 86015) and N0808olJCT, both of which expressed the TRlim trait. A two‐tier screen was applied to both populations. The initial screen was based on exposure of de‐rooted shoots to silver ions. Fifteen runner type and 12 virginia type were selected for direct measures of transpiration response to varying VPD. The results from each of the two populations showed that an effective expression of the TRlim trait occurred in about 30% of the progeny in each population. While these results do not offer a definitive index of inheritance, they do indicate that there appears to be a strong possibility of transferring the TRlim trait to progeny genotypes.

Presence of Trifolium repens Promotes Complementarity of Water Use and N Facilitation in Diverse Grass Mixtures.

Legume species promote productivity and increase the digestibility of herbage in grasslands. Considerable experimental data also indicate that communities with legumes produce more above-ground biomass than is expected from monocultures. While it has been attributed to N facilitation, evidence to identify the mechanisms involved is still lacking and the role of complementarity in soil water acquisition by vertical root differentiation remains unclear. We used a 20-months mesocosm experiment to investigate the effects of species richness (single species, two- and five-species mixtures) and functional diversity (presence of the legume Trifolium repens) on a set of traits related to light, N and water use and measured at community level. We found a positive effect of Trifolium presence and abundance on biomass production and complementarity effects in the two-species mixtures from the second year. In addition the community traits related to water and N acquisition and use (leaf area, N, water-use efficiency, and deep root growth) were higher in the presence of Trifolium. With a multiple regression approach, we showed that the traits related to water acquisition and use were with N the main determinants of biomass production and complementarity effects in diverse mixtures. At shallow soil layers, lower root mass of Trifolium and higher soil moisture should increase soil water availability for the associated grass species. Conversely at deep soil layer, higher root growth and lower soil moisture mirror soil resource use increase of mixtures. Altogether, these results highlight N facilitation but almost soil vertical differentiation and thus complementarity for water acquisition and use in mixtures with Trifolium. Contrary to grass-Trifolium mixtures, no significant over-yielding was measured for grass mixtures even those having complementary traits (short and shallow vs. tall and deep). Thus, vertical complementarity for soil resources uptake in mixtures was not only dependant on the inherent root system architecture but also on root plasticity. We also observed a time-dependence for positive complementarity effects due to the slow development of Trifolium in mixtures, possibly induced by competition with grasses. Overall, our data underlined that soil water resource was an important driver of over-yielding and complementarity effects in Trifolium-grass mixtures.

Periodicity of different phenophases in selected trees from Himalayan Terai of India

The temporal phenology of leaf bud bursting, leaf initiation, leaf maturation, leaf fall, flower initiation, fruit initiation and fruit fall was recorded for eight tree species (Ficus hispida, F. squamosa, Mallotus nudiflorus, M. philippensis, Shorea robusta, Schleichera oleosa, Pongamia pinnata and Terminalia arjuna) in the tropical moist deciduous forest of Himalayan Terai region from November 2009 to October 2012. The study revealed that a short low temperature dry period, sufficient winter rain and temperature rise are the triggering factors for summer leaf flushing and, the increased soil water availability for second leaf flushing in rains. Initiation of flowering with leaf emergence in the dry period supports higher rate of fruit setting due to maximum availability and activities of pollinators. The dispersal and post dispersal success of fruits increased by their ripening before and/or in the rainy season. In T. arjuna, no correlation has been found between the fruiting and rains. M. nudiflorus and M. philippensis initiate leaves and flowers with the first significant increase in temperature and photoperiod and are thus suggested as the potential tree species for climate change studies in tropics.

