Thomas E. Kolb

Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought?

Severe droughts have been associated with regional-scale forest mortality worldwide. Climate change is expected to exacerbate regional mortality events; however, prediction remains difficult because the physiological mechanisms underlying drought survival and mortality are poorly understood. We developed a hydraulically based theory considering carbon balance and insect resistance that allowed development and examination of hypotheses regarding survival and mortality. Multiple mechanisms may cause mortality during drought. A common mechanism for plants with isohydric regulation of water status results from avoidance of drought-induced hydraulic failure via stomatal closure, resulting in carbon starvation and a cascade of downstream effects such as reduced resistance to biotic agents. Mortality by hydraulic failure per se may occur for isohydric seedlings or trees near their maximum height. Although anisohydric plants are relatively drought-tolerant, they are predisposed to hydraulic failure because they operate with narrower hydraulic safety margins during drought. Elevated temperatures should exacerbate carbon starvation and hydraulic failure. Biotic agents may amplify and be amplified by drought-induced plant stress. Wet multidecadal climate oscillations may increase plant susceptibility to drought-induced mortality by stimulating shifts in hydraulic architecture, effectively predisposing plants to water stress. Climate warming and increased frequency of extreme events will probably cause increased regional mortality episodes. Isohydric and anisohydric water potential regulation may partition species between survival and mortality, and, as such, incorporating this hydraulic framework may be effective for modeling plant survival and mortality under future climate conditions.

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Observed increase in local cooling effect of deforestation at higher latitudes

Deforestation in mid- to high latitudes is hypothesized to have the potential to cool the Earth’s surface by altering biophysical processes. In climate models of continental-scale land clearing, the cooling is triggered by increases in surface albedo and is reinforced by a land albedo–sea ice feedback. This feedback is crucial in the model predictions; without it other biophysical processes may overwhelm the albedo effect to generate warming instead. Ongoing land-use activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback, and it is not known whether the intrinsic biophysical mechanism on its own can change the surface temperature in a consistent manner. Nor has the effect of deforestation on climate been demonstrated over large areas from direct observations. Here we show that surface air temperature is lower in open land than in nearby forested land. The effect is 0.85 ± 0.44 K (mean ± one standard deviation) northwards of 45° N and 0.21 ± 0.53 K southwards. Below 35° N there is weak evidence that deforestation leads to warming. Results are based on comparisons of temperature at forested eddy covariance towers in the USA and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectation of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models.

Physiological response to groundwater depth varies among species and with river flow regulation

We investigated the physiological response of two native riparian tree spe- cies (Populus fremontii and Salix gooddingii) and one exotic species (Tamarix chinensis) to groundwater availability along gradients of depth to groundwater at two rivers in Arizona. Depth to groundwater (DGW) at the dam-regulated Bill Williams River (BWR) was relatively constant and shallow (,4 m). Populus fremontiiat BWR did not experience reduced water availability at deeper groundwater depths, as evidenced by high predawn water potential. However, leaf gas exchange of P. fremontii was sensitive to high vapor pressure deficit where surface flow was ephemeral at BWR. Lower predawn water po- tentials of S. gooddingii at BWR suggested reduced water availability at deeper ground- water depths, but these reductions did not adversely affect net photosynthetic rate. Along the range of depth to groundwater at BWR, all three species suffered little canopy dieback, and dieback was not related to depth to groundwater. Depth to groundwater at the free- flowing Hassayampa River (HRP) was much greater and declined more rapidly in the ephemeral reaches than at BWR. Both P. fremontii and S. gooddingii experienced reduced water availability at deeper groundwater depths at HRP, as evidenced by lower predawn water potential. Both species also experienced reduced leaf gas exchange at deeper groundwater depths. Canopy dieback of all species was higher at HRP than at BWR and increased with increasing DGW, especially when DGW fell below 3 m. There was evidence to support branch sacrifice in these three riparian tree species as a means of improving water status in the surviving shoot. However, branch sacrifice was insufficient to prevent mortality in some of the native trees where DGW fell below 3 m at HRP. In contrast to the native species, T. chinensis showed no change in water availability, leaf gas exchange, or canopy dieback with increasing DGW at either river. Leaf gas exchange was lower and dieback was greater for T. chinensis at HRP where depth to groundwater was greater than at BWR, but there was no mortality at either river. Our results show that deep groundwater is more detrimental to the physiological condition of P. fremontii and S. gooddingii than it is to T. chinensis. Also, the pronounced differences in DGW and tree physiological performance between BWR and HRP suggest that dam regulation can in- crease water availability to mature trees in some desert riparian ecosystems. Finally, our study also provides estimates of the range of DGW that can maintain healthy, mature P. fremontii and S. gooddingii trees.

