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Ecological Modelling 220 (2009) 2792–2801
Contents lists available at ScienceDirect
Ecological Modelling
journa l homepage: www.e lsev ier .com/ locate /eco lmodel
ffects of different mobilities of leaf and woody litters on litter carbonynamics in arid ecosystems in Western Australia
hungo Kumada a,∗, Takuya Kawanishi a, Yoshishige Hayashi a, Hiroyuki Hamano b,atoko Kawarasaki c, Shin-ichi Aikawa c, Nobuhide Takahashi d, Yasuyuki Egashira e,iroyuki Tanouchi c, Toshinori Kojima f, Adrianne Kinnear g, Koichi Yamada f
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, JapanNational Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, JapanForestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki 305-8687, JapanFaculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda 386-8567, JapanDepartment of Chemical Science and Engineering, Osaka University, 1-3 Machikaneyama-cho, Toyonaka 560-8531, JapanFaculty of Science and Technology, Seikei University, 3-1 Kichijoji-kitamachi 3 chome, Musashino 180-8633, JapanSchool of Natural Sciences, Edith Cowan University, Joondalup Drive, Joondalup 6027, Western Australia, Australia
r t i c l e i n f o
rticle history:vailable online 17 August 2009
eywords:itter removalitter mobilityrid land
a b s t r a c t
Afforestation of arid land is a promising countermeasure against global warming. We had previouslyfound, through modeling and mass balance analyses of an arid land afforestation experimental projectin Western Australia, that a significant amount of litter could have been physically removed from thefloors of natural forests. In order to analyze litter carbon dynamics in depth, we investigated the actualmobility of litter in several natural forests in Sturt Meadows in an arid region of Western Australia, andestimated the difference between the removal rate of leaf and woody (twigs and branches) litter on theforest floor. Then we redeveloped a litter carbon dynamics model by incorporating physical removal oflitter to show the different mobilities of leaf and woody litters. We also analyzed carbon balances andthe effect of differential litter mobility on litter carbon dynamics. Except for twigs in one plot in a highdensity forest, 29–100% of leaf litter and 10–100% of woody litter was removed annually, demonstratingthe physical removal of litter in these natural forests. The main cause of litter removal was wind, notflooding. Decreases in leaf and woody litters could be approximated as first order decay functions in mostplots; first order decay or disappearance rate constants were then determined. Estimated disappearancerate constants of leaf and woody litter ranged from 0.19 to 11 and 0.11–12 year−1, respectively; most ofthe constants ranged from 0.19 to 2.0 and 0.11–0.74 year−1, respectively. Based on the disappearance rate
constant, the mobility of woody litter was estimated to be roughly 20% that of leaf litter, confirmingthat climatic factors move leaves more easily than twigs. The improved model, which took into accountthe different mobilities of leaf and woody litters, showed that annual physical removal of litter reached70–82% of the annual litter fall in Acacia aneura forests, and that roughly 40–60% of the existing litterwas removed annually from all sites. Incorporating into the model the difference in mobilities of leafand woody litters showed that the ratio of annual litter removal to annual litter fall increased about 10%e assu
points compared with th. Introduction
Forest ecosystems have a significant role in global carbon circu-ation. Arid and semi-arid land ecosystems have drawn particularttention as carbon sinks because trees can be planted over large
∗ Corresponding author. Present address: Division of Material Sciences, Graduatechool of Natural Science and Technology, Kanazawa University, Kakuma-machi,anazawa 920-1192, Japan. Tel.: +81 76 234 4810; fax: +81 76 234 4829.
E-mail address: [email protected] (S. Kumada).
304-3800/$ – see front matter © 2009 Published by Elsevier B.V.oi:10.1016/j.ecolmodel.2009.07.009
mption of that both litter types had equal mobility.© 2009 Published by Elsevier B.V.
areas without hindering food production. Thus afforestation orreforestation in arid and semi-arid lands has been considered asa possible countermeasure against global warming (Abe et al.,1997; Yamada et al., 1999). A series of arid land afforestationexperiments in the Sturt Meadows area, near Leonora, WesternAustralia has shown that engineering works such as blasting hard
pans to promote root growth and increasing available water byusing water-retaining substances can significantly increase plantbiomass in such areas (Egashira et al., 2003; Yamada et al.,2003; Hamano et al., 2006; Kojima et al., 2006; Takahashi et al.,2006).
Model
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environmental conditions. Before evaluating carbon budgets anddynamics in watersheds, quantitative clarification of the amount oflitter potentially removed from forest areas where litter is producedis needed to enable accurate estimation of carbon budgets.
Table 1Description of experimental sites.