Emerging seed enhancement technologies for overcoming barriers to restoration

Rangelands occupy over a third of global land area, and in many cases are in less than optimum condition as a result of past land use, catastrophic wildfire, and other disturbances, invasive species, or climate change. Often the only means of restoring these lands involves seeding desirable species, yet there are few cost effective‐seeding technologies, especially for the more arid rangeland types. The inability to consistently establish desired plants from seed may indicate that seeding technologies being employed are not successful in addressing the primary sources of mortality in the progression from seed to established plant. Seed enhancement technologies allow for the physical manipulation and application of materials to the seed that can enhance germination, emergence, and/or early seedling growth. In this article, we examine some of the major limiting factors impairing seedling establishment in North America's sagebrush steppe ecosystem and propose seed enhancement technologies that may have the potential to overcome these restoration barriers. We discuss specific technologies for: (1) increasing soil water availability; (2) enhancing seedling emergence in crusting soil; (3) controlling the timing of seed germination; (4) improving plantability and emergence of small‐seeded species; (5) enhancing seed coverage of broadcasted seeds; and (6) protecting seedlings from pre‐emergent herbicide. Concepts and technologies in this article for restoring the sagebrush steppe ecosystem may apply generally to semiarid and arid rangelands around the globe.

Serpentine ecosystem responses to varying water availability and prescribed fire in the U.S. Mid‐Atlantic region

Grasslands growing atop serpentine bedrock are subject to edaphic stresses and, as a result, are low productivity ecosystems. Nutrient limitations are so severe in some serpentine grasslands that plant growth is unaffected by increased water availability. However, little is known of how serpentine grasslands in eastern North America function and respond to environmental changes, including variation in water availability. Serpentine barrens of the U.S. Mid‐Atlantic region are globally rare ecosystems currently threatened by forest encroachment. Prescribed fire has been applied to maintain these ecosystems, although ecosystem responses to prescribed fire remain unquantified. Monthly measurements of CO2 exchange and leaf area index (LAI) were made at managed and unmanaged sites over two years. Environmental factors influencing CO2 exchange rates were monitored, and soil composition was also assessed. Unusually dry conditions in 2012 led to suppressed CO2 exchange rates and LAI across sites, while wet conditions in 2013 resulted in higher CO2 exchange rates and LAI. Seasonal maxima for net ecosystem production (NEP), ecosystem respiration (ER), and gross ecosystem production occurred in the late summer in 2012 and mid‐summer in 2013. Over the study period, maximum NEP and ER were 13.68 and −9.40 μmol CO2 m−2 s−1, respectively, while LAI reached 2.18 m2 m−2. Across sites, carbon storage in the top 10 cm of soil was 33.1–41.3 Mg C ha−1. Prescribed fire did not affect CO2 exchange, LAI, or soil carbon storage in the four to six years following a fire. In contrast to prior studies, Mid‐Atlantic serpentine barrens responded positively to increased soil water availability and were more productive than expected. These ecosystems were functionally similar to tallgrass prairie, suggesting that fire management practices from prairies may be applicable in Mid‐Atlantic serpentine barrens. Increased frequency of prescribed fire would likely reduce litter inputs to the soil and maintain the ecosystem's natural edaphic stessors, thereby combating forest encroachment.

Chemical and physical-hydric characterisation of a red latosol after five years of management during the summer between-crop season

Agricultural production systems that include the production of mulch for no-tillage farming and structural improvement of the soil can be considered key measures for agricultural activity in the Cerrado region without causing environmental degradation. In this respect, our work aimed to evaluate the chemical and physical-hydric properties of a dystrophic Red Latosol (Oxisol) in the municipality of Rio Verde, Goias, Brazil, under different soil management systems in the between-crop season of soybean cultivation five years after first planting. The following conditions were evaluated: Brachiaria brizantha cv. Marandu as a cover crop during the between-crop season; Second crop of maize intercropped with Brachiaria ruziziensis; Second crop of grain alone in a no-tillage system; Fallow soil after the soybean harvest; and Forest (natural vegetation) located in an adjacent area. Soil samples up to a depth of 40 cm were taken and used in the assessment of chemical properties and soil structure diagnostics. The results demonstrated that the conversion of native vegetation areas into agricultural fields altered the chemical and physical-hydric properties of the soil at all the depths evaluated, especially up to 10 cm, due to the activity of root systems in the soil structure. Cultivation of B. brizantha as a cover crop during the summer between-crop season increased soil water availability, which is important for agricultural activities in the region under study.

