Can there be one optimal ecosystem design for maximal efficiency of precipitation use?

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The answer to this question requires consideration of a number of factors, let us consider the following sub-questions to learn more…

Is precipitation use efficiency defined as the amount of biomass produced per area and amount of precipitation?

Water and soil conservation directly interact with each other and local vegetation cover, whereby improved soils and conserved ecosystem structures permit higher biological productivity given equal amounts of precipitation, because more water can infiltrate the soil, and less is running off, with partial shading or litter cover preventing evaporation (see Figure 10). The difference in biomass productivity between differently managed ecosystems in the same precipitation zone can be best expressed by the term ‘precipitation use efficiency’ (PUE), describing the amount of plant biomass produced by a given amount of precipitation. Fig. 11 gives the difference in PUE between conserved and degraded ecosystem productivities during nine years in correlation to the amount of plant material produced as deduced from a satellite derived vegetation index (NDVI). The PUE in Fig. 11 is based on the annual NDVI integral, which describes the total amount of green plant matter observed in the course of each year using a satellite based vegetation index, and is about twice in the conserved areas compared to the degraded ones. If ecosystems are assessed in more detail, biological productivity in fact is over five times higher in the most productive plots compared to the degraded areas, though satellite pixels don’t have the necessary resolution to display those differences.

Fig 11: Precipitation use efficiency deduced from satellite based vegetation indexes, calculated during 10 years in three neighboring, differently treated areas: contour trenched – contour trenched degraded plot; control – areas degraded by soil tilling and/or overgrazing; Conservation – conserved area restored by restricted grazing and tree planting. In each single year, the conserved are yielded significantly higher yields than the degraded plots indicating much higher conversion of water into biomass than in the degraded plots.

Is precipitation use efficiency a relevant parameter only in drylands?

Precipitation use efficiency may appear to be a key parameter in dryland only, but it is equally significant across all climate zones, as similar mechanisms maintaining high ecosystem productivity apply equally to all climate zones. Precipitation use efficiency can only be maximal if all other limitations are insignificant (e. g. nutrient availability, biodiversity, seed availability), and soil is able to absorb maximal amounts of rain water.

In combination these demands essentially dictate a few general characteristics for all various ecosystem types:

Deep soil layers must be built up to accommodate and store large amounts of moisture;
Soil water infiltration must be maximal for reducing runoff;
A thick layer of plant litter must be present to release nutrients into the ecosystem whenever needed;
Erosion by wind and water must be prevented to avoid loss of resources;
Over-exploitation of resources by man and animal must be avoided to prevent the loss of resources.

Fig. 12a: Rain Forest floor

Fig. 12b: Temperate Forest floor

The above requests result in the sad, but obvious fact that mankind has done and is still doing everything in its power to promote land degradation, though the requirements for maintaining resilient and productive ecosystems are understood very well. Deforestation and forest degradation, land clearing for intensive agriculture, overgrazing and even squeezing large numbers of wildlife into too small nature reserves all have the same consequences: Loss of vegetation and nutrient pools, soil erosion and compaction, reduced water infiltration and reduced biological productivity.

The technologies to restore such degraded ecosystems are equally well understood:

No till agriculture will restore proper soil profiles and litter layers in farmlands;
Reforestation and sustainable forest management including immediate replanting and enhancing forest biodiversity will restore resilience and productivity to any forest system;
Properly controlling livestock and wildlife activity will prevent ecosystem degradation by overgrazing;

In conclusion, all above phenomena are interconnected via one key parameter, the litter layer or O-horizon covering the soils in intact ecosystems resulting in maximal PUE and minimal soil erosion. It is generally accepted by farmers and foresters that such an O layer is unnecessary, or even harmful, and therefore this layer is destroyed systematically by tilling or ploughing, immediately inducing a vicious cycle of soil degradation, erosion and productivity loss that can only temporarily be stopped by fertilizer application. Returning to working with natural mechanisms as demonstrated in many Permaculture systems will restore maximal productivity and resilience to ecosystems and farmland in all climatic zones.

