Land Use Change and Mountain Biodiversity - Chapter 7 potx

and Aerial Biomass Distribution in Mountain Grasslands of Northwest Argentina Roxana Aragón, Julietta Carilla, and Luciana Cristóbal INTRODUCTION Grazing and fire are the most common dis

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and Aerial Biomass Distribution in Mountain Grasslands of Northwest Argentina

Roxana Aragón, Julietta Carilla, and Luciana Cristóbal

INTRODUCTION

Grazing and fire are the most common distur-bances in many grassland ecosystems around the world (McNaughton et al 1993; Vogl 1974

in Oesterheld et al 1999, De Baro et al 1998), and they both affect biodiversity and plant com-munity dynamics Grazing and fire influence species composition and richness, determine dominant life-forms and therefore the general structure of the community (Belsky 1992; Diaz

et al 1992; Milchunas and Lauenroth 1993;

Collins et al 1998) They can also regulate eco-system processes such as nutrient cycling (Hobbs et al 1991) and plant productivity (McNaughton 1985; Rusch and Oesterheld 1997) Importantly, grazing and fire often occur together, and they interact deeply

Grazing and fire are both consumers of plant production Herbivores feeding on forage can determine the fuel load Fire, in turn, con-sumes accumulated biomass that could be used

by herbivores (Oesterheld et al 1999) Grazing can influence fire frequency and intensity, and fire determines what is left for herbivores, not only in terms of quantity but also in terms of forage quality (Hobbs et al 1991) In addition, these disturbances provide open space for col-onization that, in turn, can modify species diversity, promote the establishment of certain species, and change the general structure of the community (Collins 1987; Pucheta et al 1998;

Valone and Kelt 1999) Grazing and fire occur naturally in many grasslands and savannas, and they also are part of many management prac-tices In addition, burning in grasslands and savannas has an important worldwide effect because it is one of the major sources of atmo-spheric methane and CO2, especially in tropical areas (Crutzen et al 1985 in Hobbs et al 1991) Livestock raising is one of the most impor-tant land uses in many montane grasslands (Eckholm 1975) Particularly in Andean grass-lands, extensive cattle grazing is often com-bined with burning of the natural vegetation (Schmidt and Verweij 1992 in Hofstede et al

1995, Grau and Brown 2000) Fire promotes resprouting and is believed to encourage the development of more palatable life-forms (Grau and Brown 2000) However, grazing and fire can also increase soil susceptibility to erosion, reduce species or functional richness (Lloret and Vila 2003), and modify community com-position (Pucheta et al 1998; Diaz et al 1992) Eventually, their positive or negative effects depend on an array of factors such as grazing intensity, fire frequency, and climate

Mountain grasslands are one of the most species-rich habitats of northwest Argentina They are important in regulating the hydric regime and in providing economic resources (e.g cattle ranching and scenic values) In spite

of their ecological and economical importance, 3523_book.fm Page 89 Tuesday, November 22, 2005 11:23 AM

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90 Land Use Change and Mountain Biodiversity

mountain grasslands are scarcely represented in the protected areas of Argentina, and little is known about their functioning The study site

of this work, the valley of Los Toldos, is located

in the upper Bermejo River basin and is con-sidered an area of high conservation priority at

a national level (Brown et al 2001) The dom-inant land use is for grazing by cattle, and this

is combined with periodic fires As was observed in other neotropical mountains, recent works suggest a decrease in land use intensity

in this area (Grau et al submitted) The reduc-tion in the density of animals may produce changes in fire frequency and intensity that can,

in turn, affect plant communities in different ways In this chapter, we describe a study on how fires affect vegetation structure in the mountain grasslands of northwest Argentina that are used for grazing More specifically, this study intends to investigate the effect that the time since the last fire event may have on plant species richness, vegetation structure, and bio-mass dynamics

