Gap dynamics in a near-natural spruce forest at Mt. Brocken, Germany
Introduction
Ongoing shifts of species ranges (Thuiller, 2007) and increasing rates of population dynamics, apparent in changes of species abundances (Parmesan and Yohe, 2004) or increasing extinction risks of populations (Keith et al., 2008), as an effect of global climate change should be mirrored in increasing rates in aspects of vegetation dynamics, e.g. structural changes (Theurillat and Guisan, 2001). However, very few studies have addressed temporal trends (but see Walther et al., 2005), and in particular not for long-lived forest vegetation as long-term observations are required for this analysis (Runkle, 1998, Runkle, 2000, Woods, 2000, Woods, 2007, Fraver et al., 2008). In addition, studies of forest gap dynamics, i.e. all processes of gap formation and closure, require mature forests where human influence is absent or negligible. Information on such old-growth forests is available from tropical rainforests to boreal spruce forests (see the review of McCarthy, 2001), but studies in Central Europe are rare, as very few forest stands can be considered natural or near-natural (Kuuluvainen, 2002, Angelstam, 1998). One of these rare examples of such old-growth forests are the Picea abies stands of Mt. Brocken, Germany. These stands are characterized by harsh climatic conditions (Glässer, 1994) and low soil pH values, resulting in low growth rates. Therefore, lower gap closure rates would be expected compared to other temperate forests with regard to long-term mean values.
However, with regard to the last decades and therefore to short-term values, gap formation might have increased in these high-montane forests as a result of climate change (Schumacher and Bugmann, 2006), involving both direct and indirect impacts. It will be the ecosystems at high latitudes and at high altitude that are the most susceptible to global change (Thuiller, 2007, Gehrig-Fasel et al., 2007). A first reason for a change in the disturbance regime might be an increase of mean temperatures. The globally observed increase in temperature over the last 150 yrs (Solomon et al., 2007) is also apparent for Mt. Brocken (Deutscher Wetterdienst). However, climate change does not only imply a change in mean climatic values, but also in frequency and magnitude of extreme weather events as another direct effect of climate change. This is already evident in daily precipitation and temperature data from 1958 to 2001 in Germany, which show that the minimum and maximum temperatures as well as magnitude and frequency of heavy precipitation events have increased in all seasons except summer (Hundecha and Bárdossy, 2005). The data also conform on the one hand to regional climate models forecasting an increase in precipitation extremes and higher temperatures on warm days (Kjellström, 2004), on the other hand to the predicted scenario of increasing summer drought for large areas of Europe north of the Alps (Schlyter et al., 2006).
Secondly, in combination with increased mean temperatures, climate extremes will most likely result in a predisposition of trees to pests and pathogen infection (Rebetz et al., 2006). These indirect effects of climate change would contribute to the overall disturbance regime (Schlyter et al., 2006). Especially for coniferous forests, an increase in summer temperatures promotes the abundance of bark beetles (Ips typographus L.; Schlyter et al., 2006, Wermelinger, 2004), which represents a major biological cause for disturbance in European spruce forests (Schelhaas et al., 2003).
The third main driver of natural forest disturbances in Europe is wind. One recent example was the winter storm Kyrill, which in January 2007 caused a timber damage of 26.5 million m3 in Germany (BMELV, 2008). Storms and gales cause direct effects by windthrow and windbreak, and thus, enhance rates of gap formation and gap expansion (Worrall et al., 2005, Splechtna et al., 2005).
However, gap dynamics are normally analyzed in long-term monitoring studies (Woods, 2000, Woods, 2007, Harcombe et al., 2002; see also the review by Bekker et al., 2007), which are essential for detecting relationships with climate. Little attention has been paid to effects of different time scales on the assessment of gap dynamic indices (gap formation rate, gap closure rate, turnover rate), i.e. of short observation periods within a long-term interval. We expect that comparing different temporal scales can provide insight into how tightly gap forming and gap closing processes are coupled.
Similarly, little is known about the influence of spatial scales of disturbances, i.e. the varying spatial extent of gaps due to mortality of single trees or of cohorts of individuals, on gap formation and gap closure rates. Although the size, the number and spatial distribution of gaps have been studied extensively (see the overview by McCarthy, 2001), research has largely been concentrated on broad-scale disturbances (overview given in Turner and Dale, 1998). Although gap sizes vary over a wide range of magnitudes, they show a certain dependence on the type of disturbance (Spies and Franklin, 1989). While windthrow can create gap patterns of different sizes depending on the disturbance intensity (Greenberg and McNab, 1998, Harcombe et al., 2002) and may cause the stepwise gap expansion due to the death of trees on the gap edge (Foster and Reiners, 1986), the age-related individual death events of canopy trees in an uneven-aged forest stand will result in small gap sizes (Stöcker, 2002, Wegener et al., 2003, Splechtna et al., 2005). Conversely, we expect that differences in gap size distribution among the different time intervals might provide hints as to the nature of the disturbance agent.
We tested our expectations in a near-natural montane spruce forest at Mt. Brocken. In particular, we hypothesised that the turnover rate and, consequently, gap formation and gap closure rates (1) are lower in our study area than in other temperate forests, (2) have increased in the last decade, and (3) are related to climate change. We also analyzed the impact of temporal and spatial resolution of our dataset to gain insight into the underlying mechanisms of gap formation and gap closure.
