Hydraulic acclimation to shading in boreal conifers of varying shade tolerance

Authors


A. L. Schoonmaker. Fax: +(780) 492 1767; e-mail: schoonma@ualberta.ca

ABSTRACT

The purpose of this study was to determine how shading affects the hydraulic and wood-anatomical characteristics of four boreal conifers (Pinus banksiana, Pinus contorta, Picea glauca and Picea mariana) that differ in shade tolerance. Plants were grown in an open field and under a deciduous-dominated overstory for 6 years. Sapwood- and leaf-area specific conductivity, vulnerability curves, and anatomical measurements (light and scanning electron microscopy) were made on leading shoots from six to nine trees of each treatment combination. There was no difference in sapwood-area specific conductivity between open-grown and understory conifers, although two of four species had larger tracheid diameters in the open. Shaded conifers appeared to compensate for small diameter tracheids by changes in pit membrane structure. Scanning electron microscopy revealed that understory conifers had thinner margo strands, greater maximum pore size in the margo, and more torus extensions. All of these trends may contribute to inadequate sealing of the torus. This is supported by the fact that all species showed increased vulnerability to cavitation when grown in the understory. Although evaporative demand in an understory environment is low, a rapid change into fully exposed conditions could be detrimental for shaded conifers.

INTRODUCTION

Shade-tolerant trees have adopted a suite of strategies to prolong their survival in shade (Walters & Reich 1999). These include changing the spatial arrangement and morphology of leaves, increasing the allocation of C towards leaf production in order to maximize light capture (Givnish 1988; Pearcy 2007), reducing allocation of C to the root system (Callaway 1992; Landhäusser & Lieffers 2001; Renninger, Gartner & Meinzer 2006; Pearcy 2007) and lowering respiration rates and photosynthetic compensation points of foliage (Callaway 1992). The sheltered environment of an understory also has reduced potential evaporative demands and wind speeds (Bladon et al. 2006). Given these changes, one also would expect the hydraulic architecture to be altered by the sheltered understory environment. To date, however, little research has actually tested this idea in a field experiment.

Angiosperm seedlings grown under artificial shade or low light have demonstrated reduced leaf-specific conductivity (Schultz & Matthews 1993; Shumway, Steiner & Kolb 1993; Brodribb, Holbrook & Hill 2005) whereas suppressed (reduction in height growth because of shading and competition by surrounding overstory trees) conifers have previously shown no reduction (Renninger, Meinzer & Gartner 2007). Shading has been associated with reduced sapwood-area specific conductivity in lower branches relative to upper sun-exposed branches of Pinus contorta (Protz, Silins & Lieffers 2000). Suppression reduced sapwood-area specific conductivity in Pseudotsuga menziesii, Tsuga heterophylla and Picea abies compared with released (previously suppressed trees where the overstory had been removed) (Renninger et al. 2006) or sun-exposed (Sellin 1993) individuals.

The xylem pressure causing 50% loss in hydraulic conductivity (P50), a proxy of cavitation resistance, was less negative in four artificially shaded European deciduous angiosperm seedlings (Barigah et al. 2006) as well as in shaded versus sun-lit branches of Fagus sylvatica (Cochard, Lemoine & Dreyer 1999; Lemoine, Cochard & Granier 2002). The relative magnitude of the decline in cavitation resistance, however, did not appear to be related to the inherent shade tolerance of the individual angiosperms measured (Barigah et al. 2006). It is unclear whether these findings are more widely applicable to conifers and especially under field conditions.

The mechanism driving cavitation resistance in conifers is not completely understood. Most recently, Cochard et al. (2009) have suggested capillary failure as a possible candidate and further speculated that incomplete contact between the torus and pit chamber or pores within the torus could allow air entry. Across a variety of northern hemisphere conifer species, resistance to cavitation and pit resistance have been positively correlated (Pittermann et al. 2006a). Higher wood density and smaller diameter tracheids have also been correlated with increased cavitation resistance (Hacke et al. 2001; Pittermann et al. 2006b; Hacke & Jansen 2009). As tracheid diameter tends to be narrower in shaded branches or suppressed individuals (Protz et al. 2000; Renninger et al. 2006), it could be inferred that shading should increase resistance to cavitation; however, this relationship was not detected in shaded angiosperms (Cochard et al. 1999; Lemoine et al. 2002; Barigah et al. 2006). There appears to be a correlation between increased cavitation resistance and declining pit aperture diameter with height in both Pseudotsuga menziesii and Sequoia sempervirens (Burgess, Pittermann & Dawson 2006; Domec, Lachenbruch & Meinzer 2006; Domec et al. 2008). Similarly, Mayr, Wolfschwenger & Bauer (2002) found that the ratio of pit aperture : pit diameter declined with increased elevation and cavitation resistance in Picea abies. There is little information, however, on how these characteristics change in understory conifers and how these are affected by the shade tolerance of the species. Given the uncertainties regarding cavitation resistance and how this relates to the tracheid anatomy of conifers, further investigation is warranted.