METHODS

S TUDY A REA

The study was performed at the valley of Los Toldos (22°30 S, 64°50 W), Santa Victoria, Salta, Argentina The study area consists of a mosaic of mountain grasslands and Alnus acuminata forest patches at an altitude of about

1700 masl This area lies in the upper altitudinal level of the phytogeographic province of the Argentinean Yungas (subtropical montane for-est) (Cabrera 1976) The original vegetation seems to have been dominated by forest patches, but a long history of grazing in the valley probably shaped the current vegetation physiognomy (Malizia 2003) The mean annual temperature is 15°C, and the average precipita-tion is 1300 mm (Ramadori 1995) The precip-itation is highly seasonal, with most of the rain falling during the summer months (Bianchi 1981)

The main disturbances at this altitudinal range are grazing, fire, and landslides (Grau, 2005), and livestock raising is the most

com-mon land use Cattle grazing is extensive with

no fences limiting individual properties There are no data on grazing intensity in Los Toldos, but information provided by national agricultural censuses for Santa Victoria shows

a decrease in the population of domestic ani-mals during the 20th century (Grau et al sub-mitted) These data, together with information provided by local people, suggest that grazing intensity in Los Toldos is currently low (between 0.5 and 1 cow per 10 hectares) The pastoral system involves transhumance, a sea-sonal movement of cattle from the highlands

to midaltitude and piedmont forests (Grau and Brown 2000) Cattle are driven up to the high-land grasshigh-lands at the beginning of the sum-mer period and, in March, they are brought back to lower ranges (piedmont forest) Dur-ing the summer period (from November to March), the animals feed mainly in grassland patches, but also browse in the Alnus forest understory Summer grazing by cattle is usu-ally prepared for by burning the vegetation in spring The extent and frequency of burning seem to depend on the proximity to settle-ments and on the weather conditions (wind, temperature, and soil humidity) when the fire

is started As a result of these management practices, the landscape consists of a mosaic

of vegetation patches, differing in the time since the last burning event occurred

S AMPLING D ESIGN AND D ATA A NALYSIS

In November 2000, we conducted a survey in the study site, looking for evidence of previous fire events Based on this survey and on infor-mation provided by local inhabitants, we iden-tified three types of vegetation patches that dif-fered in the time since the last fire event occurred:

1 Areas burned during the ongoing growing season (the last fire event probably occurred during spring 2000) These areas showed evident signs of fire, such as abundant char-coal, ashes, and burned vegetation

2 Areas burned during the previous growing season (spring 1999), with 3523_book.fm Page 90 Tuesday, November 22, 2005 11:23 AM

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Fire, Plant Species Richness and Biomass in Mountain Grasslands of NW Argentina 91

some evidence of fire (mainly the remains of charcoal)

3 Areas not burned recently In this

case, the last fire event apparently took place at least 5 years ago (spring

1995 or earlier) This information was checked with local residents

We selected three patches of each

vegeta-tion type (nine in total) The time since the last

fire event was regarded as “treatment.”

Hereaf-ter, we will refer to the different treatments as:

<1 year (areas burned during the ongoing

grow-ing season); >1 year (areas burned durgrow-ing the

previous season), and >5 years (areas not

burned for at least 5 years) Unfortunately, since

burning is a common practice in this area, we

did not have any plots that had no fire and could

have served as a control All the vegetation

patches included in our sample were no more

than 3 km apart from each other, had areas of

less than 500 m2, and were in similar

topo-graphic positions Because of the absence of

fences, vegetation patches had potentially

sim-ilar grazing pressure

In December 2000, we conducted plant

relevés in 1 m × 1 m plots with five plots per

patch The plots were placed every 10 m in a

50-m transect The transects were placed at

ran-dom in each patch (i.e 1 transect in each patch)

