Tree-shrub interactions in a subtropical savanna parkland: competition or facilitation?

Abstract. Prosopis glandulosa var. glandulosa has played a central role in the encroachment of woody plants in southern Texas, grasslands and savannas by acting as a nurse plant for various shrubs that establish in its understory. To test for continued facilitation of established understory shrubs by Prosopis and to determine if established shrubs compete with the Prosopis nucleus, selective removal experiments were conducted and monitored over a 2–5 yr period. Short-term (1–3 days) and long-term (2 yr) growth and physiological activities (midday net photosynthesis and leaf/shoot water potential) of two common understory shrubs, Zanthoxylum fagara and Berberis trifoliolata, growing with Prosopis, were generally comparable to those of individuals occurring in clusters where Prosopis was removed. Shrubs growing with an intact Prosopis occasionally showed significantly higher leaf-[N] and pre-dawn water potentials than those in clusters lacking a live Prosopis, especially under drought conditions; however, these differences did not translate into greater midday leaf gas exchange or shoot growth. By comparison, removal of understory shrubs elicited large increases in Prosopis net photosynthesis, annual trunk growth in each of the 5 yr monitored, and seed pod production in three of the four years monitored. Seven of 26 Prosopis plants in experimental clusters with an intact understory died over a 5-yr period, compared to only two of the 26 plants in clusters with the cleared understory. Results indicate that (1) the founding overstory Prosopis plant may continue to facilitate understory shrubs following their establishment, but these beneficial effects appear to be small and transitory, and (2) the understory shrubs have a pronounced negative effect on Prosopis, such that competition between overstory and understory woody plants is strongly asymmetrical. These findings suggest that understory shrubs will likely persist despite changes in microclimate and soils (potentially) that occur after the Prosopis plant, which facilitated their ingress or establishment, has died. Soil resource depletion by shallow-rooted understory shrubs appears to be a primary factor contributing to the demise of the deeply rooted, overstory Prosopis plants, especially on upland sites with duplex soils where below-ground competition is accentuated.     

Water use by woody plants on contrasting soils in a savanna parkland: assessment with δ2H and δ18O

In savanna parklands of southern Texas, patches of grassland and discrete clusters of small trees and shrubs occur on sandy loam surface soils underlain by an argillic horizon (claypan) at 40 cm. Large trees and shrubs in groves occur on deep (2 m) sandy loam soils without an argillic horizon. 2H and 18O of rainfall, groundwater, and soil and plant water were measured to: (1) determine if coexistence in woody patches occurs via vertical stratification of soil water uptake; (2) document differences in plant water acquisition on contrasting soil types; and (3) evaluate recharge and evaporative losses of soil moisture from grassland vs. wooded landscape elements. Groundwater was isotopically similar to weighted rainfall, suggesting local recharge at this site. Linear regressions of soil water 2H on 18O yielded slopes less than the meteoric water line, indicating significant evaporative losses of soil moisture in all landscape elements. Interspecific differences in root density distribution were significant; some woody species had roots well below 1.6 m, while others had few roots below 0.8 m. 2H and 18O values of stem water from all plants in groves were lower than those of soil water in the upper 1.5 m of the profile, suggesting all species obtained their water from depths >1.5 m. Deep roots of trees and shrubs at this savanna parkland site thus appeared to have a functional significance that was not revealed by biomass or density determinations. Root densities of species in discrete clusters (claypan present) were typically greater than those of the same species in groves (claypan absent), especially in the upper 80 cm of the soil profile. Consistent with rooting profiles, 2H and 18O values of plant water indicated that trees and shrubs in discrete clusters with fine- textured subsoils obtained most of their water at depths <1.5 m. As with groves, there was no indication of water resource partitioning between species. In summary, we saw no isotopic evidence that co- occurring woody plants at this savanna parkland site were partitioning soil moisture vertically during late summer/early fall, despite marked differences in their root density distributions. This supports other lines of evidence which indicate that species interactions in tree/shrub clumps are competitive, and that species composition is therefore unstable in those landscape elements.     

Foliar chemistry and tree ring δ13C of Pinus densiflora in relation to tree growth along a soil pH gradient

Background and aims

Soil acidification is known to be one of the constraints of tree growth; however, it is unclear how it affects tree growth at photosynthesis level (i.e., through affecting stomatal conductance vs. carboxylation rate) during the growth of trees. This paper studied the effects of soil acidification on Pinus densiflora foliar chemistry and tree ring C isotope ratio (¹³C/¹²C, expressed as δ¹³C) and their relationship with tree growth.

