Seasonality and Abundance of Truffles 

from Oak Woodlands to Red Fir Forests1 

Malcolm P. North2 

Abstract 

Truffles are an important food source for many small mammals in forest ecosystems; 

however, we know little about the seasonality, abundance, or diversity of the truffle 

community in the Sierra Nevada. This study examined how truffle abundance and 

diversity varied between oak woodland, ponderosa pine (Pinus ponderosa), mixed-conifer, 

and red fir (Abies magnifica) forests. Species richness (number of species) and abundance 

of truffles were highest in ponderosa pine stands, but species evenness was greatest in 

mixed-conifer stands. Truffle biomass peaked in late spring and fall, tracking precipitation 

patterns with a 1-2 month lag. At least 65 species of truffles were identified in a 1-ha 

sample of the forest. This number is still only a fraction of the fungal species present, as 

many mycorrhizae rarely produce fruiting bodies. Truffle production depends on the 

condition of the truffle’s mycorrhizal host trees. Natural or human disturbances, which 

affect the age and composition of the forest, will affect truffle abundance and the animals 

that depend on them for a substantial portion of their diet. 

Scientists and managers are becoming increasingly aware of the importance 

of the fungal community in forest ecosystems. Mycorrhizal fungi are essential for 

plant growth and survival; they provide food for many soil biota (Warnock and 

others 1982); they reduce soil pathogens and bacteria (Marx 1972); and they 

improve soil structure (Tisdall and Oades 1979). Fungi also produce fleshy 

fruiting bodies that are an important part of a forest’s food chain. Mycorrhizal 

fungi form above- and below-ground fruiting bodies known, respectively, as 

epigeous and hypogeous sporocarps, commonly called “mushrooms” and 

“truffles.” Although many forest animals are opportunistic consumers of 

sporocarps (mycophagy) (Fogel and Trappe 1978), several species of small 

mammals rely on truffles for a substantial portion of their diet (Maser and others 

1978). Animal mycophagists include many species of Geomyidae (pocket 

gophers), most Microtidae (voles), and almost all Sciuridae (squirrels and 

chipmunks) in North America (Fogel and Trappe 1978, Maser and others 1978), 

as well as many forest-dwelling marsupials in Australia (Johnson 1994, Seebeck 

and others 1989). Many truffle consumers comprise the base of the forest food 

chain for higher predators (Grenfell and Fasenfest 1979). A well-know example is 

the connection between truffles, the dominant food source of the northern flying 

squirrel (Glaucomys sabrinus) (Hall 1991; Maser and others 1985, 1986; McKeever 

1960), and the squirrel’s importance as the principal prey of the spotted owl 

(Strix occidentalis) in mesic forests (Forsman and others 1984, 1991; Verner and 

others 1992). It is essential that we understand how truffle abundance changes 

with forest conditions because of their substantial influence on small mammal 

populations and, consequently, the higher predators in the forest’s food web. 

1 An abbreviated version of this 

paper was presented at the 

Symposium on the Kings River 

Sustainable Forest Ecosystems 

Project: Progress and Current 

Status, January 26, 1998, Clovis, 

California. 

2 Research Ecologist, Pacific 

Southwest Research Station, 

USDA Forest Service, 2081 E. 

Sierra Ave., Fresno, CA 93710. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 91 
North Seasonality and Abundance of Truffles from Oak Woodlands to Red Fir Forests 

Changes in forest composition resulting from succession, disturbance, or 

timber harvesting will affect truffle abundance and diversity because truffles are 

produced by mycorrhizal fungi, which rely on carbohydrates from their tree 

hosts (Harley and Smith 1983). Other site characteristics, such as local edaphic 

conditions (North and others 1997) and the size and the decay state of coarse 

woody debris, may also influence truffle production (Amaranthus and others 

1994). North and Greenburg (1998) found a highly significant association between 

the most abundant truffle species in western hemlock (Tsuga heterophylla) forest 

and thick organic layers with a high density of fine roots. In stands that lacked 

these soil conditions because they had been clearcut and burned 60 years earlier, 

truffle biomass was only 20 percent of that found in adjacent old-growth stands. 

