Tropical Lower Montane Forest Research Articles

Further food for thought: higher tropical mountains revisited once again

D.H. Janzen argued that tropical mountains, for their heights, pose greater migration barriers than temperate mountains owing to lower temperature seasonality, and consequent greater niche and ecosystem specialization. These result in distinct altitudinal zonation under hardly varying temperature lapse rates. The Asian Far East alone provides an opportunity to test this hypothesis owing to latitudinal continuity of evergreen forests from equator to warm temperate zone. With data from four mountains, we find Janzen’s hypothesis broadly correct. The primary and most consistent mediator of forest zonation and therefore species’ habitat specificity is soil, itself indirectly mediated by climate. But declining habitat niche specificity does not proceed north along a continuum, instead in a series of latitudinal steps initiated by quite sudden habitat changes. Tropical montane forests are altitudinally floristically and structurally zoned by factors only indirectly mediated by temperature. Equatorial forests experience continuously wet climates and a frost line at c.4100 m. North of 8°N, temperature and rainfall seasonality increases and winter air frosts descend to c.2000 m. Upper montane forest becomes truncated, while lower montane forest continues to the frost line which remains constant in moist climates. The margin of the wet tropics is pushed north to 27°N by the Himalaya. Annual frost at c.2000 m here marks a narrow ecotone between tropical lower montane forest and warm temperate evergreen lucidophyll forest, a formation unique to east Asia. In equably moist South China, the lowland tropical-warm temperate (‘subtropical’) margin returns to the Tropic of Cancer and appears to be narrow at all altitudes. The temperate forests exhibit less distinct and inconsistent altitudinal zonation, instead manifesting an altitudinal floristic and structural continuum mediated by winter frost frequency and intensity.

Compositional variation in understorey fern and palm communities along a soil fertility and rainfall gradient in a lower montane tropical forest

AbstractQuestionsFerns and palms are key components of tropical wet forest understories. The distributions of these groups are influenced by variation in rainfall and soil properties, although the degree to which their resource requirements overlap remains unclear. Palms and ferns also represent extremes in a spectrum of reproductive traits, which may impact habitat requirements and compositional turnover. We asked whether: (a) environmental variables influence fern and palm distributions in a similar manner across a gradient in soil nutrient availability; and (b) larger propagule size and lower reproductive output in palms result in greater spatial aggregation of populations and, consequently, greater distance–decay rates in community similarity in palms than ferns.LocationOld‐growth premontane forest, Fortuna, western Panama.MethodsAll ferns and palms were sampled in fifteen 5 m × 5 m subplots in each of 12 one‐hectare plots. The plots span a strong gradient in soil nutrient availability related to parent material. Light, precipitation and soil variables were measured in each subplot. Species associations were analyzed using ordination and regression, as well as Procrustes and Mantel tests.ResultsParent material significantly impacted compositional similarity of both ferns and palms, with fern compositional variation related to total soil N:P ratio and red:far red (R:FR), and palms with bulk density (p < 0.05). Palm and fern abundances were not correlated; however, tree fern abundances showed an opposing abundance relationship with total soil N:P and R:FR compared to herbaceous ferns and palms. Distance–decay rates in compositional similarity were slightly higher for palms than ferns.ConclusionsFerns and palms show similar patterns of compositional variation in response to resource availability but were related to distinct environmental factors. Soil fertility and light availability emerged as key factors in predicting the abundance of plant groups, with clear partitioning of environmental gradients only apparent when tree ferns were compared with other groups.

Crecimiento diamétrico de arboles caducifolios y perennifolios del bosque mesófilo de montaña en los alrededores de Xalapa

El crecimiento diamétrico anual de los árboles es una variable esencial para eligir las especies que deben usarse para reforestación, en plantaciones, en proyectos de restauración ecológica, y aún para plantarse en jardines y calles. El objetivo del presente trabajo fue determinar las tasas de crecimiento en diámetro de algunas especies arbóreas caducifolias y perennifolias, dominantes en el bosque mesófilo de montaña de los alrededores de Xalapa, Veracruz. El crecimiento diamétrico anual de 29 especies arbóreas se midió durante cinco años en el bosque del Jardín Botánico Clavijero (112 individuos) y en el Parque Ecológico Clavijero (80 individuos), y durante un año en un bosque al norte de Banderilla (36 individuos). La tasa de crecimiento diamétrico de los árboles del bosque manejado en el Jardín Botánico (0.83 cm/año) fue significativamente mayor que la de los otros dos bosques naturales (0.29 cm/año), lo cual puede indicar el potencial de crecimiento de muchas especies. Además, la tasa de crecimiento diamétrico para el bosque natural es rápida en comparación con la estimada para otros bosques de montaña. Las especies con tasa de crecimiento más alta fueron Platanus mexicana, Quercus acutifolia, Cornus disciflora y Liquidambar styraciflua var. mexicana en el Jardín Botánico, y L. styraciflua var. mexicana, Q. xalapensis, Clethra mexicana y Q. acutifolia en el bosque natural. La tasa de crecimiento diamétrico para las especies caducifolias fue 0.94 cm/año en el Jardín Botánico y de 0.36 cm/año en el bosque natural, y para las especies perennifolias fue 0.60 y 0.15 cm/año, respectivamente.

