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Differential aboveground adventitious root production by Senecio cooperi (Asteraceae) Nathan J. Ebnet Department of Biology, Gu stavus Adolphus College ABSTRACT Previous studies have suggested that aboveground ad ventitious roots (AARs) are produced by plants in order to extract nutrients from the epiphytes they support (Nadkarni 1981 & 1984). I asked whether Senecio cooperi a neotropical plant which produces AARs, generates these roots for nutrient absorption, fragmentation establishment, or structural support. I conducted a survey in which I measured multiple characteristics of S. cooperi and their AARs in Monteverde, Costa Rica. Naturally fragmented stems produced more AARs than expected by chance (x2 = 12.80, df = 1, p < 0.05). No purely vertical stems contained AARs associated with the moss mats growing on them. However, vertical stems displayed basal AARs which increased in diameter as bo th plant height and DBH increased (R2 = 0.330, P < 0.001, N = 50, R2 = 0.402, P < 0.0001, N = 50 respectively). These findings suggest that the naturally occurring role of AARs of S. cooperi are not for nutrient absorption, but rather for structural support and fragmentation regeneration. These functions of AARs allow S. cooperi to be more successful as a pioneer species in disturbed habitats, an d highlight the need to reevaluate th e functionality of AAR s among other plants. RESUMEN Los estudios previos han sugerido que races adventicias en la superficie (AARs) son producidos por plantas para extraer alimentos nutritivos del epiphytes que ellos sostienen (Nadkarni 1981 & 1984). Pregunt si Senecio cooperi una planta de neotropical que produce AARs, engendra estas races para la absorcin de alimento nutritivo, para establecimiento de fragmentacin, o para apoyo estructural. Realic una inspeccin en la que med mltiples caractersticas de S. cooperi y su AARs en Monteverde, Costa Rica. Los tallos naturalmente fragmentados produjeron ms AARs que esperado por casualidad (x2 = 12.80, df = 1, P <0.05). Ningunos tallos puramente verticales contuvieron AARs se asoci con las esteras de musgo que crecen en ellos. Sin embargo, los tallos verticales demostraron AARs elemental que aument de dimetro como ambas altura de planta y DBH aument (R2 = 0.330, P <0.001, N = 50, R2 = 0.402, P <0.0001, N = 50 respectivamente). Estas conclusiones sugieren que el papel naturalmente ocurriendo de AARs de S. cooperi no es para la absorcin de alimento nutritivo, sino para la regeneracin estructural de apoyo y fragmentacin. Estas funciones de AARs permiten S. cooperi para ser ms exitoso como una especie de pionero en hbitates perturbado, y destaca la necesidad de reevaluar la funcionalidad de AARs entre otras plantas. INTRODUCTION An intense challenge that many neotropical tree species face is the low amount of soil nutrients available. These tropical soil conditions require plants to have efficient nutrient capturing and processing mechanisms (Na dkarni 1981). A unique nutrient pathway has been suggested in which epiphyte host pl ants produce aboveground adventitious roots (AARs) that probe beneath the collected orga nic material of the epiphytes they support (Nadkarni 1981). AARs are distinguished from other aerial roots in that they have 1
morphological features such as root hairs, quickly regenerating root tips, and contain endomycorrhizal hyphae (Nadkarni 1981). Other authors have proposed alternativ e reasons why plants may produce AARs. Kinsman (1990) experimentally fragmented stems of 22 neotropical plant species representing 14 genera Ten months after fragmentation, 35% of the fragments had survived by producing adventiti ous roots (Kinsman 1990). Scha tz et al. (1985) suggested that some plants produce AARs close to their stem base for structural support, in an effort to compensate for lacking secondary growth We iner et al. documented that intraspecific plant competition drives some plant species to grow horizontal stems in order to gain access to light resources (1990). In this horizontal form plants may produce AARs for increased structural support (Muzik & Cruzado 1956). The apparent controversy over the function of AARs is complicated by the fact that AARs may take on varying roles throughout a plants development (Smith 1936). Additionally, AAR production may occur differently under experimental conditions versus natural circumstances. Comprehensive studies exploring AAR functional versatility and adaptability are lacking. Nadkarni (1994) reported that a par ticular neoptropical tree species, Senecio