Growth adjustments of conifers to drought and to century-long irrigation

Our knowledge on tree responses to drought is mainly based on short-term manipulation experiments which do not capture any possible long-term adjustments in this response. Therefore, historical water channels in inner-Alpine dry valleys were used as century-long irrigation experiments to investigate adjustments in tree growth to contrasting water supply. This involved quantifying the tree-ring growth of irrigated and non-irrigated (control) Scots pine (Pinus sylvestris L.) in Valais (Switzerland), as well as European larch (Larix decidua Mill.) and black pine (Pinus nigra Arnold) in Vinschgau (Italy). Furthermore, the adjustments in radial growth of Scots pine and European larch to an abrupt stop in irrigation were analyzed.Irrigation promoted the radial growth of all tree species investigated compared to the control: (1) directly through increased soil water availability, and (2) indirectly through increased soil nutrients and humus contents in the irrigated plots. Irrigation led to a full elimination of growth responses to climate for European larch and black pine, but not for Scots pine, which might become more sensitive to drought with increasing tree size in Valais. For the control trees, the response of the latewood increment to water availability in July/August has decreased in recent decades for all species, but increased in May for Scots pine only. The sudden irrigation stop caused a drop in radial growth to a lower level for Scots pine or similar level for larch compared to the control for up to ten years. However, both tree species were then able to adjust to the new conditions and subsequently grew with similar (Scots pine) or even higher growth rates (larch) than the control.To estimate the impact of climate change on future forest development, the duration of manipulation experiments should be on longer time scales in order to capture adjustment processes and feedback mechanisms of forest ecosystems.

Shrub seedling survival under climate change – Comparing natural and experimental rainfall gradients

Predicting responses of plant communities to environmental changes is a key challenge in ecology. Here we examined climate-related mechanisms regulating seedling dynamics of a common Mediterranean shrub. Our objective was to analyze effects of water availability on seedling survival and to determine whether geographical gradients can serve as proxy for predicting local climate change effects. We conducted a field experiment along a natural rainfall gradient with additional long-term rainfall manipulations at a Mediterranean site, enabling the investigation of the relative importance of biotic and abiotic factors on seedling dynamics. Along the natural and artificial rainfall gradient seedling survival increased with increasing soil water availability. However, seedling survival at the Mediterranean site yielded a clear trend of decreasing seedling survival with artificially increasing aridity whereas at the dry end of the geographical gradient seedling survival was relatively high. We attribute this pattern to biotic interactions, which appeared less negative at the dry end. These findings indicate that ignoring biotic interactions is misleading when predicting shifts in the distribution of species under climate change and that because of the complex interplay between abiotic and biotic factors, environmental gradients can be poor proxies for predicting the response of plant species to climate change.

Canada bluejoint foliar δ15N and δ13C indicate changed soil N availability by litter removal and N fertilization in a 13-year-old boreal plantation

Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] outcompetes overstory tree species such as white spruce [Picea glauca (Moench) Voss] by creating a thick litter layer and competing for the available nitrogen (N). This study was conducted to investigate the effects of bluejoint litter layer (with or without litter removal) and N fertilization on soil water and N availabilities using principal component analysis (PCA) and foliar δ15N and δ13C of bluejoint in a plantation in north-central Alberta, Canada. PCA using soil properties and understory growth data demonstrated that N fertilization was more effective in changing the soil environment and resource availabilities for bluejoint growth than litter layer removal. The increase in soil N availability by N fertilization was linked with increased bluejoint foliar δ15N (by around 3‰) in fertilized plots, as a result of greater N isotopic fractionation in the fertilized plots. The more negative δ13C (by around 1‰) of bluejoint in litter layer-removed plots suggested that litter layer removal increased soil water availability, indicating that the litter layer reduced soil water availability on the site. Therefore, results from this and previous studies showed that the litter layer decreased both soil water and N availabilities. Although the exact mechanisms of the benefit of the litter layer for bluejoint remains unknown, bluejoint likely adversely impacted tree growth by competing for N due to its strong N acquisition ability under soil resource-limiting conditions.

