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Plant EcologyAn International Journal ISSN 1385-0237 Plant EcolDOI 10.1007/s11258-013-0177-y

Performance of established native seedlingsin relation to invasive Lantana camara,rainfall and species’ habitat preferences in aseasonally dry tropical forest

Geetha Ramaswami & Raman Sukumar

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Performance of established native seedlings in relationto invasive Lantana camara, rainfall and species’ habitatpreferences in a seasonally dry tropical forest

Geetha Ramaswami • Raman Sukumar

Received: 7 August 2012 / Accepted: 23 January 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Native species’ response to the presence of

invasive species is context specific. This response

cannot be studied in isolation from the prevailing

environmental stresses in invaded habitats such as

seasonal drought. We investigated the combined effects

of an invasive shrub Lantana camara L. (lantana),

seasonal rainfall and species’ microsite preferences on

the growth and survival of 1,105 naturally established

seedlings of native trees and shrubs in a seasonally dry

tropical forest. Individuals were followed from April

2008 to February 2010, and growth and survival

measured in relation to lantana density, seasonality of

rainfall and species characteristics in a 50-ha permanent

forest plot located in Mudumalai, southern India. We

used a mixed effects modelling approach to examine

seedling growth and generalized linear models to

examine seedling survival. The overall relative height

growth rate of established seedlings was found to be

very low irrespective of the presence or absence of dense

lantana. 22-month growth rate of dry forest species was

lower under dense lantana while moist forest species

were not affected by the presence of lantana thickets.

4-month growth rates of all species increased with

increasing inter-census rainfall. Community results may

be influenced by responses of the most abundant

species, Catunaregam spinosa, whose growth rates

were always lower under dense lantana. Overall

seedling survival was high, increased with increasing

rainfall and was higher for species with dry forest

preference than for species with moist forest preference.

The high survival rates of naturally established seed-

lings combined with their basal sprouting ability in this

forest could enable the persistence of woody species in

the face of invasive species.

Keywords Lantana � Mudumalai �Relative height growth rate � Seasonal rainfall �Mortality � Sprouting

Introduction

Invasive plants cause an overall increase in mortality

or decrease in regeneration of native tree seedlings by

direct competition (Gorchov and Trisel 2003; Galbra-

ith-Kent and Handel 2008; Hoffman and Haridasan

2008), by affecting below ground mutualisms (Stinson

et al. 2006; Rudgers and Orr 2009) or by releasing

potentially allelopathic compounds through leaf litter

or plant tissue (Orr et al. 2005). Recent large-scale

meta-analyses have emphasized the importance of

understanding the context dependency of the impacts

Electronic supplementary material The online version ofthis article (doi:10.1007/s11258-013-0177-y) containssupplementary material, which is available to authorized users.

G. Ramaswami � R. Sukumar (&)

Centre for Ecological Sciences, Indian Institute of

Science, Bangalore 560012, India

e-mail: rsuku@ces.iisc.ernet.in

G. Ramaswami

e-mail: geetha.ramaswami@gmail.com

123

Plant Ecol

DOI 10.1007/s11258-013-0177-y

Author's personal copy

of invasive species (Palacio-Lopez and Gianoli 2011;

Pysek et al. 2012), and, therefore, exploration of such

impacts at local scales is vital (Palacio-Lopez and

Gianoli 2011). For example, young woody plants

could already be facing stressful abiotic conditions

such as seasonal water deficit at an invaded site. How

native plants respond to a combination of invasive

species and stressful environmental conditions could

depend upon their own characteristics, such as the

degree of habitat specialization. We examined the

effects of the interaction between invasive species,

water stress via seasonal drought and species charac-

teristics on the growth and survival of young woody

plants in a seasonally dry tropical forest.

