4.7 Southern ranges CO 2 variation from 1975 to 2051
4.7.1 Plant growth
Figure 83 shows the change over time in simulated total biomass of herbaceous (TotHrb), woody (TotWdy) and shrub (TotShb) PFT’s (live+dead leaves, stems and roots) from 1975-2050 under various stocking rates. The figure displays annual plant growth to the maximum flowering period in autumn each year. Values given in the following descriptions are maximum or minimum values.
During the “warm-up“ phase and thereafter, TotWdy showed a continuous decline in peak values but stabilised after approximately 15 years of simulation at about 9.0 g m-2. It then declined again due to the introduction of heavy stocking rates until it stabilised at about 5.0 g m-2 during the last 40 years of the simulation. This pattern, except for the level of adaption, is consistent with the patterns observed for aspen and deciduous trees in the Ouarzazate base-line scenario (Figure 50). TotHrb declined during the model “warm-up“ and stabilised at annual peak values of 6.0 g m-2. It declined following the introduction of heavy stocking rates in 2004 but then increased to annual peaks of about 8 g m-2 during the last 40 years of the simulation. TotShb varied considerably in maximum total annual biomass. Even after the “warm-up“ phase, TotShb declined continuously and broke down after the introduction of heavy stocking rates in 2004. Thereafter, it increased to stabilise at an annual peak of about 18.0 g m-2. Stocking rates produced large effects on Tothrb, Totwdy and Totshb production. TotWdy stabilised after the change in stocking rates, probably due to the number of dromedaries. TotHrb and TotShb experienced a break-in period but stabilised subsequently. TotHrb eventually reached greater average total biomass under heavy grazing than under light stocking rates.
1980 1990 2000 2010 2020 2030 2040 2050 2
4 6 8 10 12 14 16 18 20 22
Year
g DWT/m2
TotHrb
TotShb
TotWdy
2004 2034
light heavy moderate
gm-2
Figure 83: Annual sum of Total (live+dead) biomass (g m-2) for the entire Drâa zone from 1975-2051 under elevated CO2: total herbaceous TotHrb (red line), total shrub TotShb (dashed blue line) and total woody (tree) TotWdy (green line) biomass.
Figure 84 shows the results described above in detail. Stocking rates, especially heavy grazing, had substantial effects on herbaceous and shrub ANPP, even though the effects were delayed for some years (2004-2007). The effects of livestock on woody ANPP were negligible, within ≤ 2.0 g m-2 of the peak amounts (see section 4.6.1). After 30 years of the simulation, however, herbaceous ANPP showed an increasing trend and finally stabilised at a peak of approximately 10 g m-2. After the transition to heavy grazing, shrub ANPP re-stabilised at a peak of approximately 20 g m-2. These maximum levels were similar to those observed during the light grazing period from 1975-2004. As seen before, (see sections 4.5.4 and 4.6.4) SysPpt was the main reason for the inter-annual fluctuations of ANPP.
Turning from PFT descriptions to more detailed descriptions of plant groups, primarily the herbaceous grass types, we now present leaf growth. Figure 85 shows the herbaceous leaf production of ”fine-”, ”coarse-” and ”alpine grass” for the period 1975-2051. The “coarse grass” leaf production, even after the “warm-up“ phase, strongly declined to an annual sum of only 2 g m-2 after 75 years of simulation. The ”fine grass” leaf production increased continuously, interrupted only by the change to heavy grazing, to approximately 12 g m-2 at the end of the simulation. This amount made ”fine grass” the single greatest leaf biomass
producer in the southern ranges. This was probably due to animal dietary preferences for
”fine grass” leaves (see Table 22), which stimulated new shoot growth from year to year at high stocking rates more than for other grass types.
1980 1990 2000 2010 2020 2030 2040 2050 0
5 10 15 20 25 30 35 40 45
Year
g DWT/m2
AnppHrb
AnppShb
AnppWdy
2004 2034
light heavy moderate
gm-2
Figure 84: Annual sum of ANPP (g m-2) for the entire Drâa catchment zone from 1975-2051 under elevated CO2: herbaceous AnppHrb (red line), shrub AnppShb (dashed blue line) and woody AnppWdy (green line).
The “alpine-” and ”fine grass” may both have benefited from the humidity and light grazing intensities of the early 1990s to increase their leaf biomass production and ground cover. But due to the lesser spatial coverage and greater animal dietary preference of ”alpine grass”
(compared to ”fine grass”), it declined strongly in dry years, e.g., after 1995. Increased stocking rates probably had the same induction effects on ”alpine grass” leaf biomass as on
”fine grass”.
We conclude that ”fine grass” leaf growth shows the greatest increase in relationship both stocking rates and precipitation thresholds, indicating that this group of grasses may perform best in spatial and environmental reclamations. This is probably due to animal dietary preferences for ”fine-” and ”alpine grass” leaves instead of ”coarse grass” leaves. These grass PFT’s are thought to benefit in terms of pollination from animal feeding and trampling.
