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Altitudinal effects on leaf morphology and their implications of plasticity in Piper species Rachel Chambers Department of Biology, University of Puget Sound _____________________________________________________________________________________ ABSTRACT Leaf morphology is affected by environmental conditions and has therefore been studied in response to many environmental factors, including altitudinal gradients, which can be used to show responses to cli matic changes of temperature and moisture. The ab ility of a plant to respond morphologically to its environment should influence the extent of its range along a gradient of changing environmental conditions. This study shows changes in width/length ratios, surface area, toughness, and petiole length of b oth young and old leaves within three Piper species along an altitudinal gradient. The ranges of two species, P. amalago and P. dotanum, extend to the end of the premontane wet forest in San Luis, Costa Rica, while the third species, P. hispidum extends in to the lower montane wet forest in the Monteverde Cloud Forest Preserve. As expected, P. hispidum shows greater morphological response to climatic changes with altitude, especially in width/length ratios 2 way ANOVA; p = 0.0012 and toughness 2 way ANOVA; p = <0.0001. This plasticity may therefore be the reason for P. hispidum's presence in the lower montane wet forest, where the other two species are not found. RESUMEN La s caractersticas de las hojas son afectadas por cambios ambientales. Muchos gradientes han sido estudiados que afectan la morfologa de la hoja, como altitud, lo que muestra una respuesta de las hojas a cambios en clima como la temperatura y la humedad. La capacidad de una planta para responder al ambiente debe influir en su distribucin donde puede existir. Este estudio muestra los cambios en la redaccin de ancho y largo de la hoja, rea, dureza y largo del peciolo de amba s ho jas jvenes y viejas de tre s especies de Piper. La distribucin altitudinal de dos especies, P. amalago y P. dotanum llegan al bosque premontane hmedo en San Luis, Costa Rica, y la tercera especie P. hispidum, llega al bosque bajo montano hmedo en la Reserva del Bosque Nuboso de Monteverde. Como se esperaba, P. hispidum muestra una respuesta mayor a cambios de clima co n el cambio en altitud, especialm ente en la relacin de ancho y largo de la hoja ANOVA 2 Vas; p = 0.0012 y dure za ANOVA 2 Vias; p = <0.0001. Es posible que esta capacidad a cambiar sea la razn por la cual puede existir en un ambiente que los otros no pueden. INTRODUCTION Leaf morphology can show how a plant has adapted to its environment, as leaf characters are not solely genetically determined, but are quite responsive to their environment Lightbody 1985; Richards 1996. There has been both convergences in morphological characters by multiple species in similar environmental conditions as well as adaptive radi ation from a common ancestor by species in different environmental conditions Dudley 1978. This is evidence for the response of certain species to environmental
selective pressures. Changes in leaf morphology have been studied along various gradients, in cluding those of canopy layers, altitude, and succession Geeske et al. 1994; Rundell & Gibson 1996; Kappelle and Leal 1996. An altitudinal gradient is useful to show a species' adaptability to certain climatic conditions, as temperature and moisture tend to change with altitude. Altitude as an influential factor of mere distance above sea level or the accompanying pressure changes seems unlikely, given, for example, that similar vegetational zones occur at very different altitudes on different mountains Dudley 1978. Changes in environmental conditions pose a problem in analyzing variation in leaf morphology with altitude, as Geeske 1994 points out, as comparisons across a wide environmental range involve comp arisons across species as well. Species appear and disappear at different altitudes with suitable conditions. The extent of a species' range may then change according to the extent to which, its leaves are able to adapt morphologically to the environment. Therefore, a species' level of plasticity will presumably influence the extent of its altitudinal range. Although individual leaf characters are seldom independent of one another, each character has expected responses to climate conditions Dudley 1978. I n her study of Piper leaves at different moisture levels, Lightbody 1985 found that leaves tend to be longer and thinner in wetter environments than in drier environments, as they shed water faster. She also found that drip tips, extended leaf tips that