Buttresses as microhabitats for leaf litter macroinvertebrates Sam uel C. Somerville Department of Environmental Studies, University of Oregon A BSTRACT The functional properties of buttresses are still debated, but hypotheses include they may be a response to mechanical strain and a method to increase of the surface area of the tree base ; t h e increased surface area may provide increased structural support and a larger area of surface soil which may give a competitive advantage for nutrient uptake and whether it supports a rich community of leaf litter fauna has not been well studied. A more abundant and species rich macroinvertebrate community in buttresses could be indicative of a more nutrient ric h microhabitat . In this study I sampled buttressed trees on hillslopes in the cloud forest of Monteverde, Costa Rica to determine if the buttress microhabitat supports a greater species richness and abundance of leaf litter macroinvertebrates . Leaf litter was collected from three zones around each tree: upslope buttresses, downslope buttresses, and an adjacent plot. B oth macroinvertebrate species richness and abundance are higher in up and downslope buttress plots than in adjacent plots, although only upslo pe and adjacent differed significantly. Also, a significantly higher number of larvae were found in the buttress plots. Together, the increased abundance and richness of both adult and larval macroinvertebrates in buttress plots suggest that tree buttresse s provide preferable habitat for these leaf litter fauna. RESUMEN Las propiedades funcionales de las gambas todavÃa se encuentran en debate, pero las hipÃ³tesis incluyen que puede deberse a una respuesta mecÃ¡nica hacia la tensiÃ³n y como un mÃ©todo para aumentar el Ã¡rea de la base del Ã¡rbol; el aumento en el Ã¡rea puede proveer un aumento en el soporte estructural y una mayor Ã¡rea para la absorciÃ³n de nutrientes. El efecto de esta Ãºltima hipÃ³tesis y de si sostiene una mayor comunidad de la fauna de hojar asca no ha sido bien estudiado. Una mayor abundancia y riqueza de especies de macroinvertebrados en las gambas puede ser indicativo de un microhabitat mÃ¡s rico en nutrientes. En este estudio muestree gambas en las laderas en el bosque nuboso de Monteverd e, Costa Rica para determinar si los micro hÃ¡bitats de las gambas contienen una mayor abundancia y riqueza de especies de macro invertebrados de hojarasca. Hojarasca fue colectada en tres zonas alrededor del Ã¡rbol, gambas con pendiente ascendente, descend ente y en un cuadrante adyacente al Ã¡rbol. Ambos riqueza y abundancia de especies son mayores en gambas ascendentes y descendente que en la cercanÃa al Ã¡rbol. AdemÃ¡s, un nÃºmero significativamente mayor de larvas fueron encontrados en los cuadrantes de la s gambas. Juntos, este aumento en la abundancia y riqueza tanto de adultos como de larvas en macroinvertebrados en las gambas sugiere que estas proveen un sitio preferible para la fauna de hojarasca. I NTRODUCTION Buttresses are a characteristic feature of tropical forests but are not restricted to them (Richards 1996). The frequency and size of buttresses tend to decrease with increasing latitude which concentrates their abundance in the tropics (Smith 1972). This leads to quest ions about their function . Buttresses are the epinastic secondary growth o f lateral surface roots that may provide a res ponse to mechanical strain due to uneven canopy weight and an i ncreased root surface area
for physical support and nutrient exploration for emergent tree species (Newberry et al. 2009, Richards 1996). The two former hypotheses have been tested thoroughly, but the latter nutrient hypothesis has not been the focus of many studies . The prevalence of buttressing in the tropics may be explained by the tendency of roots to concentrate in the richest nutrient areas of the soil, which, in the tropics is the top layer of soil and leaf litter (Richards 1996). Buttressing allows a greater s pread of surface root systems that may facilitate the acquisition of soil, nutrients, leaf litter, and water by creating barriers that act as collecting troughs (Tang et al. 2010) . This may give trees with buttresses a competitive advantage in nutrient acquisition, a very useful trait in the nutrient poor soil of the tropics (Tang et al. 2010). This indicates that buttress microhabitats provide a more resource rich environment than t he surrounding forest f loor ( Whitfield & Pierce 2005). M icrohabitat heterogeneity may indeed directly benefit the buttressed tree. On a greater scale, microhabitats also can increase forest habitat heterogeneity which may play a role in the