Site Category Main species Canopy coverage Area (m2)
S. Kumada et al. / Ecological
In order to evaluate carbon sequestration before and afterfforestation following application of these techniques, clarifica-ion of carbon dynamics in native forest ecosystems in an aridone is needed. Litter plays a particularly significant role in for-st carbon dynamics. A number of litter manipulation experimentsave indicated that litter removal changes soil physical and chemi-al characteristic (e.g., texture, temperature, moisture content andH), carbon circulation (e.g., decomposition rate and soil respira-ion rate) and nutrient circulation (e.g., nutrient availability), and,onsequently, directly and indirectly affects plant growth rate (e.g.,ayer, 2005).
In a modeling study to determine the carbon dynamics in theturt Meadows experimental area, we found that a significantmount of litter could have been physically removed (Kumada et al.,008). Thus, there is a possibility that removal of litter significantlyffected carbon circulation in forest ecosystems in this area in dif-erent ways. However, in this study, we did not directly observe litteremoval from the forest, but estimated it from material balances.lso, the model did not distinguish between the mobilities of woody
twigs and branches) and leaf litters because this information wasnavailable at the time. This might have caused considerable errorecause leaves are clearly easier to move than twigs or branches.hus, we need to improve our litter removal model (Kumada et al.,008) by taking into account the different mobilities of leaf andoody litters.
Some researchers have also referred to litter removal by naturalauses from the arid forest floor. Facelli and Pickett (1991), in theireview, pointed out that litter movement may be very important ineserts, owing to their open structure and the light weight of dry
itter. They stated that removal of fragmented litter may be a mainause of litter disappearance, that runoff water can also removeitter from higher points, and that shrub geometry may control theype and amount of litter retained. Noy-Meir (1985) stated that onlyead wood or tough stems remain on the desert surface for yearsnd finer material is removed by a combination of consumption byoil invertebrate detritivores and physical fragmentation, erosionnd transport by wind and water. However, none of these studies,or any of the other papers reviewed, discussed the removal rater quantitative comparison of the different mobilities of leaf andoody litters.
Widening our scope to the general forest environment, someapers have discussed factors affecting litter mobility. For exam-le, Orndorff and Lang (1981) investigated leaf litter movementn a steep slope in a mixed hardwood forest. They reported thatost leaf litter movement was correlated with a strong wind event
receding a storm, and they estimated the average downsloperansport distance. Boerner and Kooser (1989) investigated theedistribution of litter and found that up to 24% of litter on the slopeas redistributed downslope. Bernal et al. (2003) reported litterynamics in a riparian forest and estimated that 7.5% of the litterhat had reached the forest floor was transported by wind into thetream. However, none of these authors discussed the quantitativeifference in mobilities of leaf and woody litters.
Thus, to our knowledge, information on litter mobility is insuf-cient for use in a model to distinguish the mobilities of leaf andoody litters. The objectives of this study were:
1) To confirm experimentally the physical removal of litter on thefloor of several natural forests of the Sturt Meadows area, in anarid region of Western Australia.
2) To estimate the difference in mobility between woody and leaflitters.
3) To analyze and evaluate litter carbon dynamics when differen-tial mobilities of woody and leaf litters are incorporated in animproved litter removal model of Kumada et al. (2008).
ling 220 (2009) 2792–2801 2793
2. Methods
2.1. Site description
The research area is located at Sturt Meadows, near Leonora,600 km east-northeast of Perth, Western Australia (latitude28◦40′S, longitude 120◦58′E; Kumada et al., 2008). The area is clas-sified as a typical arid zone, with average annual precipitation ofabout 200 mm, fluctuating widely from less than 100 to more than500 mm (Yasuda et al., 2001). Also, heavy rains of several tens of mil-limeters per event are common in association with thunderstormsand cyclones, with runoff sometimes occurring during such events(Kojima et al., 2006). The topographical slope in the research areais mostly less than 1%.
The dominant vegetation in the research area is Acacia aneura,which is distributed widely and occupies 41% of the research area(Suganuma et al., 2006a). The second dominant tree species in theresearch area is Eucalyptus camaldulensis, which exists mainly alongthe wadis and occupies 0.8% of the research area. The remainingarea is mostly bare ground.
As Acacia and Eucalyptus were the two dominant tree species inthe area, we selected three sites in natural A. aneura forests, sites2, 7 and 12, and one site in a natural E. camaldulensis forest, site LP(the sites are described in Kojima et al., 2006). The canopy coverage(the ratio of vertical canopy projection area of forest canopy to theunit ground area) of these four sites was 0.74, 0.84, 0.16 and 0.20 forsites 2, 7, 12 and LP, and they were classified as semi-dense, dense,open and open forests, respectively (Suganuma et al., 2006b).
Sites 2, 7 and 12 are located on a gently inclined plain whereassite LP is located along a wadi. Appearances of the experimentalsites are shown in Fig. 1. More detailed descriptions of the researcharea and the specific sites are available elsewhere (Yamada et al.,1999; Kojima et al., 2006).
