outcrop, Southern Espinhaço Range, Brazil.*
Tadeu Guerra
1and Wesley Rodrigues Silva
21 – Programa de Pós-Graduação em Ecologia, Instituto de Biologia, Universidade Estadual de Campinas (UNICAMP), C.P. 6109, 13083970, Campinas, São Paulo, Brasil
email: [email protected]
2 – Laboratório de Interações Vertebrados-Plantas, Departamento de Biologia Animal, Instituto de Biologia, Universidade Estadual de Campinas (UNICAMP), C.P. 6109, 13083970, Campinas, São Paulo, Brasil
email: [email protected]
25
Abstract
The woody mistletoe Psittacanthus robustus Mart. (Loranthaceae) is a Neotropical hemiparasitic plant frequently associated to Vochysiaceae species in Central Brazil. In this study we investigated patterns of host infection by woody mistletoe on seven 1ha plots located in rocky outcrops, southeastern Brazil. We accessed spatial distribution, prevalence, infection intensity, and size of P. robustus individuals parasitizing four tree species. Mistletoes presented an aggregated distribution at the landscape scale, as well as within host populations. Twenty two percent of 1,108 trees were parasitized.
However, prevalence and intensity of infection varied among the four host species, with higher prevalence in Qualea cordata (57%), the rarest and largest species among host trees. Prevalence increased with host size for all tree species. Conversely, host size explained little variation in the intensity of infection. From 844 mistletoes sampled, nearly 26% were classified as seedlings, 21% as juveniles, 33.5% as adults and 19.5% as mature plants, with distribution of life stages varying among host species.
Reproductive individuals corresponded to nearly 46% of the population. Although, most infections occur on thin twigs, larger individuals were usually attached to thicker
branches. Patterns of host infection observed are similar to those reported for other mistletoe species. Psittacanthus robustus is an important component of rocky outcrops flora in Serra do Cipó.
Keywords: Distribution, hemiparasite, host infection, plant size, Serra do Cipó, Vochysiaceae.
26
Resumo A erva-de-passarinho lenhosa Psittacanthus robustus Mart. (Loranthaceae) é
uma planta hemiparasita comumente associada às hospedeiras da família Vochysiaceae no Brasil central. Neste estudo, investigamos os padrões de infecção de hospedeiros e estrutura populacional de P. robustus em sete parcelas de 1 ha localizados em uma área de campo rupestre no maciço do Espinhaço, sudeste do Brasil. Avaliamos a distribuição espacial, prevalência e intensidade de infecção e tamanho dos indivíduos parasitando quatro espécies de hospedeiras arbóreas. A erva-de-passarinho apresentou distribuição agregada na escala da paisagem, assim como nas hospedeiras. Vinte e dois por cento de 1.108 árvores examinadas estavam infectadas por P. robustus. No entanto, a prevalência e a intensidade de infestação variaram entre as espécies de hospedeiras, sendo a
prevalência e intensidade de infestação maiores para Qualea cordata (57%), a maior porem menos abundante entre as espécies de hospedeiras. A prevalência aumentou em função do tamanho para as quatro espécies de hospedeiras. Entretanto, o tamanho das hospedeiras explicou apenas uma pequena parte da variação na intensidade de infestação. De 844 ervas-de-passarinho amostradas, 26% foram classificadas com plântulas, 21% como jovens, 33.5% como adultas e 19,5% como plantas maduras, com a distribuição dos estágios variando entre as hospedeiras. Indivíduos reprodutivos representaram aproximadamente 46% da população. Embora, a maioria das infecções ocorra em ramos finos, indivíduos grandes são encontrados em ramos mais grossos. Os padrões de infecção observados são similares aos registrados para outras ervas-de- passarinho. Psittacanthus robustus é um importante componente da flora dos campos rupestres da Serra do Cipó.
Palavras-chave: Distribuição, hemiparasitas, infecção de hospedeiras, tamanho de plantas, Serra do Cipó, Vochysiaceae.