increase the rate of leaf drying, were longer in wetter environments. Water shedding is important as moisture build up on a leaf may encourage epiphyllousalgae, bryophytes, insect eggs, and larvae to collect on the leaf surface, resulting in damaging effec ts such as reduced carbon assimilation in photosynthesis as well as herbivory by larvae Richards 1996. Drip tips are commonly longer on young leaves than old leaves Rundel and Gibson 1996. In their study of leaf sizes in different Costa Rican foliar be lts, Dolph and Dilcher 1980 found surface area of leaves to be positively correlated with mean annual temperature, while annual precipitation and potential evaporation had a less defined influence. Geeske et al. 1994 suggested that the universally comm on pattern of decreased leaf size with increasing altitude may imply dependence on temperature, a factor directly correlated with altitude, rather than on precipitation and cloud cover, which are not always correlated with altitude. Leaf size differences b etween young and old leaves depend on how much growth has occurred between leaf budding and opening. Leaf toughness usually increases with increased altitude Dudley 1978; Geeske 1994. Dudley 1978 gives the possible explanation that there is some adaptive advantage for leathery leaves with higher humidity. The trend for leaf thickness will be accepted in this study, although further research is needed for the explanation. Young leaves are still toughening and therefore generally weaker than old leaves Rundel & Gibson 1996 Dudley 1978 found a positive correlation between petiole length and both leaf length and width, probably a simple correlation with overall size. He also f ound decreasing petiole lengths with thicker leaves, and suggests that these shorter petioles are more mechanically suited to support the heavier leaves. Young leaves therefore generally have shorter petioles than older leaves. Reliant on other factors, pe tiole lengths may be seen as indirectly related to climate. Climatic pressures should therefore induce narrower, longer, smaller, thicker leaves with shorter petioles with increasing altitude. Other abiotic factors as well as biotic factors have been known to influence leaf characters, such as edaphic conditions, successional stages, and herbivory Geske 1994; Kappelle 1996; Coley &Barone 1996.
Looking at differences between young and old leaves may also give clues as to the respon siveness of a plants lea ves. If an old leaf has changed to a great extent from a young leaf, this may imply great responsiveness. Also, if both young and old leaves follow the same morphological trend with altitude, this may mean that the plant is responding largely to a certain environmental pressure, since leaves of both ages are responding. However, interpretation of leaf differences may be complicated by leaf opening times. The present study observes altitudes influence on four leaf characters: width/length ratio, petiole len gth surface area, and toughness, in both young and old leaves of three species of Piper, P. amalago, P. dotanum, P. hispidum. The total range studied was 1045 1565m along the Pacific slope of the Tilarn mountain range in Costa Rica, ranging from San Luis to Monteverde, with P. amalago and P. dotanum reaching about 1500m, while that of P. hispidum extends to at least 1800m. With in species morphological differences as compared with those of the other species can help explain the differences in range extent as a function of plasticity in leaf morphology. The range of P. hispidum extends above the other two species and, therefore, w ould presumably demonstrate more adaptive responses to climate, as it is present in climatic environments which the other two species are not. MATERIALS & METHODS Study Sites For each of the three Piper species, an altimeter was used to find three 30 meter altitudinal bands along the altitudinal along a gradient were selected based on observation of species presence. Piper amalago and P. dotanum had similar ranges and therefore identical bands the first 1075 1105m in the San Luis Ecological Reserve, the second 1335 1365m and third 1435 1465m in Rafael Leitn Arces forest in San Luis. The range of P. hispidum extended to higher altitudes and therefore different bands were selected. The first 1045 1075m was in the San Luis Ecological Reserve, the sec ond 1435 1465m identical to the third band of P. amalago and P. dotanum and the third 1535 1565m behind the Estacin Biolgica de Monteverde. The region from 800 1500m is classified as premontane wet forest with annual rainfall between 2 and 2.5m with relatively higher temperatures Haber et al. 2000. There is a strong dry season from November