Review of commonly used remote sensing and ground-based technologies to measure plant water stress

This review provides an overview of the use of remote sensing data, the development of spectral reflectance indices for detecting plant water stress, and the usefulness of field measurements for ground-truthing purposes. Reliable measurements of plant water stress over large areas are often required for management applications in the fields of agriculture, forestry,conservation and land rehabilitation. The use of remote sensing  technologies and spectral reflectance data for determining spatial patterns of plant water stress is widely described in the scientific literature. Airborne, space-borne and hand-held remote sensing technologies are commonly used to investigate the spectral responses of vegetation to plant stress. Earlier studies utilised multispectral sensors which commonly collect four to seven spectral bands in the visible and near-infrared region of the electromagnetic spectrum. Advances in sensor and image processor technology over the past 3 decades now allow for the simultaneous collection of several hundred narrow spectral bands resulting in more detailed hyperspectral data. The availability of hyperspectral data has led to the identification of several spectral indices that have been shown to be useful in identifying plant stress. Such studies have revealed strong  linear relationships between plant pigment concentration and the visible (VIS) and near-infrared (NIR) reflectance, while plant water content has been linked to specific bands in the short-wave infrared (SWIR) region of the spectrum. Ground-truthing is essential to identifying useful reflectance information for detecting plant water stress, and four commonly used ground-based methods viz. predawn leaf water potential, leaf chlorophyll fluorescence, leaf pigment concentrations and leaf water content are reviewed for their, usefulness and practical application.Keywords: leaf chlorophyll fluorescence, leaf-water content, plant pigment concentrations, plant water stress, predawn leaf water potential, remote sensin

Fire as a Selective Agent for both Serotiny and Nonserotiny Over Space and Time

Acceptance date approximate as author was not able to supply

Indigenous Grasses for Rehabilitating Degraded African Drylands

Drylands provide an important livelihood stream to its inhabitants across the globe through a range of products and ecosystem services. However, these fragile ecosystems are threatened and believed to experience various degrees of land degradation. Estimates of the landmass affected by land degradation in the global drylands range from 10% to 20%, a percentage that is increasing at an annual global rate of 12 million ha of soil lost from desertification and drought. African drylands are especially highly susceptible to severe degradation because of their poor soil structure aggravated by scarce vegetation cover. Causes of degradation in these environments are both natural and anthropogenic in nature. Change in vegetation cover, decline in soil fertility, biodiversity loss and soil erosion demonstrate degradation in African drylands. Grass reseeding using indigenous species is one of the promising sustainable land management strategies to combat degradation in the drylands. Reseeding programmes are aimed at improving vegetation cover and biomass, and they conserve the soil to an extent not possible by grazing and land management alone. Indigenous drought-tolerant grasses notably African foxtail grass (Cenchrus ciliaris), bush rye grass (Enteropogon macrostachyus) and Maasai lovegrass (Eragrostis superba) have produced promising rehabilitation outcomes. Previous studies in African drylands have demonstrated the potential of such indigenous forage grasses in improving both vegetation cover (plant frequency and densities, basal cover) and soil hydrological properties (increased infiltration capacity, reduced runoff and sediment production) as indicators of rehabilitation success. Despite their comparative and widespread success, natural and anthropogenic challenges persist. This makes reseeding programmes a risky and often expensive venture, especially for the resource-poor pastoral communities in African drylands. Despite the risks, grass reseeding using indigenous pastures remains a viable sustainable land management option to combat degradation in African drylands. However, to ensure its continued success in the long term, multifaceted approaches and strategies that will integrate land and water management and seed systems suitable for African drylands need to be developed, strengthened and promoted.Peer reviewe

The Contribution of Occult Precipitation to Nutrient Deposition on the West Coast of South Africa

