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Shrublands and Grasslands of the Jornada
Long-Term Ecological Research Site:
Desertification and Plant Community
Structure in the Northern Chihuahuan
Desert
Laura Foster Huenneke
Patterns of Plant Diversity _ _ __
Abstract-Semidesert grasslands of southern New Mexico have
been replaced in this century with shrublands (chiefly creosote bush
and mesquite). The Jornada Long-Term Ecological Research Program has been monitoring aboveground plant productivity, plant
species composition, and other aspects of ecosystem function in 15
sites. Plant diversity differs significantly among vegetation types,
with grasslands being the most diverse. Aboveground net primary
productivity does not differ significantly among vegetation types.
Productivity is correlated with plant species richness in some
seasons; however, diversity has not been correlated with smaller
fluctuation of production between years in the first few years of
study. These results have implications for sustainable management
of diversity and of productivity.
First, what are the patterns of plant species diversity? In
the Jornada LTER program, we have been monitoring 15
sites scattered across the J ornada Experimental Range and
the NMSU College Ranch (Chihuahuan Desert Rangeland
Research Center): 3 grasslands, 3 grass-dominated playas,
3 creosote bush sites,3 mesquite sites, and 3 tarbush sites,
since 1989. We have compiled lists of all plant species
present in permanent study quadrats, 3 times each year
(winter, spring, and late summer), and have measured their
biomass and productivity (methods described below). In
addition, several environmental variables (precipitation,
soil moisture, etc.) are monitored on each site. We can
therefore compare the number of species present at a particular sample date, or cumulative over time-the total
species list for a site.
At all sample dates, grasslands have more plant species
present than do shrub-dominated systems (Huenneke, unpublished data). From 1989 through 1994, these numbers
are approximately 40-50 species per site (during the growing
season) for grasslands, in comparison with 20-35 species in
a creosote bush area and 10-15 in mesquite sites. Of course,
no one sample date represents all the species present in a
site; many of the plants of semi-desert regions are ephemeral
annuals or short-lived perennials, and no single sample
(even in a "good" season) can capture the entire species list.
Thus all sites have continued to "accumulate" species over
time; that is, the cumulative species list for each site continues to grow, even after 5 years of data collection. Total
species numbers are roughly 100 or so for grassland sites
thus far, compared to 50-60 in creosote bush sites and 20-40
in mesquite sites.
Over time, the graph of cumulative species numbers is
steepest for the grassland sites. That is, not only do the
grasslands have the highest species richness at any given
point in time, but they demonstrate the greatest diversity
over time. This reflects in part an impressive amount of
seasonal and annual variation. In sum, the grasslands
contain the largest and most dynamic pool of species.
Inspection of the identity of species in the various sites
reveals a great deal of overlap in the species lists. My initial
hypothesis was that grassland species might have been
displaced, replaced by distinct species' assemblages representative of Chihuahuan desert shrub systems. One would
then expect little overlap between species lists of grassland
and shrubland sites. However, nearly all plant species found
in the shrub-dominated sites are also found in grasslands.
In this symposium we have heard much about the history
of vegetation change and the alteration of ecosystem structure here in southern New Mexico (Buffington and Herbel
1965). My presentation will focus not on the causes and
mechanisms of this change, but instead on the results of that
change as seen in plant diversity and productivity. I address
the following questions.
First, how does plant community structure in remnant
grasslands of the Jornada basin compare with that in the
"desertified" shrublands?
Second, has there been a major decline in the productive
potential of the ecosystem? I assessed productivity as
aboveground net primary production (NPP) of all species,
not just economic or forage production. The conversion of
grassland to shrubland is one stage of the desertification
process (Schlesinger and others 1990),anditisimportantfor
purposes of sustainable management to determine whether
the change in vegetation has been associated with a permanent and substantial decline in the system's productivity
(Verstraete and Schwartz 1991).
Finally, I take this opportunity to address a very basic
theoretical question in ecology: is there any association
between the biological diversity of a system and its productivity? That is, is there some ecosystem "function" of plant
diversity?
I?: Barrow, Jerry R; McArthur, E. Durant; Sosebee, Ronald E.; Tausch,
Robm.J., co~ps. 1996. Proceedings: shrubland ecosystem dynamics in a
changmg envlronment; 1995 May 23-25; Las Cruces, NM. Gen. Tech. Rep.
INT-GTR-33~. Ogden, UT: U.s. Department of Agriculture, Forest Service,
Intermountam Research Station.
L. F. Huenneke is Associate Professor of Biology, Department of Biology
'
New Mexico State University, Las Cruces, NM 88003.
48
This suggests that conversion of grassland to shrubland took
place via the impoverishment of (and alteration of relative
abundances within) the grassland flora. In most of the
creosote bush and mesquite sites, we have encountered no
plant species absent from the grasslands.
that of grasslands, but these productive sites are not always
the same ones each year. Again, three sites per type allows
only a weak test, but it appears there is no gross difference
in average productivity.
Relationship Between Plant
Diversity and Productivity
Productivity of Jornada
Ecosystems _ _ _ _ _ _ _ __
Recently, we have become interested in applying our data
to a more basic ecological question: that of the relationship
between diversity and productivity of a biological system.
