Dendrochronological studies of pear trees are extremely rare. Neri
Pear trees are light-demanding, preferring xerothermic sites, rarely forming large local populations (Hofmann, 1993; Paganová, 2003). It is no problem to find single specimens or groups of 2–3 pear trees on grassy field borders, in hedges or on roadsides, but it is difficult to find local populations composed of several dozen or more specimens. The wild pear population in the military area in Biedrusko is most probably its largest Polish population, estimated at 1000 trees.
This study was aimed to assess the age of this population, and tree growth rate, to identify major climatic factors affecting tree-ring width in this species and pointer years characteristic of this population, and to analyse their relationships with meteorological conditions.
The military training area in Biedrusko was established by Prussians in 1901. In 1940, the Nazis modernized the training grounds and expanded them to include 5 villages. After the 2nd World War, The Polish authorities preserved the expanded military area. The villages were depopulated and most of the farm buildings were destroyed, but some components of the former rural cultural landscape in this area are still preserved after over 110 years of its use for military purposes. For example, it is easy to find remnants of the road network, planted trees (especially along roads), and some old buildings (Wilkaniec et al., 2012).
The military area covers over 7000 hectares and is bordered by the river Warta in the north and east; in the south its limits are near the villages of Radojewo and Złotniki, while in the east, near the villages of Złotkowo, Chludowo, Maniewo, Gołębowo and Gołaszyn. The headquarters are in Biedrusko (52°32′34,78″N 16°56′49,34″E) (Fig. 1).
The area is covered to a large extent with forests, but they are generally not of high conservation value. Most of them are planted pure pine forests or poplar forests on former farmland. Oak forests are also common there, especially acidophilous oak forests
Shrub communities are also widespread there. The largest patches, particularly in the central deforested part of the military area, are thickets dominated by the common broom,
A large proportion of the area is covered by semi-natural and anthropogenic meadow communities and pastures of the class
Most soils in the study area are podzolic. In the central part of the military area they are formed from loose and weakly clayey sands, whereas in other parts, from sands on clay (Borysiak and Brzeg, 1994 after Bartkowski, 1962).
As mentioned above, the wild pear trees in the military area in Biedrusko are most probably the largest Polish population of this species, composed of about 1000 trees. Pear trees grow primarily in the northern part of the study area, on so-called “Pola Grunwaldu”, north of the road from Biedrusko to Maniewo (Fig. 1). Morphological variation and genetic differentiation of
In August 2007 they were conducted preliminary studies of the population, a total of 21 wild pear trees were sampled with Pressler borers. The cores were placed in special strips of wood and their surface was cut with a sharp knife to obtain a clear image of tree-rings. They were measured under a stereomicroscope, from the innermost part towards the bark, to the nearest 0.01 mm (Mindur, 2000). In total, 1091 tree-rings were measured. Subsequently, a chronology was assembled using the classical dendrochronological methods and tested with the COFECHA software (DPL program package, Holmes 1983, 1994).
The RES chronology (de-trended, autocorrelation removed) constructed was subjected to the indexation (using a negative exponential curve and autoregressive modelling), in order to eliminate the age trend and to emphasize the annual changeability of the annual growth ring widths, in the ARSTAN program (Cook and Holmes, 1986). The expressed population signal (EPS) analysis was used to assess the degree, to which chronologies of each plot portrayed the perfect hypothetical chronology (Wigley
Mean annual temperature for this area is 8.5°C (in 1948–2011), varying from 6.8°C (in 1956) to 10.2°C (in 2008). The warmest month is July (mean temperature 18.5°C), whereas January is the coldest (–1.4°C). The growing season starts in mid-March and lasts till early November (225 days of the year) (Koźmiński and Michalska, 2001). Annual precipitation is on average 528 mm, decreasing to about 300 mm in the driest years (e.g. 274 mm in 1982 and 307 mm in 1984) and exceeding 700 mm in the wettest years (773 mm in 1967). Precipitation is the highest in July (76 mm) and the lowest in February (27 mm). The number of days with precipitation is lower than 95, whereas snow cover lasts less than 40 days, reaching a maximum snow cover depth of 46 cm (Koźmiński and Michalska, 2001; Ustrnul and Czekierda, 2009). The mean annual sum of hours with sun for Poznań in 1966–2011 reaches 1694, varying from 1252 (in 1966) to 2031 (in 2011). Insolation is the highest in May and July (242 h/month) and the lowest in December (35 h/month).
Chronology for pear trees from Biedrusko (BIE) spans 45 years (1963–2007), and is based on ring-width curves of 15 trees (Fig. 2). Mean tree-ring width is 1.92 mm, varying from 0.60 mm (BIE7) to 3.46 mm (BIE17). Standard deviation (ST) for the chronology is 0.91, with first-order auto-correlation (AC1) of 0.61 and mean sensitivity (MS) of 0.38, while for the indexed chronology, the corresponding values are: STD of 0.19, AC1 of 0.05, and MS of 0.33. The Expressed Population Signal (EPS) values are above the applied threshold of 0.85: in time span 1963–2007 EPS = 0.96 (Wigley
For the BIE chronology, 18 pointer years were identified, including 7 positive and 11 negative ones (Fig. 2). The analysis of meteorological conditions indicates relationships between weather conditions in the year preceding formation of the tree-ring and its width. Annual rings of pear trees were wide after years with cold and rainy summer but warm, long, sunny, and dry autumn. Growth depressions were caused by dry and hot summers in the years preceding growth. The analysis of correlation and response function indicate similar relationships between tree-ring width and climate: weather conditions in the year preceding growth affect ring width in the following year (Fig. 3). For temperature (T), correlations were detected in July (negative correlation), for insolation (IN) in July, October and December (negative correlation and regression coefficients), and for precipitation (P) in July, August and October (positive values of correlation and regression). Statistically significant values for the current growing season were also recorded for temperature (T) in August (positive values), precipitation (P) in May (negative regression coefficient) and for insolation (IN) in February (positive relationships). Multiple correlation coefficients (R) and total variance explained (r2) were the highest for precipitation (0.72 and 52%) and insolation (0.68 and 46%), while the lowest for temperature: 0.48 and 23%. F-value for precipitation was 5.48 significant at the 0.0002 level, for insolation 4.90 significant at the level and for temperature 2.85 significant at the level.
Dendrochronological studies of pear trees are very rare, probably because this species does not form dense forest stands, which are of major interest to dendrochronologists. In Poland,
Neri
Tree age over 114 years indicates that pear trees grew in this area when Prussians took it over for military purposes. Nevertheless, the major wave of wild pear “invasion” took place after the 2nd World War, when the Polish army took it over. Earlier the area was used for agriculture, as meadows and pastures, which did not allow the development of the wild pear population. The same applies to pear trees in Bielinek: tree age clearly indicates that pear trees colonized xerothermic grasslands after creation of the reserve but before the xerothermic shrub communities developed.
The preliminary analysis of relationships between tree-ring width and weather data showed strong correlations with climatic factors in the year preceding the formation of the growth ring. Growth rings of the studied pear trees were wide after cold and rainy summer and after long, warm, sunny, and dry autumn. In the current year, tree-ring width was affected by temperature in August, by precipitation in May and insolation in February. Different growth patterns and growth-climate responses for wild pear populations in Biedrusko and Bielinek indicate that habitat conditions play a significant role in the process of tree-ring formation. Further dendrochronological research of pear populations is needed in all parts of their geographic range.