Visualizing daily in situ fungal hyphal dynamics: Responses in a tropical- versus Mediterranean-ecosystem forest

Background/Question/Methods

Globally, fungi are the second largest terrestrial group of organisms and produce a large fraction of stable organic matter. But, fungal production and turnover are poorly understood, especially in different environments. I used our Soil Observatory System (a robotic automated minirhizotron coupled to a soil sensor network) to observe fungal hyphae at daily intervals and monitor the soil dynamics (temperature, water content, CO2) at 5min intervals. Here I report on comparative responses of hyphal growth and mortality in two extreme environments, a tropical rain forest (La Selva Biological Station, Costa Rica), and a semiarid mixed conifer/deciduous forest (James Reserve, California).

Results/Conclusions

Fungal hyphal dynamics were distinctly different in the two ecosystems. La Selva had high temperatures (T) and high soil water content (wc) throughout measurement period whereas the James Reserve had a wide range in both variables. In the rainforest, small variations in daily averaged T (22 to 26^oC), and wc (0.33 to 0.62) resulted in large changes in hyphal length (HL). Daily HL was negatively related to wc and T, where HL= 191 (-100wc -5.22T), r2=0.401, p<0.0001. In the temperate forest, variation was high in T (1 to 24^oC)) and wc (0.05 to 0.26). HL was positively related to wc and negatively related to T, where HL=  -1.22*(0.165wc -0.14T), r2=0.395, p<0.0001. Relative hyphal growth (RHGR) was not significantly related to any variable (soil CO2, sapflow, PAR, T, wc) at the two sites. Standing crop is a function of production and mortality. In both ecosystems, both growth and mortality predominantly occurred at moderate T and wc values, and remained stable at both high and low T and wc. Overall HL appears coupled to changing T and wc. In the rainforest, at high wc, diffusion of CO2 out and O2 into the soil is constrained at high wc by a water layer. I hypothesize that the timing and individual root plant-fungal carbon exchange on diurnal and daily time scales determines individual hyphal growth and mortality.