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Effect of Managenlent on. NII.t)~ien.t DYllamics
in SOllth.,vesterJ} Pin)70n JII.Jlipt~J· 'V()odlands
L. F. DeBano and J. M. Klopatek 1
----,--------------------------------
Abstract--Pinyon and juniper tre.~s cycle nutrients producing a moo.
saic nutrient distribution that is vulnerable to range improvement, ~Jrazing,
and fuelwood harvestin~J activities. Prescribed fire rE!leases small amounts
of availabh~ nutrl.~nts, but also volatilizes lar~Je quantities. Th.~se impacts
on nutrient cycling may affect long te!nn procluctivlty of pinyon juniper
woocllands.
Pinyon juniper woodlands occupy 62 to 79 million acres in
the weste·rn United States (Arnold et a1. 1964) and about 15
million acres in Arizona and New :Mexico (Springfield 1976).
These woodlands, which occur in the transition zone between
semiarid vegetation (chaparral, desert shrub or grasslands)
and coniferous forests, are characterized by a mosaic of
pinyon and juniper tree.s; interspace areas are occupi.ed by
sparse to dense herbaceous and shrubby vegetation.
Pinyonjuniper stands are found on a wide variety of parent
materials; soils vary in texture from stony, cobb ley, and
gravelly sandy loams to clay loams and clay, and vary in depth
from shallow to deep (Aldon and Brown 1971, Pieper 1977,
Springfield 1976). In pinyon juniper woodlands a soil nutrient
mosaic pattern develops where carbon, nitrogen (N), and
available phosphorus (P) are concentrated in the upper soil
layers under the tree canopy. This pattern reflects the accumulation of litter by different plant species (Barth 1980, Charley
and West 1975, Everett et a1. 1986, Lyons and Gifford 1980,
Klopatek 1987). Tree growth rates vary widely between sites
in close proximity to one another. Since pinyon juni.per woodlands occur in different climatic regimes, and on a wide variety
of soil types, these variable growth rates sug,gest nutrient
limitations may exist similar to those found in other forest
ecosystems. Although N is usually considered the most limiting
nutrient in forest ecosystems (Maars et al. 1983), it appears P
and potassium (K) may also be limiting (Barrow 1980, Bunderson et a1. 1985).
Past management has emphasized tree removal for range
forage improvement, but recently inte,rest has increased in
harvesting pinyon and juniper trees for fuelwood. Prescribed
fire has been used extensively during both range improvement
and slash disposal after fuelwood harvesting. Intensi.ve tree
removal and prescribed fire, coupled with continued use for
grazing, is expected to alter naturally occurring nutrient
cycling processes in these woodlands and thereby affect long
term interrelationships among site productivity, succession,
and tree growth (Miller at al.. 1981, Young and Evans 1981).
This paper presents information on distribution of nutrients
and their cycling i.n pi.nyon juniper woodlands, and assesses the
effect of different management strategies.
Nutrient Cycling and Distribution
Pinyon and juniper trees cycle nutrients both horizontally
(Tiedemann 1987) and vertically (DeBano et a1. 1987). Tre.e
roots penetrate into interspace soils between tree canopi.es
where they absorb nutrients and incorporate them into tree
biomass. A large portion of the nutrients captured from
interspaces are deposited on the soil surface under trees
during leaf fall, where they are released in an available form
by decomposition, thereby enriching the upper soil laye.rs.
Trees also translocate nutrients vertically to the soil surface
from deeper in the soil profile by a similar process. The
quantity of nutrients cycled by trees varies considerably from
one locality to another, depending upon land use, climate, soil,
and tree density and size.
Published information on nutrient patterns in pinyon juniper woodlands clearly portrays strong vertical and horiz.ontal
distribution patterns developing from the above described
nutrient cyd.ing regime. The most important vertical com partments are: above ground biomass, litter, and soil nutrients.
Nutrients are also distributed and exchanged horiz.ontally
between trees and inte.rspaces, resulting in larger amounts of
1De Bano is PrinCipal Soil Scientist. Rcd(y Mountain Fcrest and
Range Experiment Station, Forestry Scienc!~s Laboratory, Arizona State
University Campus, Tempe, AZ 85287. Station headquartE~m is in Fort
Collins, in cooperation with Colorado State University. Klopatek is Associate Professor, Department of Botany and Microbiology, Arizona State
University, Tempe, AZ 85287.
157
nutrients being present in the living biomass and litter trees
compared to interspaces. The quantities of nutrients store,d in
soils unde.r tree canopies has been reported by some authors
to be. greater than in interspaces (Everett et aJ.. 1987, Tiede··
mann 1987), while in other cases no sign.ificant differences
could be detected (DeBano et aJ.. 1987). Information presented in the literature on N, P, and K were used to develop
a model portraying vertical nutrient distribution patterns
under trees and in associated interspaces for a typical. pinyon
juniper ecosystem (table 1). Data on N presented in table 1 for
a pi.nyonjuniper ecosystem was taken from Tiedemann (1987).