cooperi (Asteraceae), responded to experimental applications of epiphytes by producing AARs This suggested that the AARs of S. cooperi assist in nutrient upt ake. In this study, I reexamine AARs of S. cooperi because initial observati ons revealed no association between moss mats and AARs. The principle objective of this study is to determine whether AARs of S. cooperi assist with nutrient uptake, structural support in horizontal or vertical growth forms, or fragmentat ion survival. I hypoth esize that AARs of S. cooperi assist with structural support and frag mentation establishment. Therefore, I expect AARs to be present at the base of tall vertical stems as well as along the stem of horizontal growth forms. I also expect AARs to be present on fragmented stems. METHODS Study Site. This study was conducted in forest e dge near a rural road within one kilometer of the Santa Elena Cloud Fore st Reserve (SECFR), Costa Rica (10 20N, 84 45W). The SECFR is located high on the Ca ribbean slopes of the Cordillera de Tilarn (1,500 m), very near Monteverde, Co sta Rica. Average annual temperature at Monteverde is 18.5 C, with a mean annual precipitation of 2519 mm (Clark et al. 2000). The study plant, S. cooperi was abundant along the forest bordering this rural road and grew well in this frequently disturbed habitat. The study was conducted from 24 October 2007 to 13 November 2007. Induced Fragmentation.Twenty adult S. cooperi individuals were identified that displayed both horizontally and vertically growing stems. Hori zontal and vertical segments were cut from each of the 20 individuals, resulting in 20 pairs of segments. Horizontal stems were classified as those stems whose angle between stem and soil was 0 to 45 degrees before being cut; while vertical stems were those whose original angle was 46 to 90 degrees. These fragmented stems were then placed on a natural soil plot (3 m x 1 m). Stems were checked every two days for AAR growth, with the last day of observation occurring ten days afte r the initial fragmentation event. 2
Natural Fragmentation. Twenty random, previously fragmented S. cooperi stems were obtained from the forest edge bordering the study site. Each stems diameter was measured with a digital caliper, along with the number of AARs per stem. Moss and Basal Associated AARs.Fifty adult S. cooperi individuals who exhibited a purely vertical growth form (90 ) were selected for the surv ey. DBH, plant height, the number and diameter of basal AARs, total amount of moss cover occurring on stems, and the number of AARs associated with moss mats were record ed for all 50 individuals. Measurements were obtained using a digi tal caliper and a 20 m measuring tape. Growth Angles and AARs. Fifty adult S. cooperi individuals who displayed both horizontal and vertical stems we re selected. Some segments (horizontal or vertical) often were continuous along the naturally curving stem of S. cooperi and therefore were documented as two different segments. Plan t height, estimated stem angle, the number of AARs associated with each stem, and th e distance between plant base and AARs were recorded using a digital calip er and a 20 m measuring tape. Statistical Analyses. A Goodness of Fit test was used to analyze significance between the number of AARs and the original angle of stems (horizontal or vertical) prior to fragmentation. Another Goodness of Fit test was used to analyze significance between the number of naturally fragmented stems w ith or without AARs. A simple linear regression was run to investig ate a relationship between stem diameter and the number of AARs present for naturally fragmented stems. To examine the relationship between vertical plant height or DBH and the average diameter of basal AARs, two simple linear regressions were run. Two more simple linear regressions were run to explore potential correlations between stem angle and the num ber of AARs and relationships between plant height and the distance between stem base and AARs. RESULTS Induced Fragmentation.Both horizontal and vertical stem segments (N = 20) cut from adult S. cooperi produced AARs ten days after the in duced fragmentation (Table 1). However, neither horizontal nor vertical stem segments displayed greater than expected AAR production (x2 = 0.94, df = 1, P > 0.05). 3
