Interspecific competition varies with hardwood species identity but has little impact on stand-level productivity in southeastern pine-hardwood mixtures after 23 years.

Background/Question/Methods

Intraspecific competition among trees can negatively affect forest health and productivity. Establishing pine-hardwood mixtures is one potential strategy for reducing intraspecific competition; however, little information exists regarding the compatibility of southeastern tree species grown in mixture, impacts of initial density on mixed stand development, or whether mixed stands can produce overyielding. To answer these questions, an experiment was established comparing the survival and growth of shade intolerant sweetgum (Liquidambar styraciflua L.), moderately shade tolerant cherrybark oak (Quercus pagoda Raf.), and shade intolerant loblolly pine (Pinus taeda L.) in two-species pine-hardwood mixtures (sweetgum:loblolly pine and cherrybark oak:loblolly pine) for 23 years on a high-quality site in northern Louisiana, USA. Initial pine-hardwood mixture densities included: low (1,482 TPH - 50pine :50hardwood), medium (1,729 TPH- 57pine:43hardwood), balanced-high (1,976 TPH - 50pine:50hardwood), and unbalanced-high (1,976 TPH - 37pine:63hardwood). We also compared loblolly pine survival and growth in mixed stands versus monocultures at a common initial density (1,482 TPH) to assess whether mixtures could produce overyielding.

Results/Conclusions

Loblolly pine mean annual diameter and basal area growth statistically exceeded sweetgum and cherrybark oak at all mixture densities in the first decade since planting. However, over the last six years of the study, loblolly pine mean survival was significantly lower than that of sweetgum. Moreover, loblolly pine mean annual diameter growth was statistically indistinguishable from sweetgum in all but the unbalanced high-density mixtures over the last 13 years of the study. In contrast, loblolly pine mean annual diameter, height, and basal area growth continued to significantly exceed cherrybark oak in low and medium-density mixtures over the last 13 years of the study. At low-density, loblolly pine survival, mean annual diameter growth, and mean annual height growth in both mixture types were similar to that found in monoculture and produced statistically less cumulative stand-level basal area growth. Stand-level survival and growth in mixture with sweetgum modestly exceeded that produced in mixture with cherrybark oak at low, medium, and balanced high-densities, but these differences were not significant. Collectively, these results indicate that neither hardwood species produced complementary interactions with loblolly pine in mixture. In addition, while limited to low-density, our results demonstrate that pine-hardwood mixtures did not produce overyielding.