As ever, choosing the manuscript for the Editor’s Choice is a difficult task given the wide range of interesting and novel papers that appear in this issue of Journal of Ecology. However, I have decided to select as my Editor’s Choice a paper by Heikki Seppä and colleagues on the Invasion of Norway spruce (Picea abies) and the rise of the boreal ecosystem in Fennoscandia.
Ecologists have long been fascinated by the study of vegetation change, and as a result there is a rich historical literature which documents how and why plant communities change over time, and how plant species can spread into new territories and become dominant. The last few years, however, have witnessed a resurgence of interest in this topic, which is largely due to the need to better predict how climate change-induced plant range shifts and species invasions influence the composition and functioning of terrestrial ecosystems. Indeed, there is mounting evidence that recent climate change is already causing dramatic shifts in both the composition and range of major terrestrial ecosystems worldwide, often with far-reaching consequences for their function and biogeochemical cycles. Despite the potential for recent climate change to influence plant species ranges, most plant range shifts occur slowly, over timescales of decades to centuries; the study of their ecological consequences therefore demands a long-term, retrospective approach.
In their paper, Seppä and colleagues used such a long-term approach to explore the ecological changes associated with one of the most dramatic alterations in vegetation that have been recorded in northern Europe over the last 7000 years, namely the invasion of Norway spruce (Picea abies) in the modern boreal zone of Europe. From its late-glacial and early-Holocene range in northern Russia, Norway spruce spread westwards through Finland, Sweden and into south-western Norway. This spread and dramatic rise in dominance of Norway spruce therefore provided an excellent opportunity to investigate the colonization and population growth patterns, and subsequent ecological changes associated with invasion by a competitive tree species on centennial to millennial time-scales. This investigation was carried out by analysing pollen accumulation rate (PAR) records (an estimate of the accumulation of pollen at the sediment surface per year) from sediment cores taken from five lakes along a 700 km long transect in the direction of Norway spruce invasion from eastern Finland to central Sweden, crossing the present southern boreal forest zone.
The analysis of PAR records revealed that at all five sites, Norway spruce invaded into dense, mixed forest composed of birch (Betula sp.), Scots pine (Pinus sylvestris) and alder (Alnus glutinosa), along with other temperate deciduous species such as small-leaved lime (Tilia cordata) and hazel (Corylus avellana). Although the rate of population growth of Norway spruce after initial invasion differed across sites, the PAR records suggest that the resident species-rich forest put up little resistance to invasion. Importantly, this finding goes against the long held view that invasive tree species do not easily spread into closed-canopy forest without disturbance (Elton 1958; Pearsall 1959), and suggests that shade-tolerant trees such as Norway spruce can invade dense, intact forest with relative ease. The other important finding relates to changes in resident forest species after invasion by Norway spruce, and especially the marked decline of small-leaved lime, which was previously a major component of the forest. Seppä and colleagues attributed this decline in small-leaved lime, and its continued rarity during the Holocene, to competitive suppression; in contrast to Norway spruce, small-leaved lime is shade-intolerant towards the northern part of its range.
As highlighted by the authors, their findings have two important implications. First, they suggest that the present-day patchy occurrence and poor performance of small-leaved lime in central and southern Scandinavia – which is generally attributed to human interference – pre-dates human influence and dates back to the initial invasion of and competitive exclusion by Norway spruce. Second, their findings raise doubt over current predictions that climate warming will promote the northward spread of temperate deciduous tree species, including small-leaved lime. On the contrary, they argue that while small-leaved lime might expand its range northwards, this is unlikely to occur in Norway spruce-dominated regions because of its poor performance in these situations. They argue that only in the event of a large-scale decline in the dominance of Norway spruce, for instance caused by increased summer drought under climate change, would small-leaved lime be able to spread northwards into the boreal zone.
The world and its ecosystems are changing fast, and perhaps the most important message of this study is that an understanding of the far past holds many clues about how global change might influence the distribution of species in the future.
Editor, Journal of Ecology
- Elton, C.S. (1958) The ecology of invasions by animals and plants. Methuen, London.
- Pearsall, W.H. (1959) The ecology of invasion: ecological stability and instability. New Biologist, 29, 95-101.
- Seppä, H., Alenius, T., Bradshaw, R.H.W., Giesecke, T., Heikkilä, M. & Muukkonen, P. (2009) Invasion of Norway spruce (Picea abies) and the rise of the boreal ecosystem in Fennoscandia. Journal of Ecology, 97, 629–640.
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