TABLE 1. Number of stems that generated aboveground ad ventitious roots (AARs) ten days after being cut from adult Senecio cooperi plants. Horizontal stems we re classified as those stems whose angle between stem and soil was 0 to 45 degrees before being cut; while vertical stems were those whose original angle was 46 to 90 degrees. After being cut from the adult plant, all stems were placed on a natural 3 m x 1 m soil plot. Original Stem Orientation Number of stems with AARs Number of stems without AARs Horizontal 3 17 Vertical 1 19 Natural Fragmentation. Of the surveyed naturally fragmented stems (N = 20), the number of stems with AARs present was signi ficantly larger than the number of stems without (x2 = 12.80, df = 1, p < 0.05). Additionally, greater fragmented stem diameter (mean in mm SD; 15.471 2.382) resulted in more AARs (10.850 9.292; Figure 1). 0 5 10 15 20 25 30 35 10121416182022 Fragmented stem diameter (mm)Number of roots FIGURE 1. Relationship between the di ameter and the number of aboveground adventitious roots found on fragmented stems of Senecio cooperi. (Y = 1.913x 18.747 R2 = 0.240, P = 0.0282, N = 20). The measured stems had all undergone previous natural fragmentation events. Moss and Basal Associated AARs.Moss cover on the vertical stems of the surveyed S. cooperi (N = 50) ranged from to complete c overage. None of these vertical growing S. cooperi individuals produced any AARs that grew into the moss mats present on their stems. Purely vertical forms of S. cooperi did produce basal AARs that probed into the soil. These basal AARs all grew be tween 0 and 100 mm along the stem of S. cooperi and were not associated with the stem moss mats. Senecio cooperi stems are thin (mean in mm SD; 18.30 2.32) a nd tall (1697.10 189.89). Greater S. cooperi DBH and height resulted in larger averag e basal AAR diameter (Figure 2). 4
There was not a significant relationshi p between plant height or DBH and the number of basal AARs present (Table 2). 0 2 4 6 8 10 12 14 16 18 20 22 131517192123252729 DBH (mm)Average Basal Root Diameter (mm) A 0 2 4 6 8 10 12 14 16 18 20 1300140015001600170018001900200021002200 Plant Height (mm)Average Basal Root Diameter (mm) B FIGURE 2. Relationship between (A ) stem DBH and average basal root diameter (Y = 1.306x 16.414, R2 = 0.402, P < 0.0001, N = 50) and (B) plant height and average basal root diameter (Y = 0.0145x 17.122, R2 = 0.330, P < 0.001, N = 50) of Senecio cooperi. The measured plants all ex hibited purely vertical growth. Growth Angle and AARs. Both horizontal and vertical S. cooperi individuals had stems that exhibited AARs. Out of all of the stems surveyed (N = 50), 42 horizontal stems contained AARs, compared to only two vertical stems. These AARs were distinguished from basal AARs in that they grew farther than 100 mm from the base of the stem. None of these AARs grew into th e soil, and even the l ongest root (429. 43 mm) was solely attached to its anchoring S. cooperi stem. Stems that exhibited a smaller 5
growth angle in relation to soil displayed a higher numb er of AARs (maximum AARs found per stem = 5; Figure 3). 0 1 2 3 4 5 6 051015202530354045505560657075808590 Stem AngleNumber of Aboveground AdventitiousRoots FIGURE 3. Relationship between stem angle and the number of aboveground adventitious roots (AARs) of Senecio cooperi Regression analyses for both horizontal stems (Y = -0.0375x + 1.81, R2 = 0.082, P = 0.0307, N = 50) and vertical stems (Y = -0.0087x + 0.6939, R2 = 0.174, P = 0.0023, N = 50) reveal a negative correlation between stem angel and the number of AARs. Horizontal stems were classified as those stems whose angle between stem and soil was 0 to 45 degrees; while vertical stems were those whose angle was 46 to 90 degrees. There was a significant correlation betw een horizontal stem segment length and the average distance between S. cooperi plant base and AARs (Figure 4), meaning longer stem segments had larger distances between plant base and thei r respective AARs. Variation was present, evident in that a st em of small length (1500 mm) had the largest distance between plant base and AARs (1236 mm). 6
0 200 400 600 800 1000 1200 1400 1600 1000120014001600180020002200240026002800 Total Length of Horizontal Stem (mm)Average Distance between Stem Base and AARs (mm) FIGURE 4. Relationship between total length of horizontal stems and the average distance between the plant base and the abovegr ound adventitious roots on that stem (Y = 0.5956x 121.4, R2 = 0.430, P = < 0.0001, N = 45) of Senecio cooperi Additional regressions were r un on other aspects of AARs, stem angle, and S. cooperi characteristics, but none revealed si gnificant relationships (Table 2). Table 2. Additional statistical regressions conc erning aboveground adven titious roots of Scenecio cooperi. None of these test yielded stat istically significant