Fig. 13: Tobosa grassland in the Chihuahuan Desert

Fig. 14: No till farmland offers huge benefits, and permanently higher, growing biological productivity, PUE, and resilience to erosion and degradation.

Densely planted forests with little soil disturbance (Fig. 12), perennial grasslands that remain untilled and are highly resilient and productive (Fig. 13), or no-till agriculture that restores soil fertility and productivity without massive fertilizer input or irrigation (Fig. 14), are all natural to fully agricultural systems that life off the benefits of a closed, permanent layer of decomposing plant matter and as such are the most productive possible systems in a given climate zone.

Can arid ecosystems maintain a stable maximal state of biological productivity?

Much has been written about the low inherent productivity of dryland areas, and it is generally accepted that productivity at the arid-semiarid interphase with 200 mm precipitation is in the range between 1-2 tons per hectare and year in functioning ecosystems, and less in degraded areas. In fact a variety of ecological models on how stable ecosystems can be maintained under such low productivity have been designed. It is claimed that such dry areas must consist of exposed crusted soil patches that provide water runoff to scattered biological patches formed around perennial plants or ant nests. However, long-term research and observation by the author and co-workers have demonstrated that such patchy shrublands are just an intermediate state of degradation, as higher productivity is easily achieved in grassland or woodland configuration contingent on a closed plant litter layer. The following a few examples of sites presenting maximal or near maximal PUE, all based on the existence of a permanent uninterrupted organic litter layer.

Fig. 15: Maximal and ‘normal’ biological productivity at the Wadi Attir site. A former goat enclosure (the green vegetated patch in the picture) retains 10-fold higher productivity (“In,” in the on the right) for at least the last 6 years, compared to the surrounding bare control areas (“Out” in the same graph) that are almost depleted of vegetation, as seen in the picture to the left.

Site 1 is a former goat enclosure at the Wadi Attir site that has been observed continuously since 2007 (Fig. 15), which has maintained 10-fold biological productivity since then (in) as compared to the exposed degraded soil surrounding it (out). This was confirmed experimentally during 2014/2015. Compost left on top of tilled degraded farmland immediately induced biological productivity of at least 10-fold over the untreated degraded soil (Fig. 16), indicating that such restoration can be achieved extremely rapidly if required.

Fig 16: Express soil and productivity restoration at Project Wadi Attir: the green, highly productive area in the middle was created by over-laying of excess compost in 2013. The good 2014/15 winter rains transformed this area into a highly productive mixed grass-land, while the control areas in front and in the back remain essentially unproductive.

The green, highly productive area in the middle was created by over-laying of excess compost The good 2014/15 winter rains transformed this area into a highly productive mixed grass-land, while the control areas in front and in the back remain essentially unproductive. This is an ideal approach to rapidly stabilize and restore erosion or desertification hotspots, though compost requirements are large.

The same effect has been achieved by long-term protection of recovering grassland from grazing and tilling. Biomass residue left on the soil spontaneously can restore a thick organic matter layer within 10 – 20 years, to result in a highly productive grassland savanna (Fig. 17).

Fig. 17: Spontaneously restored highly fertile dryland top-soil thanks to accumulation of a layer of decomposing grass litter. Similar to the examples above this soil has 5 – 10 times higher productivity that surrounding degraded plots.

This mechanism has been active in the US mid-west to form, in the course of thousands of years, meter deep layers of highly productive organic matter rich soil layers supporting the huge prairies (Fig. 18). Unfortunately this is now being degraded by ongoing intensive agriculture releasing large amounts of carbon dioxide from soil organic matter into the atmosphere while causing reduced crop yields and huge financial losses.