Trait-based community assembly of understory palms along a soil nutrient gradient in a lower montane tropical forest

Two opposing niche processes have been shown to shape the relationship between ecological traits and species distribution patterns: habitat filtering and competitive exclusion. Habitat filtering is expected to select for similar traits among coexisting species that share similar habitat conditions, whereas competitive exclusion is expected to limit the ecological similarity of coexisting species leading to trait differentiation. Here, we explore how functional traits vary among 19 understory palm species that differ in their distribution across a gradient of soil resource availability in lower montane forest in western Panama. We found evidence that habitat filtering influences species distribution patterns and shifts community-wide and intraspecific trait values. Differences in trait values among sites were more strongly related to soil nutrient availability than to variation in light or rainfall. Soil nutrient availability explained a significant amount of variation in site mean trait values for 4 of 15 functional traits. Site mean values of leaf nitrogen and phosphorus increased 37 and 64%, respectively, leaf carbon:nitrogen decreased 38%, and specific leaf area increased 29% with increasing soil nutrient availability. For Geonoma cuneata, the only species occurring at all sites, leaf phosphorus increased 34% and nitrogen:phosphorus decreased 42% with increasing soil nutrients. In addition to among-site variation, most morphological and leaf nutrient traits differed among coexisting species within sites, suggesting these traits may be important for niche differentiation. Hence, a combination of habitat filtering due to turnover in species composition and intraspecific variation along a soil nutrient gradient and site-specific niche differentiation among co-occurring species influences understory palm community structure in this lower montane forest.

Plant–soil associations in a lower montane tropical forest: physiological acclimation and herbivore‐mediated responses to nitrogen addition

Summary 1. Soil nutrients influence plant productivity and community composition in tropical forests. In lower montane tropical forests in western Panama, the distribution of understory palm species over a scale of 1–20 km correlates with differences in soil nitrogen (N). We hypothesized that soil N determines seedling performance in the forest understory, and, may therefore influence species distributions along the soil N gradient. 2. We explored the potential for N availability to generate species‐habitat associations through species‐specific differences in biomass allocation, photosynthetic capacity, N use‐efficiency, and susceptibility to herbivory. Seedlings of nine palm species from two sub‐families and four habitat types were transplanted into N‐addition and control plots at a low N site. Growth, mortality, biomass allocation, photosynthesis, foliar N content and herbivory were measured over 21 months. 3. Foliar N increased for all species (15–68%) following N addition. Most species showed strong (20–200%) increases in photosynthetic rates with N addition except two species with marginal decreases in photosynthetic rates (5–15%). However, shifts in physiological traits did not increase relative growth rate or change in biomass allocation for any species or N treatment combination. Rather, increased leaf quality contributed to greater levels of herbivory in species associated with soils of intermediate and high inorganic N availability. 4. Thus, potential increases in overall growth with N addition were masked by herbivory, resulting in no apparent growth response with increased N. We suggest that for understory palms, and potentially other montane forest plants, distribution patterns are driven by a combination of physiological and herbivore‐mediated responses to soil nutrient availability.

Radial stem variations of Tabebuia chrysantha (Bignoniaceae) in different tropical forest ecosystems of southern Ecuador

Stem diameter increments of the broadleaved deciduous tree species Tabebuia chrysantha were measured with high-resolution dendrometers in a tropical lower montane forest and in a dry forest in southern Ecuador, the latter showing a distinct dry season. Those analyses were complemented by wood anatomical studies on regularly collected microcores to determine the season of active cambial growth and the time of formation of annual growth boundaries. The length of the cambial active period varied between 3 and 7 months at the tropical lower montane forest and 2 and 4 months in the dry forest, respectively. During dry days, amplitudes of daily stem diameter variations correlated with vapour pressure deficit. During October and November, inter-annual climate variations may lead to dry and sunny conditions in the tropical lower montane forest, causing water deficit and stem diameter shrinkage in T. chrysantha. The results of the climate–growth analysis show a positive relationship between tree growth and rainfall as well as vapour pressure deficit in certain periods of the year, indicating that rainfall plays a major role for tree growth.