Methods

Tree growth (diameter, annual growth ring area, and root biomass), soil chemistry (pH, mineral N, and exchangeable Ca and Al), foliage chemistry (N, Ca/Al, and δ¹³C), and tree ring δ¹³C in P. densiflora stands along a soil pH gradient (from 4.38 to 4.83, n?=?9) in southern Korea were investigated.

Results

Overall, trees with relatively poor growth under more acidic soil conditions (low pH and Ca/Al) had lower values of foliar N concentration and δ¹³C and tree ring δ¹³C, suggesting that restricted N uptake under more acidic soil conditions caused N limitation for photosynthesis, leading to poor tree growth. In addition, relationships between mean annual area increment and carbon isotope discrimination of tree rings at five-yr intervals from 1968 to 2007 revealed that the impact of soil acidification on tree growth became severer during the last 15 yrs as negative correlations between them became significant after 1993.

Conclusions

Reduced N uptake under acidic soil conditions resulted in lower radial growth of P. densiflora via non-stomatal limitation of photosynthesis.     

Reliability of δ13C and δ15N in faeces for reconstructing savanna herbivore diet

We tested the reliability of herbivore faecal δ¹³C and δ¹⁵N values for reconstructing diet through review of an extensive database derived from a 3-year study of ungulates in South Africa's Kruger National Park. Faeces are a useful material for stable isotope studies of diet because they record dietary turnover at very short time scales, and because sampling is non-invasive. However, the validity of faecal isotope proxies may be questioned because they represent only undigested food remains. Results from Kruger Park confirm that free-ranging browsers have faecal δ¹³C consistent with C₃ feeding, grazer faeces are C₄, and mixed-feeder faeces intermediate. Although the respective ranges do not overlap, there is significant variation in faecal δ¹³C of browsers and grazers (～2.0–4.0‰) across space and through time. We demonstrate that most (～70%) of this variation can be ascribed to corresponding patterns of variation in the δ¹³C of C₃ and C₄ plants, respectively, re-enforcing the fidelity of faecal isotope proxies for diet but highlighting a need for mixing models that control for variations in plant δ¹³C in order to achieve accurate diet reconstructions. Predictions for the effects of climate (rainfall) and ecophysiology on ¹⁵N-abundance variations in mammals do not persist in faeces. Rather, faecal δ¹⁵N tracks changes in plant δ¹⁵N, with further fractionation occurring primarily due to variations in dietary protein (reflected by %N). Controlling for these effects, we show that a dual-isotope multiple source mixing model (Isosource) can extend diet reconstructions for African savanna herbivores beyond simplified C₃/C₄ distinctions, although further understanding of variations in mammal δ¹⁵N are needed for greater confidence in this approach.     

Variation in foliar δ13C in Hawaiian Metrosideros polymorpha: a case of internal resistance?

Summary Sun leaves of Metrosideros polymorpha were collected in 51 sites on 9 lava flows that represented gradients of elevation, precipitation, substrate age, and substrate texture on Mauna Loa volcano, Hawai'i. Leaf mass per unit leaf area increased with increasing elevation on all flows, while foliar nitrogen concentration decreased with increasing elevation and increased with increasing substrate age. Foliar ¹³C became less negative with increasing elevation on the wet east-side lava flows, but not the dry northwest-side flows; it did not reflect patterns of precipitation or presumed water availability. ¹³C was very strongly correlated with leaf mass per area across all of the sites. Limited gas-exchange information suggested that calculated c_i/c_a did not decrease with elevation in association with less-negative ¹³C, and photosynthesis per unit of nitrogen was significantly reduced in high-elevation plants. These results are consistent with a substantial internal resistance to CO₂ diffusion in the thick Metrosideros polymorpha leaves in high elevation sites.Published as a contribution to the International Union for Biological Sciences Tropical Mountain Ecosystems Program, and as Carnegie Institution of Washington Department of Plant Biology Publication Number 1065     

Variation in bulk tissue, fatty acid and monosaccharide δ13C values between autotrophic and heterotrophic plant organs

