Nutrient-Induced Modifications of Wood Anatomical Traits of Alchornea lojaensis (Euphorbiaceae)

Regarding woody plant responses on higher atmospheric inputs of the macronutrients nitrogen (N) and phosphorous (P) on tropical forests in the future, an adaptive modification of wood anatomical traits on the cellular level of woody plants is expected. As part of an interdisciplinary nutrient manipulation experiment (NUMEX) carried out in Southern Ecuador, we present here the first descriptive and quantitative wood anatomical analysis of the tropical evergreen tree species Alchornea lojaensis (Euphorbiaceae). We sampled branch wood of nine individual trees belonging to treatments with N fertilization, N+P fertilization, and a control group, respectively. Quantitative evaluations of eleven different vessel parameters were conducted. The results showed that this endemic tree species will be able to adapt well to the future effects of climate change and higher nutrient deposition. This was firstly implied by an increase in vessel diameter and consequently a higher theo. area-specific hydraulic conductivity with higher nutrient availability. Secondly, the percentage of small vessels (0–20 μm diameter) strongly increased with fertilization. Thirdly, the vessel arrangement (solitary vessels vs. multiple vessel groupings) changed toward a lower percentage of solitary vessel fraction (VS), and concurrently toward a higher total vessel grouping index (VG) and a higher mean group size of non-solitary vessels (VM) after N and N+P addition. We conclude that higher nutrient availability of N and N+P triggered higher foliage amount and water demand, leading to higher cavitation risk in larger vessels. This is counteracted by a stronger grouping of vessels with smaller risk of cavitation to ensure water supply during drier periods that are expected to occur in higher frequency in the near future

Tree Circumference Changes and Species-Specific Growth Recovery After Extreme Dry Events in a Montane Rainforest in Southern Ecuador

Under drought conditions, even tropical rainforests might turn from carbon sinks to sources, and tree species composition might be altered by increased mortality. We monitored stem diameter variations of 40 tree individuals with stem diameters above 10 cm belonging to eleven different tree genera and three tree life forms with high-resolution dendrometers from July 2007 to November 2010 and additionally March 2015 to December 2017 in a tropical mountain rainforest in South Ecuador, a biodiversity hotspot with more than 300 different tree species belonging to different functional types. Although our study area receives around 2200 mm of annual rainfall, dry spells occur regularly during so-called “Veranillo del Niño” (VdN) periods in October-November. In climate change scenarios, droughts are expected with higher frequency and intensity as today. We selected dry intervals with a minimum of four consecutive days to examine how different tree species respond to drought stress, raising the question if some species are better adapted to a possible higher frequency and increasing duration of dry periods. We analyzed the averaged species-specific stem shrinkage rates and recovery times during and after dry periods. The two deciduous broadleaved species Cedrela montana and Handroanthus chrysanthus showed the biggest stem shrinkage of up to 2 mm after 10 consecutive dry days. A comparison of daily circumference changes over 600 consecutive days revealed different drought responses between the families concerning the percentage of days with stem shrinkage/increment, ranging from 27.5 to 72.5% for Graffenrieda emarginata to 45–55% for Podocarpus oleifolius under same climate conditions. Moreover, we found great difference of recovery times after longer-lasting (i.e., eight to ten days) VdN drought events between the two evergreen broadleaved species Vismia cavanillesiana and Tapirira guianensis. While Vismia replenished to pre-VdN stem circumference after only 5 days, Tapirira needed 52 days on average to restore its circumference. Hence, a higher frequency of droughts might increase inter-species competition and species-specific mortality and might finally alter the species composition of the ecosystem

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

Die Implementierung eines dendroökologischen Multiparameteransatzes zur Bewertung der Auswirkungen von Umweltveränderungen auf das Wachstumsverhalten, die Ökologie sowie die Kohlenstoffisotopenfraktionierung tropischer Baumarten in Südecuador

