Cutting Trees for Water: Are Thinned Forests Wetter or Drier?

Forest thinning can be controversial.  Concerned citizens want to know when logging counts as restoration;  can thinning a forest have beneficial ecological effects beyond reducing the risk of stand-replacing wildfires?  Will cutting trees increase soil moisture because there are less "straws sucking up water", or does it decrease soil moisture due to increased windspeed and more sunlight drying out the forest understory?

April 2017 - views of Rogers Lake, AZ overlooking untreated (left) and treated (right) areas.  Photos by Conor Flynn.  Click this link to play with the slider.  

Whether thinned forests are drier or wetter is complicated.  The excellent paper "Adapting western North American forests to climate change and wildfires: 10 common questions" by Prichard et al provides a good introduction to this question:

"Decreasing canopy bulk density can change site climatic conditions (Agee and Skinner 2005). Wildfire ignition potential is largely driven by fuel moisture, which can decrease on drier sites when canopy bulk density is reduced through commercial thinning (e.g., Reinhardt et al. 2006). Reduced canopy bulk density can lead to increased surface wind speed and fuel heating, which allows for increased rates of fire spread in thinned forests (Pimont et al. 2009, Parsons et al. 2018). Other studies show no effect of thinning on surface fuel moisture (Bigelow and North 2012, Estes et al. 2012), suggesting that thinning effects on surface winds and fuel moisture are complex, site specific, and likely vary across ecoregions and seasons."

Anecdotally, some people have noticed springs beginning to flow again after thinning and prescribed fire in AZ.  My research in NM pinyon noted increased soil moisture at thinned sites (unpublished data), however this could be due to the specifics of how thinning was accomplished at those sites.  Thinned slash was chipped and the chips were left on-site without follow-up prescribed fire.

In addition to water quantity, water quality should also be considered.  Prichard et al point out that "Treatments in watersheds that are distant from the WUI and protect municipal and agricultural water supplies are critical to minimizing high-severity fire impacts that can jeopardize clean water delivery (Bladon 2018, Hallema et al. 2018). For example, post-fire erosion and debris flows may cause more detrimental and longer-term impacts to watersheds than the wildfires themselves (Jones et al. 2018, Kolden and Henson 2019)."  However, even carefully managed thinning and prescribed fire can generate excess erosion from new roads, decreased large woody debris, and increased mobility of light charred wood.  Charcoal washing into local lakes can cause fish kills, even when not generated by catastrophic wildfire.  Creating erosion-control structures as part of forest thinning work could help to mitigate these risks.  

Further research is needed to ensure that large thinning projects adequately account for water cycle restoration in addition to natural stand density and fire interval restoration.  

Restoration in hyper-arid environments?

 Is is possible to plant trees and restore grassland savanna in a hyper-arid desert?  

In Al Baydha it sometimes doesn't rain for 26 months, and apparently there are trees that can survive and thrive in that environment.

The Story of Al Baydha: A Regenerative Agriculture in the Saudi Desert.

A New Precipitation-Evapotranspiration Index to Map Global Drought

Standardized precipitation–evapotranspiration index (SPEI) is precipitation minus potential evapotranspiration (Vicente-Serrano et al 2010).  It is distinguished from other drought indices because it accounts for temperature through modelled evapotranspiration. This website maps global drought:

Drought in the US 2010-2011.

SPEI has recently been used to predict pine mortality in SW forests.  When the SPEI is below -1.68 for at least 11 months, pinyon and ponderosa pines cannot grow and mortality soon follows. (Kolb, T.E., 2015. A new drought tipping point for conifer mortality.Environmental Research Letters, 10(3), p.031002.)

Long-term drought graph for NM.

The Theory and the Reality of Shelterbelt Afforestation Projects

The Theory of Shelterbelts

The Reality

Introduction:  dust bowl, us efforts

Following the dust bowl years in the U.S. the government planted 220 million trees in a strip 100 miles wide, stretching 18,600 miles from Canada to the Brazos river.  1935-1942  Today, the growth and vigor of many trees has declined due to close spacing, age, and invasion of undesirable short-lived trees.  Wikipedia.

There are currently two major afforestation programs, one in China, and one in the Sahel.

Great Green Wall in China. 

This project aims to afforest 90 million hectares and eventually contain 100 billion trees in a 4500km belt.

A recent paper by Tan (2014) found decreased dust transport due to the plantings so far.  But independent Chinese media reported in 2013 that dust storms were increasing:  For centuries in northern China, annual sandstorms, called the Yellow Dragon, have been ripping through the city.  Wind erosion is obvious and most pronounced in spring, when sandstorms are common and the vegetation is still absent or dormant after severe winter temperatures. Sandstorms have increased in the last few years, calling into question whether the Great Green Wall is working.

