Introduction
Climatic constraints limit annual growth of woody plants in cold high-latitude environments (Havström et al . 1993; Myers-Smithet al . 2015; Weijers et al . 2018). The growth of tundra shrubs positively correlates with warmer summer thermal conditions (Pajunen 2009; Forbes et al . 2010; Li et al . 2016; Ackerman et al . 2018), implying that under warming conditions shrub growth will increase (Myers-Smith et al . 2015). On the other hand, broad-scale insect outbreaks causing defoliations of most deciduous woody plants are becoming more frequent (Jepsen et al . 2008; Tenow et al . 2004; Lund et al . 2017; Svenssonet al . 2018). Defoliations have been associated with reductions in growth (Crawley 1983, 1989; Hoogesteger & Karlsson 1992; Takahashi & Huntly 2010; Wilmking et al . 2012). The browsing of vertebrate herbivores also reduces woody plant growth in various environments (Post & Pedersen 2008; Newton et al . 2014, Vuorinen et al . 2020). At high latitudes, reindeer browsing has been found to reduce flowering and survival of entire ramets (i.e. a clonal colony or a group of genetically identical plants) or their parts (den Herder et al . 2004; Hakkarainen et al . 2007; Ravolainen et al . 2014) and long-term browsing reduces the abundance of willows (Pajunenet al . 2008, 2012, Kitti et al . 2009; Ravolainen et al . 2011). In addition, browsing by willow ptarmigan has been found to increase shoot dieback and constrain ramet growth (Hakkarainen et al . 2007; den Herder et al . 2008; Tape et al . 2010; Christie et al . 2014).
The effects of climatic factors and herbivory on growth dynamics of tundra shrubs is of growing research interest (Christie et al . 2015; Martin et al . 2017). Here a key unresolved question concerns teasing apart dynamic effects of herbivory and warming climate on willow growth. While there is experimental and observational evidence that warmer conditions are associated with higher levels of herbivory (Roy et al . 2004; Birkemoe et al . 2016; Barrio et al . 2017; Lehmann et al . 2020), it is infrequently documented that herbivory can counteract positive climate effects on plant growth (but see O’Connor 2009; Vuorinen et al . 2020). Such possible antagonistic effects of climate warming and herbivory on plant growth are further complicated by the moderating effects of plant age or browsing pressure affecting meristem activity and growth (Wijk 1986; Massad 2013). Thus, it is important when investigating the dynamic effects of insect herbivores and climate on plant growth, it is also necessary to consider plant age and browsing pressure.
The current pressing questions concern shrub growth under climatic and non-climatic drivers (Myers-Smith et al . 2015). In this study we conduct a model-based analysis of long-term shoot growth of a tundra willow, and test how it is linked to variation in thermal conditions, insect herbivory, vertebrate browsing and plant age. In particular, we hypothesized that higher temperatures in the growing season would promote shoot growth, while intensifying insect herbivory would counteract the effects of warming conditions on shoot growth. We further expected that plant age and vertebrate browsing could moderate both climatic and insect herbivore effects on shoot growth. We quantified the joint dynamic effects of insect herbivory and thermal conditions on shoot growth using empirical dynamic modelling (EDM), a recently developed method which can quantify the strength and direction of interactions in observational time series data, even when these interactions vary through time. Using these techniques, we succeeded in identifying dynamic effects of climate and herbivory on plant shoot growth in an 18-year time-series data from a browser-exclusion experiment in NW Finnish Lapland. Moreover, our analyses reveal that plant-herbivore interactions vary greatly with ecological context, including differences among experimental treatments manipulating browsing pressure (fences) and plant age (rejuvenation by pruning treatment).