Learning How Forest Stewardship Affects Our Watersheds

By Tosha Comendant & Lisa Micheli 

The primary objective of our land stewardship at Pepperwood is to cultivate healthier ecosystems. Indigenous people in this region used fire to steward the landscape for millenia. After over 150 years of fire suppression, our forests have become overstocked, meaning there are more trees per unit area than is really healthy for the forest. At Pepperwood, we estimate that without stewardship, our forests have approximately ten times more trees per acre than would have historically been here.

Research suggests that forests composed of dense stands of young trees use more water than those with fewer mature trees. This means that the overstocked forest consumes more water through a process known as evapotranspiration, which describes how plants use the water collected from their roots and then release it as vapor into the atmosphere. Based on the concept of the “water balance,” that would mean that less water remains in the ground for our soils, streams, and the riparian habitats next to streams, which nurture species needing cool, moist habitats.

Less water in the ground and in streams is bad news for species that rely on watersheds for food, shelter and water – including humans. Overstocked forests have also been shown to be less resilient forests, more vulnerable to drought, pests, pathogens and wildfire. This is in part because the competition for water in overly dense stands is not ideal for growing conditions. Redefining stewardship for a climate-changed world, using both Indigenous and western science, will be key to sustaining ecosystem services that forests provide, like clean water, clean air, carbon sequestration, nutrient cycling, wildlife habitat, and recreation.

Thanks to funding from California’s Wildlife Conservation Board, Pepperwood is working with world experts on how to make very precise measurements of the water and energy balances in our forest ecosystems. The work we are doing at Pepperwood (and at a partner site in the Northern Sierras) is filling critical data gaps by providing some of the first empirical measurements of the “exhalation” of the land – fluxes of gases including water vapor and carbon dioxide – in Northern California. We will combine these measurements with those that quantify the water in our soil and in our streams to see whether our prescription of forest thinning – wherein we remove all trees less than ten inches in trunk diameter at breast height throughout a given area – results in more water made available to the ecosystem.

We are testing the hypothesis that forest thinning may increase water yield by using a paired watershed approach. We will compare the amount of water in the air, in the ground, and in our streams in a treated versus an untreated watershed (Weimar Creek Basin vs. Redwood Creek Basin, respectively). This project is a result of a close collaboration between our Research team and our Preserve Management team. This study, investigating the relationships between forest stewardship and watershed response, is the first of its kind in our region.

What is a Flux Tower?

The most novel addition to Pepperwood’s Sentinel Site are the two “flux towers” that are part of this project. They’re designed to generate empirical measurements of evapotranspiration, contributing to a rare and critical dataset that is necessary for getting climate change projections right in our region and for the globe! The towers, each weighing in at 200 pounds and standing 70 feet tall, were hand-carried into the field and installed by our team of technicians and staff volunteers.

 

 

Sensors in the Air

High up on each tower, an anemometer measures wind speed and direction while infrared gas analyzers measure the flux of gases, including carbon dioxide and water vapor, between the terrestrial (land-based) ecosystem and the atmosphere. The towers are also equipped with radiometers to measure incoming solar radiation and outgoing thermal radiation – the inputs and outputs of the earth’s energy balance. Precipitation (rainfall), the primary input to the water balance along with fog, is measured via a set of tipping bucket rain gauges.

 

 

Sensors in the Ground

Changes in soil moisture storage and temperature is quantified using soil moisture sensors installed at multiple depths below ground. These sensors measure how much electricity can be transmitted through the soil, which is an indicator of water content. We will also measure the amount of water in fractured bedrock underlying the soil using drilled boreholes and a device called a neutron probe.

 

 

Credit – Ian A. Nelson

Sensors in Our Streams

Stream flow measurement stations (stream gauges) will be installed at the lower ends of our watersheds, allowing us to quantify the total water flowing out of the system. Each watershed will have devices installed that are robust enough to measure big winter flows and sensitive enough to measure summer base flows. We will be measuring the height and velocity of the water in order to estimate the total surface water discharge of the basin.

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