Competition and facilitation-related factors impacts on crop performance in an agro-forestry parkland system in Burkina Faso

Parkia biglobosa and Vitellaria paradoxa are known to improve soil fertility and redistribute water under their crowns in parkland systems. A field experiment was conducted to separate above and belowground interactions between these species and associated Sorghum bicolor using root trenching and crown pruning during three cropping seasons. Trenching increased soil water availability because Sorghum plants displayed higher leaf water potential (-0.73 ± 0.11 MPa) in the trenched plots than control plots (-1.32 ± 0.14 MPa). There were no significant differences in grain (315 ± 80 kg ha-1 versus 217 ± 48 kg ha-1) and straw biomass (1639 ± 295 kg ha-1 versus 1307 ± 278 kg ha-1) yields between trenched and control plots. Crown pruning increased sorghum grain yield in the trenched plots in 2008 and 2009 under P. biglobosa while the opposite happened under V. paradoxa. Better performance of Sorghum in the trenched plots under unpruned V. paradoxa trees than pruned trees could be an indication that light was less limiting under this species as previously thought but also that crown removal induced soil water evaporation and decreased soil water content under this species. An implication of this is that recommendations for including trees in cropland, or for management of existing trees within cropland, must be context and species specific. Key words: Crown pruning, plant water potential, tree-crop interactions, root trenching.

Myth and mystery of soil mycorrhiza: A review

Arbuscular mycorrhizal (AM) fungi are soil borne fungi forming symbiotic relationship with majority of higher plants providing a direct link between soil and plant roots. AM fungi undergo a multi-step colonization process before benefiting the host plant. The strong glue glomalin is a glyco protein produced by an AM fungi. The deposition of glomalin on soil particles leads to buildup and stabilization of aggregates, which leads to better soil structure which in turn leads to increased soil water availability under water stress condition. The mycorrhizal colonization is effective aggregators and therefore management of mycorrhizal fungi can be considered as a biological amendment for soil structure and other soil physical properties. Mycorrhizal plants could take up more metal nutrients via extraradical hyphae, which provide larger surface areas than the roots alone and reduce the distance for diffusion, thereby enhancing the absorption of immobile metal nutrients (especially Phosphorus and Zinc). Mycorrhizal inoculated plants produce larger biomass as a direct consequence of improved photosynthetic activities and translocate 20 to 30% of the assimilated C to the underground where mycorrhizal structures conserve soil carbon. AM symbiosis enhanced the plant growth by increasing plant access to immobile mineral ions mainly Phosphorus and Zinc, improving physical conditions and by binding heavy metals into roots that restricts their translocation into shoot tissues. The mycorrhizal symbiosis assists in biofortification of micronutrients such as Fe and Zn in maize grain irrespective of calcareous and non-calcareous soils. The mechanism involved in improved micro nutrition of maize includes acidification of rhizosphere; siderophore production enhanced physiologically active Fe and production of anti-oxidants besides synergistic interaction between P and micronutrients. Key words: Mycorrhiza, soil aggregation, micronutrients, carbon sequestration, heavy metals, enzymatic activity, glomalin, biofortification.

Sulfoximine: A novel insecticide for management of rice brown planthopper in India

Arbuscular mycorrhizal (AM) fungi are soil borne fungi forming symbiotic relationship with majority of higher plants providing a direct link between soil and plant roots. AM fungi undergo a multi-step colonization process before benefiting the host plant. The strong glue glomalin is a glyco protein produced by an AM fungi. The deposition of glomalin on soil particles leads to buildup and stabilization of aggregates, which leads to better soil structure which in turn leads to increased soil water availability under water stress condition. The mycorrhizal colonization is effective aggregators and therefore management of mycorrhizal fungi can be considered as a biological amendment for soil structure and other soil physical properties. Mycorrhizal plants could take up more metal nutrients via extraradical hyphae, which provide larger surface areas than the roots alone and reduce the distance for diffusion, thereby enhancing the absorption of immobile metal nutrients (especially Phosphorus and Zinc). Mycorrhizal inoculated plants produce larger biomass as a direct consequence of improved photosynthetic activities and translocate 20 to 30% of the assimilated C to the underground where mycorrhizal structures conserve soil carbon. AM symbiosis enhanced the plant growth by increasing plant access to immobile mineral ions mainly Phosphorus and Zinc, improving physical conditions and by binding heavy metals into roots that restricts their translocation into shoot tissues. The mycorrhizal symbiosis assists in biofortification of micronutrients such as Fe and Zn in maize grain irrespective of calcareous and non-calcareous soils. The mechanism involved in improved micro nutrition of maize includes acidification of rhizosphere; siderophore production enhanced physiologically active Fe and production of anti-oxidants besides synergistic interaction between P and micronutrients. Key words: Mycorrhiza, soil aggregation, micronutrients, carbon sequestration, heavy metals, enzymatic activity, glomalin, biofortification.

