Inter-tree competition patterns in un-debarked cork oak stands in Portugal

Context

Cork oak plantations are usually established with higher densities than those observed in adult naturally regenerated stands. Since there are no recommendations on thinning time, this is subjectively defined by the landowners. If thinning can be delayed until the second cork extraction, tree selection can be based on cork quality and not only on tree form, size and vigour. Understanding inter-tree competition and how it affects stand structure and tree growth is relevant to support the definition of optimal stand densities, thinning schedules and ultimately cork quality.

Objective

Assess inter-tree competition in permanent plots established in un-debarked, even-aged stands located in Portugal by i) comparing with the self-thinning line and ii) assessing when stand density affects cork production, iii) stand structure and iv) tree relative growth rate (RGR) pattern over tree size. Spacing coefficient (SC) values < 1.25 are assumed to affect cork production; therefore, detecting when stand density is approaching this value is important. Relative spacing (RS) is easier to evaluate, being important to find the RS value equivalent to a SC = 1.25.

Results

Young cork oak stands (< 25 years) are far from the self-thinning line, but density-dependent mortality was detected in older stands with higher densities. SC decreases with stand age, with values < 1.25 (equivalent to an RS = 0.5) observed only in the stands close to self-thinning. In young stands, RGR in diameter decreases with tree size, indicating that competition does not affect growth efficiency of small trees, but the relationship between RGR and tree diameter tends to stabilise over time due to the suppression of small trees. The RGRs of larger trees (diameter at breast height class > 20) do not decrease over time, indicating that competition is ineffective to their growth.

Recommendations

The most common stand densities of young cork oak plantations may not lead to serious inter-tree competition before the first cork extraction. When the stand is approaching values of SC = 1.25 (RS = 0.5), a thinning operation should be performed to favour trees with slower growth. If those values are not achieved, it is advisable to wait until a second cork and tree quality assessment, with poor cork quality selected for thinning. Assessment of inter-tree competition status may primarily consider RS rather than SC, since this variable is easier to measure as it is not dependent on individual tree crown measurements.

Impacts and weaknesses

Knowledge of inter-tree competition status before the first cork debarking supports the schedule and intensity of thinning operation decisions. Information on stand growth patterns can support the decision of delaying thinning until the second cork extraction, favouring high cork quality producers, to bring economic benefits.
The self-thinning line used in this study might overestimate stand density, as it was developed based on the National Forest Inventory data that assumes trees bifurcated below 1.30 m as several individual trees, while in this study, 1.0 m was assumed as the rule.

Future developments

The establishment of thinning trials in young un-debarked cork oak stands is crucial to complement the present knowledge on inter-tree competition in young stands. Information of inter-tree competition should be used to improve cork oak growth and yield models (i.e. ALCORNOQUE and SUBER). Additionally, the inclusion of size–density trajectories in those models is desirable to establish individual mortality rules, required for predicting density-dependent stand mortality.