0 2 4 6 8 10 12 14 16 18
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050
Year of simulation
g m-2
Fine grass Coarse Grass Alpine Grass
Light Heavy Moderate
2004 2034
Figure 85: Simulated annual sums of “fine-” (black line), “coarse-” (light grey line) and “alpine grass” (grey line) leaf production (g m-2) for the entire Drâa zone from 1975-2051 under elevated CO2.
Figure 86 shows the pattern of total annual leaf biomass for ”evergreen-”, ”low DMD-” and
”sage shrubs” under this scenario. “Evergreen shrubs” showed a continuous decreasing trend throughout whole simulation period, as seen in previous simulations. Again, the curves discussed indicate leaf biomass increasing to its peak in autumn. Next, the maximum and minimum values are discusses. Leaf biomass decreased by half at the end of the 20th century and again when the heavy stocking rate was introduced. Shortly after this transition, leaf biomass increased. Leaf biomass again declined under the moderate grazing regime, to
≤ 1 g m -2 by the end of the simulation. Neither “low DMD-” nor ”sage shrubs” showed an equal decreasing trend for leaf biomass production. Otherwise, both shrub PFT’s showed equal levels of leaf production. Growth was strongly influenced by precipitation rates, e.g., in the early 1980s (see Ouarzazate precipitation data, Figure 41), during the light grazing period. The introduction of heavy grazing apparently forced both “low DMD-” and ”sage shrubs” leaf biomass to decrease, but only after a delay of four to five years, as already observed for shrub ANPP (Figure 84). Subsequently, leaf growth was probably induced by grazing, reaching constant maximum levels of about 14.0 g m-2. By contrast, “evergreen shrub” leaf production declined rapidly, probably because of its high animal dietary preference. The “sage shrubs” and “low DMD shrubs” seem to be more adapted to animal
feeding and trampling, perhaps even benefitting from these conditions to increase leaf development.
1980 1990 2000 2010 2020 2030 2040 2050
5 10 15 20 25 30 35
Ye a r
gDWT/m2
Sh rub e ve rg ree
Sh rub low DMD
Sh rub - Sage
2004 2034
light heavy moderate
n
gm-2
Shrub-evergreen
Shrub-low DMD
Shrub-sage
Figure 86: Simulated annual sum of ”evergreen-” (red line), ”low DMD-” (dashed blue line) and
”sage shrub” (green line) leaf biomass (g m-2) for the entire Drâa zone from 1975-2051 under elevated CO2.
Simulated root growth of shrubs is discussed next (data not shown). Roots of “evergreen shrubs” declined from 30.0 g m-2 to <1.0 g m-2 by the end of the simulation, paralleling the trend described above (Figure 86) and demonstrating that these shrubs are not adapted to the predicted environment and stocking rates and are thus vulnerable to extinction.
The ”low DMD-” and ”sage shrubs” increased their root biomass to a maximum of 55.0 g m-2 during the first 30 years of the simulation. However, these values were strongly influenced by low precipitation. This and the heavy stocking rate together reduced shrub root biomass to 30.0 g m-2. The “sage shrub” root biomass finally stabilised at about 40.0 g m-2, whereas ”low DMD shrub” root biomass continually exceeded ”sage shrub” root biomass by about 10.0 g m-2.
Figure 87 exhibits the annual pattern of green leaf area index (Glai) for all three PFT’s in the simulated diagnostic cell. Glai represents all green parts of plants, including stem and leaves, and increases annually up to the point of flowering. Apart from a few interruptions during years with low rainfall (early 1980s and late 1990s), Glai increased and stabilised at a
maximum of about 45 m2 m-2 until about 2034. With the introduction of moderate stocking rates, however, the maximum Glai value declined. This trend in Glai was interrupted only by considerably drier years and moderate grazing. The figure shows a reliable increase in biomass, probably produced mostly by “sage-” and ”low DMD shrubs”, as indicated by Figure 86.
1980 1990 2000 2010 2020 2030 2040 2050
0 . 05 . 10 . 15 . 20 . 25 . 30 . 35 . 40 . 45
Yea r
m2 m-2
G l a i
2004 2034
light heavy moderate
1980 1990 2000 2010 2020 2030 2040 2050
0 . 05 . 10 . 15 . 20 . 25 . 30 . 35 . 40 . 45
Yea r
m2 m-2
G l a i
1980 1990 2000 2010 2020 2030 2040 2050
0 . 05 . 10 . 15 . 20 . 25 . 30 . 35 . 40 . 45
1980 1990 2000 2010 2020 2030 2040 2050
0 . 05 . 10 . 15 . 20 . 25 . 30 . 35 . 40 . 45
Yea r
m2 m-2
G l a i
2004 2034
light heavy moderate
2004 2034
light heavy moderate
Figure 87: Simulated annual sum of green leaf area index Glai (m2 m-2) for all three PFT’s in the southern ranges (based on the diagnostic cell) from 1975-2051.