The flowers of 23 species of grass and herb plants were collected from a mesotrophic grassland to assess natural variability in bulk, monosaccharide and fatty acid δ¹³C values from one plant community and were compared with previous analyses of leaves from the same species. The total mean bulk δ¹³C value of flower tissues was −28.1‰, and there was no significant difference between the mean δ¹³C_flower values for grass (−27.8‰) and herb (−28.2‰) species. On average bulk δ¹³C_flower values were 1.1‰ higher than bulk δ¹³C_leaf values, however, the δ¹³C_flower and δ¹³C_leaf values of grasses did not differ between organs suggesting that carbon isotope discrimination is different in grass and herb species. The abundance of different monosaccharides abundance varied between plant types, i.e. xylose concentrations in the grass flowers were as high as 40%, compared with up to 15% in the herb species, but the general relationship δ¹³C_arabinose > δ¹³C_xylose > δ¹³C_glucose > δ¹³C_galactose which had been observed in leaves was similar in flowers (total mean δ¹³C values = −25.9‰, −27.2‰, −28.8‰ and −28.1‰, respectively). However, the average 5.4‰ depletion in the δ¹³C values of the C_16:0, C_18:2 and C_18:3 fatty acids in flowers compared to bulk tissue was significantly greater than observed for leaves. The trend C_16:0 < C_18:2 < C_18:3 previously observed in leaves was also observed in grass flowers (δ¹³C_C16:0 = −33.8‰; δ¹³C_C18:2 = −33.1‰; δ¹³C_C18:3 = −34.2‰) but not herb flowers (δ¹³C_C16:0 = −34.1‰; δ¹³C_C18:2 = −32.4‰; δ¹³C_C18:3 = −34.5‰). We conclude: (i) that the biological processes influencing carbon isotope discrimination in grass flowers are different from herbs flowers; and, (ii) that a range of post-photosynthetic fractionation effects caused the observed differences between flower and leaf δ¹³C values, especially the significant ¹³C-depletion in flower fatty acid δ¹³C values.     

Fifty years of plant invasion dynamics in Slovakia along a 2,500 m altitudinal gradient

Knowledge on the main spatiotemporal trends in plant invasions of habitats is essential for a better understanding of the process of these invasions. The aim of this study was to determine the level of plant invasion, represented by relative richness and total cover of archaeophytes and neophytes, in 45 EUNIS habitat types along with spatiotemporal changes in invasion level with increasing altitude and time in Slovakia. In general, the most invaded habitats are those which are highly influenced by human activities. Generalized linear models and generalized linear mixed models were used to assess the associations between habitat-specific invasion level, altitude and time, respectively. There is a general decrease in the relative richness and total cover of archaeophytes and neophytes with increasing altitude in the invaded habitats. There is also an observable temporal trend in archaeophytes shifting from anthropogenic towards more natural habitats. Importantly, the relative neophyte richness has recently been increasing, predominantly in semi-natural and natural habitats, which brings about major concerns for nature conservation. This may be the manifestation of a lag phase in the dispersal of neophytes. However, accompanied with a significant increase in the relative richness of archaeophytes in some natural habitats, it may indicate more complex changes in the invaded habitats and be an early warning sign for ecological degradation of these habitats.     

How does a wetland plant respond to increasing temperature along a latitudinal gradient?

Global warming affects plant fitness through changes in functional traits and thereby ecosystem function. Wetlands are declining worldwide, and hence, ecosystem functions linked to wetlands are threatened. We use Caltha palustris “a common wetland plant” to study whether warming affects growth and reproduction differently depending on origin of source population, potentially affecting phenotypic response to local climate. We conducted a 2‐year in situ temperature manipulation experiment using clone pairs of C. palustris in four regions, along a 1300‐km latitudinal gradient of Sweden. Open‐top chambers were used to passively increase temperature, paired with controls. Growth and reproductive traits were measured from 320 plants (four regions × five sites × two treatments × eight plants) over two consecutive seasons to assess the effect of warming over time. We found that warming increased plant height, leaf area, number of leaves, and roots. High‐latitude populations responded more strongly to warming than low‐latitude populations, especially by increasing leaf area. Warming increased number of flowers in general, but only in the second year, while number of fruits increased in low‐latitude populations the first year. Prolonged warming leads to an increase in both number of leaves and flowers over time. While reproduction shows varying and regional responses to warming, impacts on plant growth, especially in high‐latitude populations, have more profound effects. Such effects could lead to changes in plant community composition with increased abundance of fast‐growing plants with larger leaves and more clones, affecting plant competition and ecological functions such as decomposition and nutrient retention. Effects of warming were highly context dependent; thus, we encourage further use of warming experiments to predict changes in growth, reproduction, and community composition across wetland types and climate gradients targeting different plant forms.     

