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  My research in NM pinyon noted increased soil moisture at thinned sites (unpublished data), however this could be due to the specifics of how thinning was accomplished at those sites.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.  

Felsic and Mafic Rock Types

 

Regenerative Agriculture Controversy?

 https://www.wri.org/blog/2020/05/regenerative-agriculture-climate-change

https://agfundernews.com/opinion-where-the-world-resources-institute-got-it-wrong-about-regenerative-agriculture.html

https://undark.org/2020/01/31/podcast-43-regenerative-agriculture/

Analysis of Soil and Vegetation Maps:  Accuracy and Utility for Describing Actual Habitats

There are four sources of landscape information from maps at the project level a few miles on a side.  Topogaphic maps, satellite maps, soil service maps, and vegetation maps.   

Comparing soil and vegetation maps at this scale is complicated by inaccuracies of both map sources and the strange ambiguity of aerial photography.  Soil was mapped by NRCS into 6 major soils.  However, two of the soils are described as compound soils, regions of undefined patches possibly intergrading continuously into one another.  For example, Pyote-Maljamar soils (PU on soil map) have a layer of fine sand everywhere, but there are unmapped bits and pieces of caliche at around 50 inches (Maljamar soils) in a matrix of deep sand (Pyote soils).

Soil map created using the NRCS Web Soil Survey showing major soil types.  PT and PU are deep sands, BH and KO are shallower silty soils, and TF is intermediate.  (PA and BA are extensions of PT and TF, respectively, in Eddy county.)

Soil Profiles:

	PT	PU		TF	BH		KO
Soil Name	Pyote	Pyote	Maljamar	Tunuco	Berino	Cacique	Kimbrough
0-10	A: Loamy fine sand	A: fine sand	A: fine sand	A: loamy fine sand	A: fine sand	A: fine sand	A: gravelly loam
10-20				AC: loamy fine sand	Btk: sandy clay loam	Bt: sandy clay loam	Bkm: cemented material
20-30				Bkm: cemented material
30-40	Bt: Fine sandy loam	Bt: fine sandy loam	Bt: sandy clay loam			Bkm: cemented material
40-50
50-60			Bkm: cemented material
Type	Sandy eolian deposits	Sandy eolian deposits	Sandy eolian deposits	Sandy eolian deposits	Sandy eolian deposits over sandy calcaereous alluvium	Calcaerous eolian deposits	Calcaerous alluvium and/or eolian deposits

Selected Soil Properties

	PT	PU	TF	BH	KO
Depth to restrictive layer	>200cm	127cm	43cm	>200cm	15cm
Calcium Carbonate (CaCO3)%	2%	2%	0%	17%	15%
% sand	75.8%	81.9%	78.6%	62.6%	43.0%
Ksat (inches/hour)	7.8	8.5	12.6	1.7	0.5

In this part of NM, depth to a restrictive soil layer indicates the presence of caliche near the surface.  These petrocalcic horizons are denoted Bkm on the soil profile.  KO has the shallowest effective soil, followed by TF.  Some parts of BH appear quite shallow, but in the table the depth to a restrictive layer is listed as greater than 200cm, possibly because some of the soil (i.e. the Berino component) lacks a caliche layer. Caliche is composed of calcium carbonate, so BH and KO are listed with the most calcium, and the least sand in their profile. 

Saturated hydraulic conductivity (Ksat) refers to the ease with which pores in a saturated soil transmit water. The estimates are expressed in inches/hour for ease of comparison to possible rainfall rates. They are based on soil characteristics observed in the field, particularly structure, porosity, and texture.

Restrictive soil layers and overall soil texture contribute to the ability of a soil to drain water.  PT, PU, and TF are listed as very well drained soils because they can all drain more than 7 inches of rain an hour, whereas BH and KO are significantly less porous, draining only 1.7 and 0.5 inches of rain an hour, respectively.  Most of the water from heavy rains probably runs off of these soil types, limiting the amount available to grow plants. 

Saturated hydraulic conductivity is considered in the design of soil drainage systems and septic tank absorption fields. It probably has the greatest impact on plant production of any soil parameter in SE NM.

Hydraulic conductivity is the rate at which a soil can absorb water.  Red areas have the least ability to absorb rainfall, while blue areas have the greatest ability to absorb rainfall. Map created using NRCS Web Soil Survey.  

Vegetation Map

Vegetation map from USGS GAP Vegetation Mapper uses NatureServe Ecological System Classification.