In the Sierra Nevada, forest managers need information on truffle biomass in 

different forest types and what forest conditions are associated with truffle 

production to assess the impact of their management decisions on truffle biomass 

and to evaluate the potential abundance of this food source for small mammals. 

This study was designed to answer two questions regarding truffles in 

forests of the Sierra Nevada: how do truffle abundance and species diversity 

vary among forest types and with seasons; and what are the biomass and species 

diversity of truffles in 1 ha of typical mixed-conifer forest? 

Methods 

We selected two stands in each of four forest types in the Sierra National Forest: 

oak woodlands, ponderosa pine, mixed-conifer, and red fir. The two oak 

woodland stands were at 320 m in elevation and dominated by blue oak (Quercus 

douglasii), interior live oak (Q. wislizenii), and a mixture of exotic grasses. The two 

ponderosa pine (Pinus ponderosa) stands were at 1,400 m in elevation, dominated 

by ponderosa pine but with a substantial density of smaller white fir (Abies 

concolor) and incense cedar (Calocedrus decurrens) stems. The two mixed-conifer 

stands, within the Teakettle Experimental Forest at 2,200 m in elevation, had 

white fir, red fir (Abies magnifica), incense cedar, Jeffrey pine (Pinus jeffreyi), and 

sugar pine (Pinus lambertiana). The red fir stands were at 2,800 m in elevation and 

dominated by red fir and occasional lodgepole pine (Pinus contorta). 

Beginning in February of 1996, all eight stands were sampled each snow-free 

month. In each stand, two parallel transects 10 m apart were randomly located, and 

4-m2 circular plots were raked for truffles every 10 m along each transect. New 

transects and plots were sampled with each stand visit because raking disturbed soil 

structure and mycorrhizae. A total of 100 m2 was sampled in each stand during each 

sample period. All truffles were labeled, bagged, cut in half, and dried for 48 hr at 60o 

C. Truffles were identified to species using a combination of visual cues and 

microscopic spore patterns against published keys. Difficult identifications were 

sent to Dr. Jim Trappe at Oregon State University. 

To investigate biomass, diversity, and stand conditions associated with 

truffles, we selected a 1-ha plot in mixed-conifer forest near Ross Crossing, at 1,500 m 

in elevation. All locations of truffles, trees, snags, logs, and shrubs were recorded 

using a surveyor’s total station. For weather data, we relied on records from two 

long-established weather stations. One, at the USDA Forest Service’s Trimmer Guard 

Station near Pine Flat Reservoir, was at the same elevation (300 m) as the oak 

woodland stands. The other was at Wishon Reservoir, about equidistant between 

the mixed-conifer and red fir sites, at an elevation of 2,400 m. 

Results 

The highest truffle abundance was in the ponderosa pine stands, where the 

biomass in June 1996 was 4.4 kg/ha (fig. 1). Truffle biomass, at 2.2 kg/ha, also 

peaked in the mixed-conifer at this time. All stands in oak woodlands and red fir 

92 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Seasonality and Abundance of Truffles from Oak Woodlands to Red Fir Forests North 

Figure 1—Truffle biomass by 

forest type and month (diamonds = 

oak woodlands, squares = 

ponderosa pine, triangles = mixed 

conifer, and circles = red fir). 

Figure 2—Truffle biomass and 

precipitation by month from April 

through October in 1996 and April 

through November in 1997. 

Table 1—Truffle species richness (number of species) and evenness in four forest types. 

