Potential and limitations of local tree ring records in estimating a priori the growth performance of short-rotation coppice plantations

Biomass and Bioenergy 92 (2016) 12e19

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Research paper

Potential and limitations of local tree ring records in estimating a priori the growth performance of short-rotation coppice plantations  a, b, Mate
j Orsa g a, Toma 
s Kyncl c, Miroslav Trnka a, b, *, Milan Fischer a, e, Lenka Barto
sova Reinhart Ceulemans d, John King e, Ulf Büntgen a, f, g Global Change Research Institute AS CR, v.v.i., B
elidla 986/4a, 603 00, Brno, Czech Republic  1, 613 00, Brno, Czech Republic Institute of Agrosystems and Bioclimatology, Mendel University in Brno, Zem
ed
elska c 
sova 37, 61600, Brno, Czech Republic DendroLab Brno, Elia d Department of Biology, Research Group of Plant and Vegetation Ecology, University of Antwerp, Universiteitsplein 1, B-2610, Wilrijk, Belgium e Department of Forestry and Environmental Resources, North Carolina State University, 2820 Faucette Dr., Raleigh, NC 27695, USA f Swiss Federal Research Institute WSL, 8903, Birmensdorf, Switzerland g Oeschger Centre for Climate Change Research, University of Bern, 3012, Bern, Switzerland a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 October 2013 Received in revised form 12 April 2016 Accepted 20 May 2016

As bioenergy plantations are a relatively new phenomenon, long-term experimental data on their productivity and tolerance to environmental stress that provides a robust framework for site selection and potential productivity assessment is still lacking. To address this need, we developed a method to correlate the productivity of bioenergy plantations with local climate using tree-ring chronologies. Treering width from 37 Populus nigra (age > 115 y) and 368 poplar hybrid (Populus nigra
Populus maximowiczii) (9e12 y) individuals were collected and analyzed at demonstration sites in the Czech Republic. The growth of mature, naturally grown solitary native trees and young congeneric hybrids grown in high density (~10,000 ha1) showed statistically significant correlations (r ¼ 0.71, p < 0.05). Further, we found significant (p < 0.05) and consistent growth responses to changes in key seasonal climatic parameters (e.g., mean air temperature, number of dry days or cumulative heat sum (degree-days) during the growing season) for both natives and their hybrids. The analysis of climate conditions and the tree-ring records revealed a gradual change of climatic conditions since the 1930s, positively affecting poplar growth and indicated that longer rather than shorter harvest cycles are preferable to ensure stable yields at our experimental site. © 2016 Published by Elsevier Ltd.

Keywords: Dendroecology Plantation site selection Populus nigra Poplar hybrid Weather-growth relationship

1. Introduction The cultivation of woody biomass crops represents one of many alternative sources of renewable energy. Although estimates suggest that the land available in Europe for bioenergy production is decreasing, there is potential that lignocellulosic woody crops will provide a more significant share of bioenergy in the future [1]. Of the potential “second-generation” bioenergy crops, poplars and willows growing in short-rotation coppice (SRC) may provide

* Corresponding author. Global Change Research Institute, AV CR v.v.i.
de
lsk Zeme a 1, Brno, 613 00, Czech Republic. E-mail addresses: [email protected] (M. Trnka), fi[email protected] g), [email protected] (M. Fischer), [email protected] (M. Orsa (T. Kyncl), [email protected] (R. Ceulemans), [email protected] (J. King), [email protected] (U. Büntgen). http://dx.doi.org/10.1016/j.biombioe.2016.05.026 0961-9534/© 2016 Published by Elsevier Ltd.

stable production of high-quality woody biomass in many regions [2e4]. These SRC bioenergy production systems are environmentally robust and provide additional ecosystem services [5,6]. The search for alternative energy sources has recently intensified due to a perceived need to decrease carbon emissions to mitigate climate change [7]; however, climate change itself may pose interesting challenges for this source of renewable energy [8,9]. According to recent studies [10e12], the Central European climate has undergone significant changes, and trends suggest that temperatures are likely to increase and that the available water supply is likely to decrease in the next few decades [13,14]. These changes create challenges to estimating the productivity of recently planted or planned bioenergy plantations (given their 25e30-year life expectancy). It is therefore even more important to understand the relationship among climate, extreme weather events, and bioenergy crop production.

