Biogoechemical evolution along a series of wave-cut terraces on the California coast gives rise to dramatic changes in forest productivity, stature and functioning over geological time. On young, fertile sites, forest productivity is substantial, home to towering Redwood trees and natural Bishop pine ecosystems. On older sites, the ecosystems undergo a series of radical changes that result in the emergence of ‘pygmy’ forest – an edaphic end-member that’s dominated by dwarf trees and shrubs of like species to the younger sites. Here we examine the causes and consequences of such shifts in forest productivity at Jug Handle State Natural Reserve. We hypothesized that the pygmy forest develops in response to severe nutrient depletion, owing to the exhaustion of rock derived nutrients from the soils over millennia. We tested this hypothesis on selected sites that shared similar bedrock geologies, climates, topographies and potential biota. We measured nitrogen, phosphorus and other elements in foliage and soils, in addition to isotopes of strontium in different components of the ecosystems.

Results/Conclusions

We observed a significant change in the strontium isotope composition of forest foliage across the sites. In young sites, forest foliage closely resembled the strontium isotope ratio of bedrock. In older, pygmy sites, strontium isotopes in foliage were similar to that in rainfall, indicating almost wholesale reliance of pygmy forests on the atmosphere for phosphorus and cations. Thus, we accept the hypothesis that pygmy forest is the product of the collapse of weatherable rock elements; rather, these forests rely on very small and dilute inputs of nutrients from rain, which is dominated by the chemistry of nearby sea water. Moreover, concurrent with this shift, soil and foliar pools of phosphorus, calcium and magnesium undergo dramatic decreases, providing additional evidence for depletion of rock-derived elements. We conclude that pygmy forests operate effectively as “rain plants”, relying almost exclusively on the atmosphere for all resources – water, nitrogen, phosphorus, and nutrient cations.