maintenance of diversity in the tropics (Whitfield & Pierce 2005). For example, buttress microhabitats with increased water and nutrient retention may play an important role in the assemblage of leaf litter fauna in and around the b uttresses themselves (Tang et al. 2010) . Increased diversity and abundance in buttress microhabitat of leaf litter fauna, specifically arthropods, has not been researched well to date and thus is the focus of this study. Leaf litter macro invertebrates are known to respond strongly to microclimate conditions such as moisture, temperature, and insolation ( Longino & Nadkarni 1990). Since buttress cavities can accumulate water (Tang et al. 2010), moisture content s in this microhabitat may have even higher soil moisture retention. The increased moisture retention along with leaf litter accumulation may increase habitat availability for soil a nd leaf litter dependent fauna (Tang et al. 2010) , potentially leading to increased species richness and abundance of arthropods in buttress microhabitats . Humidity or moisture content was not quantitatively measured in this study but previous data (Tang et al. 2010) and personal observations of buttress leaf litter confirm that this microhabitat is more moist th an the su rrounding forest floor. However, not all buttress microhabitats are created equal. In hilly terrain, uphill buttresses accumulated significantly more leaf litter, moisture, and soil nutrients such as Phosphorous, Nitrogen, and Carbon than did downhill buttresses (Tang et al. 2010). The difference is attributed to the greater accumulation of leaf litter in the cup shaped formation created by the uphill buttresses (Mack 2003). Upslope buttresses are expected to hold the greatest abundance and richness of arthropod species in the buttress microhabitat. Downslope buttresses have less capacity to accumulate leaf litter than uphill buttresses because their upside down cup shape allows for gravity and water to drag accumulated organic material down hill ( P ers. obs.) . This inherently w ould provide less suitable habitat for leaf litter macroinvertebrates. While experiencing this drainage effect, downhill buttresses are still expected to hold greater abundance and number of arthropod species than the unprotected forest floor which experie nces the same drainage effect but offers far less protection than buttresses . The purpose of this study is to see if buttresses do provide a preferable habitat for macroinvertebrates . Arthropods were the subject of study because they are important decomposers in tropical ecosystems, along with bacteria and fungi (Mack 2003) but their species richness and abundance in a specific microhabitat such as buttresses has been poorly researched .
FIGURE 1. Schematic of a sample tree. In the cross section, buttresses above the dotted line were considered upslope buttresses. Those below the line were considered downslope buttresses. M ATERIALS & METHODS This study was conducted in the primary forest near the Estaci Ã³ n Biol Ã³ gica Monteverde in Monteverde, Costa Rica. The Holdridge Life Zone for this habitat is lower montane wet forest, characterized by a 30 40m dense cloud forest canopy with frequent precipitation and cloud cover . Frequent mists maintain the lush character of the forest year round (Haber 2000). Elevation covered was a 40m band from 1530m to 1570m. Within this narrow elevation range 29 tr ees were sampled along Sendero Principal and Sendero Jilguero . Tree species was not considered in this study because there is high variability of buttressing even within species (Richards 1996). One consistent variable in buttressing is that the greatest p roportion of buttresses is found in canopy and emergent trees (Richards 1996) so a threshold diameter at breast height (DBH) was considered a better indicator of buttress occurrence. Each tree had a DBH greater than 60cm because number and size of buttress es increases with diameter ( Richards 1996). I considered tree with a DBH of 60cm to be mature and thus to have a sufficient number of buttresses for this study . Even u sing the 60 cm threshold, only eight trees sampled had a DBH less than 80 cm. Also, each tree had to be rooted on a hill so that upslope buttresses could be differentiated from downslope buttresses. When a suitable sample tree was found, enough leaf litter to fill at least half of a 1L Ziploc bag was collected from three zones per tree: upslope buttress, downslope buttress, and adjacent plot. Figure 1 represents a sample tree. Buttresses above the dotted line on the cross section were considered upslope buttresses and those below the line were considered downslope buttresses (Figure 1). The adjacent sample was taken from leaf litter on the forest floor at least 2m from the base of the sample tree. A 2m distance was considered sufficient to be outside the buttress microhabitat of even the largest tree sampled , thus providing a control sample of general forest floor leaf litter to compare to the buttress leaf litter. Macroinvertebrates were extracted from the leaf litter samples using a Berlese funnel as described by Southwood (2000). Three funnels were used for each tree constructed out of plastic 2 liter bottles , one for each zone . Enough methanol was poured in each bottle to fill the bottom . 