Forests & Climate
All plant species exist in an envelope, a niche of climate conditions. These sets of climate conditions unique to each species are often referred to as ecosystems, vegetation communities, and/ or habitat types, depending upon location and scale. They represent a way to order and rank ecotypes based on these several variables and there are at least seventeen climate variablesthat actively contribute to these various plant communities. They include:
Mean annual temperature
Mean annual precipitation
Growing season precipitation
Mean cold month temperature
Minimum cold month temperature
Mean warm month temperature
Maximum warm month temperature
Annual moisture index
Summer moisture index
Degree-days > 5 °C
Degree-days < 0 °C
Frost-free period
Last spring frost
First fall frost
Growing season degree-days > 5 °C
Summer-winter temperature differential
Date degree-days > 5 °C reaches 100
Minimum degree-days <0 °C
Light quality- amount and intensity of radiation striking the earth.
And all green plants convert carbon from the atmosphere into carbon-based energy compounds through the process of photosynthesis.
Once synthesized, these carbon-based energy compounds may be allocated to seed production (regeneration), leaf production (photosynthetic capacity), root production (water and nutrient up-take), radial growth (storing of water and carbon), or respiration (the intake of CO2 and the respiring of oxygen (O). Allocation of scarce carbon resources to any one active plant process reduces available carbon resources for these other essential growth functions.
The priority carbon allocation for most plants and trees, including conifers is respiration- the opening and closing of the stomata or gas exchange valves. These microscopic valves are typically found on the underside of leaves and needles, although some conifer species (e.g., true firs) may have them on both sides. These exchange valves enable the active process of assimilating carbon dioxide (C02) from the atmosphereand building the carbon-based energy compounds critical for all plant functions. This respiration function is a priority because only through the opening of these valves can trees uptake the essential carbon. However, there is a risk and a loss of essential water when these valves are open. Some species, (e.g., the pines) are more sensitive to water loss and will often actively close these valves to prevent damaging water loss. Closing the stomata to reduce water loss also has a cost. It reduces the all-important uptake of carbon. Other factors influence stomatal function. Solar radiation- amount and intensity also influence stomatal function. Research has shown that short wave radiation (i.e., UV light) causes early and premature stomatal closure, especially in the pine species. This closure reduces the water pressure within the tree causing the tree to emit a pheromone that attracts bark beetles. Scale this phenomena up to an ecosystem and it becomes more apparent why we lost the lodgepole pine forest from the Mexican border to the Arctic Circle. And indeed, Figure 3 indicates an upward trend in the amount and intensity of short-wave radiation striking the earth’s surface.
Carbon allocation in conifers under drought stress may force the tree to prioritize more carbon resources to be allocated to mere survival and defense at the expense of reproduction. However, the more carbon resources allocated to reproduction, the greater the likelihood of successful regeneration.
Only two things kill trees: lack of carbon intake- carbon starvation and/ or hydraulic failure. Conifers often exhibit both- carbon intake failure and hydraulic failure, whereas deciduous trees often exhibit hydraulic failure only. Hydraulic failure occurs when water losses through transpiration exceed water intake. Carbon starvation is the depletion or even partial depletion of non-structural carbon resources in response to exchange valve closure, limited carbon assimilationand long-term carbon storage dependency. However, the two are not mutually exclusive. Carbon pool depletion and reduced assimilation could have a direct impact on tree hydraulics and thus hydraulic failure.
Trees respond to stress in two ways: ‘fight’ or ‘flight’. They may ‘fight’ climate stress by allocating more carbon resources to survival functions like growth and defense mechanisms at the cost of reproduction. Because perennials reproduce over many years and any one year is typically not a reproduce or die situation, this is the path most trees are expected to take. Early seral species- like lodgepole pine, ponderosa pine, and western larch tend to fight climate stress with these mechanisms.
Alternatively, increasing reproduction increases the probability that the seeds will fall and successfully germinate on favorable sites nearby or acceptable sites in a suitable environment- flight. Climax species like red cedar, true firs, and hemlock apparently use this ‘flight’ technique in response to stress, in this case climate related stress (Fig. 1).
Figure 1: grand fir (left photo) and red cedar (right photo) produced an unusually large crop of cones in the very droughty year of 2022 in the inland northwest region of the US in what may well be an effort to adapt to climate change through ‘flight’.
Figure 2. It is very likely that the Earth will experience a faster sustained rate of climate change in the 21st century than has occurred in the previous 10,000 years. High-resolution proxy data, surface temperature records, and climate models have been used to establish the structure and magnitude of large-scale natural climate variability over the past 1,000 years. However, since the mid-20th century, surface temperatures have warmed significantly—with the greatest warming occurring since the 1970s. Evaluating all observational records and using climate models for testing various processes, this recent change in global climate temperature can only be explained by including the effects of changing greenhouse gas concentrations in the atmosphere. (Image courtesy of the National Center for Atmospheric Research.)
Figure 3. Climate data from the National Oceanic and Atmospheric Administration (NOAA) beginning in the mid 1990’s indicate an ever-increasing component of short-wave radiation (UV light) making it past the earth’s protective layers and striking the earth’s surface.
References
Hartmann, H. (2015). Carbon starvation during drought-induced tree mortality – are we chasing a myth?. Journal of Plant Hydraulics, 2, e005. https://doi.org/10.20870/jph.2015.e005
Jeffrey D., Emily V. Moran, and Stephen C. Hart. Fight or flight? Potential tradeoffs between drought defense and reproduction in conifers. 2019. Tree Physiology 39, 1071–1085.
Nagel, Linda Marie, "Even-aged and multiaged ponderosa pine: A physiological comparison of standstructure and productivity" (2000). Graduate Student Theses, Dissertations, & Professional Papers. 10602
Lacointe, André, Carbon allocation among tree organs: A review of basic processes and representationin functional-structural tree models.Annals of Forest Science. 57 (2000) 521–533.