Considered among the most threatened ecosystems

of the world (Olson and Dinerstein 2002), tropical dry

forests are increasingly being invaded by alien plant

species such as Lantana camara L. (henceforth lantana),

one of the most wide-spread invasive shrubs in the world

(Richardson and Rejmanek 2011). Seasonally dry

tropical forests receive 250–2,000 mm of rainfall

annually and experience seasonal drought of up to

6 months in a year (Murphy and Lugo 1986). Woody

species in dry forests are thus expected to face relatively

long periods of water deficit. Advanced plant regener-

ation is dependent on seasonal and spatial moisture

availability. For example, in moist forests (annual

rainfall [2,000 mm) that experience seasonality in

rainfall, growth and survival of naturally established

seedlings is higher in the wetter season and in topo-

graphically wetter habitats (Comita and Engelbrecht

2009). Interactions between species’ characteristics and

environmental factors can also result in differential

increase in growth and survival of some species guilds

over others (e.g. Nagamatsu et al. 2002; Bunker and

Carson 2005; de Gouvenain et al. 2007). For example, in

a study conducted in a seasonal moist forest in Panama,

dry forest specialists grew better than moist forest

specialists when artificially irrigated, and growth of

established seedlings was greater when light availability

at ground level was higher (Bunker and Carson 2005).

In dry seasonal forests, both light and moisture

availability are expected to co-vary with season. While

the survival of seedlings of a few dry forest species has

been found to increase with shading during the dry

season, growth has been found to be limited by light

availability in the wet season (Gerhardt 1996; McLaren

and McDonald 2003). Established seedlings of most

species in moist forests, on the other hand, seem to

prefer a mid-range of light intensity for recruiting into

higher size classes (Ruger et al. 2009). Comparable

studies of community-wide performance of established

seedlings from dry seasonal forests are lacking, where,

in addition to existing seasonal variability in light,

invasive species with shrubby habits could alter light

conditions further. Evergreen invasive shrubs else-

where have been shown to reduce light availability at

ground level (Brantley and Young 2009; Rascher et al.

2011). In a previous study conducted during the dry

season, we demonstrated that shade under lantana

increased with increasing stem density (Ramaswami

and Sukumar 2011). However, the intensity of shading

by lantana may change seasonally, with even greater

shading during high-rain months when the shrub

canopy is densest.

Given the above-mentioned trends in environmental

modification by invasive shrubs and in the growth and

survival of established seedlings under different mois-

ture and light conditions, we hypothesised the following

seedling responses in relation to lantana canopies,

seasonal drought and species habitat preferences:

1) We expected seedling growth to be light-limited

in high-rainfall months. Therefore, overall growth

under dense lantana was expected to be lower in

high-rainfall months than in low-rainfall months.

2) Dry forest preferring species, probably adapted to

growing in open-canopy and, therefore, high-light

conditions, were expected to have lower growth

rates under lantana thickets. Moist forest prefer-

ring species, probably adapted to growing in

closed-canopy and, therefore, lower light condi-

tions, were expected to show the opposite pattern.

Species with no forest habitat preferences (ubiq-

uitous) were expected to be unaffected by the

presence of lantana.

3) Seedlings of dry and moist forest preferring

species were expected to respond not only to

light conditions typical of such habitats, but also

overall moisture conditions here. Dry forest

habitat preferring species were expected to be

more drought tolerant than moist forest preferring

species. Dry forest preferring species were,

therefore, expected to show higher survival in

low-rainfall months than moist forest preferring

species.

We tested these hypotheses with a field study

conducted in the seasonally dry forest of Mudumalai,

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southern India, over a period of 2 years, with the broad

aim of understanding the effects of the presence of an

invasive plant, seasonality in rainfall and species

characteristics on the performance of native seedlings.

Methods

Study site

The Mudumalai Forest Dynamics Plot (MFDP hence-

forth), located centrally in the Mudumalai National

Park and Wildlife Sanctuary (321 km2, 11o300–11o390N,

76o270–76o430E) in the southern Indian state of Tamil

Nadu, has an area of 50 ha. The MFDP was set up in

1988–1989 in order to study the long-term dynamics

of the vegetation of a tropical dry forest (Sukumar

et al. 1992). Detailed descriptions of the vegetation

composition and fauna of this forest can be found

elsewhere (Sukumar et al. 2004; Prasad et al. 2010).

