Effects on Climate and Disturbance Regime

Herbivore-induced changes in vegetation structure likely affect soil temperature, relative humidity, erosion, soil moisture, and soil fertility (see Chapter 11). Changes in litter accumulation can affect ecosystem vulnerability or sensitivity to some disturbances, especially fire. However, relatively few studies have demonstrated herbivore effects on these variables. Herbivory increases vegetation porosity (Fig. 12.17) and penetration of light, precipitation, and wind to the understory and soil surface. Canopy opening greatly affects abiotic conditions in the understory (Chazdon and Fetcher 1984, Denslow 1995, Fernandez and Fetcher 1991). Increased soil warming as a result of penetration of sunlight may be offset to some extent by increased penetration of precipitation to the ground. Schowalter et al. (1991) reported that 20% loss of foliage mass doubled the amount of water reaching the soil surface. S. Chapman et al. (2003) also reported that herbivory by scale insects increased soil temperature and moisture. Reduced plant surface area reduces interception of precipitation and evapotranspiration (G. Parker 1983). Increased accumulation of litter resulting from herbivory in forest ecosystems may contribute to soil water retention.

Canopy opening over large areas by herbivores could affect regional climate. Although most studies of effects of canopy opening on climate have focused on anthropogenic canopy removal (see Chapter 11), herbivory may have similar effects (e.g., increased soil surface temperature, reduced evapotranspiration, and consequent regional warming and drying; J. Foley et al. 2003, Salati 1987).

Folivorous

I Increased canopy porosity resulting from herbivory. Holes chewed by folivorous insects in the large leaves of Cecropia increase the penetration of light, water, and airflow to lower strata.

FIG. 12.17

I Increased canopy porosity resulting from herbivory. Holes chewed by folivorous insects in the large leaves of Cecropia increase the penetration of light, water, and airflow to lower strata.

FIG. 12.17

Herbivory can increase or decrease the likelihood or severity of future disturbances. Herbivory in grasslands reduces the amount of standing dead material (Knapp and Seastedt 1986), potentially reducing the severity of fire but increasing soil exposure to desiccation and exacerbating effects of drought. However, reduced foliage surface area may reduce water demand and mitigate the effects of drought (Kolb et al. 1999). Herbivory in forests increases fuel accumulation in the form of fine and coarse litter material, thereby increasing the likelihood and severity of fire, especially in arid forests where litter decomposes slowly and lightning strikes are frequent (McCullough et al. 1998, Schowalter 1985). Bebi et al. (2003) concluded that spruce, Picea engelmannii, mortality to spruce beetle, Dendroctonus rufipennis, in Colorado, United States, did not increase the occurrence of subsequent fires. Pech (1993) found similar results for balsam fir, Abies balsamea, mortality to the spruce budworm, Choristoneura fumiferana, in eastern Canada. However, absence of subsequent fire in these cases may have reflected the rapid decomposition of spruce and fir litter (Bebi et al. 2003, Pech 1993).

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