I. Tanya Handa

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.

The importance of biotic factors in predicting global change effects on decomposition of temperate forest leaf litter

Increasing atmospheric CO2 and temperature are predicted to alter litter decomposition via changes in litter chemistry and environmental conditions. The extent to which these predictions are influenced by biotic factors such as litter species composition or decomposer activity, and in particular how these different factors interact, is not well understood. In a 5-week laboratory experiment we compared the decomposition of leaf litter from four temperate tree species (Fagus sylvatica, Quercus petraea, Carpinus betulus and Tilia platyphyllos) in response to four interacting factors: elevated CO2-induced changes in litter quality, a 3°C warmer environment during decomposition, changes in litter species composition, and presence/absence of a litter-feeding millipede (Glomeris marginata). Elevated CO2 and temperature had much weaker effects on decomposition than litter species composition and the presence of Glomeris. Mass loss of elevated CO2-grown leaf litter was reduced in Fagus and increased in Fagus/Tilia mixtures, but was not affected in any other leaf litter treatment. Warming increased litter mass loss in Carpinus and Tilia, but not in the other two litter species and in none of the mixtures. The CO2- and temperature-related differences in decomposition disappeared completely when Glomeris was present. Overall, fauna activity stimulated litter mass loss, but to different degrees depending on litter species composition, with a particularly strong effect on Fagus/Tilia mixtures (+58%). Higher fauna-driven mass loss was not followed by higher C mineralization over the relatively short experimental period. Apart from a strong interaction between litter species composition and fauna, the tested factors had little or no interactive effects on decomposition. We conclude that if global change were to result in substantial shifts in plant community composition and macrofauna abundance in forest ecosystems, these interacting biotic factors could have greater impacts on decomposition and biogeochemical cycles than rising atmospheric CO2 concentration and temperature.

The unseen invaders: introduced earthworms as drivers of change in plant communities in North American forests (a meta-analysis).

Abstract Globally, biological invasions can have strong impacts on biodiversity as well as ecosystem functioning. While less conspicuous than introduced aboveground organisms, introduced belowground organisms may have similarly strong effects. Here, we synthesize for the first time the impacts of introduced earthworms on plant diversity and community composition in North American forests. We conducted a meta‐analysis using a total of 645 observations to quantify mean effect sizes of associations between introduced earthworm communities and plant diversity, cover of plant functional groups, and cover of native and non‐native plants. We found that plant diversity significantly declined with increasing richness of introduced earthworm ecological groups. While plant species richness or evenness did not change with earthworm invasion, our results indicate clear changes in plant community composition: cover of graminoids and non‐native plant species significantly increased, and cover of native plant species (of all functional groups) tended to decrease, with increasing earthworm biomass. Overall, these findings support the hypothesis that introduced earthworms facilitate particular plant species adapted to the abiotic conditions of earthworm‐invaded forests. Further, our study provides evidence that introduced earthworms are associated with declines in plant diversity in North American forests. Changing plant functional composition in these forests may have long‐lasting effects on ecosystem functioning.

Near-infrared spectroscopy (NIRS) predicts non-structural carbohydrate concentrations in different tissue types of a broad range of tree species

The allocation of non-structural carbohydrates (NSCs) to reserves constitutes an important physiological mechanism associated with tree growth and survival. However, procedures for measuring NSC in plant tissue are expensive and time-consuming. Near-infrared spectroscopy (NIRS) is a high-throughput technology that has the potential to infer the concentration of organic constituents for a large number of samples in a rapid and inexpensive way based on empirical calibrations with chemical analysis. The main objectives of this study were (i) to develop a general NSC concentration calibration that integrates various forms of variation such as tree species and tissue types and (ii) to identify characteristic spectral regions associated with NSC molecules. In total, 180 samples from different tree organs (root, stem, branch, leaf) belonging to 73 tree species from tropical and temperate biomes were analysed. Statistical relationships between NSC concentration and NIRS spectra were assessed using partial least squares regression (PLSR) and a variable selection procedure (competitive adaptive reweighted sampling, CARS), in order to identify key wavelengths. Parsimonious and accurate calibration models were obtained for total NSC (r2 of 0·91, RMSE of 1·34% in external validation), followed by starch (r2 = 0·85 and RMSE = 1·20%) and sugars (r2 = 0·82 and RMSE = 1·10%). Key wavelengths coincided among these models and were mainly located in the 1740–1800, 2100–2300 and 2410–2490 nm spectral regions. This study demonstrates the ability of general calibration model to infer NSC concentrations across species and tissue types in a rapid and cost-effective way. The estimation of NSC in plants using NIRS therefore serves as a tool for functional biodiversity research, in particular for the study of the growth–survival trade-off and its implications in response to changing environmental conditions, including growth limitation and mortality.