Evenness values, using the Berger-Parker diversity index (Magurran 1988), indicate 

whether the truffle community is dominated by a single species (lower values) or if species 

are equitably distributed (higher values) 

Forest type Richness Evenness 

Oak woodlands 14 1.63 

Ponderosa pine 22 3.12 

Mixed-conifer 9 4.35 

Red fir 6 1.80 

forest had consistently low truffle biomass. The highest values for all sites occurred 

in the spring and late fall. Fluctuations in truffle biomass correlated with the 

abundance of precipitation. With a lag of 1-2 months, the peaks in truffle biomass 

during the spring and fall were closely and positively correlated with total rainfall at 

the Trimmer weather station (fig. 2). Species richness was highest in ponderosa 

stands (table 1), but species evenness was greatest in mixed-conifer stands, indicating 

a community in which no single species dominated truffle abundance. 

In the 1-ha plot, we located 869 truffles of 65-71 species, with a total dry 

biomass of 573 gm (table 2). Several individuals were immature, precluding a 

determination of whether they were new species or one already tallied. Nine 

new, undescribed species collected from the 1-ha plot now await final taxonomic 

classification at Oregon State University. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 93 
North Seasonality and Abundance of Truffles from Oak Woodlands to Red Fir Forests 

Table 2—Number of individuals and biomass (gm) of truffles by species found in a 1-ha 

plot in mixed-conifer forest. The 65-71 species (some truffles were too immature to 

identify) include nine new, undescribed species 

Sum of Number of Species 

biomass individuals 

0.01 1 

0.04 1 

0.06 1 

0.07 1 

0.16 9 

0.16 3 

0.21 8 

0.25 3 

0.25 1 

0.26 8 

0.27 2 

0.32 3 

0.36 1 

0.45 5 

0.53 1 

0.57 1 

0.57 4 

0.60 11 

0.66 1 

0.76 3 

0.81 3 

0.83 1 

0.91 2 

0.91 1 

0.93 2 

1.00 1 

1.03 5 

1.05 6 

1.19 7 

1.27 7 

Mycolevis siccigleb


Hymenogaster sp (immature)


Endogone flammicorona


Martellia brunnescens


Hymenogaster alnicola


Hymenogaster sp. nov. #19914


Hymenogaster gilkeyae


Martellia sp. (immature) 

Rhizopogon roseolus 

Genea intermedia 

Gymnomyces cinnamomeus 

Gymnomyces sp. (immature) 

Rhizopogon ellipsosporus 

Endogone lactiflua 

Hysterangium setchellii 

Alpova trappei 

Endogone sp. nov. #19927 

Rhizopogon evadens A.H. Smith 

var. evadens 

Tuber gibbosum 

Hysterangium sp. nov. #19912 

Martellia californica 

Tuber shearii Harkness 

Gymnomyces sp. nov. #22596 

Leucogaster microsporus 

Trappea darkeri 

Arcangiella crassa 

Hysterangium coriaceum 

Macowanites luteolus Smith 

and Trappe 

Martellia fallax


Hysterangium separabile


(continues on page 95) 

94 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Seasonality and Abundance of Truffles from Oak Woodlands to Red Fir Forests North 

Sum of Number of Species 

biomass individuals y g p 

1.29 2 

1.41 1 

1.47 30 

1.54 4 

1.90 14 

1.99 2 

2.39 4 

2.39 7 

2.63 3 

2.85 7 

3.66 11 

3.79 7 

3.98 2 

4.19 2 

4.76 11 

5.02 13 

5.20 20 

5.41 10 

5.60 7 

5.70 11 

6.64 28 

6.85 59 

9.56 6 

9.57 4 

12.40 8 

12.46 13 

12.75 5 

13.00 1 

14.42 9 

17.64 27 

18.16 51 

18.77 7 

19.98 7 

20.13 19 

Martellia gilkeyae


Gautieria parksiana


Genabea cerebriformis


Unknown (too immature)


Tuber sp. immature


Gautieria crispa


Hymenogaster sublilacinus


Martellia foetens


Radiigera sp. (immature)


