Small-scale spatial soil-plant relationship in semi-arid gypsum environments

Studies on soil patterning on a small scale in arid and semi-arid regions h ave rarely been conducted. Many papers implicitly assume that plant distrib ution is controlled by some soil variables acting at small scales. We have directly tackled the relationships between soil and some biotic variables i ncluding plant distribution at small scales in an Iberian semi-arid gypsum environment. This has been carried out by means of Canonical Correspondence Analysis as a hypothesis-testing tool. CCA models show that the spatial da ta matrix is able to explain a relevant fraction of the soil data set (P < 0.001). The most important variable, as firstly selected in the CCA stepwis e selection procedure, suggests the existence of a vegetation-elevation gra dient in relation to soil physical properties; the rest of selected variabl es indicates the existence of other spatial trends which may be related to certain microgeomorphological features. On the other hand, only the cover o f annuals and the cover of litter are selected in the case of the biotic da ta set as constraining matrix, but not the cover of any perennial plant. Pa rtial CCA models indicated that the remaining information explained by the spatial data set after adjusting the biotic set as covariables is also sign ificant (p < 0.001). This variability is not related to the existence of ve getation bands as shown by the two selected variables in the case of the pa rtial CCA models. The primary source of spatial soil variation is related t o the existence of three community bands and these differences are able to explain even the change of plant life forms in vegetated band. The soil par ameters controlling the changes are mainly related to texture and surface f eatures. However, we detected other sources of spatial soil variation out o f this primary model. This hierarchical spatial pattern seems to be related to some geomorphological traits of the landscape, such as soil crust stren gth, presence of gypsum crystals or bare zones, and not to the presence of mature gypsophytes (at least the five most frequent) which might ameliorate the soil environment. Furthermore, the biotic data set is not able to expl ain any new fraction of soil variability out of that already explained by t he spatial data set.