The MFDP is relatively open, with a density of 241

trees ([10 cm dbh) per hectare, as compared to an

average of 551 trees ([10 cm dbh) per ha in the moist

forest large-scale plots at Barro Colorado Island

(Panama), Sinharaja (Sri Lanka) and Pasoh (Malaysia)

(tree density estimated from data available at

www.ctfs.si.edu). This forest experiences distinct

seasonality in rain, with pre-monsoon showers during

April–May, a monsoonal phase during June–October

and a dry season typically extending from late

November to mid-April. The MFDP received an

average (± SD) of 163 ± 109 mm of seasonal rainfall

between April and October and 25 ± 40 mm between

November and March during 1990–2010. For the

duration of this study, rainfall received during the

monsoon season varied considerably between years

(Appendix 1A in supplementary material). Average

monthly maximum temperature (±SD) typically ran-

ges from 25.5 ± 2.2 to 31 ± 1.4 �C (in August and

April, respectively) while the average minimum tem-

perature (± SD) ranges from 13.9 ± 2.3 to

18.2 ± 2.9 �C (in January and April, respectively).

Study design

The MFDP is gridded into 20 9 20 m plots which are

further divided into 10 9 10 m sub plots. We chose

this site for our field study as the invasion history of

each sub plot is known from 1989 to present. The

qualitative density of lantana has been visually

estimated as being ‘absent’, ‘present’, ‘common’,

‘dense’ or ‘very dense’ at this scale annually, and the

mean biomass for each category has been estimated

once (Appendix 1B in supplementary material). In

order to compare the effects of dense lantana versus no

lantana on native species, we randomly selected a total

of forty 100 m2 plots of which 20 were lantana

‘absent’ subplots (LA plots hereon) and 20 were ‘very

dense’ (LD plots hereon) sub plots. At the time of

sampling, the lantana density map was more than a

year old and, therefore, some lantana stems were

found even in LA plots. However, these stems did not

form thickets at the time of plot selection, nor during

the entire course of the study and thicket-forming

stems were confined to LD plots. On an average

(± SE), LA plots had 6.1 ± 1.1 stems, while the LD

plots had 39.4 ± 7.2 stems of lantana. Sampled plots

were located at least 20 m apart.

Although ‘seedling’ has been defined variously in

the literature (e.g. Comita and Engelbrecht 2009; Busby

et al. 2010; Midgley et al. 2010), for the purpose of this

study we defined ‘seedlings’ as naturally established

stems of woody species that ranged from 10 to 100 cm

in height. A large proportion ([90 %) of such seedlings

in our study represent advanced regeneration while

some seedlings may have germinated more recently.

However, this is difficult to ascertain in the field without

destructive sampling, as young plants in this forest

become woody within a few weeks to months after

germinating (unpublished data). A total of 1,105 such

stems of 33 native woody species (Appendix 1B in

supplementary material) were tagged, mapped and

followed within 2-m-wide strips along the four sides of

each 10 m 9 10 m sub plot from April 2008 until

February 2010, once every 2 months. One particular

species, Catunaregam spinosa, had very high seedling

density per plot and we, therefore, tagged a maximum

of fifteen individuals per plot. Excluding seedlings of C.

spinosa, on an average (±SD) there were 20 ± 8.2 and

17 ± 8.1 native woody seedlings per 100 m2 in LA and

LD plots, respectively.

We quantified seedling response to interacting

environmental conditions in terms of their height (see

Appendix 1B in supplementary material for rationale)

and survival. The straight-line distance of the highest

living bud of an individual from the ground was

measured to the closest centimetre and was considered

as its height. Relative height growth rate (Masaka

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et al. 2012) was quantified as follows: RhGR =

(ln(htT2)-ln(htT1))/(T2-T1), where T1 and T2 are the

beginning and the end of inter-enumeration interval

measured in months. The loss of individuals from one

enumeration to the next was treated as seedling

mortality. This included instances of tissue ‘dieback’,

wherein the above-ground biomass of seedlings was

lost, usually due to desiccation. Seedlings that showed

tissue dieback were often seen to basally sprout back,

although the duration between dieback and sprouting

was highly variable. When tissue dieback was

observed, the interval for which there was no increase

in seedling height was excluded from growth analyses

as well.