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Despite its relevance, long-term empirical studies addressing the physiology and productivity of SRC and their response to climatic conditions and weather extremes, which would allow large scale upscaling and reliable mapping of the potential production, are still lacking [15]. An alternative to long-term experiments might be application of process-based models [2,16,17]. However, since the models generally require detailed (and not trivial) parameterization, they are not practically applicable for other than research purposes at the current stage. In addition, to make models a reliable tool they need to be properly validated across a broad range of soilclimate conditions, again indicating the need for a long-term empirical, experimental database [15]. Studies required for the above-mentioned purposes are hindered by the relatively longterm harvest cycle of SRC systems and by their size. Whereas plot sizes from 10 to 30 m2 are considered sufficient for traditional agricultural crops experiments, plot sizes should be at least ten times larger for tree-based experiments [18]. The harvest cycle of poplar plantations is usually between two and seven years (compared to a single year in the case of field crops), which prolongs the length of most experiments. Farmers and agronomists are used to having a wealth of available experimental data available to estimate potential agro-climatic risks and can use climate/soil suitability for traditional crops to aid in decision-making. However, as bioenergy production systems are relatively new (e.g., they have only been present in Central Europe since the end of the 1990s [19]), data on SRC productivity-climate relationships are severely lacking, although progress has been made [20e23]. Filling this gap would enable better decision making in selecting suitable energy crops species/genotypes for specific soil and climatic conditions, especially in the situation where reliable model upscaling has been not yet been realized. We suggest that finding widely grown trees of native species with the best correlation with the growth of SRC systems can be a useful way to evaluate the potential viability of SRC plantation productivity at given site. In this conceptual study, we hypothesized that the growth of trees of native species correlates well with the growth of

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congeneric hybrids in SRC, and that inter-annual growth variability can be used as a metric useful for site selection. Although the effect of growing season length cannot be controlled for, quantifying its effect on the growth response is informative since it can suggest the potential variation in yield possible. In contrast, if a native species shows high growth variability during years with similar growing season length but differences in precipitation, it suggests that water is a main constraint and the biomass productivity and plantation vitality may be at risk. To demonstrate the viability of our concept, we used over 100 years of tree-ring data from Populus nigra specimens that are relatively common across the European landscape. We first investigated a potential link between the responses of a bioenergy poplar hybrid (Populus nigra
Populus maximowiczii) SRC grown at high-density and mature solitary P. nigra trees grown under natural conditions at the same location. We show how the tree-ring response of the mature P. nigra trees to climate can be used to assess the likelihood of adverse climate events negatively affecting growth and productivity of the bioenergy plantation. Although different species might be used to relate to growth of the SRC, we reasoned that the close genetics ensured more similar resource requirements and seasonal growth patterns. 2. Materials and methods 2.1. Site characteristics The study was conducted in a typical rain-fed agricultural area of the Bohemian-Moravian Highlands (Fig. 1) near the Domanínek research station (49 310 N, 16 140 E; 530 m a.s.l.). The long-term mean (1981e2010) annual temperature is 7.2  C, the mean annual precipitation is 609.3 mm, and the mean annual reference evapotranspiration [24] is 650 mm. The mean length of the growing season (daily mean air temperature above 5  C) is 217 days, from March 30 to November 1. These data are based on daily weather variables obtained from the meteorological station at Byst
rice nad

Fig. 1. a) The spatial distribution of the sampling sites within the sampling area. Letter identifies individual sampling sets. A,G represent “dry” while B,E.F “wet” SRC sites while D,C and H indicate Populus nigra site. The first number identifies the number of specimens within the sampling while the year stands for the oldest identified tree ring at the given sampling site. The “X” site shows were the continuous growth increment measurements used in Fig. 2a took place. The arrows indicate the position of the sampling sites G and H. b) The location of the case study site. c) An image of the G (Populus nigra
P. maximowiczii) sampling site in 2003 (two years after planting) with the H (Populus nigra) site in the distance. Note: The marking A-H for tree ring sampling sites was determined based on the order of collection of the samples on the collecting day and was retained for the study. No treerings were collected at the „X“ site.

