The new issue 2018/5 of the Journal of Vegetation Science is out

The cover of the new JVS issue shows different fruits of woody plants from the island of Réunion, related to the paper by Albert, Flores, Rouget, Wilding, & Strasberg (2018)

They used a set of vegetation plots from this tropical island to describe a pattern of striking decrease in the proportion of fleshy-fruited trees and shrubs with altitude. They explored this pattern in the context of phylogenetic community structure, which tends to be overdispersed in tropical lowlands and clustered in harsher high-altitude environments. Their explanation of why there are fewer fleshy-fruited species at high altitudes is based on the hypothesis of tropical niche conservatism, i.e. inability of the tropical fleshy-fruited lineages to adapt to high-altitude environments, combined with dispersal limitation on the oceanic island, which is difficult to reach for hardy plant species from the fleshy-fruited extratropical lineages.

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.

Forest seedling community response to understorey filtering by tree ferns

James M. R. Brock, George L. W. Perry, Tynan Burkhardt & Bruce R. Burns

James Brock, surrounded by silver fern Cyathea dealbata – the symbol of New Zealand national sports teams, recording the seedling community in experimental tree fern plot no. 115 at Huapai, in west Auckland, New Zealand. Photo credit: Edin Whitehead.

Tree ferns can be a common component of the understorey of tropical and southern temperate forests. In New Zealand’s broad-leaved podocarp forests they are frequently abundant in the understorey, forming near-continuous sub-canopies representing up to 50% of the stems and 21% of biomass. Tree ferns form deep, slow to decompose leaf (frond) litter, intercept high proportions (up to 50%) of sub-canopy light, and indiscriminately destroy seedlings and saplings when up to 3 m long dead fronds disconnect from the trunk and fall to the forest floor. Despite many hypotheses considering the likely effect of these processes (e.g. strong negative effects on establishment of shade-intolerant conifers), there have been no attempts to experimentally test the response of the seedling community to these potential effects.

Our research was conducted in the forests of northern New Zealand which comprise mixed broad-leaf canopies at 10–20 m with emergent conifers supporting 27 species across 13 families. Average rainfall at the sites is 1,280 mm per year, and the soils comprise volcanic sandy loams and granular clays. In our study, we first undertook a field survey to describe where seedlings are growing in the landscape and consider what might influence where they are growing including shading, depth of leaf litter, how close nearby trees and tree ferns are, and soil moisture. We then experimentally manipulated 160 tree ferns above seedling plots, to examine how tree fern litter and shading contribute to the patterns observed in the field survey.

The field survey showed that landscape-level seedling density is affected by the presence of tree ferns. Seedling densities of both angiosperms and conifers are up to 50% lower within the drip-line of a tree fern (i.e. within the area covered by the fronds projecting from the top of the tree fern); this reduction is associated with a doubling of leaf litter depth. Our field manipulation of tree ferns removed either litter and/or fronds from within and above seedlings plots. In the litter and frond removal treatment, we found that seedlings of two species of podocarp (Podocarpus totara and Phyllocladus trichomanoides) were consistently turning up, as well as there being increased seedlings densities and species richness. Our study suggests that through macro-litter and shading, tree ferns influence seedling community composition by reducing the density of canopy angiosperm and conifer species that can establish, particularly suppressing conifers in seedling communities within their drip-lines.

This is a plain language summary for the paper of Brock et al. published in the Journal of Vegetation Science.

Slow recovery of arbuscular mycorrhizal fungi and plant community after fungicide application: An eight-year experiment

Hana Pánková, Tomáš Dostálek, Kristýna Vazačová & Zuzana Münzbergová

Study site – dry grassland in the central part of the Czech Republic. Photo credit: Hana Pánková.

Dry grasslands represent one of the most species-rich communities in Europe with a high occurrence of rare species. Some of the grasslands were changed to agricultural fields with intensive application of fertilizers or pesticides. After their abandonment, the former fields may be recolonized by dry grassland species. However, the recovery is a slow process, and many rare plant species, typical for dry grassland habitats, are absent in the formerly abandoned fields even after several decades. One of the reasons for their absence should be changes in the soil biota caused by previous intensive agriculture. The key component of soil biota for the growth of rare grassland plant species are arbuscular mycorrhizal fungi. Arbuscular mycorrhizal fungi are microscopic soil organisms, which establish a reciprocal beneficial association with plants. They help to transport nutrients and water to the plant roots and protect them against pathogens by the production of special chemicals. These fungi may be suppressed by different agricultural practices including fungicide application. Many of rare grassland species are dependent on association with these fungi, and therefore they are not able to grow on former fields, where these fungi are missing. Additionally, such changes in soil biota improve growth of large grasses, which further suppress the performance of rare plant species.

In our study, we evaluated the recovery of plant communities and arbuscular mycorrhizal fungi in dry grasslands after fungicide application. Further, the grazing was implemented on the part of the area to simulate traditional management intervention used for support of plant biodiversity. We evaluated the occurrence of particular plant species and functionality of fungi every year.

Parts of study sites were fenced to prevent grazing. Photo credit: Hana Pánková.

The results showed that the effect of fungicide application on the functionality of fungi persisted five years after the last fungicide application on ungrazed parts of the area, and even the recovery of fungi after introducing the grazing management was not sufficient for recovery of the rare plants. Grazing led to the suppression of grasses, but forbs were still largely absent with only a few exceptions of good colonizers of open habitats. This suggests that the absence of rare species could be caused by changes in the composition of the fungal community or low availability of their seeds. Direct addition of seeds of the forbs and/or adding suitable arbuscular mycorrhizal fungi may thus be tested as possible methods to support the recovery of the dry grassland community.

This is a plain language summary for the paper of Pánková et al. published in the Journal of Vegetation Science.

The assembly of a plant network in alpine vegetation

Gianalberto Losapio, Marcelino de la Cruz, Adrián Escudero, Bernhard Schmid & Christian Schöb

The study site in the Swiss Alps (Lämmerenboden, 2300 m a.s.l.). Photo credit:
Gianalberto Losapio.

Plant ecology has always focused on interactions among species within communities. Ecological research has targeted the importance of negative interactions such as competition among plants for resources. But in the last years, there is increasing interest in understanding how plants can cooperate with each other. This is particularly the case in harsh ecosystems like the alpine where some stress-tolerant plants contribute to ameliorating growing conditions for their neighbours. In our study, we analysed the spatial distribution of plants and modelled their associations to test how plant networks are formed and maintained in alpine vegetation. We collected data about the spatial location and the phenotype of thousands of individual plants and analysed them with the state-of-the-art computational model. We found that plant species were highly connected through many positive interactions. Dominant, stress-tolerant species were the most important plants for supporting the plant community network. The plant community network was more cohesive than expected by chance. This study reveals a new class of mechanisms underlying the formation of plant communities and has important implications for understanding the biodiversity of alpine vegetation.

This is a plain language summary for the paper of Losapio et al. published in the Journal of Vegetation Science.

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).