The objective of this study was to assess how understory shading influences the hydraulic architecture and tracheid anatomy of boreal conifers of varying shade tolerance, grown in either full light or under a deciduous-dominated canopy. We studied two shade intolerant pines (Pinus banksiana and Pinus contorta) and two shade tolerant spruces (Picea mariana and Picea glauca). In the boreal forests of western Canada, both pines are early-successional species that typically establish after stand-replacing disturbances. Picea glauca commonly regenerates underneath an overstory of aspen (Populus tremuloides) and will eventually overtop the aspen and become a dominant tree. In contrast, Picea mariana is a common inhabitant of low-lying areas that are often poorly drained. However, it is also often found regenerating in high densities underneath a canopy dominated by Pinus contorta and sparsely in the understory of aspen-dominated stands. We are hypothesizing that understory grown conifers will have: (1) increased vulnerability to embolism compared with open-grown conifers; (2) lower sapwood-area specific conductivity driven by smaller tracheid diameters; and (3) lower leaf-area specific conductivity and soil-to-plant hydraulic conductance compared with open-grown conifers. Given that Picea glauca has shown greater morphological plasticity (relative to Pinus contorta) when grown in this type of shade (Landhäusser & Lieffers 2001) we are expecting that the differences between open and understory conifers will be larger in the shade-tolerant spruces than the shade-intolerant pines.

METHODS

Study location and experimental design

One-year-old Pinus banksiana Lamb, Pinus contorta Dougl. Ex Loud., Picea mariana (Mill) B.S.P. and Picea glauca (Moench) Voss seedlings (nursery-grown container stock grown from local seed sources) were planted in the spring of 2000 at the University of Alberta Farm (Ellerslie), Edmonton, Alberta (53°N 113°W). Forty seedlings of each species were planted in an open field at 1.0 m spacing. Another set of seedlings (50 of each species) were planted in a nearby aspen-dominated forest. Average leaf area index (LAI) in the stand was 1.82 ± 0.24 (mean ± standard deviation) (LAI-2000, Licor, Lincoln, NE, USA) with an average light transmission of 21.5 ± 5.5%. Seedlings were arranged at 0.5 m spacing in 10 separate plots (five seedlings of each species) to account for the variability in light conditions in the understory environment. After the first establishment year, two seedlings of each species in each understory plot and 10 seedlings of each species in the open area were caged to prevent browsing over the following years. Seedlings were grown for a total of 6 years throughout which they were kept weed free. For the first 3 years all seedlings were watered during periods of summer drought (June–August) and fertilized with a slow release fertilizer (Osmocote 14-14-14 N-P-K, Scotts Miracle-Gro, Marysville, OH, USA) in order to ensure establishment. After the third year, no additional watering or fertilization took place.

Growth measurements

Six trees of each species growing in the open field and deciduous understory were harvested in August 2007. Total height and diameter at the root base were measured to the nearest 0.5 mm. To determine whole tree leaf area (Al), projected leaf area was determined on a sub-sample of fresh needles collected from each tree using Winfolia (Regent Instruments Inc., Quebec City, Canada) image analysis software. Measured needles were oven-dried and weighed. The remaining needles on each tree were removed, oven-dried and weighed. The relationship between needle dry weight and projected leaf area from the sub-samples was used to calculate Al for each tree.

Soil-to-plant hydraulic conductance

To determine soil-to-plant hydraulic conductance (Ks−p), 12 trees of each species growing in the open field and deciduous understory were selected. Measurements were conducted over 4 d (August 1, 2, 15 and 16, 2007) and six pine and spruce trees from the understory and open were measured each day (Saliendra, Sperry & Comstock 1995; Andrade et al. 1998; Sellin 2001). A summary of ambient conditions during the 4 d period (Environment Canada 2009) is given in Table 1. The afternoon prior to the measurement day, a current year branch from upper crown position was covered with foil and a plastic bag to reduce cuticular transpiration. Pre-dawn (3:00–6:00) and midday (11:30–15:00) water potential (Ψplant) measurements were made on current year shoots using a Scholander pressure chamber (PMS Instruments, Corvallis, OR, USA). Midday measurements of transpiration (E) were taken with a steady-state porometer (Li-cor 1600, Lincoln, NE, USA) concurrently with the Ψplant measurements. Using these measurements, Ks−p was determined as:

Table 1.  Average air temperature (Temp), photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) during the period 11:00–15:00
DateTemp (°C)PAR (µmol s−1 m−2)VPD (kPa)
  1. PAR was measured in open conditions.

August 1 200721.31634.21.14
August 2 200725.31306.51.74
August 15 200718.6745.70.96
August 16 200721.61264.21.35
image(1)

where ΔP = ψmidday − ψpredawn.

