par values and their evaluation
The basis for the use of the PAR as a proxy for past population size is the assumption that the PAR of a given pollen type is positively correlated with the size of the population of the plant taxa around the study site. In the case of P. abies, this assumption can be tested by comparing modern PAR and modern biomass values from eight sites in northern Europe (Table 1). At sites located outside the current range of P. abies the modern P. abies PAR is about 5–30 grains cm−2 year−1. This agrees with the find of 50 P. abies grains cm−2 year−1 coinciding with the first firm evidence for the presence of P. abies near (within few km) a site (Giesecke 2005). In the study sites located within the range of P. abies in the southern boreal zone, the modern PAR value ranges from about 610 to 790 cm−2 year−1 (Table 1). Although the number of observations is too low to estimate statistically the correlation between the modern volume and PAR values, it appears that the presence of sparse or dense P. abies forest can be reliably identified from the PAR data and that the modern P. abies PAR values are generally larger with larger modern P. abies biomass. Values above 50 grains cm−2 year−1 reflect the local presence of a sparse P. abies population and values above 500 grains cm−2 year−1 are typical in the P. abies- and P. sylvestris-dominated forest, where the P. abies above-ground biomass usually varies between 20 and 50 t ha−1.
The Holocene P. abies PAR records are shown in Fig. 2. The results reflect the east-to-west spread of P. abies, indicating an approximate time difference of 4000 years between the local establishment at Lake Kirkkolampi in eastern Finland and at Lake Klotjärnen in Sweden and an average spreading rate of 0.2 km year−1 from eastern to western Finland. The rise of PAR values was abrupt at Lakes Orijärvi, Nautajärvi and Klotjärnen and more gradual at Lakes Kirkkolampi and Laihalampi. At all Finnish sites the period of highest PAR values, and thus the period of highest biomass, dates roughly to 4000–1500 cal. years BP, when P. abies PAR at Lakes Kirkkolampi, Orijärvi and Nautajärvi is consistently 1400–1600 grains cm−2 year−1. Lake Laihalampi has PAR values of over 2000 grains cm−2 year−1, probably because local edaphic conditions are more favourable for P. abies and because the proportion of lakes and bogs in the vicinity of the lake is smaller than at other sites. At Lake Klotjärnen the average PAR value at 2500–1500 cal. years BP is about 1500 grains cm−2 year−1, which is consistent with the majority of the Finnish sites.
Figure 2. The general PAR values of Picea abies at the five study sites, indicated on the right-hand side of the y-axis. The sites are arranged along the direction of the spread from the right (east) to the left (west). The PAR values of Tilia cordata are shown on the left-hand side of the y-axis to reflect its population dynamics during and after the colonization process of P. abies in Fennoscandia. The arrows indicate when the P. abies PAR reached the value comparable with the modern at each site and the light and dark vertical lines point to the periods of maximum P. abies and T. cordata populations. Two curves indicating the Holocene climate trends are shown on the left. The NGRIP δ18O record is derived from an ice core in central Greenland and indicates a general temperature pattern in the North Atlantic region (Johnsen et al. 2001). Other temperature records confirm that this curve in general is valid for northern Europe as well. The southern Swedish δ18O record is obtained from isotopic composition of a calcareous lake sediment core and reflects main trends both in temperature and hydrological conditions (Hammarlund et al. 2003; Seppäet al. 2005). HTM denotes the approximate end of the warm, dry and relatively stable mid-Holocene period in northern Europe.