Each plot was divided into four 0.5 m × 0.5 m

quadrats, and all the plant species present were

recorded In addition, we collected all the aerial

biomass in ten 0.2 m × 0.2 m plots in each

patch Whenever possible, the plots were placed

in two 50-m transects that were separated by

10 m If the vegetation patches were not big

enough, we placed plots in shorter transects, but

always used the same number of plots We

col-lected biomass in December 2000 and in

Janu-ary, FebruJanu-ary, March, and August 2001 (before

the next burning event) The biomass was

clas-sified into live biomass, standing dead, and

lit-ter Live biomass was further classified for

dif-ferent life-forms (i.e graminoids, tussock

grasses, erect species, rosettes, prostrate

spe-cies, ferns, and woody species) All the material

was classified, dried to constant weight at 70°C,

and weighted

We used ANOVA tests for the comparisons

between treatments (both for total biomass and

proportions) In the case of species richness and total number of species, we used Kruskal–Wal-lis tests, a nonparametric technique, because the assumptions required for parametric tests were not met Differences in the biomass collected throughout the year were tested through repeated measures ANOVA Species frequency was computed as the number of plots per treat-ment in which each species was recorded Equativity was computed as:

E m = –Σ (p i log p i)/log N

where p i is the proportion of the species recorded in transect m that belong to life-form i, and N is the total number of different life-forms in that transect Small values of E

imply that one or a few life-forms are dominant

in the community; in other words, this index

is an indication of evenness (O’Neill et al 1988) To measure compositional similarity among plots, we performed a detrended corre-spondence analysis (DCA) with downweight-ing of rare species Only the species that were recorded in at least two plots were considered

We computed a nonparametric Kendall’s tau correlation between plot scores in the ordina-tion space and the time since the last fire event The analyses were performed in Statistica (StatSoft 1993) and PCORD (McCune and Mefford 1997)

RESULTS

We recorded a total of 149 species in the study area In Table 7.1, we have included only the species that had a frequency 0.4 in at least one

of the treatments Among these 45 species, 32 were common to all the treatments The number

of species per square meter did not differ between the treatments (Kruskal-Wallis test,

KW 3.29, p = 19) (Table 7.1), and the most common species were present in all the patches independently of their fire history The most common grasses were Elionurus muticus and

Paspalum notatum; Stevia yaconensis was the most frequent woody species, and the Cuphea

sp was the dominant prostrate species The ordination of plots in the DCA was not clearly linked to the treatments The first axis 3523_book.fm Page 91 Tuesday, November 22, 2005 11:23 AM

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of the ordination explained approximately 30%

of the overall variance in species data λ1=

0.259, total inertia = 0.859), and plot scores

were not significantly correlated with the time

since the last fire event (Kendall’s tau = 0.48 p

= 07) However, there seemed to be some

minor changes in species composition in

response to the treatments because we found

that some species were differentially recorded

in certain plots Anemone decapetala and

Tes-saria fastigiata were recorded only in plots that

were recently burned Eupatorium

bupleurifo-lium and Ophioglossum sp were more abundant

in the >5-year treatment, whereas Baccharis

tridentata and Setaria sp were predominantly

recorded in <1-year treatment (Table 7.1) In

addition, the equativity of life-forms showed a

slight tendency to decrease in the patches that

were not burned for 5 years (KW 5.42, p = 06)

(Table 7.1) The decrease in equativity in areas

that were not burned for 5 years was related to

the increasing dominance of woody species in

comparison to other life-forms that were less

frequently recorded, such as erect species and

rosettes

The total aerial biomass was significantly

higher in the patches that were not recently

burned (F = 69.59, p < 001) The biomass in

the >1-year treatment was almost twice as high

as the biomass in the <1-year treatment, and the

biomass in the >5-year treatment was more than

3 times the biomass in <1-year treatment

(431.48 ± 27.95, 738.09 ± 63.64, and 1303.25

± 58.79 g m−2 for <1-year, >1-year, and >5-year

treatments, respectively, Figure 7.1) There was

no difference between <1-year and >1-year

treatments with respect to total live biomass,

but live biomass was highest in the >5-year

plots (F = 22.02, p < 01) There was no

differ-ence in total standing dead material (F = 3.33,

p = 10), but the total amount of litter differed

between the treatments (F = 71.17, p <.001)