Hydraulic conductivity and vulnerability curves

Hydraulic conductivity (kh) was measured on 2-year-old apical stem segments from late September to mid-October 2007. For the open-grown pines, however, 2-year-old segments were too large for the apparatus so 1-year-old stem segments were used. In order to ensure that one and 2-year-old stems of open-grown pines gave comparable results we measured conductivity and vulnerability curves on smaller Pinus contorta saplings that were growing adjacent to the open-grown individuals in this study. Vulnerability curves constructed from 1- and 2-year-old stems of these smaller trees were not significantly different from one another (data not shown). We followed a similar measurement procedure as in Pittermann et al. (2006a) and Hacke & Jansen (2009) where a 14.2 cm segment was sealed to hoses on both ends and a small pressure head of filtered (0.2 µm) 20 mm KCl + 1 mm CaCl2 solution was applied with outflow being measured every 10 s on a balance (CP225D, Sartorius, Göttingen, Germany). When outflow stabilized (within 2–5 min), we used the average outflow over the previous 40 s in order to calculate hydraulic conductivity as expressed in:

image(2)

Hydraulic conductivity was measured both before and after flushing the stems with the same solution for 20 min at a pressure of 10 kPa in order to remove any native embolism. Overall, there was little or no change between pre- and post-flushing measurements; thus the maximum conductivity was taken as the larger of the two values obtained. Cross-sectional sapwood area (Aleader) was measured with a stereomicroscope (MS5, Leica, Wetzlar, Germany) and image-analysis software (ImagePro Plus 6.1, Media Cybernetics, Silver Spring, MD, USA). All needles distal to the measured segment were collected and distal leaf area (Adistal leaf) was determined similarly as in whole-plant leaf area measurement. Thus, sapwood-area specific conductivity (ks) and leaf-area specific conductivity (kl) were expressed as:

image(3)
image(4)

Vulnerability to cavitation was determined by centrifuging a 14.2 cm stem segment to a known negative pressure for 10 min and then measuring the resulting conductivity. Each segment was repeatedly measured through a series of pressures (5–9 pressures depending on the species × treatment; for more details see Pittermann et al. 2006a; Hacke & Jansen 2009). The relationship between loss of conductivity and xylem pressure was fitted to two functions commonly used for this type of data. The first was a sigmoidal-exponential function (Pammenter & Vander Willigen 1998):

image(5)

where x is the xylem pressure (MPa) at a corresponding PLC and a and b are coefficients that describe the slope and xylem pressure at 50% loss of conductivity (P50). The second was a Weibull function (Li et al. 2008):

image(6)

where b determines the xylem pressure at 63.2% loss of hydraulic conductivity and c the steepness of the slope at b. Of the two fitted functions, we chose the one resulting in the best fit for each curve. The vulnerability curves fitting the data for each species (Fig. 1) were determined based on the best fit for all of the data for all stems measured. Additionally, individual curves for each measured stem were fitted in order to estimate individual P50 measures that were averaged between species and growing environment (Fig. 2a).

Figure 1.

Vulnerability curves of (a) Piba (Pinus banksiana), (b) Pico (Pinus contorta), (c) Pima (Picea mariana) and (d) Pigl (Picea glauca) grown in an open field (open) or in an aspen-dominated understory (understory). Error bars are 1 standard error of the mean. (n = 6–9).

Figure 2.

Average (a) xylem pressure causing a 50% loss of conductivity (P50) and (b) sapwood-area specific conductivity (ks) for Piba (Pinus banksiana), Pico (Pinus contorta), Pima (Picea mariana) and Pigl (Picea glauca) grown in an open field (open) or under an aspen-dominated understory (understory). Error bars are 1 standard error of the mean (n = 6). * indicates the means of within species comparison are significant at α ≤ 0.05.

Xylem anatomical measurements: light microscopy

The anatomical measurements and parameters derived from these measurements follow the methodology and modeling in Pittermann et al. (2006a,b) and Sperry et al. (2007). The same apical stem segments used in ks and vulnerability measurements were also used for the anatomical measurements. Tracheid diameters (Dm) were measured on three radial files (two to three cells wide) on thin cross-sections of the entire stem with a Leica DM3000 microscope at 200× magnification and analyzed with image analysis software (ImagePro). This generated between 500–1100 individual diameter measurements per stem that included early and latewood tracheids. Average lumen resistivity (RL) and the average lumen diameter (D) corresponding to RL for each stem were expressed as:

image(7)
image(8)

where η is the viscosity of water at 20 °C and Dm is the measured diameters of individual tracheids. Additionally, all of the tracheids measured in a species × treatment combination were pooled and summarized in a frequency histogram (Supporting Information Appendix S2)