2.2. Brief description of litter carbon dynamics in the study site
Previous modeling of litter carbon dynamics in this area(Kumada et al., 2008), suggested that significant amounts of litterwere removed from the sites of 400–4000 m2 scale shown in Table 1by physical processes such as winds and floods. There is a possibil-ity that this litter removal considerably affects soil physical andchemical characteristic, carbon circulation and nutrient circulationon the forest floor, particularly in arid areas which are susceptibleto water runoff and strong winds. Also, vegetation distribution inarid land areas shows strong heterogeneity, and part of the litterremoved by wind and flood may be deposited in different envi-ronments where different decomposition process prevail (e.g., saltlakes and watersheds). In order to evaluate the carbon dynamics inlarge areas such as watersheds, it is important to investigate howlitter is distributed inside these areas including where and howmuch litter is deposited and how litter decomposes under different
2 Semi-dense forest Acacia aneura 0.74 16 × 807 Dense forest A. aneura 0.84 20 × 2012 Open forest A. aneura 0.16 40 × 100LP Open forest Eucalyptus camaldulensis 0.20 50 × 50
All data taken from Suganuma et al. (2006b).
2794 S. Kumada et al. / Ecological Modelling 220 (2009) 2792–2801
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Fig. 1. (a–d) Forest p
In analyzing our model, the amounts of litter removal based onaterial balances from the observed or experimentally determined
alues of above-ground biomass, litter fall, litter decomposition andmount of litter on the forest floor were calculated. Although weid not directly measure litter removal, the data used in the calcu-
ations implies that carbon dynamics including litter removal in thetudy, for example the amount of existing litter on the forest floor,an be determined from the rate of litter fall, litter decomposition,itter removal and litter brought into the area. With respect to datauality, we considered that it is reasonable to assess litter carbonynamics in the study sites without distinguishing between theobilities of woody and leaf litters. However, in general, leaf litter
s easier to move than woody litter. Thus, we needed to improve ouritter removal model (Kumada et al., 2008) by taking into accounthe different mobilities of leaf and woody litters to enable moreccurate estimation of carbon budgets. To obtain information onitter mobility, we conducted the quadrat experiments described inhe next section.
.3. Monitoring of litter removal
Litter abundance on the forest floor in the studied sites was notniform. We chose three plots in each site according to litter abun-ance on the forest floor; namely, a plot having dense litter in theicinity of a tree (DL), a plot having sparse litter in the vicinity oftree (SL) and a plot on bare soil (BS). For example, we named thelot having dense litter in site 2 as 2-DL. The quadrat of 1 m2 waset up in each plot.
One hundred leaves and 10 twigs marked with colored paintere placed randomly in each quadrat. The mean(±S.D.) diameter
nd length of the twigs were 0.7(±0.2) and 18(±4) cm, respectively.e did not include large woody debris such as tree trunks and large
ranches for this mobility investigation because, in the model cal-ulations, we used the woody decomposition parameters obtained
rom the decomposition experiments using twigs and branchesor the woody pool including tree trunks and other woody debris.ecause the leaves and twigs placed in the quadrat were collectedrom the vegetation present at each site the types of leaves andwigs placed in each site differed among the sites. Acacia narrow
nomy of study sites.
leaves for site 2, Acacia broad leaves for sites 7 and 12 and Eucalyptusbroad leaves for site LP were utilized.
Litter removal was monitored as the numerical losses of leavesand twigs in the quadrat. Numbers of remaining leaves and twigswere counted nine times during the period December 2004 toDecember 2006. We measured numbers of leaves and twigs only sixor seven times for plots in site 2 and plot LP-BS, because establishedquadrats in those plots were completely destroyed by livestock orwild animals during the measurement and could not be replaced.
2.4. Litter disappearance rate constants and mobility ratio
We fitted litter removal to the exponential or first order decaymodel; the litter disappearance rate constants, k, were then calcu-lated as an index of the mobility of litter. To clarify this, we plottedthe logarithm of the remaining number of pieces of leaf and woodylitter against time. For some plots, the experimental period wasdivided into sub-periods in which the slopes could be seen in thesemi-logarithmic plot as constant values.
To compare the mobilities of leaf and woody litters, we focusedon the ratio of the disappearance rate constants of woody litter toleaf litter rather than the values themselves. We denoted the ratioas mobility ratio, which was evaluated as follows. We plotted theresidence time, that is, the inverse of the disappearance rate con-stant, of woody litter against that of leaf litter, and then estimatedthe mobility ratio from the regression line. We used the residencetime instead of the disappearance rate constant itself, because bydoing so we could include data on the immediate disappearanceof leaf litter. Because we are more interested in fast-moving litterthan very slow-moving litter, we considered this treatment to bereasonable.