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All PAR records show a marked declining trend during the late Holocene, starting at Lake Kirkkolampi 1700 cal. years BP and at the other sites about 1000 cal. years BP. This decline coincides with the beginning of intense agriculture and forest clearance, especially in the form of slash-and-burn cultivation in the boreal zone of Scandinavia (Nilsson 1997; Lindbladh et al. 2000; Pitkänen et al. 2002; Seppäet al. 2009). The records suggest that these new land-use practices particularly impacted the fire-intolerant P. abies (Pitkänen et al. 2002; Vanha-Majamaa et al. 2007; Seppäet al. 2009). The P. abies PAR values fell below 600 cm−2 year−1 at each site 600 cal. years BP, suggesting the smallest regional P. abies population. A slight recovery has taken place at most sites during the last 100–150 years, apparently due to reforestation, cessation of slash-and-burn cultivation and decreased fire frequency.
picea colonization and population growth
Once the relationship between the P. abies PAR and biomass values has been approximately established, it can be used to reconstruct past population and forest community dynamics during the colonization and subsequent population growth of P. abies in Europe (Fig. 3). Detailed PAR records from the lakes unambiguously reflect the site-specific patterns of the colonization. Picea abies colonized the easternmost site, Lake Kirkkolampi, 6600 cal. years BP. The modern PAR level, about 700 grains cm−1 year−1 , was reached 6050 cal. years BP, but was followed by a transient decline starting 5800–5500 cal. years BP. The maximum PAR level, about 1300–2000 grains cm−2 year−1, was attained 4800 cal. years BP, 1800 years after the initial colonization (Fig. 2). At the second easternmost site, Lake Orijärvi, colonization started 6100 cal. years BP and was followed by rapid population growth. By 6000 cal. years BP, within 100 years, the P. abies PAR values had grown to about 600 grains cm−2 year−1, corresponding to the modern P. abies PAR in the vicinity of the lake (Fig. 3). The third site, Lake Laihalampi, 57 km west of Lake Orijärvi, was colonized by P. abies 6050 cal. years BP, but the modern PAR level of about 800 grains cm−2 year−1 was attained 450 years later, thus suggesting slower population growth here. At the fourth site in Finland, Lake Nautajärvi, 67 km west of Lake Laihalampi, P. abies immigrated 5400 cal. years BP, and attained a PAR value comparable to the modern (700 cm−2 year−1) in 200 years, followed by a slower population growth and a PAR level of about 1000 grains cm−2 year−1 about 4800 cal. years BP. At Lake Klotjärnen in Sweden, P. abies colonized 2700 cal. years BP, followed by rapid population growth. The modern population size was reached within 300 years, thus corresponding roughly with the growth rate recorded at Lake Nautajärvi in Finland.
Figure 3. The PAR records as indicators of forest population dynamics during and after the Picea abies colonization at the five study sites. The stippled line indicates the modern P. abies PAR value at each site and the light-blue columns reflect the period from the colonization (the onset of the rise of the PAR curve) to the modern level.
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PAR records also provide evidence of the resident forest composition and structure before and during P. abies colonization. All sites show relatively comparable patterns of species composition and their PAR values. The dominant tree taxa at all sites during the colonization were Pinus, Betula and Alnus, while Tilia and Corylus were much more common than at present (Fig. 3). The PAR values of all these taxa are stable and high before the onset of the colonization. Along with the lack of peaks in the charcoal records (Fig. 3) and pollen from species that would indicate opening of the vegetation, such as Poaceae (see Heikkilä & Seppä 2003; Alenius et al. 2007), this suggests that P. abies invaded a dense, closed-canopy Pinus–Betula–Alnus–Tilia–Corylus forest without any large-scale disturbance-related openings during the colonization. Once established, P. abies populations at some sites grew rapidly, apparently without constrains on population growth, whereas at other sites population growth was a slower process constrained either by internal or external factors. We can only speculate about the reasons for the slower and more fluctuating population growth at some sites. The composition of the resident forest cannot be the key factor because it was remarkably similar at all sites. The reason may be related to biological factors, such as variable intensity and frequency of release events or small disturbances that may be necessary for the seedlings of the invasive species to recruit into the canopy (Martin & Marks 2006), but it may also be related to changes in the physical environment such as changing soil moisture, paludification and peatland expansion in the vicinity of the lakes which can cause episodic decreases and expansions of the local P. abies populations.