Using the same cross-sections as earlier, tracheid density was also estimated by counting the number of tracheids in each radial profile (which contained no rays) and multiplying it by the fraction of tracheid-occupied sapwood. Tracheid length (L) and pit measurements were determined by digesting wood sections in a 1:1 mixture of glacial acetic acid (80%) and hydrogen peroxide (30%) for 48 h at 60 °C (Chaffey 2002). Macerated tissues were mounted on slides and average tracheid length determined by measuring a minimum of 50 tracheids using a light microscope at 25× magnification with image analysis software. One-sided tracheid surface area and the total number and area of inter-tracheid pits were measured on five tracheids per stem. Given that inter-tracheid pits will occupy only the radial walls of a tracheids we divided the total area occupied by pits on one radial wall by two times the area of a single tracheid wall in order to obtain the fraction of tracheid surface area (Fp) occupied by inter-tracheid pits:

image(9)

If it is assumed that a tracheid may approximate a rectangle then the total inter-tracheid pit area per tracheid (Ap) can by given as:

image(10)

Tracheid resistivity (Rc) is given as:

image(11)

where Rw is the end wall resistivity and Rxa is the inverse of ks. Pit-area resistance (rp) is given as:

image(12)

where L′ is the distance between tracheid end walls, a parameter that is assumed to approximate half the tracheid length (L), because tracheids will overlap by half on average (Lancashire & Ennos 2002; Pittermann et al. 2006a).

Xylem anatomical measurements: scanning electron microscopy (SEM)

Because of the labor-intensive nature of SEM measurements, we focused on describing two species (Pinus contorta and Picea mariana). We measured pit anatomical parameters on the same plant material used in hydraulic and vulnerability curve measurements (n = 6). Samples were prepared for SEM measurements following a modified procedure from Jansen, Pletsers & Sano (2008). Specifically, 1-cm-long frozen wood sections were split in half and thawed in distilled water for 5 d. After soaking, they were subjected to an ethanol dehydration series (30, 50, 70 and 90% for 0.5 h in each concentration), immersed for 12 h in 100% ethanol and finally air-dried for at least 24 h. We found this procedure was very effective in minimizing aspiration of bordered pits. Dried samples were split again, mounted on aluminum stubs with silver paint (Ted Pella Inc., Redding, CA, USA) and a thin layer of chromium (1–2 nm) was applied with a sputter coater for 1 min. Photographs were taken with a JEOL 6301F field-emission scanning electron microscope (JEOL Ltd., Tokyo, Japan) at an accelerating voltage of 2.5 kV. Only earlywood pits were used for measurements. In the further discussion we describe the measurement procedure for a single stem segment (24 stem segments in total were measured from each species × light treatment). For measurement of pit, aperture and torus area and diameter, photographs were taken at 1000× magnification, resulting in the use of 6–20 pits per image. Approximately 25–60 pits were measured and averaged for each individual stem. For measurement of the margo parameters (strand length, width, pore size, pore fraction and extended torus) 17–21 individual pits were photographed at 5000–11000× magnification. For each pit, we randomly measured the length of 4–8 margo strands and 8–12 strand widths and obtained an average for each pit. We selected a representative section of intact margo (approximately one-fourth to one-half of the entire margo area) and measured the area of each pore and determined the mean and maximum pore area for each pit. The pore fraction represents the area taken up by pores relative to the whole margo (strands and pores). The presence of an extended torus was defined as a bridge of amorphous material that continuously connects the central torus with the pit border (Sano, Kawakami & Ohtani 1999). Individual pit-level measurements were averaged for each stem segment analysed. Image analysis software (ImagePro) was used in the measurement of all pit structures.

Wood density

Wood density (ρ) was measured by water displacement. Specifically, the bark and pith was removed with tweezers and a razor blade from a 2.5-cm-long stem section that was subsequently inserted with a pin into a beaker of water on a balance to determine the fresh volume of wood. Stems were then oven-dried at 80 °C for 48 h. Wood density was expressed as the dry mass divided by the fresh volume.

Statistical analysis

R (R Development Core Team 2006) statistical environment for statistical computing and graphics was used for all statistical analyses performed. The non-linear regression function nls was used to estimate parameters of the sigmoidal-exponential and Weibull functions used to fit the loss of conductivity versus xylem pressure data for each species × environment and individual stem segments. Plots of predicted-y versus residuals and calculated r2 values were compared in order to determine which function best fit the data. T-tests (function t.test) were performed within species (not between) in order to separate means of open-grown and understory saplings within each species. Means were considered significantly different at α ≤ 0.05. Assumptions of normality and homogeneity of variances were assessed with diagnostic plots, normality tests (function ad.test, cvm.test, lillie.test, pearson.test, sf.test) and an F-test (function var.test). Where variances were unequal, we used Welch's two sample t-test to account for unequal variances. Results of within species t-tests (p-values and degrees of freedom) are presented in supplemental materials (Supporting Information Appendix S3).

