Browsing by Subject "Tree growth"

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Radial tree growth dynamics and xylem anatomy along an elevational gradient in the El Sira Mountains, Peru
(2019) Niessner, Armin; Küppers, Manfred
The explicit purpose of this study was to (1) characterize climate and vegetation along the western slope of the Cerros del Sira (Peru), (2) evaluate radial tree growth along this slope in response to seasonal rainfall anomalies, (3) reveal how the meteorological environment controls tree radial stem growth, and (4) to investigate how xylem anatomy relates to dynamics of tree growth. From May 2011 until September 2015, radial stem growth of 67 trees was monitored using point-dendrometers, and meteorological parameters were measured within five sites along an elevational gradient. Additionally, tree dimensions (stem diameter, stem height) and xylem anatomical traits (mean vessel diameter, vessel frequency, cumulative vessel area and potential hydraulic conductivity) were determined. The transect extends from lowland terra firme forests (ca. 250 m asl) over submontane forests, late and mid successional montane cloud forests up to exposed elfin forests (ca. 2200 m asl). Continuous rainfall records for remote tropical areas are extremely rare and measurements along this transect are also incomplete. Monthly rainfall estimates by the TRMM PR satellite ("product 3B42") were highly correlated with rain gauge observations, although they underestimate rainfall at high elevations. Different intra-annual tree growth patterns could be identified within each elevational forest type, showing species/individuals with seasonally independent continuous or alternating growth patterns and strictly seasonal growth. Stem growth at each site was typically higher during rainy seasons, except for in the elfin forest. The rainy season from October 2013 to March 2014 was extraordinarily dry, with only 73 % of long-term mean precipitation received, which resulted in reduced tree radial growth rates, again with the exception of the elfin forest. Different analytical approaches revealed that precipitation is the main growth-controlling factor at lower elevations, especially during rainy seasons. Growth within montane and cloud forests positively correlates to solar radiation. Tree growth within the elfin forest is only weakly correlated to meteorological parameters, but dry conditions during dry seasons promote growth. It was hypothesized that (1) individuals/species with large vessel diameters and low vessel frequencies have higher radial growth rates, but are more vulnerable to relatively dry periods. Therefore, (2) they are more likely to grow seasonally and predominantly during rainy seasons, (3) their growth during the exceptionally dry rainy season 2013/14 was more constrained, and (4) their growth is generally closer correlated to meteorological parameters. Larger trees tend to have larger vessel diameters, which positively relates to radial growth rates, and they also tend to grow more seasonal. As hypothesized, trees having large vessel diameters are more sensitive to droughts, as indicated by stronger positive correlations with rainfall and negative with solar radiation. However, in mountainous forests, relations between xylem anatomical traits and tree growth dynamics seem to be more complex. In late-successional cloud forests, growth of trees with large vessel diameter is positively, while of trees with small vessel diameter negatively related to solar radiation. Sensitivity to the dry rainy season 2013/14, expressed as relative reduction in growth compared to "normal" rainy seasons, could not be explained by xylem anatomical traits, contradicting the preceded hypothesis. Tropical lowland rainforest species, especially individuals with large vessels, will likely suffer from increasing drought frequencies and intensities. How montane forest ecosystems will react to a (globally) changing climate is rather uncertain, especially in exposed elfin forests. Results of this study suggest that species of late-successional tropical montane forests may profit from higher temperatures. While montane tropical rain forests may also suffer from prolonged droughts, exposed ridges covered by elfin forests still receive plenty of precipitation and may benefit from receiving more solar radiation for photosynthesis and, thus, grow faster.
