Fine-scale classification and mapping units for California vernal pools

Background/Question/Methods When vegetation consists of a fine-scale mosaic of many communities, individual communities often cannot be mapped because of their small size. Instead, the minimal mapping units can be complexes that re-occur in particular habitats. Finding and defining these stable complexes is an additional process necessary for linking classification and mapping.  Our objective in studying Californian vernal pools was to classify individual community types and then to define their complexes that can be mapped. We sampled >2000 plots in >700 pools throughout California.  One finding was that a typical pool consists of several plant communities that differ in duration of inundation.  Each community within the pool was sampled with 10 m² plots.  Using TURBOVEG, JUICE, and TWINSPAN software, and following the Braun-Blanquet technique, we defined one class (Downingio-Lasthenietea), three orders (Downingio-Lasthenietalia, Frankenio-Lasthenietalia, and Lasthenietalia glaberrimae), five alliances, and >30 associations. The class includes all types of hardpan, claypan, volcanic, terrace, freshwater, and saline pools in California, Oregon, Washington, and Baja California (Barbour et al. 2007).

Results/Conclusions The associations were not mappable because the pools themselves are generally <500 m² and dominated by ephemerals that change seasonally and annually.  We applied the same technique for defining mappable complexes as we used for defining associations.  Mappable complexes consist of 2-3 associations with high constancy and additional associations with lower constancy, and they are local -- restricted to one vernal pool regions.  We found two principal types of complexes, one with hardpan and one with claypan pools, each of which was subdivided to multiple locally restricted complexes.  Each complex was defined based on its own set of associations.  At a fine scale, diversity of complexes increases with pool depth.  At a coarser scale, diversity of complexes depends upon the uniqueness of the set of locally restricted associations.  Most associations are restricted to only one complex, while a few associations, such as Downingio bicornutae-Lasthenietum glaberrimae, occur in many complexes. Transformation of ground-based vegetation classification to mapping units permits us to address fine-scale patchiness and to incorporate valuable ecological information into a vegetation map. We suggest using this process as an additional step of on-the-ground data analysis for mapping other vegetation types that consist of fine-scale mosaic of plant communities.