Summary Aboveground xylem hydraulic conductance was determined in Scots pine (Pinus sylvestris L.) trees and stands from 7 to about 60 years of age. At the stand scale, leaf area index and net primary productivity (NPP, above- plus below- ground) increased and reached a plateau at about 25--30 and 15--20 years, respectively; both parameters declined in mature stands. Stand hydraulic conductance followed a similar trend to NPP, with a maximum at about 15--20 years and a pro- nounced reduction in old stands. At the tree scale, annual biomass growth per unit of leaf area (growth efficiency) de- clined with tree age, whereas aboveground sapwood volume per unit leaf area, which is linearly related to maintenance respiration costs, steadily increased. Radiation interception per unit leaf area increased significantly with reduced leaf area index of mature stands, despite increased foliage clumping in the canopies of mature trees. Needle nutrient concentration did not change in the chronosequence. Tree hydraulic conductance per unit leaf area was strongly and positively correlated with growth efficiency. We discuss our findings in the context of growth reductions in mature and old trees, and suggest that increased hydraulic resistance and maintenance respiration costs may be the main causes of reduced carbon gain in mature and old trees. Keywords: hydraulic architecture, Pinus sylvestris, respiration costs, transpiration. Introduction The pattern of variation in net primary productivity (NPP) of forest stands over the life cycle of the forest is well established (Waring and Schlesinger 1985). In a young stand, NPP first increases until canopy closure when a plateau is reached. Later, there is a steady decline in NPP with the progressive opening of the canopy. Part of the decline in NPP is attributable to reductions in stand density and leaf area index; however, large variations in growth efficiency of trees, E g (i.e., biomass or volume growth per unit leaf area; Waring 1983) also occur with age (Kaufmann and Ryan 1986, Kuuluvainen 1991). Traditionally, declines in stand NPP and E g have been re- lated to the progressive increase in growth and maintenance respiration associated with nonphotosynthetic tissues in ma- ture and old stands (Yoda et al. 1965, Kira and Shidei 1967), but alternative explanations have also been proposed including a reduction in the efficiency of radiation interception (Kuulu- vainen 1991), a change in the allocation patterns (Long and Smith 1992), or a reduction in the needle photosynthetic ca- pacity of old trees (Kull and Koppel 1987). When a tree ages, the pathway for water transport from the soil to the top of the canopy increases greatly. According to a simple Ohm’s Law analogy of water flow (Richter 1973), if height growth is not exactly matched by a steady increase in transport capacities per unit of tree leaf area, the difference in water potential between the soil and leaves, ∆Ψ, will have to increase to maintain a constant transpiration rate (Jarvis 1975, Zimmermann 1983). Increases in ∆Ψ during the tree life cycle may be detrimental, particularly if tissue water potentials fall below species-specific cavitation thresholds (Tyree and Sperry 1989, Tyree and Ewers 1991). As a consequence of this trade- off, old trees might have a lower time-averaged transpiration rate than young trees. However, few experiments have been performed to test whether the hydraulic conductance over the whole pathway changes with tree age (Yang and Tyree 1994, Saliendra et al. 1995, Mencuccini and Grace 1996). A lower transpiration rate may represent a constraint for tree growth because of stomatal limitations to photosynthesis (Jones 1992). Recently, Mencuccini and Grace (unpublished observa- tions) estimated aboveground xylem hydraulic conductance for Scots pine (Pinus sylvestris L.) trees ranging from 7 to about 60 years of age. We now use those results to estimate: (a) at the stand scale, the hydraulic conductance for the same stands; and (b) at the tree scale, the relationship between tree hydraulic conductance per unit of leaf area and growth effi- ciency, E g . Parameters related to sapwood maintenance respi- ration (i.e., tree sapwood volume), photosynthetic capacity (needle nitrogen concentration) and radiation interception (light extinction coefficients) are also presented. Hydraulic conductance, light interception and needle nutrient concentration in Scots pine stands and their relations with net primary productivity MAURIZIO MENCUCCINI 1,2 and JOHN GRACE 3 1 Istituto di Ecologia Forestale e Selvicoltura, Università degli studi di Firenze, Via S. Bonaventur a, 13, 50145 Firenze, Italy 2 Present address: Boyce Thompson Institute for Plant Research, Cornell University, Tower Road, Ithac a, NY 14853-1801, USA 3 Institute of Ecology and Resource Management, University of Edinburgh, Darwin Building, Mayfield Ro ad, EH9 3JU Edinburgh, U.K. Received June 8, 1995 Tree Physiology 16, 459--468 © 1996 Heron Publishing----Victoria, Canada by guest on July 14, 2011 treephys.oxfordjournals.org Downloaded from