The apparent migration of the sun creates predictable differences in the seasonal intensity and angle of incoming radiation (red arrows). Incoming blue arrows represent the magnitude of precipitation (snow or rain) for the (A) spring, (B) pre-monsoon, (C) monsoon, (D) fall and (E) winter periods, with no input represented during the pre-monsoon dry summer. Both incoming radiation and input of water contribute to differences in soil moisture on opposing aspects. Hypothesized water loss and carbon uptake is shown for all seasons, with environmentally driven responses differing on each aspect. Arrows are not drawn to scale.
Semiarid forests in the southwestern USA are generally restricted to mountain regions where complex terrain adds to the challenge of characterizing stand productivity. Among the heterogeneous features of these ecosystems, topography represents an important control on system-level processes including snow accumulation and melt. This basic relationship between geology and hydrology affects radiation and water balances within the forests, with implications for canopy structure and function across a range of spatial scales. In this study, we quantify the effect of topographic aspect on primary productivity by observing the response of two codominant native tree species to seasonal changes in the timing and magnitude of energy and water inputs throughout a montane headwater catchment in Arizona, USA. On average, soil moisture on north-facing aspects remained higher during the spring and early summer compared with south-facing aspects. Repeated measurements of net carbon assimilation (Anet) showed that Pinus ponderosa C. Lawson was sensitive to this difference, while Pseudotsuga menziesii (Mirb.) Franco was not. Irrespective of aspect, we observed seasonally divergent patterns at the species level where P. ponderosa maintained significantly greater Anet into the fall despite more efficient water use by P. menziesii individuals during that time. As a result, this study at the southern extent of the geographical P. menziesii distribution suggests that this species could increase water-use efficiency as a response to future warming and/or drying, but at lower rates of production relative to the more drought-adapted P. ponderosa. At the sub-landscape scale, opposing aspects served as a mesocosm of current versus anticipated climate conditions. In this way, these results also constrain the potential for changing carbon sequestration patterns from Pinus-dominated landscapes due to forecasted changes in seasonal moisture availability.
Murphy P.C., Knowles J.F., Moore D.J.P., Anchukaitis K., Potts D.L., Barron-Gafford G.A. (2020): Topography influences species-specific patterns of seasonal primary productivity in a semiarid montane forest. Tree Physiology 40(10): 1343–1354. DOI: 10.1093/treephys/tpaa083
This Paper/Book acknowledges NSF CZO grant support.