This is not a pine: a fieldwork story

The post provided by Emilie Champagne

One of my field assistant (Lorraine Lessard) carefully recording browsing in a 4 m2 plot, in Outaouais (Québec, Canada). Photo credit: Emilie Champagne.

This post refers to the article Forage diversity, type and abundance influence winter resource selection by white‐tailed deer by Emilie Champagne, André Dumont, Jean‐Pierre Tremblay and Steeve D. Côté, published in the Journal of Vegetation Science: https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12643.

This is the story of a major fieldwork problem and a desesperate Ph.D. student, with the academic happy ending: an article.

It was the second year of my Ph.D. project. My team of assistants and I were set to collect data in a new region, never exploited by my supervisors nor by me. Our objective was to determine how diversity in plant communities influenced browsing on white pine (Pinus strobus) by white-tailed deer (Odocoileus virginianus). The core of my thesis related to the effects of neighbouring plants on browsing (i.e. associational effects). Most of my thesis’ data were collected on Anticosti Island (Québec, Canada), where plant diversity is very low because of deer overbrowsing in a boreal forest (Potvin et al. 2003; Tremblay et al. 2006). By measuring browsing in the Outaouais region, we would be able to test the effects of plant diversity on browsing, as this is one of the most diverse regions for plants in Québec. Although many studies had investigated the relationship between diversity and invertebrate herbivores (Kambach et al. 2016; Moreira et al. 2016), few had investigated the relationship with large herbivores, especially in natural environments. Is was most probably a question of scale: it is easier to manipulate the composition of patches of plants for invertebrates than to do the same thing for cervids.

A white-tailed deer (Odocoileus virginianus) on Anticosti Island (Québec, Canada). Photo credit: Florent Déry.

With the help of my co-author (A. Dumont), we had selected a white pine plantation site. It was a small area (less than 1 km2), where we could do an almost complete mapping of the stems. The first day in the field, we found the site easily and saw that there were plenty of pines within deer’s reach. Several plants presented signs of browsing in the area. Browsing studies are often characterized by a lot of ‘non-event’, so it was an encouraging sign! My goal for this first day was to train my assistants, which is an essential task. Measuring browsing is not complicated, but it does require a bit of training and lots of rigour. We had done an entire morning of work, finished two complete plots, when I realized there was a problem.

These were not white pines, but rather red pines (Pinus rubens). Now those two species are easy to discriminate, but I was so concentrated on training my assistants that I had not realized my mistake. A bit anxiously, I told my assistants to wait, while I walked the site, in a quest of white pines. Not a white pine in sight. Both white and red pines were supposedly planted on that site, but for an unknown reason, only red pines remained. And, more tragically for my project, red pines are not browsed by deer. I will never know whether it was a mistake on the forest map or whether deer had completely browsed the poor white pine, but the result was the same: no data to collect.

Two of the elusive white pine (Pinus strobus, the right side of the picture), accompanied by a highly browsed balsam fir (Abies balsamea, left). Photo credit: Emilie Champagne.

For an experienced researcher, this could be a minor issue. For a Ph.D. student, it was a panicking event. I frantically tried to contact my supervisor, in a region with low cell phone coverage, but he was unreachable. I then turned to my collaborator, who suggested contacting a forestry technician working at the nearby office. With his invaluable help, I was able to obtain detailed maps of the region. I selected 13 forest stands that had received a forestry treatment in the last 23 years, and planned a new sampling design. I would use transects in all those stands, and each time we encountered a white pine, we would set a circular plot around it.