RESULTS

Xylem pressure corresponding to a 50% loss in conductivity (P50) was consistently less negative (more vulnerable) in all species grown in the understory (Figs 1 & 2a). However, the greatest shift was observed in understory Picea mariana where the average P50 was 50% of open-grown saplings (Fig. 2a). No statistical difference was detected in the average sapwood-area specific conductivity (ks) between open-grown and understory conifers (Fig. 2b). Leaf area specific conductivity (kl) was significantly lower in all understory conifers compared with their open-grown counterparts (Fig. 3a). Soil-to-plant hydraulic conductance (Ks−p) increased in open-grown Pinus contorta and Picea glauca compared with understory saplings whereas no change was observed between open and understory Pinus banksiana and Picea mariana (Fig. 3b).

Figure 3.

Average (a) leaf area specific conductivity (kl) and (b) soil-to-plant hydraulic conductance (Ks−p) for Piba (Pinus banksiana), Pico (Pinus contorta), Pima (Picea mariana) and Pigl (Picea glauca) grown in an open field (open) or under an aspen-dominated understory (understory). Error bars are 1 standard error of the mean (n = 6 or 12). * indicates the means of within species comparison are significant at α ≤ 0.05.

Wood density (ρ) was significantly higher in all understory saplings compared with open-grown conifers (Fig. 4a). There was no difference in tracheid length between open and understory conifers (Fig. 4b). Average lumen diameter (D) was significantly larger in open-grown Pinus contorta and Picea glauca relative to their understory counterparts (Fig. 4c). Corresponding with trends in ρ and D, tracheid density was also higher in understory Pinus contorta, Picea mariana and Picea glauca saplings relative to open-grown saplings (Fig. 4d).

Figure 4.

Average (a) wood density, (b) tracheid length, (c) lumen diameter and d) tracheid density for Piba (Pinus banksiana), Pico (Pinus contorta), Pima (Picea mariana) and Pigl (Picea glauca) grown in an open field (open) or under an aspen-dominated understory (understory). Error bars are 1 standard error of the mean (n = 6). * indicates the means of within species comparison are significant at α ≤ 0.05.

On average, the width of margo strands was significantly reduced, whereas the maximum pore area was significantly larger in understory Pinus contorta and Picea mariana compared with open-grown individuals (Figs 5 & 6a,c). There was no difference in margo strand length between open and understory conifers (Table 2). Mean pore area, fraction of area occupied by pores (pore fraction), extended torus area and the occurrence of torus extensions were significantly higher in Picea mariana grown in the understory compared with the open (Fig. 6b,d–f). No difference was detected in these parameters between understory and open-grown Pinus contorta (Fig. 6b,d–f). Open-grown Pinus contorta had significantly larger tori and pit aperture diameters compared with understory saplings (Table 2). Tori and overall pit diameters were also significantly larger in open-grown Picea mariana (Table 2). The ratio of torus : pit area was also larger in open-grown individuals of Pinus contorta, whereas the torus : aperture area ratio was significantly larger in open-grown Picea mariana compared with understory saplings (Table 2).

Figure 5.

Scanning electron microscopy images showing typical bordered pit structures of (a) open-grown Pinus contorta, (b) understory Pinus contorta, (c) open-grown Picea mariana and (d) understory Picea mariana. Close-up view of margo in (e) open-grown Picea mariana and (f) understory Picea mariana. Scale (white bar) represents 1 µm.

Figure 6.

Average (a) margo strand width, (b) mean pore area, (c) maximum pore area, (d) pore fraction, (e) extended torus area and (f) occurrence of extended tori for Pinus contorta and Picea mariana grown in an open field (open) or under an aspen-dominated understory (understory). Error bars are 1 standard error of the mean (n = 6). * indicates the means of within species comparison are significant at α ≤ 0.05.

Table 2.  Results from scanning electron microscopy measurements: Mean strand length, torus/pit area, aperture/pit area, torus/aperture area, torus diameter, pit diameter and aperture diameter for Picea contorta and Picea mariana grown in an open-field (open) or in an aspen-dominated understory (understory)
SpeciesStrand length (µm)Torus/ pit areaAperture/ pit areaTorus/aperture areaTorus diameter (µm)Pit diameter (µm)Aperture diameter (µm)
  1. Values in brackets are 1 standard error of the mean (n = 6). Bold values indicate the means of within species comparison are significant at α ≤ 0.05.

Pinus contorta       
 Open2.61 (0.10)0.29 (0.01)0.12 (0.01)2.37 (0.16)6.37 (0.21)11.77 (0.34)4.15 (0.11)
 Understory2.59 (0.04)0.27 (0.00)0.11 (0.01)2.42 (0.22)5.63 (0.22)10.93 (0.30)3.66 (0.15)
Picea mariana       
 Open2.51 (0.07)0.23 (0.01)0.09 (0.00)2.55 (0.10)5.21 (0.09)10.88 (0.15)3.27 (0.08)
 Understory2.40 (0.07)0.23 (0.00)0.10 (0.00)2.14 (0.10)4.65 (0.11)9.88 (0.17)3.19 (0.10)

Tracheid resistivity (Rc) showed no consistent trend between open and understory saplings (Table 3). Corresponding with larger values of D, open-grown Pinus contorta and Picea glauca had lower lumen resistivities (RL) (Table 3). The importance of Rw is clearly seen in the wall fraction, as the end wall component comprised upwards of 75% of total resistivity across all open-grown conifers but was 72% in shaded conditions for both pines and 61–62% in the spruces (Table 3). Pit resistance (rp) was higher in open-grown individuals of Pinus contorta compared with those in the understory (Table 3). The number of pits per tracheid was higher in open-grown Picea mariana compared with understory but remained unchanged in all other conifers (Table 3).