2.5. Litter carbon dynamics model
We analyzed litter carbon dynamics by using a revised model(Fig. 2) that adapted different mobilities of litter to the model devel-oped by Kumada et al. (2008). The model consisted of three majorcompartments of carbon pools: plant body (WP), litter (WL) andsoil (WS). Litter is divided into two categories, leaf litter and woody
S. Kumada et al. / Ecological Modelling 220 (2009) 2792–2801 2795
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L
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Table 2Carbon pools, litter fall rate constant, and leaf litter and woody litters as fractions oflitter production in Sturt Meadows, as described in Kumada et al. (2008).
Site WP(AG)(kg-C m−2)
WL (kg-C m−2) kLF (year−1) f1 f2
Fig. 2. Revised litter carbon dynamics model.
itter. The leaf litter and woody litter are also divided into two sub-ategories, according to their rate of decomposition. Thus, we haveour sub-compartments for the litter pool, denoted as WLij, whererepresents the type of litter (i = 1 for leaf and i = 2 for woody), andrepresents the decomposition rate (j = 1 for fast, j = 2 for slow).
Carbon flows between the compartments were as follows: netrimary production (NPP) is put into the plant body, and the plantody produces litter as litter fall (LF). Then, a part of the litter isemoved physically by floodwater or wind (LR). We represent thishysical removal as “runoff” regardless of the causes, and the sub-cript “run” is used. Another part of the litter is decomposed (LD).ne part of decomposed litter is transformed into soil organic mat-
er (humus), and the other part is lost to respiration. For soil carbon,arbon input is transformation from litter, and output is the respi-ation loss.
The annual litter production or litter fall, LF, was assumed to beroportional to the amount of carbon in plant body and calculateds
F = kLF WP (1)
here WP is the amount of carbon in the plant (kg-C m−2) and kLF
s the rate constant for litter fall (year−1). Annual litter productionLFWP was then divided into two parts according to the type of lit-er. We denote the mass fraction of either leaf or woody litter in theitter fall as fi (i = 1 for leaf and i = 2 for the woody litter); thus, annualeaf litter production is f1kLFWP and annual woody litter productions f2kLFWP. For the mass ratio of the fast- and slow-decompositionractions, we use xij. The mass fraction of the jth component inhe ith litter is described as xij. For example, the mass fraction ofhe slow-decomposing component (j = 2) in the leaf litter (i = 1) is12. Note that x11 + x12 = 1 and x21 + x22 = 1, not x11 + x12 + x21 + x22 = 1.hus, fixijkLFWP of carbon was put into the ijth sub-compartmentf litter every year, and the following relationship holds:
F =2∑
j=1
2∑i=1
fixijkLF WP (2)
The dynamics of the ijth component of the litter was describeds follows:
dWLij
dt= fixijkLF WP − (kLdec ij + krun i)WLij (3)
2 2.94 0.5070.125 0.798 0.2027 3.41 0.749
12 0.500 0.0855
The first term on the right hand side refers to the input and thesecond term represents the decomposition and physical removal,where kLdec ij, denotes the first order decomposition rate constantand krun i describes the physical removal rate constants of leaf (i = 1)and woody (i = 2) litters.
Total annual amount of litter removal is
LR =2∑
j=1
2∑i=1
krun iWLij =2∑
j=1
krun 1(WL1j + rTLWL2j) (4)
Then the steady state litter carbon can be described as follows:
WL(run)∞ =
2∑j=1
2∑i=1
fixijkLF WP
kLdec ij + krun i
=2∑
j=1
(f1x1jkLF WP
kLdec 1j + krun 1+ f2x2jkLF WP
kLdec 2j + rTLkrun 1
)(5)
We also introduced the mobility ratio rTL, namely, the ratio ofthe removal rate constants of woody litter to leaf litter:
rTL = krun 2
krun 1(6)
For the value of the mobility ratio, we used the ratio of the dis-appearance rate constants of woody litter to leaf litter, which isdetermined by the method described above:
rTL ≈ kwoody
kleaf(7)
There is no guarantee that rTL can be assumed as a constant, orthat the approximation-like Eq. (7) is valid. However, for the pur-pose of incorporating the differences in mobility of leaf and woodylitter, we believe that the assumption expressed as Eq. (7) is rea-sonable as a first approximation. Note that our previous estimate(Kumada et al., 2008) corresponds to rTL = 1.
We assumed that in the experimental sites the amount of lit-ter had reached the steady state value. We calculated the annualamount of litter removal and litter removal rate constant foreach sub-compartment from the observed or experimentally deter-mined values of above-ground biomass, litter fall rate constants,ratio of leaf in litter fall, ratio of fast decomposable fraction ineach litter, litter decomposition parameters, amount of litter on theforest floor, and mobility ratio of litter. Parameters used for the cal-culation are tabulated in Tables 2 and 3, which are the same as thoseused by Kumada et al. (2008), except for the removal rate constants.
