Stephan Hättenschwiler

A TEST OF THE TREELINE CARBON LIMITATION HYPOTHESIS BY IN SITU CO2 ENRICHMENT AND DEFOLIATION

Historically, carbon limitation, through a shortage of photoassimilates has been argued to limit the growth of trees at the upper altitudinal treeline. In a three-year free-air CO2 enrichment (FACE) experiment, two species of 30-year-old alpine conifers (Larix decidua and Pinus uncinata) were studied to test this hypothesis in situ in the Swiss Central Alps (2180 m above sea level). CO2 enrichment was combined with foliage removal to test the effect of altered source-sink relationships on tree growth and leaf level responses. Elevated CO2 enhanced photosynthesis and increased nonstructural carbohydrate (NSC) concentrations in the needles of both species. While the deciduous larch trees showed longer needles and a stimulation of shoot growth over all three seasons when grown in situ under elevated CO2, pine trees showed no such responses. Irrespective of CO2 concentration, defoliation in both species stimulated photosynthesis and increased stomatal conductance in remaining current-year needles in the treatment year and reduced leaf nitrogen concentration in the year following defoliation. Defoliated larch trees had fewer and shorter needles with reduced NSC concentrations in the year following defoliation and showed no stimulation in shoot elongation when exposed to elevated CO2. In contrast, defoliation of evergreen pine trees had no effect on needle NSC concentrations, but stimulated shoot elongation when defoliated trees were exposed to elevated CO2. After three years, our results suggest that deciduous larch is carbon limited at treeline, while evergreen pine is not. However, as indicated by the defoliation treatment, the carbon economy of these trees can clearly be modified by extreme events. The expected changes in growth of these treeline trees with improving carbon availability as atmospheric CO2 continues to increase will thus depend on both the interplay between biotic and abiotic processes, and the species or tree functional types involved.

Seedling occurrence in alpine treeline conifers: A case study from the central Rocky Mountains, USA

Abstract Information concerning the occurrence of very young (1- to 10-year-old) tree seedlings in the alpine treeline ecotone is rare. Seedling occurrence of the dominant conifers Picea engelmannii and Abies lasiocarpa was measured in the treeline ecotone of the Medicine Bow Mountains, Wyoming (central Rocky Mountains, USA), an area composed of elongated tree islands separated by open meadows (ribbon forest) that grade into the closed forest. No seedlings were found on the windward sides of tree islands, while a mean of 0.6xa0seedlings.m–2 occurred on the leeward (downward) sides. These values compared to the 4.2xa0seedlings.m–2 in the closed forest. In addition, a strong correspondence was found between snowpack depth and seedling abundance, with depths that were either too shallow ( 1.5xa0m) associated with fewer or no seedlings. A. lasiocarpa seedlings made up much less of the overall seedling population in the ribbon forest (6xa0%) than in the closed forest (22xa0%). Seedling establishment in this portion of the alpine treeline ecotone appears to be occurring at a low rate that differs between the two dominant species and may be strongly influenced by wind-driven snow accumulation patterns.

Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2

The mechanisms for species-specific growth responses to changes in atmospheric CO2 concentration within narrow ecological groups of species, such as shade-tolerant, late-successional trees, have rarely been addressed and are not well understood. In this study the underlying functional traits for interspecific variation in the biomass response to elevated CO2 were explored for seedlings of five late-successional temperate forest tree species (Fagus sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba). The seedlings were grown in the natural forest understorey in very low and low light microsites (an average of 1.3% and 3.4% full sun in this experiment), and were exposed to either current ambient CO2 concentrations, 500, or 660xa0µlxa0CO2xa0l–1 in 36 open-top chambers (OTC) over two growing seasons. Even across the narrow range of successional status and shade tolerance, the study species varied greatly in photosynthesis, light compensation point, leaf dark respiration (Rd), leaf nitrogen concentration, specific leaf area (SLA), leaf area ratio (LAR), and biomass allocation among different plant parts, and showed distinct responses to CO2 in these traits. No single species combined all characteristics traditionally considered as adaptive to low light conditions. At very low light, the CO2 stimulation of seedling biomass was related to increased LAR and decreased Rd, responses that were observed only in Fagus and Taxus. At slightly higher light levels, interspecific differences in the biomass response to elevated CO2 were reversed and correlated best with leaf photosynthesis. The data provided here contribute to a mechanistic process-based understanding of distinct response patterns in co-occurring tree species to elevated CO2 in natural deep shade. I conclude that the high variation in physiological and morphological traits among late-successional species, and the consequences for their responses to slight changes in resource availability, have previously been underestimated. The commonly used broad definitions of functional groups of species may not be sufficient for the understanding of recruitment success and dynamic changes in species composition of old-growth forests in response to rising concentrations of atmospheric CO2.

