Planting density affects growth and water-use efficiency depending on site in Populus deltoides × P. nigra

Highlights

• Planting density affects growth and WUE depending on pedoclimatic conditions.

• Under favorable conditions, increasing planting density accentuates plant competition for light.

• Under limiting water availability increasing planting density accentuates plant competition for water acquisition.

• No trade-off between growth potential and WUE.

Abstract

Poplar coppice plantations for biomass production can be conducted under either short rotation coppice (SRC) or short rotation forestry (SRF) systems, depending on planting density and rotation length. It is likely that differences in planting density affect tree physiology through competition for resource acquisition, including light, water and nutrients. In this paper, we hypothesized that the effects of planting density on growth and water-use efficiency (assessed through bulk leaf carbon isotope discrimination, Δ¹³C) in poplar depend on site characteristics in terms of soil fertility and water availability. To test this hypothesis, 56 Populus deltoides × P. nigra genotypes were planted under both SRC and SRF and replicated at two sites differing for pedoclimatic conditions. At the most favorable site for growth, trees grown at the higher density (SRC) displayed higher stem height, lower stem circumference, higher specific leaf area, higher mass-based leaf nitrogen contents and higher Δ¹³C, indicating that increased tree density mainly accentuated competition for light. Under less favorable conditions, trees grown under SRC still displayed lower stem circumference, higher specific leaf area and higher mass-based leaf nitrogen contents. However, stem height remained unaffected by increasing planting density while Δ¹³C was lower, likely because of increased competition for water availability. Genotypic rankings across planting densities were overall conserved while they were significantly modified across sites, suggesting that rankings for genotypic performances were much less affected by planting density than by site. Realized growth measured after 2 years (height and circumference) was weakly correlated with Δ¹³C, but a negative relationship between Δ¹³C and growing season leaf increment rate was observed in most cases. The absence of trade-off between growth and water-use efficiency combined with the large genotypic variations observed for these traits confirms the potential for selecting genotypes with high water-use efficiency without counter-selecting on biomass production in P. deltoides × P. nigra.

Introduction

Renewable resources such as woody biomass have received renewed interest with regards to the increasing world energetic demand (Karp and Shield, 2008). Poplars (Populus spp.) are part of the species recommended for bioenergy plantations under temperate climate (Karp and Shield, 2008) mainly because of their high productivity and their good coppicing ability (Dillen et al., 2011a). However, although poplars are known to consume a large amount of water and to be very sensitive to water deficit (Liang et al., 2006), irrigation of large scale plantations is not conceivable. Optimizing the carbon/water trade-off is thus important for such plantations (King et al., 2013), and the identification of plant material combining satisfactory growth with higher water-use efficiency (WUE) may be one step to achieve this goal.

Water-use efficiency describes the rate of CO₂ uptake or plant dry matter production for a given rate of plant water loss; therefore, WUE can be defined at different biological and time scales. Whole-plant WUE (transpiration efficiency), i.e. the ratio of total biomass produced to total water used during the same period, has been shown to vary among species and genotypes (Cernusak et al., 2011, Clifton-Brown and Lewandowski, 2000, Linderson et al., 2007, Raper et al., 1992). Water-use efficiency can also be assessed instantaneously through leaf gas exchange as the ratio between net CO₂ assimilation rate (A) and stomatal conductance to water vapor (g_s) (WUE_i, intrinsic water-use efficiency). In poplar, bulk leaf carbon isotope discrimination (Δ¹³C) has been shown to scale negatively with WUE_i at leaf level (Fichot et al., 2011, Monclus et al., 2006, Ripullone et al., 2004) and more recently with transpiration efficiency at the whole-plant level (Rasheed et al., 2013). Significant genetic variations in Δ¹³C have been reported in poplar, both under controlled and field conditions (Bonhomme et al., 2008, Dillen et al., 2011b; Gornall and Guy, 2007, Marron et al., 2005, Monclus et al., 2009, Monclus et al., 2012, Soolanayakanahally et al., 2009). In addition, WUE assessed through Δ¹³C has been shown to be modulated by water availability (DesRochers et al., 2007, Larchevêque et al., 2011, Monclus et al., 2006, Xu et al., 2008), nutrient availability (DesRochers et al., 2006, DesRochers et al., 2007, Siegwolf et al., 2001), age and developmental stage (Leffler and Evans, 2001, Marron et al., 2005, Rasheed et al., 2011), or site (Bonhomme et al., 2008, Chamaillard et al., 2011, Dillen et al., 2011b).