3. Results and discussion
3.1. Trends of litter disappearance
After 1 year, the relative quantities of leaves remaining in sites
2, 7, 12 and LP ranged from 0 to 60, 12–71, 0–57 and 0–53%, respec-tively (Table 4). The remaining quantities of twigs in sites 2, 7, 12 andLP ranged from 50 to 90, 70–100, 20–80 and 0–60%, respectively.Thus, the physical removal as predicted in the model was quali-tatively verified in this study. In the model calculation, we estimated
2796 S. Kumada et al. / Ecological Modelling 220 (2009) 2792–2801
Table 3Litter decomposition parameters (from Kumada et al., 2008).
Site x11 x21 kLdec 11 (year−1) kLdec 12 (year−1) kLdec 21 (year−1) kLdec 22 (year−1)
2 0.18 0.051 Infinitea
7 0.15 0.061 Infinitea
12 0.22 0.037 1.2
a The fast-decomposing fraction was assumed to decompose instantaneously after litte
Table 4Relative litter remaining after 1 and 2 years.
Site Plot Relative remaining litter (%)
Leaf Twig
After 1 year After 2 years After 1 year After 2 years
2DL 60 – 90 –SL 14 – 50 –BS 0 – 50 –
7DL 60 30 70 50SL 71 37 100 100BS 12 0 80 70
12DL 57 47 80 80SL 1 0 60 40BS 0 0 20 10
LDL 53 29 60 70
Fp
P SL 35 6 40 10BS 0 – 0 –
ig. 3. (a–d)Leaf decreases in a semi-logarithmic plot with fitted an exponential functionlot SL; open triangle with dashed-dotted line: plot BS; bar: daily rainfall.
0.27 6.9 0.0130.15 4.3 0.0300.12 Infinitea 0.025
r fall.
that the annual rate of physical removal of litter was 38–55% ofexisting litter, on a carbon mass basis (Kumada et al., 2008). On theother hand, in this research, the observed annual litter removal rateranged from 29 to 100% for leaves and from 10 to 100% for twigs,except for twigs at plot 7-SL, on a numerical basis. As the modelpredictions is based on mass and the experimental observations onnumbers, we cannot directly compare the results; however, boththe model and the experimental results show that some tens ofpercentage points of existing litter are annually removed from theforest floor.
Rate of litter removal differed among the plots, sites and lit-ter types. Loss was most rapid from plots on bare soil in openforests (plots 12-BS and LP-BS); all leaf litter placed in the quadratswas removed after 80 days. At plot 12-BS, even twigs were rapidlyremoved from the quadrat (90% of the placed twigs were gone after80 days). On the other hand, at plot 7-SL, in a dense Acacia forest,
no twigs were removed from the quadrat during the 2-year obser-vation period. The rate of decrease of woody litter was slower thanthat for leaf litter at each site.As mentioned by Facelli and Pickett (1991) and Noy-Meir (1985),litter was indeed removed from the forest floor in the study area in
for each plot. Open circle with solid line: plot DL; open square with dashed line:
S. Kumada et al. / Ecological Modelling 220 (2009) 2792–2801 2797
F unctiop
alir
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tie71wbr
sHSp2def
ig. 4. (a–d)Twig decreases in a semi-logarithmic plot with fitted an exponential flot SL; open triangle with dashed-dotted line: plot BS; bar: daily rainfall.
n arid region of Western Australia. Also, the removal rate of leafitter in the study area (more than 30%) was much faster than thatn a steep-sided valley (24%; Boerner and Kooser, 1989) and in aiparian forest (7.5%; Bernal et al., 2003).
.2. Disappearance rate constants and difference in mobilities ofeaf and woody litters
Because leaf and twig litter appears to have decreased exponen-ially with time, the relative amount of remaining litter was plottedn semi-logarithmic form (Figs. 3 and 4). Leaf litter decreasedxponentially throughout the periods in plots 2-DL, 2-SL, 2-BS,-SL, LP-DL and LP-SL (Fig. 3). For plots 7-DL, 7-BS, 12-DL and2-SL, the period can be divided into two or three sub-periods inhich the decreases are exponential. No such apparent trend can
e seen for 12-BS and LP-BS plots because of the almost immediateemoval of all litter.