The anthropogenic impact on global climatic and environmental changes appears obvious. In addition to increasing emissions of greenhouse gases into the atmosphere (global warming), steadily increasing nutrient depositions (N and P) are evident factors causing changes in ecosystems. In particular, the tropical humid and dry forests of South Ecuador are extremely vulnerable to these influences as their unique bio- and structural-diversity with high nutrient limitation are characteristics. The influence of nutrient depositions is highly debated as increasing N and P soil stocks are thought to increase carbon fixation of tropical tree species, which in turn may actually lead to beneficial effects on tree growth. On the contrary, extreme climatic events such as severe droughts influence tree physiology possibly leading to reductions of growth rates. However, available data provide limited information regarding growth rates of different tree species along the ecological gradient – from the evergreen lower tropical montane forest to the seasonally dry tropical forest. The main aim of the present thesis was to conduct tree physiological, wood anatomical, and physical isotope investigations of different tree species inhabiting within these two ecosystems. Further, the influence of environmental change on seasonal growth patterns, wood anatomical traits, and carbon isotope ratios was evaluated to identify risk factors for vulnerability and predictors of adaptation mechanisms. Funding of the current investigations was supplied by the projects PAK 823-825 and FOR 816/2 of the German research foundation (Deutsche Forschungsgemeinschaft, DFG). Study sites comprised both locations, i.e. (i) seasonally dry tropical forest (~600 m a.s.l.) and (ii) an interdisciplinary fertilization experiment (NUMEX) in the evergreen lower montane forest. Starting in 2008, selected forest plots (2000-2150 m a.s.l.) were fertilized biannually applying moderate amounts of limited nutrients (N, P and N+P). The following methods were part of the innovative dendroecological multi-parameter approach: • high-resolution dendrometer measurements to assess short- and long term radial growth dynamics related to extreme climatic events and increased nutrient supply (dry forest and NUMEX), • preparation of microscopic thin sections (branch and stem, light microscopy), descriptive 3Danalyses of wood structures, and quantitative assessment of eleven different wood anatomical parameters to evaluate nutrient-induced modifications of the hydraulic system (NUMEX), • evaluation of high-resolution intra-annually δ13C isotopic values and annual growth rates to assess nutrient-induced modifications in carbon fixation (NUMEX). The present results showed that within both ecosystems radial growth fluctuations of the investigated tree species were controlled and restricted by water availability. In particular, this relationship was established by the tree species Loxopterygium huasango of the dry forest showing an increased vulnerability, which was documented by diminished radial growth during the strong La Niña event 2011 (severe drought). The prognosticated increase of climatic drought events leads to an even greater scarcity of water negatively affecting carbon cycle and carbon sequestration of this tree species. In contrast, within the evergreen lower montane forest the present findings support the hypothesis that a higher nutrient availability may reduce vulnerability towards droughts and favors overall growth rates in most tree species (Alchornea lojaensis, Podocarpus oleifolius und Prunus sp.). However, the influence of nutrient supply showed species-specific characteristics as the tree species with the highest abundance within the study area (Graffenrieda emarginata) did not respond or only responded in a delayed fashion towards nutrient addition. For the investigated individuals of the tree species