Species differences in crown structure and growth performance of juvenile loblolly and slash pine

Abstract

Few comparative studies exist with southern pines that examine species variation in crown structure in relation to growth performance for a range of silvicultural treatments and site types. Establishing these relationships at the interspecific level is essential for improving our understanding of growth strategies, development of crop ideotypes and species-site deployment decisions. In north-central Florida, USA, crown structure and tree growth were compared among three pine taxa—genetically improved loblolly pine (PTI; Pinus taeda L.), genetically improved slash pine (PEI; P. elliottii Engelm. var. elliottii) and unimproved slash pine (PEU)—that were planted in two replicated field experiments and each managed under two silvicultural treatment regimes (low intensity—no fertilizer additions or weed control; high intensity—sustained weed control and fertilizer additions). In comparison to slash pine, PTI consistently accumulated more crown biomass (5.3 kg per tree versus 3.3 kg per tree) and total leaf area (year 3>35 m 2 per tree, year 4>50 m 2 per tree versus year 3<20 m 2 per tree, year 4<35 m 2 per tree), and had greater live crown lengths (>4.0 versus <4.0 m) and crown widths (>2.0 m versus <2.0 m) across locations and treatments. Significant differences in crown structural traits (total branch number per tree, crown width and length) were associated with varying levels of growth performance. During this early phase of stand development, PTI was the most productive and it maintained more branches per tree and allocated more biomass to branches (>2.5 kg per tree) than either slash pine taxa (<2.0 kg per tree). The leaf:branch biomass ratio, used as an expression of crown development, varied significantly among taxa (1.5, 1.9, and 2.0, respectively, for PTI, PEI, and PEU). The lower leaf:branch biomass ratios for PTI appeared to be an important strategy that facilitated spacious crown development, higher leaf area carrying capacity and faster growth rates compared to PEI and PEU. Although the intensive silvicultural treatment significantly increased total leaf area accumulation compared to the less intensive treatment (i.e. 45, 51, and 50% more, respectively, for PTI, PEI, and PEU, averaged across sites at the age of 4 years), it had little impact on the relative distribution of leaf area along the crown profile. Significant differences in growth efficiency (volume produced per unit leaf area per year) were also detected among taxa (the respective values of PTI, PEI and PEU are 804, 1106, 1173 cm 3 stem wood m −2 leaf area), and the early advantage found for slash pine diminished as leaf area levels of PTI approached 55–65 m 2 per tree.