Table 3.  Results from light microscope measurements: Mean tracheid resistivity (Rc), lumen resistivity (RL), end-wall resistivity (Rw), wall fraction, pit area resistance (rp), number of pits per tracheid and pit fraction (Fp) for Pinus banksiana, P. contorta, Picea mariana and P. glauca grown in an open-field (open) or in an aspen-dominated understory (understory)
SpeciesRc (MPa s mm−4)RL (MPa s mm−4)Rw (MPa s mm−4)Wall fraction (Rw Rc−1)rp (MPa s m−1)# pits/ tracheidFp
  1. Values in brackets are 1 standard error of the mean (n = 6). Bold values indicate the means of within species comparison are significant at α ≤ 0.05.

Pinus banksiana       
 Open4.47 (0.34)1.06 (0.11)3.41 (0.32)0.76 (0.02)2.43 (0.31)18.78 (1.03)0.06 (0.01)
 Understory6.53 (1.51)1.63 (0.29)4.89 (1.30)0.72 (0.03)2.39 (0.45)20.18 (1.68)0.07 (0.01)
Pinus contorta       
 Open7.09 (1.45)0.96 (0.15)6.14 (1.31)0.86 (0.01)4.53 (0.85)17.58 (1.57)0.07 (0.00)
 Understory7.19 (0.87)2.02 (0.19)5.18 (0.65)0.72 (0.02)2.84 (0.43)16.03 (0.93)0.07 (0.01)
Picea mariana       
 Open11.28 (1.96)2.39 (0.19)8.89 (1.83)0.76 (0.03)2.86 (0.54)21.03 (1.14)0.06 (0.00)
 Understory9.94 (1.38)3.58 (0.68)6.36 (1.49)0.61 (0.07)2.31 (0.49)15.20 (1.72)0.06 (0.01)
Picea glauca       
 Open6.30 (0.49)1.32 (0.20)4.98 (0.52)0.79 (0.03)3.15 (0.18)25.65 (2.11)0.07 (0.00)
 Understory6.96 (1.00)2.46 (0.17)4.51 (1.06)0.62 (0.05)2.48 (0.41)22.51 (1.73)0.06 (0.00)

Average height, root collar diameter and total leaf area on leading shoots were significantly larger in all open-grown conifers compared with understory conifers (Table 4). Leader leaf area to xylem area (LA : SA) was consistently higher in shaded conditions. When LA : SA was expressed on a whole plant basis, however, the same trend observed in leading shoots was only expressed in Picea mariana. Pinus contorta showed the opposite pattern (significantly higher LA : SA in open compared with understory) and no difference was observed between open and understory grown Pinus banksiana or Picea glauca (Table 4).

Table 4.  Mean height, root collar diameter, total leaf area (Al) and leader LA : SA for Piba (Pinus banksiana), Pico (P. contorta), Pima (Picea mariana) and Pigl (P. glauca) grown in an open-field (open) or under an aspen-dominated understory (understory) in August 2007
TreatmentHeight (m)Root collar diameter (mm)Al (m2)Leader LA : SA (cm2 mm−2)aWhole-plant LA : SA
  • Values in brackets are 1 standard error of the mean (n = 6 except Ks−p where n = 12). Bold values indicate the means of within species comparison are significant at α ≤ 0.05.

  • a

    Leader LA : SA was measured from the 1- (open-grown pines) or 2- (all other treatments) year-old shoots used in hydraulic measurements.

  • LA : SA, leaf area to xylem area.

Pinus banksiana     
 Open2.68 (0.15)41.9 (3.6)3.1 (0.4)9.2 (1.4)2212 (186)
 Understory0.78 (0.10)10.0 (0.5)0.2 (0.02)34 (3.4)1714 (183)
Pinus contorta     
 Open2.49 (0.17)48.4 (4.6)5.3 (0.9)8.8 (0.7)2829 (245)
 Understory0.72 (0.07)10.0 (0.6)0.1 (0.03)28 (3.0)1644 (220)
Picea mariana     
 Open1.83 (0.16)33.8 (2.9)1.7 (0.3)5.3 (0.7)1854 (128)
 Understory0.78 (0.06)9.3 (0.5)0.2 (0.02)11 (1.1)2950 (230)
Picea glauca     
 Open1.82 (0.20)35.7 (2.9)2.6 (0.5)4.6 (1.2)2432 (226)
 Understory0.68 (0.07)10.0 (0.5)0.2 (0.04)11 (0.6)2713 (236)