Forestry as well can provide the means for low input soil restoration and ecosystem rehabilitation. Normally tree productivity is additional to annual productivity, and thanks to litter fall and partial shading even induces a synergistic enhancement of ecosystem productivity. An A. victoria woodland in the Wadi Attir area (Fig. 19) is the most productive system nearby with up to 10 tons of annual biomass production at annual mean precipitation of 220 mm.

Fig. 18: Prairie soils are huge layers of organic matter rich deposits derived from decomposition of grass and plant roots, and are highly productive and resilient with maximal PUE. Ongoing tilling and conversion of such soils to unsustainable agriculture leads to immediate loss of fertility, erosion and massive greenhouse gas emissions.

The data presented in figures 15 – 19 all confirm that the climate zone around Project Wadi Attir was initially rich and productive rangeland or woodland, and only overexploitation together with farming activities such as tilling and deforestation have created the desert like unproductive state encountered at present, which is directly correlated with low PUE, and high water runoff and evaporation. Experimental approaches to restore this original productivity and agricultural prosperity have shown highly promising immediate results and imply that sustainable and profitable low input agriculture can be restored to the benefit of farmers, biodiversity and climate protection.

Fig. 19: Planting of appropriate tree species will rapidly provide the necessary litter layers to enhance and maximize PUE. Combining productivity of woody vegetation with annual herbaceous productivity, litter fall, and root biomass yields an average biological productivity of 8 – 10m tons of dry matter per hectare and year, compared to about 1 ton in exposed degraded land nearby.

Is low PUE a sign of degraded ecosystems and farming systems?

It is. The amount of biomass produced per unit of rainfall can be expressed as precipitation use efficiency (PUE) and can strongly vary between differently managed plots within the same area. Degraded soils are characterized by low PUE, meaning only a low proportion of precipitation is being exploited by vegetation for the production of biomass with the remainder running off or evaporating. In the Wadi Attir area PUE can vary by up to 10 fold, as determined experimentally. Maximal productivity, resilience and PUE can be achieved only if a closed organic litter layer captures precipitation, mediates high water infiltration and releases nutrients as required. According to recent studies, experiments and field studies by the authors, the productivity potential of the Northern Negev is far underestimated. True biological productivity in the area is anywhere between five and ten tons per hectare and year (Fig. 20).

Fig. 20: An experimental agroforestry system has evolved into a highly productive savanna within 20 years, featuring a variety of dryland fruit trees and nitrogen fixing Acacia. Based on this experience a 20 year period of development and conservation suffices to restore completely degraded farmland to productive rich savanna with maximal water use efficiency and minimal erosion.

This indicates and confirms that ecosystems even at the border to arid climates can be extremely productive (Fig. 20), and any systems of lower productivity must be considered degraded.

The shrubland area assessed by the authors recently does however reveal another aspect of land management, biodiversity. This conserved shrubland plot (Fig. 21) displays only about 30% of the productivity of the most productive woodland system nearby, but seems to feature an optimal status concerning plant biodiversity with many protected geophytes abundant together with a wide variety of shrubs and grasses. A full-scale restoration program must take this in account and avoid complete extermination of such biodiversity hotspots. Project Wadi Attir has therefore reserved a significant proportion of various habitats for conservation, avoiding any external interference or disturbance.

Fig. 21: A diverse, restored shrubland displays less than maximal productivity, but justifies its existence to high biodiversity of plants and animals, and resilience to floods and erosion. Such systems will further recover spontaneously while more degraded ecosystems can never recover due to too low PUE.

In conclusion, we have solidly established that enhancing PUE to its maximum in arid drylands, by means of creating and maintaining a closed organic litter layer, will rapidly create a far more productive and resilient ecosystem. Such ecosystems will have strongly reduced erosion, higher carbon sequestration, and provide large amounts of fodder or biomass, by exploiting most precipitation to the benefit of growing vegetation. Restoring such highly productive drylands worldwide would address a large number of urgent problems including food and water security, global warming and development of marginal areas, all by enhancing precipitation use efficiency by enhancing soil and water conservation.

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