To determine the effect of species characteristics on

growth and mortality, observed seedlings were

grouped into one of three habitat preference catego-

ries—dry forest, moist forest or ubiquitous (no pref-

erence for either dry or moist forests). The functional

classification of species on the basis of their habitat

preference has been defined in detail elsewhere

(Ramaswami and Sukumar 2011). To determine the

effects of temporal availability of moisture on seedling

growth and mortality, we used daily rainfall (in mm)

data recorded at an automatic weather station in

Kargudi, about 3.4 km from the MFDP, maintained by

the Centre for Ecological Sciences (Indian Institute of

Science, Bangalore, India). For the seedling growth

and survival analyses, total rainfall received between

two consecutive growth intervals (inter-census rainfall

hereon) was considered.

We examined variation in light availability in LA and

LD plots between May 2009 and March 2010 (with the

exception of August and November 2009) using light

sensing data loggers (UA-002-08, HOBO Pendant�

Temperature/Light loggers). One pair of loggers per plot

was kept at two levels (*50 cm and *3 m) from the

ground and placed at one of the four corners of the plot.

In LD plots, *50 cm represented light received below

the lantana canopy and*3 m was sufficiently above the

lantana thickets to capture ambient light, while in LA

plots both levels were expected to record ambient light.

Instantaneous light availability measured in lux was

recorded every 10 minutes in a plot for 24 h, across

7–10 days per month. Since the total solar energy

received at each logger was highly variable between

days of the same month, we used the proportion of light

received at*50 cm to that received at*3 m in order to

make measurements comparable between plots. The

amount of light at a plot was thus computed as a

proportion of the total light received from 9:00 am to

7:00 pm, at *50 cm to that at *3 m. Proportion light

filtered at *50 cm in LA and LD plots was arcsine-

square root transformed before further analyses. We

used t-tests to compare per-month light filtered at

*50 cm from ground level between LA and LD plots.

We also ascertained whether global light availability

varied with precipitation by comparing the monthly

averaged solar insolation with total monthly rainfall for

the years 2005–2008. Thus the shading effects of

lantana, if any, were expected to compound the variation

in seasonal light availability.

Statistical analyses

Community patterns of seedling growth—RhGR was

computed at a growth interval of 2 months (the

frequency of height measurement), 4 months (roughly

corresponding to a seasonal rate of growth) and

22 months (the duration of the study). Given the

longitudinal and unbalanced nature of the data, we

chose a mixed effects modelling approach to examine

seedling growth. Lantana density (LA or LD), species’

forest habitat preference (dry, moist or ubiquitous) and

inter-census rainfall were specified as fixed effects while

Plot ID was specified as a random effect (see Appendix

1C in supplementary material for model details).

Analyses were performed using function ‘lmer’ of the

lme4 package (Bates et al. 2011) of the statistical

software R version 2.13.1 (R Development Core Team,

R Foundation for Statistical Computing, Vienna, 2011,

Austria). Our aim was to look at the relative effects of

each factor considered, and, therefore, we did not

simplify the global models. We generated non-para-

metric bootstrap confidence intervals (95 % CI hereon)

of model coefficients by randomly sub-sampling from

the data and then refitting the model 10,000 times.

Coefficients with 95 % CI non-overlapping with zero

were considered statistically significant (for details, see

Appendix 1C in supplementary material).

Species level patterns of seedling growth—the

most abundant species in the plot were likely to

influence community-level patterns. We, therefore,

examined the individual growth rates of seedlings of

the four most abundant species (Appendix 1B in

supplementary material) in our plots—C. spinosa,

Syzygium cumini, Grewia tiliifolia and Diospyros

montana—with respect to rainfall and lantana density

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in a mixed effects modelling framework (Appendix

1C in supplementary material) and significance of

model parameters was based on 95 % CIs. We also

checked whether the most abundant species in the

observed size class was by itself influencing the

growth of other species. We, therefore, correlated plot-

wise abundances of C. spinosa (estimated in a sepa-

rate study conducted in late 2007; Ramaswami and

Sukumar 2011) with the average 22-month growth

rates of all other species.