Variation in logging debris cover influences competitor abundance, resource availability, and early growth of planted Douglas-fir

Logging debris remaining after timber harvest can modify the microclimate and growing conditions for forest regeneration. Debris also can influence tree seedlings indirectly through its effects on development of competing vegetation, although the mechanisms are poorly understood. At two sites in Washington and Oregon (USA) that differed in availability of soil water and nutrients, mechanisms were studied by which logging debris and competing vegetation interacted to influence performance of planted Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) seedlings. In a split-plot design, two levels of competing vegetation (presence and absence) and three covers of logging debris (0%, 40%, and 80%) were replicated eight times at each site on 2×2-m areas centered on individual Douglas-fir seedlings. Vegetation abundance, seedling growth, and resource availability were monitored for 4years (2005–2008). Soil water depletion was lower and Douglas-fir water potential and foliar nitrogen were higher in the absence of competing vegetation, resulting in increased seedling growth. The highest seedling growth rates and foliar nitrogen contents occurred where absence of vegetation was combined with 80% debris cover. Where competing vegetation was present, 40% debris cover was associated with decreases in herb cover and soil water depletion and increases in seedling growth relative to 0% or 80% debris covers. At the Washington site where soil quality was lower, the combination of presence of vegetation and 80% debris cover was associated with a 2.4°C average reduction in summer soil temperatures at 15cm depth, reduced foliar nitrogen content, and the slowest rates of seedling growth. Potential effects of logging debris, such as mulching (i.e., reduced evaporation of soil water) and interception loss (i.e., reduced precipitation inputs), were minor to non-detectable from sensors buried at 20–40cm soil depth. Results of the research suggest that retention of moderate levels of logging debris (i.e., 40% cover) after forest harvesting in the Pacific Northwest is likely to increase early growth of Douglas-fir by increasing soil water availability through reduced herb abundance. Where intensive vegetation control is practiced, retention of higher debris levels (i.e., 80% cover) may provide further benefits to seedling growth.

Effects of soil water and nitrogen availability on photosynthesis and water use efficiency of Robinia pseudoacacia seedlings

The efficient use of water and nitrogen (N) to promote growth and increase yield of fruit trees and crops is well studied. However, little is known about their effects on woody plants growing in arid and semiarid areas with limited water and N availability. To examine the effects of water and N supply on early growth and water use efficiency (WUE) of trees on dry soils, one-year-old seedlings of Robinia pseudoacacia were exposed to three soil water contents (non-limiting, medium drought, and severe drought) as well as to low and high N levels, for four months. Photosynthetic parameters, leaf instantaneous WUE (WUEi) and whole tree WUE (WUEb) were determined. Results showed that, independent of N levels, increasing soil water content enhanced the tree transpiration rate (Tr), stomatal conductance (Gs), intercellular CO2 concentration (Ci), maximum net assimilation rate (Amax), apparent quantum yield (AQY), the range of photosynthetically active radiation (PAR) due to both reduced light compensation point and enhanced light saturation point, and dark respiration rate (Rd), resulting in a higher net photosynthetic rate (Pn) and a significantly increased whole tree biomass. Consequently, WUEi and WUEb were reduced at low N, whereas WUEi was enhanced at high N levels. Irrespective of soil water availability, N supply enhanced Pn in association with an increase of Gs and Ci and a decrease of the stomatal limitation value (Ls), while Tr remained unchanged. Biomass and WUEi increased under non-limiting water conditions and medium drought, as well as WUEb under all water conditions; but under severe drought, WUEi and biomass were not affected by N application. In conclusion, increasing soil water availability improves photosynthetic capacity and biomass accumulation under low and high N levels, but its effects on WUE vary with soil N levels. N supply increased Pn and WUE, but under severe drought, N supply did not enhance WUEi and biomass.

