Abstract
The water storage capacity of the root zone determines whether plants
survive dry periods and controls the partitioning of precipitation into
streamflow and evapotranspiration. It is currently thought that
top-down, climatic factors are the primary control on this capacity via
their interaction with plant rooting adaptations. However, it remains
unclear to what extent bottom-up, geologic factors can provide an
additional constraint on storage capacity. Here we use a machine
learning approach to identify regions with lower than climatically
expected apparent storage capacity. We find that in seasonally dry
California these regions overlap with particular geologic substrates. We
hypothesize that these patterns reflect diverse mechanisms by which
substrate can limit storage capacity, and highlight case studies
consistent with limited weathered bedrock extent (melange in the
Northern Coast Range), toxicity (ultramafic substrates in the
Klamath-Siskiyou region), nutrient limitation (phosphorus-poor plutons
in the southern Sierra Nevada), and low porosity capable of retaining
water (volcanic formations in the southern Cascades). The observation
that at regional scales climate alone does not ‘size’ the root zone has
implications for the parameterization of storage capacity in models of
plant dynamics (and the interrelated carbon and water cycles), and also
underscores the importance of geology in considerations of
climate-change induced biome migration and habitat suitability.