Seedling survival—2-month interval seedling sur-

vival was pooled for dry forest habitat, moist forest

habitat and ubiquitous categories of habitat prefer-

ences across all LA and all LD plots. Seedling survival

was modelled as a function of lantana density, species’

forest habitat preference, inter-census rainfall and all

two-way interactions in a generalized linear modelling

framework. Survival was modelled using two data-

sets—with and without the most abundantly sampled

species C. spinosa (?CS and -CS models, respec-

tively). We specified a quasibinomial error structure in

order to control for overdispersion in the data (Craw-

ley 2007). We used graphical methods to check for

model assumptions and simplified the model by

sequentially dropping terms. Simplified models were

compared with the global model using a likelihood

ratio test (LRT, see Crawley 2007) and terms were

retained when the p value for the LRT statistic was

\0.05. Analyses were performed using the glm

(package ‘stats’) function in R.

Results

The relative height growth rates of established seed-

lings were highly variable, with most seedlings

showing either negative or no growth during the study

duration of 22 months both under and outside dense

lantana (Fig. 1). Of the total of 1,105 seedlings tagged

in April 2008 at the beginning of this study, 845

survived to the end of the study in February 2010.

Between May 2009 and February 2010, the proportion

of light received at ground level in LA and LD plots

was 76.3 and 57.2 %, respectively. Though the

difference in light availability in LA and LD plots

varied considerably in time, the overall means were

lower in LD plots than in LA plots (Fig. 2). The high

variance in light could be due to the fact that the

sensors of the light loggers were recording reflected

and diffuse light as well as incident light. The

proportion of light received at ground level was

significantly more in LA plots than in LD plots only in

the months of June (t = 2.12, df = 31.9, p = 0.042),

July (t = 2.62, df = 18.9, p = 0.016), September

2009 (t = 2.3, df = 18.5, p = 0.032) and January

2010 (t = 3.79, df = 24.8, p = 0.001).

Seedling growth

Community growth patterns: The 2-month growth

rates of dry forest preferring species were significantly

lower growth under dense lantana. 4-month growth

rate of dry forest species were significantly more than

that of moist forest species (Table. 1; Fig. 3). 4-month

growth seemed to increase with increasing inter-

census rainfall. However, the parameter estimates as

well as the 95 % CIs for the models were too small to

infer this conclusively (Table. 1). The difference in

22-month growth rates of moist forest species in LA

and LD plots was less than that of dry forest species

(inferred from the significant interaction term in

Table. 1), indicating that moist forest species were

less affected by the presence of dense lantana than dry

forest species (Fig. 3).

Species growth patterns: 2-, 4- and 22-month

growth rate of C. spinosa was lower in LD plots than

in LA plots. 2-, 4- and 22-month growth rates of S.

cumini and D. montana were not affected by either of

the fixed effects. While 2- and 22-month growth rates

of G. tiliifolia were not affected by any fixed effect,

4-month growth rate increased with increasing inter-

census rainfall. We also found no correlation between

the average growth rates of all species with abundance

of C. spinosa (Spearman’s rho = 0.007, p = 0.84).

Seedling survival

The observed seedling survival from one enumeration

to the next was very high. While an average of 97.6

and 96.3 % of dry forest habitat preferring species

survived in LA and LD plots, respectively, 94.9 and

95.6 % of moist forest habitat preferring species

survived in LA and LD plots from one enumeration

to the next. Survival of ubiquitous species was 97.1

and 95.3 % in LA and LD plots, respectively. The best

fit model explaining probability of seedling survival

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had inter-census rainfall and species’ habitat prefer-

ences but not lantana density. The best fit model did

not change on excluding C. spinosa. The probability of

seedling survival for all species groups increased

significantly with increasing inter-census rainfall

(Table 2; Fig 4). The log odds of survival of dry

forest habitat preferring species was significantly

greater than that of moist forest habitat preferring

species, but not different from that of ubiquitous

species (orthogonal contrasts, p (?CS model) \ 0.01,

p (-CS model) \ 0.05; Table 2).