While there are theoretical and conceptual reasons to suppose that greater diversity in plant growth forms and species, for example, might allow more efficient production in a
system over time, there are few empirical studies of these
relationships. There are at least two contrasting views
(Schulze and Mooney 1993): one suggests that the more
diverse a system is, the more productive and efficient it will
be (with more complete use of resources, alternative species
succeeding under varying conditions, and so on). The alternative view is that above some minimum number, extra
species do not add much to ecosystem function - that is, there
may be some "redundancy" in ecological communities. Our
data afford a preliminary opportunity to address this topic.
When one arranges the 15 sites in order of species richness
during a given sample period, there is a strong positive
correlation with aboveground productivity at some (but not
all) sample dates. Sites do receive different amounts of
rainfall, even in a single season, because of the patchiness of
summer storms. We have precipitation data for individual
sites, so I was able to calculate the ratio between productivity and rainfall as an index of efficiency of production. The
hypothesis was that a larger number of species in a system
should allow more efficient use of rainfall (that is, a positive
correlation between species number and efficiency of production in a given season). But there was no significant
correlation.
Also, when one calculates the average annual production
at a site, and plots it against the cumulative species number
for that site over the first few years of sampling, there is no
significant correlation. I also hypothesized that where conditions vary greatly from year to year (as in deserts), perhaps
having a large species pool means a greater ability to
produce SOMETHING each year. That is, species may
substitute for one another from year to year, and NPP (net
primary production) may remain more constant than it
would for a less diverse system. The hypothesis, then, was
that the range or fluctuation in NPP values should decrease
as number of species in system increases - a negative
correlation or slope. However, this was not the case! The
most species-rich systems, if anything, had larger (not
smaller) ranges ofNPP values.
Given that these ecosystems have altered substantially in
structure, how much has this change in structure altered
ecosystem function, especially productivity? While there is
an undoubted decline in forage and economic productivity as
grassland is replaced by desert scrub, it is not obvious
whether there has been a permanent decline (as of yet) in the
productive capacity of the landscape. The LTER program is
interested in determining whether there has been any decrease in net primary productivity - the amount of carbon
fixed by plants in photosynthesis per unit area in a fixed time
period.
The 15 sites being followed in detail provide an opportunity to assess whether aboveground productivity differs
between shrub- and grass-dominated systems. We have
sampled each site three times per year, in winter, spring,
and late summer or fall, since 1989. Non-destructive measures of aboveground plant size are made for all species, in
49 permanent 1_m2 quadrats per site. Then an estimate of
live biomass for each species in each quadrat is made, based
on regressions developed for individual species from harvests over the past several years. Productivity is measured
as the increases in biomass from one sample period to the
next, summed for all species in a quadrat. This figure is
definitely an underestimate, as we miss some production
and loss of tissue over the season, but has proven to be a
useful index for between-site comparisons. Our approach
allows us to sample shrublands and grasslands with equivalent methods and equal intensity; it also yields non-destructive estimates that include a measure of variation over
space-that is, variation from one square meter to another.
Thus far we have found few significant differences among
ecosystem types in the average aboveground standing crop
(g/m 2 ), except in winter when those systems with shrubs
definitely have more living tissue aboveground. (However, a
sample size of three sites per ecosystem type means we have
a fairly weak ability to detect differences.) Of course,
aboveground biomass is distributed on the surface differently; in grasslands, there is not much variation in biomass
from one quadrat to another (low range of values), but in
shrublands there is tremendous variation. Some patches
have very high biomass, while others are completely bare (on
the scale of 1_m2 quadrats).
Does net aboveground productivity differ between grasslands and shrublands? On average, there is no meaningful
difference among ecosystem types. In some seasons, playas
have very high productivity (when they have filled with
water and herbaceous growth has been great); in other
seasons, they may have virtually zero productivity
aboveground. Other than when playas have extremely low
productivity, however, there are rarely significant differences among vegetation types. At least one or two shrubdominated sites each season have productivity as high as
Conclusions
-------------------------------
In the J ornada basin of southern New Mexico, desert
shrublands contain fewer species than do the remnant semidesert grasslands. In general, the shrub sites contain only
plant species that are also present in grasslands, while the
grasslands contain many unique species. Thus the grasslands are important reservoirs of plant diversity.
49
There is no large-scale difference in the mean biomass or
average primary productivity of grasslands and shrublands
(although of course these are distributed differently across
the surface). These results suggest that as of yet there has
been no severe impact of desertification on the productive
capacity of the system.
While there is some tendency for more species-rich systems to have higher productivity than others, this higher
species richness does NOT seem to provide the buffering
against year-to-year variability one would expect. Further
analysis may focus on growth form diversity or on some
index of diversity more sophisticated than simple species
richness. Certainly the LTER data will be a rich source of
information on the relationships between ecosystem structure and function in this diverse and complex landscape.
References
---------------------------------
Buffington, L. C.; Herbel, C. H. 1965. Vegetational changes on
a semidesert grassland range from 1858 to 1963. Ecological
Monographs. 35: 139-164.
Schlesinger, W. H.; Reynolds, J. F.; Cunningham, G. L.; Huenneke,
L. F.; Jarrell, W. M. Virginia, R. A; Whitford, W. G. 1990.
Biological feedbacks in global desertification. Science. 247:
1043-1048.
Schulze, E.-D.; Mooney, H .. A 1993. Biodiversity and ecosystem
function. Ecological Studies 99. Berlin: Spring-Verlag. 525 p.
Verstraete, M. M.; Schwartz, S. A 1991. Desertification and global
change. Vegetatio. 91: 3-13.
50