A common garden experiment examining light use efficiency and heat sum to explain growth differences in native and exotic Pinus taeda

TL;DR: Examining the hypotheses that growth, light use efficiency, and volume growth per unit heat sum is the same for native and exotic plantations found that Pinus taeda grows faster and has a higher carrying capacity when grown outside its native range.

Abstract: Previous work indicates that Pinus taeda L. grows faster and has a higher carrying capacity when grown outside its native range. We were interested in examining the hypotheses that growth, light use efficiency (volume growth and absorbed photosynthetically active radiation relationship, LUE) and volume growth per unit heat sum is the same for native and exotic plantations. To test these hypotheses, we installed a common garden experiment where the same six genetic entries of P. taeda (four clonal varieties, one open pollinated family and one control mass pollinated family) were planted at three densities (618, 1235, and 1853 stems ha−1) with three or four replications at three sites (Virginia (VA), and North Carolina (NC) in the United States and Parana State in Brazil (BR)). The VA and BR sites were outside the native range of P. taeda. After five years of growth, the BR site had larger trees and stand scale basal area and volume were increasing faster than the other sites. Site did not affect LUE but density and genetic entry did. The sites were at different latitudes but the average photosynthetically active radiation at the top of the canopy was similar for the years when all sites were operational, likely because the BR site receives more rain annually and the cloudiness associated with the rain may have reduced available light. We estimated an hourly heat sum where the daytime temperature was between 5 and 38 °C, hours where vapor pressure deficit exceeded 1.5 kPa and days following nights where nighttime temperatures were less than 0 °C were excluded. Site was significant for the cumulative volume and heat sum relationship, for a given level of cumulative degree hours the sites ranked BR > VA > NC in cumulative volume. The different growth per unit of degree hours for each site indicated that something other than the heat sum was causing the observed difference in growth. Other factors including respiration and extreme climatic conditions may contribute to growth differences per unit degree hour and including these differences in the analysis would require a more detailed modeling effort to examine. The sites used in this study are ideally suited to continue testing additional hypotheses to explain the different growth between native and exotic P. taeda plantations because they have the same genotypes at all sites and consequently eliminate differences in genetics as a potential explanation for observed growth differences.

Summary

1. Introduction

• Environmental variables have large effects on tree growth.
• At the same time, identifying driving factors or relationships similar to LUE and heat sums that influence growth will make this analysis applicable to other species.

2.1. Experimental design

• The authors installed a split split-plot design with three or four replications at three sites (Vickers et al., 2011).
• The second site (VA) had four replications and was in the Piedmont of Virginia, United States at the Reynolds Homestead (36.64232°, −80.1546138°) in an area where P. taeda grows successfully but is outside the native range of the species.
• Plots with different initial density or genotypes were adjacent to each other.
• When on-site data were not available, the nearest meteorological station available from CRONOS (2015) was used to fill in data for the VA and NC sites.

2.2. Statistical analyses

• To examine their first hypothesis, the authors used a mixed model approach (PROC MIXED (SAS-Institute 2002)) to test for treatment effects for all sites after five years for diameter, diameter increment, height, height increment, basal area, basal area increment, volume, volume increment, and stand density.
• Random effects were block and genetic entry by block (Schabenberger, 2013).
• Non-significant terms were dropped from the model until all terms in the model were significant.
• For a given hour, if the ambient temperature was 5 °C, it was during the day and the previous nighttime temperatures were above zero then the heat sum for that hour was 5–5 or 0.
• = ∗CV DH S DH S where CV was cumulative volume in m3 ha−1 for each site at year end as an average of all individual plot estimates of volume, DH was the cumulative degree hour statistic and S was a class variable indicating each site, also known as The full model was.

3. Results

• Site effects were significant for all growth metrics (diameter, diameter increment, height, height increment, basal area, basal area increment, volume, volume increment, stand density, peak and off-peak leaf area index) (Table 2).
• Early survival was less for these genetic entries likely because they were planted as bare root seedlings whereas the other genetic entries at the VT and NC sites were containerized seedlings.

4. Discussion

• Site did affect growth and, consequently, the authors rejected their first hypothesis.
• At the same time, the diameter increment at the BR site was growing on a larger tree indicating that the total amount of stem wood required to produce this amount of diameter increment was much greater than that at the VA and NC sites and this was reflected in the stand scale measurements.
• Density significantly influenced the intercept of the LUE relationship where increasing the number of trees per hectare increased the volume growth per unit of absorbed light (Fig. 2a and Table 4).
• Clearly, it was warmer at the NC site (Table 1 and Fig. 1) and when examining only degree hours and accounting for potential loss of growth from cold temperatures, the NC site had considerably more degrees hours than the other sites.
• Respiration increases with increasing temperature (Maier, 2001), which could reduce the carbon available for stem growth with the generally higher temperature at the NC site.