Discussion

Our study shows that the in situ impacts of an invasive

species cannot be studied without taking into consid-

eration prevalent environmental conditions such as

seasonality in rainfall and characteristics of native

species. The effects of lantana on native communities

and woody species have generally been reported as

being detrimental (Gentle and Duggin 1997; Sharma

and Raghubanshi 2007; Prasad 2010; Sundaram and

Hiremath 2011). It must be noted that we could not

ascertain the actual age of the plants defined as

seedlings. Nevertheless, in a forest like Mudumalai,

where seed germination is most likely highly episodic

(pers. obs.), established seedlings may be the best

indicators of future community composition. For the

range of heights examined, our study indicates that

woody seedlings of most species may be unaffected by

the presence of this invasive alien.

Seedling growth

We found that light was consistently lower under

dense lantana and significantly so in 4 out of the

11 months in which light was measured (Fig. 2). We

expected this decrease in light availability to affect dry

forest preferring species negatively. As expected,

dense lantana seemed to negatively affect the growth

of dry forest species more than moist forest species.

However, the community-level patterns could be

influenced by the response of the most abundant

species in a habitat preference guild. For example,

when considered separately, C. spinosa showed sig-

nificantly lower 2-, 4- and 22 month growth rates in

Fig. 1 Histograms of the

observed relative height

growth rates (RhGR) of

woody seedlings in lantana

absent (LA) and lantana

dense (LD) plots for the

period from April 2008 to

Dec 2009 (22 months).

Most seedlings showed

negative or no growth both

under and outside dense

lantana

Fig. 2 Light received at ground level under (LD) and outside(LA) dense lantana from May 2009 to March 2010. Points

represent mean arcsine-square root transformed proportion of

solar energy (in lux) received at a distance of 50 cm from the

ground to that received at *3 m from the ground, for a month.

Error bars indicate ± standard error. The proportion light

received outside dense lantana is significantly more for the

months of June, July, September 2009 and January 2010 (t test).

***p \ 0.001, *p \ 0.05

Plant Ecol

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Plant Ecol

123

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LD than in LA plots. The second most abundant dry

forest species, D. montana, on the other hand was not

affected by the presence of dense lantana in any of the

growth intervals. S. cumini and G. tiliifolia, both moist

forest preferring species, seemed to perform equally

well in LA and LD plots. We found that seedlings were

benefitting from seasonal rainfall, with 4-month height

growth rates increasing with increasing rainfall. This

result is congruent with findings from other seasonal

forests (Comita and Engelbrecht 2009). Seedling

performance in seasonal forests has been shown to

benefit from the spatial availability of moisture (e.g.

Nagamatsu et al. 2002). Our study lacks in the

temporal characterisation of plot-level variables such

as soil nutrients and moisture. We infer that unmea-

sured microsite characters, or the ability of these

plants to resprout, could be over-riding the effects of

the presence of an invasive species in determining

woody seedling performance.

Seedling survival

Contrary to expectation, seedling survival was not

different between LA and LD plots. Since our defini-

tion of mortality included tissue dieback, with most

individuals sprouting during the wet season, these

results may still be inconclusive about the effects of

lantana on native seedlings. As expected, increasing

availability of moisture in time increased survival,

congruent with studies from other seasonal forests (e.g.

Comita and Engelbrecht 2009). In a study conducted in

Barro Colorado Island, Panama, mortality of seedlings

was not affected by increasing moisture availability,

but was found to be density dependent (Bunker and

Carson 2005). Although we have not explicitly tested

for density dependence in our study, we speculate that

owing to low seedling densities at the study site

(average of only 1.5 seedlings m-2 compared with 44

seedlings m-2 in BCI) mortality may be driven more

by environmental factors rather than competition. We

also found no correlation between the most abundant

species in the observed size class C. spinosa and the

growth of less abundant species pooled together

indicating that this particular species was not, at least

for the time being, out-competing other native species.

