Evaluation of Walnut Tree Flowering and Frost Occurrence Probability During 1961-2012

Strength and direction of the potential effect of climate change on walnuts is regionally specific (Gauthier and Jacobs, 2011) as climate change will probably affect the spatial distribution of the walnut. This paper evaluated the long-term phenological series (1961-2012) of the beginning of flowering, end of flowering and duration of flowering in walnut tree (Juglans regia) at two phenological stations located in different geographical locations of the Czech Republic but in the same climatic conditions (warm region). Phenological stages were analyzed in relation to growing degree days and to spring frosts occurrence. Onset of the beginning and end of flowering occurred earlier at Velké Pavlovice station (-2.1 and -1.3 days), and conversely occurred later at Doksany station (+1.8 and +1.0). Period of flowering shortened at Doksany station (-0.8 day) and prolonged at Velké Pavlovice station (+1.2 day). The occurrence of days with minimum air temperature < 0 oC during walnut tree flowering was more frequent at Doksany station (in total, 29 days) with absolute minimum value -5.5 oC. At Velké Pavlovice station 5 days with minimum air temperature below 0 oC were found during examined period with absolute minimum value -3.8 oC. The negative trend in number of frost days occurrence during flowering period was found at both stations. Pearson correlation coefficient between phenophase onset (and flowering period) and sums of growing degree days was stronger at Velké Pavlovice station, and the highest value was between period of flowering and temperature sums (0.782). The results confirmed our hypothesis of changes in phenophase onsets and duration of flowering including spring frosts occurrence according to west-east gradient (maritime climate-continental climate).O

Present-day vegetation helps quantifying past land cover in selected regions of the Czech Republic.

The REVEALS model is a tool for recalculating pollen data into vegetation abundances on a regional scale. We explored the general effect of selected parameters by performing simulations and ascertained the best model setting for the Czech Republic using the shallowest samples from 120 fossil sites and data on actual regional vegetation (60 km radius). Vegetation proportions of 17 taxa were obtained by combining the CORINE Land Cover map with forest inventories, agricultural statistics and habitat mapping data. Our simulation shows that changing the site radius for all taxa substantially affects REVEALS estimates of taxa with heavy or light pollen grains. Decreasing the site radius has a similar effect as increasing the wind speed parameter. However, adjusting the site radius to 1 m for local taxa only (even taxa with light pollen) yields lower, more correct estimates despite their high pollen signal. Increasing the background radius does not affect the estimates significantly. Our comparison of estimates with actual vegetation in seven regions shows that the most accurate relative pollen productivity estimates (PPEs) come from Central Europe and Southern Sweden. The initial simulation and pollen data yielded unrealistic estimates for Abies under the default setting of the wind speed parameter (3 m/s). We therefore propose the setting of 4 m/s, which corresponds to the spring average in most regions of the Czech Republic studied. Ad hoc adjustment of PPEs with this setting improves the match 3-4-fold. We consider these values (apart from four exceptions) to be appropriate, because they are within the ranges of standard errors, so they are related to original PPEs. Setting a 1 m radius for local taxa (Alnus, Salix, Poaceae) significantly improves the match between estimates and actual vegetation. However, further adjustments to PPEs exceed the ranges of original values, so their relevance is uncertain

Vegetation cover in selected regions.

<p>Vegetation cover in selected regions.</p

Histograms of sizes of sedimentation basins in different regions.

<p>The number of sites included is indicated at top right.</p

Comparison of actual regional vegetation with REVEALS estimates.

<p>Settings: original PPEs under wind speed of 3 m/s - S3MS (black squares, confidence intervals show their standard errors); adjusted PPEs under wind speed of 4 m/s - S4MS_P (red dots). Both settings deal with original sizes of the sedimentation basin. The diagonal line shows the position of the optimal fit of the model to expected values.</p

Fall speed of pollen (<i>v_g</i>), relative pollen productivity estimates (PPE) and their standard error (SE).

<p>*) in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Mazier1" target="_blank">[22]</a>, **) in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Fredh1" target="_blank">[7]</a>, ***) recalculated to Poaceae.</p><p>Footnotes by original PPEs indicate the source of the values. PPEs were adjusted according to the results of REVEALS model under higher wind speed (<i>u</i>) – S4MS_P and lower radius of the sedimentation basin (R) for local taxa – S4MSR_P.</p

Map of the Czech Republic with pollen sites (yellow crosses) and assigned regions (red circles).

<p>Northern Bohemia <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Pokorn1" target="_blank">[49]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Pea1" target="_blank">[53]</a>, Northeastern Bohemia <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Pokorn1" target="_blank">[49]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Peichlov1" target="_blank">[54]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Skrzypek1" target="_blank">[63]</a>, Polabí <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Petr2" target="_blank">[64]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Kozkov1" target="_blank">[67]</a>, Southern Bohemia <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Pokorn2" target="_blank">[68]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Jankovsk4" target="_blank">[73]</a>, Southern Moravia <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Bzov2" target="_blank">[74]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Svobodov5" target="_blank">[79]</a>, Bohemian-Moravian Highland <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Rybnkov3" target="_blank">[80]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Jankovsk6" target="_blank">[85]</a>, Šumava Mts. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Svobodov6" target="_blank">[86]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Svobodov7" target="_blank">[88]</a>. Yellow lines and arrows show the allocation of sites to regions in cases when sites are situated in two overlapping regions.</p

Two measures of dissimilarity between actual vegetation and REVEALS estimates or pollen proportion.

<p>Note the improvement of sums against previous steps. Abbreviations code the REVEALS settings: S - wind speed, R - radius of local taxa decreased to 1 m, _P - pollen adjusted.</p

Comparison of the area derived from the CLC map [30] and numerical data sources.

<p>(A) Forest area within municipalities with extended competence <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Forest1" target="_blank">[32]</a>. (B) Arable land within regions <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone.0100117-Czech1" target="_blank">[31]</a>.</p

Input parameters for simulations of REVEALS vegetation estimates.

<p>*) includes calculation of standard errors.</p><p>All calculations were performed with original PPEs (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100117#pone-0100117-t001" target="_blank">Table 1</a>) using the Prentice model.</p