The transition of the relative numbers of remaining twigs lackedmoothness because the number of twigs added was small (Fig. 4).owever, the results for plots 2-DL, 2-SL, 7-DL, 7-BS, 12-DL, 12-L, 12-BS, LP-DL and LP-SL decreased exponentially throughout the
eriods, as did results for the initial period for plot LP-BS. For plot-BS, the period can be divided into two sub-periods in which theecrease or a part of a period is exponential. For 7-SL, no such appar-nt trend can be seen. The transfer rate constants can be calculatedrom these results, as discussed in the next section.n for each plot. Open circle with solid line: plot DL; open square with dashed line:
As discussed above, litter removal can be represented by anexponential or first order decay model. We calculated the litter dis-appearance rate constants, k (year−1), as an index of the mobility oflitter by fitting to those models. Fitted first order decay models areshown in Figs. 3 and 4. Estimated disappearance rate constants, k(year−1), are summarized in Table 5. Litter disappearance rate con-stants for leaves in 12-BS and LP-BS, and for twigs in 7-SL, were notobtained because of litter movement rates that were too fast or tooslow. Except for these plots, the transfer rate constants of leaves andtwigs ranged from 0.19 to 11 and 0.11–12 year−1, respectively; mostconstants ranged from 0.19 to 2.0 and 0.11–0.74 year−1, respectively.All of the estimated rate constants were significant, and most of thecoefficients of determination were high (Table 5).
The values for the disappearance rate, k, varied widely. Also,because estimated k values are based on numbers not on mass, it isnot practicable to estimate k values directly from the results. Insteadwe tried to estimate the ratio of the disappearance rate constants.For this, we compared the residence time, that is, the inverse ofthe transfer rate constant, of leaf and woody litters. The results areshown in Fig. 5. Using the residence time (1/k) instead of the disap-pearance rate constant enabled us to include data for the immediatedisappearance of leaf litter in plots 12-BS and LP-BS as 1/k = 0. We
could not include the information for twigs in plot 7-SL; but, as wewere interested in fast-moving litter, we used 1/k rather than k.As seen from Fig. 5, data for plots 7-DL (2004.12.1–2005.9.4) and12-DL (2005.3.12–2006.12.1) were significantly different from thetrends for the other plots. The residence times for leaf litter at these
2798 S. Kumada et al. / Ecological Modelling 220 (2009) 2792–2801
Table 5Estimated disappearance rate constant, k, for each plot.
Site Plot Leaf Twig
k (year−1) Calculation period R2 k (year−1) Calculation period R2
2
DL 0.58*** 2004.12.1–2006.2.25 0.88 0.11** 2004.12.1–2006.2.25 0.53SL 1.8*** 2004.12.1–2006.7.16 0.99 0.57*** 2004.12.1–2006.7.16 0.86
BS 6.7*** 2004.12.1–2006.7.16 1.01.8*** 2004.12.1–2005.3.12 1.01.0** 2005.9.6–2006.7.16 0.95
7DL
0.19** 2004.12.1–2005.9.4 0.850.34*** 2004.12.1–2006.12.1 0.761.0*** 2005.9.4–2006.12.1 0.95
SL 0.38*** 2004.12.1–2006.12.1 0.95 0b 2004.12.1–2006.12.1 –
BS1.2** 2004.12.1–2005.6.27 0.97
0.21*** 2004.12.1–2006.12.1 0.771.3* 2005.9.4–2006.12.1 0.76
12
DL1.6*** 2004.12.1–2005.3.12 1.0
0.16*** 2004.12.1–2006.12.1 0.420.19*** 2005.3.12–2006.12.1 0.84
SL11** 2004.12.1–2005.3.12 1.0
0.53*** 2004.12.1–2006.12.1 0.822.0** 2005.3.12–2006.12.1 0.91BS Infinitea 2004.12.1–2006.12.1 – 12* 2004.12.1–2006.12.1 0.95
LPDL 0.67*** 2004.12.1–2006.12.1 0.97 0.21*** 2004.12.1–2006.12.1 0.47SL 1.3*** 2004.12.1–2006.12.1 0.98 0.74*** 2004.12.1–2006.12.1 0.81BS Infinitea 2004.12.1–2006.12.1 – 0.73** 2004.12.1–2005.6.27 0.92
a Litter was moved immediately.b
pstdw
3
iipw(w
oAt
No decrease in the number of twigs was observed.* P < 0.05.
** P < 0.01.*** P < 0.001.
lots were the longest among those observed. Except for these twolow-moving leaf litter plots, we found good correlations betweenhe residence times of leaf and woody litters (R2 = 0.64, below theotted line in Fig. 5); the ratio of the mobility of woody to leaf litteras 0.19.
.3. Cause of the decrease in litter
We consider that physical removal, not chemical or biochem-cal degradation, was the major cause of the observed decreasen the amount of litter. The reasons were: (1) we counted anyartly degraded leaves and twigs; (2) the observed rates of decreaseere much faster than can be explained by litter degradation rates
Kumada et al., 2008); and (3) some of the colored leaves and twigs
ere found outside the quadrats.As Facelli and Pickett (1991) and Noy-Meir (1985) stated, onef the possible causes of physical litter removal is flooding runoff.ccording to local inhabitants of the study area, runoff is expected
o occur during heavy rainfall events of more than 20 mm. Events
Fig. 5. Relationship between residence times of leaf and twig litters.
of more than 20 mm rain occurred on 2005.8.23–24 (total rainfall:29 mm), 2006.1.10 (60 mm), 2006.3.1 (44 mm), 2006.4.1 (24 mm)and 2006.10.26–27 (total rainfall: 35 mm).