As hypothesised, we found that the probability of

survival of seedlings of dry forest species is greater

than that of moist forest habitat species but not

different from ubiquitous species. However, this result

could be influenced by the higher sprouting ability of

dry forest preferring and generalist species as com-

pared to moist forest preferring species. This has been

demonstrated for woody recruits in Hawaii, where the

relative density and basal area of sprouts were found to

be greater in dry forest recruits than in moist forests

(Busby et al. 2010).

Seedling persistence in dry forests

Plants that encounter frequent disturbances such as

fires resulting in the total loss of above-ground

Table 2 Model outputs for generalized linear models with quasibinomial error structure for woody seedling survival with respect to

inter-census rainfall and species’ habitat preferences for data with (?CS model) and without (-CS model) Catunaregum spinosa

?CS model -CS model

Coefficient (± SE) t-value p Coefficient (± SE) t-value p

Intercept 3.23 (0.14) 23.4 \ 0.001 *** 3.16 (0.16) 19.3 \0.001***

Moist forest habitat Preferring species -0.53 (0.16) -3.3 0.0016 ** -0.44 (0.17) -2.6 0.013*

Ubiquitous species -0.12 (0.42) -0.3 0.7 -0.04 (0.39) -0.1 0.92

Intercensus rainfall 0.0013(0.0005) 2.7 0.009 ** 0.0011 (0.0005) 2.2 0.029*

Coefficients reported are on the logit scale. The ‘Intercept’ term corresponds to the survival of dry forest habitat preferring species

when rainfall is held constant. The coefficients for the other two habitat preference categories—moist forest and ubiquitous, are

differences from the ‘Intercept’ term

*p \ 0.05; **p \ 0.01; ***p \ 0.001

Fig. 3 Effects of lantana density and species habitat prefer-

ences on the observed 2-, 4- and 22-month relative height

growth rate (RhGR) of seedlings of a all species and b the four

most abundantly samples species—C. spinosa, S. cumini, G.tiliifolia and D. montana. Error bars indicate ± standard error.

Differences in means were inferred as being significant if the

95 % confidence intervals of the coefficients estimated from the

mixed effects models did not overlap with zero and are indicated

by a ‘*’ in this figure

b

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123

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biomass, often persist by regenerating basally (Bond

and Midgley 2001). Woody plants in the dry decid-

uous forests at Mudumalai experience seasonal

drought and annual rainfall deficit, fires that recur

every 3–4 years on an average (Kodandapani et al.

2004), as well as mechanical damage by large

herbivorous mammals (Sukumar et al. 2004). We,

therefore, expected established seedlings of woody

species’ to show adaptive sprouting, and which was

indeed observed. Only 33 of the 1,105 initially tagged

seedlings failed to sprout back even once after being

top-killed during the course of enumeration from April

2008 to February 2010. Additionally, a plot-wide fire

occurred in March 2010. A post-fire enumeration in

June 2010 (during the onset of the summer Indian

monsoon) revealed that at least 671 of the 845 tagged

seedlings alive in the February 2010 had basally

sprouted back. Woody species that adopt sprouting as

a persistence strategy have also been shown to have

slower growth rates than the alternate strategy of

regenerating from seeds (Bond and Van Wilgen 1996).

Although we refer to the woody plants in the

observed size class as ‘seedlings’ to indicate advanced

regeneration, ‘recruits’ in Mudumalai are classified as

woody stems[1 cm dbh (Sukumar et al. 2004). Given

the low growth rates, how established seedlings reach

the[1 cm dbh size class is still not understood well.

Whether the low numbers of saplings observed in dry

forests can be attributed entirely to the presence of

lantana as reported by other studies (Prasad 2010;

Sundaram and Hiremath 2011) or to overall low

growth rates of woody species is an issue that warrants

further exploration.