We found the new design plan easy to follow, and we completed fieldwork without any more trouble. Buried under the more pressing needs of other chapters of my thesis, it took me months to analyze the data and a couple of years to have a manuscript. My misfortune was forgotten by almost everyone, and even for me, the adventure felt like an old story. The results brought me an additional surprise. I was expecting results similar to the previous studies, where browsing by moose (Alces alces) increases with plant diversity (Vehviläinen & Koricheva 2006; Milligan & Koricheva 2013). In my data, however, browsing by deer decreased with diversity indices such as the Shannon index, although browsing did increase with plant richness. By using multivariate analyses (principal component analysis) and an index of plant selection, it became clearer that herbivore choice was influenced by the relative abundance of species. We measured diversity using indices, such as the richness or the Shannon index, but herbivores do not perceive the environment in this manner. To them, the identity and relative abundance of the different plant resources is what matters. The key message of this article is that while diversity indices are interesting proxies for several ecological functions, they might not be as useful in plant-herbivore interactions.

If my original sampling plan had worked, I would have sampled one forest stand very precisely. That would have been interesting, but I am quite sure that diversity would have been lower than in the realized sampling plan. The results would also have been less representative of the entire area. In the end, I am not sure we would have learned as much. Perhaps more importantly for my personal development, I learned how to react to unexpected issues. So, to all the early career researchers reading this…please, don’t panic!

References:

  • Kambach, S., Kühn, I., Castagneyrol, B. & Bruelheide, H. (2016). The impact of tree diversity on different aspects of insect herbivory along a global temperature gradient – a meta-analysis. Plos One, 11, e0165815. https://doi.org/10.1371/journal.pone.0165815
  • Milligan, H.T. & Koricheva, J. (2013). Effects of tree species richness and composition on moose winter browsing damage and foraging selectivity: an experimental study. Journal of Animal Ecology, 82, 739-748. https://doi.org/10.1111/1365-2656.12049
  • Moreira, X., Abdala-Roberts, L., Rasmann, S., Castagneyrol, B. & Mooney, K.A. (2016). Plant diversity effects on insect herbivores and their natural enemies: current thinking, recent findings, and future directions. Current Opinion in Insect Science, 14, 1-7.
  • Potvin, F., Beaupré, P. & Laprise, G. (2003). The eradication of balsam fir stands by white-tailed deer on Anticosti Island, Québec: a 150-year process. Ecoscience, 10, 487-495. https://doi.org/10.1080/11956860.2003.11682796
  • Tremblay, J.-P., Huot, J. & Potvin, F. (2006). Divergent nonlinear responses of the boreal forest field layer along an experimental gradient of deer densities. Oecologia, 150, 78-88. https://doi.org/10.1007/s00442-006-0504-2
  • Vehviläinen, H. & Koricheva, J. (2006). Moose and vole browsing patterns in experimentally assembled pure and mixed forest stands. Ecography, 29, 497-506. https://doi.org/10.1111/j.0906-7590.2006.04457.x

Emilie Champagne is a specialist of plant-herbivore relationships. She completed a PhD in biology at Université Laval (Canada) in 2017 and is currently the recipient of a Mitacs Accelerate fellowship, resulting from a partnership between Université Laval, Ouranos and Ministère des Forêts, de la Faune et de Parcs du Québec.

In the quest of designing grassland communities resistant to invasions during ecological restoration

The post provided by Florencia A. Yannelli

Controlled conditions in greenhouse experiments enable us to test a plethora of hypotheses in community or invasion ecology by reducing the effect confounding factors. Photo credit: Florencia Yannelli.

This post refers to the article Seed density is more effective than multi‐trait limiting similarity in controlling grassland resistance against plant invasions in mesocosms by Yannelli et al. published in Applied Vegetation Science: https://onlinelibrary.wiley.com/doi/abs/10.1111/avsc.12373

Semi-natural grasslands are biodiversity hotspots in Central Europe but they are highly threatened by land-use change and impacts of human activity. This urgency calls for a usage of any ‘available’ area (e.g. roadsides, abandoned mines or set-aside fields) for grassland restoration and for increasing the connectivity between remaining patches. Yet, these areas are exposed to frequent disturbance and are susceptible to the colonization by invasive alien plant species. In this context, the focus of our study was to design seed mixtures that suppress invasive species during grassland restoration due to a high similarity of native plants in the community and the invader. Following the limiting similarity hypothesis, we expected that invasive species will be unsuccessful to establish in communities where native plants share similar niches with the invader, as judged by species functional traits.