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Pern
stejnem (49 310 N, 15 150 E; 560 m a.s.l.) within 1 km of the experimental site. The locality itself is considered suitable for plantation forestry [25]. The site is characterized by relatively deep soils and mild slopes ranging from 3 to 8%, and it generally has a cool and relatively wet climate. Soil conditions at the sampling sites are representative of the wider region, with Cambisols at wet sites influenced by gleyic processes. In April 2000, the first of the investigated operational highdensity monoclonal plantations was established followed by further plantings in 2002 and 2003 using the hybrid poplar clone J105 (Populus nigra
P. maximowiczii), hereafter referred to as the “hybrid poplar.” The plantation was planted on agricultural land previously cropped predominantly for cereals and potatoes. In the vicinity of the newly established plantations we located three sites where mature P. nigra specimens had grown (C, H) or were still growing ((D), Fig. 1). The trees at sites C (1896e2009) and H (1892e2011) fell down after a severe wind storm in late spring of 2012 (but after measurements for the current study had been taken). 2.2. Tree-ring data A total of 37 mature trees and 368 younger hybrid poplars were sampled in March 2012 at the different neighboring sites (Fig. 1). Whereas two cores were collected from the mature trees, a single core was considered sufficient in the case of the hybrids due to their small size. Core samples were extracted with five millimeter €f increment drilling devices (so called Pressler-borer by Haglo Comp., Langesele, Sweeden), the samples were glued to wood

holders and the surface was consecutively sanded with progressively finer grit (80e400) to ensure optimal visibility of growthring boundaries. Ring widths of all individual core samples were measured to an accuracy of 0.01 mm using a LinTab measuring table (Rinntech; http://www.rinntech.de). The subsequent measurement series were visually cross-dated under a light table, as well as by performing the automatic cross-dating programs PAST4 (SCIEM: http://www.sciem.com/download/) and COFECHA (http://web.utk. edu/~grissino/software.htm). Since standardization of the raw measurement series results in dimensionless indices, analyses were based on the un-detrended data to allow comparison of absolute growth levels between the different sites and age classes (Fig. 2bec). The comparison used both visual assessment and Pearson’s r was used to calculate correlation coefficients between tree ring series and between climate indicators and tree ring increments. In addition paired t-test was applied to test the null hypothesis of no difference between diameter growth between different sites or time intervals. The sites where the hybrid poplars were sampled can be divided into those where no effect of the water table was obvious (sites A and G), with 170 sampled trees, and those (B, E, F) where either the water table or lateral flow of soil water provided continuous water availability. Conditions of the mature tree sites were typical for the relatively wet soils of the site, originating from either lateral flow from up-slope pastures (C, H) or from immediate proximity to a small pond (D) fed by a local spring. Sample preparation, ring width measurements, subsequent cross-dating, and chronology development followed standard dendrochronological procedures ensured by the applied software

Fig. 2. a) Monthly running means of relative daily stem diameter increments at the Populus nigra
P. maximowiczii plantation (taken at near or at G and X sampling sites) indicating the duration of the growing season gray line depicts the particular 16 individuals and black line their average). b) The solid black line shows the tree ring diameter increment for old P. nigra trees (mean of 37 specimens from C, D and H sampling sites) with thin-lines showing the mean increment for the individual sites; the colored lines represent 368 individuals of Populus nigra
P. maximowiczii hybrids divided into dry (soil red i.e. A,G with 170 samples) and wet (solid blue i.e. B,E,F with 198 samples) sites. c) Captures the 2000e2011 period and provides the correlation between the diameter increment of young-dry/young-wet and old trees during the period from 2005 to 2011 in red/blue color. Closure of canopy in 2005 is shown. The top row of inserted figures shows the plantation in the early years with apparent strong weed competition up to the point when the trees fully outgrew the understory competition (2005). The lower row shows three sites where the old P. nigra trees were sampled as well as the habitus of the trees at the time of sampling.