Needle δ13C and mobile carbohydrates in Pinus koraiensis in relation to decreased temperature and increased moisture along an elevational gradient in NE China

A tree’s crown interacts with atmospheric variables such as CO₂, temperature, and humidity. Physioecology of leaves/needles (e.g. δ¹³C, mobile carbohydrates, and nitrogen) is, therefore, strongly affected by microclimate in and surrounding a tree crown. To understand the physiological responses of leaves to changes in air temperature and moisture, we measured δ¹³C, soluble sugars, starch, and total nitrogen (N) concentrations in current year and 1-yr-old needles of Pinus koraiensis trees, and compared the growing season air temperature and relative humidity within and outside P. koraiensis crowns along an elevational gradient from 760 to 1,420 m a.s.l. on Changbai Mountain, NE China. Our results indicated that needle N and mobile carbohydrates concentrations, as well as needle δ¹³C values changed continuously with increasing elevation, corresponding to a continuous decrease in air temperature and an increase in relative humidity. Needle carbon and nitrogen status is highly significantly negatively correlated with temperature, but positively correlated with relative humidity. These results indicate that increases in air temperature in combination with decreases in relative humidity may result in lower levels of N and mobile carbohydrates in P. koraiensis trees, suggesting that future climate changes such as global warming and changes in precipitation patterns will directly influence the N and carbon physiology at P. koraiensis individual level, and indirectly affect the competitive ability, species composition, productivity and functioning at the stand and ecosystem level in NE China. Due to the relatively limited range of the transect (760–1,420 m) studied, further research is needed to explain whether the present results are applicable to scales across large elevational gradients.     

Variation in hair δ13C and δ15N values in long-tailed macaques (Macaca fascicularis) from Singapore

Much of the primatology literature on stable isotope ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) has focused on African and New World species, with comparatively little research published on Asian primates. Here we present hair δ¹³C and δ¹⁵N isotope values for a sample of 33 long-tailed macaques from Singapore. We evaluate the suggestion by a previous researcher that forest degradation and biodiversity loss in Singapore have led to a decline in macaque trophic level. The results of our analysis indicated significant spatial variability in δ¹³C but not δ¹⁵N. The range of variation in δ¹³C was consistent with a diet based on C₃ resources, with one group exhibiting low values consistent with a closed canopy environment. Relative to other macaque species from Europe and Asia, the macaques from Singapore exhibited a low mean δ¹³C value but mid-range mean δ¹⁵N value. Previous research suggesting a decline in macaque trophic level is not supported by the results of our study.     

Can we use plant traits and soil characteristics to predict leaf damage in savanna woody species?

Although several hypotheses aim to explain insect herbivory on plants, the relative importance of plant traits, environment, and organizational scale (i.e., individual or community) to herbivory damage level is not well understood. We used an approach based on a local scale, divided into individual and community levels, to test if plant traits, soil characteristics, and plant density explain leaf damage. We sampled 983 individuals in 49 plots distributed over dense and open savanna formations in Emas National Park. In order to explain plant damage, we performed a multi-model inference analysis of four plant traits associated with plant damage, five soil characteristics, and plant density. We did not find any support to plant vigor or plant stress hypotheses at individual plant level. However, the resource concentration hypothesis and plant stress hypothesis explained leaf damage at the community level. We found that increased availability of calcium (Ca) in soils reduced plant damage at the community level. Because soil Ca concentration is a major constraint to plant development in the Brazilian savanna, we postulated that its increasing availability permits plants to invest more in defense strategies. We demonstrate that plant density, Ca soil concentration, and leaf size can be used to predict the plant damage suffered by woody species in savannas at community level.     

