Topography influences species-specific patterns of seasonal primary productivity in a semiarid montane forest

Authors: MURPHY, P. - University Of Arizona; Knowles, John; MOORE, D.J.P. - University Of Arizona; ANCHUKAITIS, K. - University Of Arizona; POTTS, D.L. - Buffalo State College; BARRON-GAFFORD, G.A. - University Of Arizona

Submitted to: Tree Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2020
Publication Date: 6/29/2020
Citation: Murphy, P., Knowles, J.F., Moore, D., Anchukaitis, K., Potts, D., Barron-Gafford, G. 2020. Topography influences species-specific patterns of seasonal primary productivity in a semiarid montane forest. Tree Physiology. 40(10:1343-1354. https://doi.org/10.1093/treephys/tpaa083.
DOI: https://doi.org/10.1093/treephys/tpaa083

Interpretive Summary: Montane forests are responsible for the majority of atmospheric carbon dioxide removal (via photosynthesis) in the western United States. However, future trends in the health of these forests are unknown and large-scale forest monitoring is complicated by steep slopes and variable topography in complex mountain terrain. In particular, small-scale differences in slope orientation determine patterns of snow accumulation and melt that affect moisture availability to forest vegetation throughout the year. As a result, this research measured seasonal photosynthesis from two co-dominant tree species located on opposing slopes in Arizona, USA. Soil moisture was higher on north-facing slopes that receive less sunlight and therefore retain moisture for longer periods of time. This translated to higher photosynthesis rates on north- versus south-facing slopes by ponderosa pine trees, but not Douglas fir trees, during the spring and summer. Regardless of landscape position, ponderosa pine trees maintained higher photosynthesis rates into the dry fall season relative to Douglas fir trees, potentially due to differences in drought adaptation between species. Notwithstanding, Douglas fir trees used water more efficiently during this time. As a result, this research contributes to current understanding of when and where mountain forests may respond to forecasted warming and/or drying via opportunistic (ponderosa pine) versus conservative (Douglas fir) growth strategies with respect to moisture availability on the landscape.

Technical Abstract: Semiarid forests found in the southwestern United States 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 seasonal response of two co-dominant native tree species to topographic variation in the timing and magnitude of energy and water inputs throughout a montane zero-order basin in Arizona, USA. On average, soil moisture was 34% higher on north- versus south-facing aspects during the spring and summer, and repeated measurements of net carbon assimilation (Anet) showed that Pinus ponderosa was sensitive to this difference, while Pseudotseuga menziesii 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 supports the potential for increased water use efficiency in response to future warming and/or drying, but at lower rates of production relative to more drought adapted species. At the sub-landscape scale, opposing aspects served as a mesocosm of current versus anticipated climate conditions. These results constrain both aspect- and warming-driven carbon sequestration reductions from Pinus-dominated landscapes due to forecasted changes in seasonal moisture availability.