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tools. The growth of a subset of hybrid poplars was monitored in detail to obtain information about the growing season duration. To estimate tree trunk diameter increment, 15 manual (DB 20) and 5 automated (DRL 26) band dendrometers manufactured by EMS Brno, Czech Republic, were used. The automated band dendrometers provided detailed long-term monitoring of tree trunk circumference in a short time step (hourly) with a measurement resolution of 1 mm. The average growth dynamics based on these five automated dendrometer records allowed for the interpolation and conversion of the manually collected diameter measurements from a weekly time step to daily values. These daily diameter changes were finally used for assessment of the relative diameter increments (in % of the entire seasonal increment) to identify the most important periods of growth. The taxonomy of the mature P. nigra trees was determined by using dendrology keys for deciduous woody species [26] to ensure that the mature trees were not hybrids. The P. nigra specimens used in our study were intentionlly planted, as this species is typical in periodically flooded areas of floodplain forests, especially in the southern part of the Czech Republic [27]. The taxonomy of the hybrids is known because only certified material was used for the establishment of the plantations, as required by Czech Republic forestry legislation. 2.3. Climate data and analysis Daily long-term (from 1961 to 2011) weather data (including minimum and maximum air temperature, global radiation, precipitation, wind speed, and air humidity) were selected as agroclimatic indicators. These indicators included the mean air

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temperature from April to June, the difference between the precipitation and reference evapotranspiration calculated from September to August, the number of degree days above 5  C accumulated on days when the rate of the actual and the reference evapotranspiration was higher than 0.2 mm d1, and the number of dry days, i.e., days when the ratio of daily actual and reference evapotranspiration was below 0.4. The reference evapotranspiration was calculated using the Penman-Monteith approach [24]. The values of the reference and the actual evapotranspiration of closed poplar canopy were estimated using SoilClim software [28] and were used to calculate the number of dry days. Statistical analyses were performed using Unistat 5.6.08 software package (UNISTAT Ltd., U.K). The significance of linear relationships were assessed using Pearson’s product-moment correlation coefficients considered significant at 0.05 level (2-tailed test). 3. Results Stem diameter growth in the hybrid poplars started in early April and continued until late September (Fig. 2a). However, the majority of the stem diameter increments occurred between midMay and the end of August. This seems to be a typical growth pattern of P. nigra
P. maximowiczii when soil water availability is not limiting, as in the study year 2009 (Fig. 2a). The analysis of the diameter increments of mature trees (Fig. 2b) revealed much slower current growth rates (Fig. 2b) compared to the hybrids, with growth rate steadily decreasing to approximately 30% of the initial rate 60 years after planting. However, we did observe an increase in the growth rate of mature poplars during the 1980s. The trend toward a more rapid growth of mature trees during the 1981e2011

Fig. 3. The relationship between environmental factors found to be the most relevant (and significantly correlated) with poplar growth and the response of the native P. nigra (gray) at sites C,D,H and P. nigra
P. maximowiczii (black) hybrids at the dry (A,G) sites. The lines represent linear regression line fitted to the data. a) The mean air temperature during the April to June period. b) The difference between precipitation and the reference evapotranspiration calculated over the agricultural year (OctobereSeptember) period. c) Degree days above 5  C accumulated on days when the ratio of actual and reference evapotranspiration was higher than 0.4 during the given year. d) The number of days with a daily ratio of actual and reference evapotranspiration below 0.2 during MarcheMay period. Note: the statistical significance (a ¼ 0.05) of the relationship is marked by asterisks.

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period was statistically significant (p ¼ 0.002). The first step of the dendroclimatic analysis was to determine the relationship between the annual diameter increment of mature trees and the hybrid poplars (Fig. 2c). Although hybrid poplars were planted between 2000 and 2002, we only used data beginning in 2005, when herbaceous competition was finally suppressed due to light limitation by the growing poplar stands. At this time, the poplar canopy was closed (Fig. 2) for the entire growing season, with mid-summer leaf area index values above 5, effectively eliminating all understory plants. Within the period from 2005 to 2011, the diameter increment of hybrid poplars and mature trees were averaged for each year and compared. Both series were significantly correlated (r ¼ 0.711; p ¼ 0.037) when both wet and dry hybrid sites were used. When the mean annual growth of hybrids from the dry sites was compared to the mature trees, an even closer relationship (r ¼ 0.77; p ¼ 0.02) was found. However, hybrids grown at the wetter sites only showed marginally significant relationships (r ¼ 0.61; p ¼ 0.07) with the mature trees. The difference in mean diameter increment of hybrids between the wet and dry sites (Fig. 2c) was pronounced (p ¼ 0.007), but the series were still closely correlated (r ¼ 0.77; p ¼ 0.02). Some features in hybrid growth were not present in mature individuals, especially a growth decline in 2010 and 2011 (Fig 2c). This can be explained by the pattern of growth of the high density hybrid plantation, where annual increment peaked between 5th and 6th year after planting and then declined due to inter-tree competition. Despite a relatively short period of overlap, hybrid poplars in our