Elevated atmospheric CO2 fuels leaching of old dissolved organic matter at the alpine treeline

[1]xa0Dissolved organic matter (DOM), the mobile form of soil organic matter (SOM), plays an important role in soil C cycling and in nutrient transport. We investigated the effects of 5 years of CO2 enrichment (370 versus 570 μmol CO2 mol−1) on DOM dynamics at the alpine treeline, including the analysis of fast-cycling components such as low molecular weight organic acids (LMWOAs), dissolved organic carbon (DOC) biodegradability, and the decomposition of 14C-labeled oxalate. Concentrations of DOC in canopy throughfall were 20% higher at elevated CO2, probably driven by higher carbohydrate concentrations in leaves. In the organic soil layer, 5 years of CO2 enrichment increased water-extractable organic C by 17% and soil solution DOC at 5 cm depth by 20%. The 13C tracing of recently assimilated CO2 revealed that the input of recent plant-derived C (<15% of total DOC) was smaller than the CO2-induced increase in DOC. This strongly suggests that CO2 enrichment enhanced the mobilization of native DOC, which is supported by significant increases in dissolved organic nitrogen (DON). We mainly attribute these increases to a stimulated microbial activity as indicated by higher basal and soil respiration rates (+27%). The 14C-labeled oxalate was more rapidly mineralized from high CO2 soils. The concentrations of LMWOAs, but also those of “hydrophilic” DOC and biodegradable DOC (6% of total DOC), were, however, not affected by elevated CO2, suggesting that production and consumption of “labile” DOC were in balance. In summary, our data suggest that 5 years of CO2 enrichment speeded up the cycling of “labile” DOM and SOM in a late successional treeline ecosystem and increased the mobilization of older DOM through a stimulated microbial activity. Such a “priming effect” implies that elevated CO2 can accelerate the turnover of native SOM, and thus, it may induce increasing losses of old C from thick organic layers.

Morphological adjustments of mature Quercus ilex trees to elevated CO2

On ne sait toujours pas si les arbres forestiers matures repondent a des concentrations accrues du CO 2 atmospherique dune maniere similaire a celle des jeunes plants. Des chenes mediterraneens matures (Quercus ilex), poussant dans une atmosphere enrichie en CO 2 autour darrivees naturelles de CO 2 depuis leur stade de plantule presentent une augmentation moderee, liee a lâge, de la production de biomasse du tronc; mais ils presentent une biomasse significativement plus faible des branches de 6 ans, une ramification reduite, et une plus faible surface foliaire par unite de biomasse de branche que des arbres temoins sur un site proche non-enrichi. Nos donnees indiquent que les arbres, dans des parcelles forestieres naturelles, sadaptent morphologiquement a une augmentation du CO 2 et reduisent les stimulations initiales de croissance induites par le CO 2 . Des ajustements allometriques tels que la reduction de la surface foliaire peuvent etre consideres comme des regulations « a la baisse » de la photosynthese de la canopee et pourraient constituer un mecanisme efficace deconomie de leau.

Growth and reproduction of the alpine grasshopper Miramella alpina feeding on CO2-enriched dwarf shrubs at treeline

