Do cork oak trees benefit from a surrounding legume-rich, permanent pasture?

Background/Question/Methods

Biodiverse permanent pastures rich in legumes have been implemented in Portugal as a management tool to increase soil fertility, grassland productivity and animal population carrying capacity, and were later selected as a national land-use change strategy for increasing carbon sequestration and climate change mitigation. Sown, legume-rich improved pastures are commonly found in the understory of Mediterranean-type agroforestry systems – the Montados - with evergreen oaks, such as cork oak (Quercus suber L.), as the dominant tree species. However, little is known about the effects of these pastures on the physiology and productivity of co-occurring trees. Understanding the impact of sown, biodiverse pastures on cork oak, in particular assessing potential benefits from increased legume-nitrogen-fixation, can provide important information for future land-management and policy decisions. We examined a suite of leaf-level physiological and morphological traits in trees from a pair of adjacent natural and improved pastures at a cork-production site in South-Central Portugal. Nitrogen concentration, C/N, δ¹⁵N, δ¹³C, specific leaf area, photosynthetic and stomatal conductance rates, and leaf water potential were measured in cork oak trees across both land-use scenarios and different seasons.

Results/Conclusions

Both δ¹⁵N and nitrogen concentration in cork oak leaves were significantly different between pasture types (treatment) across time. δ¹⁵N was consistently higher (less negative) in the improved pasture than in the natural pasture, consistent with a higher percentage of legumes on the improved side. While nitrogen concentration was similar in both treatments at the beginning of the study, significant differences emerged later on with leaf-level N being higher in cork oaks growing on the improved pasture side. Despite these differences, and contrary to our initial hypothesis, there was no evidence of higher photosynthetic rates in cork oak from the improved pasture land-use. On the other hand, both photosynthetic rates and foliar nitrogen concentration results show a significant effect of time regardless of pasture type. Similarly, the difference in δ15N between pastures oscillated seasonally and between years. These observations together with measurements of water relations suggest a potential interaction between nitrogen and water availability in these systems that can modulate the impact of legume-rich pastures on cork oak physiology.