Tuber rufum var. nitidum


Gymnomyces sp. nov. #19913


Rhizopogon vulgaris


Tuber californicum


Rhizopogon subcaerulescens


Melanogaster tuberiformis


Tuber murinum


Rhizopogon sp. immature


Balsamia magnata Harkness


Rhizopogon pedicellus


Martellia subochracea


Tuber monticola


Zelleromyces sp. nov. #19929


Rhizopogon sp. nov. w/pink stain


Rhizopogon subgelatinosus


Rhizopogon variabilisporus


Hydnotryopsis setchellii


Balsamia nigrens


Rhizopogon sp. nov. #19920


Gautieria gautierioides


Hydnotryopsis sp. nov. #19890


Tuber rufum var. rufum


Gautieria graveolens


Geopora cooperi f. gilkeyae


Hydnotrya cerebriformis


(continues on page 96) 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 95 
North Seasonality and Abundance of Truffles from Oak Woodlands to Red Fir Forests 

Sum of Number of Species 

biomass individuals 

20.13 19 Hydnotrya cerebriformis 

21.60 23 Rhizopogon brunnescens 

22.63 10 Gautieria caudate 

22.68 50 Leucophleps spinispora 

24.57 100 Hydnoplicata sp. nov. #19923 

35.28 12 Gautieria monticola 

35.28 57 Leucogaster rubescens 

96.45 74 Radiigera taylori 

573.58 869 Total 

Discussion 

Forest types with the highest densities of truffle consumers also contain the 

greatest truffle abundance. Ponderosa pine and mixed-conifer forests are home 

to most Sierra forest mycophagists, including the northern flying squirrel. Low 

truffle abundance in oak woodlands may correspond to long dry seasons or a 

low density of tree hosts. In red fir forests, long, cold winters probably reduce the 

duration of available soil moisture and may depress truffle production. 

The observed relation between truffle productivity, with peaks in late spring 

and fall, and precipitation patterns in the Sierra Nevada, is consistent with other 

studies of both epigeous (Richardson 1970) and hypogeous fruiting bodies (Hunt 

and Trappe 1987). It is reasonable to infer from these results that fungal fruiting 

is strongly conditioned on soil moisture, which is certainly influenced by a 

variety of stand factors, including canopy cover, litter depth, and root density. 

Truffle production should follow peak periods of nutrient and moisture uptake 

because mycorrhizal fungi require carbohydrates from their host plant to produce 

fruiting bodies. 

The observed patterns of species diversity are consistent with theories of 

fungal community competition. In ponderosa pine stands, which may have the 

longest period of available soil moisture, a high number of species may occur in 

the soil, but truffle production is dominated by a few superior competitors. In 

mixed-conifer forests, fewer species occur but dominance is less pronounced. 

The 1-ha plot was searched in June of 1997, after an exceptionally dry spring. 

Even under these conditions, 65-71 species were collected from the plot. Much of 

the Sierra Nevada has not yet been sampled for truffles, so probably many new 

species of truffles are as yet undescribed. Furthermore, because truffles are 

produced by only a fraction of the fungal species in the soil, even the 65 identified 

species in this sample comprise only a portion of the species present. As such, the 

data suggest that the soil fungal community may have even more species than 

the invertebrate community. We have yet to identify many of these species and 

to understand their role in “healthy” ecosystem functions. 

The observation that truffles are most abundant in the ponderosa pine and 

mixed-conifer forests is reason for extra care in planning for forest management, 

as most stand altering projects occur in these forest types. Further research is 

needed to understand the effects of thinning, burning, soil scarification, and 

compaction on this important below-ground food source. 

96 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
Seasonality and Abundance of Truffles from Oak Woodlands to Red Fir Forests 

Acknowledgments 

My thanks to Dr. Jim Trappe, Oregon State University, for introducing me to the 

world of truffles and making the difficult identifications. This work would not 

have been possible without the many field crews of trufflehounds who did much 

of the field sampling. Brian Oakley and Bill Laudenslayer provided helpful 

review comments on the draft manuscript. 

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