Contribution of shade trees to wind dynamics and pathogen dispersal on the edge of coffee agroforestry systems: A functional traits approach

Through microclimate modifications, coffee agroforestry systems have both facilitative and inhibitory effects on the persistence of Hemileia vastatrix, a major fungal pathogen. Wind, a key element in the dispersal of H. vastatrix uredospores, is affected by shade tree presence, especially at the edge of agroforestry systems. However, little is known on how shade tree leaf functional and canopy-level architectural traits impact wind dynamics and subsequent airborne uredospore dispersal in this transition zone. In this study, we determine the contribution of shade tree leaf functional and canopy traits to changes in throughflow wind speeds and H. vastatrix uredospore dispersal at the edge of coffee agroforests across three size classes of the shade tree Erythrina poeppigiana (sparse (pruned shade trees), medium (conventional management) and dense (mature, larger, unpruned shade trees)). As expected, dense shade trees reduced throughflow speeds into the farm more frequently than the other size classes. Shade tree leaf functional traits (e.g. specific leaf area, leaf thickness, leaf angle) expressed high variability (coefficient of variations: 13.67%–89.48%) across the size classes and significantly predicted throughflow speed reductions (r² = 0.891, P = 0.002). Emerging mature uredospore counts were significantly lower (P < 0.001) on the leeward side of dense shade trees, yet high reductions in throughflow speed into the farm were related to increased airborne uredospore capture across all size classes in these agroforestry systems. Seemingly, trade-offs exist between reduced wind speeds that decrease uredospore dispersal and reduced wind speeds that encourage uredospore settling into the coffee canopy at the edge of farms. This suggests a level of “fluidity” in desirable shade tree traits throughout the agroforestry system design. Our findings highlight the complexity of uredospore movement in agroforestry systems and the important role of shade tree canopy and leaf functional traits in wind dynamics.