178 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Radiation Microclimate 



Very little information is yet available on the 

 detailed effects of stock on radiation microcli- 

 mate. However, a number of effects can be antici- 

 pated following the reduction in vegetation cover, 

 a change in dominant species, and modification 

 of the soil surface. The implications of these 

 effects are examined in relation to net radiation, 

 the partition of net radiation and diversity of 

 microclimate. 



Net Radiation 



The albedo, or surface reflectivity a is known 

 to be the main contributor to variations in net 

 radiation R x of similarly exposed surfaces (59) 

 where 



R N -=R G (l-a)+R NL (5) 



with R G the incoming shortwave radiation and 

 R NL the net longwave radiation. 



Net radiation will then alter with changes in 

 the proportion of ground occupied by vegetation, 

 changes in the dominant species and modification 

 of the soil surface depending on albedo differ- 

 ences between plant species and soil surfaces. 

 These changes were examined in shrub steppe 

 communities with the shrub species Kochia sedi- 

 folia, K. pyramidata, Atriplex vesicaria, and A. 

 nummular) a Lindl. present at a site 64 km. 

 NNW. of Balranald, N.S.W. in December 1970. 

 Summertime values of albedo were obtained for 

 the shrub species, the perennial grass Danthonia 

 caespitosa, samphire Pachycornia tenuis (Benth.) 

 J. M. Black, and compacted and uncompacted 

 soil (table 4). 



The albedos for the plant species in table 4 

 vary comparatively little. From this it can be 

 inferred that stock-induced changes in species- 

 dominance are unlikely to produce a marked 

 shift in surface net radiation. The maximum 

 possible change, that is from Kochia sedifolia 

 to K. pyramidata, would result, given equal re- 

 placement of one by the other, in a local increase 

 in net radiation of 4 percent. When the change 

 in the net radiation of the entire surface is con- 

 sidered, this is likely to be lower because of the 

 unchanging albedo of the intervening surface. 



Changes in vegetation cover are likely to re- 

 sult in a still smaller change in surface net 

 radiation, at least for the situations examined. 



Sample — 







No. ' 



Mean 



S.E. 



14 



0.208 



0.0229 



11 



.165 



.0076 



8 



.186 



.0216 



7 



.183 



.0151 



11 



.179 



.0132 



2 



.174 



— 



14 



.175 



.0169 



6 



.135 



.0108 



j loam : 







21 



.276 



.0289 



6 



.186 



.0145 



Table 4. — Summertime albedos of some range- 

 land plants and soils (solar elevation^SO ) 



Albedo 



Sample 



Item 



Plant species : 

 Kochia sedifolia 

 K. pyramidata 

 Atriplex vesicaria 

 A. nummularia 

 Danthonia caespitosa 



(50 percent dry leaves) 

 Pachycornia tenuis 

 Soils : 

 Uneroded red sandy loam 



Dry 



Wet 

 Eroded compacted red clay loam 



Dry 



Wet 



1 20 readings per sample. 



Thus, the change expected from a reduction in 

 the cover of Kochia sedifolia (a = 0.208) from the 

 commonly encountered value of 33 percent to a 

 bare soil surface (a = 0.175) would only be a 1.1 

 percent increase in surface net radiation. 



The small change due to a reduction in vege- 

 tation can. however, be more than offset by 

 changes in soil surface albedo due to compaction 

 and surface erosion. The albedo of an uneroded 

 red sandy loam was found to be much lower 

 than that for an adjacent eroded red. compacted, 

 clay loam surface (table 4). The expected net 

 radiation over the eroded soil surface was con- 

 sequently 10.1 and 5.1 percent lower for dry and 

 wet conditions, respectively. 



Reflection coefficients of several Australian 

 rangeland species are reported by Sinclair and 

 Thomas (56), but they are generally higher than 

 the values obtained for the in situ measurements 

 on whole plants in table 4 because they are for 

 single leaves or mosaics of leaves. 



Partition of Net Radiation 



The partition of net radiation at a surface 

 can be expressed, in the absence of advection and 

 neglecting the photosynthesis term, as 