Ponderosa pine mortality following fire in northern Arizona

Sampling of 1367 trees was conducted in the Side wildfire (4 May 1996), Bridger-Knoll wildfire (20 June 1996) and Dauber prescribed fire (9 September 1995) in northern Arizona ponderosa pine forests (Pinus ponderosa). Tree mortality was assessed for 3 years after each fire. Three-year post-fire mortality was 32.4% in the Side wildfire, 18.0% in the Dauber prescribed fire, and 13.9% in the Bridger-Knoll wildfire. In the Dauber and Side fires, 95% and 94% of 3-year post-fire mortality occurred by year 2, versus 76% in the Bridger-Knoll wildfire. Compared with trees that lived for 3 years after fire, dead trees in all fires had more crown scorch, crown consumption, bole scorch, ground char, and bark beetle attacks. Logistic regression models were used to provide insight on factors associated with tree mortality after fire. A model using total crown damage by fire (scorch + consumption) and bole char severity as independent variables was the best two-variable model for predicting individual tree mortality for all fires. The amount of total crown damage associated with the onset of tree mortality decreased as bole char severity increased. Models using diameter at breast height (dbh) and crown volume damage suggested that tree mortality decreased as dbh increased in the Dauber prescribed fire where trees were smallest, and tree mortality increased as dbh increased in the Side and Bridger-Knoll wildfires where trees were largest. Moreover, a U-shaped dbh–mortality distribution for all fires suggested higher mortality for the smallest and largest trees compared with intermediate-size trees. We concluded that tree mortality is strongly influenced by interaction between crown damage and bole char severity, and differences in resistance to fire among different-sized trees can vary among sites.

Bark Beetle Attacks on Ponderosa Pine Following Fire in Northern Arizona

Abstract There is little quantitative information on relationships between insect attacks and fire damage for ponderosa pine, Pinus ponderosa Douglas ex Lawson, in the southwestern United States. Tree mortality and insect attacks were measured on 1,367 trees for three years after a spring wildfire (4 May 1996), a summer wildfire (20 June 1996), and a fall prescribed fire (9 September 1995) in northern Arizona. Western pine beetle, Dendroctonus brevicomis LeConte, mountain pine beetle, D. ponderosae Hopkins, roundheaded pine beetle, D. adjunctus Blandford, red turpentine beetle, D. valens LeConte, Ips species, and wood borers in the Buprestidae and Cerambycidae families were found in fire-damaged trees. The most frequently occurring insects, listed from most to least frequent, were wood borers, red turpentine beetle, Ips spp., western pine beetle, roundheaded pine beetle, and mountain pine beetle. Trees attacked by Dendroctonus and Ips spp. as a group had more crown damage from fire than unattacked trees. The percentage of trees attacked by Dendroctonus and Ips species was lowest during the fall fire (11%, 25 of 222 trees), intermediate during the summer fire (19%, 154 of 833 trees), and highest during the spring fire (41%, 127 of 312 trees). More than one-half of all wood borer colonization (58%) and attacks by western pine beetle (68%), roundheaded pine beetle (56%), and Ips spp. (66%) occurred in the first year after the fire. Measures of tree damage from fire and insect attacks were used to develop logistic regression models of tree mortality to quantitatively investigate factors that influenced tree mortality. Tree mortality 3 yr postfire was low until crown damage by fire exceeded 70–80% for unattacked trees, 40–50% for trees with partial attacks by Dendroctonus and Ips species, and 30–40% for trees with mass attacks. We concluded that several Dendroctonus and Ips species colonize fire-damaged ponderosa pines in northern Arizona and colonization is promoted by heavy crown damage from fire.

Land management and land-cover change have impacts of similar magnitude on surface temperature

The direct effects of land-cover change on surface climate are increasingly well understood, but fewer studies have investigated the consequences of the trend towards more intensive land management practices. Now, research investigating the biophysical effects of temperate land-management changes reveals a net warming effect of similar magnitude to that driven by changing land cover.