Cover crops alter phosphorus soil fractions and organic matter accumulation in a Peruvian cacao agroforestry system

In many tropical soils, excessive weath- ering of primary minerals confounded by intense agricultural production has resulted in the depletion of organic matter and plant available forms of phosphorus (P). Long-term growth of cover crops in tropical agroforestry systems have been shown to influence nutrient cycling, and soil organic matter pools. The objective of this experiment was to assess the affect of 2 years of cover-crop cultivation on organic matter accumulation and P bioavailability using Mehlich I and sequential fractionation methods. The experiment included six treatments in the understory of a cacao-plantain agroforestry system adjacent to lower montane tropical forests of the San Martin district of Eastern Peru. Cacao and plantain formed the primary canopy on otherwise abandoned agricultural land. The treatments consisted of four perennial leguminous cover crops (Arachis pintoi, Calopogonium mucunoides, Canavalia ensiformis, and Centrosema macrocarpum), a non-legume cover crop (Callisia repens), and a control treatment (no cover crop). After only 2 years of cultivation, results suggest that all cover crop species accessed residual P pools in 0-5 cm soil depths as indicated by a decrease in the 0.5 M HCl extractable P pools when compared to control. Additional use of residual P pools by A. pintoi and C. macrocarpum were indicated by significant reduction in the 6.0 M HCl extractable P pool. Relative to control, there was no treatment effect on soil organic matter content; however significant differences occurred between treatments. The C. ensiformis, C. mucunoides and C. repens treatments in 5-15 cm soil depths contained significantly more organic matter than the A. pintoi treatment. In 15-30 cm soil depths the C. ensiformis treatment contains significantly more organic matter than the A. pintoi treatment. Continued research should focus on monitoring the long-term effects of cover crop cultivation on the bioavailability of soil P pools in surface soil horizons, development of organic matter pools and the productivity of the agroforestry species.

Soil‐based habitat partitioning in understorey palms in lower montane tropical forests

AbstractAim Dispersal assembly and niche assembly are two competing theories proposed to explain the maintenance of species diversity in tropical forests. Dispersal theory emphasizes the role of chance colonization events and distance‐limited seed dispersal in explaining species abundance and distribution, whereas niche theory emphasizes differences among species in requirements for potentially limiting resources. Species distribution patterns in tropical forests often correlate with geology and topography, but tests of the relative importance of dispersal and niche partitioning have been hampered by an inadequate characterization of resource availability. The aim of this study was to explore how soil chemical and physical properties, climate, and geographic distance affect understorey palm communities in lower montane forests.Location Fortuna Forest Reserve, Chiriqui Province, and Palo Seco Forest Reserve, Bocas del Toro Province, in western Panama.Methods Understorey palms and soil nutrient concentrations were surveyed within 10 sites on different soil types across a 13‐km transect. Variation in palm community composition was examined in relation to spatial and environmental variables.Results The 25 understorey palm species recorded in the study were non‐randomly distributed among forests differing in soil nutrient availability. In support of dispersal theory, floristic similarity decreased predictably with increasing geographic distance. However, environmental and soil variables were also correlated with geographic distance. Floristic similarity was also highly associated with a subset of environmental variables. Variation in palm community similarity was most strongly correlated with inorganic nitrogen availability and cation concentration. A subset of soil variables had a stronger relationship with floristic similarity when geographic distance was controlled for than did geographic distance when differences in soils were controlled for.Main conclusions Both dispersal and niche processes affect palm species distribution patterns. Although spatially limited dispersal may influence species distribution patterns, soil‐based habitat associations, particularly with respect to soil nitrogen, cation availability and aluminium concentrations, remain important factors influencing palm community composition at the mesoscale level in this tropical montane forest.

Habitat divergence in sympatric Fagaceae tree species of a tropical montane forest in northern Thailand

Spatial distributions of many tropical trees are skewed to specific habitats, i.e. habitat specialization. However, habitats of specialist species must be divergent, i.e. habitat divergence, to coexist in a local community. When a pair of species specialize in the same habitat, i.e. habitat convergence, they could not coexist by way of habitat specialization. Thus, analyses of habitat divergence, in addition to habitat specialization, are necessary to discuss coexistence mechanisms of sympatric species. In this study, the habitat specialization and habitat divergence along topographic gradients of eight sympatric tree species of the Fagaceae were studied in a 15-ha study plot in a tropical lower montane forest in northern Thailand. A statistical test with torus shift randomizations for 9673 trees of Fagaceae revealed significantly biased distributions for all of the species, for at least one of the four topographic variables used: elevation, slope inclination, aspect and convexity. Slope convexity was the most critical topographic variable, along which all but one species had significantly skewed distributions. Out of 112 possible combinations of species pairs and topographic variables, 18 (16%) and two pairs (1.8%) showed significant habitat divergence and habitat convergence, respectively. The observed habitat divergence alone could not completely explain the coexistence of the eight species. There was a gradation in the habitat position of each species, with relatively large overlaps between species distributed in similar habitats, and small overlaps between species associated with contrasting habitats, respectively. The gradual changes in the habitats of the species suggested that dividing the species into a small number of distinct habitat groups, such as ridge and valley specialists, would not be straightforward.

