Tree growth and tree regeneration in two East African rain forests as related to the abiotic environment after human disturbance

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.Thema dieser Studie ist das Stammwachstum und die Regeneration verschiedener einheimischer Baumarten auch in Bezugnahme auf räumlich und zeitlich variable Umwelteinflüssse in zwei durch menschliche Eingriffe beeinflusste ostafrikanische Regenwälder in Kenia (Kakamega Forest) und Uganda (Budongo Forest). Im unteren-montanen Regenwald (1500 bis 1700 mNN) Kakamega Forest (KF) wurden daher Bestandsaufnahmen von Bäumen ≥ 5 cm Brusthöhendurchmesser (BHD) auf insgesamt 2,08 ha (1 ha Fläche und 27 Flächen à 400 m2, 2007 Individuen, was 965 ha-1 entspricht, 3 Bestandsaufnahmen in 7 Jahren) durchgeführt. Dabei wurden 91 Baumarten aus 39 Pflanzenfamilien identifiziert. In Subplots innerhalb der Dauerflächen wurden außerdem 8441 kleine Bäume und Keimlinge ≤ 5 cm BHD (auf 432 m2, in 6 Jahren) aufgenommen, die sich auf 59 Baumarten aus 29 Pflanzenfamilien verteilten. Im Tieflandregenwald (1000 bis 1100 mNN) Budongo Forest (BF) wurden auf insgesamt 0,72 ha Bestandsaufnahmen von (18 Flächen à 400 m2, 2 Inventuren in 5 Jahren) 1010 Individuen, was 1403 ha-1 entspricht, durchgeführt, die sich auf 91 Arten aus 30 Pflanzenfamilien verteilten. Während des Observationszeitraums wurden in Subplots kleine Bäume und Keimlinge (auf 72 m2, 708 Individuen, in 3 Jahren) zu 38 Baumarten aus 18 Pflanzenfamilien zugehörig, erfasst. Die Untersuchungsflächen lagen in Waldgebieten, die sich in verschiedenen Sukzessionsstadien befanden. Diese Stadien haben ihre Ursache in unterschiedlich starken Störungseinflüssen in Form von menschlichen Eingriffen mit kommerziellem Interesse in der jüngeren Vergangenheit. Da diese Arbeit im Kontext eines Projektverbundes steht, standen fü einige der Dauerflächen kleinräumig erhobene Faktoren wie Bodenwerte, Klimawerte, Kronencharakteristik und Störungsindizes, z.B. der Commercial Disturbance Index (CDI), zur Verfügung. Es wurde festgestellt, dass Artenreichtum und -diversität niedrig in Gebieten mit keiner und sehr hoher vorangegangener kommerzieller menschlicher Störung (z.B. Kahlschlag) waren, während Gebiete mit mittlerer Störung als artenreicher bzw. diverser identifiziert wurden. Auch die Verteilung der β-Diversität spiegelte einen Störungsgradienten wieder. Die Nachwirkungen, in den durch menschliche Störung entstandenen Sukzessionsstadien sind auch in der Struktur der Waldgebiete ersichtlich. Das mittlere Stammvolumen der Bäume in BF nahm mit abnehmendem Störungsindex zu, wohingegen in KF das höchste Stammvolumen in Gebieten mit mittlerem CDI auftrat. Die Baumpopulationen in Gebieten mit geringer Störung in BF zeichneten sich durch hohe Volumen bei geringerer Stammzahl pro Fläche und waren stabiler, was auch durch ihren geringen Turnover gezeigt wurde. Auch die Öffnung des Kronendachs war verschieden in den unterschiedlichen stark gestörten Gebieten und hatte einen positiven Einfluss auf die Keimlingsdichte in BF, welche wiederum die Bedeutung des Lichts für die Regeneration der Keimlinge aufzeigte. Um die Regenerationsfähigkeit von einzelnen Baumarten zu testen, wurde hauptsächlich in den intermediären Sukzessionsstadien der beiden Wälder an 1198 Bäumen (≥ 10 cm BHD) monatlich der Stammzuwachs mit Dendrometerbändern gemessen. Zusätzlich wurde die Phänologie der Blätter, Blüten und Früchte der Bäume über einen Zeitraum von drei (in BF) bzw. fünf Jahren (in KF) erhoben. Außerdem wurde die Regenerations- und Etablierungsdynamik analysiert, indem Keimlinge (≤ 100 cm Höhe) in den Subplots vierteljahrlich gezählt,identifiziert und deren Höhe gemessen wurden. Die Baumarten konnten je nach gefundenen Charakteristiken in das Pionier-Klimax Kontinuum eingeordnet werden. Die früh-sukzessionellen Pionierarten wuchsen eher schnell, besaßen hohe Mortalität, hatten keine bis wenige Keimlinge im Unterwuchs und waren schattenintolerant. Die spät-sukzessionellen Klimaxarten hingegen zeigten langsameres Wachstum, wenig Mortalität, waren als Keimlinge im Unterwuchs gut etabliert und schattentolerant. Zwischen diesen Extremen lagen die meisten Arten, die entweder dazwischenliegende Charakteristiken oder sowohl Charakteristiken von früh- als auch von spät-sukzessionellen Arten aufwiesen und daher als intermediär klassifiziert wurden. Der jährliche Stammzuwachs zwischen den Baumarten war sehr variabel von 0,6 bis 2,5 mm in BF und von 0,6 bis 4,4 mm in KF und bei relativem Durchmesserwachstum zwischen 0,3 und 1 % in BF und zwischen 0,25 und 1,58 % in KF. Die Variabilität der Wachstumsraten innerhalb der Baumarten war noch größer als zwischen den Baumarten. Außerdem war die jährliche Variabilität des Wachstums innerhalb einer Art bei dünneren höher als bei dickeren Individuen. Einzelne Baumindividuen dagegen wuchsen relativ konstant. Monatliche klimatischen Änderungen hatten Einfluss auf das monatliche Wachstum einiger Baumarten in KF und aller Arten im stärker saisonalen BF. In der warmen Trockenzeit war das Wachstum reduziert und nahm mit einkehrender Regenzeit wieder zu. Positive Korrelationen zwischen monatlichem Wachstum und dem über 2 bzw. 3 Monate aufsummierten Niederschlag bzw. stark negative Korrelationen mit der Maximaltemperatur bestätigten dies. Die Stärke dieser Beziehungen war artspezifisch verschieden. Auch die phänologischen Merkmale der meisten Arten waren abhängig vom Klima, wie z.B. Blattabwurf und Blühen in der Trockenzeit und Fruchtbildung in der Regenzeit. Die Dynamik der Keimlinge zeigte tendenziell eine Zunahme der Populationen in der Regenzeit und eine Abnahme in der Trockenzeit, je nach Art verschieden. Die Verteilung der Baumparameter zeigte weniger klare Zusammenhänge in Relation zu der räumlichen Verteilung der Bodenparameter. Es gab einige wenige Korrelationen zwischen Baumdiversität und Bodenparametern in KF, positive Korrelationen mit säureextrahiertem Ca, C und N und negative zur elektrischer Leitfähigkeit. Weiterhin wurden signifikante Korrelationen in KF zwischen Baumwachstum und Ca, C und N, sowie negative zu Mn und Mg im Boden festgestellt, wobei die Intensität dieser Beziehung artspezifisch war. Die Diversität der Keimlinge korrelierte ebenfalls positiv mit Ca, C und N, sowie negativ mit Mn. Die Verteilung und Dichte der Keimlinge hingegen wurde eher von unregelmäßig wiederkehrenden Schüben von neu gekeimten Individuen bestimmt als durch abiotische Umweltfaktoren. Die Studie liefert einen Einblick in die ökologischen Regenerationsvorgänge der Baumgesellschaften von Baumarten und Waldgesellschaften nach menschlichen Eingriffen und kann als Grundlage für Forstwirtschaft und Waldmodelle dienen

