Climate & Plants

Climate & Plants

1. Temperature & Biochemistry

   • Life depends on biochemical reactions, which are regulated by temperature. Although there is an optimum temperature for most biochemical reactions, relatively few places on Earth are consistently warm enough for this to be achieved. Therefore, if we increase the global temperatures, we'd expect the rate of biological processes to increase.
   • Plant growth
     • In many cases, higher temps have been shown to increase plant growth rates
       • Most research has focused on trees
         • Spruce saplings in Yukon (Danby & Hick 2007)
           • They raised temperatures by about 1.8°c around naturally-established white spruce saplings in the Yukon. Found that warmed saplings had higher photosynthetic rates & grew taller than unheated controls.
         • Spruce & beech saplings (Bruhn 1998)
           • Shows how elevated temperature increased the growth rate of 1yr old European spruce & beech saplings. The response varies between the two species with spruce having a greater range & magnitude of responses than beech. This could have consequences for competition
   • Advancing Tree Lines
     • In addition to effects on sp. composition in forests, increasing temperature may also influence forest range.
     • The main barrier to forest regeneration is thought to be seedling mortality in exposed, cold conditions
       • E.g. at high altitudes & latitudes, the climate often becomes too cold for trees to survive. At this boundary, a treeline occurs
         • Any increase in sapling growth/survival due to increased temp is likely to see an advance in treelines
     • Harsch et al (2009) used M-A on 166 sites to look at treeline dynamics since 1900.
       • Air temp had increased at 111 of 166 sites - avg of 0.013°c/yr
         • Treeslines had: Advanced at 87 sites ; remained stable at 77 ; retreated at 2
           • Most response due to warmer winters as it affects sapling survival more than warmer summers
           • Diffuse treelines much more likely to advance than abrupt ones
   • In the tropics . . .
     • Freely et al (2007) examined tree growth in Panama & Malaysia
       • Found that decreasing growth rates were common - up to 71% of Panamanian tree sp. & 95% of Malaysian sp. had reduced growth rates
         • Change in growth was negatively correlated with increases in mean daily minimum temperature, an effect linked to photorespiration
   • Photosynthesis may be reduced due to WATER LOSS via transpiration.
     • If stomata are open to cool the leaf, excessive water loss may reduce photosynthesis
     • C4 plants have low stomatal conductance as they are adapted to hot/dry conditions so they may be particularly sensitive to heat stress
   • Photorespiration
     • During photosynthesis, RUBISCO adds CO2 to RuBP but as temperature increases, RUBISCO tends to add more O2 to RUBP (photorespiration)
     • Photorespiration effect depends on photosynthetic pathway
       • C3 plants evolved when there was more CO2 and photoresp was less of a problem. So it is likely to affect them more (Ehleringer & Bjorkman 1977)
         • Reported that photorespiration was stimulated much more in the C3 plant than a C4 plant
         • However, eCO2 increases photosynthesis rates so this might alleviate the impacts of temperature (Wong et al 2008)
           • Wong looked at the effects of eCO2 & heat stress on C3 & C4 plants and found that photosynthesis in C3 plants was much higher than in C4 plants
       • C4 plants are more recent & have evolved ways of reducing photoresp that allow the Calvin cycle to proceed as normal. Therefore they should be less affected.

2. Precipitation

   • Drought
     • According to various climate models (IPCC 2014) drought will be commonplace - particularly in Mediterranean-climates
       • Investigated rainfall and growth of Pinus brutia on 3 Greek islands (Sarris et al 2007)
         • Rainfall declined rapidly from 1970 & associated with this decline they found reduced tree-ring width (i.e. growth) in the species - the lowest annual stem increment on record. There was a close correlation between post-1960s precipitation & tree growth for all ages classes studied.
       • Unless saplings in these climates reach water quickly, they succumb to drought (Padilla & Pugnaire 2007)
         • Reduced rainfall & lowered water tables increase the possibility of succumbing to drought.  P & P report seedling survival is higher for sp. with deeper roots & so plant recruitment is differentially affected by drought. This has consequences for community composition
     • Changes in soil moisture caused by climate change will likely affect species distributions, community composition & diversity
     • Distributions of tropical trees in Panama (Engelbrecht et al 2007)
       • Distributions were greatly influenced by drought sensitivity which is species specific. This variation is likely to cause local extinctions & changes to community composition
   • Flooding
     • Increased rainfall in currently water-stressed regions could positively influence plant productivity, but not all plants would respond similarly, so outside of monocultures its hard to predict
     • Flooding has obvious ecophysiological effects on vegetation, including reduced O2 availability - especially when temperatures are high
       • Flood sensitive plants are severely damaged by 24hrs of anoxia - limits nutrient availability & increases soil toxicity due to reducing conditions
         • Some plants get around this problem by developing adventitious roots or other morphological adaptations
     • Flooding & Distribution (Van Eck et al 2004)
       • Flooding during the growing season is an important determinant of distribution along floodplains
         • Plants that are able to survive do so through variety of physiological mechanisms
         • Plants vary in response & so distribution is dictated by flooding scenarios