plantations and local mature trees exhibited good agreement in growth dynamics. Mature trees and hybrids reacted in a very similar, coherent manner to four distinct environmental factors that affected growth during the 2005e2011 period (Fig. 3). In general, diameter increment for both hybrids and mature trees was favored by a warm AprileJune period, and in growing seasons with a high number of accumulated growing degree days. Similarly, a positive water balance during the period from September of the previous year to August of the current year led to higher yields. In contrast, a dry spring (MarcheMay period) caused a significant growth decrease in both young and mature trees. The year with the highest increment growth in both hybrid and mature trees (2009) was notable due to a positive water balance and a correspondingly low number of dry days in the spring. The growing season was comparatively warm and long as compared to the long-term mean growing conditions. In contrast, years with low tree-ring growth (2005, 2006, and 2010) were characterized by AprileJune temperatures below 12  C, a low number of cumulative degree days during the entire season, and a higher number of dry days during the spring. The strong relationship between increment growth of the mature, naturally grown trees and the young hybrid trees in plantations, and the fact that both tree populations responded similarly to climate drivers, allowed us to investigate the climatic suitability of the study location for bioenergy production. We selected climate predictors that showed a statistically significant correlation with seasonal growth (p < 0.05) for both mature and hybrid poplars: i)

Fig. 4. Changes in the values of the three bioclimatic factors found to be significantly associated with diameter increment between 1961e1990 and 1991e2011 periods for: a) The mean air temperature during the April to June period; b) Degree days above 5  C accumulated on days when the daily ratio of actual and reference evapotranspiration was higher than 0.4; c) The number of days with the daily ratio of actual and reference evapotranspiration below 0.2. The first column shows the time dynamics of the three predictors from 1961 to 2011. The second column shows the proportion of the days in individual categories over the period from 1961 to 1990 and the last 21 years (1991e2011). The third column represents the correlation between the environmental growth factors and the diameter increment during 1961e1990 (black) and 1991e2011 (gray) with values of the Pearson’s correlation coefficient calculated for the entire period using both annual values and first-order differences. Note: the statistical significance (a ¼ 0.05) of the relationship is marked by asterisks.

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mean air temperature during the period from AprileJune, ii) cumulative degree days above 5  C on days when the ratio of actual and reference evapotranspiration was higher than 0.4, and iii) the number of days with a daily ratio of actual and reference evapotranspiration below 0.2. A time-series analysis of these three climate predictors of growth revealed a trend toward increased AprileMay temperatures and an overall increase in the growing season degree days, especially during the past 20 years. These statistically significant trends were accompanied by a progressively wetter period from MarcheMay. Significantly, there has been a marked shift in the distribution of these climate predictor values between the 1961e1990 and 1991e2011 periods (Fig. 4) which corresponds with significantly higher growth of mature poplars. 4. Discussion Significant increase in the growth rate of mature poplars during the 1980s (Fig. 5a), is not unique to our research site and has been reported for several species across Europe [29]. The close relationship between all tree rings (young and old, both dry and wet sites) indicates that because the ratio of precipitation to reference

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evapotranspiration ratio is close to unity, the inter-annual growth variability of poplars must not primarily driven by water availability. Recent studies have shown that the water use of poplars at our site is slightly lower than that of a reference grass cover [30] and that the biomass productivity is mostly limited by the growing degree days (i.e. length of growing season) and partly by spring temperatures. The highest growth rates were observed in years with the highest cumulative heat sum and AprileMay temperatures, together with a lower than usual number of dry days (Fig. 4). The positive and significant correlation between the early growing season temperature and growth may be explained by the fact that during this spring period, soil temperature controls the physiological activity and development of fine roots [31e33]. Rapid fine root development during this period, the so-called “spring flush” (occurring before, and soon after, budbreak), is enhanced if the soil moisture level is high [34]. This pattern is in line with our findings that a dry spring (MarcheMay period) significantly limited growth. The analysis of the first differences (Fig. 2b) showed that the recent increase in growth rates at the site can be partially explained by improving environmental conditions. As each coppice cycle includes several growing seasons, we

Fig. 5. a) The annual increment of old trees (together with three and seven-year mean increments) between 1931 and 2001. b) The correlation between the mean air temperature from April to June and the mean increment over seven consecutive seven-year periods (starting with 1962e1969 and ending with 2005e2011). c) The same as (b) but for degree days above 5  C accumulated on days when the ratio of the daily actual and the reference evapotranspiration was higher than 0.4. Note: the value of the Pearson correlation coefficient is provided, and the significance (a ¼ 0.05) is marked by asterisks.

