Black spruce (Picea mariana) and eastern larch (Larix laricina) are two widespread coniferous boreal tree species with distinct life strategies, crown architecture, and resource use. Black spruce is a slow growing late successional species with evergreen phenology, while eastern larch is a fast growing early successional species with deciduous phenology. The crown architecture of black spruce is relatively narrow while eastern larch is characterized by wider canopy and lower degree of foliage clumping compared to black spruce. Moreover, the two species display contrasting stomata regulation strategies, with eastern larch being relatively isohydric while black spruce relatively anisohydric. Here, we used a unique dataset across the distributional range of these two boreal tree species to quantify inter-specific differences in their functional and structural traits and to investigate trade-offs in their resource allocation and use. We analysed the co-variation and inter-specific differences in functional foliage traits (e.g., specific leaf area, SLA; maximum photosynthetic capacity, Amax; water use efficiency, WUE) and structural traits (e.g., tree height, H; projected crown area, CA) collected across a 2000-km latitudinal climate and permafrost gradient in northwestern Canada, spanning from the southern- to the northern-edge of the boreal forest.
Results/Conclusions
Our results reveal significant inter-specific differences in foliage traits with black spruce having lower SLA, Amax and needle nitrogen concentration in comparison to eastern larch. Moreover, WUE, quantified with two methods, instantaneous needle gas exchange measurements and isotopic data, was lower for black spruce in comparison to eastern larch. Structural traits also varied significantly for the two species, with black spruce displaying lower H and CA in comparison to eastern larch. However, no significant interspecific differences were found in their Huber values (HV, i.e., the ratio of xylem sapwood area to the total needle area). To further investigate this pattern, we derived an analytical expression relating HV to the examined functional and structural traits. This framework allowed us to explain trade-offs in HV variability as a result of whole-plant functional and structural traits coordination. HV is a critical parameter that describes tree water transport and resource allocation, since it quantifies tree investments in conductive tissues (xylem) relative to evaporative tissues (foliage). These results shed light on whole-plant traits coordination at the boreal forest and provide novel insight into inter-specific differences in boreal tree species resource allocation and use.