to May, when little rain falls and wind borne mist at higher altitudes is blocked by the peaks of the Cordillera Tilarn to the northeast of Monteverde. The low er montane wet region from 1500m to about 1835m receives 3m of rainfall annually. It also receives wind borne mists and cloud cover blown in from the Atlantic side, even during the dry season, and experiences relatively lower temperatures than the premonta ne wet forest. Gradual precipitation increases and temperature decreases along the entire gradient are assumed as well. Data Collection The four leaf characters, width/length ratio, surface area, and toughness, and petiole length, were measured from sample s of each species at each altitudinal band. Sample size was 30 leaves, a young and old leaf taken from each of fifteen plants. For consistency, sample plant heights ranged from 1 to 1.5 meters. Leaves were taken from the branch off the third node from the top of the plant. New leaves were the first fully opened leaf at the branch tip and old leaves the third leaf in from the tip. If the desired leaves were damaged or missing leaves were damaged or missing, leaves were taken from the fourth branch from the t op. Width,
length, and petiole lengths were measured using calipers. Width was measured at the widest point of the leaf and length from petiole base to leaf tip. In this study, drip tips are only indirectly measured through leaf length, as determining drip tip length seemed rather subjective. Petiole length was measured from the base to the widest point of attachment. Surface area was measured by placing leaves underneath a transparent squared centimeter grid and counting the squares within the leaf margin For toughness, leaves were placed in a penetrometer, and the amount of water applied by a constant stream of water using a plastic spray bottle was recorded. Data Analysis To test for statistically significant differences in within species responses by leaf character with changing altitude, both for new and old leaves, a two way ANOVA was run for each leaf character of each species, along with Fishers PLSD post hoc tests. RESULTS Neither P. amalago nor P. dotanum showed the expected trends of longer, narrower, smaller, tougher leaves with shorter petioles as altitude increased. In P. amalago there were significant differences in width/length ratios, specifically between narrower, longer leaves at 1075 1105m and wider, shorter leaves at 1435 1465 m Table 1, Figure 1A. As for P. dotanum width/length ratios did not change with altitude Table 1, Figure 2A. For P. amalago, the effects on surface area depended on leaf age Table 1, Figure 1B. Surface area of P dotanum was unchanged by altitude Table 1, Figure 2B. Toughness was not significantly different between altitudes for either P. amalago or P. dotanum Table 1, Figures 1C, 2C. Petiole lengths of P. amalago did not change with altitude overall, but wer e significantly longer at 1075 1105m than at 1335 1365m Table 1, Figure 1D. For P. dotanum petiole length changed significantly with altitude but only between 1335 1365m and 1435 1465m, where petioles were shorter at the highest altitude Table 1, F igure 2 D. In contrast to the other two species, P. hispidum leaf characters did show responses that would be expected with the changing climatic changes. Width/length ratios were smaller and therefore leaves were narrower and longer at 1535 1535 m than at either of the two lower altitudes Figure 3 A. Piper hispidum leaves were significantly smaller at 1435 1465 m than both the other altitudes, which were not significantly different than one another Figure 3B. Significantly tougher leaves were foun d at 1435 1465 m than at 1075 1105 m and leaves at 1535 1565 m were significantly tougher than those at 1045 1075 m Figure 3C. Petioles were significantly different at all altitudes, with the longest at 1435 1465 m followed by 1535 1565 m and shortest petioles at the 1045 1075m Figure 3 D. In terms of differences with leaf age, all characters between new and old leaves were statistically significant for both P. amalago and P. dotanum Table 1. Young leaves were longer and narrower than old leaves Figures 1A, 2A. Young leaves were smaller than old leaves for both species, although in P. amalago there was an interaction between leaf age and altitude due to changes in the size of youn g leaves, while old leaves were relatively the same size at all altitudes Table 1, Figure 1. Young leaves were significantly larger at 1075 1105 m as compared to the new leaves of the two higher altitudes Table 1, Figure 1. Older leaves were tougher than younger leaves for both