Section snippets
Study area
The study area is located in the centre of Germany on the north-eastern slope of Mt. Brocken (Saxony-Anhalt, Germany, 10°37′15″E, 51°48′6″N, 900–1050 m a.s.l.) in the core zone of the Harz National Park. The study area covers 225.2 ha of a near-natural spruce forest. According to Stöcker (1997) the structure of this forest is comparable to virgin forests known from the boreal zone. The tree line is very low for this latitude (1100 m a.s.l.), which is caused by strong winds at the peak (1142 m
Results
General characteristics and the temporal development of gap area are shown in Table 2. These data reflect a high variability in gap area between the different years but they do not show a clear trend with time.
Are the characteristics of gap dynamics comparable to those of other temperate forests?
In contrast to our first hypothesis, we encountered gap formation and gap closure rates of considerable magnitude, which were in the range of other studies comprising evergreen, broad-leaved, deciduous and conifer forests (see Table 4 and the overview given by McCarthy, 2001). For example, the area-based gap formation rate of 0.1% yr−1, which included interim transitions and was calculated for our long-term observation period, is perfectly within the 0.1–0.2% yr−1 range reported by Holeksa and
Conclusion
In summary, indices of gap dynamics are of comparable magnitude to other temperate forests. Despite possible confounding effects of different length of time intervals between aerial photographs, indices of gap dynamics increased with time, especially in the last few decades. Combining these findings with climate data, temperature-related climate change effects might be the main drivers of forest gap formation and turnover rates at Mt. Brocken.
Acknowledgments
We are very grateful to the team of the Harz National Park for providing aerial photographs and for the permission to work in the core zone. The help of Daniel McCluskey with the final linguistic check is greatly acknowledged. The manuscript was much improved by comments from three anonymous referees. This project was funded by a grant from ESRI free GIS scholarship, the graduated scholarship of Saxony-Anhalt as well as the German Research Foundation DFG (BR 1698/6).
References (77)
- et al.
A gap-based approach for development of silvicultural systems to address ecosystem management objectives
For. Ecol. Manage.
(1997) - et al.
Forest disturbance in hurricane-related downbursts in the Appalachian mountains of North Carolina
For. Ecol. Manage.
(1998) - et al.
Canopy height changes of an old-growth evergreen broad-leaved forest analyzed with digital elevation models
For. Ecol. Manage.
(2004) - et al.
Impacts of climate change on the population dynamics of Ips typographus in southern Sweden
Agric. For. Meteorol.
(2007) - et al.
The response of the pseudoannual species Trientalis europaea L. to forest gap dynamics in a near-natural spruce forest
For. Ecol. Manage.
(2009) - et al.
Response of Picea abies populations from elevational transects in the Polish Sudety and Carpathian mountains to simulated drought stress
For. Ecol. Manage.
(2002) - et al.
Norway spruce (Picea abies): Bayesian analysis of the relationship between temperature and bud burst
Agric. For. Meteorol.
(2008) - et al.
Impact of bark beetle (Ips typographus L.) disturbance on timber production and carbon sequestration in different management strategies under climate change
For. Ecol. Manage.
(2008) The effects of windthrow on forests at different spatial scales: a review
For. Ecol. Manage.
(2000)Ecology and management of the spruce bark beetle Ips typographus – a review of recent research
For. Ecol. Manage.
(2004)
Natural disturbance and patch dynamics: an introduction
Maintaining and restoring biodiversity in European boreal forests by developing natural disturbance regimes
J. Veg. Sci.
Gap disturbances in northern old-growth forests of British Columbia, Canada
J. Veg. Sci.
Spruce decline as a disturbance event in the subalpine forests of the northeastern United States
Can. J. For. Res.
Long-term datasets: from descriptive to predictive data using ecoinformatics
J. Veg. Sci.
Gap-phase regeneration in a tropical forest
Ecology
Wasseraufnahme und artspezifische hydraulische Eigenschaften der Feinwurzeln von Buche, Eiche und Fichte: In situ-Messungen an Altbäumen
Berichte des Forschungszentzums Waldökosysteme – Reihe A
Size distribution and expansion of canopy gaps in a northern Appalachian spruce-fir forest
Vegetatio
Demographics and disturbance history of a boreal old-growth Picea abies forest
J. Veg. Sci.
Long-term canopy dynamics analysed by aerial photographs in a temperate old-growth evergreen broad-leaved forest
J. Ecol.
Tree line shifts in the Swiss Alps: climate change or land abandonment?
J. Veg. Sci.
Das Klima des Harzes
Stand dynamics over 18 years in a southern mixed hardwood forest, Texas, USA
J. Ecol.
Progress in the study of climate extremes in northern and central Europe
Clim. Chang.
Long-term canopy dynamics in a large area of temperate old-growth beech (Fagus crenata) forest: analysis by aerial photographs and digital elevation models
J. Ecol.
Long-term canopy dynamics analyzed by aerial photographs and digital elevation data in a subalpine old-growth coniferous forest
Ecoscience
50 years of change in a Swedish boreal old-growth Picea abies forest
J. Veg. Sci.
Canopy gaps in a Carpathian subalpine spruce forest
Forstwiss. Cent.bl.
Trends in daily precipitation and temperature extremes across western Germany in the second half of the 20th century
Int. J. Climatol.
Natural disturbance and gap dynamics in a Swedish boreal forest
Predicting extinction risks under climate change: coupling stochastic population models with dynamic bioclimatic habitat models
Biol. Lett.
Recent and future signatures of climate change in Europe
Ambio
Canopy gap characteristics and tree replacement in the southeastern boreal forest
Ecology
Forest decline in Europe: development and possible causes
Water Air Soil Pollut.
Disturbance dynamics in boreal forests: defining the ecological basis of restoration and management of biodiversity
Silva Fenn.
Gap-phase structure of a subalpine old-growth forest
Can. J. For. Res.
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