Inverse gradients in leaf wax δD and δ13C values along grass blades of Miscanthus sinensis: Implications for leaf wax reproduction and plant physiology

Compound specific hydrogen and carbon isotopic ratios of higher plant leaf waxes have been extensively used in paleoclimate and paleoenvironmental reconstructions. However, studies so far have focused on the comparison of leaf wax isotopic differences in bulk leaf samples between different plant species. We sampled three different varieties of tall grasses (Miscanthus sinensis) in six segments from base to tip and determined hydrogen and carbon isotopic ratios of leaf waxes, as well as hydrogen and oxygen isotopic ratios of leaf water samples. We found an increasing, base-to-tip hydrogen isotopic gradient along the grass blades that can probably be attributed to active leaf wax regeneration over the growth season. Carbon isotopic ratios, on the other hand, show opposite trends to hydrogen isotopic ratios along the grass blades, which may reflect different photosynthetic efficiencies at different blade locales.     

C3 woody plant expansion in a C4 grassland: are grasses and shrubs functionally distinct?

The expansion of C(3) shrubs into C(4)-dominated tallgrass prairies represents a fundamental shift in growth-form dominance accompanied by changes in resource acquisition and use. We assessed these changes by comparing the ecophysiological traits of the dominant C(4) grass Andropogon gerardii, with traits of three C(3) invasive shrub species, Cornus drummondii, Prunus americana, and Rhus glabra. We tested the hypothesis that ecophysiological traits of the shrubs would be similar within this growth form but distinct from grasses and that these species would conform to the two-layer soil water model. Photosynthetic rates in R. glabra were similar to A. gerardii and higher than in the other two shrubs, while water use efficiency was markedly greater in A. gerardii. Among all species, midday xylem pressure potentials (XPP) were distinctly lower (70%) for P. americana, but were similar among the other species. Predawn XPP was related to soil water at shallow depths for A. gerardii (r(2) = 0.59) and P. americana (r(2) = 0.62), and to deeper soil moisture for R. glabra (r(2) = 0.63); there was no relationship for C. drummondii at any soil depth. Thus, a simple two-layer soil water model for partitioning shrub/grass resource acquisition was not appropriate for this grassland. We conclude that these shrubs could not be considered functional equivalents from an ecophysiological perspective, nor were they, as a group, distinct from A. gerardii in resource acquisition and use.     

A 13C NMR spectrometric method for the determination of intramolecular δ13C values in fructose from plant sucrose samples

Recent developments in (13) C NMR spectrometry have allowed the determination of intramolecular (13) C/(12) C ratios with high precision. However, the analysis of carbohydrates requires their derivatization to constrain the anomeric carbon. Fructose has proved to be particularly problematic because of a byproduct occurring during derivatization and the complexity of the NMR spectrum of the derivative. Here, we describe a method to determine the intramolecular (13) C/(12) C ratios in fructose by (13) C NMR analysis of the acetyl-isopropylidene derivative. We have applied this method to measure the intramolecular (13) C/(12) C distribution in the fructosyl moiety of sucrose and have compared this with that in the glucosyl moiety. Three prominent features stand out. First, in sucrose from both C(3) and C(4) plants, the C-1 and C-2 positions of the glucosyl and fructosyl moieties are markedly different. Second, these positions in C(3) and C(4) plants show a similar profile. Third, the glucosyl and fructosyl moieties of sucrose from Crassulacean acid metabolism (CAM) metabolism have a different profile. These contrasting values can be interpreted as a result of the isotopic selectivity of enzymes that break or make covalent bonds in glucose metabolism, whereas the distinctive (13) C pattern in CAM sucrose probably indicates a substantial contribution of gluconeogenesis to glucose synthesis.     

Changes in species density along the soil pH gradient — evidence from German plant communities

The relationship between small-scale species richness and soil pH in plant communities was reviewed using 40 (mainly phytosociological) studies primarily from the northern parts of Germany. Four formations were distinguished (forest, grassland, mire and sand). To examine the above relationship, two approaches were applied: (a) regression analysis using linear and quadratic models, and (b) calculation of the mean pH of the five plots with the highest species density. Despite methodological problems (lack of single plot data in some studies, varying plot sizes, different ways of measuring pH) some general patterns could be identified. In forests, the regressions in most cases indicated a linear or curved increase in species density with increasing pH, whereas maximum species density was, except in one case, found at an intermediate pH of around 5. In contrast, in grasslands and mires, most studies showed hump-backed relationships between species density and soil acidity, and, accordingly, maximum species density was found at moderately high pH between 5 and 6. For sand vegetation, the few studies available revealed an increase in species density with increasing pH. The findings imply that the predominance of calciphilous speciesvs. acidophilous species in the Central European flora does not always translate into positive relationships between plant species density and pH on a local plot scale. The hump-backed species density-pH pattern may be explained by confounding effects of other environmental factors and of productivity, but also by the large overlap of species tolerances in the middle of the acidity gradient.     