Tree growth and tree regeneration in two East African rain forests as related to the abiotic environment after human disturbance
(2010) Gliniars, Robert; Küppers, Manfred
This study deals with the stem growth and seedling regeneration of different native tree species in two East African rainforests influenced by human disturbance in Kenya (Kakamega Forest) and Uganda (Budongo Forest), also considering spatially and temporally variable environmental influences. In the lower montane rainforest (1500 to 1700 m a.s.l.) Kakamega Forest (KF) surveys were conducted on trees ≥ 5 cm diameter at breast height (DBH) on an overall area of 2.08 ha (1 ha plot and 27 plots x 400 m2, 2007 individuals, equalling 965 ha-1), 3 inventories in 7 years). At the same time 91 tree species from 39 families were identified. In addition, in subplots within the permanent plots, 8441 saplings and seedlings ≤ 5m DBH distributed among 59 tree species from 29 families were recorded (on 432 m2, over 6 years). In the lowland rainforest (1000 to 1100 m a.s.l.) Budongo Forest (BF) a total of 1010 trees (1403 ha-1), distributed among 91 tree species from 30 families, were recorded in surveys on an overall area of 0.72 ha (18 plots à 400 m2, 2 inventories in 5 years). During the observation period saplings and seedlings (on 72 m2, 708 individuals, in 3 years) belonging to 38 tree species from 18 families were recorded. The study areas were situated in forest areas characterised by different successional stages. These stages have emerged as a result of differing degrees of human disturbance for commercial purposes in the recent past. As this research was part of a joint project, highly resolved data on factors such as soil values, climate values, canopy characteristics and disturbance indices e.g. the Commercial Disturbance Index (CDI) was also available for a few permanent plots. It was established that the level of species richness and species diversity is low in areas with either no or very high levels of past commercial human disturbance (e.g. after clear-cutting), whereas areas with intermediate disturbance were seen to be richer or more diverse in species. The distribution of β-diversity also indicated a disturbance gradient. The degree of human disturbance was as well reflected in the successional stages within the structure of the forest areas. The mean stem volume of trees in BF increased as the disturbance index decreased, in KF however the highest stem volume occurred in areas with intermediate CDI. The tree populations in areas with a low level of disturbance in BF were characterised by high volumes together with lower stem numbers per plot and were more stable, as manifested in the lower turnover. The openness of the canopy varied in the differently disturbed areas and had a positive effect on seedling density in BF, which in turn underlines the importance of light for the regeneration of seedlings. In order to test the regeneration capacity of individual tree species for the most part in the intermediate stages of succession in both forests, the stem increment of 1198 trees (≥ 10 cm DBH) was measured monthly using dendrometer bands. At the same time the phenology of leaves, flowers and fruits of these trees was monitored over a period of three (in BF) and five years (in KF). In addition regeneration and establishment dynamics were analysed by counting, identifying and measuring seedlings (≤ 100 cm high) in the subplots every three months. The tree species could then be classified and allotted to their position along the pioneer-climax continuum. The early successional pioneer species were characterised by rapid growth and higher mortality rates, had no to few seedlings in the understorey and were shade intolerant. The late successional climax species however grew more slowly, had a low mortality rate, were well-established as seedlings in the understorey and shade tolerant. Most species possessed either characteristics somewhere in between these extremes or both characteristics of early and late successional species and were thus classified as intermediate. The annual stem increment varied considerably between tree species ranging from 0.6 to 2.5 mm in BF and from 0.6 to 4.4 mm in KF, and the relative diameter growth ranged from 0.3 to 1% in BF and from 0.25 to 1.58% in KF. The variability in the growth rates within tree species was even higher than that between species. Moreover the annual variability in growth within a species was higher in thinner than in thicker individuals. Individual trees however grew at a relatively constant rate. Beside the inherent growth potential, monthly changes in the climatic conditions also had an influence on the growth pattern of many of the tree species in KF and on all of the species in the more strongly seasonal BF. During the warm dry season the growth rate decreased and then increased again when the rainy season set in. This was confirmed by positive correlations between monthly growth rates and the sum of rainfall over two or three months respectively and also by strongly negative correlations between monthly growth and maximum temperature. The intensity of these relations varied according to species. The phenological features of most species were also dependent on the climate, e.g. leaf shedding or flowering in the dry season and fruiting in the rainy season. Seedling dynamics showed a tendency towards an increase in the populations in the rainy season and a decrease in the dry season depending on the species. The distribution of the tree parameters was not as clearly related to the spatial distribution of soil parameters. There were a few correlations between tree diversity and soil parameters in KF, positive correlations with acid-extracted Ca, C and N and negative with electrical conductivity. Furthermore significant correlations were established in KF between tree growth and Ca, C and N, as well as negative ones with Mn and Mg in the soil, whereby the intensity of the relationship was species specific. Seedling diversity also correlated positively with Ca, C and N, as well as negatively with Mn. The distribution and density of the seedlings was however more highly dependent on recurring pulses of newly recruited individuals at irregular intervals than on the abiotic environment. The study provides a first, precise insight into ecological regeneration processes of tree species populations and forest communities after human disturbance that have for the most part not yet been the subject of investigation, and can serve as a basis for forest management and forest models.