Historical anthropogenic disturbances influence patterns of non-native earthworm and plant invasions in a temperate primary forest

Time lags are of potentially great importance during biological invasions. For example, significant delays can occur between the human activities permitting the arrival of an invader, the establishment of this new species, and the manifestation of its impacts. In this context, to assess the influence of anthropogenic disturbances, it may become necessary to include a historical perspective. In this study, we reconstructed the history of human activities in a temperate forest now protected as a nature reserve to evaluate the magnitude and duration of the impact of human disturbances (e.g. trails, old quarries), as well as environmental factors, in explaining the probability of occurrence and the intensity of invasion by non-native earthworms and plants. The present-day patterns of distribution and intensity of earthworms and plants were better explained by proximity to the oldest human disturbances (initiated more than a century ago) than by proximity to more recent disturbances or to all disturbances combined. We conclude that understanding present-day patterns of non-native species invasions may often require reconstructing the history of human disturbances that occurred decades or even centuries in the past.

Springtail community structure is influenced by functional traits but not biogeographic origin of leaf litter in soils of novel forest ecosystems

With ongoing global change, shifts in the ranges of non-native species and resulting novel communities can modify biotic interactions and ecosystem processes. We hypothesized that traits and not biogeographic origin of novel plant communities will determine community structure of organisms that depend on plants for habitat or as a food resource. We tested the functional redundancy of novel tree communities by verifying if six pairs of congeneric European and North American tree species bearing similar leaf litter traits resulted in similar ecological filters influencing the assembly of springtail (Collembola) communities at two sites. Litter biogeographic origin (native versus non-native) did not influence springtail community structure, but litter genus, which generally reflected trait differences, did. Our empirical evidence suggests that a functional trait approach may be indeed as relevant as, and complementary to, studying biogeographic origin to understand the ecological consequences of non-native tree species in soils of novel forest ecosystems.

First Record In Canada of Onthophilus pluricostatus LeConte (Coleoptera: Histeridae) and a New Mention for the Rare Species Lordithon niger (Gravenhorst) (Coleoptera: Staphylinidae)

We report the first record of the species Onthophilus pluricostatus LeConte (Coleoptera: Histeridae) in Canada, new country record, and a new mention for the rare species Lordithon niger (Gravenhorst) (Coleoptera: Staphylinidae). One specimen of O. pluricostatus was caught in a pitfall trap in a red oak (Quercus rubra L.; Fagaceae) stand in the Parc national du MontSaint-Bruno (N 45°33′3.5′′ W 73°19′27.8′′) in southern Québec. The trap used a solution of 40% ethanol and 5% vinegar as preservative and was active during 1–23 August 2011. Our record represents the northernmost known range limit for O. pluricostatus. Its previously reported range extended northwards to Massachusetts, with the southern limit occurring in northern Florida and the western limit in Michigan (Bousquet and Laplante 2006). Although the species has not been reported from Ontario, a specimen reported from Detroit, Michigan indicates it may be found in that province (Helava 1978; Bousquet and Laplante 2006). Two specimens of L. niger were observed together (one collected) under the bark of a midfallen dead deciduous tree (most likely sugar maple (Acer saccharum Marshall; Aceraceae) or red oak) on 18 May 2013, also in the Parc national du Mont-Saint-Bruno. The species is considered potentially endangered in New England (McCollough 1997), and potentially extirpated in Pennsylvania (Rawlins and Bier 1998) and Virginia (Roble 2003). The status of this species is unknown in Canada, but seems less critical. Campbell (1982) reports only seven records of L. niger after 1942, but four of these were in Québec. Furthermore, Webster et al. (2012) report four records in New Brunswick between 2006–2009 and one in Québec for SaintRaphaël in 2006. The other records of this species in Québec were for Québec City (four records) and West Brome (Campbell 1982). The identification of O. pluricostatus was confirmed by Serge Laplante from the Canadian National Collection of Insects, Arachnids and Nematodes in Ottawa, Ontario where the voucher is kept. The specimens of L. niger were identified by the first author, and the voucher is kept in the arthropod collection in the laboratory of I. Tanya Handa at the Université du Québec à Montréal.