The Strandveld mediterranean-ecosystem of the west coast of South Africa supports floristically diverse vegetation growing on mostly nutrient-poor aeolian sands and extending from the Atlantic Ocean tens of kilometers inland. The cold Benguela current upwelling interacts with warm onshore southerly winds in summer causing coastal fogs in this region. We hypothesized that fog and other forms of occult precipitation contribute moisture and nutrients to the vegetation. We measured occult precipitation over one year along a transect running inland in the direction of the prevailing wind and compared the nutrient concentrations with those in rainwater. Occult deposition rates of P, N, K, Mg, Ca, Na, Al and Fe all decreased with distance from the ocean. Furthermore, ratios of cations to Na were similar to those of seawater, suggesting a marine origin for these. In contrast, N and P ratios in occult precipitation were higher than in seawater. We speculate that this is due to marine foam contributing to occult precipitation. Nutrient loss in leaf litter from dominant shrub species was measured to indicate nutrient demand. We estimated that occult precipitation could meet the demand of the dominant shrubby species for annual N, P, K and Ca. Of these species, those with small leaves intercepted more moisture and nutrients than those with larger leaves and could take up foliar deposits of glycine, NO3-, NH4 + and Li (as tracer for K) through leaf surfaces. We conclude that occult deposition together with rainfall deposition are potentially important nutrient and moisture sources for the Strandveld vegetation that contribute to this vegetation being floristically distinct from neighbouring nutrient-poor Fynbos vegetation

Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation

In savannas, the tree–grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation

Nitrogen fixation ability explains leaf chemistry and arbuscular mycorrhizal responses to fertilization

Atmospheric nitrogen (N) and phosphorus (P) deposition rates are predicted to drastically increase in the coming decades. The ecosystem level consequences of these increases will depend on how plant tissue nutrient concentrations, stoichiometry and investment in nutrient uptake mechanisms such as arbuscular mycorrhizal fungi (AMF) change in response to increased nutrient availability, and how responses differ between plant functional types. Using a factorial nutrient addition experiment with seedlings of multiple N-fixing and non-N-fixing tree species, we examined whether leaf chemistry and AMF responses differ between these dominant woody plant functional groups of tropical savanna and dry forest ecosystems. We found that N-fixers have remarkably stable foliar chemistry that stays constant with external input of nutrients. Non-N-fixers responded to N and N + P addition by increasing both concentrations and total amounts of foliar N, but showed a corresponding decrease in P concentrations while total amounts of foliar P stayed constant, suggesting a ‘dilution’ of tissue P with increased N availability. Non-N-fixers also showed an increase in N:P ratios with N and N + P addition, probably driven by both an increase in N and a decrease in P concentrations. AMF colonization decreased with N + P addition in non-N-fixers and increased with N and N + P addition in N-fixers, suggesting differences in their nutrient acquisition roles in the two plant functional groups. Our results suggest that N-fixers and non-N-fixers can differ significantly in their responses to N and P deposition, with potential consequences for future nutrient and carbon cycling in savanna and dry forest ecosystems

African honeybees as a mitigation method for elephant impact on trees

Conservation managers are concerned about the impact that African elephants (Loxodonta africana) have on large tree species, necessitating the need for mitigation methods. Elephants actively avoid contact with African honeybees (Apis mellifera subsp. scutellata), staying clear of crop fields surrounded by beehive fence-lines and moving away from the sounds of swarming honeybees. Therefore, our objectives were to test whether the presence of beehives in trees influenced the likelihood of the tree receiving elephant impact, and compare these results to wire-netted (method used to prevent bark-stripping) and control (no treatment) trees. We selected a tree highly sought after by elephant, the marula tree (Sclerocarya birrea subsp. caffra), as our study species. We also assessed whether elephants avoided areas with marula trees containing beehives. Finally we provide a comparison of the financial costs of the beehive and wire-netting mitigation methods. We hung 50 active beehives in 50 trees, with 50 dummy beehives hung from branches on the opposite ends of each tree's main stem. We wire-netted another 50 trees and then assigned 50 trees as a control. Elephant impact on all 150 trees was measured prior to the addition of treatments and then post-treatment addition for 9 months. 54% of the control trees received some form of elephant impact, in comparison to 28% of the wire-netted trees and only 2% of the beehive trees. Wire-netting protected trees against bark-stripping but did not prevent elephants from breaking branches. Beehives proved to be the more effective mitigation method for elephant impact on large trees, although the presence of beehives did not prevent elephants from moving through the study site. The financial cost and maintenance required for the beehive mitigation method are greater than that of wire-netting, but the beehives can provide honey as an additive benefit on a small-scale usage level