When a new tree species invades an intact closed-canopy forest, competition for space and resources will inevitably lead to changes in the resident plant community (Davis et al. 1998; Bradshaw & Lindbladh 2005). The impact the invasion of P. abies had on the structure and composition of the mid-Holocene mixed forest can be assessed by examining the PAR values of the resident tree taxa during and after the invasion. All five detailed records unambiguously demonstrate that the tree species that most clearly declined was T. cordata (Figs 2–4). In Finland the T. cordata populations started to decline abruptly or more gradually about 200 years after the onset of the population growth of P. abies, when the size of the P. abies population around the lakes was comparable to the modern. This decline was particularly dramatic at Lake Orijärvi, where T. cordata attained a PAR value of about 400 grains cm−2 year−1, suggesting that it was an abundant, possibly dominant tree species around the lake before the P. abies invasion. At all four sites T. cordata PAR values remained at about 50–100 grains cm−2 year−1 after the rise of P. abies, until a gradual decrease towards the present trace values during the last 3000–4000 years (Fig. 2).
Figure 4. The cross-correlations showing the influence of the colonization and population growth of Picea abies on the population dynamics of the most important tree taxa of the resident forest. The analyses were carried out with 50-year time lags before and after the P. abies colonization at each site. The y-axis indicates the correlation coefficient between the P. abies PAR and the each species. The solid horizontal lines show the limits of statistically significant values.
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The westernmost record from Lake Klotjärnen in Sweden is remarkable because there the T. cordata population started to decline 4000 cal. years BP, about 1300 years before the immigration of P. abies (Fig. 2). Hence, this decline was not caused by interspecific competition with P. abies but by other internal or external factors. The T. cordata population around the lake was near to the northern distribution limit of the species during the mid-Holocene (Giesecke 2005) and the likely reason for the decline was a climatic cooling that started about 4000 cal. years BP in northern Europe (Hammarlund et al. 2003; Seppäet al. 2005) (Fig. 2). However, after the decrease the T. cordata PAR value remained at 70–100 grains cm−2 year−1, corresponding to the modern PAR value of T. cordata recorded at the northern part of the temperate zone where it is present (Eide et al. 2006). Thus, the cooling that started about 4000 cal. years BP reduced the local T. cordata population but the species remained present as scattered stands at favourable sites in the region, in a way comparable to the present northernmost scattered T. cordata stands, until a total disappearance when P. abies invaded the region 2700 cal. years BP.
The results of the cross-correlation analysis show that the other main tree taxa show a less clear population decline during the P. abies colonization (Fig. 4). Pinus sylvestris shows no or little change during the P. abies colonization and population growth, probably because P. sylvestris predominantly grows in drier and more nutrient-poor soils than P. abies, thus avoiding major competitive exclusion by the more shade-tolerant P. abies. Betula has been observed to have declined in central Sweden as a result of P. abies invasion (Giesecke 2005). This is supported by the results of the cross-correlation analysis, showing that the reduction of Betula PAR with the rise of P. abies was significant at Lakes Kirkkolampi, Orijärvi and Klotjärnen. Corylus avellana and Ulmus also indicate significant negative correlations with P. abies at most sites, although with a varying time offset before and after the rise of P. abies. In contrast, the population of Alnus was least affected by the P. abies invasion. This feature, also observed in central Sweden (Giesecke, 2005), is unexpected because both Alnus species, A. incana and A. glutinosa, are relatively light-demanding and have overlapping soil requirements with P. abies (Dahl 1998).
The positive and negative time lags in the cross-correlation analysis can be interpreted in different ways. A positive lag will be seen if the P. abies population starts to expand in a particular habitat and slowly moves into locations with different soil and light conditions, thereby affecting some species later than others. A negative lag indicates that a species declined before the expansion of P. abies had started. Thus environmental conditions such as climate or disturbance regime may have lead to the decline of one species and possibly facilitated the expansion of P. abies. Another interpretation is that the expansion of a P. abies population at some distance from the site led to competitive replacement by a species with a pollen type that is more easily airborne (e.g. Betula) than the heavy P. abies pollen. The site would therefore receive less pollen from long-distance sources before P. abies was established at the site and had started flowering.