Figures

Table 4 Parameter estimates for significant terms in the light use efficiency equation where I is absorbed photosynthetically active radiation, G is genetic entry (clones 1-4 (C1-C4), mass control-pollinated (MP), open pollinated (OP)), D is density (618, 1236, 1854 trees ha−1) and * denotes an interaction. These equations were used to generate regression lines in Fig. 2.

Table 2 Statistical summary (p values) for stand and growth increment metrics for three sites (Virginia (VA), North Carolina (NC) and Brazil (BR)) where Pinus taeda was planted using the same six genetic entries and three initial density levels. Growth data for all sites are after 5 years and the fifth growing season increment. Leaf area data for the BR site are from the fifth year, leaf area data for the VA and NC sites are for the eighth year.

Fig. 2. Light use efficiency (volume growth per unit absorbed photosynthetically active radiation (PAR)) for the eighth growth season at the Virginia (VA) and North Carolina (NC) sites and the fifth growing season at the Brazil (BR) site where the same Pinus taeda genetic entries were planted at three initial densities. Regression lines are for density main effects (a) where 618, 1236 and 1854 represent the corresponding density treatments (in trees ha−1) with the open pollinated genetic entry, and genetic entry main effects (b) (Clones 1-4 (C1–C4), MP (mass control pollinated), and OP (open pollinated)) with 1854 tree ha-1. Site was not a significant factor in the relationship. Each point represents one plot.

Table 3b Genetic entry treatment least square means for stand and growth increment metrics for three sites (Virginia (VA), North Carolina (NC) and Brazil (BR)) where Pinus taeda was planted using the same six genetic entries (4 clones (C1, C2, C3, C4), a mass-control pollinated family (MP), and an open pollinated family (OP)) and three initial density levels (618, 1236, 1854 trees ha−1). Growth data for all sites are after five years and the fifth growing season increment. Leaf area data for the BR site are from the fifth year, leaf area data for the VA and NC sites are for the eighth year. Lower case letters indicate means separation for the main effect treatment within a site.

Table 3b Genetic entry treatment least square means for stand and growth increment metrics for three sites (Virginia (VA), North Carolina (NC) and Brazil (BR)) where Pinus taeda was planted using the same six genetic entries (4 clones (C1, C2, C3, C4), a mass-control pollinated family (MP), and an open pollinated family (OP)) and three initial density levels (618, 1236, 1854 trees ha−1). Growth data for all sites are after five years and the fifth growing season increment. Leaf area data for the BR site are from the fifth year, leaf area data for the VA and NC sites are for the eighth year. Lower case letters indicate means separation for the main effect treatment within a site.

Fig. 3. Cumulative volume and cumulative degree hours (> 5 °C) where ambient temperature was between 5 and 38 °C, photosynthetically active radiation (PAR) was greater than zero (daytime) and if nighttime temperatures were below 0 °C no hours are counted the next day for three sites (Virginia (VA), North Carolina (NC) and Brazil (BR)) where the same Pinus taeda genetic entries were planted at three initial densities (a). In Panel b cumulative degree hours were estimated as in Panel a and included an adjustment for vapor pressure deficit (VPD) where hours when VPD exceeded 1.5 kPa were not included. Regression lines for each sites are shown. Each point is a site mean.

Table 1 Summary of environmental data including average annual air temperature, average daily maximum and minimum air temperature, annual precipitation and annul incident photosynthetically active radiation for three sites (VA=Virginia, NC=North Carolina, BR=Brazil) where the same genetic material of Pinus taeda was planted at all sites.

Fig. 1. Mean monthly air temperature (a), maximum air temperature (b), minimum air temperature (c), mean daily vapor pressure deficit (VPD) (d), mean monthly photosynthetically active radiation (PAR) (e), and mean monthly precipitation (f) for the sites in Virginia (VA), North Carolina (NC) and Brazil (BR) where the same Pinus taeda genetic entries were planted at three initial densities.

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Other factors including respiration and extreme climatic conditions may contribute to growth differences per unit degree hour and including these differences in the analysis would require a more detailed modeling effort to examine. The sites used in this study are ideally suited to continue testing additional hypotheses to explain the different growth between native and exotic P. taeda plantations because they have the same genotypes at all sites and consequently eliminate differences in genetics as a potential explanation for observed growth differences.