Hydraulic characteristics and leaf water use efficiency in trees from tropical montane habitats

This study was carried out in pioneer and successional forest tree species in a lower montane tropical forest with seasonal rains. We tested whether pioneer species feature high hydraulic conductance allowing them to use water profusely at leaf level. Conversely, forest species may have relatively low hydraulic conductance accompanied with better control over water use. This may lead in turn to pioneer species being at a relatively higher risk of shoot water potential falling below the threshold value at which cavitations occur compared to forest. Specific hydraulic conductance (K s) measured during the wet season was comparable between pioneers and forest species. During drought, K s was significantly reduced, and species of both plant groups responded to this by modifying the relationship between conducting area and leaf area (Huver value), such that leaf specific conductivity (K l) was unaffected. Thus, leaf area seemed to be adjusted to maintain constant hydraulic sufficiency during drought. Pioneer species were more efficient in conducting water to their leaves but had low control over water use compared to forest species. A trade-off between water transport and leaf water use efficiency was suggested. These ecophysiological differences may have an impact on the performance of the species occupying contrasting habitats. Nonetheless, drought-induced embolisms occurred in trees growing in both open and forest habitats. Overall, during drought, adjustment of leaf area occurred in order to maintain a homeostasis of some physiological traits (leaf-specific conductivity and carbon assimilation).

Habitat differentiation of Lauraceae species in a tropical lower montane forest in northern Thailand

Dependency on topographical habitat was examined for Lauraceae tree species in a lower montane forest using a large‐scale research plot established at Doi Inthanon National Park, northern Thailand. Twenty species of 10 genera of Lauraceae were recorded in a 7.5‐ha part of the plot; Lauraceae accounted for 18% of the total basal area. Lauraceae was the most species‐rich and most abundant family in the plot. In a cluster analysis based on the matrix of spatial associations between species, two clusters were recognized. Members of one cluster seemed to associate with lower‐elevation habitats, and members of the other associated with habitats on ridges. By subdividing the study plot into 20 m × 20 m squares, a discriminant analysis could be applied to the presence–absence data for the 17 species that had sufficient population density. The predictor variables used were the relative elevation, slope inclination, slope direction (transformed to deviation from SSW) and slope convexity for each of the squares. The discriminant models were tested statistically by applying the random shift technique. The models were significant for 11 of the species (65% of the species examined) and were associated with the topographical condition of the habitat. Stepwise selection of the predictor variables for these 11 species revealed that relative elevation and slope convexity were the most important factors for predicting the presence or absence of the Lauraceae species. Both these variables were considered to indicate the hydrological condition of the habitat.

Latitudinal pattern of mountain vegetation zonation in southern and eastern Asia

AbstractA new scheme of altitudinal and latitudinal vegetation zonation is proposed for eastern Asia. The latitudinal patterns of mountain vegetation zonation show a clear boundary at ca. 20°–30° N. For the tropical mountains south of 20° N, the altitudinal series includes tropical lowland, tropical lower montane, and tropical upper montane zones. For the temperate mountains north of 30° N, the series includes temperate lowland, temperate lower montane, and temperate upper montane zones. The mountains located between 20° and 30° N show a transitional zonation pattern; the lower two zones are comparable to the lower two of the tropical zonation (tropical lowland and tropical lower montane), and the upper two zones are comparable to those of the temperate zonation (temperate lower montane and temperate upper montane). The tropical upper montane zone is not found north of 20°–30° N, while the tropical lower montane zone reaches down to sea level and constitutes the temperate lowland zone. Thus the zonation between 20° and 30° N includes tropical lowland, tropical lower montane/temperate lowland, temperate lower montane, and temperate upper montane zones.The latitudinal series of lowland rain forests follows the scheme of climatic division into tropical, subtropical/warm‐temperate, cool‐temperate and cold‐temperate, with a shift of the respective life forms, evergreen, evergreen notophyllous, deciduous, and evergreen needle‐leaved. The tropical lower montane forest can be correlated to the horizontal subtropical/ warm‐temperate zone. The temperate altitudinal and latitudinal zonations above 30° N are correlated and show an inclined parallel pattern from high altitudes in the south to low altitudes down to sea level in the north.