The consequences for plant-insect interactions of atmospheric changes in alpine ecosystems are not well understood. Here, we tested the effects of elevated CO2 on leaf quality in two dwarf shrub species (Vaccinium myrtillus and V. uliginosum) and the response of the alpine grasshopper (Miramella alpina) feeding on these plants in a field experiment at the alpine treeline (2,180xa0m a.s.l.) in Davos, Switzerland. Relative growth rates (RGR) of M. alpina nymphs were lower when they were feeding on V. myrtillus compared to V. uliginosum, and were affected by elevated CO2 depending on plant species and nymph developmental stage. Changes in RGR correlated with CO2-induced changes in leaf water, nitrogen, and starch concentrations. Elevated CO2 resulted in reduced female adult weight irrespective of plant species, and prolonged development time on V. uliginosum only, but there were no significant differences in nymphal mortality. Newly molted adults of M. alpina produced lighter eggs and less secretion (serving as egg protection) under elevated CO2. When grasshoppers had a choice among four different plant species grown either under ambient or elevated CO2, V. myrtillus and V. uliginosum consumption increased under elevated CO2 in females while it decreased in males compared to ambient CO2-grown leaves. Our findings suggest that rising atmospheric CO2 distinctly affects leaf chemistry in two important dwarf shrub species at the alpine treeline, leading to changes in feeding behavior, growth, and reproduction of the most important insect herbivore in this system. Changes in plant-grasshopper interactions might have significant long-term impacts on herbivore pressure, community dynamics and ecosystem stability in the alpine treeline ecotone.

Liana seedling growth in response to fertilisation in a neotropical forest understorey

Summary I tested the hypothesis that lianas grow taller and/or produce more biomass with higher soil nutrient availability despite severe light limitation in the forest understorey. Young seedlings of Callichlamys latifolia (Bignoniaceae), Doliocarpus major , and D. olivaceus (Dilleniaceae) were grown along a light gradient (0.8–2.2% of full sun) in the undisturbed soil of a lowland tropical rain forest on Barro Colorado Island (Panama) for 19 months. An equal number of seedlings was either supplied with NPK fertiliser equivalent to 220 kg N ha −1 year −1 or remained without fertilisation. Seedling survival was not significantly different among species and was not affected by either light availability or fertilisation. Biomass and height increased linearly with increasing light availability in seedlings of all species. Under fertilisation, biomass production was 50% higher in D. olivaceus irrespective of light availability, increased in C. latifolia only in relatively bright microsites, and was not affected in D. major across the entire light gradient. Seedling height was generally less affected by fertilisation with only C. latifolia seedlings growing taller in fertilised plots of a relatively high light availability. Biomass allocation changed little across the light gradient in any species. The strong biomass response to fertilisation in D. olivaceus was correlated with an increased biomass allocation to leaves at the expense of stems with only small changes in root mass fraction. The other two species showed no fertilisation effects on biomass allocation. I conclude that extreme light limitation does not preclude growth responses to nutrient addition in tropical lianas, but species differ in this respect. Ich testete die Hypothese, dass das Wachstum junger Lianen-Samlinge trotz extremem Lichtmangel im Waldunterwuchs durch eine erhohte Nahrstoffverfugbarkeit stimuliert werden kann. Dazu wurden Jungpflanzen von den drei Lianenarten Callichlamys latifolia (Bignoniaceae), Doliocarpus major und D. olivaceus (Dilleniaceae) mit NPK (220 kg N ha −1 Jahr −1 ) gedungt. Die Lianen-Samlinge wurden entlang eines naturlichen Lichtgradienten im Unterwuchs (0.8 bis 2.2% des vollen Sonnenlichts) gepflanzt. Die Uberlebensrate der Samlinge unterschied sich nicht zwischen den drei Arten und war auch nicht durch eine unterschiedliche Licht- oder Nahrstoffverfugbarkeit beeinflusst. Bei allen drei Arten nahmen die Biomasseproduktion und der Hohenzuwachs linear mit dem Lichtangebot zu. Unabhangig vom Lichtangebot produzierten D. olivaceus Samlinge 50% mehr Biomasse unter Dungerzugabe. Bei C. latifolia war eine positive Dungewirkung nur in relativ lichtreichen Mikrostandorten zu beobachten, und D. major reagierte uberhaupt nicht auf Dungung. Der Hohenzuwachs der Lianen-Samlinge war generell weniger beeinflusst durch Dungerzugabe. Nur C. latifolia Samlinge wurden etwas hoher im oberen Bereich des Lichtgradienten unter erhohter Nahrstoffverfugbarkeit. In keiner der drei Arten veranderte sich die Biomasseallokation in Abhangigkeit vom Lichtangebot. Die Zunahme der Biomasse unter Dungerzugabe bei D. olivaceus war aber von einer verstarkten Allokation in Blattbiomasse, auf Kosten der Stammbiomasse, begleitet. Die anderen beiden Arten zeigten keinen Einfluss der Nahrstoffverfugbarkeit auf die Biomasseallokation. Diese Daten weisen darauf hin, dass eine Wachstumsstimulation durch erhohte Nahrstoffverfugbarkeit bei tropischen Lianen selbst unter extremem Lichtmangel moglich ist. Verschiedene Arten reagieren allerdings ganz unterschiedlich.