Vegetation Map Key and Attributes

Table 3. Vegetation Map Key and Attributes

Color	ReGAP Community Name	Vegetation Type	Dominant Species	Accuracy
	Great Plains Shortgrass Prairie	Grassland	Biennial wormwood, Russian thistle	Low – should be mapped as disturbed area
	Mesquite Upland	Thornscrub	Mesquite, Catclaw Acacia, Mimosa, Yucca	High - mesquite dominant
	Sandhill Shrubland	Shrub	Shinnery oak, Catclaw acacia, Giant dropseed	Medium – not all dune
N/A	Sandy Plains Semi-Desert Grassland	Grassland	Purple three-awn, Sand dropseed, Sand muhly	Low – not mapped

The GAP national land cover data, based on the NatureServe Ecological Systems Classification, are the foundation of the most detailed, consistent map of vegetative associations available for the United States.  The soil map is interpolated based on soil pits and vegetation patterns, so in a way it functions as a hand-drawn vegetation map.  Vegetation patterns have changed from the time the soil survey was completed (1960’s?) to now.  This GAP high-resolution vegetation map was produced via satellite mapping and computer algorithms. 

The prairies of the southern Great Plains are also called the Llano Estacado, a region where vast flat to rolling uplands are covered with blue grama grass.  However, this vegetation type is misclassified.  GAP maps roads and disturbed areas with low grass as shortgrass prairie (brown on image) because these areas look similar to prairie in multispectral satellite imagery.  It maps the rest of the project area as a fractal pattern of mesquite upland (mauve) patches and sandhill shrubland (green) patches. 

Mesquite has spread throughout areas with deep sandy soils and now forms the default vegetation community across much of the area. Mesquite can also invade sandhills and desert washes and other coarse-textured soil areas. It is especially invasive in grasslands such as Sandy Plains Semi-Desert Grasslands, Great Plains Shortgrass Prairie, and Chihuahan Semi-Desert Grasslands. 

Mesquite grows best when soils are deep, lacking the caliche or clay pan that would limit infiltration and storage of winter precipitation in deeper soils layers. Mesquite and other deep-rooted shrubs exploit the deep soil moisture that is unavailable to cacti or grasses. 

The effects of major soil boundaries are evident: deep sand (PT and PU) soils support more sandhill shrubland, whereas soils with shallow restrictive horizons (BH and KO) tend to have more mesquite upland patches.  The vegetation map fails to identify patches of Lehman lovegrass grasslands, or catclaw acacia shrublands, but it does correctly identify shinnery oak areas as sandhill shrubland. 

However, the map misses out on an important intermediate community, sandy plains semi-desert grassland.  Sandy plains grasslands are actually the dominant community throughout much of the project area.  It is distinguishable on the ground by the greater proportion of grass than shrubs on sandy soils, often with Aristida purpurea, Muhlenbergia arenicola, and especially Sporobolus flexuousus.  However, this community has been invaded by mesquite shrubs (some areas of which have been recently killed with herbicides) so these grassland patches can be difficult to distinguish from true shrublands.

Topo Map

A topo map shows that areas with accumulating sand are typically uplands, especially breaks in slope where winds drop eolian deposits.  Eroding, exposed slopes reveal deeper, more-developed paleosoils, possibly Pleistocene clays (Steve Hall, 2006 Geomorphology of Mescalero Sand Dunes).  

On top of soil and geomorphic landscape-determined vegetation patterns, local populations of invasive species have overlaid an unpredictable pattern of monocultures of Lehman Lovegrass, Artemisia biennis, and occasional plants of Salsola tragus around wellpads.  Note that none of these invasive species are NM state-listed noxious weeds.  There are also surprising areas of intact, diverse Chihuahuan grasslands with healthy stands of black grama , muhly arenicola, and sporobolus cryptandrus.  All of the sand dunes here, despite presence of shinnery oak, and even some Artemisia filifolia, are coppice or hummock dunes that formed around shrubs during historical time (Hall, 2006). 

 Conclusion

Unfortunately, there are no map sources of reliable data on habitats and vegetation communities at field- or project area-scale.  Each source provides valuable clues along with misleading simplifications, errors, and obfuscations of actual on-the-ground conditions. 

Appendix: Soil Properties Maps

Depth to a restrictive soil layer:

Percent sand:

Percent calcium carbonate:

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)

Two Soils from the Manzano Mountains

Overview SOIL 1

Apparently frost -heaving has raised very weak physical (WP) crust-mounds.

