AIACC Biodiversity course Ecosystem function modelling Bob Scholes CSIR Environmentek, Pretoria

AIACC Biodiversity course Aspects and levels of biodiversity Composition: what it contains Structure:what it looks like Function: how it works Gene Population Ecosystem Biome envelope models Species niche models Ecosystem function models

AIACC Biodiversity course Dynamic Vegetation Models All the major Global Circulation Models have ‘coupled’ models of the land surface –Simulate carbon uptake/loss, albedo, bulk stomatal conductivity, surface roughness –Have a crude representation of biomes or ‘functional types’ –Some of the better ones (LPJ, Sheffield) have fire and mammals in them

AIACC Biodiversity course DGVMs continued… Problems: –Very hard to use unless you have a supercomputer –Results are not freely available, unlike the GCM outputs –Mostly not optimised for Africa –Scale is inappropriate for protected areas –Equations are complex and untransparent

AIACC Biodiversity course A ‘reduced form’ ecosystem model for savannas under climate change ‘Functional types’ are restricted to those occuring in savannas (but are expanded beyond the generic global types) Includes effects of temperature, rainfall, seasonality, CO2, soil texture, fire and megaherbivores ‘Quasi-mechanistic’ equations –Simple, reduced forms based on emergent properties at ecosystem scale Timestep of one year (‘implicit seasonality’) and ‘patch’ spatial scale

AIACC Biodiversity course Temperature CO 2 Rainfall Sand % Tree ht & BA Sour grass Tree prodn Mixed Browsers Coarse graz Fine grazers Carnivore Fire intens Sweet grass Elephants Fire freq Basic savanna system model

AIACC Biodiversity course Water balance modelling G =  Rain/E 0 * days month if Rain<E 0, else R/E= 1 months E0 = open water evaporation ~ 0.8 x {700(T+0.006A)/(100-L)+15(0.0023A+0.37T*0.53(TxTn) +0.35Tann-10.9))/(80-Tx)} T = mean air temperature ( C) Tx= max air temp Tn=min air temp A=altitude (m) L=latitude (deg) [Linacre 1984]

AIACC Biodiversity course Controls on grass growth at the annual timescale Rainfall in the current growing season –Actually, it is the duration of growth opportunity that matters –This is affected by evaporation as well as rain, and is mediated by soil texture The fertility of the soil The amount of tree cover Daytime temperature [CO 2 ]

AIACC Biodiversity course Linear relation between grass production and rainfall

AIACC Biodiversity course Slope: Rain Use Efficiency (g/m 2 /mm)

AIACC Biodiversity course Intercept dependent on soil water holding capacity covaries with the rain use efficiency

AIACC Biodiversity course Effect of trees on grass P = P 0 * e -0.1BA

AIACC Biodiversity course Maximum tree basal area

AIACC Biodiversity course What controls the growth rate of trees? Size of the tree Competition with other trees P tree = e -0.2d * e -3(A/Amax) where A = basal area, d = diameter (cm) Shackleton data

AIACC Biodiversity course Effect of CO 2 on NEP F (CO 2 ) = 1+  ln([CO 2 ]/[CO 2ref ]  ~ 0.4 for trees, 0.2 for grass [CO 2ref ] = 360 ppm

AIACC Biodiversity course Effects of temperature on NEP ƒ[T] = e c*(1-{[(b-T)/(b-a)]^d }/d *(b-T)/(b-a) c a = position of optimum ~ 28°C for trees, ~33°C for grasses b =temperature below which no growth occurs ~5C trees, 10C grass c = steepness of curve below optimum ~3 d = steepness of curve above optimum ~7

AIACC Biodiversity course What controls tree mortality? Fire Elephants

AIACC Biodiversity course Mammal dynamics dN/dt = rN - offtake r = r max * fN/cPrey biomass Ln(r max ) = Body mass f = food requirement (kg/head/d) C = fraction of prey biomass than can be consumed in a year

AIACC Biodiversity course Keeping it together! Complete competitors cannot coexist –Give each herbivore a partly unique resource The faster-growing prey must be more prefered by predators –Preference = N 2 /  N 2 Predators must grow slower than prey

AIACC Biodiversity course Test 1: trees, grass and fire

AIACC Biodiversity course Test 2: +herbivores, carnivores

AIACC Biodiversity course Test 3: + elephants

AIACC Biodiversity course The experiment design B2 Scenario Hadley model 550 ppm A2 Scenario Hadley Model Upper estimate +5C, -6% rain B2 Scenario CCModel Lower estimate +2.2C, -1.2% A2 Scenario CCModel 700 ppm

AIACC Biodiversity course Change in production drivers

AIACC Biodiversity course Change in vegetation structure

AIACC Biodiversity course Change in herbivores

AIACC Biodiversity course Preliminary conclusions Water and temperature effects can overwhelm the CO 2 effect Substantial changes in herbivore stocking rate are possible in the future Elephants at high density put the tree cover into a stable coppice state The outcome of climate-change induced habitat change depends on how you manage fires and elephants