Annual CO2 budget of spruce model ecosystems in the third year of exposure to elevated CO2

Des clones depiceas (Picea abies) âges de 4 ans ont ete cultives en competition dans des eco-systemes modeles sur un sol forestier naturel pauvre en nutriments et avec une vegetation basse naturelle; ils ont ete exposes a 3 concentrations en CO 2 (280, 420 et 560 μmol mol -1 ) pendant 3 ans. Le prelevement diurne net en CO 2 de lecosysteme (NEC d ), la perte nocturne nette en CO 2 de lecosysteme (NEC n ) et le degagement de CO 2 du sol ont ete mesures a plusieurs reprises au cours de la troisieme annee dexposition au CO 2 et ont ete utilises pour estimer un budget annuel de lecosysteme en CO 2 . La stimulation de NEC d induite par le CO 2 varie au cours de lannee sans stimulation mesu-rable au printemps et a lautomne mais avec une forte stimulation du CO 2 en milieu de saison. Seuls les pertes respiratoires de lecosysteme entier et le degagement de CO 2 du sol augmentent progressivement avec une elevation du CO 2 , contrecarrant ainsi la stimulation de la photosynthese par le CO 2 par unite de surface. En consequence, le prelevement net annuel de lecosysteme en CO 2 nest stimule que moderement et de faconnon-lineaire par le CO 2 (+8 % = 84 g C m -2 a -1 a 420 et +9 = 90 g C m -2 a -1 a 560 a comparer avec 280 μmol CO 2 mol -1 ). Nous en concluons que laugmentation de la concentration en CO 2 atmospherique peut mener a un accroissement de laccumulation annuelle nette en carbone de lecosysteme de communautes depiceas plutot pauvres en nutriments. Nos resultats suggerent de plus que les effets de la fertilisation par le CO 2 pourraient etre tres importants pour des concentrations courantes en CO 2 et que les augmentations relatives du prelevement net de lecosysteme en CO 2 deviendront relativement plus faibles a mesure que le CO 2 atmospherique continuera daugmenter.

Growth of autotrophic and root-hemiparasitic understory plants under elevated CO2 and increased N deposition

Nous avons etudie les effets dun enrichissement en CO 2 atmospherique (280, 420 et 560 μmol CO 2 mol -1 ) et dune augmentation de lapport en N (0, 30 et 90 kg ha -1 a -1 ) sur Oxalis acetosella, Homogyne alpina et Melampyrum sylvaticum cultives dans des ecosystemes modeles sous des epiceas. La biomasse epigee dHomogyne (la moins tolerante a lombre) et de lhemiparasite annuelle Melampyrum saccroit fortement avec une augmentation du CO 2 mais ce nest pas le cas dOxalis qui est mieux adaptee a lombre. En revanche, laugmentation de lapport en azote stimule la croissance dOxalis, mais na pas deffet sur Homogyne et Melampyrum. En raison de la fermeture de la canopee des epiceas, Homogyne a subi une limitation de lapport en lumiere et sa survie etait fortement correlee au LAI de la canopee depiceas au cours de la seconde annee dexperimentation. Nos resultats suggerent quune elevation du CO 2 facilite lexpansion dHomogyne vers des micro-habitats moins favorables (ombre plus dense) et quune augmentation de lapport en azote permet a des especes plus vigoureuses, comme Oxalis, de devenir plus abondantes. La croissance de lhemiparasite Melampyrum est stimulee indirectement par lapport accru de carbone heterotrophique (donnees isotopes du carbone) en provenance de lhote (Picea abies) en consequence, cette espece pourrait aussi devenir plus abondante avec une augmentation du CO 2 . Neanmoins, de possibles effets indirects (retroactions de la canopee) rendent difficiles des predictions a long terme sur le developpement du sous-bois.

Natural CO2 springs: obstacle or opportunity?

Plant Responses to Elevated Carbon Dioxide: Evidence from Natural Springs edited by A. Raschi, F. Miglietta, R. Tognetti and P.R. van Gardingen Cambridge University Press, 1997. £40.00/