This greenhouse experiment was related to my PhD project but also devised as an exercise for the Training School ‘Controlling common ragweed by vegetation management’ within the COST-Action SMARTER (Sustainable management of Ambrosia artemisiifolia in Europe). We established the experiment before the summer school but collected all data and discussed the preliminary results together with the participants. The idea was to create realistic communities, so we used grassland species that commonly occur in our study area, southern Germany, and two target invasive species that are present in urban disturbed areas, i.e. common ragweed (Ambrosia artemisiifolia) and giant goldenrod (Solidago gigantea). We depicted the niche of all study species based on ten functional traits related to establishment success, persistence and competitive ability, and extracted these properties from two trait databases. Using trait information on native and invasive species, we designed specific seed mixtures to suppress each invasive species. We created a greenhouse experiment to test the resistance of two communities in response to the two invasive species, respectively, and set up a control that consisted of invasive species monocultures. This procedure allowed us to check for more general patterns explaining the potential biotic resistance of the two native communities.

Participants of the Training School ‘Controlling common ragweed by vegetation management’ part of the COST-Action SMARTER performing final measurements in July 2015. Photo credit: Johannes Kollmann.

The design of resistant communities to target invasive species – making use of math!

The novelty of our paper’s method was the use of a system of linear equations to design the native plant communities to suppress invasive species, as suggested by Laughlin (2014). This offers the advantage of a multi-trait approach to maximize the similarity between the native community and the invasive species, but also of having communities composed of native species with different abundances. As a result, the two communities were mostly different in terms of species dominating the seed mixture and their seed size. That is, while the seed mixture designed for ragweed was dominated by more large-seeded species such as Centaurea scabiosa, the one for goldenrod was dominated by the smaller-seeded Achillea millefolium.

Is the limiting similarity hypothesis a good predictor of early stage biotic resistance in grassland communities?

We found that the community designed to be resistant against goldenrod invasion was actually the one that presented a higher suppression effect on both invasive species. Although this result could be interpreted as partial support for the limiting similarity hypothesis in the context of our experiment, we found that plant density was a better predictor of resistance (Fig. 1). This can be explained by the fact that most species included in the community designed for goldenrod had smaller seeds, which resulted in a high density of individuals and reflected in the measured leaf area index. This result is consistent with another study, where we also found that higher sowing density increases the chances of native species establishment, resulting in biotic resistance due to fewer resources for invading species (Yannelli et al. 2017a).

Another explanation for our results is related to the dominance of Achillea millefolium in the successful community. This species has a fast germination and shows an early growth form in form of multi-leaves rosettes that quickly covered the available space. Results from another study showed that this species is closely related to both invasive species, which suggests more functional similarity that might have not been captured by our trait selection (Yannelli et al. 2017b).

Fig. 1. Distinct decrease in the total aboveground biomass measured for the two invasive species ragweed and goldenrod (i.e. Ambrosia artemisiifolia and Solidago gigantea, respectively) when grown under competition with the community designed for goldenrod (SG), compared to the other community and the control. Figures from Yannelli et al. 2018; photo credit Florencia Yannelli.

Some limitations that could explain our lack of unequivocal support for the limiting similarity hypothesis are related to the pool of species included in the design of our communities and the use of traits obtained from databases. In practice, the species we choose to include for a restoration project might not be the most suitable in terms of matching similarity between the communities and the invasive species. Also while traits from databases are measured for adult plants, our experiment only covered the establishment phase and thus might not best portray resource acquisition or competitive ability during an early successional stage.

To sum up, we did not find clear support for the limiting similarity hypothesis in the context of our experiment. Instead, from our results at this early stage, increased seed density and fast vegetative growth seem to be better predictors of biotic resistance of grassland communities.

