DOI: 10.5061/dryad.wdbrv15mp
title: Dynamic effects of insect herbivory and climate on tundra shrub growth: roles of browsing and ramet age

description: 1. To predict shrub responses under climate change in tundra, we need to
 understand how thermal conditions and herbivory contribute to growth. We
 hypothesise that shrub growth increases with thermal conditions and
 precipitation, but that this increase is counteracted by insect herbivory,
 and that these climate-insect herbivory relationships are modified by both
 browsing and plant age. 2. We use empirical dynamic modelling (EDM) to
 analyse a 20-year time series on willow (Salix phylicifolia) shoot growth,
 growing degree days, summer precipitation and herbivory from an experiment
 at forest-tundra ecotone. The experiment includes manipulations of avian
 and mammal browsing (fences) and ramet age (pruning to rejuvenate
 willows). 3. Negative effects of insect herbivory on willow shoot growth
 were intensified during warmer years, whereas increasing precipitation led
 to reduced effects. Moreover, the effect of insect herbivores on shoot
 growth varied with ramet age and vertebrate browsing: Younger ramets
 generally experienced less negative insect herbivore effects, whereas
 Ptarmigan browsing was associated with more positive temperature effect on
 shoot growth, and reindeer browsing with more negative effects of insect
 herbivory and precipitation. 4. Synthesis. Our findings show that the
 negative effects of insect herbivory on shoot growth likely intensify
 under warmer thermal conditions, but that increasing precipitation can
 counteract these effects. Moreover, changing thermal conditions,
 precipitation and vertebrate browsers all have predictable, albeit complex
 and nonlinear, effects on shrub growth, highlighting the importance of
 long-term experimental data and flexible analytical methods such as EDM
 for characterizing climate and community interactions in artic systems.
description: The study area is located in NW Finnish Lapland at a transitional
 area between boreal mountain birch forests and treeless oroarctic and
 arctic tundra heaths at higher elevations (Virtanen et
 al.<i>,</i> 2016). The study site lies at an elevation of
 550–570 m a.s.l. (68.92° N, 20.97° E). In total, 40 willow genets in two
 forest-tundra ecotone sites were selected for the experiment in 1997. In
 these two sites lying c. 800 m apart, the genets were selected within
 areas of c. 50 × 200 m and 50 × 250 m. Genets were 30–50 cm tall and
 consisted of 10–25 ramets each. All genets were initially accessible to
 both reindeer and ptarmigan. The genets were then assigned into blocks of
 four genets. In 1997–1998, two random genets within each block were
 rejuvenated by pruning ramets five cm above the ground level. The pruned
 genets produced sprouts that initially grew vigorously for 1–3 years and
 within a few years reached the size of unpruned genets. Thus, each block
 consisted of two old genets and two rejuvenated genets. Within each block
 one of the old and rejuvenated genets were protected by reindeer fences
 (2×2 m area). In 2000, reindeer fences were replaced by ptarmigan proof
 fences in half of the blocks, selected at random. Final treatments
 therefore included rejuvination ("J"), reindeer browsing
 ("R"), and ptarmagin browsing ("P").
description: Data are in .csv format, with decimal points indicated by
 ".". "GDD5MJJ" shows summed daily mean temperature in
 excess of + 5 °C in May–July. "PJune" and "PJuly" show
 summed annual  precipitation in June and July, in mm. Lastly, columns
 "s1.cm" through "s11.cm" show shoot lengths -- the
 mean taken across these columns was the primary response variable in our
 paper. Finally, the ".R" file includes the source code used to
 analyse these data.