species Table 1, Figures 1C, 2C. Petioles were significantly longer in older leaves for both species Table 1, Figures 1D, 2D. As for P. hispidum young and old leaves followed the same trends. With the exception of width/l ength ratio, all characters were significantly different in young and old leaves Table 1. Young leaves were smaller than old leaves Figure 3B; old leaves were tougher than young leaves Figure 3 C; and young leaves had smaller petioles than old leaves Figure 3D. DISCUSSION If leaves of Piper were responding to climatic variation at different altitudes, they should have become longer, narrower, smaller, and tougher with shorter petioles as altitude increased. Because the leaf morphology of P. amalag o and P. dotanum either did not change with altitude or showed trends other than expected, they did not seem to respond to climatic changes. They are then either unresponsive to environment or responsive to other abiotic or biotic conditions. Piper amalago had increasingly wider and shorter leaves with increasing altitude, opposite what was expected Figure 1A. Perhaps this response is due to edaphic conditions, as they have been known to affect leaf shape Dudley 1978. Thus soil nutrients and moisture ma y be responsible, but further investigation of the soil conditions at the different altitudes is needed before conclusions can be made. In P. dotanum, width to length ratios were unaffected by altitude and corresponding climate changes Figure 2A. The siz e of old leaves of P. amalago did not respond significantly to changing elevations, while young leaves became smaller Figure 1B. Possible explanations for this are discussed later. The size of P. dotanums leaves did not change with altitude Figure 2 B Toughness did not change for either P. amalago or P. dotanum although there was a leaf age altitude interaction for P. dotanum that will be discussed later Figures 1C and 2C. Despite significantly similar size and toughness of P. amalago leaves at dif ferent altitudes, petioles were shorter at the middle altitude Figure 1D. Petiole length may then be individually related to width and length, significantly longer with longer leaf length at the lowest altitude and again longer with significantly wider l eaves at the highest altitude, and shorter when neither leaf width nor length are largely pronounced Dudley 1978. The petiole length trend for P. dotanum likewise does not correspond with unchanging leaf size and toughness, as petioles were shortest at t he highest elevation Figure 2D. This could be due to the significantly tougher young leaves at the highest elevation, which would have required relatively shorter petioles to support heavier leaves as speculated by Dudley 1978. Piper hispidum leaves followed the expected trend, becoming longer and narrower with increased altitude and moisture Table 1, Figure 3A. Thus, the species may be responding to environmental changes in rainfall and moisture as needed to prevent damaging effects by collected wa ter on leaf surfaces Richards 1996. The longer, narrower leaves at higher altitudes should allow for faster water drainage as found by Lightbody 1985. This ability of P. hispidum to respond to moisture may therefore be vital to its presence in the lower montane wet forest, given the higher moisture levels with the transition from premontane wet forest Although leaf size of P. hispidum begins to follow the expected trend with climatic change, decreasing between the lowest and middle altitudes, leave s again becomes larger at the
highest altitude Figure 3B. Leaf size may have therefore been affected by two opposing factors, temperature being the factor between the first two elevations, and a number of possible factors differing between the premontane wet forest and the lower montane wet forest life zones. Smaller leaves can result from decreased soil fertility and dryness Dudley 1978. Greater amounts of organic material as well as the volcanic parent material of the lower montane wet forest do give this zone more fertility than lower regions, and thus, fertility might outweigh the factor of temperature on leaf size between middle and highest altitudes Clark et al. 2000. Leaves of P. hispidum also follow the expected trend of increasing toughness wi th increasing altitude Figure 3C. Thus, they may be responding to increased humidity, although the exact advantage of this is unknown Dudley 1978. The species P. amalago and P. dotanum, then, may be neither suited nor able to adjust morphologically to tolerate higher humidity. Petioles of P. hispidum only make the expected decrease in length between middle and highest altitudes Figure 3D. The effects on petiole length may be unclear due to its dependence on many factors, of width, length, size and thi ckness