References

  • Laughlin, D.C. (2014). Applying trait-based models to achieve functional targets for theory-driven ecological restoration. Ecology Letters, 17, 771-784. https://doi.org/10.1111/ele.12288
  • Yannelli, F.A., Hughes, P., & Kollmann, J. (2017a). Preventing plant invasions at early stages of revegetation: the role of limiting similarity in seed size and seed density. Ecological Engineering, 100, 286-290. https://doi.org/10.1016/j.ecoleng.2016.12.001
  • Yannelli, F.A., Koch, C., Jeschke, J.M., & Kollmann, J. (2017b). Limiting similarity and Darwin’s naturalization hypothesis: understanding the drivers of biotic resistance against invasive plant species. Oecologia, 183, 775–784. https://doi.org/10.1007/s00442-016-3798-8

Florencia Yannelli is a community ecologist, with a strong interest in invasion and restoration ecology.  While this work was carried out during her doctoral studies at the Chair of Restoration ecology, at the Technical University of Munich, she is currently a postdoc at the Centre for Invasion Biology (C·I·B), Stellenbosch University.

The symmetry of competition: a battle crown-to-crown or roots-to-roots?

The post provided by Marco Mina

Stems, leaves and crowns is only half of the picture. A darker battle for resources happens below the surface where roots work hard to extract water and nutrients from the soil, and compete for space. Photo credit: https://pixabay.com/

This post refers to the article The symmetry of competitive interactions in mixed Norway spruce, silver fir and European beech forests published in the Journal of Vegetation Science: https://onlinelibrary.wiley.com/doi/full/10.1111/jvs.12664

Forests rich in tree species are not only known for providing higher levels of ecosystem services but also to be prompter to cope with unexpected disturbances and climatic changes. However, the mechanisms of competitions in multi-species forests are all but clear. Scientists are still puzzled about which combinations of tree species grow better in a particular environment or what factors promote or reduce a positive growth complementarity in secondary forests and/or plantations. A recently published JVS paper (Mina et al. 2018, Journal of Vegetation Science 29: 775-787) tackled this question for the three economically most important species growing in Central Europe: Norway spruce (Picea abies), silver fir (Abies alba) and European beech (Fagus sylvatica).

The study was carried out in the framework of the recently completed project Integrating Species Mixtures in Tree Growth Functions for Forest Development Models in Switzerland – Swiss-SpeMixMod, by researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. Overall, the project focused of evaluating patterns of tree growth complementarity for the major central European tree species growing in a total of 19 mixture types. In a a follow-up, the researchers focused on mixed forests composed of Norway spruce (Picea abies), silver fir (Abies alba) and European beech (Fagus sylvatica). The aim of the study was to better understand the different modes of intra‐ and inter‐specific competition in these two‐ and three‐species mixed forests. The main goal was to disentangle if species interactions in spruce‐fir‐beech forests are more associated with size‐symmetric (i.e., competition for belowground) or size‐asymmetric competition (i.e., for aboveground resources). In other words:

Are tree interactions a battle crown-to-crown or roots-to-roots?

The researchers took advantage of the extensive database available in the context of the Swiss National Forest Inventory (NFI). The Swiss NFI is based on terrestrial sampling on a 1.4×1.4 km grid of permanent plots covering the entire country of Switzerland with measurements taken since the early 1980s until now. Large-scale forest inventories are a great source of data for scientists with the strength of being representative for a very broad range of environmental conditions, stand development, stand densities, forest types, silvicultural regimes and species compositions. As shown in the figure below, spruce, fir and beech grow in many locations across the country, and they grow whether in monospecific stands or in two- and three-species mixture stands.

Distribution of the spruce-fir-beech sample plots in Switzerland used in our study. Plots were categorized based on their species composition and level of mixture. Credit: Mina et al. 2018.

Marco Mina, the lead author of the JVS article, shared his summary of the study: “We applied a similar approach as in one of our previous study and examined the individual-tree growth of Norway spruce, silver fir and European beech using non-linear mixed effect models to assess tree growth. This time, however, we implemented distance-independent competition indices in the models. These indices were used as a proxy for size-symmetric (competition for belowground resources) and size-asymmetric (competition for light) competition. To explore the influence of species mixture on tree growth, and thus to analyse the size-symmetric and size-asymmetric mixing effects, we split these two indices into species-specific components. After a process of model selection, we obtained three final models that were used to evaluate the symmetry of competition”.