Implications for dry forests

Our study demonstrates that in the size class exam-

ined, C. spinosa is currently the species most affected

Fig. 4 Probability of

survival for seedlings of dry

forest, moist forest habitat

preferring and ubiquitous

woody species with

increasing inter-census

rainfall (mm) when C.spinosa is included in the

analyses. The presence of

dense lantana did not affect

the survival of seedlings.

Survival increased with

increasing rainfall

(p = 0.008). Survival of dry

forest habitat preferring

species was significantly

higher than that of moist

forest preferring species

(p = 0.0016), but not

different from ubiquitous

species. The inferences were

the same for both ?CS and

-CS models (with and

without C. spinosa,

respectively)

Plant Ecol

123

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by the presence of dense lantana. C. spinosa had an

average of *42 stems of 1–5 cm dbh per hectare and

*7 adult trees ([10 cm dbh) per hectare in 2008 in the

MFDP. Assuming that the negative impacts of lantana

were long-lasting enough to alter the demography of

these slow growing trees, one may expect a shift in

community composition at the[10 cm dbh stratum of

this forest in the future. Interestingly, the most

dominant species in the [10 cm dbh size class—

Lagestroemia microcarpa (*67 stems/ha) and Ter-

minalia crenulata (*46 stems/ha) were not repre-

sented in the 10 cm–1 m size class at all while Tectona

grandis (*39 stems/ha) was represented by only 2

individuals in this study (Appendix 1B in supplemen-

tary material). Species-specific properties such as the

episodic recruitment of some species over others still

remain unexplored and need to be understood well

before the effects of extraneous factors such as lantana

can be assessed objectively.

The non-woody understorey of this forest may be

more severely affected by lantana than woody species.

For the duration of this study, we observed grasses

being consistently excluded from LD plots. If dense

lantana continues to persist, grazing mammals may be

affected in the long run (Prasad 2010). In addition, the

replacement of the grassy understorey with woody

lantana that can climb into the canopy of trees has been

speculated to result in alteration of fire regimes

(Sundaram and Hiremath 2011). Fires that typically

occur only at the ground level may now travel up into

the canopy, resulting in mortality of adult trees, a

phenomenon that was previously highly unlikely.

The presence of dense lantana may also influence

the composition of the dry forest vegetation indirectly,

by influencing plant-animal mutualisms. In an earlier

study, we had found that mammal-dispersed dry forest

habitat preferring species were less abundant under

dense lantana (Ramaswami and Sukumar 2011) sug-

gesting that mammalian dispersers (and herbivores)

were avoiding lantana infested areas. Sundaram and

Hiremath (2011) hypothesise that the exclusion of

mammals from lantana infested areas make seedlings

in non-lantana areas more susceptible to herbivory.

Dense lantana also seems to adversely affect bird

communities, while supporting larger numbers of

certain frugivorous birds only (Aravind et al. 2010).

Indirect and long-term effects of invasive species

may be difficult to capture at short temporal scales

such as the duration of this study. Invasion history has

been implicated as a key factor in determining current

impacts on native communities (Strayer et al. 2006).

Invasive species also leave ‘legacies’—alterations to

ecosystem processes and functioning such that native

species cannot recover even after the invader has been

controlled or removed (Corbin and D’ Antonio 2011).

Legacy effects of lantana have not been explored.

Considering that the management of lantana across

three continents and over a period of almost a century

has been largely ineffective (Bhagwat et al. 2012),

more site-specific, contextualized solutions need to be

devised based on rigorous monitoring and experimen-

tal rather than purely observational studies.

Acknowledgments We are thankful to the Ministry of

Environment and Forests, Government of India, for funding

this study and to the Tamil Nadu Forest Department for granting

us permissions to carry out this study. The data collection for

this study was done with the help of our field assistants

M. Bomman, Kunmari, Paulimara, B. Bomman and Krishna.

Dr H.S. Suresh provided the biomass data for the different

lantana density classes. We thank Dr. Soumya Prasad and

Dr. Kavita Isvaran for help with statistical analysis and manu-

script preparation. Thanks to Sandeep Pulla, Karpagam Chelliah

and Nandita Mondal for many insightful discussions and inputs.

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