Does forest diversity matter?

By Julia Koricheva and Harriet Milligan, School of Biological Sciences.

Prof. Julia Koricheva and PhD student Harriet Milligan report on their work in the collaborative EU project which examines links between forest diversity and ecosystem functioning. This project takes them on a journey across Europe, from Finland to Spain, to assess the effects of browsing by moose and deer on different forest types.   

Julia Koricheva:

Forests are the most species-rich of all terrestrial ecosystems, they also control a large portion of the carbon, nutrient and water balances of the Earth. But are these two aspects interlinked? Do more diverse forests deliver better ecosystem services? This is an important question given that the majority of European forests are subjected to some degree of human intervention and the way that forests are managed has a direct impact on their biodiversity. Quantifying the role of forest biodiversity for ecosystem functioning is not an easy task. First, as forest ecosystems provide many different functions and services, we need to measure many variables (e.g. carbon sequestration, water cycling, nutrient retention, habitat provision, timber production etc), which requires joint efforts of specialists from different fields. In addition, the assessment should not be restricted to a single country or forest type but cover as many forest types as possible. Our team at the School of Biological Sciences at Royal Holloway, University of London (RHUL) is currently involved in an ambitious large-scale collaborative project which is funded by the EU and aimed at meeting the above challenges. The FunDivEUROPE (Functional significance of forest biodiversity in Europe) project represents a consortium of 24 partner institutions from 15 European countries.

Harriet assessing browsing of oak seedlings

Harriet assessing browsing of oak seedlings

The RHUL’s FunDivEUROPE team has two important missions in the project. First, we provide one of the experimental platforms for testing the effects of tree diversity on ecosystem functioning in forest ecosystems – the Satakunta forest diversity experiment which I have established in SW Finland in 1999. Last summer we have hosted in Satakunta numerous FunDivEUROPE research teams which measured everything from photosynthesis, tree growth and sap flow, soil chemistry and soil fauna to understory vegetation and herbivory in mixed and pure stands in order to assess how stand diversity and species composition affect ecosystem functioning. This summer it is our turn to travel and to do our own data collection, which brings me to the second mission of the RHUL’s team – assessment of forest resistance to mammalian herbivory.

Ability to regulate and reduce damage by pests is one of the services provided by forest ecosystems, and it is commonly believed that diverse ecosystems are able to regulate pest damage better than species-poor ecosystems and communities. This principle is widely used in agriculture; for instance, intercropping (planting several crop species together) has been shown to successfully reduce crop losses to a variety of insect pests. But does the same mechanism work for mammalian herbivores in forests? Damage by moose, deer, voles and hare is an important source of seedling and sapling mortality. Could it be reduced by planting or maintaining mixed-species tree stands? The answer is not straightforward because one important difference between insect and mammalian herbivores is that while majority of insect pests are specialists and feed on only one or a few closely related plant species, majority of mammalian herbivores are generalists and have a fairly broad diet. Therefore, instead of reducing herbivore damage, diverse tree stands may actually attract more mammalian herbivores by providing a kind of ‘smorgasbord’. I call on Harriet Milligan, who is doing her PhD on effects of forest diversity on mammalian herbivory within the FunDivEUROPE project.

Harriet Milligan:

Assessing herbivore browsing in forests: from bites to poo

To investigate if and how mammalian herbivores respond to forest diversity, I am assessing the degree of browsing on woody plant species in forest plots of differing tree species number and composition across 3 key European forest ecosystems (boreal, temperate, and Mediterranean). My PhD looks at what mammalian herbivores in these forests like to eat, and will attempt to understand how the structure of the forest influences their meal choices. Does a species-rich forest, for example, provide protection for regenerating woody plants or do herbivores prefer a more species-rich environment to make sure they get their 5-a-day? Levels of nutrients and secondary chemicals vary among tree species, and generalist mammalian herbivores have been shown to include various plant species in their ration to obtain a balanced diet and avoid too many toxins. In the field we identify and quantify the understory vegetation and subsequently assess levels of browsing damage on individual plants. These data provide information on the dominant woody species, occurrence of repellent plant species, and preference of different species by browsing mammals.

Faecal pellets of roe deer

Faecal pellets of roe deer

Looking for evidence of mammal visits to our plots (poo!) is another important aspect of data collection across Europe as it helps to assess the habitat use by mammalian herbivores. One thing I have noticed since starting my fieldwork is how excited I get when I spot a group of deer fecal pellets. I’m still unsure as to whether I should be concerned about this, or if this is a healthy level of enthusiasm for my PhD data. Hopefully these data will help us answer questions such as:

  • Are there common factors that determine deer distributions in forested areas?
  • Is this consistent across different regions in Europe?
  • How important is vegetation? Is it the main driver of deer distribution or second consideration to other factors such as distance to roads/water, slope etc.

Last summer I did field work in the boreal forests of Finland assessing browsing by moose in the Satakunta forest diversity experiment. First results suggest that tree species diversity and composition have significant influence on moose browsing behaviour. The experimental set up is obviously different from the natural systems, but we found greater degree of moose damage in plots with larger number of tree species.  Selectivity of moose also decreased with stand diversity – they were less picky of food items in these plots. Another important factor to come from the study was the importance of preferred tree species, Scots pine and silver birch in the case of moose. Moose know what they like and browse more from plots which have higher abundance of preferred species.

Alto Tajo National Park

Alto Tajo National Park

Currently I am working with Julia in the Mediterranean oak-pine woodlands of the Alto Tajo National Park in Spain. The landscapes here are beautiful and remote, the forests are as I expected from a Mediterranean system; open, shrub-like, dry and hot. There is no moose here, but plenty of deer (Red, Fallow, and Roe deer), as well as wild boar. We often hear the ‘bark’ warning call of the deer while in the forest, and one highlight of the field trip so far was coming across a Roe deer fawn on a hike through the mountains. The region is also home to a diversity of plants as well as several species of eagles and vultures that can be spotted ominously circling as we work in the field.

After collecting data in Spain, the rest of my summer will be spent visiting temperate forests of Romania, Poland and Germany. This project hopes to gain a holistic view of the processes of forest systems in Europe and so my data will allow across-country and across-European comparison. You can follow my travels on twitter @harriettui

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About rhulscience

The Faculty of Science at Royal Holloway, University of London
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