Variations in leaf β13C along a vertical profile of irradiance in a temperate Japanese forest

The vertical profile of stable carbon isotope ratios (δ¹³C) of leaves was analyzed for 13 tree species in a cool-temperate deciduous forest in Japan. The vertical distribution of long-term averaged δ¹³C in atmospheric CO₂ (δ_a) was estimated from δ¹³C of dry matter from NADP-malic enzyme type C₄ plant (Zea mays L. var. saccharata Sturt.) grown at a tower in the forest for 32␣days, assuming constant Δ value (3.3‰) in Z. mays against height. The δ_a value obtained from δ¹³C in Z.␣mays was lowest at the forest floor (−9.30 ± 0.03‰), increased with height, and was almost constant above 10␣m (−7.14 ± 0.14‰). Then leaf Δ values for the tree species were calculated from tree leaf δ¹³ C andδ_a. Mean leaf Δ values for the three tall deciduous species (Fraxinus mandshurica, Ulmus davidiana, and Alnus hirsuta) were significantly different among three height levels in the forest: 23.1 ± 0.7‰ at the forest floor (understory), 21.4 ± 0.5‰ in lower canopy, and 20.5 ± 0.3‰ in upper canopy. The true difference in tree leaf Δ among the forest height levels might be even greater, because Δ in Z. mays probably increased with shading by up to ∼‰. The difference in tree leaf Δ among the forest height levels would be mainly due to decreasing intercellular CO₂ (C _i) with the increase in irradiance. Potential assimilation rate for the three tree species probably increased with height, since leaf nitrogen content on an area basis for these species also increased with height. However, the increase in stomatal conductance for these tree species would fail to meet the increase in potential assimilation rate, which might lead to increasing the degree of stomatal limitation in photosynthesis with height. Received: 30 September 1995 / Accepted: 25 October 1996     

Predator–prey ratios on cocoa along a land-use gradient in Indonesia

Tropical landscapes are dominated by agroecosystems, but the potential value of agroecosystems for the survival of species is often overlooked. In agroecosystems, species conservation is especially important when functional groups such as predators are affected. In Central Sulawesi, we sampled arthropods on cocoa in a gradient of land-use intensity from extensively used forest gardens to intensively used agroforestry systems. The abundance and diversity of all arthropods did not correlate with land-use intensity, so human impact was not followed by high species losses. However, the number of species and abundance of the phytophagous arthropods increased and that of the entomophagous arthropods decreased with land-use intensity. The reduced predator–prey ratio in intensified systems can be related to their reduced species richness of shade trees and the changed microclimate (increased temperature, decreased humidity and canopy cover). In conclusion, transformation of traditional into intensified agroforestry systems had a great impact on arthropod community structure on cocoa. Since predator–prey ratios decreased with increasing land-use intensity, local farmers should have least pest problems in the traditionally diversified agroforestry systems.     

Ecological speciation along an elevational gradient in a tropical passerine bird?

Local adaptation of populations along elevational gradients is well known, but conclusive evidence that such divergence has resulted in the origin of distinct species in parapatry remains lacking. We integrated morphological, vocal, genetic and behavioural data to test predictions pertaining to the hypothesis of parapatric ecological speciation associated with elevation in populations of a tropical montane songbird, the Grey‐breasted Wood‐wren (Henicorhina leucophrys: Troglodytidae), from the Sierra Nevada de Santa Marta, Colombia. We confirmed that two distinct populations exist along the elevational gradient. Phylogenetic analyses tentatively indicate that the two populations are not sister taxa, suggesting they did not differentiate from a single ancestor along the gradient, but rather resulted from separate colonization events. The populations showed marked divergence in morphometrics, vocalizations and genetic variation in mitochondrial and nuclear loci, and little to no evidence of hybridization. Individuals of both populations responded more strongly to their own local songs than to songs from another elevation. Although the two forms do not appear to have differentiated locally in parapatry, morphological and vocal divergence along the elevational gradient is consistent with adaptation, suggesting a possible link between adaptive evolution in morphology and songs and the origin of reproductive isolation via a behavioural barrier to gene flow. The adaptive value of phenotypic differences between populations requires additional study.