Plantations for woody biomass production can be adapted depending on planting density and rotation length. Two coppice systems can be typically used for poplars (Eppler and Petersen, 2007): short-rotation forestry (SRF) which is based on planting densities ranging from 1000 to 2000 trees per hectare (ha) and rotation lengths varying between 6 and 8 years, and short-rotation coppice (SRC) which is based on denser planting densities (6000–15000 trees per ha) but shorter rotation lengths (typically between 2 and 4 years). To our knowledge, the effect of planting density on WUE remains so far undocumented in poplar. Only few studies have investigated how Δ¹³C responds to planting density in other woody species, leading to conflicting results: no differences in Δ¹³C values were evidenced between low and high planting densities in Pseudotsuga menziesii (Mirb.) (Woodruff et al., 2002), higher planting densities have been shown to be associated with lower Δ¹³C in Pinus halepensis (Mill.) (Querejeta et al., 2008), and Pinus ponderosa (Lawson) (McDowell et al., 2003), but the opposite trend has also been observed in Pinus radiata (Don) and Pinus nigra (Arnold) trees (Martin-Benito et al., 2010, Walcroft et al., 1996, Warren et al., 2001). Comparatively, the effects of planting density on growth have already been documented. A decrease in stem circumference is a commonly observed response to increasing planting density in poplar (Benomar et al., 2012, DeBell et al., 1996). However, in fast growing hardwoods, tree height has been shown to increase, decrease, or remain unchanged with increasing planting density (Alcorn et al., 2007, Benomar et al., 2012, DeBell et al., 1996, Fang et al., 1999, Kerr, 2003, Pinkard and Neilsen, 2003).

The effect of planting density on tree physiology is fundamentally mediated by competition for resource acquisition, including light, water and nutrients (Benomar et al., 2012, Bullard et al., 2002a, Bullard et al., 2002b, DeBell et al., 1996, Green et al., 2001). In this paper, we hypothesize that the effects of planting density on growth and WUE in poplar depend on site characteristics in terms of soil fertility and water availability. Specifically, we hypothesize that when soil conditions are favorable in terms of water and nutrients, increasing planting density would primarily accentuate plant competition for light; expected results on plant physiology would be on one hand, an increase of the ratio between shoot primary and secondary growth and on the other hand, a decrease in WUE as a consequence of a decrease in net assimilation rate due to light limitation (increase in Δ¹³C; Buchmann et al., 1997). Favorable environmental conditions are known to maximize the expression of genetic variations for growth (Monclus et al., 2006, Monclus et al., 2009, Chamaillard et al., 2011); in these conditions, genotypes displaying maximum biomass production, increased canopy size, and consequently higher water demand may also exhibit better stomatal regulation in order to limit water losses, resulting into a higher intrinsic WUE and therefore a positive relationship between growth and WUE. In contrast, when soil conditions are limited for water and nutrients, we hypothesize that growth would be reduced and increasing planting density would primarily accentuate plant competition for soil resources. Expected result on plant physiology would be a more moderate increase in the ratio between shoot primary and secondary growth and an increase in WUE as a consequence of water limitation (decrease in Δ¹³C; Chamaillard et al., 2011, Fichot et al., 2010, Larchevêque et al., 2011, Monclus et al., 2006). Less favorable environmental conditions may limit the expression of genetic variations for growth (Monclus et al., 2006, Monclus et al., 2009), and may therefore hamper the detection of a relationship between growth and WUE. In order to test these hypotheses, 56 P. deltoides × P. nigra genotypes were planted under SRF and SRC systems and replicated at two sites contrasted for soil fertility and soil water availability. Given that most of the studied genotypes were new and still under test, a corollary to this study was to characterize the extent of genotypic variations for the traits of interest.