Closeup of a crust mound:  The surface has been softened and the top centimeter has filled in with fine sediments.  This top layer actually has more structure than the frost-heaved material below it, which readily crumbles into its constituent soil particles.  Note the plant root, at top center of the photo, growing in the fine sediment layer.

Where litter is present, but too discontinuous to form duff, the action of frost heaving rapidly incorporates pine and juniper litter into the mineral horizon.

Overview SOIL 2

Some areas have much more biological crust than others.  In these photos, the blue grama grass Bouteloua gracilis (BOGR) is more abundant with denser cryptobiotic crust.  The darkened biological soil crust (BSC) consists of free-living blue-green algae such as Nostoc. 

Compact SP (strong physical) crust with roots and moisture evident underneath.  These cracked peds come up in 5-8 inch radius plates.  Note how different this crust is from the previous “frost-heaved” crust.  This crust has significant structure to it and doesn’t immediately crumble into constituent particles.  

To Burn or Decay? What is the best management practice to deal with excess biomass?

 Conventional forest restoration in Western pine ecosystems involves reduction of biomass through thinning, which is sometimes followed by prescribed burning to further reduce fuels.  Burning slash piles sterilizes soil patches and doesn't decrease overall site litter, so broad-scale prescribed burns have traditionally been the best management practice to reduce fuel loads.

Passing over the discussion of what was historically "natural," is fire the best tool for increasing site productivity?

Apparently not.  Duff burning kills fungi, small roots, and (obviously) removes duff. “EMF mortality and complete duff reduction after fire have been implicated with poor tree survival and slow stand recovery in forest ecosystems world-wide.”  (Smith, McKay, Brenner, mcIver, Spatafora.  2005: Early impacts of forest restoration treatments on the ectomycorrhizal fungal community and fine root biomass in a mixed conifer forest.  (PDF)

As Stametz and others have pointed out, burning is not the best use of available resources: fire volatilizes stored nutrients such as nitrogen and organic carbon (N and SOC).  Fire can also form hydrophobic soil crusts, kill flora and fauna, decrease soil microbiota (important for decomposition), destroy tree roots, mycelial networks, and sometimes mature trees.  In contrast, decomposing organic material could increase site productivity.


Permaculture forest restoration?
Restoration projects proceed with multiple goals, either explicit or implicit.  One such goal has been the return of historical fire to degraded forests.  According to a large body of research, at least some pine forests historically experienced short return-interval, low-intensity fire.  However, using this to justify current prescribed fire approaches assumes that we can --and should-- attempt to replicate historical ecosystems.  I believe it is a fallacy to assume that ecosystems, like species, must be maintained in the face of changing environmental conditions; paleoecology clearly shows that species migrated independently throughout prehistory, indicating that the ecosystems we see today are only contingent associations of species; there may be better arrangements of species and better ways of managing ecosystems than relying on historical norms.

That being said, there are even better reasons to question prescribed fire in forest restoration.  If we abandon the idea of mimicking natural disturbances we are free to innovate more productive restoration methods.  For example, Permaculture-inspired ideas of maximizing species diversity and ecosystem services could inspire a new type of forest restoration.  I envisage a restoration program designed to optimize current site conditions rather than recreate history...

What Controls Decomposition Rate in Conifer Forests?

Source: Litter decomposition, climate, and litter quality. Link.

Nutrients:  Mn and N

Manganese and Nitrogen control forest decomposition, but in an unexpected way. Mn is an essential component of ligninolytic enzymes important for degrading litter in the later stages of decomposition.  But high available N can limit decomposition. The most efficient degraders of lignin and humic acids are wood-rotting or litter-decomposing white-rot fungi (Hintikka 1970; Hatakka 2001).  For several of the lignin-degrading white-rot fungi, high concentrations of low-molecular weight N compounds suppress the synthesis of the lignin-degrading enzymes (Keyser et al. 1978; Eriksson et al. 1990; Carreiro et al. 2000). 

Further, N has repeatedly been reported to react with remains of degrading lignin to form recalcitrant condensation products. Such products form chemically (Vahtras 1982; Stevenson 1982) rather than biologically.  Spaccini (1999) suggested that such bonds create a hydrophobic surface thereby resisting decomposition. The higher the concentrations of lignin and N in a litter material the more likely it seems that such covalent bonds will be formed. Using 106 sets of foliar litter comprising 21 tree species (both coniferous and deciduous) representing a wide range in chemical composition, Berg (2000) found a highly significant negative relationship between limit values and initial N concentrations in litter.

