STEM ANATOMY AND RELATIVE GROWTH-RATE IN SEEDLINGS OF A WIDE-RANGE OFWOODY PLANT-SPECIES AND TYPES

Stem traits were analysed in laboratory-grown seedlings of 80 European woody and semiwoody species of known potential relative growth rate ( RGR) and of similar ontogenetic phase. The objectives were, firstly, t o assess the relation between stem structure and plant growth potentia l and, secondly, to explore how stem structure varies among species di ffering in life form and leaf habit. Hydraulic conductance was represe nted by the mean diameter of the widest xylem conduits (Dmax), and str uctural strength by the percentage of xylem tissue occupied by cell wa ll material (CWx) or stem tissue density (SD). Across all species RGR showed a weak positive correlation with Dmax and weak negative ones wi th CWx and SD, with slow-growers showing great dispersion of stem trai t values. In the RGR-Dmax relationship this dispersion disappeared whe n trees were removed from the analysis. None of the relationships were significant among tree species alone. It was suggested that fast-grow ers require a xylem with wide conduits thigh (Dmax) to achieve high hy draulic conductivity, and ''cheaply'' constructed stems (low CWx and S D) to maximise allocation to leaves. However, the possession of such t raits does not guarantee fast growth, as other factors may constrain R GR elsewhere in the plant. Deciduous seedlings showed higher Dmax and lower CWx than evergreens. Higher Dmax could reflect an innate higher tolerance of conductivity loss by freeze-induced embolism in deciduous plants, which are not burdened by the maintenance of foliage in winte r. In contrast, life forms were differentiated most clearly by SD. For instance, shrub seedlings had less dense stem tissues than tree seedl ings, possibly because they need less investment in long-term strength and stature.