Do Norway spruce, silver fir and European beech compete more for aboveground or belowground resources? And what is the most competitive species?

The study demonstrated that species-specific competition indices could be integrated into individual tree growth models to express the different modes of competition among species in mixed forests. Clear differences between intra– and inter-specific competition among these three important species were found. This highlight the presence of mixing effects in two- and three-species mixtures of spruce, fir and beech. These effects, however, seems to differ whether competition for aboveground or belowground resources is considered.

In the case of mixtures of Norway spruce and silver fir, results showed that tree growth of both species is larger in spruce-fir mixtures than in the respective monocultures.

The positive competitive interactions found between these two species might be due to more efficient use of belowground rather than aboveground resources. A possible explanation could be the complementary root systems of the species, which is shallow for spruce and deep rooting for fir.

However, when beech grows together spruce and fir, negative effects of increasing proportions of beech on individual tree growth of both conifers were detected. “It is not a surprise that beech is a highly competitive species”, continues Marco Mina. “Our results indicated that the negative effects of beech on the growth of the two conifers could be due to the competition of rooting systems and belowground use rather than for aboveground resources.” It is important to remark that, even if the growth of spruce and fir is reduced by the presence of beech, beech’s growth is highly improved by the presence of the two conifers rather than competing against an individual of the same species. In summary: beech is highly self-competitive species and it grows better when mixed with spruce and fir rather than in monospecific stands.

The figure below shows how different the growth of a single beech tree is depending on the increasing number of competitors of different species (x-axis: basal area of trees larger than the target one, BAL). The indices BAL indicates competition for light, thus a battle crown-to-crown. It is noticeable from the lower line (“All beech”) that the higher reduction of growth occurs when a beech tree is surrounded by bigger trees of the same species. Instead, the lowest reduction in growth occurs when the beech is surrounded by spruce and fir trees (lines “All spruce”, “Spruce and fir”).

Predicted effect of increasing size-asymmetric competition (BAL) for beech when larger competitors are composed of one species (solid lines), spruce and fir (dotted lines); beech-spruce-fir (dashed lines). Credit: Marco Mina.

To sum up, this study demonstrates that it is possible to further understand the symmetry of competition (i.e., how trees compete against each other) alongside with species competitive interactions. This analysis also highlights that forest inventories are a great source of data for performing such analysis.

Improved modelling of competitive interactions can help to better evaluate adaptation measures for mixed forests under global change stressors. Although it has been widely demonstrated that planting and restoring forests using multiple tree species brings only advantages, in many parts of the world monospecific plantations are still the rule. In the face of uncertainty such as climate change and unexpected disturbances, mixed species forests are believed to be a suitable option to build resilient forests and mitigate climate change by stocking more carbon.

This post is based on a contribution that appeared first in the blog Forest Monitor on July 16th, 2018.

Marco Mina is a forest ecologist. He is currently a postdoc at the Centre for Forest Research at the Université du Québec à Montréal (Canada) and a research associated at the Swiss Federal Institute for Forest, Snow and Landscape Research WSL (Switzerland).

A virtual tour from the Carpathian Basin to the Far East – an overview and synthesis of Eurasian forest-steppes

The post provided by László Erdős

Forest steppe in Northern Hungary. Photo credit: Péter Török.

According to the biophilia hypothesis of Edward O. Wilson, certain biological patterns evoke positive feelings in humans. As a considerable part of our evolution took part in savannas, so the argument runs, we are genetically determined to enjoy ecosystems with a savanna-like mosaic pattern of trees and grasslands. Some analyses have in fact shown that humans have an aesthetic preference for woody-herbaceous mosaics.