regression (N = 50 for all regressions). Comparison Regression Equation R2 Pvalue DBH vs. number of basal adven titious roots (vertical plants only) Y = 0.1076x + 0.671 0.013 0.42 Plant height vs. number of basal adventitious roots (vertical plants only) Y = 0.0011x + 0.8075 0.009 0.51 Stem angle vs. average above ground adventitious root (AAR) length Y = 2.1655x + 102.59 0.018 0.40 Total number of stem segments per individual vs. number of AARs per individual Y = 0.0527x + 1.5378 0.002 0.74 Length of stem vs. total number of AARs per stem Y = -0.0003x + 2.0726 0.009 0.52 Average distance between stem base and AARs vs. number of roots per stem Y = 0.0008x + 0.7231 0.030 0.92 Average distance between stem base vs. average AAR length Y = -0.058x + 167.52 0.024 0.32 7
DISCUSSION Prior to this study, I hypothesized that the AARs generated by S. cooperi were principally important for regeneration after fragmentation and structural support. As expected, some fragmented stems did produce AARs. However, there were only a small number of experimentally fragmented stems with AARs, resulting in no significant difference between the numbers of AARs produced by fr agmented stems that were originally horizontal or vertical (Table 1). The sma ll amount of AAR production can be explained by the short time period (ten da ys) allotted for fragmented stems to produce new AARs. Many AARs have been documented to take as long as one month to grow out of fragmented stems of neotropi cal species (Kinsman 1990). Naturally fragmented stems had more AARs present than was expected by chance. This supports the idea that AARs assist in fragment ation survival and establishment. Structurally, S. cooperi stems are weak and thus are susceptible to fragmentation. It follows from this that S. cooperi stems would gain a fitness advantage if fragmented stems could reestablish, provi ding a form of asexual reproduction. This ability to fragment successfully, as a result of AAR production, explains S. cooperi abundance in disturbed areas. A larger stem diameter resulted in a greater number of AAR s (Figure 1). AARs likely originate from the cam bium of fragmented stems (Smith 1936). Thicker stem fragments would have a greater amount of cambium and necessary nutrients for root growth, allowing them to generate more AAR s. It is important to note that the fragmented stems that were collected were taken across a range of abiotic conditions. The varying amounts of light and nutrients across this range may explain how several stems with smaller diameters had a numerous AARs (Figure 1). No vertical S. cooperi stems had AARs that probed in to the moss mats they were supporting. This information suggests that AARs do not offer an alternative nutrient pathway for S. cooperi individuals. Furthermore, th is datum suggests that the AARs produced by S. cooperi in response to experimental epi phyte treatments (Nadkarni 1994) do not reflect normal ecological growth patterns. As expected, vertical S. cooperi stems did display basal AARs. Stems that were taller and exhibited a greater DBH had basal roots with thicker diameters (Figure 2). These results begin to suggest th at basal roots offer structural support to vertical stems. It is also possible that these roots were primar ily in place for nutrient absorption. If these basal AARs were produced for nutrient absorp tion, it would be more advantageous for larger plants to produce a gr eater number of AARs (inste ad of AARs with greater diameters) in order to maximize surface area. There was not a significant relationship between plant height or DBH and the number of basal AARs. This further supports the idea that basal AARs are used for structural support. S. cooperi stems are slim yet tall, which results from their rapid growth pattern characteristic of pion eer species. Producing thick basal AARs allows S. cooperi individuals to grow quick ly to maximize sunlight absorption, while offering increased structural stability (S chatz et al.1985). As stem angle increased towards 90 the number of AARs decreased (Figure 3). Horizontal stems experience more gravita tional pull (Wilkins 1977), and AAR growth may be facilitated by this increase. In terestingly, none of these AARs present on horizontal stems reached the soil, and therefore did not offer these stems additional 8