DISCUSSION

All four conifer species demonstrated significantly less negative average P50 values when grown in the shade, indicating that they were all more susceptible to drought-induced cavitation compared with their open-grown counterparts. This result supports our first hypothesis that understory trees will show increased vulnerability to embolism and is in line with earlier studies on angiosperms (Cochard et al. 1999; Barigah et al. 2006). Interestingly, the magnitude of this response, both in terms of the average P50 (Fig. 2a) and shifting of the entire vulnerability curve (Fig. 1) was greater in the shade-tolerant spruces, Picea mariana in particular, compared with the shade-intolerant pines. This indicates that the spruces appear to have greater plasticity in cavitation resistance than the pines. In contrast, Barigah et al. (2006) did not observe a change in the magnitude of cavitation resistance in angiosperms of differing shade tolerance.

Although understory trees had significantly greater wood densities and reduced tracheid diameters (D), they also had more vulnerable xylem, which contrasts with previous work where increased cavitation resistance was positively correlated with increasing wood density and decreasing D (Hacke et al. 2001; Hacke & Jansen 2009). Based on our SEM measurements we suggest three possible mechanisms (that may also operate simultaneously) by which understory conifers could become more susceptible to cavitation. Mechanism 1: the width of margo strands was significantly smaller in both understory conifers (Pinus contorta and Picea mariana) measured (Fig. 6a). From this and the fact that there were fewer margo strands in understory conifers, we could infer that that these strands would be more likely to tear and prevent the torus from sealing properly. Similarly, Domec et al. (2006) found that margo strand thickness was related to vulnerability in roots, trunkwood and branches of Pseudotsuga menziesii. Mechanism 2: increased vulnerability could be driven by increased occurrence and size of torus extensions (Fig. 6e,f). It is possible that these extensions are less flexible than the ‘regular’ margo strands and may prevent complete sealing of the torus. Our SEM measurements support this idea as both the occurrence and the area of extensions were significantly larger in shaded Picea mariana trees (Fig. 5d). Although we did not observe a statistical difference in torus extensions for Pinus contorta, the average occurrence and size of torus extensions were in the same direction. Also, the difference in vulnerability was much larger between open and understory Picea mariana than it was in Pinus contorta, thus it would make sense that the magnitude of the response would be less in Pinus contorta. In support of this mechanism, Cochard et al. (2009) have suggested that capillary failure is a strong candidate driving cavitation resistance in conifers. They further speculated that air could seed in through lack of contact between the torus and pit wall. Mechanism 3: the ratio of torus : aperture area was significantly larger in open-grown Picea mariana compared with understory individuals (Table 2). Given that both open and understory saplings had similar sized apertures, the smaller tori observed in understory Picea mariana would be more easily dislodged or not completely cover the pit aperture, allowing air entry to adjacent tracheids. Domec et al. (2008) also found an increased ratio of torus to aperture diameter in Pseudotsuga menziesii corresponding with increased height and cavitation resistance. However, in their case, this ratio appeared to be mainly driven by the pit aperture size. Interestingly, both the average torus diameter and pit aperture diameters were significantly larger in open-grown Pinus contorta relative to understory individuals suggesting that Mechanism 3 cannot explain the differences in P50 found in this particular species.

We observed no decline in ks as a result of understory shading (Fig. 2 b). This was an unexpected result as increased transport capacity typically corresponds with larger diameter tracheids (Pittermann et al. 2006a,b; Hacke & Jansen 2009). Given these results, how can understory conifers with significantly smaller diameter tracheids (Pinus contorta and Picea glauca) still have comparably efficient xylem to open-grown conifers? It is possible that longer tracheids could offset smaller diameter tracheids to some extent. Hacke & Jansen (2009) actually observed a positive relationship between tracheid length and ks. However, we did not observe a significant difference in tracheid length between open and understory conifers (Fig. 4b). A more probable explanation relates to the reduced proportion of total resistivity attributed to the end wall of the tracheid; the end-wall fraction tended to be reduced in all understory conifers (significantly so for Pinus contorta and Picea glauca; Table 3). However, it was not the area or number of pits driving a decrease in end-wall resistivity as there was no statistical difference in these parameters between open and understory conifers (Table 3). We did observe within the margo that the mean pore area in Picea mariana and maximum pore area in Pinus contorta and Picea mariana were significantly greater in the understory compared with the open (Figs 5 & 6b,c); larger pores would allow water to pass more easily through the bordered pit and would decrease end-wall resistivity (Hacke, Sperry & Pittermann 2004; Wilson et al. 2008). This agrees with a tendency for higher pit resistances in open-grown conifers (Table 3). Similarly, Domec et al. (2006) observed a tradeoff between pit conductivity and P50 in Pseudotsuga menziesii.