Patches that were burned in the ongoing

grow-ing season (<1-year) had considerably less litter

than both >1-year and >5-year patches (Figure

7.1)

The relative contribution of the different

biomass categories differed between the

treat-ments Live biomass had a high contribution to

the total biomass in the patches that were

burned during the ongoing growing season

(<1-yr) (60 ± 0.5%), whereas the proportion of litter was minimum in this treatment (15, 27, and 41% in <1-year, >1-year, and > 5-year treat-ments, respectively) (F = 5.14, p = 04 for ANOVA on live biomass and F = 73.33, p < 0.001 for ANOVA on litter) (Figure 7.2) The contribution of standing dead material was reduced in the patches that were not burned for

5 years (22, 29, and 10%, respectively) (F = 9.77, p < 01)

The proportion of live biomass differed between <1-year and >1-year treatments, but there was no difference between these two treat-ments and the >5-year treatment But impor-tantly, although the overall proportion of live biomass was similar between the <1-year and

>5-year treatments, the relative contribution of the different life-forms to the total of live bio-mass was quite distinct Patches that were recently burned (<1 year) had a high proportion

of erect species and ferns compared to the other treatments (Table 7.2) The proportion of tus-sock grasses plus graminoids did not differ between <1-year and >1-year treatments but their contribution was significantly smaller in the patches that were not burned for 5 years (28 and 33% in <1-year and >1-year and 15%

in >5-year treatments) (Table 7.2) This differ-ence was mainly due to tussock grasses that were reduced in >5-year patches (23, 25, and 9%, respectively, in the <1-year, >1-year, and

>5-year treatments) The biomass of grami-noids was similar in all three treatments Woody species accounted for 72 ± 9% of the live bio-mass in the >5-year treatment (Table 7.2) The seasonal dynamics of the total live bio-mass, standing dead, and litter showed some similarities between the treatments Biomass assigned to the standing dead compartment showed a peak in August in all three treatments (Table 7.3) Similarly, litter had its maximum value in August in the <1-year and >5-year treatments, but we did not detect any seasonal trend in the >1-year patches Live biomass showed a significant decrease in August in the

>1-year patches, and a small peak in March and December; however, no similar trend was detected in the other two treatments (Table 7.3) Interestingly, the relative contribution of live biomass throughout the year strongly differed between the treatments Patches that were 3523_book.fm Page 92 Tuesday, November 22, 2005 11:23 AM

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TABLE 7.1

Species with frequencies ≥ 0.4 in at least one of the different treatments a

<1 year >1 year >5 years p Values b

Species Life-forms <1 year >1 year >5 years

treatment is also shown.

3523_book.fm Page 93 Tuesday, November 22, 2005 11:23 AM

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FIGURE 7.1 Variation of biomass according to different treatments Live biomass, standing dead, litter, and

total biomass (g m –2 ) in patches that were burned in the ongoing growing season (<1 year), burned in the

previous season (>1 year), or not burned for 5 years (>5 years) Different letters denote significant differences

at p < 05 according to an ANOVA test, and ns indicates no significant differences The comparisons were

made between treatments within each biomass category.

FIGURE 7.2 Relative contribution of live biomass, standing dead, and litter to the total biomass in the different

treatments: patches that were burned in the ongoing growing season (<1 year), in the previous season (>1 year),

or not burned for 5 years (>5 years) Different letters stand for significant differences at p < 05 according to

an ANOVA test The comparisons were made between treatments within each biomass category.

a

b

ns

a

b

c

a

b

c

0

200

400

600

800

100 0

120 0

140 0

160 0

< 1 > 1 > 5 y e ar s Time since last fire event (years)

-2 )

Live biomass Standing dead Litter

Total biomass

c

b

a

b

a

cd

bd

ac

0

0 2

0 4

0 6

0 8

1

<1 >1 >5 Time since last fire event (years)

