Students installing resin-based collectors in the soil to measure nutrient changes after slash pile burning

Ceramic cup lysimeter (on left) used to collect soil water, and a runoff collector (right) used to collect water that runs over the soil surface.

Collecting and measuring soil solution

Dale Johnson

Dale Johnson
Biogeochemical cycling in the Sierra Nevada
My research in the Whittell Forest over the last four decades has contributed greatly to our knowledge of biogeochemical cycling the in Sierran forest ecosystems. Such studies have shown that the long-standing, textbook paradigms for biogeochemical cycling in forest ecosystems, which were developed for humid systems, simply do not apply in forest ecosystems with a Mediterranean climate.

The Mediterranean climate imposes a much higher degree of spatial and temporal variability on biogeochemical cycling processes than is the case for more humid systems. Whereas textbooks describe intense competition between plant roots and soil microbes for nutrients in humid systems, such competition is almost entirely lacking in forests of Little Valley and the eastern Sierra Nevada Mountains: because extremely dry summers, most litter decomposition takes place beneath winter snowpack, even at near freezing temperatures. The implications for climate change, with predictions of reduced snowpack duration, are significant. This spatial discoupling allows nutrients released by decomposition to enrich surface runoff, which we believe has been exacerbated by O horizon buildups during the 20th century era of fire suppression. Finally, and perhaps most importantly, conceptual models of long-term biogeochemical budgets of more humid forests which emphasize inputs and outputs via the hydrologic cycle are completely inadequate to describe cycling in forests of Little Valley and the eastern Sierra Nevada, where, for example, nitrogen exports by volatilization during fire and inputs by post-fire biological nitrogen fixation exceed inputs by precipitation and outputs via leaching and stream flow by several orders of magnitude (see the nitrogen cycle diagram).

Nitrogen Flux

The nitrogen cycle for a pine forest in Little Valley comparing nutrient budgets dominated by the hydrologic cycle with those dominated by fire and post-fire vegetation.