50g of bulk leaf litter from each zone was placed in its corresponding Berlese funnel. Fine mesh mosquito netting was placed over the exposed leaf litter and held tight by a rubber band around t he bottle to ensure no macroinvertebrates from the samples escaped and no outsiders got in. Funnels from each zone were placed under their own 100 watt bulb fo r 24 hours. RESULTS During the study 280 individual macroinvertebrates and 27 morp hospecies from two phyla and seven classes were collected (Table 1). The most individuals were found in upslope buttresses with 130 followed by downslope with 96 and the adjacent plots had the least number of
individuals with 54. The most common taxa found were Cole optera, Arachnida, Isopoda, and Formicidae . TABLE 1. List of morphospecies found in upslope buttresses, downslope buttresses, and adjacent plots from 29 trees in the Monteverde Cloud For est in October and November 2010. Samples were collected between 1530 and 1570 m elevation. Abundance for each morphospecies is given by zone : adjacent , down, and up. Total number of individuals (N), species richness (S), and Shannon also shown . Taxa Abundance by Zone Adjacent Down Up Annelida Annelida 1 0 1 0 Arthropoda Arachnida Red Acarina 0 2 3 Acarina 2 0 1 0 Acarina 3 0 0 1 Arachnida 1 0 0 5 Arachnida 2 0 3 1 Arachnida 3 2 3 1 Arachnida 4 0 0 1 Chilopoda 2 1 0 Diplopoda Diplopoda 1 0 1 0 Diplopoda 2 1 0 0 Insecta Coleoptera Coleoptera 1 9 10 21 Coleoptera 2 0 3 0 Coleoptera 3 2 1 4 Coleoptera 4 1 1 2 Coleoptera 5 0 1 0 Staphylinidae 5 3 4 Curculionidae 5 3 7 Hymenoptera Formicidae 9 26 20 Isoptera 1 0 2 Diptera 2 0 2 Neuroptera Myrmeleontidae 1 0 1 Phasmatodea 1 0 0 Holometabolous Insect Larvae 8 17 28 Malacostraca Isopoda
TABLE 1 continued . Taxa Abundance by Zone Adjacent Down Up Isopoda 1 0 5 10 Isopoda 2 4 10 12 Collembola 1 4 5 N 54 96 130 S 16 19 19 2.22 2.18 1.55 There were significantly more species (S) in the upslope buttress area than in the adjacent plots (ANOVA, F = 6.34, df = 2, p = 0 .0027) However, no significant difference in S was found within buttress microhabitats or between downslope buttresses and adjacent plots (Figure 2). Upslope buttresses had the lowest Shannon Weiner diversity index and differed significantly from the other two zones (Modified t test, t = 3.57, p< 0 .0001). downslope buttresses and the adjacent plots was 2 .18 and 2.22, respectively. These values did not differ significantly (Modified t test, t = 3.57, p< 0 .0001). A species accumulation curve shows there were equal spec ies richness found in upslope buttresses and downslope buttresses (S = 19) but S was slightly lower with 16 species found in the adjacent plot s (Figure 3). Each zone had a similar rate of species discover y but were higher in both buttress zones . The lines do not appear to asymptote after 29 trees had been sampled. A AB B 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Adjacent Down Up Species Richness FIGURE 2. Mean ( + SD) macroinvertebrate species richness (S) by buttress micro habitat zone and adjacent plot in the Monteverde Cloud Forest. Up refers to upslope buttresses, down to downslope buttresses, and adjacent refers to the adjacent plot. Different letters indicate significant differences between means (p<0.05, post hoc Tukey test)