Homeostatic Maintenance Of Ponderosa Pine Gas Exchange In Response To Stand Density Changes

Homeostatic maintenance of gas exchange optimizes carbon gain per water loss. Homeostasis is regulated by short-term physiological and long-term structural mechanisms, both of which may respond to changes in resource availability associated with competition. Therefore, stand density regulation via silvicultural manipulations may facilitate growth and survival through mechanisms operating at both short and long timescales. We investigated the responses of ponderosa pine (Pinus ponderosa) to stand basal area manipulations in Arizona, USA. Stand basal area was manipulated to seven replicated levels in 1962 and was maintained for four decades by decadal thinning. We measured basal area increment (BAI) to assess the response and sustainability of wood growth, carbon isotope discrimination (A) inferred from annual rings to assess the response of crown gas exchange, and ratios of leaf area to sapwood area (A(l):A(s)) to assess longer term structural acclimation. Basal area treatments increased soil water potential (r2 = 0.99) but did not affect photosynthetic capacity. BAI increased within two years of thinning, and the 40-year mean BAI was negatively correlated with stand basal area (r2 = 0.98). delta was negatively correlated with stand basal area for years 5 through 12 after thinning (r2 = 0.90). However, delta was relatively invariant with basal area for the period 13-40 years after initial thinning despite maintenance of treatment basal areas via repeated decadal thinnings. Independent gas exchange measurements verified that the ratio of photosynthesis to stomatal conductance was invariant with basal area, but absolute values of both were elevated at lower basal areas. A(l):A(s) was negatively correlated with basal area (r2 = 0.93). We hypothesize that increased A(l):A(s) is a homeostatic response to increased water availability that maximizes water-use efficiency and whole-tree carbon uptake. Elevated A(l):A(s) of trees at low basal areas was associated with greater resilience to climate, i.e., greater absolute BAI during drought; however, trees with high A(l):A(s) in low basal area stands also exhibited the greatest sensitivity to drought, i.e., greater relative decline in BAI.

Drought predisposes piñon–juniper woodlands to insect attacks and mortality

To test the hypothesis that drought predisposes trees to insect attacks, we quantified the effects of water availability on insect attacks, tree resistance mechanisms, and mortality of mature piñon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma) using an experimental drought study in New Mexico, USA. The study had four replicated treatments (40 × 40 m plot/replicate): removal of 45% of ambient annual precipitation (H2 O-); irrigation to produce 125% of ambient annual precipitation (H2 O+); a drought control (C) to quantify the impact of the drought infrastructure; and ambient precipitation (A). Piñon began dying 1 yr after drought initiation, with higher mortality in the H2 O- treatment relative to other treatments. Beetles (bark/twig) were present in 92% of dead trees. Resin duct density and area were more strongly affected by treatments and more strongly associated with piñon mortality than direct measurements of resin flow. For juniper, treatments had no effect on insect resistance or attacks, but needle browning was highest in the H2 O- treatment. Our results provide strong evidence that ≥ 1 yr of severe drought predisposes piñon to insect attacks and increases mortality, whereas 3 yr of the same drought causes partial canopy loss in juniper.

Carbon and water fluxes from ponderosa pine forests disturbed by wildfire and thinning

Disturbances alter ecosystem carbon dynamics, often by reducing carbon uptake and stocks. We compared the impact of two types of disturbances that represent the most likely future conditions of currently dense ponderosa pine forests of the southwestern United States: (1) high-intensity fire and (2) thinning, designed to reduce fire intensity. High-severity fire had a larger impact on ecosystem carbon uptake and storage than thinning. Total ecosystem carbon was 42% lower at the intensely burned site, 10 years after burning, than at the undisturbed site. Eddy covariance measurements over two years showed that the burned site was a net annual source of carbon to the atmosphere whereas the undisturbed site was a sink. Net primary production (NPP), evapotranspiration (ET), and water use efficiency were lower at the burned site than at the undisturbed site. In contrast, thinning decreased total ecosystem carbon by 18%, and changed the site from a carbon sink to a source in the first posttreatment year. Thinning also decreased ET, reduced the limitation of drought on carbon uptake during summer, and did not change water use efficiency. Both disturbances reduced ecosystem carbon uptake by decreasing gross primary production (55% by burning, 30% by thinning) more than total ecosystem respiration (TER; 33-47% by burning, 18% by thinning), and increased the contribution of soil carbon dioxide efflux to TER. The relationship between TER and temperature was not affected by either disturbance. Efforts to accurately estimate regional carbon budgets should consider impacts on carbon dynamics of both large disturbances, such as high-intensity fire, and the partial disturbance of thinning that is often used to prevent intense burning. Our results show that thinned forests of ponderosa pine in the southwestern United States are a desirable alternative to intensively burned forests to maintain carbon stocks and primary production.