Our results contrast a series of earlier studies where shading actually resulted in decreased sapwood-area conductivity in conifers (Sellin 1993, 2001; Protz et al. 2000; Reid, Silins & Lieffers 2003; Renninger et al. 2007). We are suggesting three possibilities that could account for the discrepancy between our study and the previous investigations. Firstly, the absolute amount of shade could have influenced previous results. All studies had coniferous overstories that probably cast deeper and more continuous shade relative to our deciduous stand. Secondly, we examined apical shoots, whereas these studies measured either lateral branches or stem sections from lower sections in the crown; it is possible that competition for resources between upper and lower branches or even along the primary stem could have had an impact on ks. Thirdly, genetic differences between suppressed and dominant trees could have influenced ks in previous studies (Sellin 1993, 2001; Reid et al. 2003) as suppressed trees may simply be genetically inferiorand slower growing compared with dominant trees in the same stand.

The significant decline of leaf area specific conductivity (kl) in all four shade grown conifers and the decline of soil-to-plant hydraulic conductance (Ks−p) in shade grown Pinus contorta and Picea glauca (Fig. 3) support our final (third) hypothesis. The lack of response in Ks−p in Picea mariana could be the result of a relatively greater sensitivity to drought (in the open-grown environment) inducing reduced transpiration and subsequently lower Ks−p (Zwiazek & Blake 1989). In the case of Pinus banksiana, we do not have any direct evidence but this species may be similarly sensitive to drought as in Picea mariana. The magnitude of decrease in kl in the shade ranged from 35–72% for the four conifers measured. This is consistent with Shumway et al. (1993), Schultz & Matthews (1993) and Sellin (2001) who reported reductions in kl in shaded plants. However, Renninger et al. (2007) reported no difference in kl between suppressed and released Pseudotsuga menziesii and Tsuga heterophylla. The suppressed trees in that particular study also had reduced LA : SA relative to released trees. In our study, the shoots we used to measure kl all had higher LA : SA in the shade relative to the open.

Our original hypothesis had suggested that shade tolerant species (as opposed to shade intolerant species) would show greater differences in hydraulic parameters between open and understory environments. We found this was true in terms of cavitation resistance and in various characteristics of bordered pit anatomy. However, we saw little difference between shade tolerant and intolerant species in terms of water transport efficiency and most other structural anatomical observations (wood density, tracheid length and diameter, etc.). In fact, it seems more likely that differences in their ecological distributions appeared to dictate their plasticity in wood development. Pinus banksiana, for example, typically showed the least hydraulic and anatomical responses to understory shading; it is typically a dominant tree on dry habitats. On the other hand, Picea mariana, was probably the most variable in its responses and as such appeared to have the greatest plasticity. This species inhabits a wide range of habitat types that are likely to vary enormously in water availability. Thus, a strategy that could presumably reduce the investment of carbon into wood production (i.e. producing wood that is less cavitation resistant when grown in conditions not prone to high potential evapotranspiration) could be adaptive.

CONCLUSIONS

Overall, for plants growing in an understory, both evaporative demand and light availability decline. Lower evaporative demand results in reduced need for water whereas low light levels limit the amount of carbon fixation. The decline in kl and Ks−p in the understory environment supports the notion of reduced need for water under these circumstances. Increased LA : SA in the leading shoots are indicative of a shift in allocation from stem and root growth (hydraulics) to leaf area (light capture) development in the understory. In terms of the type of xylem produced, our data show that understory trees tended to produce narrower tracheids. Surprisingly, these tracheids were still capable of comparable flow with that of open-grown trees with larger diameter tracheids. This was probably driven by changes in pit structure, which is supported from our observations of larger maximum pore sizes in the margo of shaded Pinus contorta and Picea mariana. Having relatively efficient sapwood water transport means that understory conifers could invest less carbon into wood production by producing bordered pits with a more porous but fragile structure, corresponding with increased xylem vulnerability. A potential drawback of this strategy is that a rapid change in evaporative demand caused by the formation of a canopy gap could be detrimental for some shaded conifers. For an open-grown conifer, the risk of losing the capacity to transport water in order to ‘save’ carbon is too large, thus they have employed the strategy of a more conservative bordered pit design and compensated for increased pit resistance by larger lumen diameters.

ACKNOWLEDGMENTS

We gratefully acknowledge field and lab assistance from Jennifer Langhorst, Jessica Snedden, Kevin Renkema, Dominique Deshaies, Kelci Mohr, Caroline Lecoutier, Kristine Dahl and Kim Stang. We thank George Braybrook and De-Ann Rollings for assistance with the SEM work. An NSERC PGSM and CGSD and Alberta Ingenuity Scholarship to ALS, NSERC and Mixedwood Management Association grants to VJL and support from the Canada Research Chair program and the Canada Foundation for Innovation to UGH were all greatly appreciated.

Ancillary