Difference in m ean macroinvertebrate abundance (N) was statistically significant between the upslope buttress zone and the adjacent plot ( ANOVA, F = 7.33, df = 2, p = 0 .0012 ; Figure 4 ). There was no significant difference in average macroinvertebrate abundance within buttress microhabitats and downslope buttresses did not differ significantly from adjacent plots. Abundance of holometabolous insect larvae followed the same pattern as general abundance. Abundance of larvae was significantly higher in upslope buttresses and adjacent plots (means = 0 .93 +/ 1.3, 0 .28 +/ 0 .53 respectively). Average N of larvae did not differ signi ficantly between downslope buttresses (mean = 0 .62 +/ 0. 86) and either of the other two zones. DISCUSSION This study has shown that there is a higher abundance and species richness of invertebrates in upslope buttress microhabitats than in the unprotected leaf litter on the forest floor. The increased presence of macroinvertebrates in buttresses aligns with the hypothesis that buttresses 0 2 4 6 8 10 12 14 16 18 20 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Number of Species Tree Up Down Adjacent A AB B 0 1 2 3 4 5 6 7 8 9 Adjacent Down Up Number of Individuals FIGURE 3. Macroinvertebrate s pecies accumulation shows that a greater species richness (S = 19) was found in both buttress microhabitats. Sixteen different species w ere found in the adjacent plot. FIGURE 4. Mean ( + SD) number of individual macroinvertebrate s in each zone. Up denotes individuals found in upslope buttresses, down indicates those found in downslope buttresses, and adjacent indicates individuals found in adjacent plots. Different letters indicate significant differences between means (p<0.05, post hoc Tukey test).
create a more resource rich environment than the surrounding forest floor (Whitfield & Pierce 2005) . T he tree benefits from concentrated soil nutrients and moisture at the roots and the leaf litter fauna benefit from shelter provided by the buttresses as well as the accumulation of scarce soil nutrients, moisture, and leaf litte r . The increased soil moisture, leaf litter accumulation, and soil nutrients found in uphill buttresses also reinforce this idea and offer an explanation for the increased abundance and number of species (Tang et al. 2010). Humidity, moisture , and soil nut rient content were not tested quantitatively in this study but the study by Tang et al. found significantly higher amounts of moisture and soil nitrogen and carbon in upslope buttresses (2010). Also, among the sampled trees leaf litter and soil in the buttress microhabitat was noticeably more damp than the surrounding forest floor (Pers. obs.) . While the data did not show significant differences between downslope buttresses and the adjacent leaf litter, both species richness and abundance were higher in the buttress. This suggests that while gravity and water drainage drag accumulated leaf litter away from the downslope buttresses (P ers. obs.) the shelter buttress roots provide creates a more preferable habitat for macroinvertebrates than the surrounding forest floor. The species accumulation curve does not appear to asymptote, suggesting that new species were still being collected at the end of the study. The rate of new species collection is similar for all three zones but higher for both buttress plots. With a larger sample size, this trend could show significance between buttress plots and the adjacent leaf litter, as well as revealing the presence of buttress specialists. The data does not show strong trends of buttress specialists that are not f ound at all in the leaf litter . This suggests that the taxa found do not depend on buttress microhabita ts for survival but do prefer them when they are present . Eleven adult macroinvertebrate species were found only in the buttress microhabitat but their a bundances were so low that they could not be considered specialists . However, larvae abundances in upslope buttresses were found to be significantly higher than abundance in adjacent plots. This suggests that the accumulated leaf litter and moisture retention found in buttresses is not only a preferable habitat for adult macroinvertebrates but also a good place for them to lay eggs. This means that l eaf litter macroinvertebrates may complete their whole life cycle in a buttress microhabitat , suggesting buttresses are very important to holometabolous leaf litter fauna in tropical ecosystems. The variety of taxa found in this study is very similar to previous research o n leaf litter fauna in tropical montane regions (Nadkarni & Longino 1990). Coleoptera, Isopo da, and Formicidae and holometabolous insect larvae were the most common species overall. These detritivore taxa are the major forces of leaf litter fragmentation (Nadkarni & Longino 1990) and their increased abundance in buttresses can be explained by a great accumulation of leaf litter. However, the greatest numbers of arthropod richness and abundance in upslope buttresses was accompanied by a Shannon Weiner diversity index value sig nificantly lower than the other two zones . This means that the upslope buttress h abitat had low species evenness ; o nly f our out of 19 species found in upslope buttresses had abundances greater than seven individuals . This reveals an upslope buttress microhabitat dominated mainly by ants and beetles. While other detritivores are perfectly at home in buttresses the dominance of ants and beetles is likely attributed to their mobility. Among the species found, ants and beetles are some of the largest and most mobile. Their greater numbers in buttresses may be attributed to their ability to disperse there. M acroinvertebrates like mites and annelids may lack the dispersal ability to reach buttress cavities. A biased species composition in buttresses towards macroinvertebrates that have greater dispersal abilities could explain the lack of community evenness found there.