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analyzed seven seven-year periods between 1932 and 2011. The tree-ring record indicated that our location is suitable for growing poplars and likely also poplar hybrids. When this seven-year sum of climate predictors is correlated with the actual growth between 1931 and 2011, the growth is relatively well predicted (Fig. 5), and recent climate conditions have been far more suitable for poplar production than in the 1930s.We also found that there was high inter-annual variability in the seasonal growth of mature trees, as partly reflected in the growth of the young hybrids, and this variability increased over the past two decades (Figs. 2c and 5). However, when the growth rates were averaged over the seven-year periods, the overall production seems to be far more robust. Therefore shorter harvesting periods (e.g., three-year harvesting periods or those even shorter), which are sometimes recommended for SRC [35], might lead to higher variability in the total production by individual growers in different harvest seasons, and also for other reason longer harvest cycles are sometimes recomended [36,37]. This would be undesirable given the required space to store surplus harvest or, in the case of shortfall, trees would have to be imported from elsewhere, reducing the competitiveness of local growers. In case of the high density plantations (unlike for solitary trees), prolonging the rotation will lead to increased stand competition and associated decline in productivity [38]. While a compromise between these two factors should be sought, the suggested concept of implementing dendroclimatic methodology to estimate productivity/site suitability might also provide useful guidance for determining the optimal rotation length of an SRC system based on local climatic conditions. While using dendroclimatogical methods as another source of relevant information on the given site suitability and likely effect of long-terms climatic trends on the SRC plantations there are caveats that need to be studied. This study considered only one particular Populus nigra
P. maximowiczii clone and compared it with native Populus nigra trees. It might be argued that as all trees growing naturally in the area have relatively similar requirements (light, water, an adequate growing season), and that similar correlations between growth of hybrid poplar and any native tree species would be found. However, Populus nigra
P. maximowiczii showed very uniform stem diameter growth throughout the entire season which differs from other species, such as oaks, that are characterized by most of the diameter growth during the April to June period [39], which supports the concept of choosing reference native species with similar growth patterns as those shown by the bioenergy crop of interest. In addition, there is marked difference between some of the SRC clones in their phenology, resource-use efficiencies and other key physiological properties [22,40e42]. Despite the fact that our current study does not provide a way to estimate the absolute value of the yields, but rather the hypothetical yield variability and relative productivity between sampled sites, the absolute values may be obtained by further testing of our proposed concept at larger scale units (country or regions) when combined with the available data on SRC yields as well as with site-specific allometric relationships and/or stand inventory data. 5. Conclusions This study highlights the possibility of using dendrochronological information to estimate potential inter-annual yield variability, estimate low yield potential, and determine the most appropriate harvest period to limit variability in yearly biomass output. As dendrochronological data are easily available at many sites, they represent quick and inexpensive data useful to combine with other traditional methods (e.g. modelling and/or field experiments) to estimate site potential. The presented method can potentially be used for any species grown for woody biomass that has

distinguishable growth rings but its true potential needs to be tested across multiple sites and clones, ideally of several species used in the SRCs. Acknowledgements Support from Jan Balek (programming and data handling) is greatly appreciated. We acknowledge funding from Ministry of Education, Youth and Sports of CR within the National Sustainability Program I (NPU I), grant number LO1415 and of the KONTAKT project LD13030. The funding body had no influence on study design, collection, analysis or interpretation of data, writing or the decision to submit the article for publication beyond providing general financial support for the experimental work. The authors express their sincere thanks for the long-term technical support of this research by the ZEMSERVIS Zku
sební stanice Domanínek Ltd., , as well as to the township of in particular M. Trnka Sr. and J. Fialova Byst
rice nad Pern
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