Live biomass Standing dead Litter

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burned during the ongoing growing season

(<1 year) had 70 to 80% of their biomass as

live biomass during December and January

(Figure 7.3), whereas in the >1-year and

>5-year treatments, this proportion hardly

approached 50% The contribution of live

bio-mass to the total was higher in the <1-year

patches almost throughout the year, which may

represent substantial changes in the seasonal

pattern of forage availability

DISCUSSION

Time since the last fire event affected the total aerial biomass, the proportion of live biomass, standing dead, and litter, and the contribution

of the different life-forms, both in terms of bio-mass and life-form frequency Nevertheless, species richness was similar among all treat-ments, and species composition showed only small variations Our results differ from those

of Collins (1987) and Pucheta et al (1998), who

TABLE 7.2

Relative contribution (mean and standard deviation) of the different life-forms to the

live biomass

Life-Forms <1 year a >1 year a >5 year a p-Values b

burned for 5 years

TABLE 7.3

Seasonal dynamics of total live biomass, standing dead, and litter (mean and standard error) a

Biomass

Compartment December January February March August F b p

<1 year

>1 year

>5 years

in 5 years (>5 years).

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found that disturbances such as grazing and fire

increased both species richness and diversity

In many cases, the increment in the number of

species results from the colonization by exotic

species or from the predominance of

small-sized species, which are tolerant to disturbance

(Belsky 1992; Pucheta et al 1998) In our study

site, we did not record exotic species, and

because all our patches have a long history of

grazing, most of these species may indeed be

tolerant to disturbances Fire and grazing may

produce the same kind of selective pressure,

and they can both favor fast-growing or

small-sized species, especially tussock grasses and

annuals For these reasons, fire suppression may

not cause compositional changes in areas such

as our study site, where grazing occurs

simul-taneously

Although species richness remained similar

in the different treatments, we detected changes

in the life-form spectrum and in the distribution

of aerial biomass The reduction in functional

or species diversity as a consequence of a

decrease in disturbance frequency has been

observed in many cases (e.g Pucheta et al

1998; Valone and Kelt 1999) and is often attrib-uted to a strengthening in species competition

In the grasslands of Los Toldos, fire suppression caused an increase in the dominance of woody species; many of these species were present in burned plots, but they became more abundant and of a bigger size in plots that were not recently burned Tussock grasses were favored

by fire, but their contribution was reduced in

>5-year treatment This change in the domi-nance of woody species alters site flammability that might reduce fire frequency in these plots

in the future

In addition to the changes in life-form con-tribution, there was a change in the distribution

of aerial biomass Fire reduced the total bio-mass by more than two-thirds (1303 gm–2 in areas not burned for 5 years compared to 431

gm–2 in areas that were recently burned), and the amount of litter was reduced in a similar way This reduction in aboveground biomass that is associated with changes in the contribu-tion of different life-forms results in changes in vegetation structure that may alter soil cover Modification of soil cover can, in turn, affect

FIGURE 7.3 Relative contribution of live biomass to the total biomass throughout the year in patches that

were burned in the ongoing growing season (<1 year), in the previous season (>1 year), or not burned for

5 years (>5 year).

0 0.2 0.4 0.6 0.8

1

Dec Jan Feb Mar Aug

<1 year

>1 year

>5 years

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erosion hazards that may have further