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Introduction
The group of obligate parasitic flowering plants, popularly known as mistletoes, attach to the host’s aerial shoots via a specialized root called haustorium (Kuijt, 1969; Calder and Bernardt, 1983). Most mistletoe species are hemiparasites, obtaining water and mineral nutrients from the host’s xylem, but also photosynthesizing most of their own carbohydrates (Ehleringer et al. 1985, Marshall et al., 1994). Host compatibility is related to physiological and morphological adaptations that allow mistletoe seeds overcome hosts' chemical and mechanical resistance traits to establish effective vascular connection (Hoffmann et al., 1986; Lichter and Berry, 1991; Yan, 1993; Medel, 2000). There is a continuum in host specificity among mistletoes. Some species are host specific and parasitize only few host species within one genera or family, whereas others are highly generalists infecting a wide range of host species (Yan, 1990; Norton and De Lande, 1999). According to Norton and Carpenter (1998) relative abundance of hosts is the key to understand evolution of specificity. The authors suggested that specialization should be more advantageous when profitable hosts are frequently encountered, and parasites are able to make the best use of those abundant hosts. In heterogeneous communities where host species become increasingly scarce, generalization should be favored. However, host specificity must be viewed as a dynamic and evolving condition, variable in space and time (Thompson, 1994).
Distribution of mistletoes on landscapes is directly related to the presence and abundance of compatible hosts (Norton et al., 1995; Aukema, 2004). Combinations of processes involving parasite, vector and host interactions affect establishment and survival of mistletoes, modulating the structure of populations and distribution within
28 host species. The crucial step on mistletoe life cycle is seed deposition on a live branch within restricted diameter range of a compatible host (Sargent, 1995). Seed dispersal and deposition is intimately related to abundance and behavior of bird dispersers (Martinez del Rio et al., 1996; Aukema and Martinez del Rio, 2002a,b; Medel et al., 2004; Roxburgh and Nicolson, 2005; Ward, 2005). Establishment rates can vary according to host species compatibility (López de Buen and Ornelas, 2002; Norton et al., 2002). Survival is directly affected by abiotic factors (Lamount, 1983; Lichter and Berry, 1991; Back et. al. 2005), by seed predation (Sargent, 1995; Yan and Reid, 1995), and by intra-specific competition due to clumped deposition (Davidar, 1983).
Regular patterns of host infection have been observed for different species in distinct ecosystems. Like most animal macroparasites (Shaw et al., 1998), distribution of mistletoes within host populations is highly clumped (Thomson and Mahall, 1983; Monteiro et al., 1992; Overton, 1994; Donohue, 1995; Aukema and Martinez del Rio, 2002a; Medel et al., 2004). Infection prevalence determined by fraction of infected hosts on a given population, as well as infection intensity, the mean number of parasites per host, is usually low or intermediate (Thomson and Mahall, 1983; Hoffman et al., 1986; Monteiro et al., 1992; Donohue, 1995; Bannister and Strong, 2001; Aukema and Martinez del Rio, 2004). The fraction of hosts parasitized often increases for larger older hosts (Overton, 1994; Norton et al., 1997; Reid and Stafford-Smith, 2000;
Bannister and Strong, 2001;Aukema and Martinez del Rio, 2002b; Carlo and Aukema, 2005; Roxburgh and Nicolson, 2007). Mistletoes usually establish on small twigs of their hosts (Hoffman et al., 1986; Sargent, 1995; Bannister and Strong, 2001; López de Buen and Ornelas, 2002), but older mistletoes are generally found on large branches (Norton et al., 1997; Reid and Stafford-Smith, 2000). Conversely, patterns of population
29 structure were described for few mistletoe species in New Zealand, Australia and North America (Dawson et al., 1990; Norton et al., 1997; Reid and Stafford-Smith, 2000;
Bannister and Strong, 2001).