Downslope buttresses show a similar yet weaker trend. While Coleoptera and Formicidae are most abundant, species evenness is higher downslope than in upslope buttresses, while the A similar amount of total species was found in all three zones but in the adjacent plot there were no more than ten individuals per species , which gives this zone the highest evenness . This may also create a more accurate picture of the relative abundances and species composition of the general forest floor. Overall, buttress roots are an important microhabitat for leaf litter macroinvertebrates. B uttressing in trees across many families suggests their presence or absence is not determined by species. Even individuals of the same species do not always have buttresses (Richards 1996) . Also, it indicates that their function is for something many tropi cal trees need , both structural support and nutrient acquisition. The spreading of surface roots may allow a tree to exploit a larger surface area for nutrient acquisition (Newberry et al. 200 9 ). At the same time, these buttress barriers can collect leaf litter, moisture, and leaf litter fauna. An increased abundance of macroinvertebrates in and around buttresses, as was found in this study, may benefit the tree incidentally by concentrating the on e of the main agents of leaf litter decomposition right at the (Smith 1972) . T he usefulness of buttresses for tropical trees is hard to pinpoint but many studies suggest they benefit the tree in several ways. ACKNOWLEDGE MENTS Thank you to my advisor, Pablo Allen , for helping me a nalyze my data, help with macroinvertebrate identification, and guidance throughout this study . Thanks to Moncho Calder Ã³ n for fixing my lamps , helping me gather materials, and pointing out the obvious flaws in my Berlese funnel design. Gracias to Ala n Masters for providing insight that helped me get my project rolling. Thanks to the whole CIEE staff and my peers who were very helpful and supportive during my research. Muchas gracias to the Chavarria Morera family for housing me during my research. And a special thank you to the Estaci Ã³ n Biol Ã³ gica Monteverde for allowing the use of their land for this study. LITERATURE CITED Haber, B. 2000. Plants and vegetation. In: Monteverde: ecolog y and conservation of a tropical cloud forest. Editors: Nadkarni, N.M. & Wheelwright, N.T. Oxford University Press, New York, NY. Pages 41 44 . Longino, J. T. & Nadkarni, N. M. 1990. A comparison of ground and canopy leaf litter ants (Hymenoptera: Formicid ae) in a neotropical montane forest. Psyche 97: 81 93. Mack, A. 2003. Effects of tree buttresses on nutrient availability and macroinvertebrate species richness. CIEE Fall. Monteverde, Costa Rica. Pages 21 27. Nadkarni, N. M. & Longino, J. T. 1990. Invertebrates in canopy and ground organic matter in a neotropical montane forest, Costa Rica. Biotropica 22: 286 289. Newberry, D. M., Schwan, S., Chuyong, G. B., van der Burgt, X. M. 2009. Buttress form of the central African rain forest tree Microberlinia bisulcata , and its possible role in nutrient acquisition. Trees Structure and Function 23: 219 234. Richards, P. W. 1996. The tropical rainforest: an ecological study. Cambridge University Press. Cambridge, MA. Pages 76 88. Smit h, A. P. 1972. Buttressing of tropical trees: a descriptive model and new hypotheses. The American Naturalist 106: 32. Southwood, T. R. E. & Henderson, P. A. 2000. Biological methods. Blackwell Publishing. Malden, MA. Pages 230 231. Tang, Y., Yang, X., Cao, M., Baskin, C. C. & Baskin, J. M. 2010. Buttress trees elevate soil
heterogeneity and regulate seedling diversity in a tropical forest. Plant Soil . Published online DOI: 10.1007/s11104 010 0546 4. Voris, H. K. 1977. Comparison of herpetofanual diversity in tree buttresses of evergreen tropical forests. Herpeteologica 33: 375 380. Whitfield, S. M. & Pierce, M. S. F. 2005. Tree buttress microhabitat use by a neotropical leaf litter herpet o fauna. Journal of Herpetology 39: 192 198.