implica-tions on the hydrology and nutrient dynamics

of the system (Hofstede et al 1995) The

dif-ferential allocation of biomass to the distinct

biomass compartments and, especially, the

vari-ation in the amount of dead material that

reaches the soil, can alter the decomposition

rate and, consequently, the nutrient pools

(Hobbs et al 1991) Unfortunately, due to the

lack of sound information, at present we can

only hypothesize about these effects in the

grasslands of Los Toldos On the other hand,

the short-term effect of fire on forage

availabil-ity to herbivores in this site is easier to

appre-ciate

Patches that were burned in springtime had

more than 70% of their total biomass as live

biomass in the following summer (December

and January) Therefore, fire modifies the

sea-sonal dynamics of aerial biomass and changes

forage availability at least for the summer

period, when livestock is brought up to these

mountain grasslands The availability of green

forage, especially in the form of highly

palat-able grasses, is particularly important for cattle

after a period when they have had access only

to low-quality winter forage This means that

cattle obtain, in proportion, more green biomass

per bite in the patches that were recently

burned This can explain why these patches are

often preferred (Coppock and Detling 1986;

Hobbs et al 1991) Consequently, the

propor-tion of live biomass can have important effects

on livestock energy budgets and determine their

local movements Importantly, fire promotes

more palatable life-forms (grasses instead of

woody species), and this makes the effect of

fire even more meaningful to herbivores

Our results indicate that changes in the fire

frequency strongly affect vegetation dynamics

in the montane grasslands of northwest

Argen-tina However, it is worth pointing out some

limitations of this study First, we were unable

to find areas that were not burned and,

there-fore, we lacked a true control for our treatments

Our conclusions refer to the effects of a change

in the fire frequency from once a year to once

in 5 years We do not know if there is a threshold

after which a reduction in fire frequency

pro-duces no further changes in plant communities,

so we cannot say if our >5-year treatment

patches represent a transitional or a steady state Second, our sample size was rather small, espe-cially with regard to species composition This

is why we gave major emphasis to the results referred to biomass distribution, the variability

of which seems to have been sufficiently accounted for by our samples Third, an assumption of the present study is that our sam-ple patches experience a similar grazing pres-sure Even though there are no fences or other obstacles, and livestock have free access to all patches in the study area, which are also very close to one another, animals, as mentioned ear-lier, may prefer recently burned grasslands As

a consequence, these patches may receive higher grazing pressure All these limitations have to be taken into account when considering our conclusions To overcome these inherent difficulties, we are currently carrying out a con-trolled experimental study with a bigger sample size in the Los Toldos grasslands, which aims

at separating the effects of fire and grazing The preliminary results of this new experimental setup, which has been running for more than

2 years, seem to support the findings reported here

SUMMARY

Fire and grazing are the most common distur-bances in the mountain grasslands of northwest Argentina They can affect species composition and richness, determine dominant life-form, and the general structure of the community This work aims to determine the effect of burn-ing on species richness, vegetation structure, and aerial biomass distribution in the grasslands

of northwest Argentina that are subjected to grazing We performed a comparative study at Los Toldos, Salta, Argentina (22º30 S, 64º50 W) at 1700 masl and surveyed patches that differed in the time since the last fire event

We considered three treatments: patches that were burned during the ongoing growing son (in spring 2000), burned the previous sea-son, or not burned for at least 5 years Treat-ments did not cause differences in species richness, and caused only small changes in spe-cies composition The equativity of life-forms showed a tendency to decrease with fire sup-pression, with woody species becoming more

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dominant in plots that were not recently burned

Total biomass and the proportions of live

bio-mass, standing dead, and litter varied among

treatments Fire caused a reduction in total

mass, but increased the contribution of live

bio-mass and encouraged the development of more

palatable growth forms (mainly grasses)

Patches that were burned during the ongoing

growing season had 80% of their biomass as

live biomass in December and January In these

months, livestock are moved from forests at

lower altitudinal levels to these highland

grass-lands This modification in the seasonal

dynam-ics of aerial biomass may represent a substantial

change in the pattern of forage availability,

especially at this time of the year

ACKNOWLEDGMENTS

We are grateful to phytogeography students of

Universidad Nacional de Tucumán for the

assis-tance during fieldwork The manuscript

bene-fited from suggestions from three anonymous

reviewers and from colleagues from LIEY

International Foundation for Science and

Fun-dación PROYUNGAS provided financial

sup-port for this study

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