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Tree Saplings (tree + sapling)
Selected AbstractsInterannual changes in folivory and bird insectivory along a natural productivity gradient in northern Patagonian forestsECOGRAPHY, Issue 1 2004C. Noemi Mazía Trophic regulation models suggest that the magnitude of herbivory and predation (top-down forces) should vary predictably with habitat productivity. Theory also indicates that temporal abiotic variation and within-trophic level heterogeneity both affect trophic dynamics, but few studies addressed how these factors interact over broad-scale environmental gradients. Here we document herbivory from leaf-feeding insects along a natural rainfall/productivity gradient in Nothofagus pumilio forests of northern Patagonia, Argentina, and evaluate the impact of insectivorous birds on foliar damage experienced by tree saplings at each end of the gradient. The study ran over three years (1997,2000) comprising a severe drought (1998,1999), which allowed us to test how climatic events alter top-down forces. Foliar damage tended to increase towards the xeric, least productive forests. However, we found a predictable change of insect guild prevalence across the forest gradient. Leaf miners accounted for the greater damage recorded in xeric sites, whereas leaf chewers dominated in the more humid and productive forests. Interannual folivory patterns depended strongly on the feeding guild and forest site. Whereas leaf-miner damage decreased during the drought in xeric sites, chewer damage increased after the drought in the wettest site. Excluding birds did not affect leaf damage from miners, but generally increased chewer herbivory on hydric and xeric forest saplings. Indirect effects elicited by bird exclusion became most significant after the drought, when total folivory levels were higher. Thus, interannual abiotic heterogeneity markedly influenced the amount of folivory and strength of top-down control observed across the forest gradient. Moreover, our results suggest that spatial turnovers between major feeding guilds may need be considered to predict the dynamics of insect herbivory along environmental gradients. [source] The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannasGLOBAL CHANGE BIOLOGY, Issue 7 2003W. J. BOND Abstract The distribution and abundance of trees can be strongly affected by disturbance such as fire. In mixed tree/grass ecosystems, recurrent grass-fuelled fires can strongly suppress tree saplings and therefore control tree dominance. We propose that changes in atmospheric [CO2] could influence tree cover in such metastable ecosystems by altering their postburn recovery rates relative to flammable herbaceous growth forms such as grasses. Slow sapling recovery rates at low [CO2] would favour the spread of grasses and a reduction of tree cover. To test the possible importance of [CO2]/fire interactions, we first used a Dynamic Global Vegetation Model (DGVM) to simulate biomass in grassy ecosystems in South Africa with and without fire. The results indicate that fire has a major effect under higher rainfall conditions suggesting an important role for fire/[CO2] interactions. We then used a demographic model of the effects of fire on mesic savanna trees to test the importance of grass/tree differences in postburn recovery rates. We adjusted grass and tree growth in the model according to the DGVM output of net primary production at different [CO2] relative to current conditions. The simulations predicted elimination of trees at [CO2] typical of the last glacial period (180 ppm) because tree growth rate is too slow (15 years) to grow to a fire-proof size of ca. 3 m. Simulated grass growth would produce an adequate fuel load for a burn in only 2 years. Simulations of preindustrial [CO2] (270 ppm) predict occurrence of trees but at low densities. The greatest increase in trees occurs from preindustrial to current [CO2] (360 ppm). The simulations are consistent with palaeo-records which indicate that trees disappeared from sites that are currently savannas in South Africa in the last glacial. Savanna trees reappeared in the Holocene. There has also been a large increase in trees over the last 50,100 years. We suggest that slow tree recovery after fire, rather than differential photosynthetic efficiencies in C3 and C4 plants, might have been the significant factor in the Late Tertiary spread of flammable grasslands under low [CO2] because open, high light environments would have been a prerequisite for the spread of C4 grasses. Our simulations suggest further that low [CO2] could have been a significant factor in the reduction of trees during glacial times, because of their slower regrowth after disturbance, with fire favouring the spread of grasses. [source] Facilitation of tree saplings by nurse plants: Microhabitat amelioration or protection against herbivores?JOURNAL OF VEGETATION SCIENCE, Issue 2 2008Lorena Gómez-Aparicio Abstract Question: Positive interactions are predicted to be common in communities developing under high physical stress or high herbivory pressure due to neighbour amelioration of limiting physical and consumer stresses, respectively. However, when both stress sources meet in the same community, the relative importance of the two facilitation mechanisms is poorly understood. We ask: What is the relative importance of abiotic vs. biotic mechanisms of facilitation of tree saplings by shrubs in Mediterranean mountain forests? Location: Sierra Nevada, SE Spain (1800,1850 m a.s.l.) Methods: Saplings of four tree taxa (Acer opalus ssp. grana-tense, Quercus ilex, Pinus nigra ssp. salzmanii and P. sylvestris var. nevadensis) were planted following a 2 × 2 factorial design: two levels of herbivory (control and ungulate exclusion) and two microhabitats (under shrubs and in open areas). Sapling survival and growth were monitored for five years. Results: Shrubs had positive effects on sapling survival both in control and ungulate excluded plots. This effect was species-specific, with shrubs increasing the survival of Acer opalus and Quercus ilex three and twofold, respectively, but having a minor effect on the Pinus species. Herbivory damage was also species-specific, being much higher for Acer opalus than for any other species. Shrubs did not protect saplings of any species against ungulates. Thus, all Acer saplings (the most damaged species) suffered herbivory outside the exclosures, which largely reduced sapling height. Conclusions: Protection from abiotic stress (summer drought and winter frost) was much more relevant than protection from biotic stress (herbivory). However, we propose that the final balance between the two mechanisms can be expected to vary strongly between sites, depending on the relative magnitude of the different sources of stress and the intrinsic traits (e.g. palatability) of the species interacting. [source] | ||||||||||