Wet years and farmers’ practices may offset the benefits of residue retention on runoff and yield in cotton fields in the Sudan–Sahelian zone

Cotton (Gossypium hirsutum L.) is sensitive to water shortage during its establishment phase. In the Sudan–Sahelian zone, a significant part of rainfall events can be lost through runoff, particularly as the rainy season begins. Using residues from previous crops as soil cover is expected to reduce runoff and thus improve water availability to young cotton plants. To test this hypothesis, we set up a trial in Sikasso, Southern Mali, with a two-year rotation of cotton and sorghum (Sorghum bicolor L.). The conventional cropping system (CS) was compared with two no-tillage, mulch-based cropping systems, where cotton was grown on straw residues of a sorghum crop (DMC1) or sorghum/brachiaria (Brachiaria ruziziensis) intercropping (DMC2). Soils in this region are classified as ferruginous types with hydromorphic characteristics at depths below 0.7m (Plinthic Haplustalfs). Measurements included runoff, soil water content, biomass, cotton yield, and soil surface status (i.e. covered, open with visible macropores, or closed with no visible macropores). On DMC systems, the amounts of residues from the preceding crop kept on the soil surface at the beginning of the cotton cycle ranged from 4.4 to 9.3×103kgha−1 of dry matter, depending on the year and preceding crop. The soil surface status was more open under DMC1 and DMC2 (38 and 62%, respectively) than under CS (30%). Crop residues reduced runoff at the beginning of the rainy season in both years (−26 and −56mm between CS and DMC2 in 2006 and 2007, respectively) and increased soil water availability for cotton (+26mm and +50mm, respectively). This gain in soil water did not result in higher cotton yields, probably because 2006 and 2007 rainfall amounts were unusually high, with a low gap between water supply and cotton requirements. Row ridges made perpendicular to the slope and after sowing in CS, created an effective obstacle to runoff and increased soil water content. The effects of crop residues on rainwater infiltration accumulated over the entire growing season were minimal. In conclusion, during wetter periods, the potential benefit of DMC systems on water balance and cotton yield in these types of soil may be low when compared with conventional systems if ridging done perpendicular to the slope is added.

Conservation tillage and water availability for wheat in the dryland of central Chile

The dryland areas of Central Chile are associated to Mediterranean climate. Under these conditions, water availability during crop growth is a crucial factor for crop productivity. Conservation tillage systems play an important role in this area, increasing soil water availability; however, crop yield responses can be variable. Soil management should be aimed at reducing water loss and promoting water use by crops. The aim of this review is to analyze and discuss the factors affecting water availability in the Mediterranean drylands of Central Chile, as well as to study the effect of tillage systems on the water use efficiency of wheat.

Adaptive management to climate change for Norway spruce forests along a regional gradient in Finland

We hypothesized that the responses of boreal Norway spruce (Picea abies) forests to climate change would be region-specific due to regional differences in temperature and water availability. In this context, we analyzed the adaptive effects of varied thinning intensities on the gross primary production (GPP), total stem wood growth, and timber yield over a 100-year period using a process-based ecosystem model. Our simulations represented Norway spruce forests for five different bioclimatic zones spanning southern to northern Finland (61–67oN). Ten thinning regimes with thinning intensities ranging from 5 to 50 %, as well as an unthinned regime, were included in the calculations. The results showed that at the southern sites without thinning, the cumulative GPP and total stem wood growth were lower under the changing climate than in the current climate over the simulation period due to greater water depletion via evapotranspiration and reduced soil water availability. At the central and the northern sites, the climate changes increasingly enhanced the GPP and total stem wood growth due to the mitigation of low-temperature limitation and the improved soil water availability. Thinning generally mitigated the soil water deficit by reducing water evaporation and led to a reduction of the natural mortality. At the southern sites, light and moderate thinning intensities increased the GPP and total stem wood growth relative to sites with a changing climate that experienced no thinning. Moreover, moderate thinning resulted in the greatest timber yield. Heavy thinning, in which a large proportion of standing trees were removed, reduced the GPP and total stem wood growth despite allowing increased soil water availability. At the northern sites, all levels of thinning, including light thinning, decreased the GPP and stem wood growth, indicating that soil water availability was not a limiting factor for growth prior to thinning.