Alchornea lojaensis diverging adaptation strategies were present in response to enhanced nutrient availability. Wood anatomical investigations at the branch level, in contrast to the stem level data (contradictory effects after nutrient supply), showed a significant increase in vessel diameters and vessels groupings accounting for an improved hydraulic system in response to increased nutrient supply. Due to the application of an innovative multi-method approach, including wood anatomical traits, intraannual δ13C-values, and annual growth rate measurements, it was possible to reproduce the exact timing of fertilization within A. lojaensis. For this tree species significant nutrient-induced alterations of carbon fixation were documented, opposing the starting hypothesis (higher nitrogen availability enrichment in δ13Cplant), as the lighter carbon isotope 12C was significantly preferred by the N-fertilized trees. Whether these significant changes are additionally induced by climatic conditions requires further research. In light of the unexpected findings presented here, this dissertation serves as important contribution to enhance the understanding of tropical forest ecosystems. The present thesis illustrates the complexity of tree physiological processes reacting to changing environmental conditions. Adaptation strategies not only vary clearly between tree species, but more interestingly adaptation mechanisms even vary at tree anatomical levels of the same species. Thus, on the long term, significant changes in species composition and diversity may be expected within the studied forest ecosystems.Der Einfluss anthropogenen Handelns auf weltweite Klima- und Umweltveränderungen ist mittlerweile unbestritten. Neben einem erhöhten Ausstoß von Treibhausgasen an die Atmosphäre und dessen Folgen für die globale Erwärmung, sind stetig steigende Nährstoffdepositionen (N und P) evidente Faktoren, die zu einem Wandel terrestrischer Ökosysteme beitragen. Hierbei gelten insbesondere die tropischen Feucht- und Trockenwälder Südecuadors aufgrund ihrer einzigartigen Biodiversität, Strukturvielfalt und starken Nährstofflimitierung als besonders vulnerabel. Obwohl in der Literatur kontrovers diskutiert, kann davon ausgegangen werden, dass durch eine Erhöhung der N- und P-Vorräte im Boden eine Steigerung der Kohlenstofffixierung tropischer Baumarten zu erwarten ist und dass sich diese förderlich auf das Dickenwachstum der Bäume auswirkt. Zusätzlich aber beeinflussen klimatische Extremereignisse, wie beispielsweise starke Trockenheit, baumphysiologische Prozesse erheblich und führen zu einer Verringerung der Wuchsleistung. Bisherige Untersuchungen erlauben jedoch keine Einschätzung darüber, wie sich die Wuchsleistung verschiedener Baumarten entlang eines ökologischen Gradienten von der immergrünen Bergwaldstufe bis in die saisonal laubwerfende Trockenwaldstufe Südecuadors ändert. Für diese Dissertation war es daher von besonderem Interesse, baumphysiologische, holzanatomische und isotopenphysikalische Untersuchungen an verschiedenen Baumarten zweier Ökosysteme durchzuführen und den Einfluss veränderter Umweltbedingungen auf deren saisonales Dickenwachstum, ihrer holzanatomischen Eigenschaften sowie ihrer Kohlenstoff-Isotopenverhältnisse (δ13C) im Holz zu beurteilen und mögliche Auswirkungen auf deren Vulnerabilitätsrisiko und Adaptionspotenzial herauszuarbeiten. Die Untersuchungen wurden im Rahmen der DFG geförderten Projekte PAK 823-825 sowie FOR 816/2 in Südecuador durchgeführt. Die Projektgebiete umfassen sowohl (i) einen Standort im saisonalen Trockenwald (~600 m a.s.l.) als auch (ii) ein interdisziplinäres Düngungsexperiment (NUMEX) im immergrünen tropischen Bergregenwald. Seit 2008 werden dort ausgewählte Waldparzellen (2000 – 2150 m a.s.l.) mit einer moderaten Düngung an limitierenden Nährstoffen (N, P und N+P) halbjährlich angereichert. Folgende Methoden waren Teil