Apart from the sudden loss of leaves in plot 7-BS and twigs inplot LP-BS on 2005.8.23–24, no apparent relationship was observedbetween heavy rain events and decreases in the remaining amountsof leaf and woody litters. The litter remaining decreased ratherexponentially during some intervals or throughout the experimen-tal period. From these data, we concluded that runoff during heavyrain events was not a major cause of litter removal.
Wind is another likely cause of litter removal. Wind erosionaffects large parts of the arid and semi-arid zones of Australia,including south-west Western Australia, which is indeed cat-egorized as a wind-erosion-prone area (e.g., McTainsh, 1998).
Compared with flooding, wind blows are frequent and we expectedthat a certain percentage, in a probability sense, of litter would beblown off during a certain intensity of wind. What we observedwere the results of frequent events of varying intensity. It appearsFig. 6. The estimated ratio of physically removed litter to litter fall, LR/LF, with var-ious mobility ratios, rTL . Open circle: site 2; open square site 7; open triangle: site12.
S. Kumada et al. / Ecological Modelling 220 (2009) 2792–2801 2799
Table 6Estimated value of the steady state amount of litter carbon, rate constant for physical removal, krun i , and ratio of physically removed litter to litter fall, compared with thecalculations of Kumada et al. (2008).
Site rTL krun 1 (year−1) krun 2 (year−1) WL1∞ (kg-C m−2) WL2∞ (kg-C m−2) LR (kg-C m−2 year−1) LR1/LF1 LR2/LF2 LR/LF
20.19 1.07 0.202 0.179 0.328 0.258 0.65 0.89 0.701a 0.425 0.345 0.161 0.215 0.50 0.92 0.59
70.19 0.801 0.152 0.304 0.445 0.311 0.72 0.79 0.73
a 0.200
0.0500.021
rnp
3b
c
F
1 0.377 0.549
120.19 1.15 0.218 0.03531a 0.550 0.0643
a rTL = 1 are data from Kumada et al. (2008).
easonable that the consequences of all these events revealed expo-ential relationships. Thus, we consider that the major cause of thehysical removal of litter in the research area was wind.
.4. Estimation of the amount of removal litter and carbonalances
From the estimated value of the ratio of litter mobility, we re-alculated the litter carbon balances by taking into account the
ig. 7. (a–c)Carbon balances in three sites for natural acacia forests (box: kg-C m−2; flow: k
0.282 0.61 0.87 0.66
0 0.051 0.81 0.86 0.821 0.047 0.71 0.92 0.75
different mobilities of woody and leaf litters. The results (rTL = 0.19)are shown in Table 6, together with our previous estimates, thecalculation of which did not distinguish between the mobilities ofwoody and leaf litters (rTL = 1).
Obtained krun 1 values for leaf litter were 1.07, 0.801 and1.15 year−1 for sites 2, 7 and 12, respectively; krun 2 values for woodylitter were 0.202, 0.152 and 0.218 year−1 for sites 2, 7 and 12, respec-tively. The values of WL1∞ and WL2∞ were 0.179, 0.304, 0.0353 and0.328, 0.445, 0.05 kg-C m−2 for sites 2, 7 and 12, respectively. That
g-C m−2 year−1). aThis fraction of litter was assumed to decompose instantaneously.
2 Mode
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wttsarcTel
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800 S. Kumada et al. / Ecological
s, the steady state amount of woody litter is roughly from 1.4 to 1.8imes higher than that of leaf litter.
Incorporating the difference in mobility between the two littersignificantly influenced the estimated composition of the remain-ng litter. We previously estimated that from 68 to 75% of theemaining litter was from leaf litter (Kumada et al., 2008), but ourew estimates are 35–41%. That is, the new estimates indicate thatoody litter comprised the majority the existing litter on the for-
st floor. This agrees with Noy-Meir (1985), who noted that in aridand ecosystems most of the remaining detritus is woody litter. Inhe study area, soil fauna rapidly decomposed leaf litter (Kumadat al., 2008), and, as shown in this research, wind removes leafitter more rapidly than woody litter. Thus, we consider that thealculated estimates reported here are more accurate than thosereviously published (Kumada et al., 2008).
The estimated value of the annual amount of litter removal, LR,as increased by taking into account the different mobilities of the
wo litters: from 0.215 to 0.258, from 0.282 to 0.311 and from 0.047o 0.051 kg-C m−2 year−1 for sites 2, 7 and 12, respectively. Corre-pondingly, ratios of annual amount of litter removal increased,nd were estimated to be 0.70, 0.73 and 0.82 for sites 2, 7 and 12espectively. The estimated values increased from 7 to 11% pointsompared with those reported previously (Kumada et al., 2008).hus, we can see that the differences in litter mobilities can influ-nce the composition of the remaining litter, and also how muchitter is removed from the forest floor.
