Functional variation and resource-usage strategies of seven tropical dry forest tree species: Wood and foliar traits

Background/Question/Methods

Information regarding plant functional traits is essential to detect resources-usage strategies employed by different species to define functional groups. Seven tropical dry forest tree species of Michoacán, México were studied to relate vessel diameter and frequency with (i) relative hydraulic conductivity (RC), (ii) vulnerability to cavitation (VC), (iii) wood density, and (iv) leaf traits. Wood and leaf samples were collected from tree species abundant in the study area, with a diameter at breast height (1.3 m high) of > 6 cm. Fresh leaves were weighed and scanned; leaf area, length and width of leaves were obtained from images using the ImageJ program and finally, we obtain leaf dry mass. With this data we were able to calculate: (i) leaf mass per area, (ii) leaf dry matter content, (iii) eccentricity and (iv) leaf area. Petiole length was measured with a caliper. Wood density (dry mass/green volume) was estimated with the water displacement method. Wood anatomical traits (vessels diameter and frequency) were obtained from transversal segments (20-30 µm) of wood samples cut on a slide microtome. RC and VC were estimated in each plant species as: RC=r⁴VF and VC=VD/VF, where r is vessel diameter, VF is vessel frequency in a square mm and VD is vessel diameter in square mm.

Results/Conclusions

A principal component analysis was conducted to determine functional variation axis and resources-usage strategies. Major axes of functional variation strategies in the seven studied species were related to attribute traits to define the resource-usage. Wood density showed a relationship with VC, RC, and petiole length. Vessel diameter and its frequency influenced VC and RC, while petiole length was related to the smaller vessel diameter. We identified two axis of functional variation. The first one was a drought tolerant axis related to leaf dry matter content; while the second was associated with an exploitation–competition strategy and linked with the leaf area. By the other hand, RC and VC were associated with the drought tolerant axis. Drought resistance is associated to species with dense tissues (e.g., Caesalpinia platyloba and Apoplanesia paniculata), while species that avoided drought showed higher RC and VC due to softwood (e.g., Spondias purpurea and Bursera fagaroides). We detected a relationship between leaf area and the axis of exploitation–competition strategy. Our results suggest that tropical dry forest wood density is a RC and VC indicator to determine resource-usage strategies in the studied species.