des innovativen dendroökologischen Multiparameteransatzes: Hochauflösende Dendrometermessungen zur Beurteilung kurz- und langfristiger radialer Wachstumsdynamiken unter Einfluss klimatischer Extremereignisse und erhöhter Nährstoffzufuhren (Trockenwald und NUMEX), Präparation mikroskopischer Dünnschnitte (Ast, Stamm), deskriptive 3D-Analysen des Holzes sowie eine quantitative Auswertung elf verschiedener holzanatomischer Parameter, um nährstoffbedingte Modifikationen des hydraulischen Leitsystems zu bewerten (NUMEX), Evaluierung hochauflösender intra-annueller δ13C Isotopenwerte sowie jährlicher Wachstumsraten, um nährstoffbedingte Veränderungen der Kohlenstofffixierung zu bewerten (NUMEX). Die Ergebnisse zeigen, dass in beiden Ökosytemen radiale Wachstumsschwankungen der untersuchten Baumarten maßgeblich von der Wasserverfügbarkeit gesteuert und beschränkt werden. Insbesondere im Trockenwald ist dieser Zusammenhang signifikant und ein hohes Vulnerabilitätsrisiko wird für die Baumart Loxopterygium huasango bestätigt, da während des starken La Niña Ereignisses 2011 (extreme Trockenheit) kein Wachstum erfolgte. Mit der prognostizierten Zunahme klimatischer Trockenereignisse wird die Ressource Wasser noch knapper vorhanden sein und sich insbesondere auf den terrestrischen Kohlenstoffkreislauf und somit auch auf die Kohlenstoffaufnahme dieser Baumart negativ auswirken. Im immergrünen Bergregenwald hingegen wird deutlich, dass eine vermehrte Nährstoffverfügbarkeit bei einem Großteil der Baumarten (Alchornea lojaensis, Podocarpus oleifolius und Prunus sp.) die Vulnerabilität gegenüber Trockenheit abschwächt und insgesamt das Dickenwachstum begünstigt (Ausgangshypothese). Jedoch sind hinsichtlich der Nährstofflimitierung große artspezifische Unterschiede festzustellen, da entgegen der Erwartungen genau die Baumart mit der höchsten Abundanz im Gebiet (Graffenrieda emarginata) ein gegensätzliches Muster zeigte und nicht bzw. erst verspätet auf die Zugabe von Nährstoffen reagierte. Für die Baumart Alchornea lojaensis können insgesamt sehr unterschiedliche Anpassungsstrategien der untersuchten Baumindividuen gegenüber einer vermehrten Nährstoffverfügbarkeit festgestellt werden. Während holzanatomische Untersuchungen auf der Ast-Ebene ergaben, dass deutlich vergrößerte Gefäßdurchmesser sowie vergrößerte Gefäßgruppen auf ein verbessertes hydraulisches Leitsystem durch einen erhöhten Nährstoffeintrag hindeuten, wurde dies auf der Stammebene nicht bestätigt. Durch den innovativen Methodenverbund aus holzanatomischen Parametern, intra-annuellen δ13C Werten sowie jährlichen Wachstumsraten war es möglich, den exakten Zeitpunkt der Düngung im Holz von A. lojaensis festzustellen. Somit kann für die Baumart eine deutliche Veränderung der Kohlenstofffixierung nachgewiesen werden, jedoch entgegen der Ausgangshypothese (erhöhte Nährstoffverfügbarkeit Anreicherung δ13CPflanze), da zeitgleich mit dem Beginn der Düngung deutlich vermehrt das leichtere Kohlenstoffisotop 12C eingebaut wurde. Ob diese signifikanten Veränderungen zusätzlich klimatisch bedingt sind, bedarf weiterer Untersuchungen. Insgesamt liefert diese Dissertation, trotz oder gerade wegen vieler unerwarteter Ergebnisse, einen sehr wichtigen Beitrag für das bessere Verständnis tropischer Waldökosysteme. Die Arbeit verdeutlicht gleichzeitig die Komplexität baumphysiologischer Prozesse im Hinblick auf veränderte Umweltbedingungen, da sich die jeweiligen Anpassungsstrategien sowohl zwischen verschiedenen Arten als auch auf der Einzelbaumebene einer Art deutlich unterscheiden. Somit ist, auf lange Sicht gesehen, mit einer deutlichen Veränderung der Artenzusammensetzung in den untersuchten Waldgebieten zu rechnen

WagnerClimateSeasonalityLimits.pdf

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

WagnerClimateSeasonalityLimitsSupplementalTablesandFigures.pdf

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