Distributions of P and K were tak~n from DeBano et a1. (1987),
where. soils data for the 0 3.8 cm depth was extrapolated
linearly to 60 cm. Important features of the vertical. di.stribution pattern are: (1) a small percentage of the, total nutrient
pool resides in plant biomass and litter, and (2) the three
nutrients differed in the proportion of a nutrient stored in
living biomass and liUer. For example, under tree canopies
higher proportions of N are present in litter and above ground
biomass compaIed to P and K. About 98 percent of total P in
the tree ecosystem is contai.ned in soil, compared to 93 percent
for K, and 82 percent for N. Horizontally, N, P, and K are
concentrated in a mosaic pattern corresponding to litter and
canopy distribution. Although total. N, P, and K in the soil may
or may not differ significantly beneath tree canopies and
interspaces, more total N, P, and K accumulates in live tree
boles, stems, and leaves and in litter under tree canopies than
in interspace vegetation (DeBano et a1. 1987).
Nutrient Availability
Available and total nutrients are delicately balanced because only a small percent of the total nutrient pool is in a
readily available form. Vertical and horiz.ontal distribution
patterns of available and total nutrients are similar. Horizontal
patterns of nitrate N and ammonia N are influenced by tree
canopy distribution, with higher concentrations of ammonia N
being found in the surface soil under tree canopies compared
to interspaces (DeBano et a1.1987). In contrast, nitrate N may
or may not differ between trees and interspace areas (DeBano
et a1. 1987, Klopatek 1987, Thran and Everett 1987).
Management Implications
Information on nutrient dynamics can be used for assess··
ing the effect of different management strategies on the
nutrient status and productivity of pinyon juniper woodlands.
Important strategies include: grazing, fuehvood harvesting,
and using prescribed fire either for type conversion or slash
disposal following fuelwood harvesting. Tiedemann (1987)
estimated N losses over a 100 year peri.od would be 340 kg/ha
by grazing and 856 kgjha if chaiJling and burning were used for
cover conversion. In contrast, fuelwood harvesting over the
same 100 year rotation would remove only about 133 kglha of
158
Table 1.--Arnounts of nitrogen, I)hosphorus, and potasslurn (kg/ha) In
above-ground blornass and the eO-cm soli depth under trees and
associated InterSI)aCeS In 1)lnyon-)unll)er WOOdlands, and percent In each ecosystem COml)artrnent.
Ecosystem
cornpartment
N1
p2
K2
Trees
Foliage
Twigs
Wood
Litter
Soil (0-60 ern)
Total
108
184
133
1,000
6,615
8,040
(1 )
(2)
(2)
(12)
(82)
20
11
5
44
3,963
4,043
« 1)
« 1)
« 1)
(1 )
(98)
147
5!5
21
65
3,584
3,872
(4)
(1 )
« 1)
(2)
(93)
InterSI)aCeS
Foliage
Soil
4 « 1)
0.2 « 1)
4,527 (>99) 3,963 (>99)
0.2 « 1)
3,584 (>99)
1Data from Tiedemann 1987.
20ata from DeBano et al. 1987.
3percent of total nutrient pool made up by a particular ecosystem
compartment.
N in the woody material. However, if prescribed fire was used
for slash disposal following harvesting, an additional 277 kg/
ha of N would be volatilize,d from twigs and leaves (if 95
percent of the N is volati1iz.ed) i.n addition to variable amounts
of the 1000 kglha of N contained in the litter. If large amounts
of litter were consumed by fire during slash disposal ope.rations, then an additiona.1400 500 kg/ha of N could be lost. It is
not known how grazing would affect P and K pools.
Fuelwood harvesting would remove only a small percentage of the P (5 kg/ha) and K (21 kg/ha) nutrient pools during
a 100 year rotation. Substantial P would also be lost if the leaves
a.nd twigs were burned following fue.lwood harvesting. Nonparticulate losses up to 50% of the total P (16 kg/ha) could
occur if these fine materials we.re totally consume.d during
burning (Raison et al. 1985). A variable amount of the P
contained in the litter could also be lost, depending on the
intensity of the fiIe. Similar percentages of K may also be lost
because it volatilizes at the same temperature as P (Raison et
a1. 1985).
The effect of different management activities on available
nutrients is not as well understood as on total nutrient pools,
although there is some information available on the effect of
fire on nutrient availability. Studies in pinyon juniper and other
ecosystems show fire acts as a rapid mineralizing agent, and
releases ammonia N which is later converted to nitra.te N when
conditions are favorable for nitrification (Klopatek 1987). The
release of highly available forms of N by fire portrays the
impression that burning increa.ses soil fertility on a site.