Many examples exist in which addition of N to a N-deficient system slows down decomposition, especially where organic matter with high lignin content is present (Verhoef and Brussaard, 1990; Carreiro et al., 2000). Nitrogen and other fertilizers may negatively influence specific groups of organisms, particularly microbes.  Any shift in microbial composition can have a negative effect on other soil fauna.  As a result, decomposition and mineralization may decrease. Additionally, plants can compete successfully with decomposers for nutrients.  (Neher 2003.)

What about pH?

In a review of 58 studies, Wardle (1998) found that temporal variability in soil carbon (C) was related most closely to soil N content in forests and soil pH in arable and grassland ecosystems. (Neher 2003.)

But a study by Parn et al adding ash to a pine forest to increase soil pH did not observe increased decomposition.  A large meta-review noted that 22-85% of treatment trials have failed to affect decomposition: “This analysis shows that Ca additions are not universally beneficial and provides insight into when Ca additions to forest soils are likely to be most effective." (Evaluating the effects of liming and wood-ash treatment on forest ecosystems through systematic meta-analysis. Carolyn Reid, Shaun A. Watmough)

Decomposition Rate and Ecosystem Integrity

Neher 2003 "Effects of disturbance and ecosystem on decomposition." is a great review of this topic.  (PDF)  They conclude that "decomposition of organic matter is a useful indicator of soil condition because it is measured easily and serves as integrator of the collective activities of organisms within the soil food web."  For example, slowing rates of decomposition serve as an early warning sign of pathology in forest ecosystems (Bormann and Likens 1979),  A difference in decomposition rate between similar sites implies either a change in the decomposer community or quality of the biotic and abiotic resources at a site. 

This WSU website has a great review of pine forest decomposition.

Biological Soil Crust Restoration?

The biotic soil crust in Red Rock Canyon, Lake Mead National Recreation Area, NV is showing clumps of the dominant moss species, Syntrichia caninervis. Each black dot represents a separate plant. This extreme arid-dwelling moss inhabits the loose sandy soils of the Mojave Desert. [Credit: Alexis Wartelle]

1.  What Biological Soil Crusts (BSC) occur in the desert? Gallery of BSC: from Canyonlands Research Station

Major types (easily observable):
(NB: A misting of water can make crustal organisms more visible...)

Three soil lichens dominate crusts of both the Sonoran and Grea Basin deserts:

Collema: a genus of gelatinous lichen (blackish, jelly-like when moist). First to colonize.

Placidium: a genus of squamulose lichen (discrete rounded flakes, convex or concave). Usually a secondary successional species.

Psora: a genus of squamulose lichen (discrete rounded flakes, convex or concave), a late-successional stage lichen.

Also:
Short moss:  Mosses <10mm and="" b="" bryum="" nbsp="" spp.="">Certodon purpureus 
Heterocystic cyanobacteria: (Notoc, Schizothrix)
Large thalloid liverworts:

Source:  Biological Soil Crusts: Ecology and Management.   Technical Reference 1730-2 2001.

2.  Where do BSC occur?
Soils having high electrical conductivity, high phosphorous, and high salt contents facilitate the
formation of cryptogam crusts. Shrink-swell clays (smectites, montmorillonites dervied from volcanic ash) are the worst.

Sonoran and Chihuahan deserts have more heterocystic cyanobacteria lower lichen, but lichen like Collema, Placidium, and Peltula do occur.  Ecoregions that receive summer monsoons (e.g., the Sonoran Desert) tend to have a greater diversity of heterocystic cyanobacteria (such as Lyngbya, Calothrix, Schizothrix, and Nostoc) and lower lichen abundance. Lichens in these areas generally include the gelatinous genus Collema and squamulose genera Placidium and Peltula.  Large thalloid liverworts are more common in warm deserts than cool deserts.

Sonoran:  heterocystic cyanobacteria (Notoc, Schizothrix), gelatinous (nitrogen-fixing) lichens (e.g. Collema), squamulose lichens, short mosses
Chihuahan:  heterocystic cyanobacteria (Notoc, Schizothrix), short moss


3. Resources for mapping BSC distribution?

Map shrink/swell clays!
Some mosses are on Seinet
Lichen image gallery (from Europe) organized by structure.
Lichens on LichenPortal
Lichens on iNaturalist
USFS Database