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The Sierra Nevada Adaptive Management Experiment (AMEX) is a large-scale, replicated experiment utilizing progressive, scientifically-supported silvicultural treatments to increase resilience, resistance, and adaptation capacity of California's Sierra Nevada mixed conifer forests. The experiment is designed to generate and track long-term changes in forest composition, structure, and function under ongoing and future climate change, and treatments represent a basic suite of plausible approaches that managers may feasibly take to address ongoing and novel stresses to forest ecosystems.

This project is a collaborative effort amongst foresters and ecologists in academia and state and federal land management agencies, with fully-replicated treatments installed on state (LaTour and Mountain Home Demonstration State Forests) and private (UC Berkeley's Grouse Ridge Forest) forests paired with companion sites on federal land (Stanislaus-Tuolumne and Eshom Forests).

Over four degrees latitude from the northern to southern Sierra Nevada (Shasta to Tulare counties), this long-term, multi-decade study will compare treatments representing fundamentally different climate change impact scenarios and a suite of potential approaches forest managers may take to mitigate impacts on the ecological, economic, and social services provided by forest ecosystems.

Ongoing and predicted impacts to forests include regeneration failures, shifts in species’ ranges, drought mortality, and increasing severity of disturbances, such as bark beetle outbreaks and/or fire. These changes are creating uncertainty in how forests will function in the future and concerns around sustaining ecosystem goods and services. This cooperative, interagency plan will utilize a diverse range of silvicultural tools to reduce carbon loss and to return drought- and beetle-impacted Sierra forests from carbon sources to carbon sinks.

The foresters leading this project are dedicated to maintaining experimental sites to facilitate continued carbon sequestration and other ecosystem services while also tracking effectiveness of these varying intensity treatments.

Resilience

Resilience treatments closely mimic forest structure under historic fire conditions of the Sierra Nevada and are designed to prepare the forest for disturbance by creating stand conditions that will facilitate recovery of pre-disturbance forest conditions. These treatments mitigate climate change effects and reduce the likelihood of requiring assisted recovering following disturbance or catastrophic climate impact through retention of diverse species and structures. Resilience treatments are comparable to conventional density management in the Sierra Nevada and will create a patchy matrix with high structural heterogeneity and species diversity while retaining locally rare species (e.g. giant sequoia at southern properties)

resistance

Resistance treatments are aimed at reducing fuel loading and will prepare the forest to resist a disturbance by creating stand structure that is open, park-like, and forces fire to stay on the ground. While some change following a future disturbance may occur, it is assumed that change will be small enough in these treated stands so that the fundamental structure and composition needed to sustainably resist future disturbances remain intact. These treatments favor large trees that can rapidly respond to release and increase in average diameter. Treatments further change stand structure by removing ladder fuels and increasing spacing among trees. Opening of the understory will allow for utilization of these materials while also supporting the establishment of new cohorts of species at varying intervals. Resistance treatments lead to low stocking with retention focused on large, healthy trees across a diversity of species. Foresters will retain the largest trees of diverse species, using high leaf area, as opposed to stem form, as a deciding factor in marking (i.e. high leaf area = assumed more resistant to stress).

transition

Transition treatments will work to actively help the forest adapt to changing climate, representing the scenario where resistance and resilience treatments are not effective and the forest cannot recover without intervention. Treatments mimic a disturbance that fundamentally changes the composition and structure of the forest. In this treatment, foresters will use group selection to favor large, live trees but also create large gaps for reforestation. Canopy gaps will cover 10% of transition treatment area, with gaps ranging from 0.25ac to 1ac openings. A provenance test of seed sources will be used to reforest the properties, which will require annual visits to track success and maintain treatments. Foresters and property managers are recommitted to protecting seedlings and treating as needed. A diversity of species and seed sources (provenances) will be selected for reforestation, which will include both local populations and those predicted to be better adapted to current and future climate conditions. Available seed by species and population for each seed zone have already been identified in collaboration with the Cal Fire State Seed Bank at the L.A. Moran Reforestation Center. Transition treatments will create a low stocking matrix with large canopy openings to facilitate planting trials of seed from different provenances and species that are predicted to be better adapted to future climate conditions. These seedlings are the future forest and will need to endure ongoing changes in climate while growing into the canopy.

CONTROL

Control treatments will maintain untreated areas so that the relative effects of treatments can be assessed. Importantly, controls also represent the plausible alternative that a hands-off approach may in some cases be appropriate given certain climate scenarios. While the context of this study primarily assumes that pro-active management will likely be necessary to sustain forest values into the future, it is also critical to actively test a broad set of approaches- including a hands-off approach.