Effects of compost on water availability and gas exchange in tomato during drought and recovery

Compost can increase soil water availability and nutrient uptake by plants, but it is not clear whether it can also improve the ability of plants to recover after drought stress. Tomato plants (Lycopersicon esculentum L.) were grown in sandy soil without compost or with compost either incorporated or mulched. There were two water treatments: (i) plants grown under sufficient water supply throughout the experiment and (ii) plants grown with sufficient water supply until day 33 after which water was withheld until stomatal conductance was close to zero. Compost addition increased water content at both field capacity and permanent wilting point, but only incorporated compost increased total available water. Compost addition increased shoot and root growth under well-watered and drought stressed conditions with a greater effect by incorporated compost. At sufficient water supply, the rates of photosynthesis and transpiration were similar in all treatments. Drought stressed plants with incorporated compost wilted earlier than control plants, whereas mulched compost increased water availability to plants and hence the number of days until wilting. Photosynthesis and transpiration recovered faster in plants grown with incorporated compost compared to other treatments. The rapid recovery of plants after drought with incorporated compost could be due to their greater root length.

Complexity and coexistence in a simple spatial model for arid savanna ecosystems

Tree–grass coexistence is broadly observed in tropical savannas. Recent studies indicate that, in arid savannas, such coexistence is stable and related to water availability. The role of different factors (from niche separation to demographic structure) has been explored. Nevertheless, spatial mechanisms of water–vegetation interactions have been rarely taken into account, despite their well-known importance for vegetation distribution. Here, we introduce a spatial model including tree and grass biomass dynamics, together with soil and surface water dynamics. We consider two water–vegetation feedbacks. Grasses increase water infiltration into the soil, while tree shadow limits evaporation, and both mechanisms increase soil water availability, leading to positive feedbacks. The infiltration feedback can also lead to spatial pattern formation. Despite the fact that trees and grasses compete for the same resource, namely water, we observe stable coexistence as a possible model outcome. The system displays a complex behavior, with multiple stable states and possible catastrophic shifts between states, e.g., patterned grassland, bare soil and forest. In our model, coexistence is always linked with multi-stability and spatial pattern formation, driven by grass infiltration feedback. Given such complex model solutions, we expect that, under real conditions, heterogeneities and disturbances, acting on the multi-stable states, may further foster coexistence.

Effects of water availability on carbon and water exchange in a young ponderosa pine forest: Above- and belowground responses

Changes in the hydrological cycle, as predicted and currently observed, are expected to significantly impact the water and carbon balance of water-limited forest ecosystems. However, differences in the water-sensitivity of component processes make carbon balance predictions challenging. To examine responses of ecosystem components to water limitations, we conducted a study of tree, soil and ecosystem-level processes in a young ponderosa pine stand under natural summer drought (control) and increased soil water conditions (watered). Weekly-averaged tree transpiration (Ttree), gross ecosystem photosynthesis (GPP) and soil CO2 efflux (Rstree; nearby trees) were related with soil water content (SWC; polynomial form: TtreeR2=0.98 and RstreeR2=0.91, logarithmic form: GPP R2=0.86) and declined rapidly when relative extractable soil water (REW) was <50%. The sensitivity of daily variations in canopy conductance (Gs) to vapor pressure deficit was affected by SWC (R2=0.97; logarithmic function), decreasing at REW <50%. Watering maintained REW at about 70% in July and August but positively affected tree carbon and water dynamics only at the end of summer when fluxes in the control treatment were strongly water-limited. A tight coupling of above- and belowground fluxes became apparent. In the control treatment, root-rhizosphere respiration (Rr) decreased along with GPP and Ttree (R2=0.58) as drought progressed, while watering maintained Rr, Ttree and Gs at a significantly higher level than those of the unwatered trees in late summer. In contrast, microbial respiration responded instantaneously and strongly to the watering compared to the control treatment. The net effect was that increased soil water availability during the typical dry growing season has a negative effect on the short-term seasonal ecosystem C balance due to a larger increase in decomposition than photosynthesis. However, longer-term effects remain uncertain. In summary, our study highlights that understanding the dissimilar response of tree dynamics and soil decomposition to water availability is a key component in predicting future C sequestration in water-limited forest ecosystems.