Dudley 1978. The petiole lengths found may be explained by relating petiole length with area, as intermediate altitude had the smallest leaves and the longest petioles. This could be due to leaf weight, the larger leaves weighing more and needing shorter petioles for support Dudley 1978. The longer petioles at the highest altitude versus lowest altitude may be due to a relationship between petiole length and leaf length, which could be outweighed at the middle altitude by leaf size Dudley 1978. Differences between young and old leaves of P. amalago and P. dotanum also do not seem to suggest responses to climate. In P. amalago, width/length ratios for young leaves follow the same trend as the old leaves, suggesting a possible response to edaphic conditions Figure 1 A. Young leaves of P. dotanum show the same unresponsiveness as o ld leaves, with unchanging wi d th/l ength ratios at different altitudes Figure 2A. Young leaves of P. amalago became significantly smaller with increased altitude, altho ugh older leaves remained unchanged Figure 1 B. Thus younger leaves at the highest altitudes, although small, are becoming just as large as the larger young leaves at lower altitudes. Thus, there seems to more effort to become large at the highest altitud e, which should not occur if the leaves are responding to climate and should be becoming smaller at high altitudes. Perhaps this is a response to greater herbivory in the warmer, drier conditions of lower altitudes. Reducing the overall expansion period re duces damage to vulnerable young leaves, as exposure time to generalist herbivores is shortened and specialists are constrained to find hosts Coley & Barone 1996. If expansion times are not different, the young leaves at lower altitudes may be responding to higher herbivory by delaying opening times until they are tough enough to resist herbivores Coley & Barone 1996. Surface area of young P. dotanum leaves, like old leaves, does not show a response to environment Figure 2B. Toughness of P. amalago ar e unresponsive to climatic changes Figure 1C. For P. dotanum, there is an altitude leaf age interaction caused by the tougher younger leaves at the highest elevation Figure 2C. Perhaps humidity or edaphic features are involved, although it is unknown whether such features would affect young and old leaves differently Dudley 1978. It is unlikely that such a great change in toughness would result from relatively small humidity changes, especially given the lack of change between the lower two altitudes and because it only exists in young leaves. Petioles of young leaves showed no change with altitude in both species although there were significant changes in older leaves Figures 1D and 2D. This may be due to delayed petiole response to environmental conditions. In P. hispidum young leaves and old leaves showed the same trends for all characters. This may therefore show the high responsiveness of the species to environmental conditions, since leaves of bot h ages are affected. New and old leaves were not significantly different from each other in width/length ratio, possibly due to unique, and perhaps advantageous, trait of the species allowing it to maintain a favorable width/length ratio throughout a leaf s lifetime Figure 3A. This may be because of the high importance of water shedding capabilities in the
lower montane wet forest. Overall, P. hispidum seems to respond more to the altitudinal effects of temperature and precipitation than either P. amalago or P. dotanum The longer, narrower leaves of P. hispidum at higher altitudes especially give support to the species greater adaptability to increasing moisture, as they showed both a clear trend with a clear explanation. Piper hispidums response to cli mate coincides with the presumed higher plasticity of P. hispidum that allows it to occur in the lower montane wet forest while the others do not. Given the lack of response to climatic changes in the premontane wet region, P. amalago and P. dotanum may no t be plastic enough to adapt to a more drastic temperature and moisture difference between the premontane and lower montane wet zones, and thus explain its absence there. AKNOWLEDGEMENTS I would like to give a big thank you to Mauricio Garcia for being the re to help with my project design, run tests, answer questions, and get my glasses untangled from my hair. Thanks to William Wieder for all the help with making corrections. Thank you so much to Willow Zuchowski and Bill Haber for giving their expertise in identifying plant species, the San Luis and Monteverde Biological Stations and Rafael Leitn Arce for letting me work in their forests. Thanks to Andrew, Alan, and Karen for answering all the little questions. __________________________________________________________________________________________________ LITERATURE CITED C lark, K.L, Lawton, R.O., and But ler, P.R. 2000. The Physical Environment. In: Monteverde: Ecology and Conservation of a Tropical Cloud Fore st, N.M. Nadkarni and N.T. Wheelwright, eds. Oxford University Press, Oxford, New York, pp. 15 38. Coley, P.D and J.A. Barone. 