The effect of large-scale piñon mortality on tree-level photosynthetic capacity in a piñon-juniper woodland ecosystem
Abstract
Anthropogenic climate forcing is expected to increase temperature and decrease precipitation, leading to an increase in drought events throughout the southwest United States. An extended drought in this region between 1999 and 2002 resulted in widespread tree mortality, particularly affecting piñon-juniper (PJ) woodlands, where 40-95% of Pinus edulis (piñon) trees died across 1.5 million ha. To study the change in carbon dynamics triggered by these abrupt mortality events, we simulated the observed drought-related mortality by girdling ~1600 large piñon trees in a 4 ha PJ woodland site in September 2009. We have used a fully instrumented eddy covariance system to quantify how whole ecosystem-atmosphere carbon fluxes respond to and recover from this widespread mortality event. To resolve the ecosystem carbon dynamics post-mortality on a finer scale, we used leaf-level gas exchange measurements made bi-weekly from Fall 2011- Fall 2012 to compare the photosynthetic capacity of the remaining piñon and Juniperus monosperma (juniper) trees at the girdled site to a nearby control site. Our objectives were to 1) determine if these species are exhibiting competitive release in response to the mortality of piñon trees and 2) compare the contributions of each species to the overall carbon dynamics of the ecosystem. We did not find a significant difference between light-saturated measurements of photosynthetic rate (Amax) of either species in the girdled site compared to the control site over the course of the year. Similarly, we found no significant difference between the maximum carboxylation velocity of Rubisco (Vcmax) between small piñon trees in the girdled and control sites, although the Vcmax values were significantly lower for the juniper trees in the girdled site compared to the control site. These measured photosynthetic rates (both Amax and Vcmax) suggest that competitive release is not occurring in response to the mortality of the large piñon pines, despite the decreased canopy cover and expected reduction in competition for resources. The lower or comparable photosynthetic rates in the remaining trees at the girdled site mitigates the recovery trajectory of carbon dynamics in PJ woodlands following widespread mortality events. The seasonal variation in photosynthetic rates in both species matched up closely with flux tower data; periods of high photosynthetic rates for the individual trees corresponded to periods of high carbon uptake at the ecosystem level. However, juniper had higher photosynthetic capacity and maintained higher photosynthetic rates at high temperature and low soil water content compared to piñon. By combining high resolution structural data and these differences in leaf level gas exchange characteristics between the species, we are able to explain the overall functional differences in the response of ecosystem carbon dynamics to temperature and soil water availability observed between intact and disturbed PJ woodlands.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B53B0669L
- Keywords:
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- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0429 BIOGEOSCIENCES / Climate dynamics