The woody mistletoe Psittacanthus robustus Mart. (Loranthaceae) is a hemiparasitic plant restricted to South America, and widely distributed in Cerrado biome in central Brazil, also occurring on borders of gallery forests in Venezuela (Rizzini, 1980). It has succulent leaves that produce their own carbohydrates. In fact, the amounts of photosynthate produced by P. robustus are nearly the same of adjacent branches of infected hosts (Luttge et al., 1998). This mistletoe presents host specificity within species of two angiosperm families (Monteiro et al., 1992). To date, 11 species in three genera of Vochysiaceae and two species in two genera of Melastomataceae were reported as hosts of P. robustus at seven sites in central Brazil (Monteiro et al., 1992). These authors observed that P. robustus presents clumped distribution on hosts and grows preferentially on Vochysiaceae species in tall Cerrado vegetation at Mogi Guaçú, state of São Paulo, southeastern Brazil. Nevertheless, there is no detailed information on host specificity and distribution within host populations in other localities along its geographic range, as well as basic information on plant size or host branch infection.
In this study we investigated patterns of host infection by P. robustus at a rocky outcrop in Serra do Cipó, Espinhaço range, southeastern Brazil. We presented data on host range, spatial distribution, prevalence, intensity of infection, and population structure of P. robustus parasitizing four tree species. More specifically, we addressed the following questions: 1) Does P. robustus infect preferentially some host species at rocky outcrops? 2) Do prevalence and intensity of infection vary among host species? 3)
30 Are prevalence and intensity of infection positively related to host plant size? 4) How mistletoes in different life stages are distributed among host species? 5) Does diameter of branches infected by P. robustus differ among life stages and hosts species?
Methods
Area description
We collected data between June and December of 2007 at Reserva Particular do Patrimônio Natural (RPPN) Vellozia, a private area of “Campos Rupestre” varying from 1100 to 1400 m above see level, integrated to Área de Preservação Ambiental (APA) Morro da Pedreira in the vicinity of Serra do Cipó National Park, Minas Gerais, southeastern Brazil (43º 35’W, 19º 17’S, Fig. 1). The “Campos Rupestres” or rocky outcrop fields along Espinhaço Range comprise an important center of plant diversity and conservation (Giulietti et al., 1997; Mendonça and Lins, 2000). Species richness is high and concentration of narrow endemics is remarkable (Giulietti et al., 1997). Unfortunately, many plant species found in rocky outcrops are vulnerable to extinction (Mendonça and Lins, 2000). According to Rapini et al. (2008) the extreme
environmental conditions and the great spatial heterogeneity are responsible for the huge beta diversity found in Campos Rupestres. These ecosystems are closely related to Cerrado Biome along western slopes of Espinhaço range (Giulietti et al., 1997). The vegetation is associated to quartzite rocks, with shallow, acid, sandy and nutrient poor soils in areas higher than 1000 m above sea level (Ribeiro and Fernandes, 2000; Rapini et al., 2008). Climate is mesotermic (Cwb of Köppen) with cold dry winters from May to September and hot wet summers from October to April. Mean monthly temperature varying from 17 to 23 Co and mean monthly precipitation varying from 11.9 mm to
31 281.1 mm, with annual precipitation around 1370 mm (Madeira and Fernandes, 1999). The study site is characterized by a mosaic of open fields dominated by grasses and small bushes sparsely distributed, and rock outcrops where small tortuous trees grow between a variety of bush species and a developed herbaceous strata (Fig. 2). Many species are found at the study site, with shrubs and trees from many families, especially Velloziaceae, Compositae, Melastomataceae, Myrtaceae, Vochysiaceae, Labiatae, Leguminosae, Rubiaceae, Lythraceae, Malpighiaceae.