We used the value of the ratio of the mobility of woody to leafitter of 0.19, although clearly this value may be prone to significantrror. Our final goal was to estimate approximately the carbon bal-nces in the region. Because the values of LR or LR/LF are the mostmportant for this estimation, we investigated the sensitivity of LRo the woody/leaf litter mobility ratio.
As can be seen from Fig. 6, as the woody/leaf litter mobility rationcreases, the annual ratio of physical litter removal decreased. Theensitivity differs among the sites, owing to the difference in param-ters such as decomposition rates and magnitude of disappearanceate constant. The semi-log plots can be approximated as a straightine near the mobility ratio of 0.19, and the slope of the line was esti-
ated to be a change in LR/LF divided by a change in log(mobilityatio), and the results are −0.139 (R2 = 0.99), −0.0866 (R2 = 0.99) and0.106 (R2 = 1.00) for sites 2, 7 and 12 respectively. Fig. 6 shows that
f the mobility ratio is changed from 0.02 to 1 (i.e., from one-tenth ofhe estimated mobility ratio to no difference), the difference in thealue of LR/LF changes from 14 to 23% points. That means change inhe mobility ratio affects the litter removal to litter fall ratio, but byo more than 23% points against change in one order of the mobilityatio from 0.02 to 1.
The re-estimated carbon balance is shown in Fig. 7. For sites 2,and 12, 0.37, 0.43 and 0.063 kg-C m−2 year−1 of leaves and woody
omponents fell annually from the above-ground plant to the forestoor, of which 0.26, 0.31 and 0.05 kg-C m−2 year−1 were estimated
o be removed physically from the floor, and 0.11, 0.12 and 0.011 kg-m−2 year−1 were incorporated into soil or lost to respiration. By
aking into account the different mobilities of leaf and woody lit-ers, estimated annual litter removal by physical causes increasednd thus the annual fluxes from the litter to soil or to respirationecreased. The latter flux is one third lower than previously esti-ated by Kumada et al. (2008) (0.15, 0.14 and 0.016 kg-C m−2 year−1
or sites 2, 7 and 12, respectively). We observed more change inarbon balance than in LR values because of the change in the com-osition of leaf and woody litters on the forest floor.
Because measuring the amounts of litter on the forest floorerived from leaves and woody components is not practicable, ver-
fying the reported results experimentally is very difficult. Also, inhis study, the assumption that the mobility ratio – the ratio of dis-ppearance rate of leaf litter to woody litter – was a constant is not
lling 220 (2009) 2792–2801
guaranteed. Within these limitations we consider that by takinginto account the difference in the mobilities of leaf and woody lit-ters for describing the easier removal of leaf litter than woody litter,we were able to derive a more realistic picture of the litter dynam-ics of the arid zone forest than provided by our previous estimates(Kumada et al., 2008).
4. Conclusions
(1) It was shown experimentally that a significant amount of litterwas physically removed from forest floors in arid land ecosys-tem of the Sturt Meadows area of Western Australia. From 29to 100% of leaf litter and from 10 to 100% of twig litter wereremoved annually on a numerical basis, except for woody litterat one plot in a highly dense forest.
(2) The main cause of litter removal at the experimental sites wasestimated to be wind, not floods.
(3) Litter disappearance was represented approximately by expo-nential functions, and the estimated removal rate constants ofleaf and woody litters ranged from 0.19 to 11 and 0.11–12 year−1,respectively; most of these constants ranged from 0.19 to 2.0and 0.11–0.74 year−1, respectively.
(4) Climatic factors move leaves more easily than twigs. The mobil-ity of twig litter was estimated to be roughly one-fifth that ofleaf litter.
(5) Incorporating the difference in mobilities between woody andleaf litters significantly changed the estimated ratio of existingwoody and leaf litters, and also increased the ratio of annuallitter removal to annual litter fall by about 10% points comparedwith the assumption of equal mobilities for both types of litter.
(6) The improved model, which took into account the differentmobilities of woody and leaf litters, showed that annual physi-cal removal of litter reached 70, 73 and 82% of the annual litterfall for sites 2, 7 and 12, respectively, in A. aneura forests: roughly40–60% of the existing litter was estimated to be removed annu-ally from all sites.
Acknowledgments
This work was conducted with the support of the Global Envi-ronment Research Fund of The Ministry of Environment (GHG-SSCPProject) and a Grant-in-Aid for JSPS Fellows of the Ministry of Edu-cation, Culture, Sports, Science and Technology of Japan.
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