Liu Tuo, head of the desertification control office in the state forestry administration, is of the opinion that there are huge gaps in the country's efforts to reclaim the land that has become desert. At present there are around 1.73 million sq kilometers that have become desert in China, of which 530,000 km2 are treatable. But at the present rate of treating 1,717 km2 per year, it would take 300 years to reclaim the land that has become desert.  

Background
In early times, Korqin was not a semi-desert, but savannah-type woodland, in transition between dense forest and the steppe zone. The rolling sand-sheet was deposited during the last glacial period (12000 years BP). During 10,000 years of vegetation growth, thick dark topsoil developed. Since historical times, the region has gone through several cycles of man-induced desertification and subsequent recovery, when human pressure lessened. Fertile dark topsoil vanished and extensive dune fields gradually build up.  Overgrazing (by cattle, goats, sheep, camels, horses), clearing of land for agriculture and over-cutting of trees and shrubs in this vulnerable ecosystem have resulted in an increasingly severe land degradation and desertification.

Other Approaches?

There are many who do not believe the Green Wall is an appropriate solution to China’s desertification problems. Gao Yuchuan, the Forest Bureau head of Jingbian County, Shanxi, stated that “planting for 10 years is not as good as enclosure for one year,” referring to the alternative non-invasive restoration technique that fences off (encloses) a degraded area for two years to allow the land to restore itself.  Soil fertility, already critically low, has shown a sharp decline as all organic residues from crops are removed for fuel and fodder during wintertime. Willow and poplar stands are pollarded in autumn, before leaf fall, for the same purpose. The continuous removal of potential nutrients to the soil is not balanced by the relatively small amounts of manure and inorganic fertiliser applied to crops.

Problems

 Jiang Gaoming, an ecologist from the Chinese Academy of Sciences and proponent of enclosure, says that “planting trees in arid and semi-arid land violates [ecological] principles”.The worry is that the fragile land cannot support such massive, forced growth. Tree growth in Korqin is largely dependent on the presence of a high groundwater table, fed by percolation and inflow from the western and southern mountainous areas. The long-term trend of a decreasing depth of the groundwater table is due to an increasing demand for water to irrigate crops and for human and industrial needs. If the trees succeed in taking root, they could soak up large amounts of groundwater, which would be extremely problematic for arid regions like northern China.  For example, in Minqin, an area in north-western China, studies showed that groundwater levels have dropped by 12–19 metres since the advent of the project.

Progress So Far

As of 2009 China’s planted forest covered more than 500,000 square kilometers (increasing tree cover from 12% to 18%) – the largest artificial forest in the world.However, of the 53,000 hectares planted that year, a quarter died. In 2008 winter storms destroyed 10% of the new forest stock, causing the World Bank to advise China to focus more on quality rather than quantity in its stock species.  FAO report

But the program’s widespread tree planting campaigns typically allot only one or two species of tree to an area. Professor Jiang wrote in a 2009 Epoch Times article, “In Ningxia, for example, 70 percent of the trees planted were poplar and willow. In 2000, one billion poplar trees were lost to a disease (Anoplophora), wiping out 20 years of planting efforts.”  FAO report followup

More criticisms:  Wikipedia.

Great Green Wall in Africa - the Sahel

The Great Green Wall initiative is much more nuanced than simply planting a belt of trees across the continent: “Behind the name or the brand ‘Great Green Wall,’ different people see different things. Some people saw just a stripe of trees from east to west, but that has never been our vision,” he says. “In Niger, Mali, and Burkina Faso . . . natural regeneration managed by farmers has yielded great results. We want to replicate and scale up these achievements across the region. It’s very possible to restore trees to a landscape and to restore agroforestry practices without planting any trees. This is also a sustainable way of regenerating agroforestry and parkland.”

But it should be noted that the Great Green Wall is not designed to prevent the Sahara Desert from expanding. “We are not fighting the desert,” he says. “In the majority of the areas we are working in these 11 countries, the desert is not advancing. The [Sahara] Desert is a very stable ecosystem. Of course, there are some areas on the margins—for instance in Senegal, Mauritania, and Nigeria—where there are some sand movements. But from a geographic perspective, over time the desert has been relatively stable in this area.” (Source)

But some authors advocate  "a shift from planting trees in the GGW to utilizing shrubs (e.g., Leptospermum scoparium, Boscia senegalensis, Grewia flava, Euclea undulata or Diospyros lycioides), which would have multiple benefits, including having a faster growth rate and proving the basis for silvo-pastoral livelihoods based on bee-keeping and honey production.” (Connors and Ford, 2014 Sustainability)