Effects of reindeer browsing on tundra willow and its associated insect herbivores

Summary

In northern subarctic Fennoscandia tea-leaved willow Salix phylicifolia is one of the common willow species browsed by reindeer Rangifer tarandus. An objective of this study was to compare the growth and reproductive response of S.phylicifolia to reindeer summer browsing. Growth characteristics inside and outside exclosures 4 and 5 years after establishment were measured. The impact of reindeer feeding on herbivorous insects on willows was also investigated to test the hypothesis that summer browsing would adversely affect both willow and associated insect herbivores.

Study area: The study was undertaken near Mt. Muotkatakkavaara, 20 km south of the village of Kilpisjärvi in north-west Finland. Birch woodland (dominated by mountain birch Betula pubescens ssp. czerepanovii) is the predominant vegetation below the treeline, with heath at higher altitudes. On drier sites, subarctic heath vegetation dominated by dwarf birch Betula nana is frequent. On moister sites thickets of willow Salix spp. scrub dominate, S.phylicifolia is common in areas not subjected to flooding, while downy willow S.lapponum is dominant in areas subject to spring flooding.

Semi-domesticated reindeer are the dominant large herbivores. In winter they mainly feed on lichens, in summer on a variety of vascular plants. The number of reindeer in the study area decreased between 1994 and 1998 from 7,609 to 7,489 (c. 1.6 reindeer/km²). However, the local reindeer owner maintained an above-average stocking rate of approximately 2–3 animals/km². Mountain hare Lepus timidus and moose Alces alces densities during were low.

Folivorous insects: The most common folivorous insects are Gonioctena leaf beetles (Coleoptera: Chrysomelidae). There were also three gall-inducing sawflies (Hymenoptera: Tenthredinidae) on experimental plants: a leaf-folder genus Phyllocolpa, the leaf-galler Eupontania arcticornis and the bud-galler Euura mucronata.

Experimental design: Twenty blocks with four willow genets (new plants that have developed from seed, as opposed to clonal/vegetative reproduction) in each were established in 1997–98 on two sites (2 km apart) of similar habitat type (10 blocks in each). In each block, two randomly chosen genets were cut at ground level in 1997 to stimulate vegetative growth (young), and the remaining two genets were left uncut (old). In each block, one of the young and one of the old willow genets were fenced to prevent large mammal browsing. The fences (commercially available reindeer fence attached to four fence posts) measured 2 × 2 m square and 2 m in height. The fences did not prevent access by insects, small mammals (e.g. Microtus and Clethrionomys voles and lemmings Lemmus lemmus) or willow grouse. Plants that were cut in 1997 started rejuvenating by forming new basal sprouts in 1998, hereafter referred to as rejuvenated). Plants that remained uncut are hereafter referred to as old. In each plot, four ramets belonging to the same genet were selected at random and identified with a tag. All measurements were taken on these ramets throughout the study.

Measurements on effects of reindeer herbivory: Effects of summer browsing were examined from spring 2001 to autumn 2002. Ramets were measured in late August to early September 2001 and 2002 at the end of the growing season to record maximum annual growth. The length of the leading shoots was recorded to quantify the growth of the main stem. By measuring the distance between the internodes, annual shoot lengths back to 1998 were recorded. In 2001, dieback of the annual shoots back to 1998 was measured and in 2002, the previous year's die back was recorded. This enabled reconstruction of the development of the plants from 1997 onwards. The number of shoots, buds per shoot and leaves eaten by reindeer per shoot were counted. Reindeer feed on willow throughout the growing season but browsing was highest in August and had stopped almost completely by the measurement dates.

Insect herbivory: A herbivory index was used to give an estimate of feeding by folivorous insects. Each leaf of the three leading shoots was given an index number based on the proportion of leaf removed by insects, where 0 = no damage, 1 = 0–1%, 2 = 1–5%, 3 = 5–25% and 4 = 25–100% of the leaf area eaten; an average herbivory index number for the whole ramet was calculated. The number of Gonioctena on ramets was counted in 2001. In 2001 and 2002, the number of galls of the three sawfly species (Phyllocolpa spp., E.arcticornis and E. mucronata) were recorded.

In the third week of June 2001 and 2002, the number of catkins on each ramet was counted as a measure of plant reproduction to assess the reproductive response to herbivory.

Reindeer herbivory: Reindeer ate a higher proportion of available leaves from young rejuvenated bushes compared with old bushes. Furthermore, in 2002 the feeding intensity was lower than in 2001. No browsing damage from mountain hare was noted.

Effects of reindeer browsing on willows: The effect of fencing increased over time due to continued willow growth in fenced plots. From 1997 to 2002, reindeer exclusion resulted in a large increase in height of rejuvenated plants, and in 2002 rejuvenated plants reached the same height as the old plants. In contrast, unprotected rejuvenated plants showed a height increase of less than half of that of the fenced plants.

Initially, rejuvenated plants had much longer annual shoots compared with old plants. A large effect of browsing (i.e. in 1998–99) on shoot length levelled off with increasing age of the plants (i.e. in 2000–02). Reindeer browsing resulted in the following: increased shoot dieback, with rejuvenated plants showing much greater dieback compared with old plants; decreased branching; a tendency for rejuvenated willows protected from browsing to have more branches; a marked decrease in the number of buds per shoot (rejuvenated plants had significantly more buds per shoot) in 2001; a significant decrease in the number of catkins per ramet. Old ramets had more catkins compared with rejuvenated ramets and hardly any rejuvenated bushes that were browsed flowered.

Insect herbivory and abundance: There were no differences found in the amount of defoliation by insects between fenced and unfenced blocks or between rejuvenated and old bushes. However, there was considerable variation between different habitats and blocks and from 2001 to 2002 the amount of insect-feeding on willows increased markedly.

In 2001, there were fewer Gonioctena leaf beetles on browsed willows and there was no significant difference between rejuvenated and old bushes. Eupontania galls were much more numerous in 2001 than in 2002 and also decreased strongly through reindeer browsing. The number of Phyllocolpa galls was higher on old willows, whilst browsing reduced their number. Similarly, browsed willows had fewer Euura galls than those in exclosures.

Conclusions: The results of this study indicates that reindeer browsing in summer reduces biomass and diminishes reproductive success of willow. It also lowers the numbers of its associated insect herbivores. This effect is likely to be most evident in low-productivity tundra heaths where alternative forage plants, such as relatively palatable and productive graminoids, are scarce. The authors advise that reindeer should be maintained below the present levels of 2–3 reindeer/km² to sustain the long-term persistence of important forage plants such as willow in these low productivity habitats.


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