Pan-Tropical Analysis of Climate Effects on Seasonal Tree Growth

Climate models predict a range of changes in tropical forest regions, including increased average temperatures, decreased total precipitation, reduced soil moisture and alterations in seasonal climate variations. These changes are directly related to the increase in anthropogenic greenhouse gas concentrations, primarily CO2. Assessing seasonal forest growth responses to climate is of utmost importance because woody tissues, produced by photosynthesis from atmospheric CO2, water and light, constitute the main component of carbon sequestration in the forest ecosystem. In this paper, we combine intra-annual tree growth measurements from published tree growth data and the corresponding monthly climate data for 25 pan-tropical forest sites. This meta-analysis is designed to find the shared climate drivers of tree growth and their relative importance across pan-tropical forests in order to improve carbon uptake models in a global change context. Tree growth reveals significant intra-annual seasonality at seasonally dry sites or in wet tropical forests. Of the overall variation in tree growth, 28.7% was explained by the site effect, i.e. the tree growth average per site. The best predictive model included four climate variables: precipitation, solar radiation (estimated with extrasolar radiation reaching the atmosphere), temperature amplitude and relative soil water content. This model explained more than 50% of the tree growth variations across tropical forests. Precipitation and solar radiation are the main seasonal drivers of tree growth, causing 19.8% and 16.3% of the tree growth variations. Both have a significant positive association with tree growth. These findings suggest that forest productivity due to tropical tree growth will be reduced in the future if climate extremes, such as droughts, become more frequent

Climate seasonality limits leaf carbon assimilation and wood productivity in tropical forests

The seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is  < 2000 mm yr⁻¹ (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall  < 2000 mm yr⁻¹

Pan-Tropical Analysis of Climate Effects on Seasonal Tree Growth

Climate models predict a range of changes in tropical forest regions, including increased average temperatures, decreased total precipitation, reduced soil moisture and alterations in seasonal climate variations. These changes are directly related to the increase in anthropogenic greenhouse gas concentrations, primarily CO2. Assessing seasonal forest growth responses to climate is of utmost importance because woody tissues, produced by photosynthesis from atmospheric CO2, water and light, constitute the main component of carbon sequestration in the forest ecosystem. In this paper, we combine intra-annual tree growth measurements from published tree growth data and the corresponding monthly climate data for 25 pan-tropical forest sites. This meta-analysis is designed to find the shared climate drivers of tree growth and their relative importance across pan-tropical forests in order to improve carbon uptake models in a global change context. Tree growth reveals significant intra-annual seasonality at seasonally dry sites or in wet tropical forests. Of the overall variation in tree growth, 28.7% was explained by the site effect, i.e. the tree growth average per site. The best predictive model included four climate variables: precipitation, solar radiation (estimated with extrasolar radiation reaching the atmosphere), temperature amplitude and relative soil water content. This model explained more than 50% of the tree growth variations across tropical forests. Precipitation and solar radiation are the main seasonal drivers of tree growth, causing 19.8% and 16.3% of the tree growth variations. Both have a significant positive association with tree growth. These findings suggest that forest productivity due to tropical tree growth will be reduced in the future if climate extremes, such as droughts, become more frequent.status: publishe

Model parameters, standard errors, t values and posterior densities of the univariate () and complete (<i>m_BIC</i>) analyses.

<p>Model parameters, standard errors and t-values. The parameter values of posterior parameters densities (MCMC median and MCMC mean) and their 95% confidence intervals (Highest posterior density at 95% [HPD95] lower and upper) and associated p-value are estimated from a distribution of 10000 parameter samples computed with Monte Carlo Markov Chains from the model parameters. Goodness-of-fit criterion of the growth models are root mean square error of prediction (RMSEP), Bayesian Information Criterion (BIC), R² of the fixed part of the model () and R² of the complete model (fixed plus random part).</p

Climate datasets used to model seasonal tropical tree growth at a pantropical scale.

a<p>: <i>sol</i> per day (mm/day equivalent) is calculated using the methodology presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092337#pone.0092337-Allen2" target="_blank">[39]</a> specifically for the 15<i>^th</i> day of each month to describe averages per month. Total <i>sol</i> per month (mm/month equivalent) is calculated by multiplying the value of sol for the 15th day of the month by the number of days in the month, 1 mm.day⁻¹ equivalent of evaporation  =  2.45 MJ.m⁻².day⁻¹.</p

Locations of the 25 study sites and their countries (grey areas).

<p>1: Attapadi; 2: Budongo; 3: CPM; 4: El Palmar; 5: FLONA SFP; 6: Guanacaste; 7: Ibicatu; 8: Kakamega; 9: La Barcinera; 10: La Selva; 11: Lamto; 12: Luki forest; 13: Marajoara; 14: Muara Bungo; 15: Munessa-Shashamene Forest; 16: Paracou; 17: Pinkwae; 18: RBSF; 19: RFC; 20: Rio Cachoiera; 21: Selangor plantation; 22: SERS; 23: Tapajos; 24: Tinte Bepo; 25: ZF-2.</p

Predicted diameter at breast height (dbh) growth under the model <i>m_BIC</i> and relationship with precipitation and extra solar radiation.

<p>Predictions were computed using <i>pre</i>, <i>sol</i>, mean <i>dtr</i>, mean <i>swc</i> and mean of the random effect. The extra solar radiation unit is equivalent of evaporation in mm.month⁻¹, 1 mm.month⁻¹ equivalent of evaporation  =  2.45 MJ.m⁻².month⁻¹.</p

Observed versus predicted diameter at breast height (dbh) growth under the model m<i>_BIC</i>.

<p>The red line is the identity line y  =  x. Note that the model underestimated the diameter growth above 1 mm.month⁻¹.</p

Expected tropical tree growth response to climate variables.

a<p>: expected growth response to the climate variable: (+) trees are expected to grow faster with high values of the climate variable, (−) trees are expected to grow slower with high values of the climate variable. <i>^b</i>: biological processes involve in the tree growth response to a given climate variation. <i>VPD</i> is vapour pressure deficit, and Friction velocity (<i>U*</i>) is a climate variable provided by eddy flux data, which is correlated with wind speed. Relative extractable water (REW), is a daily value between 0 and 1; when , the amount of extractable water by the tree is at its maximum and when , no water is available for the trees <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0092337#pone.0092337-Wagner1" target="_blank">[28]</a>.</p