The original ideas and stimulus for the study came up from observations of heavy ptarmigan browsing on tundra willows at several arctic sites (H. Roininen and R. Virtanen during the Swedish-Russian tundra expedition to arctic Eurasia in the mid 1990s). These observations were at odds with the dominant idea that primarily abiotic stresses constrained tundra willow abundance and growth. We found it necessary to establish ptarmigan exclusion experiments to measure the browser impacts on tundra willows.
We thus established experiments at a tundra landscape in NW Finnish Lapland close to Kilpisjärvi Biological Station. By setting up some pilot experiments, we soon realized that reindeer summer browsing was pretty strong on willows, and this had to be considered too. Because many willow genets in that area had stunted growth-form due to browsing, we thought it would be of interest to rejuvenate willows by initially cutting a subset of experimental genets, and let them develop vigorously growing ramets without or with browsing. The eventual experimental design thus included small mesh cages excluding ptarmigan and reindeer and large mesh allowing ptarmigan browsing but excluding reindeer, and these treatments for both rejuvenated and old genets. Indeed, our initial interest and experimental design was set up to find out how ptarmigan and reindeer effects on willow growth and survival of willows and insect herbivores.
In the first years of the experiment, reindeer effects on willow shrubs were pretty strong and have been published earlier (den Herder et al. 2004 [1]). Because of rather slow growth of willows (just c. 2-3 cm /year) the intention was to continue monitoring over several years. In that region, autumnal moth outbreaks had not been seen for decades and we did not expect them to happen. However, a couple of years after the initiation of the experiment things changed: the first autumnal moth (Epirrita autumnata) outbreak took place in 2004-2005 (as most intense) which caused virtually complete defoliations of willow (and nearby mountain birch forests), and obviously negative effects on growth. The second outbreak came in 2011-2013 (a bit less severe than the first) with nearly as strong effects on willow as during the first outbreak. Some background insect herbivory occurred every year, and there were marked among-year variations in key climate variables thought to impact willow growth. We thought that the long-term experimental monitoring data would allow us to look at how insect herbivory, summer thermal and precipitation conditions influence annual shoot growth and even how these depend on browsing pressure.
The analyses of relations between annual shoot growth increments, climatic and biotic drivers turned out to be challenging, and we found empirical dynamic modelling (EDM) to be the most powerful approach for the purpose. The EDM analyses on the 20-year time series showed that negative effects of insect herbivory on shoot growth became stronger in warmer summers, whereas precipitation effects tended to become less negative effects with increasing precipitation, and this latter effect seems to moderate warming effect. These effects further depended on browsing pressure and ramet age.
The results establish that local growth dynamics of willows at a forest tundra ecotone is impacted by multiple herbivores that moderate growth responses to climate. The research continues to look the long-term cumulative impacts of treatments, and also the effects of changes in reindeer management, on the survival and growth of willows.
You can find the full text here.
Reference: Virtanen, R., Clark, A.T., den Herder, M., Roininen, H. (2020) Dynamic effects of insect herbivory and climate on tundra shrub growth: roles of browsing and ramet age. Journal of Ecology 109(3):1250-1262
https://doi.org/10.1111/1365-2745.13551
[1] den Herder, M., Virtanen, R., & Roininen, H. (2004). Effects of reindeer browsing on tundra willow and its associated insect herbivores. Journal of Applied Ecology, 41(5), 870-879. [link to publisher]
Text by Risto Virtanen, University of Oulu
Picture: An experimental exclosure with reindeer fence and a willow genet (Salix phylicifolia) inside the fence (taken in 2019 N of Lake Haukijärvi, tundra like habitat with other species Betula nana, Salix glauca, and close to Betula pubescens ssp. pumila treeline) (photo: Risto Virtanen, University of Oulu, 2020)