Hillside Starvation

Snow-on-the-mountain (Euphorbia marginata)

These luscious plants don't appear to have been affected by the drought except positively and in an ironic sense: they managed to have lived next to some helpless blackberry plants that require every-third-day watering by the sad gardener. If it weren't for the drought, there'd be little chance such a beautiful plant would be so tall and stout.

Imagine a continuous train of water molecules extending from the tips of root hairs all the way up a tree's internal system of hollow columns through the trunk, limbs, and twigs to the undersides of leaves where one after another molecule of water is allowed to evaporate from tiny openings called stomata. Interrupt this train at the leaf, and the whole train all the way down to the root hairs stops.
When a tree battles for life during a drought, the odds of winning are greatly decreased if temperatures are high. So in these hot and dry times, the trees attempt to reduce the rate of water evaporation through transpiration by closing the pores in their leaves. But when they do this, they cannot bring in necessary carbon dioxide and end up dying a death of carbon starvation. It's the carbon dioxide that provides one of the necessary ingredients to photosynthesis.
One recent report showed that drought-stricken trees (pinon pine trees, specifically in this study) died at a rate 28% faster if they were also subjected to temperatures 7 degrees higher than normal. Depending on the source, July in the Austin area averages a temperature of 84.3, but this July the average was about 5 degrees higher at 89.7 degrees. Beating the average by 5 degrees over the span of a month is a big deal. We set a heat record for the month of July. Combine that with the driest twelve-month period on record, and the stage is set for brown-out here on Whitman's Rough.
The leaves wilt because of loss of turgor pressure in the leaf's blade and petiole, but they lose water from more than just the stomata pores on the underside of the leaves. It's probably safe to say that many of the trees up on the hillside have long since initiated stomatal control, so now they are losing water directly through the leaf surface, the twigs' lenticels, the stems, and the roots. Now we are to the point with some of the trees that if we were to receive rains from off a passing hurricane, many of the leaves' stomata will never open again, meaning that food production within the plant will be delayed or denied permanently. All the parts of the plant that contribute to its photosynthetic ability (chloroplasts, for example) may themselves be damaged and then take too long to recover before the tree dies of starvation.
Tiny roots just beneath the ground surface die and then the plant couldn't suck up moisture if you poured barrels of water on it. As a thriving plant draws up moisture, it carries nutrients along with the water. So shutting off the water supply also shuts off the nutrient supply.
Some trees shed their leaves in a last ditch effort to save the plant and will grow new, if somewhat stunted, leaves when the drought ends. Others lose the leaves and die. But even if they live, next year's growth will be retarded by this year's drought. Cambial growth slows when water supplies run low, and it's this year's cambial growth that helps set the limits of next year's growth potential.

Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought

Abstract

Large-scale biogeographical shifts in vegetation are predicted in response to the altered precipitation and temperature regimes associated with global climate change. Vegetation shifts have profound ecological impacts and are an important climate-ecosystem feedback through their alteration of carbon, water, and energy exchanges of the land surface. Of particular concern is the potential for warmer temperatures to compound the effects of increasingly severe droughts by triggering widespread vegetation shifts via woody plant mortality. The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates—temperature-sensitive carbon starvation in response to a period of protracted water stress or temperature-insensitive sudden hydraulic failure under extreme water stress (cavitation). Here we show that experimentally induced warmer temperatures (≈4 °C) shortened the time to drought-induced mortality in Pinus edulis (piñon shortened pine) trees by nearly a third, with temperature-dependent differences in cumulative respiration costs implicating carbon starvation as the primary mechanism of mortality. Extrapolating this temperature effect to the historic frequency of water deficit in the southwestern United States predicts a 5-fold increase in the frequency of regional-scale tree die-off events for this species due to temperature alone. Projected increases in drought frequency due to changes in precipitation and increases in stress from biotic agents (e.g., bark beetles) would further exacerbate mortality. Our results demonstrate the mechanism by which warmer temperatures have exacerbated recent regional die-off events and background mortality rates. Because of pervasive projected increases in temperature, our results portend widespread increases in the extent and frequency of vegetation die-off.