Influences of canopy photosynthesis and summer rain pulses on root dynamics and soil respiration in a young ponderosa pine forest

Abstract

Our first objective was to link the seasonality of fine root dynamics with soil respiration in a ponderosa pine (Pinus ponderosa P. & C. Lawson) plantation located in the Sierra Nevada of California. The second objective was to examine how canopy photosynthesis influences fine root initiation, growth and mortality in this ecosystem. We compared CO2 flux measurements with aboveground and belowground root dynamics. Initiation of fine root growth coincided with tree stem thickening and shoot elongation, preceding new needle growth. In the spring, root, shoot and stem growth occurred simultaneously with the increase in canopy photosynthesis. Compared with the other tree components, initial growth rate of fine roots was the highest and their growing period was the shortest. Both above and belowground components completed 90% of their growth by the end of July and the growing season lasted approximately 80 days. The period for optimal growth is short at the study site because of low soil temperatures during winter and low soil water content during summer. High photosynthetic rates were observed following unusual late-summer rains, but tree growth did not resume. The autotrophic contribution to soil respiration was 49% over the whole season, with daily contributions ranging between 18 and 87%. Increases in soil and ecosystem respiration were observed during spring growth; however, the largest variation in soil respiration occurred during summer rain events when no growth was observed. Both the magnitude and persistence of the soil respiration pulses were positively correlated with the amount of rain. These pulses accounted for 16.5% of soil respiration between Days 130 and 329.

Process-based modelling of biogenic monoterpene emissions combining production and release from storage

“…Simulated gross primary productivity (GPP, Fig. 1b) also agreed well with observations (Misson et al, 2006), except for a short period in late July 2003, for which the simulated daily GPP was approximately reduced by 25% compared to the values derived from eddy flux data, likely due to an overestimation of drought stress in the model during that period. Measurements from other years indicate that Blodgett Forest experiences drought stress during summer, but the extent is less than in other Ponderosa pine forests with comparable climatic circumstances (Panek, 2004;Misson et al, 2004).…”

Looking deeper into the soil: biophysical controls and seasonal lags of soil CO2 production and efflux

“…Previous studies have shown that fine roots and mycorrhizal rhizomorph dynamics (Misson et al 2006, Heinemeyer et al 2007, Vargas and Allen 2008c and substrate supply and nutrient availability (Schimel et al 1994, Ruess et al 2003) are other biological drivers that influence P s . To date we do not have a common biological variable (other than GPP) among sites (e.g., root production, microbial biomass) that could allow us to test this hypothesis.…”

How do variations in the temporal distribution of rainfall events affect ecosystem fluxes in seasonally water-limited Northern Hemisphere shrublands and forests?

“…Other studies in arid and semi-arid ecosystems have observed differing responses of GPP and RE to rainfall pulse size (Huxman et al, 2004b;Potts et al, 2006;Arneth et al, 2006;Williams et al, 2009). The structure of respiration response has been attributed to the high sensitivity of soil microbes to rainfall events following drought conditions (Inglima et al, 2009;Misson et al, 2006;Xu et al, 2004;Lee et al, 2004;Jenerette et al, 2008), although the relationship between pulse size and duration of active soil respiration seems to saturate at moderate event sizes (Huxman et al, 2004b;Sponseller, 2007), perhaps due to exhaustion of labile carbon sources. The response of vascular plant photosynthetic activity to rainfall events is generally of longer duration than the response of microbial respiration, with both the magnitude of the response (Ignace et al, 2007;Chen et al, 2009;Williams et al, 2009;Scott et al, 2006) and the duration of physiological activity (Huxman et al, 2004b;Williams et al, 2009) increasing following larger rainfall pulses.…”