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Somerville, Samuel, C.
Gambas en micro hbitats de macroinvertebrados de hojarasca
Buttresses as microhabitats for leaf-litter macroinvertebrates
The functional properties of buttresses are still debated, but hypotheses include they may be a response to mechanical strain and a method to increase the surface area of the tree base; the increased surface area may
provide increased structural support and a larger area of surface soil which may give a competitive advantage for nutrient uptake. The effects of the latter nutrient hypothesis and whether it supports a rich community of leaf-litter fauna has not been well-studied. A more abundant and species rich macroinvertebrate community in buttresses could be indicative of a more nutrient rich microhabitat. In this study I sampled buttressed trees on hillslopes in the cloud forest of Monteverde, Costa Rica to determine if the buttress microhabitat supports a greater species richness and
abundance of leaf-litter macroinvertebrates. Leaf litter was collected from three zones around each tree: upslope buttresses, downslope buttresses, and an adjacent plot. Both macroinvertebrate species richness and abundance are higher in up and downslope buttress plots than in adjacent plots, although only upslope and adjacent differed
significantly. Also, a significantly higher number of larvae were found in the buttress plots. Together, the increased abundance and richness of both adult and larval macroinvertebrates in buttress plots suggest that tree buttresses provide preferable habitat for these leaf-litter fauna.
Las propiedades funcionales de las gambas todava se encuentran en debate, pero las hiptesis incluyen que puede deberse a una respuesta mecnica hacia la tensin y como un mtodo para aumentar el rea de la base del rbol; el aumento en el rea puede proveer un aumento en el soporte estructural y una mayor rea para la absorcin de nutrientes. El efecto de esta ltima hiptesis y de si sostiene una mayor comunidad de la fauna de hojarasca no ha sido bien estudiado. Una mayor abundancia y riqueza de especies de macroinvertebrados en las gambas puede ser un indicativo de un micro hbitat ms rico en nutrientes. En este estudio muestree las gambas en las laderas en el bosque nuboso de Monteverde, Costa Rica para determinar si los micro hbitats de las gambas contienen una mayor abundancia y riqueza de especies de macro-invertebrados de hojarasca. La hojarasca fue colectada en tres zonas alrededor del rbol, las gambas con pendiente ascendente, descendente y en un cuadrante adyacente al rbol. Ambas riqueza y abundancia de especies son mayores en gambas ascendentes y descendente que en la cercana al rbol. Adems, un nmero significativamente mayor de larvas fue encontrado en los cuadrantes de las gambas. Juntos, este aumento en la abundancia y riqueza tanto de adultos como de larvas en macroinvertebrados en las gambas sugiere que estas proveen un sitio preferible para la fauna de hojarasca.
Text in English.
Monteverde Biological Station (Costa Rica)
Diversidad de especies
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Fall 2010
Ecologa Tropical Otoo 2010
t Monteverde Institute : Tropical Ecology