Field procedures
We searched extensively for hosts and mistletoes inside seven 1 ha (100 x 100 m) plots located at least 100 m apart from each other, randomly assigned along an approximately 2 km trail. For each host taller than 0.5 m, we recorded tree identity, presence and number of alive P. robustus, trunk base diameter (TBD hereafter) using a diameter tape (Forestry Suppliers ©), tree height using a measuring pole. For each mistletoe we recorded height, expressed as the distance from the haustorial base to the apex of the largest branch using a measuring tape or pole, and presence of reproductive shoots. For mistletoes located in the first four plots sampled, besides mistletoe height we also recorded the haustorial base diameter (HBD hereafter) and the diameter of host branch just below the attachment point using a stainless caliper (0.05 mm). To evaluate population structure, mistletoes were classified in four life stages according to height: seedlings, those plants smaller or equal to 5 cm (mean ± SD, height: 3.1 ± 1.4 cm, HBD: 0.7 ± 0.2 cm, n = 134), juveniles, between 5.1 and 25 cm (height: 12.0 ± 6.4, HBD:1.6 ± 0.5, n = 104), adults, between 25.1 and 75 cm (height: 48.6 ± 15.0, HBD: 4.6 ± 2.0, n = 151) and matures, individuals larger than 75 cm (height: 93.7 ± 18.7, HBD: 7.6 ± 2.1, n = 56).
32 Statistical analyzes
To determine the degree of mistletoes aggregation on hosts we fitted observed distribution to expected negative binomial distribution for each host species, using goodness of fit Chi-square test according to Krebs (1989). This distribution function is commonly used to evaluate distribution of parasites on hosts (Shaw et al., 1998). It has two parameters: the mean intensity of infection (μ) and the inverse measure of
aggregation (k). Low values of k (< 1.0) indicate clumped distributions (Krebs, 1989). We utilized qui-square test to evaluate whether distribution of infected trees
corresponded to the expected abundance of hosts species at the study site. To determine differences in height, TBD and intensity of infection among host species we utilized the Kruskal-Wallis test, because data did not meet the assumptions of parametric statistic tests, even after data transformation. The relationship between TBD and height of host species were evaluated using simple linear regressions, after log transformations. The relationship between HBD and height of mistletoes growing on different hosts were calculated using simple linear regressions. The relationship among infection intensity and TBD was evaluated using non-parametric Spearman correlation rank for each host species. We utilized logistic regressions to access the effect of plant size (TBD) on probability of infection for each host species. We utilized Williams G test to evaluate whether distribution of mistletoes in different life stages differed among host species. To evaluate differences in branch diameter among mistletoe life stages and host species we utilized two-way ANOVA, using log transformed data. The significance level adopted for all analyzes was α = 0.05. All analyzes were according to Zar (1996).
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Results
Host range
We observed P. robustus growing on eight host species in Serra do Cipó rock outcrops. Four tree species were commonly recorded as hosts: Vochysia thyrsoidea Pohl, Qualea cordata Spreng. (Vochysiaceae), Trembleia laniflora Triana, Miconia ferruginata DC. (Melastomataceae). We also found P. robustus less frequently on other four shrub species, including Vochysia elliptica Mart. (Vochysiaceae), Campomanesia sp. (Myrtaceae), Dyplusodon sp. (Lythraceae) and Coccoloba cerifera Schawcke (Polygonaceae). Six V. elliptica were parasitized by P. robustus, including seedlings, juveniles and reproductive mistletoes. We recorded only three Campomanesia sp. infected by P. robustus, mostly by juveniles but also by large reproductive mistletoe. A single seedling was recorded on Dyplusodon sp., and two C. cereifera were parasitized by one reproductive P. robustus each. Three seedlings of P. robustus grown as self- parasite on stems of adult reproductive plants. From 844 P. robustus sampled, nine plants (1%) were infected by the mistletoe Struthanthus flexicaulis Mart.
(Loranthaceae), growing as hyperparasites on branches of adult woody mistletoes.
Mistletoe distribution
Density of P. robustus on seven plots varied from two to 342 individuals (mean ± SD, 120.5 ± 113.1, plants/ha -1). Variance / mean ratio (12,801.9 / 120.5 = 106.1) were significantly larger than one (P < 0.0001), indicating that mistletoe distribution in rock outcrops is highly clumped. Distribution on hosts was also aggregated. Most hosts were uninfected, most infected hosts were parasitized by few mistletoes and just a small fraction of hosts was intensely parasitized (Fig. 3). The low k values in the negative
34 binomial distribution indicated highly clumped distribution of P. robustus on four host species. Distribution of mistletoes on hosts fitted negative binomial distribution for V. thyrsoidea (μ = 0.57, k = 0.150, χ2 = 9.1, d.f. = 9, P = 0.422), Q. cordata (μ = 3.74, k =
0.312, χ2 = 5.9, d.f. = 10, P = 0.816), and for T. laniflora (μ = 0.61, k = 0.138, χ2 = 9.3,
d.f. = 5, P = 0.093). Data for M. ferruginata also indicated clumped distribution (μ = 0.11, k = 0.081), but was insufficient for fitting negative binomial distribution using qui- square analyzes.