The hypothesis may be debated, but it certainly applies to me and the whole authorial team of the paper “The edge of two worlds: A new review and synthesis on Eurasian forest-steppes”, published in Applied Vegetation Science.

Granted, I like extensive forests and endless grasslands, but mixtures of these two habitats have always fascinated me. That is why I study forest-steppes. But what exactly are forest-steppes, and where can we find them? As it turned out, it is not very easy to answer these questions. Definitions and distribution maps abound in the literature, but they are usually contradictory. There are many case studies and even some reviews on national or regional scales, but a synthesis at the scale of Eurasia has been lacking.

After a few months of work spent in evaluating hundreds of forest-steppe publications, the situation that seemed difficult at the beginning became even more confusing. So I contacted some experts who are familiar with forest-steppes, and finally an international team emerged, formed by fourteen ecologists from six countries. I would have never imagined that so many different opinions exist regarding forest-steppes. Sometimes it seemed hopeless that co-authors could ever reach a compromise.

Finally, however, we accepted a definition that we think is broad yet accurate. It includes all types of forest-steppes. We firmly believe this definition works well, but we do not deny that it is somewhat arbitrary. Indeed, an exact forest-steppe definition is complicated by inherent ambiguity. It is clear that in nature a continuum exists, ranging from totally treeless grasslands to closed-canopy forests. Forest-steppes lie somewhere between the two extremes. The middle of the continuum is clearly a forest-steppe, but lower and upper thresholds can always be debated. Other problems of this kind are numerous. Constructive criticism and alternative definitions are welcome!

The largest part of our paper is about forest-steppe distribution and the delineation and brief description of the main forest-steppe regions. We hope this part can be used for education (for example, it may prove useful for biogeography courses).

If I try to explain why I enjoy walking and working in forest-steppes, the answer is biodiversity. First of all, there are so many kinds of forest-steppes, in lowlands and mountains, on sand and on rocky surfaces, some with an almost mesic character, others like a semi-desert. The high diversity of habitats is a core feature of all forest-steppes, but it is particularly conspicuous in the Carpathian Basin. You are in a shady and cool forest stand. Take a few steps, and you find yourself on a baking sand dune with sparse vegetation. A few meters away, in the dune slack, there is a small fen. Walk a bit farther, and you will be standing on the shore of an alkaline lake. Exceptional habitat diversity is accompanied by high species diversity and a remarkable number of endemics. The variety of life-forms and the colours of the flowers add to the beauty of forest-steppes. During botanical studies. I especially enjoy the company of animals. The European roller, bee-eater, and hoopoe are among my favourites, but I equally like the insect world: the swallowtail, the cone-headed grasshopper, the predatory bush cricket, and all the others. Sometimes I can hear the golden jackal, although I have not yet been lucky enough to meet one in person.

Forest steppe in the Kiskunság sand region. Photo credit: Laszló Erdős.

All in all, biodiversity explains why forest-steppes are so exciting. At the same time, they belong to the most threatened ecosystems on Earth, as pointed out in the final sections of our paper. Conversion to croplands, the spread of invasive species, afforestation, and the inadequate legal protection are the main factors that determine the present status of forest-steppes. The impact of climate change, combined with negative local or regional processes, may result in further forest-steppe decline.

However, there is some reason for hope. Forest-steppes have a long history of human presence, which proves that their sustainable use is possible. The scenic beauty of forest-steppes can be utilized in ecotourism. In some countries, the abandonment of former croplands provides a unique opportunity for grassland restoration. Grazing can be re-established in forest-steppe areas that are currently not grazed; grazing can be beneficial for biodiversity, and consumer demand for healthy products from free-ranging animals is expected to increase.

We hope that our paper will be useful for those who study forest-steppes. In addition, it is our hope that our review will encourage everyone to visit forest-steppes and discover their beauty.

László Erdős is a researcher at the Hungarian Academy of Sciences. The full Synthesis paper The edge of two worlds: A new review and synthesis on Eurasian forest‐steppes by Erdős et al. can be read for free in Applied Vegetation Science.