Factors controlling Eucalyptus productivity: How water availability and stand structure alter production and carbon allocation

Abstract

Wood production varies substantially with resource availability, and the variation in wood production can result from several mechanisms: increased photosynthesis, and changes in partitioning of photosynthesis to wood production, belowground flux, foliage production or respiration. An understanding of the mechanistic basis for patterns in wood production within a stand and across landscapes requires a complete annual carbon budget. We measured annual carbon flows to wood production, foliage production and total belowground carbon flux (the sum of root production, root respiration, and mycorrhizal production and respiration) from ages three to five years in clonal Eucalyptus plantations at four sites in Brazil to test if fertility, water availability and stand structure changed wood production and by what mechanism. We also quantified the patterns in light interception and the efficiency of light use to provide additional mechanistic insights into growth responses and to determine if light-use efficiency was related to changes in flux and partitioning.

The routine level of forest fertilization at these four sites was high enough that further increases in nutrient supply did not increase wood growth. Irrigation increased wood net primary productivity (age three to five) from 1.45 to 1.84 kg m⁻² year⁻¹ of C (27%), because of increases in light interception (5%), photosynthetic efficiency (from 0.028 to 0.031 mol C/mol photons absorbed, 11%), gross primary productivity (from 3.62 to 4.28 m⁻² year⁻¹ of C, 18%), and partitioning to wood (from 0.397 to 0.430 of photosynthesis, 8%). These changes increased light-use efficiency by 20%. Annual flux belowground varied among sites from 0.43 to 1.0 m⁻² year⁻¹ of C but did not vary with water availability. Across the four sites for the irrigated and unirrigated treatments, light-use efficiency was positively correlated with gross primary productivity and partitioning to wood production. Increasing heterogeneity of stand structure (resulting from staggered timing of planting within plots) led to a 14% loss in wood biomass relative to uniform stand structure at age six. Light-use efficiency, gross primary productivity, and wood net primary productivity were lower, but not significantly so, in heterogeneous compared to uniform stands.

Introduction

The growth of wood in forests varies by more than a factor of two across local landscapes, and by more than 50% during the development of individual stands. The supply of resources (particularly water and nutrients) strongly influences wood production, but predicting how forest growth and ecosystem carbon storage respond to changes in resource supplies remains a challenge, particularly because the controls over carbon allocation are poorly understood (Landsberg, 2003, Trumbore, 2006). Considerable progress has been made for modeling the effects of climate and resources on wood production, and in some cases, other ecosystem carbon fluxes. However, lack of understanding of carbon allocation currently limits the capacity to model the forest carbon cycle, accurately predict the effects of global change on carbon cycling, and accurately predict forest productivity for new climates, sites and genotypes (Gower et al., 1997, Ryan et al., 1997, Friedlingstein et al., 1999, Landsberg, 2003, Litton et al., 2007).

Annual production per unit photosynthetically active light absorbed by the canopy (light-use efficiency, Monteith, 1972, Monteith, 1977) provides simple, basic insights into changes in productivity and carbon allocation. The ‘production’ in light-use efficiency has been defined as crop or dry matter yield (Monteith, 1977), gross primary production or photosynthesis (for example, Drolet et al., 2005), and wood production (Linder, 1985), but we will use wood production in this paper. Differences in light-use efficiency indicate differences in canopy photosynthesis, partitioning of the annual photosynthesis to different sinks or respiration, or both.

Measuring canopy photosynthesis is very challenging, and the three methods commonly used (leaf measurements plus models, eddy covariance, carbon budget) have limitations. Photosynthesis can be estimated by measuring photosynthetic capacity of the canopy, the responses of photosynthesis and stomatal conductance to the environment, and using simple (for example, Landsberg and Waring, 1997) or complex models (for example, Williams et al., 2001, Medlyn, 2004) to extrapolate to the canopy. However this requires careful sampling that adequately represents the sources of variation within the canopy and through time. The limitations of this approach are the difficulty in obtaining the measurements in a tree canopy, spatial and temporal variability in photosynthetic capacity and the responses of photosynthesis and stomatal conductance to the environment, and the accuracy of any model used. A second way to estimate photosynthesis is to use net ecosystem exchange measurements from eddy covariance (Curtis et al., 2005, Sacks et al., 2007). Respiration at night is adjusted to temperatures during the day using a temperature response function, and added to the net ecosystem carbon exchange in the day. The strength of this approach is that it is derived from whole-canopy measurements. The limitations are that eddy covariance often underestimates ecosystem respiration because of advective flow and lack of turbulence (Lavigne et al., 1997), temperatures at night are rarely encountered during the day (at least in the same season), foliar respiration during the day is likely less than at night (Kirschbaum and Farquhar, 1984), and eddy covariance requires a large area with uniform vegetation (∼0.5 km²), which makes assessing treatments and replication difficult. A third way to estimate photosynthesis is to measure the sinks and fluxes resulting from photosynthesis and sum them to get photosynthesis (Möller et al., 1954, Ryan, 1991). Estimation of total belowground carbon flux using soil respiration, litterfall, and carbon pool changes (Giardina and Ryan, 2002) has greatly aided this mass balance approach. The strengths of this approach are that it can be applied to small plots to assess treatment effects and that variability in respiration is lower than that for photosynthesis. The limitations are that much work is required to sample temporal and spatial variability and the accuracy of the models used to extrapolate measurements to the stand.

For this study, we used the third method of estimating photosynthesis, which has the additional advantage of providing estimates of the components of the carbon budget (Giardina et al., 2003, Maier et al., 2004, Ryan et al., 2004, Forrester et al., 2006, Litton et al., 2007, Stape et al., 2008, Bown et al., 2009). We considered carbon flux for five major components: foliage respiration, foliage net primary production, wood respiration, wood net primary production, and total belowground carbon flux (carbon flux to root growth and respiration, exudates and mycorrhizae). We also manipulated resources (Linder, 1981, Raison and Myers, 1992) and assess how efficiency, and the three components of carbon allocation (biomass, flux, partitioning, Litton et al., 2007) change when resources and structure change. These manipulations were done over a six-year rotation (where final tree height reached ∼60% of the site maximum for the clone) for four locations with different climates for fast-growth Eucalyptus in Brazil. Our objectives were to measure changes in the C budget across sites to (1) increases in nutrient and water supply and (2) the uniformity of tree sizes within plots (stand structure). A third objective was to assess the importance of changes in flux, partitioning, light capture, and light-use efficiency in driving these responses.