However, total N i.s reduced, and increases in ammonia and
nitrate N are short lived because these nutrients are rapidly
immobilized biologically. Inorganic P also is released by
burning, but it too is quickly i.mmobilized chemically (DeBano
and Klopate.k, In press) and may no longer be readily availa.ble
for plants. Harvesting also increases the concentration of
nitrate N in soil surface layers (DeBano et al. 1987). Nitrate N
presumably increases because harvesting reduces inhibition of
nitrification, eliminates trees which assimilate any nitrate N
being formed, or changes the microclimate. Both s(;il moisture
and temperature can be increased by harvesting (Everett and
Sharrow 1985) which in turn may affect microbial relationships.
Bunderson, E. D.; Weber, D.J.; Davis,J.N.1985.Soilmineral
composition and nutrient uptake in Juniperus osteosperma in 17 Utah sites. Soil Sc.ienc.e. 139: 148.
Charley, J. L.; 'Vest, N. E. 1975. Plant-induced soil chemical
patterns in some shrub-dominated semi-desert ecosystems jn Utah. Journal of Ecology. 63: 945-963.
DeBano, L. F.; Klopatek, J.M. (In press.) Phosphorus dynamics of pinyon-juniper soils following simulated burning.
Soil Science Society of Ameri.c.a Journal.
DeBano, L. F.; Pe.rry, H.M.; Overby, S. 1987. Effe.cts of
fuelwood harvesting and slash burning on biomass and
nutrient relation.c;hips in a pinyon juniper stand. I'n: Proceedings--pinyon juniper conference; 1986 January 1316; Reno, NV.; R. L. Everett (compiler); Gen. Tech. Rep.
INT -215. Ogden, UT: U. S. Department of Agriculture,
Forest Service, Intermountain Research Station. 382-386.
Summary and Conclusions
Pinyon and juniper trees enrich the surface soil beneath
their canopies by cycling plant nutdents, both vertically from
the subsoil and hodzontally from adjacent interspaces. This
produces a spatial distribution. of nutrients roughly corresponding to existing tree canopy cover. Both Nand P undergo
extensive horizontal and vertical translocation. Important
features of this vertical distribution pattern are: (1) a small
percentage of the total nutdent pool resides in plant biomass
and litter, and (2) N, P, and K differ in the proportion of a
nutrient stored in Hving bioma~ and litter. The location of
active nutrient pools beneath tree canopies makes them vulnerable to different management practices such as range
improveme.nt and fuelwood harvesting. Prescribed fire is used
in both range improvement and slash disposal after fuelwood
harvesting. Range improvement practices designed to perma··
nently convert pinyon juniper stands to grassl.ands may have a
major impact on the storage and cycling of both above and
below ground nutrients, particularly if chaining and burning
are used as part of the treatment. Fuelwood harvesting not
only removes nutrie.nts directly from the site, but also affects
mineralization and the release of available nutrients remaining
on the site. Prescribed fire acts as a rapid mine,ralizing agent,
making part of the nutdent pool more readily available but at
the same time volatilizing substantial amounts of nutrients
from the litter and above ground biomass. These losses and
changes in nutrient pools may affect the productivity of pinyon
juniper woodlands.
EvereU, R. L.; Sharrow, S. 1985. Soil water and temperature
in harvested and on harvested pinyon-juniper stands. Res.
Pap. INT-342. Ogden, UT: U.S. De.partment of Agriculture, Forest Service, Intermountain Forest and Range
Experiment Station.
EvereU, R. L.; Sharrow, S.; Thran, D. 1986. Soil nutrient
distribution under and adjacent to singleleaf pinyon
crowns. Soil Science Society of America Journal. 50: 788792.
Klopatek, J. M.1987. Nitrogen mineralization and nitrification
in mineral soils of pinyon-juniper ecosystems. Soil Science
Society of America Jotunal. 51: 453-457.
Lyons, S.l\1.; Gifford, G. F. 1980. Impact of incremental
surface soil depths on plant production, transpiration
ratios, and nitrogen mineralization rates. Journal of
Range J\ianagement. 33: 189-196.
Maars, R. H.; Roberts, R. D.; Skeffington, R. A.; Bradshaw,
A. D. 1983. Nitrogen in the development of ecosystems.
In: J. A. Lee et a.1., ed. Nitrogen as an ecologica.l factor.
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Miller, H. G .1981. Forest fertilization: some guiding concepts.
Journal of Forestry. 54: 152-167.
Pieper, R. D. 1977: The southwestern pinyon-juniper
ecosystem.I'n: Ecology, uses and management of pinyonjuniper woodlands. Gen. Tech. Rep. RJ\I-39. Fort Collins,
CO: U.S. Department of Agriculture, Forest Service,
Rocky l\lountain Forest and Range Expe.riment Station.
3-10.
Raison, ILr.; Khanna, P. K.; "'ooels, P. V.1985. Mechanisms
of element transfer to the a.tmosphere during vegetation
fires. Canadian Journal of Forestry. 15: 132-140.
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H."'.
Springfield,
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