1996. Herbivory and Plant Defenses in Tropical Forests. Ann. Rev. Ecol. Syst. 27:305 335. Dolph, G.E a nd D.L. Dilcher. 1980. Variation in Leaf Size with Respect to Climate in Costa Rica. Biotropica 12:2, 91 98. Dudley, E .C. 1978. Adaptive Radiation in the Melastomataceae along an Altitudinal Gradient in Peru. Biotropica 10:2,134 143. Haber, W.A., W. Zuchowski, and E. Bell o. 2000. An Introduction To Cloud Forest Trees. Monteverde, Costa Rica. Mountain Gem Publications, Monteverde de Puntarenas, Costa Rica. Geeske, J., G. Aplet, and P.M. Vitousek. 1994. Leaf Morphology along Environmental Gradients in Hawaiian Metrosideros pol ymorpha. Biotropica 26: l, 17 22. Kappelle, M. and M.E. Leal. 1996. Changes in Leaf Morphology and Foliar Nutrient Status Along a Successional Gradient in a Costa Rican Upper Montane Quercus Forest. Biotropica 28:3, 331 334. Lightbody, J.P. 1985. Distribution of Leaf Shapes of Piper sp. In a Tropical Cloud Forest: Evidence for the Role of Drip tips. Biotropica 17:4,339 342. Richards, P.W. 1996. The Tropical Rain Forest. The University of Cambridge Press, New York, New York, USA. Ru ndel, P.W. and A.C. Gibson. 1996. Adaptive Strategies of Growth Forms and Physiological Ecology in Neotropical Lowland Rain Forest Plants. In : Gibson, A.C. ed.. Neotropical Biodiversity and Conservation, Uni versity of California Los Angele s
Figure 1. The eff ect of altitude on leaf characters of P. amalago for both young and old leaves mean +/ 1 SD. Sample size was thirty leaves at each altitude, fifteen new and fifteen old. Width/length ratios A between the lowest and highest elevations were significantl y different, while those between lowest and middle and middle and highest were not Fishers PLSD post hoc test; p = 0.0200, 0.537, 0.6798. Surface area B was significantly different between lowest and m altitudes and lowest and highest altitudes, but n ot between middle and highest Fishers PLSD post hoc test; p = 0.0218, 0.0050, 0.5855. Toughness C was not significantly different between lowest and either altitude nor between middle and highest altitudes Fishers PLSD post hoc test; p = 0.6780, 0.1 330, 0.2743. Petiole lengths D were significantly different between lowest and middle altitudes, but not between lowest and highest nor middle and highest Fishers PLSD post hoc test; p = 0.0399, 0.0987, 0.6775. All pairs with the same letter are sig nificantly different.
Figure 2. The effect of altitude on leaf characters of P. dotanum for both young and old leaves mean +/ 1 SD. Sample size was thirty leaves at each altitude, fifteen new and fifteen old. Width/length ratios A were not significantly different between the lowest and middle, lowest and highest, nor between middle and highest altitudes Fishers PLSD post hoc test; p = 0.6192, 0.846, 0.4902. Su rface area B was not significantly different between altitudes Fishers PLSD post hoc test; p = 0.3097, 0.4220, 0.8465. Toughness C was not significantly different either Fishers PLSD post hoc test; p = 0.9074, 0.0756 0.0588. Petiole lengths D were significantly different between middle and highest altitudes, but not between lowest and middle nor highest Fishers PLSD post hoc test; p = 0.0020, 0.2103, 0.0569. All pairs with the same letter are significantly diff erent.
Figure 3. The effect of altitude on leaf characters of P. hispidum for both young and old leaves mean +/ 1 SD. Sample size was thirty leaves at each altitude, fifteen new and fifteen old. Width/length ratios A were significantly different between lowest and highest but not between lowest and middle altitudes Fishers PLSD post hoc test; p = 0.0004, 0.0093, 0.2972. Surface area B was significantly different between lowest and middle altitudes and between middle and highest elevations but not between lowest and highest Fishers PLSD post hoc test; p = 0.0119, 0.0010, 0.3975. Toughness C was significantly different between lowest and both middle and highest altitudes, but not between middle and highest altitudes Fishers PLSD post hoc test; p = 0.0038, < 0.0001, 0.1118. Petiole lengths D were significantly different between lowest and both middle and highest altitudes as well as between middle and highest Fishers PLSD post hoc test; p = < 0.0001, 0.0013, 0.0348. All pairs with the same letter are significantly different.