9 support. AARs may be produced on horizontal stems in order to anticipate fragmentation, which indeed is more likely to occur compared to vertically oriented stems (Wilkins 1977). Having a few AARs produ ced prior to fragmentation would allow more immediate nutrient uptake to occur in the fragmented stem. Longer horizontal stems had a greater di stance between their stem base and AARs (Figure 4). Again, horizontal stems are more likely to experience fragmentation, and the probability increases with distance along the stem. Therefore, longer stems would be benefited by producing AARs closer to a potential fragmentation area. These results question previous studies that suggested AARs produced by S. cooperi were for epiphyte associated nutrient uptake (Nadkarni 1994) by highlighting their important role in fragmentation recovery and structural support. However, it is still unknown whether AARs of S. cooperi change function based upon microhabitat and other abiotic conditions. Furthermore, the ab sorptive capacities and rate of growth of AARs are poorly documented. Still, my results show the importance of reevaluating the functional role of AARs beyond nutrient absorpti on for many neotropical plant species. ACKNOWLEDEMENTS Many thanks to Karen Masters for her exceptional project guidance and Alan Masters for his numerous contributions. I also thank the creative think-tank team of Pablo Allen and Taegen McMahon. Finally, I thank Heladio Cruz for his help in identifying Senecio cooperi. Literature Cited Clark, K.L., R.O. Lawton and P.R. Butler. 2000. The physical environment. In: Monteverde: Ecology and Conserva tion of a Tropical Cloud Forest N.M. Nadkarni and N.T. Wheelwright. Oxfo rd University Press, Oxford, pp. 15-20 Kinsman, S., 1990. Regeneration by fragmentation in tropical montane forest shrubs. American Journal of Botany 77: 1626-1633. Muzik, T.J., and Cruzado, H.J., 1956. Formati on and rooting of adventitious shoots in Hevea brasiliensis. American Journal of Botany 43: 503-508 Nadkarni, N.M., 1981. Canopy roots: convergen t evolution in rainforest nutrient cycles. Science 214: 1023-1024. --------1994. Factors affe cting the initiation and growth of aboveground adventitious roots in a tropical cloud forest tree: and experimental appr oach. Oecologia 100: 94-97. Schatz, S.E., Williamson, G.B., Cogswell, C. M., and Stam, A.C., 1985. Stilt roots and growth of arboreal palms. Biotropica 17: 206-209. Smith, A.I., 1936. Adventitious roots in stem cuttings of Begonia maculate and B. semperflorens. American Journal of Botany 23: 511-515. Wilkins, M.B., 1977. Lightand gravity-sensing guidance systems in plants. Biological Sciences 1999: 513-524.
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Ebnet, Nathan J.
Produccin diferencial de las races adventicias sobre el suelo por Senecio cooperi (Asteraceae)
Differential aboveground adventitious root production by Senecio cooperi (Asteraceae)
Previous studies have suggested that aboveground adventitious roots (AARs) are produced by plants in order to extract nutrients from the epiphytes they support (Nadkarni 1981 & 1984). I asked whether Senecio cooperi, a neotropical plant which produces AARs, generates these roots for nutrient absorption, fragmentation establishment, or structural support. I conducted a survey in which I measured multiple characteristics of S. cooperi and their AARs in Monteverde, Costa Rica. Naturally fragmented stems produced more AARs than expected by chance (x2 = 12.80, df = 1, p < 0.05). No purely vertical stems contained AARs associated with the moss mats growing on them. However, vertical stems displayed basal AARs which increased in diameter as both plant height and DBH increased (R2 = 0.330, P < 0.001, N = 50, R2 = 0.402, P < 0.0001, N = 50 respectively). These findings suggest that the naturally occurring role of AARs of S. cooperi are not for nutrient absorption, but rather for structural support and fragmentation regeneration. These functions of AARs allow S. cooperi to be more successful as a pioneer species in disturbed habitats, and highlight the need to reevaluate the functionality of AARs among other plants.
Estudios previos han sugerido que las races adventicias en la superficie (AARs) son producidos por las plantas para extraer alimentos nutritivos de las epifitas que ellas sostienen (Nadkarni 1981 & 1984). Pregunte si Senecio cooperi, una planta neotropical que produce AARs, engendra estas races para la absorcin del alimento nutritivo, para el establecimiento de la fragmentacin o para el apoyo estructural. Realice una encuesta en la que med las mltiples caractersticas de S. cooperi y de su AARs en Monteverde, Costa Rica.
Text in English.
Tropical Ecology 2007
Aboveground adventitious roots
Ecologa Tropical 2007
Races adventicias sobre el suelo
t Monteverde Institute : Tropical Ecology