Prevalence of infection
Overall prevalence was 22% for 1,108 trees examined (Table 1). However, prevalence varied among four host species and differed from expected by the relative abundance of tree species (χ2 = 46.7, d.f. = 3, P < 0.001, Fig. 4). Vochysia thyrsoidea (mean ± SD, 95.8 ± 26.8 plants/ha -1) was the most common host at the study site, representing nearly 61% of plants sampled and 59% of infected hosts. Trembleia laniflora (26.8 ± 12.1 plants/ha -1) represented nearly 17% of hosts and 16% of infected plants. However, M. ferruginata (23.1 ± 30.2 plants/ha -1) represented nearly 16% of hosts, but only 4.5% of infected trees. On the other hand, Q. cordata was more scarce (12.4 ± 14.5 plants/ha -1), comprising only 7.7% of trees sampled, but nearly 20% of parasitized hosts.
Host species varied greatly in height (H3,1108 = 45.4, P < 0.001, Table 1) and in
TBD (H3,1108 = 72.9, P < 0.001, Table 1), with significant statistical results. We
examined size/infection relationship considering TBD, since it was positively correlated to height for all host species (log transformed data; V. thyrsoidea: R2 = 0.65, P < 0.001,
35 188; M. ferruginata: R2 = 0.62, P < 0.001, n = 162). Considering size structure of host populations, our data indicated that prevalence increases for plants with larger TBD (Fig. 5). In addition, logistic regression results showed that probabilities of infection by P. robustus greatly increase with TBD for all host species. (V. thyrsoidea: Logit (P) = - 2.878 + 0.135 (TBD), χ2 = 52.5, P < 0.001, n = 671; Q. cordata: Logit (p) = -1.531 +
0.143 (TBD), χ2 = 12.9, P = 0.0003, n = 87; T. laniflora: Logit (p) = -3.547 + 0.235
(TBD), χ2 = 36.6, P < 0.001, n = 188; M. ferruginata: Logit (p) = -7.891 + 0.415 (TBD), χ2 = 30.1, P < 0.001, n = 162). However, the relationship between plant size and
probability of infection was variable among host species. Probability of infection was higher for small Q. cordata individuals than for other species, and lower for large V. thyrsoidea trees than for other hosts (Fig. 6).
Intensity of infection
Intensity of infection varied among hosts species with statistically significant results (H3, 243 = 20.7, P = 0.0001). Post hoc Dunn test indicated that it was larger for Q.
cordata than for V. thyrsoidea and M. ferruginata, with T. laniflora not differing from other species (Table 1). Number of mistletoes per infected host varied from 1 to 58 (median = 3) for Q. cordata and was positively correlated to host TBD (rs = 0.33, P =
0.018, n = 49, Fig. 7). For V. thyrsoidea this number varied from 1 to 30 (median = 1) and was weekly positively correlated to TBD (rs = 0.27, P < 0.001, n = 144, Fig. 7). For
T. laniflora varied from 1 to 10 (median = 2) and was not correlated to TBD (rs = 0.30,
P = 0.06, n = 39, Fig. 7). For M. ferruginata varied from 1 to 4 (median = 1) and was not correlated to TBD (rs = -0.10, P = 0.75, n = 11, Fig. 7).
36 Mistletoe population structure
Mistletoe height was positively correlated to HBD for mistletoes growing on different host species (log transformed data; V. thyrsoidea: R2 = 0.83, P < 0.001, n =
149; Q. cordata: R2 = 0.91, P < 0.001, n = 253; T. laniflora, R2 = 0.85, P < 0.001, n =
42; M. ferruginata: R2 = 0.93, P < 0.001, n = 18, Fig. 8). Indeed, the height of the
smallest reproductive P. robustus found on V. thyrsoidea was 27 cm (2.2 cm, HBD), 22 cm (1.8 cm, HBD) on Q. cordata, 20 cm (1.5 cm, HBD) on T. laniflora, and 48 cm (3.0 cm, HBD) on M. ferruginata. Population structure was slightly biased toward adult individuals (χ2 = 40.9, d.f. = 3, P < 0.001, Fig.9). Seedlings represented nearly 26% of 844 mistletoes sampled, juveniles 21%, adults 33.5% and mature plants 19.5%. Seedlings were not reproductive, while 18% of juveniles, 70% of adults and 96% of mature plants presented reproductive shoots. Reproductive individuals corresponded to nearly 46% of the population (Fig. 9). Distribution of mistletoes in distinct life stages among host species was not random (G = 89.5, d.f. = 9, P < 0.001). Most plants growing on V. thyrsoidea were adults (35%) and mature plants (28%). Conversely, seedlings (36%) and juveniles (34%) represented most mistletoes parasitizing T. laniflora. Mistletoes infecting Q. cordata were mostly seedlings (33%) and adults (32%), whereas only 10% of the mistletoes parasitizing M. ferruginata were seedlings with most individuals (47%) classified as adult plants (Fig. 10).
Branch diameter was positively correlated to HBD (log transformed data; R2 = 0.64, P < 0.001, n = 445). Branch diameter where seedlings established varied from 0.41 to 3.65 cm (mean ± SD, 1.52 ± 0.66, n = 136). Branch diameter varied from 0.48 to 7.45 cm for juveniles (2.02 ± 1.02, n = 106), from 1.29 to 8.52 cm for adults (3.76 ±
37 1.58, n = 159) and from 2.75 to 12.0 cm for mature plants (5.25 ± 1.73, n = 60).
However, diameter of branches infected by P. robustus differed among life stages (F3, 445 = 32.6, P < 0.001) and among host species (F3, 445 = 2.65, P = 0.04) with significant
interaction between two factors (F9, 445 = 4.39, P < 0.001). Post hoc planned
comparisons indicated no statistical differences in branch diameter for seedlings and juveniles growing on distinct hosts. However, adult and mature plants found on T. laniflora usually infected thinner branches than those found on other host species, whereas mature plants attached to thicker branches when parasitizing V. thyrsoidea and Q. cordata (Fig. 11).
Discussion
Nearly 85% of woody mistletoes were found on Vochysiaceae species,
confirming close association with species within this family as reported by Monteiro et al. (1992). However, in rock outcrops this mistletoe parasitized different species from those reported as hosts in São Paulo state. Woody mistletoes seem to infect few species within plant community, but present an intermediary degree of host specificity infecting at least 15 host species within five families along its geographic range. Monteiro et al. (1992) recorded four host species for P. robustus in tall cerrado area: Qualea
grandiflora, Qualea multiflora, Vochysia cinnamomea, and Miconia albicans. However, they reported four different host species from Serra do Cipó, including Q. cordata, Q. dichotoma, V. thyrsoidea and T. laniflora. Indeed, we extended host range of P. robustus, adding four new host species to the list: M. ferruginata, Campomanesia sp., Dyplusodon sp., and Cocoloba cerifera. Even though infrequent, P. robustus also infected species from three angiosperm families not previously reported. Families
38 Myrtaceae and Lythraceae, like Vochysiaceae and Melastomataceae, are included in order Myrtales (APG, 2003). These data indicate that host compatibility of P. robustus could be related to characteristics shared by species within this clade. In fact, many mistletoe species infect closely related species (Norton and Carpenter, 1998). For example, Psittacanthus angustifolius has been reported almost exclusively as parasite of Pinus spp. in Central America (Howell and Mathiasen, 2004). However, Psittacanthus schiedeanus infects at least 19 host species in 11 families in cloud forests in Mexico (López de Buen and Ornelas, 1999), indicating that the degree of host specificity is